nutrient value of digestate

Upload: tony-deligiorgis

Post on 02-Jun-2018

222 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/10/2019 Nutrient Value of Digestate

    1/44

    Nutrient Value of Digestate from Farm-Based BiogasPlants in Scotland.

    Report for Scottish Executive Environment and

    Rural Affairs Department - ADA/009/06.

  • 8/10/2019 Nutrient Value of Digestate

    2/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Contents

    1 Executive Summary.......................................................................................................... 3

    2 Introduction ....................................................................................................................... 42.1 Project purpose .................................................................................................................. 42.2 Project background............................................................................................................. 42.3 Technical background ........................................................................................................ 42.4 Nutrient benefit ................................................................................................................... 52.5 Project objectives ............................................................................................................... 6

    3 Methodology and Approaches.............................................................................................. 73.1 Review of existing research ............................................................................................... 73.2 Chemical analysis of input slurry and digestate output...................................................... 73.3 Further investigation of the effectiveness of plant nutrients in digested slurry.................... 8

    4 Review of existing research.................................................................................................. 94.1 Digestate nutrient content .................................................................................................. 94.2 Nitrogen emissions during storage and following land application .................................. 124.3 Nitrogen fertiliser replacement value (following land application).................................... 14

    5 Chemical analysis of input slurry and digestate output....................................................... 175.1 Site 1. Ryes Farm............................................................................................................. 175.2 Site 2. Corsock Farm........................................................................................................ 185.3 Digester operation ............................................................................................................ 205.4 Analysis and nutrient content of digester feedstock and digestate.................................. 215.5 Conclusions on chemical analysis of slurry and digestate............................................... 26

    6 Further investigation of the effectiveness of plant nutrients in digested slurry ................... 286.1 Proposals for field assessment of digestate nutrient value.............................................. 28

    6.2 Proposed modelling appraisal of nutrient fluxes following land application of digestate. 316.3 Other suggestions for action ............................................................................................ 31

    Acknowledgements ................................................................................................................. 32

    7 References...................................................................................................................... 33

    Annex A Anaerobic Digestion and Digestate Analysis..................................................... 36

    Annex B Glossary of Terms ............................................................................................. 37

    Annex C Nutrient content of livestock slurries before and after anaerobic digestion....... 39

  • 8/10/2019 Nutrient Value of Digestate

    3/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    1 Executive Summary

    There is a strong body of opinion that, among the claimed benefits of anaerobicdigestion, there are improvements in the effluent (digestate) quality as a result of thedigestion process. This project considered this aspect of anaerobic digestion via:

    a detailed technical review of published and unpublished research data, and

    a short-term study of two farm-scale digesters in SW Scotland.

    Results of the detailed study of the farm-scale plant were reviewed in the context of the

    main findings within the technical review. During anaerobic digestion (AD), organiccompounds are broken down by bacteria resulting in the production of methane andcarbon dioxide. As a result of the digestion process a number of changes in slurryanalysis can be expected. These include a substantial reduction (up to 25%) in solidscontent and a consequential increase in ash content, due to the conservation of mineralsand reduced slurry carbon (and organic matter content). Increases in slurry pH (up to 0.5pH units) and ammonium nitrogen (N) content (up to 25%) may also occur, though thesechanges are less consistent than the reduction in solids content and organic mattercontent, and may be transient or dependent on digester operating conditions and theanalysis of the feedstock slurries.

    Because of the increase in slurry ammonium-N content, usually with increased pH andreduced solids content, there is a risk of increased emissions of ammonia during post-digestion storage. Such increased emissions have been confirmed by Danish researchbut have been shown to be effectively controlled by a range of store coverings. Althoughthe increased pH and ammonium-N content might be expected to increase risk of

    ammonia emissions following application of slurry to the land, the reduced solids contentwould be expected to improve surface infiltration of the slurry which should help toconserve slurry N. Low emission application techniques are recommended for ADtreated slurries.

    Increased ammonium-N content of slurries, even with reduced ammonia emissions, doesnot guarantee improved crop recovery and utilisation of slurry N and increased savings infertiliser N. The limited research covering agronomic assessments has generated mixed

    results with small, short term, or inconsistent benefits. On the basis of availableevidence, it is recommended that farmers with AD slurries should at least have anoccasional laboratory analysis of digestate quality; this should include dry matter content,total and ammonium-N content, for which rapid field assessment techniques are alsoavailable.

    Th i t id f th lit t d f th t h t t

  • 8/10/2019 Nutrient Value of Digestate

    4/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    2 Introduction

    2.1 Project purpose

    The purpose of this project is to identify whether there is any greater nutrient benefitderived from farm slurry that has been processed through anaerobic digestion (as someempirical farm work in South West Scotland and work undertaken by the DanishAgricultural Advisory Service has suggested), and to assess the likely costs of furtherresearch work on its use as a fertiliser.

    2.2 Project background

    As part of a strategy1to improve the quality of Scottish bathing waters, the ScottishExecutive funded a number of projects to pilot innovative approaches to reducing theimpact on bathing waters from diffuse agricultural pollution. One of these projects hasbeen the installation of anaerobic digestion (biogas) plants on farms in the Sandyhills andSaltcoats catchments, both in the South West of Scotland2. Its purpose has been toexamine the potential of anaerobic digestion as a tool to reduce the bacterial content of

    slurry prior to it being applied to land. Initial results have shown significant reductions.This would indicate that spreading the resulting digestate on the land would present areduced risk of Faecal Indicator Organism (FIO) contamination of bathing waters. Thisstudy is to seek further clarification on whether there are other environmental benefitsthat may accrue through changes in the chemical composition of the major plantnutrients.

    The aim of this project is to review existing relevant research, to analyse the chemical

    characteristics of slurry before and after it has been through an anaerobic digestion plant;and to advise on the means to measure the effectiveness (benefits and risks to theenvironment) of anaerobically digested farm slurry as a fertiliser.

    This research will complement earlier farm biogas pilot studies undertaken by theExecutive to examine the potential of digesting livestock slurry to reduce the risk ofbacterial pollution to bathing waters.

    2.3 Technical backgroundDuring the anaerobic digestion process, organic compounds are broken down, firstly viaacetogenic bacteria to methane precursors, largely volatile fatty acids (VFAs) and then tomethane and other products via methanogenic bacteria. Under anaerobic conditions,organic forms of nitrogen (N) are converted into ammonium-N (NH4-N), i.e. readilyavailable nitrogen The readily available nitrogen (RAN) content of cattle slurry is

  • 8/10/2019 Nutrient Value of Digestate

    5/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    A number of studies have demonstrated apparently significant changes in slurrycomposition following anaerobic digestion (Hobson et al.,1974; Baldwin 1993). The

    results of some early research in Germany (Vetter et al., 1987) showed a small reductionin slurry solids content, a decline in organic N content and an increase in NH4-N content(from 50% to about 60% of total N). However, without detailed information about therepresentative nature of the sampling (doubts about balancing of daily slurry input,sedimentation within store and whether the slurry input can be considered comparable tothe digester output) such analyses can be misleading. The current project provided anopportunity for the study of nutrient transformations following anaerobic digestion of cattleslurries, with samples collected according to a rigorous protocol. The aim was to ensurecomparability of raw slurry with the digestate and to contribute a robust and reliable set ofresults to the critical review process.

    Nitrogen can be taken up directly by plants as NH4-N or, more rapidly, as nitrate-N (NO3-N) following nitrification, a process which occurs very rapidly in fertile soils underfavourable conditions. The plant uptake of N from digested (readily available Nenriched) manures might therefore be expected to be closer to that from commercialfertilisers, as a result of the digestion process and may be regarded as a morepredictable source of N than raw slurry of lower RAN content. However, it must also beremembered that in the NH4-N form, slurry N is more vulnerable to environmental losses.Substantial losses may occur to the atmosphere, as NH3gas, both during slurry storageand, especially following land application. Furthermore, following the rapid conversion ofNH4-N to NO3-N in the soil, further losses to surface and ground waters can readily occurthrough nitrate leaching and, to the atmosphere, as nitrous oxide gas (N2O) followingdenitrification.

    Changes in slurry P availability may also occur as a result of the release of P fromorganic forms during digestion, leading to an increase in the water-soluble P fraction.This may increase the vulnerability of slurry P to losses by surface run-off or via by-passflow through field drainage systems, unless application practices are carefully managedand controlled.

    This study will highlight the extent of nutrient transformations within cattle slurries duringanaerobic digestion, taking account of other evidence within the published literature and

    in unpublished research reports accessed via national and international contacts. Thefindings of this work will inform proposals on the need for, and the structure of, potentialfuture research on the nutrient benefits and dis-benefits of AD in Scotland.

    2.4 Nutrient benefit

    The nutrient parameters included in the study were dry matter (DM) (solids content)

  • 8/10/2019 Nutrient Value of Digestate

    6/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Improved crop responses, increased yield and savings in inorganic fertiliser nutrientinputs;

    Improved predictability of manure nutrients for utilisation by crops;

    Reduced risk of scorch to growing crops.

    Assessment of these potential benefits will be based upon the available evidence in thescientific and research literature and on the monitoring undertaken within the project.However, nutrient benefit will ultimately depend upon overall manure management on theproducing unit; for example:

    Nature of slurry, e.g. dilute slurry of low solids content and high NH4-N content, will bechanged to a lesser extent by digestion than a high DM slurry;

    Losses of ammonia following land application may be impacted by other componentsof slurry analysis, e.g. DM content, pH; and by timing and method of application;

    Availability of adequate slurry storage on the unit (influencing timeliness ofapplication);

    Post-digestion physical treatment, such as solids-liquids separation;

    Range and extent of cropping on the farm;

    Crop growth stage at the time of application;

    Soil type and land accessibility.

    The research outputs will include recommendations on the best way to maximise

    potential benefits and on the need for future research.2.5 Project objectives

    The overall objectives of the project were, thus, to provide:

    (1) review of existing research on the environmental benefits and the nutrient value offarm slurry digestate from anaerobic biogas systems;

    (2) comparative chemical analysis of farm slurry and the digestate resulting from the

    anaerobic digestion of that slurry; and

    (3) proposals for field trials to evaluate crop response to farm slurry and biogas digestateand, thus, to determine the potential chemical fertiliser replacement value of digestatecompared to untreated slurry; also for a modelling approach to undertake a range ofscenario analyses on the nutrient benefit and the likely wider environmental impacts of

  • 8/10/2019 Nutrient Value of Digestate

    7/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    3 Methodology and Approaches

    ADAS and SAC Commercial Ltd met representatives of SEERAD at a Project InceptionMeeting to establish working links and agree project approaches, including the selectionof preferred farm-scale digester sites for sampling and monitoring and the detailedsampling protocol.

    3.1 Review of existing research

    In order to identify suitable information sources, some preliminary networking and initial

    scoping of known reference material was undertaken, to identify further key referencedata. This also included an outline internet search of known research organisations inEurope and USA, for example, the FAO RAMIRAN network conference proceedings andresearch database (www.ramiran.net). Follow-up requests for papers and reports weremade, initially largely via existing relevant contacts; RAMIRAN network, N Europeannetwork of specialists (Danish Agricultural Advisory Centre), EU-AGRO-BIOGAS STREPproject (T Amon, University of Natural Resources and Applied Life Sciences, Vienna).Relevant analytical data and technical information were also collected from recent

    projects in the UK (e.g. the Holsworthy project, Devon).

    3.2 Chemical analysis of input slurry and digestate output

    Careful site selection of two representative farm-scale digesters was agreed using localknowledge (SAC) and in consultation with the installing company (Greenfinch Ltd,Bishops Castle, Shrops) and the project Steering Group. Selection criteria includedconsideration of:

    Range and type of livestock;

    Livestock feeding system;

    Match of digester with livestock slurry production and calculated retention times;

    Potential for homogeneous and consistent feedstock and representative sampling;

    Location, management and capacity of the farm to accommodate sampling visits.

    Sampling and analysis costs

    The detailed work plan included provision for sampling of the two farm sites on twooccasions per week over a four-week period. Two samples were collected on eachoccasion (input and outlet samples), i.e. a total of 18 samples per site and 38 samples intotal, including separate samples from each of the digestate stores.

  • 8/10/2019 Nutrient Value of Digestate

    8/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Analyses included: DM (solids content), organic matter, pH, total N, NH4-N, NO3-N, totalP, bio-available P (water soluble),total K, total S, total Mg, total Na.

    The sampling was carried out by experienced scientific staff from SAC, using standardoperating procedures and within the agreed protocol. On collection, the samples werecooled and refrigerated, then submitted for analysis within 24 hours to SAC, AnalyticalServices Department, a designated UKAS accredited laboratory.

    3.3 Further investigation of the effectiveness of plant nutrients in digested slurry

    The analysis dataset was considered in relation to information on digester operating

    conditions. The potential implications of these results were evaluated in the context ofother research results and published information and further research needs considered.

  • 8/10/2019 Nutrient Value of Digestate

    9/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    4 Review of existing research

    The anaerobic degradation of organic substances to the most reduced form of methane(CH4) is a microbial process. The energy released in the process is mostly recovered inthe methane. The degradation of organic substances is a complex process, involving (i)(slow) enzymatic hydrolysis and the formation of sugars, amino acids and fatty acids; (ii)(fast) acetogenesis of volatile fatty acids (VFAs) and (iii) methane (and CO2) formation. Anumber of groups of bacteria are involved in the various stages. Details of the processare available from a number of sources (Hobson et al.,1974; Mller, H.B., 2001; Burtonand Turner, 2003) and an appreciation of at least part of the biochemistry will assist inunderstanding the nutrient transformations occurring during digestion and the nutrientcontent of the final digestate product.

    pH and buffer capacity the equilibrium of CO2and bicarbonate (HCO3-) with ammonium

    cations (NH4+), exerts substantial buffering on digestate pH, the breakdown of organic

    acids generating CO2and, hence, carbonic acid in solution:

    CO2+ H2OH2CO3HCO3- + H+

    VFAs decrease the buffering capacity of the bicarbonate ions:RCOO-H + NH4HCO3RCOO-NH4+ H2CO3

    While the addition of NH3will increase bicarbonate in balancing the generation of CO2:

    CO2+ H2O + NH3NH+

    4+ HCO-3

    The higher the bicarbonate concentration, the greater the buffering in solution andresistance to changes in pH. The optimum pH varies according to the stage in thedegradation process.

    Volatile fatty acids the effect of VFA levels on the micro-organisms involved in theprocess is complicated by their impact on pH; with near neutral pH, the VFAs have notoxic effect on the methanogenic bacteria at concentrations < 10,000 mg/l.

    Ammonia is formed during the breakdown of proteins and, where free NH3is formed, canact as a potent inhibitor of methanogenesis. Thus, it can be seen that pH and

    temperature (via its effect on pH) can have a strong effect on the NH3concentrations andthe stability of CH4generation. It is reported that up to 1500 mg/l as NH4+can be

    tolerated though, with acclimatization, stable operation has been demonstrated at NH4-Nconcentrations of up to 8000 mg/l (van Velsen, 1979).

    Against this background, the evidence available from the research community and in theliterature and also the supporting analytical data collected from the farm plant

  • 8/10/2019 Nutrient Value of Digestate

    10/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    been mixed feedstocks (e.g. slurry from both cattle and pigs) and this is particularly thecase in centralised anaerobic digesters (CADs) where feedstock materials such as

    abattoir and food processing wastes have been widely used. These materials will have asubstantial impact on digestate analysis. In some cases, mean results from farmdigester sites were for different numbers of digested and undigested slurry samples.

    Where it is thought that meaningful and reliable comparisons of digester input and outputanalyses can be made, the changes in analyses have been summarised in Table 1, withreference to further information in the relevant appendix tables.

    Table 1: Change in nutrient content as a result of anaerobic digestion (comparison

    between digester input and output expressed as % except for pH units)

    Table1 Location Substrate DM N-total NH4-N P2O5 pH COD

    A1 Suffolk Cattle & pig -10.0 13.0 15.0 18.0 0.45 -38A2 Yorks Pig -21.0 - 40.0 -6.2 0.5 -41A3 Kent Dairy cattle -29.5 -11.5 -12.4 -12.7 -0.09 -33.3A4 N Ireland Beef cattle -26.1 -14.3 8.7 - 0.4 -24.4A5 Scotland Dairy cattle -19.2 - 10.2 - - -17.2

    A8 Denmark Cattle - -7.0 32.0 - - - Denmark Pig - 0 14.0 - - - Denmark Pig - 0 13.0 - - - Denmark Pig - 0 42.0 - - - Denmark Sep. solids - 0 45.0 - - - Denmark Sep. solids - 3.0 52.0 - - -A10 USA (NY) Dairy cattle -25.2 10.4 33.3 3.2 0.5 -41.9A11 USA (Wisc.) Dairy cattle -35.4 -6.6 24.9 -8.4 0.6 -38.5

    A13 USA (NY) Dairy cattle -27.3 6.7 36.5 2.1 0.7 -30.3 USA (NY) Dairy cattle -25.1 0.9 27.7 0 0.18 -9.3 USA (NY) Dairy cattle -60.3 -4.6 11.3 -6.2 0.3 -61.3 USA (NY) Dairy cattle -11.1 3.5 37.7 5.9 0.29 -9.0 USA (NY) Dairy cattle -16.4 -5.5 31.1 10.9 0.22 -14.3

    Mean -25.6 -0.8 25.7 0.7 0.4 -29.9 Median -25.15 0 29.4 1.05 0.4 -31.8 Observations 12 16 18 10 11 12

    1 Note source data from each of these sites presented in the appendix tables listed.

    Data from all of these research sites showed a reduction in slurry dry matter (DM)content as a result of anaerobic digestion with, overall, a difference of c.25% betweeninput and output slurry DM content (Table 1). This reflects the breakdown of organic

  • 8/10/2019 Nutrient Value of Digestate

    11/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    when averaged across the range of the more reliable data, they disappear (Table 1).The consistency in total content of N, P2O5and K2O (confirmation of the anticipated lack

    of change), in fact, gives greater confidence in the reliability of the changes observed inDM, NH4-N and in pH.

    While much of the recent research data relate to performance of farm scale digesterplants, some much earlier pilot-scale research has also provided valuable insight, e.g.studies on the impact of factors on the efficiency of digester performance includedretention times (Summers and Bousfield, 1978). In these experiments at the RowettResearch Institute, optimum retention time for pig slurry digestion proved to be 10 days.

    Although this work showed generally increasing reduction in slurry DM content, BOD andCOD, with increasing retention time, in contrast to much of the other research reportedabove, a short retention time resulted in increased slurry NH4-N content, with theopposite effect apparent with longer retention times (Fig. 1).

    Although this result at first appears contradictory to other evidence, this reflects thecomplexity of the process, with many different bacteria demanding N as well as energyfrom the mix of substrate materials available. In general, livestock manures supply a

    surplus of N, so there will usually be an increase in digestate NH4-N content as proteinsare broken down in digestion. However, this state of flux will also depend upon thebalance of nutrients including carbon supply, C:N ratio and the extent of bacterial growthand N utilisation.

    0

    1

    2

    3

    4

    5

    3 days 5 days 7 days 10 days

    SlurryDMc

    ontent%

    0

    0.5

    1

    1.5

    2

    2.5

    Ammonium-Ng/l

    DM input

    DM outputNH4-N input

    NH4-N output

  • 8/10/2019 Nutrient Value of Digestate

    12/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    content. This reflects the solubilisation of some of the organic P as a result of thedigestion process.

    In recent years, there has been increasing interest in centralised anaerobic digestion(CAD) plants. While comparative data on the analysis of feedstock and the digestate aregenerally unavailable, it is of interest to note the often high nutrient content of the output.Data from two CAD plants are presented in Table 2.

    Table 2: Comparison of digestate analysis for two centralised anaerobic digestion(CAD) plants

    CAD Plant Total-N

    kg/m3

    NH4-N

    kg/m3

    NH4-N/N

    % total

    P2O5

    kg/m3

    K2O

    kg/m3

    DM

    %

    Holsworthy1

    (England)

    6.6 5.0 75.8 3.3 4.5 5.8

    Ribe average

    1992-96 (Denmark)

    4.9 3.2 65.3 2.4 4.2 5.8

    Ribe2

    1992(Denmark) 4.6 3.1 67.4 2.1 4.2 6.4

    Cattle slurry(Denmark)

    4.7 2.7 57.4 1.4 5.3 8.5

    Pig slurry (Denmark) 5.3 3.7 69.8 3.4 2.8 6.0

    1Feedstocks by volume 57% dairy cow slurry, 19% blood, 11% food waste, 8% chicken manure, 5%other non-farm waste. Results relate to May 2004.2

    Feedstocks (1992) by volume 84% from 71 farms (56 dairy, 7 pig, 3 mixed, 5 mink or poultry),16% from industry (mostly from an abattoir)Sources: Holsworthy - OSullivan, C.M. and Cumby, T.R. (2004).

    Ribe - Holm-Nielsen et al., 1997

    The relatively high total N and P2O5 content of the Holsworthy CAD digestate is likely tobe due to the blood used as a feedstock. Blood has a high total N content (>15 kg/m3)compared to dairy cow slurry (c.4 kg/m3undiluted). For the Ribe CAD in 1992, 14% ofthe total N in the feedstock came from industry. An estimate of total N from blood for theHolsworthy CAD is >40%.

    4.2 Nitrogen emissions during storage and following land application

    One of the possible consequences of the increase in slurry pH and NH4-N contentfollowing anaerobic digestion is an increased risk of NH3 losses during storage and after

  • 8/10/2019 Nutrient Value of Digestate

    13/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table 3: Analysis of pig slurry used in storage and application method experiments

    Dry matter Total N NH4-N NH4-NYear Slurry type

    %

    pH

    kg/m3

    % of total2002 Undigested 3.4 7.4 4.3 3.1 72

    2002 Digested 3.2 8.1 5.2 3.7 71

    2002 Digestedseparated

    2.1 8.3 4.8 3.6 75

    2003 Undigested 3.3 7.2 3.7 2.4 65

    2003 Undigested

    separated

    1.5 8.6 4.9 3.9 80

    2003 Digested 2.8 8.1 4.3 2.9 67

    2003 Digestedseparated

    2.2 8.2 4.2 3.4 81

    In the first storage season, the slurry stores were covered with a 15 cm layer of Leca(lightweight-expanded clay aggregates) which resulted in low nitrogen losses from allslurry types. In 2003, however, the stores were left uncovered and, as anticipated, NH3losses increased from digested and separated slurries, with the greatest loss being fromseparated undigested slurry (Table 4). These results were attributed to the elevated pHand low DM content in these slurries (Table 3).

    Table 4: Monthly relative loss of nitrogen from covered and non-covered storeswith the four slurry types indicated as percentage of the initial nitrogen content.

    Storage

    period

    Cover

    treatment

    Undigested

    slurry

    Digested

    slurry

    Separated

    undigestedslurry

    Separated

    digestedslurry

    09/01-01/05/2002

    Covered* 0.8 0.9 - -0.1

    20/03-06/05/2003

    Uncovered 2.5 4.4 6.1 4.4

    * Slurry stores for each slurry type covered with a 15 cm layer of Leca (lightweight-expandedclay aggregates)

    Ammonia losses were measured following application of 30 m3/ha of the slurries bytrailing hoses to spring barley. The lowest losses following application were from thedigested and separated slurries particularly in 2003 (Table 5). The reduced loss fromdigested and/or separated slurries reflects the likely quicker infiltration into the soil as a

    lt f th l DM t t

  • 8/10/2019 Nutrient Value of Digestate

    14/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table 5: Ammonia loss following land application via trailing hoses to springbarley, losses expressed as a % of NH4-N applied

    Year Undigested slurry Digested slurry Separatedundigested slurry(liquid fraction)

    Separateddigested slurry

    (liquid fraction)

    2002 27 22 - 26

    2003 46 34 23 18

    Some recent work in Finland has compared NH3and greenhouse gas (N2O & CH4)

    emissions following undigested and digested pig and cattle slurry applications (Reginaand Perl, 2006). In field experiments in 2005-06 pig slurry was applied on barley withtarget soluble-N application rates of 100 kg/ha. With injected (undigested or digested)pig slurry, NH3emissions were undetectable. Where the slurries were band spreadbefore sowing the barley crop NH3emissions continued until the slurry was incorporated(one hour after application). There were no statistical differences between emissionsfrom the different slurries. When the slurries were band spread into the growing crop twoweeks after sowing, NH

    3emissions were higher than those for band spreading on the

    day of sowing. The digested slurry gave higher NH3emissions than the undigestedslurry, probably as a result of the high pH. The digested slurry was also separated andemissions from solid fraction gave higher emissions than the liquid fraction because ofthe lack of infiltration into the soil.

    Considering nitrous oxide (N2O) emissions over the first month after sowing, emissionswere lower from the solid fraction of digested slurry than from the liquid slurries.Because the solid fraction could not infiltrate the soil, denitrification from this fraction wasnot likely. There were no clear effects of slurry digestion on the annual emissions of N2O.Digestion seemed to lower emissions compared to undigested slurry one month afterinjection, but later there were no marked differences between treatments. Digestionappeared to reduce CH4emissions from slurry spreading.

    In cattle slurry experiments on grass, NH3emissions were higher from digested than fromundigested slurry (Perl and Regina, 2006). Slurry injection decreased NH3emissionsbut less for digested than for undigested slurry. Considering nitrous oxide emissions in

    the cattle slurry experiment, cumulative total emissions over the first four months werelowest from band spread digested slurry. Emissions from both digested and undigestedslurry were much higher when the slurry was injected into the soil, with undigested beingthe highest. More CH4was emitted from injected digested slurry than from band spreaddigested slurry, possibly indicating that there was some CH4production in the soil in

    dditi t th l f di l d CH f th l C l ti CH i i ft

  • 8/10/2019 Nutrient Value of Digestate

    15/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    In Denmark, field assessments on the utilisation of slurries following a range oftreatments, in particular including anaerobic digestion and separation, have been carried

    out for a number of years. In fact, it is claimed that the utilisation of N in manure hasincreased dramatically and the use of mineral fertiliser N has decreased by 50%(Sommer and Birkmose, 2007). These authors presented results from several years ofresearch at a national crop production seminar including data from 11 trials with digestedslurry and 15 trials with pig and cattle slurry (Fig. 2) and the Danish Advisory Service arenow actively promoting the benefits of increased NH4-N content and improved utilisationof fertiliser N in digested slurries.

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    trailing hose injection

    SlurryNutilization%

    Digested slurry Pig slurry Cattle slurry

    Figure 2: Utilisation of N in digested slurry compared with pig and cattle slurry infield trials with the Danish Advisory Service. (Sommer and Birkmose, 2007).

    These Danish results (Fig. 2) suggest an overall 15-30% increase in slurry N efficiency,

    depending on slurry type and application technique. However, the results of individualexperiments are not always consistent, with sometimes only marginal benefit apparentfrom digestion, or higher efficiencies following slurry separation treatment (Pedersen,2002).

    Schrder and Uenk (2006) studied the nitrogen fertiliser replacement value (NFRV) or N

  • 8/10/2019 Nutrient Value of Digestate

    16/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    undigested slurry by 5%. However this initial advantage was completely offset whenresidual N effects in years 2, 3 & 4 were taken into account, yielding similar long term

    NFRVs for both types of slurry (Fig. 3).

    0

    10

    20

    30

    40

    50

    60

    70

    Year 1 Year 2 Year 3 Year 4

    NFRV%

    Untreated

    Digested

    Figure 3: Impact of anaerobic digestion on the nitrogen fertiliser replacement valueof cattle slurry over four years following application (Dutch experiments) (Schrderand Uenk, 2006).

  • 8/10/2019 Nutrient Value of Digestate

    17/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    5 Chemical analysis of input slurry and digestate output

    As a result of the consultation process, two farm sites from within the Sandyhillscatchment in Dumfries and Galloway, were selected for the detailed sampling andmonitoring work, which after an introductory letter from the Scottish Executive, involvedthe full cooperation of the host farmers. The basic layout of the biogas plant, comprisingholding tank for raw slurry, digester and digestate storage tank, is the same at bothfarms. Other relevant details of the two sites are presented below.

    5.1 Site 1. Ryes Farm

    Slurry is sourced from 110 dairy cows plus dairy waste, but the digester was scaled totake slurry from young stock also. Currently, the digester (Fig. 4) is working belowcapacity, with a c.40 day retention period (the design retention period is 21 days).

    The raw slurry holding tank of 84m3has sufficient capacity for once a week loading bytractor pump. The digester volume is 251m3and the storage tank volume, 1000m3, oraround 3 months capacity. A small open yard area collects rainwater via a slatted tank,but the slurry cellars are known to admit groundwater. Maintaining a high level of slurryin the cellars minimises water ingress.

  • 8/10/2019 Nutrient Value of Digestate

    18/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    hour. The reactor tank is agitated by gas recirculation. Reactor temperature, pump runtimes and tank contents (holding, digester and storage) can all be read from the control

    panel. In addition, a diary is provided for the farmer to log events such asloading/unloading, changes in control settings and power failures.

    Gas production:in addition to maintaining digestion temperature, surplus gas is piped toa domestic heating boiler in the farmhouse. In general, gas production has exceededdemand and, rather than allowing surplus gas to vent to atmosphere the digester hasbeen operating at a higher than normal temperature, around 42oC. This was simply inorder to burn off the excess gas.

    Sampling procedures:Both feed and delivery screw pumps have sample portsaccessible from inside the control room. It was not feasible to sample both during thehourly timed run periods due to the limited run time. Therefore the pumps were operatedmanually while sampling, keeping the time down to a minimum. The aim was for therecirculation pump to be run for a short period prior to sampling, with the reactor tank(digestate) sampled first, to avoid mixing with raw slurry, then the holding tank(feedstock). Two bucketfuls were taken, the first (to purge pipework) discarded, withsamples drawn from the second. On the basis that further activity is known to occur instorage facilities post digestion, an attempt was made to also sample the storage tank toevaluate any possible further impact on slurry nutrient content. Provision was made forthis using a weighted bucket on a rope lowered from the access ladder, using the bucketto give some local mixing no agitation is provided.

    Unloading:normal farm practice is for the vacuum tanker to be coupled direct topipework connected from the base of the storage tank.

    (a) (b)

  • 8/10/2019 Nutrient Value of Digestate

    19/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    according to rainfall; this is added once a week along with slurry when loading theholding tank. The slurry transfer rate is calculated to retain sufficient raw material to

    continue gas production through the summer.

    Figure 6: General view of digester facility at Corsock Farm

    (a) (b)

    Fi C k F ( ) l h ldi k d d d b ildi f

  • 8/10/2019 Nutrient Value of Digestate

    20/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Sampling procedures: identical to Ryes in every respect. The storage tank level sensorproved unreliable, so an attempt was made to assess the volume of contents by counting

    internal panel bolt heads visible from the access ladder. A similar event log is kept in thecontrol room.

    5.3 Digester operation

    An attempt was made to record mass flow of digester input and output. This was basedon farm diary entries for amounts added and removed, plus tank levels recorded at eachvisit. The control panels included a read-out of levels in each tank (reception, digester,storage), although there was a fault in the storage tank level sensor at Corsock Farm, sothat level was manually assessed by counting rivet heads visible from the top of the tank.Subsequently it was apparent that digester levels are affected by the heat exchangercirculation pumps, such that the level falls when the pumps cut in. As this was not knownin advance, it was not possible to take account of this in assessing tank levels at CorsockFarm and, consequently, the recorded volumes were unreliable. Digester temperaturesfor both sites and estimated digester output volumes for Ryes Farm are shown in Fig. 8.

    0

    10

    20

    30

    40

    50

    60

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    Slurryvolumem

    3

    38

    39

    40

    41

    42

    43

    44

    TemperaturedegC

    temp

    0

    10

    20

    30

    40

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    Slurryvolumem

    3

    20

    25

    30

    35

    40

    Tempera

    turedegC

    Dig temp

    (a)

    (b)

  • 8/10/2019 Nutrient Value of Digestate

    21/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    diesel boiler restored digester temperature to 35 C on March 5th, and to 39 C on March8th. Average operating temperature at Corsock Farm was 35.3 C, which is typical for

    mesophilic anaerobic digestion. Because of the surplus gas production at Ryes Farm,the digester was operated at an elevated temperature, averaging 41.6 C.

    5.4 Analysis and nutrient content of digester feedstock and digestate

    Samples were collected for analysis from the two digesters on two occasions each week,commencing February 12thand finishing March 12th(9 sampling dates), thus covering bysome margin the estimated hydraulic retention time of the digesters. Mean input and

    output analyses, with overall differences for the two sites are presented in Table 6. Thedetails of these analyses, for both the input feedstock and the digestate output, arepresented in Tables 7 and 8.

    Table 6: Mean digester input and digestate analyses with overall differences andestimated statistical significance.

    Difference P value2

    Ryes Farm Input Output %1

    Dry matter % 8.1 6.23 -21.2 0.005

    pH 7.68 7.84 0.16 0.264

    Ash %DM 34.84 40.29 16.7 0.017

    Total N % 0.29 0.29 -0.87 0.608

    NH4-N % 0.11 0.13 17.7 0.092

    NO3-N % 0.03 0.03 - -

    Total P2O5 % 0.111 0.108 -2.49 0.347Water sol P %DM 0.012 0.012 - 0.299

    Total K2O % 0.43 0.42 -1.9 0.383Corsock Farm

    Dry matter % 7.68 6.76 -11.8 0.002

    pH 7.35 7.58 0.22 0.008

    Ash %DM 16.32 19.41 19.1

  • 8/10/2019 Nutrient Value of Digestate

    22/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    22

    Table 7: Digester input and output analyses with sampling date Ryes Farm

    Date 12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar 08-Mar

    Digester input Mean St.dev. cv % Store1

    Dry matter % 9.1 9 9.1 7.9 8.6 7.9 7.7 7.5 6.1 8.1 0.979 12.08 -

    pH 7.41 8.12 7.82 7.62 7.66 7.66 7.3 7.51 8.05 7.68 0.274 3.57 -

    Ash %DM 34.5 34.9 35.3 33.7 34 32.9 33.7 32.1 42.5 34.84 3.033 8.70 -

    Total N % 0.31 0.3 0.32 0.27 0.29 0.29 0.28 0.27 0.29 0.29 0.017 5.81 -

    NH4-N % 0.104 0.111 0.083 0.106 0.115 0.104 0.115 0.111 0.157 0.11 0.020 17.47 -NO3-N % 0.028 < 0.010 0.051 0.013 < 0.010 0.022 0.018 0.018 < 0.010 0.03 - - -

    Total P2O5 % 0.12 0.11 0.12 0.1 0.12 0.11 0.1 0.11 0.11 0.11 0.008 7.04 -

    Water sol P %DM 0.015 0.011 0.012 0.012 0.013 0.012 0.013 0.013 0. 009 0.012 0.002 13.43 -

    Total K2O % 0.46 0.43 0.41 0.39 0.43 0.41 0.45 0.45 0.47 0.43 0.026 6.11 -

    Digester output

    Dry matter % 5.8 5.9 6.1 6.3 6.2 6 6 6.1 7.7 6.23 0.570 9.15 10.1

    pH 7.77 8.07 7.86 7.87 8 7.88 8.05 7.78 7.32 7.84 0.225 2.87 7.44

    Ash %DM 40.4 40.7 41 42 41.5 40.8 42.3 40.2 33.7 40.29 2.569 6.38 39.4

    Total N % 0.27 0.29 0.3 0.28 0.29 0.3 0.29 0.29 0.28 0.29 0.010 3.38 0.30

    NH4-N % 0.138 0.114 0.095 0.147 0.15 0.149 0.132 0.122 0.114 0.13 0.019 14.78 0.111

    NO3-N % 0.015 0.035 0.056 < 0.010 0.011 < 0.010 0.018 0.036 0.015 0.03 - - < 0.010

    Total P2O5 % 0.1 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.1 0.11 0.004 4.09 0.22

    Water sol P %DM 0.012 0.012 0. 01 0.011 0.011 0.011 0.013 0.012 0.012 0.012 0.001 7.63 0.024

    Total K2O % 0.41 0.37 0.41 0.41 0.44 0.43 0.45 0.45 0.45 0.42 0.027 6.36 0.3

    1Large digestate store sampled only on one occasion, 8th March.

  • 8/10/2019 Nutrient Value of Digestate

    23/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    23

    Table 8: Digester input and output analyses with sampling date Corsock Farm

    Date 12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar 08-Mar

    Digester input Mean St.dev. cv % Store1

    Dry matter % 7.2 7.5 8.3 8.3 8.5 7.8 7.3 7.5 6.7 7.68 0.579 7.78 -

    pH 7.07 7.32 7.55 7.37 7.36 7.45 7.39 7.31 7.3 7.35 0.130 1.77 -

    Ash %DM 16.6 16.2 16.2 16.6 16 18.4 16 15.4 15.5 16.32 0.883 5.41 -

    Total N % 0.3 0.3 0.29 0.3 0.32 0.31 0.27 0.29 0.25 0.29 0.021 7.21 -

    NH4-N % 0.118 0.095 0.136 0.124 0.128 0.13 0.095 0.103 0.095 0.11 0.017 14.76 -NO3-N % 0.022 0.048 < 0.010 0.021 0.015

  • 8/10/2019 Nutrient Value of Digestate

    24/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    24

    (a) Ryes Farm (b) Corsock Farm

    0

    0.05

    0.1

    0.15

    0.2

    0.25

    0.3

    0.35

    0.4

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    SlurryN

    content%

    0

    1

    2

    3

    4

    5

    6

    7

    8

    9

    pH

    Total N-in

    NH4-N-in

    Total N-out

    NH4-N-out

    pH-in

    pH-out

    0

    0.05

    0.1

    0.15

    0.2

    0.25

    0.3

    0.35

    0.4

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    SlurryNcon

    tent%

    0

    1

    2

    3

    4

    5

    6

    7

    8

    9

    pH

    Total N-in

    NH4-N-in

    Total N-out

    NH4-N-out

    pH-out

    pH-in

    0

    10

    20

    30

    40

    50

    60

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    Slurryvolumem

    3

    38

    39

    40

    41

    42

    43

    44

    TemperaturedegC

    temp

    0

    10

    20

    30

    40

    12-Feb 15-Feb 19-Feb 22-Feb 26-Feb 01-Mar 05-Mar 08-Mar 12-Mar

    Slurryvolume

    20

    25

    30

    35

    40

    Temperature

    Dig temp

    Figure 9: Changes in slurry pH, total N and NH4-N over the monitoring period in digester input and output at (a) Ryes and(b) Corsock Farms; digester temperature for both sites and output volumes for Ryes, also shown.

  • 8/10/2019 Nutrient Value of Digestate

    25/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    In general the results followed the pattern of those identified within the technical

    review, with a substantial and highly significant reduction (P

  • 8/10/2019 Nutrient Value of Digestate

    26/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    subject to rapid transformation during digestion, as a result of microbial activity andionic balance.

    Water soluble P content was very low in the slurry at both sites (1.5 4.2% of total P)with no difference between input and digestate at Ryes, but an apparent decrease inwater soluble P in the digestate at Corsock. It must be noted, however, that thelaboratory detection limit for this particular determination is at 0.0065%DM anddifferences between observations only 2-3x the detection limit must be regarded asunreliable and probably within assessment variability.

    The analysis of the slurry on the single occasion (March 8th) from the large storage

    tank, was closely similar, in both cases, to the overall mean of the digestateanalyses, except for a reduced DM content at Corsock, in this case reflecting thelikely extra dilution by rainfall within the store and the normal in-store settlement ofsolids.

    Unfortunately, the data on digester input and output volumes were not consideredsufficiently reliable to allow calculation of a nutrient mass balance for the monitoringperiod.

    5.5 Conclusions on chemical analysis of slurry and digestate

    There is a significant body of opinion that, among the claimed benefits of anaerobicdigestion, are improvements in the effluent (digestate) quality, as a result of thedigestion process. Based on both the findings of the technical review and the resultsof the farm studies, a number of observations and conclusions can be drawnconcerning the impact of digestion on slurry nutrient content.

    As a result of the digestion process a number of changes in slurry analysis canbe expected. These include a substantial reduction (up to 25%) in solidscontent and a consequential increase in ash content, due to the conservation ofminerals against a background of reducing slurry carbon (and organic mattercontent).

    Increases in slurry pH (up to 0.5 pH units) and NH4-N content (up to 25%) mayalso occur, though these changes are less consistent than the reductions in

    solids content and BOD and may be transient, or dependent on digesteroperating conditions and the analysis of the feedstock slurries. Thus, althoughan increase in Nmin/Ntotalis expected, AD treated slurries should not be regardedas mineral fertiliser solutions, as has sometimes been reported.

    To address another occasional misconception about AD, although the

  • 8/10/2019 Nutrient Value of Digestate

    27/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    the reduced solids content would be expected to improve surface infiltration ofthe slurry, which should help to conserve slurry N. Recent Danish research has

    shown reduced NH3emissions from AD slurries when band applied via trailinghose application. Low emission application techniques are recommended forAD treated slurries.

    Increased mineral N content of slurries, even with reduced NH3emissions, doesnot guarantee improved crop recovery of slurry N content and savings infertiliser N, with maintained crop yields. The limited research coveringagronomic assessments presented in the review have generated mixed results

    with Dutch experiments showing small and short term benefits only; Danishresearch has produced more encouraging results, though with not alwaysconsistent benefits.

    There is strong evidence, from the literature and from other recent research, tosuggest that an increased availability factor for the phosphate content of ADslurries should be considered. Although the digester monitoring study failed toshow an increase in the water soluble P content of the digestate, several otherrecent studies have indicated significant potential. Depending on the location ofsuitable P responsive field sites, this aspect could be included within theproposed field experiments (see section 6.2).

    Following on from these conclusions and, from element (3) of section 2.5Project Objectives,a number of recommendations for further action and forfurther research are drawn together within Chapter 6 of this report.

    N t i t V l f Di t t f F B d Bi Pl t i S tl d

  • 8/10/2019 Nutrient Value of Digestate

    28/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    6 Further investigation of the effectiveness of plant nutrients indigested slurry

    In addition to the significant changes in slurry analysis with digestion highlighted bythe technical review and site monitoring data, benefits in terms of reduced risk ofcrop scorch would be anticipated as a result of reduced solids content and VFAconcentrations in the digestate (Smith and Chambers, 1992; Smith et al.,1995).Thus, although the potential nutrient benefits of slurry digestate are unproven, theseare of sufficient number and magnitude to justify further research (i) on emissionsfrom digestate storage and following land application; and (ii) on agronomic impactsand nutrient recycling.

    6.1 Proposals for field assessment of digestate nutrient value

    Based on the findings of the literature review and the monitoring of farm-basedbiogas plants, it is recommended that carefully designed, replicated field experimentsare undertaken to assess the real potential of digested slurries under practical farmconditions.

    It is well known that it is often difficult to demonstrate measurable agronomic effectsarising from differential treatment or application techniques for organic manures.This is because the potential magnitude of such treatment effects is likely to berelatively small, particularly when considered against the likely background ofsubstantial in-field soil and sward variability. Thus, it is proposed that sites are verycarefully selected for uniformity and a robust experimental design, with goodreplication and a large number of treatment degrees of freedom is adopted.

    It is proposed that the trial design should include the following elements and carefulconsideration of the following guidelines:

    An N response curve with a minimum of 7 N levels, including nil N and rangingup to a maximum ensuring the optimum N is exceeded. The levels might be upto 330 kg/ha N, split between applications on 1stand 2ndcuts (up to say 180kg/ha on 1stcut and 150 kg/ha on 2ndcut), though final decision will depend onsite, soil type and grass growth potential.

    Slurry treatments will include digested and undigested slurry at a rate aimed atsupplying a significant rate of mineral N (say minimum of 80 kg/ha NH4-N,which will mean a total N rate of c. 160 kg/ha for cattle slurry), so that thechances of measuring treatment differences are enhanced.

    Nutrient Value of Digestate from Farm Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    29/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    small differences in slurry N supply are often difficult to measure, significantdifferences in emissions from land application practices are more often

    observed (e.g. Smith et al.,2000). The residual N effects of the slurry are of interest because of the different

    analysis of digested and undigested slurries. It is therefore proposed that a 3rdcut should be taken to allow an indication of the shorter term residual effectsfollowing the slurry applications before 1st and 2ndcuts.

    To ensure that crop responses are to N only, plots should receive phosphate(as triple superphosphate) and potash (as sulphate of potash) according to soilanalysis. Sulphate of potash to be used as potash source rather than muriateof potash in view of the likely response of grass to sulphur across much ofScotland. This will apply to all treatments except for the possible P evaluationtreatments outlined below.

    If a low soil P status site can be located, a set of additional treatments shouldbe included to evaluate the potential difference in P availability between AD and

    raw slurry. This would require only surface applied digested and raw slurrytreatments (at the same rate as the main experimental treatments) for 1stcut(when greatest response to fresh P is likely).

    Treatments should also be included to evaluate the proposed reduced risk ofscorch associated with AD slurry this should include surface broadcastdigested and undigested slurries only, applied before 1stcut in association witha high rate of fertiliser N, say 150 kg/ha, which would remove the grass

    response to slurry N, leaving the potential scorch effect to be examined againstthe equivalent fertiliser N treatment.

    The selected site(s) should be on a uniform soil type, on a short term ryegrassley (i.e. excluding clover), with a known history, which should exclude intensivegrazing or heavy slurry/manure applications and with none applied since theprevious spring. An early site, with good grass growth potential to be preferred.

    An experiment of this type is unsuited to the application of treatments usingfield-scale equipment. For the slurry treatments, the ADAS purpose-built slurryplot applicator will allow careful control of the slurry treatments within a small-plot design (Basford et al,1996).

    Consideration should be given to running an associated, pot-based experiment

  • 8/10/2019 Nutrient Value of Digestate

    30/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    31/44

    Nutrient Value of Digestate from Farm Based Biogas Plants in Scotland

    Ammonia emissions Ammonia emissions over a period of 5-6 days followingapplication of digested and untreated slurries, from both injection and surface

    application treatments, using small wind tunnels (Lockyer, 1984; Smith et al.,2000).Grass recording- Yield of fresh and DM;- Analysis for dry matter, total-N, total P, K, Mg & S;- Assessment of any visual herbage scorch effects following surface applicationbefore 1stcut, with comparison of digested and undigested slurry and againstequivalent control (no slurry) plots.

    Weather recordsParticularly for the day and period after slurry application.

    ReportIncluding statistical analysis of grass yields and analytical data. Assessment of slurryN use efficiency to be based on (1) fertiliser N equivalent of slurry by interpolationagainst the fitted fertiliser N response curves for 1stand 2ndcuts and (2) grass N

    recovery from slurry treatments relative to control (nil fertiliser N treatment).

    6.2 Proposed modelling appraisal of nutrient fluxes following land applicationof digestate.

    Following on from the findings of this report it is proposed that a modellingassessment of nutrient fluxes following land application be carried out using therecently developed MANNER-PSM (MANure Nitrogen Evaluation Routine- PolicySupport Model) software. A range of slurry management scenarios relevant tofarmers in the south-west of Scotland should be evaluated. For example the impactof increasing slurry NH4-N content, but decreasing slurry solids content, which willtend to act in opposing directions on NH3emissions following surface applications ofslurry to land. Also the impact of digestate application to land across a range oftimings and climatic conditions via different application techniques, on gaseous Nemissions and nitrate leaching losses. This approach will allow the potentiallybeneficial contribution of digestate to be evaluated more widely, in the context of

    Scottish farming conditions and practical issues. The modelling work will also help toidentify the likely optimum range of digestate quality, in terms of improved Nutilisation efficiency and reduced environmental emissions.

    This work offers scope for a detailed analysis of possible impacts in the SW region ofScotland and depending upon the quality of information available, includes the

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    32/44

    g g

    Carefully designed, replicated, field experiments should be undertaken tocompare crop response to anaerobically digested and untreated slurries and the

    potential for fertiliser savings. The results should be used to inform and updatecurrent advice on N availability of livestock slurries and other effluent/wastestreams, e.g. output of mixed digestate materials from CAD plants. Evidencefrom this work would contribute to the revision of published advice, includingRB209; and also current manure nutrient DSSs such as MANNER andPLANET. Recommendations for such experiments are included in section 5.2.

    In view of the lack of well documented research data in the scientific literature

    on the impact of anaerobic digestion on slurry/effluent nutrient content, it isproposed that an edited version of the results should be prepared for publicationin a suitable peer reviewed scientific journal.

    The main highlights and conclusions of this research should be promoted viaappropriate industry/farm events and via the agricultural press.

    AcknowledgementsThe authors of this report and the Scottish Executive gratefully acknowledge the co-operation and assistance of the host farmers in the monitoring and sampling of thefarm digesters used in this study:

    Mr Wesley Millar, Ryes Farm;

    Mr Brian Smallwood, Corsock Farm;

    We also acknowledge the advice and technical assistance provided by Mr JamieGascoigne, field engineer, Greenfinch Ltd.

  • 8/10/2019 Nutrient Value of Digestate

    33/44

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    34/44

    Forschungsprojekt Nr. 811941/8539 SCK/SAI, Auftraggeber: sterreichischeForschungsfrderungsgesellschaft mbH, Wien. In press.

    Lockyer, D.R. (1984). A system for the measurement in the field of losses of ammonia,through volatilisation. Journal of the Science of Food and Agriculture, 35, pp 837-848.

    Martin, J.H.Jr. (2004). A Comparison of Dairy Cattle Manure Management with and withoutAnaerobic Digestion and Biogas Utilization. Report to U.S. Environmental Protection Agencyby Eastern Research Group Inc. 17 June 2004. EPA Contract No. 68-W7-0068, Task OrderNo. 400.

    Martin, J.H.Jr. (2005). An Evaluation of a Mesophilic, Modified Plug Flow Anaerobic Digesterfor Dairy Cattle Manure. Submitted to Kurt Roos AgSTAR Program U.S. EnvironmentalProtection Agency by Eastern Research Group Inc. 20 July 2005. EPA Contract No. GS10F-0036K Work Assignment/Task Order No. 9.

    Mller, H.B. (2001). Anaerobic digestion and separation of livestock slurry Danishexperiences. Report to MATRESA 2nd edition, Danish Institute of Agricultural Sciences, DeptAgricultural Engineering, Research Centre, Bygholm, Horsens, Denmark.

    Mller, H.B. (2006). Kvaelstofomsaetning I biogasanlaeg (Turnover of nitrogen in ADplants). Forskning i Bioenergi. Nr 17. December 2006.

    OSullivan, C.M. and Cumby, T.R. (2004). Analytical assessment of digestate samples fromHolsworthy Biogas PLC. Silsoe Research Institute Contract Report CR/1534/04/3516.

    Pedersen, C.A. (2002). Annual Report of the National Field Trials Animal Manure. Extractof the fertiliser chapter, pp 25-29. Dansk Landbrugsraadgivning Landscentret. www.lr.dk

    Perl, P. and Regina, K. (2006). The effect of cow slurry fermentation and applicationtechnique on greenhouse gas and ammonia emissions from a grass field. In 12thRAMIRANInternational Conference, Technology for Recycling of Manure and Organic Residues in aWhole Farm Perspective Vol. II. Paper P-306 pp245-247.

    Regina, K. and Perl, P. (2006). Ammonia and greenhouse gas emissions from pig slurry the effect of slurry fermentation, separation of the fermentation product and applicationtechnique. In 12thRAMIRAN International Conference, Technology for Recycling of Manureand Organic Residues in a Whole Farm Perspective Vol. II. Paper P-305, pp241-243.

    Schrder, J and Uenk, D. (2006). Cattle slurry digestion does not improve the long termnitrogen use efficiency of farms. In 12thRAMIRAN International Conference, Technology for

    Recycling of Manure and Organic Residues in a Whole Farm Perspective, RAMIRAN 2006,Aarhus, Denmark. Vol. II, pp 9-11.

    Smith, K. A. & Chambers, B. J. (1992). Improved utilisation of slurry nitrogen for arablecropping. In Nitrate and Farming Systems. Aspects of Applied Biology 30, 1992 p127-134.

    Smith K A Jackson D R Unwin R J Bailey G & Hodgson I (1995) Negative effects of

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    35/44

    Sood, D. (2006). Waste to Watts: Anaerobic Digestion of Livestock Manures (World BankFunded Study) Workshop Monitoring nutrient related pollution reduction from diffuseagricultural sources and from agro-industrial point sources, Moldova September 2006.

    Summers, R. and Bousfield, S. (1978). Anaerobic digestion of farm wastes experimentalexperiences. (Rowett Research Institute, Bucksburn, Aberdeen). In Anaerobic Digestion ofFarm Wastes Seminar, 18-20 October, 1978. pp. 30-42. ADAS report.

    Van Velsen, A.F.M. (1979). Anaerobic digestion of wet piggery waste. In Hawkins,J.C. (ed.)Engineering problems with effluent from livestock. CEC, Luxembourg, pp. 476 489.

    Vetter, H., Steffens, G. and Schrpel, R. (1987). The influence of different processingmethods for slurry upon its fertiliser value on grassland. In HG van der Meer, et al(eds).Animal Manure on Grassland and Fodder Crops. Fertiliser or waste ? Martinus Nijhoff,Dordrecht, pp 73-86.

    Webb, J. and Misselbrook, T.H. (2004). A mass-flow model of ammonia emissions from UKlivestock production. Atmospheric Environment 38, pp 399-406.

    Wright, P et al. (2004). Preliminary Comparison of Five Anaerobic Digestion Systems onDairy Farms in New York State. Department of Biological and Environmental Engineering,Cornell University, Ithaca, NY 14853. 2004 ASAE/CSAE Annual International Meeting.

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    36/44

    Annex A Anaerobic Digestion and Digestate Analysis

    Anaerobic digestion (AD) is a microbial process via which organic substances arebroken down, the major ultimate products of which include carbon dioxide (CO2),methane (CH4) and water (H2O). Significant by-products include ammonia(NH4

    +/NH3) and hydrogen sulphide (H2S).

    Although the degradation of organic compounds is a complex process, involving

    many groups of bacteria, the process comprises three main steps:(i) enzymatic hydrolysis of organic compounds with the formation of sugars andamino acids (slow);

    (ii) conversion of sugars and amino acids to volatile fatty acids (VFAs) (acetogenesis)(rapid);

    (iii) formation of CH4+ CO2+ H2O (methanogenesis).

    The resulting biogasis a mixture of the gases CH4and CO2, with smallerconcentrations of other gases, in particular NH3and H2S, the latter resulting from thebreakdown of proteins. Anaerobic digestion is a dynamic process in which theanalysis of the digestate will depend on the dominant phase at the time of samplingand a number of buffering reactions in solution. The equilibrium of CO2andbicarbonate (HCO3

    -) with ammonium cations (NH4+), exerts substantial buffering on

    digestate pH, the breakdown of organic acids generating CO2and, hence, carbonic

    acid in solution:CO2+ H2OH2CO3HCO3

    - + H+

    VFAs decrease the buffering capacity of the bicarbonate ions:

    RCOO-H + NH4HCO3RCOO-NH4+ H2CO3

    The formation of NH3will increase bicarbonate in balancing the generation of CO2:

    CO2+ H2O + NH3NH+

    4+ HCO-3

    The higher the bicarbonate concentration, the greater the buffering in solution andresistance to changes in pH. The optimum pH varies according to the stage in thedegradation process. Overall, the breakdown of organic substances in AD will resultin a reduction in organic matter (OM) content and, hence, a reduction in BOD, CODand in solids (DM) Minerals are retained and thus ash content (expressed on DM

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    37/44

    Annex B Glossary of Terms

    To facilitate understanding and reduce the risk of ambiguity or confusion over termswhich have been used in this report, a number of technical terms and acronyms havebeen listed below with simple definitions or explanations.

    ANAEROBIC Containing no free oxygen (or not requiring free oxygen such asANAEROBIC BACTERIA) or chemically bound oxygen such as

    nitrates (NO3).

    AD Anaerobic digestion

    ASH Product remaining after incineration in laboratory combustion.

    BIOLOGICAL OXYGENDEMAND (BOD)

    Together with the COD, BOD is the measure of the pollutionpotential in water bodies and organic wastes. A laboratory test isused to measure the amount of dissolved oxygen consumed bychemical and biological action when a sample is incubated at 20C

    for a given number of days (five for BOD5).

    CAD Centralised anaerobic digester: plant designed to receiveorganic substrates from several sources (e.g. SLURRIES fromneighbouring farms, wastes from abattoirs, food processingfactories etc.), so offering economies of scale in investment andoperating costs.

    CH4 Methane; a greenhouse gas produced during anaerobic

    fermentation of organic matter, especially from entericfermentation in ruminants and storage of liquid manure. Aconstituent of biogas

    CHEMICAL OXYGENDEMAND (COD)

    A measure of the amount of oxygen consumed in the microbialoxidation of decomposable and inert organic matter and theoxidation of reduced substances in water. The COD is alwayshigher than the BOD, but measurements can be made in a fewhours while BOD measurements take five days.

    C:N RATIO The amount of total carbon divided by the amount of totalnitrogen contained in livestock manures. Manures with a highC:N RATIO such as FARMYARD MANURE usually take longerto break down, or mineralise, in the soil than those such asslurry with a lower C:N RATIO

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    38/44

    HCO3- Bicarbonate ion; from the dissolution of CO2in solution.

    K Chemical symbol for potassium.

    ORGANIC MATTER (OM) Residues derived from plants, animals and micro-organisms invarious stages of decomposition.

    pH A measure of the hydrogen ion concentration of a solution andan indication of its` acidity or alkalinity. Expressed on a scalefrom 0 to 14, 7 is neutral, higher values more alkaline, lowervalues more acid.

    MANNER MANure Nutrient Evaluation Routine: DSS tool designed to

    assess the fate of manure nutrients following application to land.Mg Chemical symbol for magnesium.

    Ntotal Total nitrogen content; also Total Kjeldahl Nitrogen.

    Nmin/ Ntotal The proportion of mineral nitrogen (usually ammonium-N)content of total nitrogen; provides an indication of the readilyavailable N content of manures.

    Na Chemical symbol for sodiumNH3 Ammonia (gas)

    NH4+ Ammonium; ionic form following dissolution of ammonia gas in

    aqueous solution.

    NO3- Nitrate; oxidised form of nitrogen in solution, readily available for

    uptake by plant roots, but vulnerable to loss via leaching indrainage water.

    N2O Nitrous oxide; powerful greenhouse gas.

    P Chemical symbol for phosphorus.

    PLANET Nutrient management software to facilitate the planning offertiliser and manure nutrient inputs

    RB209 Reference Book 209; Fertiliser recommendations for Agriculturaland Horticultural Crops. MAFF Reference Book RB209, 7thEdition, Dec 2000. Published by The Stationery Office, Norwich.

    RCOO-H Generic formula for short chain fatty acid, where the radical Rmay represent an alkyl radical containing from one to fourcarbon atoms, e.g. methyl (CH3), ethyl (C2H5) etc. (see also VFAbelow)

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    39/44

    39

    Annex C Nutrient content of livestock slurries before and after anaerobic digestion.

    Table C1: Comparison of analysis results for undigested and digested dairy cattle/pig slurry mix1, Suffolk, 1979-80

    (Nielsen, 1980).

    Total N NH4-N NH4-N P2O5 K2O DM pH COD BOD

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % mg/I %

    Feedstock1 3.0 (8) 2.0 (8) 66.7 1.4 (7) 3.5 (7) 4.7 (14) 7.30 (17) 49,800 (17) 4.7 (14)

    Digestate 3.4 (10) 2.3 10) 67.6 1.6 (8) 3.2 (9) 4.2 (16) 7.75 (20) 30,400 (19) 4.2 (16)

    Change %2 +13 +15 - +18 -7 -10 +0.452 -38 -101The results relate to digestion of slurry from 200 dairy cows and 3,000 fattening pigs.( )Figure in brackets = number of samples from which mean derived2All changes expressed as % except for pH units.

    Source: Nielsen, V.C. (1980) Internal ADAS R&D Report, ADAS Farm Waste Unit.

    Table C2: Comparison of analysis results for undigested and digested pig slurry1, Yorkshire, 1981 (Friman, 1981).

    Total N NH4-N NH4-N P2O5 K2O DM pH COD BOD

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % mg/I %

    Feedstock1(5 samples) 7.6 3.5 46 0.65 1.3 2.33 7.6 (13) 36,200 (3) nd

    Digestate (3 samples) nr 4.9 - 0.61 nr 1.84 8.1 (7) 21,400 (2) nd

    Change % - +40 -6.2 -21 +0.52 -40.91Slurry from 900 sows with progeny to bacon weight; includes cleaning water. Slurry separated and liquid fraction digested.( )Figure in brackets = number of samples from which mean derived; nr not reported (unreliable data); nd not determined.2All changes expressed as % except for pH units.

    Source: Friman, R. (1981) Internal ADAS R&D Report, ADAS Farm Waste Unit.

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table C3: Comparison of analysis results for undigested and digested dairy cattle slurry1

    Kent January February 1982

  • 8/10/2019 Nutrient Value of Digestate

    40/44

    40

    Table C3: Comparison of analysis results for undigested and digested dairy cattle slurry1, Kent, January February 1982

    (Friman, 1982).

    Total N NH4-N NH4-N P2O5 K2O DM pH COD BOD

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % mg/I %

    Feedstock1(16 samples) 4.45 1.69 38.0 1.57 5.86 10.5 8.1 72,000 8,630Digestate (28 samples) 3.94 1.48 37.6 1.37 5.07 7.4 8.01 48,000 5,138

    Change %2 -11.5 -12.4 -12.7 -13.5 -29.5 -0.092 -33.3 -67.51Slurry from 900 sows with progeny to bacon weight; includes cleaning water. Slurry separated and liquid fraction digested.( )Figure in brackets = number of samples from which mean derived; nr not reported (unreliable data); nd not determined.2All changes expressed as % except for pH units.

    Source: Friman, R. (1982) Internal ADAS R&D Report, ADAS Farm Waste Unit.

    Table C4: Comparison of analysis results for undigested and digested beef cattle slurry1, Northern Ireland, 1989-90(Clarkson, 1990).

    Total N NH4-N NH4-N P2O5 K2O DM pH COD BOD

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % mg/I %

    Feedstock1 4.9 2.3 46.9 nd nd 8.8 7.2-7.5 82,000 12,600

    Digestate 4.2 2.5 59.5 nd nd 6.5 7.7-7.8 62,000 4,100

    Change %2

    -14.3 +8.7 - - - -26.1 +0.42 -24.4 -67.51The results relate to digestion of slurry from beef cattle housed on slats.nd not determined

    Source: Clarkson, C.R. (1990) Long Term Performance of Anaerobic Digester at Bethlehem Abbey, Portglenone, Northern Ireland. ADAS ResearchReport, 1990.

    Table C5: Comparison of analysis results for undigested and digested dairy cattle slurry1, Scotland, 1981 (Anon, 1981).

    Total N NH4-N NH4-N P2O5 K2O DM pH COD BODkg/m

    3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % mg/I %

    Feedstock1 - 1.04 - - - 7.3 - 80,280 18,470

    Digestate - 1.15 - - - 5.9 - 66,490 3,840

    Change % +10.2 -19.2 -17.2 -79.21Dairy cattle slurry.

    Source: Anon (1981). ETSU Report ETSU B 1052 (1987). Report on Anaerobic Digestion of Dairy Wastes to October 1981. Microbiology Department,Rowett Research Institute Engineering Division, North of Scotland College of Agriculture.

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

  • 8/10/2019 Nutrient Value of Digestate

    41/44

    41

    Table C6: Comparison of analysis results for undigested livestock slurries and mixed digestate, Austria (Hopfner-Sixt, et

    al., 2007).

    Total N NH4-N NH4-N P2O5 K2O DM pH

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 %

    Cattle slurry (47) 2.63 1.27 48.3 0.98 3.21 4.18 7.96

    Pig slurry (16) 3.72 2.20 59.1 1.37 2.28 3.03 7.95

    Mixed slurry (11) 3.19 1.83 57.4 1.19 2.88 3.34 8.14

    Digestate1(6) 4.12 2.24 54.4 1.90 3.31 4.87 7.791Digester feedstock average 62% cow/pig slurry.

    Source: Hopfner-Sixt, et al.,(2007). Data from Biogas Forum Austria, University of Natural Resources and Applied Life Sciences,Vienna.

    Table C7: Comparison of analysis results before and after anaerobic digestion, from studies in Germany1(Bauermeister et

    al., 2006).

    Total N NH4-N NH4-N P2O5 K2O DM pH

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 %

    Pre-digestion 4.85 2.18 44.9 11.3 6.71

    Post digestion 4.24 2.69 63.5 5.58 7.90

    Change %2 -12.6 +23.4 - - -50.6 +1.192

    1Data represent average from 43 biogas plants in Thuringia.2

    All changes expressed as % except for pH units.Source: Bauermeister et al., (2006). (Reference for this data given as Reinhold, G. , Eigenschaften und Einsatz der Grreste inPflanzenproduction; Vortrag zum ZONARO-Fachgesprch am 26.10.05 an der LLG in Bernburg).

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table C8: Comparison of nitrogen in different types of biomass before and after anaerobic digestion from studies in

  • 8/10/2019 Nutrient Value of Digestate

    42/44

    42

    Table C8: Comparison of nitrogen in different types of biomass before and after anaerobic digestion, from studies inDenmark (Moller, 2006).

    Feedstock Total N kg/m3 % NH4-N kg/m

    3 % NH4-N/tot N (%)

    Pre- dig Post-dig Change Pre- dig Post-dig Change Pre- dig Post-digCattle slurry 3.0 2.8 -7 1.6 2.1 +32 53 75

    Pig slurry (thermophilic AD) 4.4 4.4 0 3.6 4.1 +14 82 93

    Pig slurry (thermophilic AD) 5.6 5.6 0 4.4 5.0 +13 79 89

    Pig slurry (mesophilic AD) 4.0 4.0 0 2.2 3.1 +42 55 78

    Solids from decanter centrifuge 12.0 12.0 0 5.0 7.3 +45 42 60

    Solids from separation1 7.6 7.8 +3 4.0 6.3 +52 53 81160% solid fraction from separation (Kemira) and 40% untreated slurry.

    Source: Moller, H.B. (2006). Kvaelstofomsaetning I biogasanlaeg (Turnover of nitrogen in AD plants). Forskning i Bioenergi. Nr 17. December 2006.

    Table C9: Nutrient concentrations in animal manure before land application (sampled by the Danish Advisory Service)1.NTotal

    NH4-N P2O5 K2O DM NH4-N

    kg/m3 kg/m3 kg/m3 kg/m3 % % of total

    Slurry after anaerobic digestion(mean 44 samples)

    4.75 3.67 2.11 2.75 4.49 82

    Slurry (means 228-238 samplesdepending on determination)

    3.56 2.49 1.67 3.10 5.26 70

    Acidified slurry(mean 10 samples)

    4.33 3.07 1.90 2.99 5.15 71

    Cattle slurry

    (mean 104 samples)

    3.62 2.07 1.69 3.67 7.36 57

    Pig slurry finishing pigs(mean 24 samples)

    4.06 3.19 1.99 2.96 4.26 79

    Pig slurry(mean 107 samples)

    4.27 3.37 2.29 2.89 4.55 79

    1Ref: Data provided by Torkild Birkmose, Danish Advisory Service personal communication.

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table C10: Comparison of digested (digestate) and undigested (feedstock) cow slurry analysis results in New York State,

  • 8/10/2019 Nutrient Value of Digestate

    43/44

    43

    Table C10: Comparison of digested (digestate) and undigested (feedstock) cow slurry analysis results in New York State,USA (mean results for semi-monthly sampling late May 2001 to early June 2002)

    1.

    Total N NH4-N NH4-N P2O5total P2O5 ortho P2O5 Ortho pH COD DM

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % of total mg/I %

    Feedstock 4.63 2.16 46.7 1.86 1.05 56.5 7.4 153,496 11.32Digestate 5.11 2.88 56.4 1.92 1.29 67.2 7.9 89,144 8.47

    Change %2 +10.4 +33.3 +3.2 +22.3 +0.52 -41.9 -25.21source: Martin (2004).2All changes expressed as % except for pH units.

    Table C11: Comparison of digested (digestate) and undigested (feedstock) cow slurry analysis results in Wisconsin, USA

    (mean results for semi-monthly sampling late January to December 2004)1.Total N NH4-N NH4-N P2O5total P2O5 ortho P2O5 Ortho pH COD DM

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % of total mg/I %

    Feedstock 3.48 1.70 48.9 1.79 0.017 0.9 7.6 69,923 8.81

    Digestate 3.25 2.12 65.2 1.64 0.011 0.7 8.2 43,000 5.69

    Change %2 -6.6 +24.9 -8.4 -35.3 +0.62 -38.5 -35.41source: Martin (2005).

    2All changes expressed as % except for pH units.

    Table C12: Comparison of digested (digestate) and undigested waste water from pig housing (farrowing and gestation)analysis results from sampling in N Carolina, USA (mean results 1998)

    1.

    Total N NH4-N NH4-N P2O5total P2O5 ortho P2O5 Ortho pH COD DM

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % of total mg/I %

    Farrowing house2 1.31 0.79 60.3 0.85 0.43 50.6 6.88 14,847 0.94

    Gestation2

    1.42 0.85 59.9 1.09 0.51 46.8 7.21 15,621 1.10Digestate 0.92 0.78 84.8 0.24 0.20 83.3 7.48 897 0.24

    Change %3 -32.9 -5.3 -75.6 -57.9 +0.44 -94.1 -76.71source: Cheng et al. (1999).2The authors report that reduction in N and P2O5 in digestate compared to farrowing and gestation wastes likely to be due to precipitation in the covereddigestion lagoon.3Change based on the waste water digested being composed of 43.8% farrowing and 56.2% gestation.4All changes expressed as % except for pH units.

    Nutrient Value of Digestate from Farm-Based Biogas Plants in Scotland

    Table C13: Comparison of digested (digestate) and undigested (feedstock) cow slurry analysis results for five digester

  • 8/10/2019 Nutrient Value of Digestate

    44/44

    44

    p g ( g ) g ( ) y y gsystems in New York State, USA (mean monthly sampling 2002-03)

    1.

    Total N NH4-N NH4-N P2O5total P2O5 ortho P2O5 Ortho pH COD DMDigester

    kg/m3 kg/m

    3 % of total kg/m

    3 kg/m

    3 % of total mg/I %

    Site 1 (63 samples)

    Feedstock 4.96 1.92 38.7 1.92 1.08 56.3 7.21 134,695 11.42

    Digestate 5.29 2.62 49.5 1.96 1.26 64.3 7.92 94,148 8.30

    Change %3 +6.7 +36.5 +2.1 +16.7 +0.71 -30.3 -27.3

    Site 2 (16 samples)

    Feedstock 3.43 1.73 50.4 1.17 0.59 50.4 7.45 121,987 9.01

    Digestate 3.46 2.21 63.9 1.17 0.67 57.3 7.63 110,658 6.75

    Change %3

    +0.9 +27.7 0 +13.6 +0.18 -9.3 -25.1Site 3 (12 samples)

    Feedstock 3.89 2.22 57.1 1.45 0.88 60.7 7.45 109,723 9.58

    Digestate 3.71 2.47 66.6 1.36 0.95 69.9 7.75 42,416 3.80

    Change %3 -4.6 +11.3 -6.2 +8.0 +0.3 -61.3 -60.3

    Site 4 (12 samples)

    Feedstock 3.38 1.35 39.9 1.54 0.92 59.7 5.64 137,547 12.46

    Food waste2.59 0.72 27.8 1.19 0.59 46.6 4.15 271,945 17.60

    Digestate 3.27 1.47 45.0 1.34 0.80 59.7 7.61 63,996 5.50

    Change %2 - - - - - - -

    Site 5 (9 samples)

    Feedstock 4.01 1.67 41.6 1.01 0.41 40.2 7.45 72,100 8.99

    Digestate 1 4.15 2.30 55.4 1.07 0.66 61.7 7.74 65,627 7.99

    Digestate 2 3.79 2.19 57.8 1.12 0.60 53.6 7.67 61,823 7.52

    Change 1 %3 +3.5 +37.7 +5.9 +61.1 +0.29 -9.0 -11.1

    Change 2 %3 -5.5 +31.1 +10.9 +46.3 +0.22 -14.3 -16.41source: Wright, et al. (2004).2Change not calculated since proportion of cattle slurry/food waste not known.3All changes expressed as % except for pH units.