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15th

European Biosolids and Organic Resources Conference and Exhibition

www.european-biosolids.com

Organised by Aqua Enviro Technology Transfer

OPTIMISATION OF THE ADVANCED DIGESTION PLANT AT AVONMOUTH

S. Bungay1,

, L. O’Hara2, M. I. Baloch

3

1 Principal Process Engineer, Monsal Ltd / Director, Helix Environmental Consultancy Ltd

2 Process Scientist, Geneco - Wessex Water Ltd

3 Senior Process Engineer, Jacobs Engineering, 1180 Eskdale Road, Wokingham, RG41 5TU

Corresponding Author Tel: 07796 172670 Email: [email protected]

Abstract

Anaerobic digestion involves a consortium of bacteria, with the degradation of complex

particulate sludge solids being described as a multi-step process of serial and parallel reactions.

Acid phase digestion (APD) or Enzymic Hydrolysis (EH) separates out the hydrolysis and

acidogenesis stages from the methanogenic stage, providing optimal conditions for hydrolysis

and acidification. The different microbial groups have different environmental and nutritional

requirements, and this is the fundamental premise for enzymic hydrolysis or two stage acid

phase digestion. Hydrolysis, acidogenesis, and acetogenesis proceed faster in an acidic

environment, and methanogenesis proceeds faster in a neutral environment. Under these

conditions, hydrolysis is no longer the rate-limiting reaction, and digestion becomes more

efficient.

In 2007, Wessex Water installed an APD plant upstream of six existing conventional Mesophilic

Anaerobic Digesters at their wastewater treatment works at Avonmouth, Bristol to maximise the

generation of renewable energy at the site. At the time of construction it was the largest

advanced digestion plant in the UK.

This paper discusses the integration and optimisation of the APD plant with the methane phase

plant at Avonmouth.

Key Words

Acid Phase Digestion, Methane Phase Digestion, Advanced Anaerobic Digestion, Biological

Hydrolysis, Enzymic Hydrolysis, Two-Phase Digestion.

Introduction

The Sludge Treatment Centre (STC) at Avonmouth treats a mixture of indigenous primary and

secondary sludge, imported liquid municipal sludge, and imported liquid commercial waste. The

STC treats the sludge using Mesophilic Anaerobic Digestion (MAD), before recycling to

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agriculture. The STC has undergone a number of expansions over the last few years. In 2007,

Wessex Water installed an Acid Phase Digestion (APD) plant upstream of six existing

conventional Mesophilic Anaerobic Digesters at the STC at Avonmouth, Bristol to maximise the

generation of renewable energy at the site, and to produce a conventionally treated sludge. At

the time of construction it was the largest advanced digestion plant in the UK.

Anaerobic digestion is unique amongst current treatment technologies in that it stabilises

sludge, reduces volume and odour, and generates biogas that can be used as a renewable

energy source. The flow sheet for a conventional anaerobic digestion plant is shown in Figure 1

below, and is a fair representative of the STC at Avonmouth before the acid phase pre-

treatment stage was installed.

Figure 1: Conventional Anaerobic Digestion

Anaerobic bioreactors generally comprise of four major components; a closed vessel; a mixing

system, a heating system; and a gas-liquid-solids separation system. The first tank is used for

digestion and is heated and mixed. The second tank is usually unheated and used principally for

storage and degassing of the digested sludge. In some installations the secondary digester is

covered and connected to the biogas system. The terminology when describing digesters varies

between America, Europe, and the UK. The flow sheet shown in Figure 1 is described in Metcalf

& Eddy (2003) as a two-stage digestion plant, where a high-rate digester is coupled in series with

a secondary digester or post-digestion tank. In the UK this flow sheet would be referred to as a

conventional digestion plant; not high-rate; and not always two-stage. For the purposes of this

paper, two-phase or phased digestion is used to describe a process where the digestion process

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is separated into different reactors (or phases) to optimise the process, such as acid phase

digestion, or enzymic hydrolysis.

As shown in Figure 1, Avonmouth is a fairly typical anaerobic digestion plant for treating

municipal sludge arising from wastewater and sewage treatment. The plant comprises of

primary and secondary digesters, biogas storage, and Combined Heat & Power (CHP). Digested

sludge is dewatered and recycled to agriculture. Prior to the APD plant being installed the

dewatered cake was limed to achieve the desired pathogen kill. The primary digesters are

operated at 35oC and utilise a combination of pump and gas mixing. The digesters are heated by

recovering heat from the CHP plant.

Two-Phase Anaerobic Digestion

The performance of anaerobic digestion can be improved by adding advanced pre-treatment

methods. Biological hydrolysis (enzymic or acid phase) using two-phase digestion enables the

hydraulic retention time (HRT) to be reduced; the digesters can be operated with a solids

loading as high as 4-6 kg VS/m3/d; biogas yields are increased; and reliable pathogen inactivation

can be achieved. This paper discusses the optimisation of the two-phase digestion plant at

Avonmouth. Figure 2 shows the configuration of a typical acid phase digestion plant.

Figure 2: Acid Phase Digestion

Phased biological hydrolysis; Acid Phase Digestion or Enzymic Hydrolysis separates out the

hydrolysis and acidogenesis stages from the methanogenic stage, providing optimal conditions

for hydrolysis and acidification. As shown in Figure 2, an additional reactor is installed upstream

of a conventional MAD. The hydraulic retention time of this acid-phase reactor is in the order of

3 days. Technically, Acid Phase Digestion is acid driven hydrolysis, and Enzymic or Enzymatic

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Hydrolysis is enzyme driven hydrolysis. However, sewage sludge involves a consortium of

bacteria, so in the context of anaerobic digestion they are both the separation of the hydrolysis

stage from the acidogenesis and methanogenesis stages, and the terms are almost

interchangeable. The APD plant installed at the Avonmouth STC was a Monsal Enzymic Hydrolyis

(EH) plant. The Monsal Enzymic Hydrolysis process utilises multiple CSTRs in series to harness

the benefits of plug flow batch treatment prevents short-circuiting. An advantage of the Monsal

EH Process is that in using multiple tanks a hydrolysis profile across the reactors develops

making the configuration more robust when treating variable sludge loads. The flow sheet for

Enzymic Hydrolysis is shown in Figure 3 below.

Figure 3: Avonmouth Acid Phase Digestion Plant

The APD plant at Avonmouth utilises six serial reactor vessels, with an overall retention time of

2-3 days upstream of MAD. The plant was designed to operate as a mesophilic system at 42oC

for optimum enzyme activity. Each APD vessel is mixed using gas mixing, and sludge is moved

through the plant in a reverse cascaded batch, via high and low-level gas lifts. Electrical energy is

generated via 5 no. biogas powered CHPs, and heat recovered from the CHP plant is used to

provide all the heating requirements of the process stream.

Avonmouth Sludge Treatment Centre

The Sludge Treatment Centre has undergone a number of expansions over the last few years;

before and after the installation of the APD. Sludge from the various locations is treated in two

process streams; Stream 1 (MAD1); and Stream 2 (MAD2). It is MAD1 that includes the APD

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plant upstream of six anaerobic digesters, treating up to 84tds/d to conventional treated

standards, and generating renewable energy via biogas. MAD2 consists of four conventional

anaerobic digesters, with the utilisation of MAD2 being a recent upgrade to the site. Four

redundant secondary digesters have been converted to primary digesters and now treat up to

33tds/d. Currently the site has the capacity to treat up to 84tds/d in MAD1, and 33 tds/d in

MAD2, giving a combined capacity of 117tds/d.

The STC is operated such that MAD1 takes priority over MAD2, with MAD1 treating a consistent

daily sludge load, and MAD2 treating the excess sludge not treated in MAD1, as MAD2 still

requires additional treatment using lime to achieve a conventionally treated sludge. However,

Wessex Water are currently reviewing the option to upgrade the STC such that all sludge can be

treated in the APD pre-treatment plant prior to anaerobic digestion so that additional treatment

is not required to achieve the desired pathogen destruction.

In practice MAD1 is fed 1,400m3/d at approximately 5.5 % d.s. (% w/w) giving an actual average

daily load of 77tds/d, and MAD2 treats 10 to 30tds/d at approximately 5.5 to 6% d.s. (w/w)

depending whether there is sludge available in the catchment. Therefore, the actual catchment

load is in the range 87 to 104tds/d. Table 1 below shows a comparison between the two

streams.

Table 1: MAD1 & MAD2 Comparison

MAD1 MAD2

No. of Digesters 6 4

Digester Vol (m3) 2,700 2,200

Digestion Volume (m3) 16,200 8,800

Feed Flow (m3/d) 1,400 500

Dry Solids (% d.s.) 5.5 6.0

Dry Solids (kg/d) 77,000 30,000

VM (%) 75 75

Volatile Solids (kg/d) 57,750 22,500

VS Load (kg VS/m3) 3.56 2.56

HRT (days) 11.57 17.60

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Stream 1 (MAD1)

MAD1 comprises of an APD Thickener Feed Tank; three Ashbrook Simon-Hartley gravity belt

thickeners; an APD Buffer Tank; six APD Reactors; six MPD reactors (Digs 5 to 10); a Post-

digestion Storage Tank; an External Buffer Tank; and three Centrifuges.

Stream 2 (MAD2)

MAD2 comprises of a Bellmer Thickener Feed Tank; two Bellmer gravity belt thickeners; a

Common Sludge Tank; four conventional MADs (Digs 1 to 4); a Post-digestion Storage Tank; a

Digested Sludge Break Tank; and three Baker Hughes Centrifuges. The dewatered cake is treated

in a Euroby Belt Drier. There is a facility to transfer the sludge from either stream (MAD1 or

MAD2) to the existing liming plant. Because the refurbishment of MAD2 is fairly recent, this

paper does not cover the performance and operation of Stream 2, but just the APD & MPD in

Stream 1.

Avonmouth APD & MPD Digestion Plant

The Monsal Enzymic Hydrolysis plant at Avonmouth was commissioned in October 2007. The EH

plant has become known as the APD, and existing MADs have become known as the methane

phase digesters (MPDs). The main drivers behind the APD installation were to maximise the

biogas production in order to generate more electrical power, and to achieve a conventionally

treated sludge to reduce the cost of liming the dewatered cake.

The APD plant is shown in Figure 4 below. At the time of construction, Avonmouth was the

largest advanced anaerobic digestion plant in the UK. The HRT in the APD is 3-days at maximum

flow, and the HRT of the MPD is 12-days, giving a combined minimum HRT of 15-days.

APD Process Description

Primary sludge gravitates to an internal pumping station sludge pit, and is pumped to the STC.

Secondary sludge is thickened and is either pumped to the STC; or gravitates to an internal

pumping station foul pit, and pumped to the primary tanks for co-settlement. If co-settled, the

secondary sludge is mixed with the primary sludge.

The imported liquid municipal sludge is discharged into the internal pumping station sludge pit

and mixed with the indigenous primary sludge. Liquid commercial waste is discharged into the

head of the main wastewater treatment works. Therefore, sludge arising from the commercial

waste stream will manifest itself in admixture with the indigenous primary and secondary

sludge.

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The APD is fed from an APD buffer tank. This tank receives thickened primary sludge; thickened

co-settled sludge, and thickened SAS. The APD consists of six completely mixed stirred tank

reactors (CSTRs) in series to approximate a plug flow reactor. All six reactors can be operated

within the mesophilic range between 32˚C and 42˚C. The reactors operate in a reverse cascade

batch system with 24 batches per day. This method of operation prevents short circuiting and

allows individual biological environments to be established in each reactor. The mesophilic stage

of APD commences the hydrolysis element of the plant. The first reactor vessel (APD1) is heated

by an external heat exchanger to (nominally) 42˚C and the hydrolysing sludge is batched in a

reverse cascade once per hour from APD5 to APD6, then APD4 to APD5 and so on to APD1 to

APD2. Following this final transfer a fresh batch of sludge is then fed to APD1 from APD buffer

tank. This batching process is split into hourly intervals with an approximate batch transfer of

58.3m3 at peak flow.

Figure 4: Avonmouth APD Plant

The reactors operate between a top sludge level and bottom sludge level of approximately

11.8m and 10.8m respectively. The difference in height between the top sludge level (TSL) and

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bottom sludge level (BSL) equates to the hourly batch flow volume. In effect each reactor acts in

a similar way to a sequential batch reactor. The system basically comprises of 7 sequences:

1. A batch of sludge is pumped from AP6 reducing the level from TSL to BSL. The pump rate

is such that it will take approximately 40 minutes to draw APD6 down from TSL to BSL,

providing an (almost) continuous feed to the digesters. This sequence is carried out in

parallel to Sequences 3, 4, 5, 6 & 7

2. A batch of Sludge is transferred from APD5 to APD6




6. A batch of Sludge is transferred from APD1 to APD2. This operation is only carried out if

APD1 is operating at set point temperature

7. A fresh batch of sludge is fed to APD1 from APD buffer tank and heated to the set point

temperature (32˚C to 42˚C) by the Stage 1 heat exchanger.

The system waits for the end of a 1 hour cycle time and repeats.

Performance & Optimisation

The initial commissioning of the APD plant was successful with the APD & MPD plant achieving

on average 52% volatile solids reduction (maximum 56%) during the months between February

2008 to July 2008, with an average and maximum gas production of 390 and 430 m3/tds fed

respectively. The feed rate to APD during this period was 78tds/d. However, after about 10-

months operation a number of operational problems were encountered. To remedy this a

programme of extensive investigation was undertaken to overcome these problems and to

subsequently optimise the performance of the plant. The operational problems were two-fold;

firstly there was a problem with the control of the volume of sludge fed from the APD to the

MPDs; and secondly there was a problem with a stable foam or mousse was forming in the

MPDs, which was restricting the volumetric throughput of the STC.

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APD Flow Control

The throughput of the APD is controlled via a Human Machine Interface (HMI) manually

inputted value. The target daily throughput is entered to the APD HMI, and the software

converts this figure to a batch volume, and sets the fill level in APD1 depending on the flow. As

the inputted flow goes up, the fill level of APD1 goes up accordingly until a top sludge level is

reached, at which point the APD is at its maximum hydraulic throughput. This bottom-up control

logic is exactly the same to that used in many wastewater treatment sequencing batch reactors.

At Avonmouth, there is a flow meter installed between the APD and MPD to measure the flow

pumped to the MPD. This flow meter would monitor the flow over 24-hours and a daily

adjustment would be made to the target level set-point in APD1 to compensate for any gain or

drift in the measured level in APD1. However, there was never parity between the calculated

flow based on level change in APD1, and the measured flow at the flow meter. The result of this

was that the flow pumped to the MPDs was always hunting around the set-point, and consistent

flows forward could not be guaranteed. After investigating the levels in the individual APD

reactors, it was discovered that there was actually a gain in volume of approximately 3.5 to

5.0%, across the six APD reactors as hydrolysis was starting, and the sludge was starting to gas,

and was becoming less dense. This phenomena is not unique to enzymic hydrolysis; a similar

phenomena is observed with the aeration of activated sludge. Figure 5(a) to 5(f) show the

individual APD levels and Figure 6 shows gain in volume across the APD process.

Figure 5: (a) (b)

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Figure 5: (c) (d)

Figure 5: (e) (f)

Figure 6: APD Sludge Volume Increase

The problem was that the increase in volume was not consistent, so a simple offset could not be

used in the flow trimming calculation. To overcome this, the daily adjustment calculation was

simply removed, and the flow meter between the APD and MPD was used for flow

measurement only, and not flow control. Once this feature had been disabled, the flow through

the APD stabilised, and the daily flow targets were achieved.

APD & MPD Foam Control

The installation of the APD upstream of the six existing digesters facilitated an increase in the

hydraulic and organic loading of Stream 1, improved the gas yield of the system, and produces a

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treated sludge suitable for recycling to agriculture. Therefore it was essential for Wessex Water

to overcome the operational problems and maintain reliable and robust treatment at all times.

Extensive investigations were initiated across APD and MPD plant. Initially anti-foam was used

to control the mousse, and a strategy was put in place to investigate the organic loading rate,

the hydraulic loading rate, the mixing regime, operational temperature, and any potential

inhibitory inputs to the STC. It was very difficult to establish any positive relationship between

any analytical sampling data, and the onset of the formation of foam in the MPD. The plant was

monitored for pH, total VFAs, individual VFAs, alkalinity, acid:alkalinity, dry solids, volatile solids,

and temperature. Results of the analysis are shown in Figure 7 below.

Figure 7: (a) pH (b) VFAs

(c) Alkalinity (d) TSS

(e) VSS (f) VSS & VSR

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(g) DS Feed v VSR (h) DS, VSR, Biogas

(i) VSR, Biogas (j) VFAs v Temp

The key to the solution was to try and find a relationship between the analytical data and the

operation and performance of the plant. As well as sampling the APD and MPD, the mixing,

feeding, and heating of stages were monitored and adjusted; microscopic analysis of the MPD

was undertaken; batch biogas testing was undertaken; and the liquid commercial waste treated

at the STC was scrutinised.

(k) MPD Alkalinity (l) MPD Acid:Alkalinity

Figure 7: Operating data and analytical parameters of APD and MPD

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The sample analysis was relatively inconclusive. The formation of the mousse in the digesters

coincided with rising VFA concentrations in the MPD. So the trigger in the elevated VFAs had to

be found. In accordance with the normal response to rising VFAs in a digester, the hydraulic and

organic loading to the digesters was reduced. At maximum flow to the APD, the HRT in the

MPDs assuming perfect mixing was actually under 12 days. Therefore, it was considered that the

organic and hydraulic loading to the MPDs may be the limiting factor in the process, so they

should be reduced in order for the digesters to recover. However, in practice, this actually made

the situation worse, and the digesters became more unstable.

MPD Mixing & Feeding Regime

During the investigations various changes were made to the mixing and feeding regime of the

digesters. The design of the MPDs is such that a variety of operational changes can be made to

them in order to optimise their performance. First of all the MPDs can be fed either direct from

the APD, or fed into the existing heating recirculation loop. Then via either route, the feed can

be directed to the surface, the middle, or the base of the digester. The MPDs are operated as

fill-and-spill reactors, and digested sludge spilling from the MPD can be drawn from near the

surface, the middle, or the base of the digester. Each digester then has two mixing systems; a

pumped system turning over the digester contents, and a gas mixing system using draught

tubes. The mixing systems are operated independently of each other, and independently of the

feeding regime. Both mixing systems operate on a run and dwell time. Details of the individual

MPD feeding and mixing configurations are illustrated in Figure 8 below.

The six MPDs are identified on-site as Digs 5, 6, 7, 8, 9, and 10. They are fed in a sequential cycle

based on a batch feed and daily volume. During the period that hydrolysed sludge is being

pumped from APD6 to the MPD, a batch volume is fed to the first digester, the second digester,

and so on, in a cycle until the APD batch has been transferred. Changes were made to the

feeding and mixing regime to try and maximise the effective volume of the digesters. Initially the

target was to mix all MPDs constantly, feed the fresh hydrolysed sludge into the recirculation

line, and to feed into the middle of the digesters to avoid ‘hot’ sludge being pumped on top of

the digester. This was to prevent a layer of VFA rich sludge accumulating at the surface of the

digester, as it was suspected that this might be the cause of the foam or mousse.

Varying the run and dwell times of the mixing didn’t have any marked affect on the situation;

either improving or exacerbating it. Whereas, re-directing the fresh ‘hot’ sludge away from the

surface of the MPD actually made the situation worse. The benefit of the surface feed was that

each MPD was fed in four locations, and the action of the fresh sludge hitting the surface

actually helped to break up any surface foam.

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Figure 8: Primary Digester Schematic

So although it was felt that operating the MPDs at an HRT of less than 12-days might have been

the cause of the formation of the foam, ultimately, varying the operation of the MPDs made

very little difference to the overall performance of the STC.

APD Retention Time, Temperature and Microbial Reactions

The APD plant was designed to maximise the performance of the STC in terms of both

maximising the biogas and subsequent renewable energy generated in the plant, and to produce

a conventionally treated sludge product. The initial development of the Enzymic Hydrolysis

process by United Utilities demonstrated maximum enzyme activity at 42oC. This operating

temperature was the basis of the APD plant at Avonmouth. With regard hydraulic retention time

(HRT), the APD plant was designed to give an overall system HRT of 15 days. As the MPD HRT

was actually just under 12 days, the HRT of the APD was designed at 3 days to achieve the

overall target of 15 days. This meant that the HRT in the hydrolysis stage was longer than the

normal design standard.

Analysing the data, although there were no clear relationships between results and the onset of

foam formation other than the elevated VFA levels, there was inference that there might be a

relationship between the dry solids feed, and the VFA instability. It was surmised that at higher

MPD Feed

Recirc Pumps

HEX

Outlet Box

Gas Mixing

Digester Feed

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dry solids loading rates, hydrolysis was becoming more efficient, and consequently the APD

plant was actually working too efficiently, and that hydrolysis may be proceeding as far as

acteogenesis. From this it was postulated that the effectiveness of the APD could be controlled

by varying the operational temperature and HRT of plant.

Where there is process instability, there must be a breakdown somewhere in the chain of

biochemical reactions during digestion.. There are a number of microbial groups involved in

digestion:

1. Hydrolytic Bacteria

Produce extra-cellular enzymes to break down carbohydrates, proteins, and lipids.

2. Acid Forming Bacteria

The break down products of hydrolysis is fermented to acetate, propionate, butyrate,

and hydrogen by the acid-forming bacteria. There are two-groups of acid-forming

bacteria.

i. Acidogenic Bacteria

Acidogenic bacteria metabolise amino acids and sugars to the intermediary

products acetate, hydrogen, and carbon dioxide.

ii. Acetogenic Bacteria

Two distinct groups of acetogenic bacteria can be distinguished on the basis of

their metabolism. The first group, the obligate hydrogen producing acetogens

(OHPA) produce acetic acid, carbon dioxide (CO2) and hydrogen (H2) from the

major fatty acid intermediates (proprionate and butyrate), alcohols and other

higher fatty acids (valerate, isovalerate, stearate, palmitate, and myristate). The

second group are homoacetogens catalyzing the formation of acetate from

hydrogen and carbon dioxide.

3. Methanogenic Archea

Methanogens are divided into two groups.

i. Acetoclastic Methanogens

Which cleave acetic acid into methane and carbon dioxide

ii. Hydrogen Utilising Methanogens

Which utilise hydrogen and carbon dioxide to produce methane.

In simplistic terms, as one group of bacteria produce soluble compounds, they are quickly

degraded as the substrate by another group of bacteria, giving rise to an anaerobic food chain.

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This chain starts with carbohydrates, proteins, and lipids, and ends with methane, carbon

dioxide, and water. The difficulty begins with the interaction of the different microbial groups

which have different environmental and nutritional requirements. This is the fundamental

premise for acid-phase digestion. The theory is that hydrolysis, acidogenesis, and acetogenesis

proceed faster in an acidic environment, and that methanogenesis proceeds faster in a neutral

environment. Therefore, hydrolysis is no longer the rate-limiting reaction, and digestion

becomes more efficient, and digester can be smaller.

The separation of the hydrolysis and acid forming stages from methanogenesis must not

compromise the obligate syntrophy between the acetogens and the methanogens. This relates

to the production and consumption of hydrogen. Acetogens are hydrogen producers, and

methanogens are hydrogen consumers. Hydrogen is central to the production of acetic acid as

the major end product of acidogenesis. However, when the partial pressure of H2 is high, the

reactions leading from long chain fatty acids, volatile acids, amino acids and carbohydrates to

acetic acid will not proceed, and instead fermentation will occur. The bacteria that produce H2

are obligately linked to the methanogens that use it. Only when the methanogens continually

remove H2 by forming methane will the H2 partial pressure be kept low enough to allow

production of acetic acid and H2 as the end products of acidogenesis.

The OHPAs produce acetate and hydrogen. However hydrogen inhibits the activity of these very

same OHPAs. So for the biochemical pathway to follow acidogenesis instead of fermentation,

hydrogen has to be consumed at the same rate it is being produced, i.e., the methanogens have

to consume hydrogen as fast as the acetogens produce it. If acidogenesis is occurring in an

upstream reactor, H2 will accumulate and inhibition of the acetogens will occur.

So theoretically, if the retention time in the acid phase is too long, it is possible that anaerobic

digestion could proceed too far and intermittent inhibition of the acetogens occurs as described

above. This would drive some of the biochemical reactions to fermentation. However, it is very

unlikely that hydrogen inhibition actually occurs in practice, as the pH in both the acid stage and

the methane stage would be driven down, and digester souring would occur. Whereas, acid

phase digestion generates very high levels of alkalinity which will buffer any severe pH changes,

and in practice, the operating pH in the MPD will actually be higher than traditional mesophilic

anaerobic digesters.

Although, inhibition of acetogens is unlikely to occur, unstable anaerobic digestion could occur

with acid phase digestion. If the hydraulic retention time of the acid stage too long, transient

acteogenesis may occur i.e. the reactions in the acid phase proceed as far to allow intermittent

growth of acidogenic and acetogenic bacteria, resulting in unstable digestion. Therefore, two-

phase digestion should be operated such that it forces acetogenesis into the MPDs to prevent

this. This is achieved by keeping the retention time shorter than the doubling time of the

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acetogens, and if the feed sludge hydrolyses very rapidly, reducing the operating temperature to

increase the reaction time of hydrolysis.

The APD plant at Avonmouth is six vessels in series, and it was impractical to take one vessel out

of the flow sheet to try reducing the overall HRT. Therefore, instead of reducing the HRT, the

operational temperature was reduced, to reduce the biological activity within the APD, and to

force acetogenesis into the MPDs. The operational temperature of the APD was reduced to

33oC. In doing this, it actually improved the overall heat balance of the STC, as the sludge to be

treated no longer had to be raised to 42oC. The MPDs now required heating from 33

oC to 35

oC,

but this is easily achieved using the existing hot water ring main and heat exchangers.

Following the reduction of the operational temperature in the APD, the performance of the APD

and MPD stabilised, the foam or mousse in the MPDs disappeared, and the STC could

consistently treat 1,400m3/d. Figure 9 (a, b, and c) show the performance of the APD from

commissioning to the present day. The optimisation of the process took place during the period

June 2008 to January 2009, and throughout that period a lot has been learnt regarding the

hydraulic and organic loading of both the APD and MPD plant.

Figure 9: (a) APD Alkalinity

Although optimal enzyme production occurs at 42oC, this optimisation process at Avonmouth

has demonstrated that the operational temperature is site specific and ideally the APD plant

should be operated at the minimum temperature required for volatile fatty acid production

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(VFA) and stressing pathogen indicator organisms. So now the APD plant is operated at 33oC,

and the MPD plant is operated at 35oC. This has not affected the overall performance of the

plant with regard to volatile solids destruction, biogas production, or pathogen reduction.

In an effort to ‘sweat the asset’, Wessex Water are always looking to push the process harder.

Operating the APD at 33oC has given them a good benchmark to do this from. Wessex Water is

always looking to increase the throughput of the plant. Currently the average dry solids in the

feed to the APD is 5.5% d.s. w/w corresponding to an average daily solids load of 77tds/d. From

this average the APD can tolerate peaks up to 84tds/d, but if the peak is prolonged for a period

of time, the plants starts to suffer from elevated VFA levels again.

(b) APD pH

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(c) APD Volatile Acids

Figure 9: APD Alkalinity, pH and Volatile Acids

So at Avonmouth, given the reactivity of the sludge, the optimum operating temperature of the

APD is 33oC, and given a hydraulic throughput of 1,400m

3/d, and the HRT of the MPDs, the

optimum dry solids feeding the APD is 5 to 6% dry solids.

There is scope to combine the Streams MAD1 and MAD2. By prorating the sludge between the

two streams, the performance of MAD1 could improve further, as the VS load and the HRT are

adjusted back to within the normal design range for anaerobic digesters. The HRT of the APD

plant would be reduced so it could be fed with a higher dry solids feed, and it would also give

Wessex Water the facility to take one digesters out-of-service for routine maintenance.

Conclusions

• The optimisation process of Acid Phase Digestion (enzymic hydrolysis, biological

hydrolysis, or acid-phase) at the STC at Avonmouth, Bristol, has provided a useful insight

on the operation of APD and MPD process.

• The volume of the sludge fed to the APD increases by about 5% across the APD reactors.

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• At Avonmouth, changing the mixing and feeding regime on MPDs did not affect the

overall efficacy of the digestion process.

• Two-phase digestion should be operated such that it forces acetogenesis into the

methane phase digestion, to prevent the onset of transient acetogenesis.

• The performance of acid phase digestion plants should be optimised by adjusting the

hydraulic retention time, the operational temperature, and solids inventory to suit the

sludge feedstock on a site-by-site basis.

• The APD plant was optimised at Avonmouth by reducing the operational temperature to

33oC, and restricting the dry solids feed to 5 to 6% d.s. w/w.

Acknowledgements

The opinions expressed in this report are those of the author(s), and do not necessarily reflect

the views of the organisations involved. The author(s) would like to offer sincere thanks to

Wessex Water and Geneco for their co-operation in researching and producing this paper.

References

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Biosolids Conference. Aqua Enviro, Leeds.

Bungay. S. R., Abdelwahab. M. (2008) Monsal Enzymic Hydrolysis – New Ideas and Lessons

Learnt. 13th

European Biosolids Conference and Workshop. Aqua Enviro, Manchester.

De Lemos Chernicharo. C.A. Biological Wastewater Treatment Series, Volume 4, Anaerobic

Reactors, IWA, 2007.

Leslie Grady. C.P., Daigger. G.T., Lim. H.C. Biological Wastewater Treatment. 2nd Ed, CRC, 1999.

Speece. R.E. Anaerobic Biotechnology – For Industrial Wastewaters. Vanderbilt University, 1996.

Tchnobanoglous. G., Burton. F.L., Stensel. H.D. Wastewater - Engineering Treatment and Reuse.

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Ed, McGraw Hill, 2003.

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