materials flow and possibilities

Bioresource Technology 41 (1992) 235-245

Materials Flow and Possibilities of Treating Liquid and Solid Wastes from Slaughterhouses in Germany. A Review W. P. Tritt & F. Schuchardt

Institute of Technology, Federal Research Centre of Agriculture, Bundesallee 50, W-3300 Braunschweig, Germany

(Received 18 August 1991 ; revised version received 30 October 1991 ; accepted 5 November 1991 )

Abstract

The task of treating and disposing of residues and wastes and~or of purifying wastewater is faced by slaughterhouses worldwide. Legislation on regu- lating the treatment of refuse and wastewater is not (yet) uniform, but the aims of protecting and preserving the environment can be seen clearly in all laws and regulations.

Germany makes high demands in environ- mental matters, so that an account of future concepts Jbllowing from them may serve as an example .for all countries, at least regarding the essential features, notwithstanding a possible standardiza- tion (~r example in the framework of the Euro- pean Community)..

Key words: Slaughterhouse, wastewater, wastes, anaerobic treatment, composting.

INTRODUCTION

In the Federal Republic of Germany some 4-86 million head of cattle, 0.64 million calves and 38.93 million pigs were slaughtered in the year 1988 (Wiethrlter, 1989). Based on an average live weight of 550 kg in cattle, 175 kg in calves and 100 kg in pigs, this meant a total weight of slaughtered animals of 7.07 million tonnes. There are some 1.77 million tonnes of residues and wastes produced during slaughter and further processing, which corresponds to 25% of the total weight of slaughtering animals (Baller et al., 1982). Legisla- tion on the disposal of carcasses (Anon., 1975) provides, in principle, for the orderly disposal of the slaughtering wastes, slaughtering byproducts

Bioresource Technology 0960-8524/92/S05.00 © 1992 Great Britain

and confiscates (materials that must (eyes, stomach, genitals, etc.) or may be (diseased tis- sues, etc., condemned after slaughter) removed for separate disposal via carcass-disposal plants). Under this legislation not only carcasses but also parts of them (such as blood, bristles, feathers, hides, skins, horns, claws, bones and wool) are to be disposed of by the carcass-disposal plants. However, since it is impossible for technical and organizational reasons to retain these residues completely, some of them get into the wastewater system together with the process water. The residues separated in the wastewater by physical means, such as screenings, fat and flotation tailings, as well as the stomach and intestinal contents and the manure, are materials that the slaughterhouses bear sole responsibility for disposing of.

The classification of slaughterhouse wastes as hazardous wastes, valid since 1990 (Anon., 1990b, c,d), and the tightening of wastewater regulations from 1991 by resetting the contamination parameters and increasing payment rates (Anon., 1990a) will lead to a sharp increase in the costs of treatment and disposal. For many slaughterhouses this means in the short and medium term that wastewater purification plants will have to be reviewed or newly planned. In addition to this there is the task of developing economically viable and ecologically acceptable concepts for processes, process combinations and/or disposal plants for treating the wastes and disposing of the solids-rich substrates.

The necessary substrate-specific values have been drawn up both for the primary liquid and solid residues and wastes occurring during slaughter and processing and for the secondary solids-rich substrates separated from the wastewater.

235 Elsevier Science Publishers Ltd. England. Printed in

236 W. P. Tritt, F, Schuchardt

For the tasks of t rea tment and disposal in the f r amework of wastewater purif icat ion and waste t rea tment possible combina t ions of processes and al ternative pr incipal chains are shown, special a t tent ion being given to new concepts whose individual processes have p roved successful with o ther substrates but are not yet the s tandard for s laughterhouses .

W A S T E P R O D U C T S

Description of the liquid and solid residues and wastes T h e liquid and solid residues and wastes occurring in a s laughterhouse are l u m p e d together, somet imes with some simplification, as 'materials flow'. Besides the waste flows occur r ing in the vehicle washing, animal sheds, s laughtering and tr ipery depar tmen t s , the res idues f r o m triping, and the residues f rom screening, f lotation, grease

removal and rumenal - res idues dehydra t ion are descr ibed with regard to a m o u n t and compos i t ion in Table 1.

Residues from vehicle washing and from the sheds T h e residues occurr ing in the t ranspor t vehicles and in the sheds are c o m p o s e d of excrement , ur ine and bedd ing (e.g. straw). It is especially on these materials that very few data, if any, have been received, so that the animal-specific amoun t s and the compos i t ion of exc remen t and urine had to be assessed on the basis of figures f rom animal keeping. Based on an a m o u n t of excrement and urine of approximate ly 3"9 and 7.0 kg /d in the case of pigs and approximate ly 15 and 60 kg /d in the case of cattle (fattened oxen, fa t tened cows) (Bailer et al., 1982) and an average waiting t ime for the animals at the s laughterhouse of 12 h (Coenen & Kamphues , 1981), figures result as in Table 1.

Table 1. Amount and composition of liquid and solid residues and wastes from a slaughterhouse

Substrate Amount (a) per pig (b) per head of cattle

Composition

Straw/manure

Excrement and urine (liquid manure)

Washing down and cleaning water

Slaughtering water

Blood

0"5-3 kg/animal

(a) 2"0-3"5 kg"

(b) 7-5-30 kg"

Approx. 100 litres per delivery vehicle (3 min cleaning time), water consumption high-pressure cleaner approx. 0"5 litre/s

(a) 100-300 litres a.' (b) 500-1000 litres a,'

Total: (a) 4-6 litres ~,',J (b) 20-35 litres~,',~ Retained: (a) 3.4 litres J (b) 15-3 litres a Unavoidable losses: (a) Approx. 0"5 litre' (b) Approx. 2.0 litres'

C/Nratio: approx. 30

DM: 6-10%;ODM: 77-84%ofDM" C/N ratio: 9-15" BODs: approx. 30 000 mg/litre b Nitrogen: 6.7 g/litre" Potassium: 3-7 g/litre" Phosphorus: 5'8 g/litre" Calcium: 4.5 g/litre a Magnesium: 0-8 g/litre"

DM: 10-12%;ODM: 77-85%ofDM" C/N ratio: 9-15" BODs: approx. 15.000 mg/litreh Nitrogen: 4.7 g/litre" Potassium: 5"9 g/litre" Phosphorus: 2"4 g/litre" Calcium: 2"5 g/litre" Magnesium: 0.6 g/litre a

BODs: 10 000-20 000 mg/litre'

BODs: 1 000-2 800mg/litre' BODs: 1 500-3 250mg/litre a COD: 375 000 mg/litreg BODs: 150 000-200 000 mg/litre",',J DM: 18-20%;ODM: 96% of DM Crude protein: 680-790g/kgDM Crudefat: <50g/kgDM Nutrient salts: 100-110 g/kg DM Organic acids: 80 mg/litre Organic N: 30 g/kg NH4-N: 2 g/kg

Materials f low and possibilities of treating liquid and solid wastes 237

Table 1 . - - contd.


( 'omposition

Slaughtering wastes/ slaughtering byproducts

Confiscates

Bristles, claws

Bones

Tripery wastewater: Intestinal contents

Fat sludge

Intestinal mucus

Wastewater

Rumenal and pig stomach contents

Total wastewater

Screenings ( 1-0-25 mm screen aperture)

Fat/flotation tailings (from flotation)

(a) Approx. 22 kg" (b) Approx. 77 kg ~ (a) Approx. 0.91 kg ~ (b) Approx. 4"76 kg '

(a) 0 '4-0 '5 kg je Bristles: 0.25-0.6 kg(depending on season) (b) 1.8-2.4 kg j,¢

(a) Approx. 9'5 kg ' (b) Approx. 66"2 kg"

(a) 2.5-2-8 kg k-~ (b) Approx. 18 kgJ (a) Approx. 0"4-0"6 kg (b) 1.2-1.8 kg ~ (a) Approx. 0-75 kg' ib) Approx. 1.8 kg ' Approx. 5000 litres/100 intestines

(a) 0-4-1.6 kg ...... (b) 40-80 kg"

(a) 200-600 litres"P,~ 58-620 litres'~

(b) 1000-1500 litres"P,,~

400-3100 litre'~

(a) and (b)

Pig slaughter: DM: 0.15-0.30 g/litre

Cattle slaughter: DM: 13.0-15.0 g/litre

(a) 0.5-4-5 litres (b) 4.0-24.0 litres

DM: 20%; fat: 15%'

Bristles: DM: 76%; ODM: 98"/,,of DM Crude protein: 950 g/kg DM Crude fat: 20 g/kg DM

DM: 57%:ODM: 63% of DM Crude protein: 360 g/kg DM Crude fat: 220 g /kgDM

COD~ea: approx. 2"800 mg/litrc BODs,~,~: approx. 2-000 mg/litrc e Settlable solids: approx. 14 ml/litre

DM: 12-15%;ODM: 80-84% of DM ...... DM: l l - 1 3 % ; O D M : 8 0 - 8 7 % o f D M " C/N ratio: 17-21 Crude protein: 105- 173 g/kg I )M" Crude fat: 15-31 g/kg DM" Crude fibre: 256-391 g/kg DM" Nitrogen: 20-22 g/kg DM" Phosphorus: 5-6 g/kg DM" Potassium: 4-5 g/kg DM ~' Calcium: 6-8 g / k g D M " Sodium: 9-15 g/kg DM" Magnesium: 0.8-1 g/kg DM"

COD: 500-10 500 mg/litres'~ BODs: 300-6 100 mg/litrc'~ Sed. solids: 1-60 ml/litre'~ COD: 300-600 g'~'/ BODs: 200-350 gJ '¢ Sed. solids: 8-425 g'~

COD: 600-25 000 mg/litres (max. 60 000 mg/litre)'~,v

BODs: 500-11 '500 mg/litre'~ Sed. solids: 30-150 ml/litre'~ COD: 1400-5000 g J'~ BODs: 1000-3500 g,~,,l Sed. solids: 60-4800 g '~

COD: 1000-6000 mg/litrc (max. 20 000 mg/litre) 'e ......

BODs: 1000-4000 mg/litre (max. 10 000 mg/litre) '~ ....

pH: 6-5-10 '~- 't ~' T: 20-35°C/'v~ TKN: 250-700 mg/litre(max. 950 mg/litre) NH4-N: 200-300 mg/litre P: 80-120 mg/litre Sed. solids:: approx. 10 ml/litre

DM: 10-20%; '.,~ ODM: 95-99 '%of DM COD: 300 000-450 000 mg/kg NH4-N: 0"5 g/kg

DM: 5 -24%;ODM: 83-98% of DM COD: 95 000-400 000 mg/kg

238

Table 1 . - contd.

W. P. Tritt, F. Schuchardt


Composition

Fat/flotation tailings (contd)

Fat (from greasetrap)

Liquid phase (fresh rumenal contents

Solids (fresh rumenal contents

(a) and (b) 10-20 litres/m 3 wastewater

1% of live weight enters wastewater as grease,J of which 60% is retained in grease trap (= 6 g/kg live weight)

(b) 0'5-0'6 m3/m 3 rumenal contents ~,u

(b) 0.4-0.5 m~/m 3 rumenal contents g,"

Crude protein: 200-548 g/kg DM Crude fat: 177-440 g/kgDM Organic acids: 20 g/kg DM NH4-N: 0.2 g/kg Phosphorus: 9 g/kg DM Potassium: 0"5 g/kg DM Calcium: 6 g/kg DM Sodium: 2.5 g/kg DM Magnesium: 0"6 g/kg DM

COD: 600 000-800 000 mg/kg DM: 35-70%;ODM: 96% of DM Organic acids: 22 g/kg DM NH4-N: 0'7 g/kg Sand: 20%' Wasteflesh: 15% (20% bounded fat)' Free fat: 15%' Soapable oils and fats: 250 mg/litreq

DM: 1.4-6.5%;ODM: 1.1-5.2%ofDMg," COD: 10 000-80 000mg/litre",~ BODs: 2'500-3-500 mg/litre",g Protein: 300 g/kg DM w Phosphorus: 300mg/litreW Organic acids: 7.000 mg/litre NH4-N: 300 mg/litre Potassium: 1000-3000 mg/litre w Calcium: 90 mg/litre " Sodium: 2000-3000 mg/litre ~ Magnesium: 40 mg/litre "

DM: 25-30%;ODM: 80-90%ofDMs," C:N ratio: 11-20 Minerals: 53-103 g/kgDMg Fat: 25-83 g/kgDMU Protein: 71-111 g/kg DM Crude fibre: 757-825 g/kgDMg Phosphorus: 4.6 g/kgDMW Potassium: 4.9 g/kg DM w Calcium: 5"3 g/kg DM ~ Sodium: 9"5 g/kgDM ~ Magnesium: 0"8 g/kgDMW

"Anon. (1989). hStrauch et al. (1977). ' Jfippelt & Neumann (i 985). 'JATV Leaflet M767, (1988). ~Grosse Frie (1984). fLiebmann (1970). gTritt (1989b). iMiinch (1978). iBaller etaL (1982). kStephan (1983). /Neumann ( 1986 b ).

mSchuchardt (1989). "Tritt (1989a). "Tritt & Kang ( 1991 ). 'Sixt (1979). qSchiissler (1985). rSteiner & Kandler (1983). ~'G6rner (1980). 'B6hm (1990a, b). "Zimmermann & Eggersgliil3 (1986). "Eggersgliil3 & Zimmermann (1986). WKamphues (1980).

Total wastewater Th e total was tewater of a s laughterhouse is in most cases made up of the slaughtering wastewater, the tripery was tewater and the washing- d o w n and cleaning water (see Fig. 1 ). The specific amounts of was tewater and contaminat ion loads

related to the animals s laughtered and processed vary widely among slaughterhouses: they depend on the one hand on the degree of fur ther processing of the slaughtered animals, in part icular on the degree of processing of the s tomachs, rumen and intestines in the tripery, and on the o ther hand

Materials flow and possibilities of treating fiquid and solid wastes 2 3 9

~LX:jhtedng

. . . . . j p : ~ s ~ t o r byp~od - - -

- ~ J - [ S 1 o m a c t ) conte~ts --{ Press water

Pressed solids ]

~ . ~ . . I 1 I ~ , ~ e I . . . . .

' ~ _

Fig. 1. Materials flow of liquid and solid residues and wastes in slaughterhouses, and possible disposal systems: r'--"l process step. ~ ~ primary and intermediate matter, • ramification.

on the quality of measures to retain the solid and liquid slaughter residues.

The greatest part of the contamination is caused by blood (even with proper handling blood losses of about 2 litres per head of cattle and 0-5 litre per pig can be expected) and by stomach and intestinal mucus. Whereas the rumenal contents are retained as a rule in the tripery, the intestinal contents and pig stomach contents are mostly washed down and get into the wastewater. From the tripery comes 30% of the total wastewater and of the organic contamination.

Loaded with solid and dissolved organic substances the total wastewater of a slaughterhouse is characterized, in particular, by fats and proteins and their degradation products, such as volatile organic acids, amines and other organic nitrogen compounds. Carbohydrates (glucose, cellulose), too, arc present in the wastewater in dissolved, or colloidally dissolved, forms. The COD/BODs ratio is about 2"7 to 2"9 (Bailer etal., 1982; B6hm, 1989). Since slaughterhouse wastewater, in principle, also contains disease-causing agents, it must give rise to concern from the epidemiological point ~f view (B6hm, 1990a). Especially on account of the increased temperatures, the wastewater shows a strong tendency towards microbial decomposition and is a good breeding ground for germs. The increases in wastewater temperatures (see Table 1) occur especially when the scalding tanks are emptied. With regard to an aerobic/anaerobic wastewater treatment particular attention must be paid to the presence of disinfectant and cleaning agents in the wastewater.

Slaughter waste, slaughter byproducts and confiscates Contrary to the definitions contained in the legislation above of slaughter wastes, slaughter byproducts and confiscates, blood, bristles, claws and bones are listed separately (Fig. 1). The amount of confiscates contained in Table 1 do not include animal loss in transport. Transport losses are put at 0-4% in the case of pigs and 0"02% in the case of cattle (Grosse Frie, 1984). Only in exceptional cases should disposal of the above residue and waste material in a carcass-disposal plant or special processing plant be dispensed with.

Blood To largely retain the blood is by far the most important internal measure of the slaughterhouses in reducing wastewater pollution. It requires technical precautions to ensure sufficient bleeding of the animals (Borowski & Michel, 1986). The blood is first collected in blood tanks and then transported by special vehicles for further processing in carcass-disposal plants or special processing plants. The data given in Table 1 on composition refer to fresh blood. What is said later on slaughtering wastes, slaughtering byproducts and confiscates applies in general to the anaerobic treatment of blood.

Rumen and stomach contents The rumen and stomach contents are, together with the blood, at the focus of the disposal prob-

240 W. P. Tritt, F. Schuchardt

lems. Processing them into animal fodder, for example via the carcass-disposal plants, is forbid- den under the law on fodder (Anon., 1987) and the regulations governing fodder (Anon., 1981) in the Federal Republic of Germany. The problems of contamination of the residues and wastes to be processed in carcass-disposal plants by rumen, stomach and intestine contents have been pointed out (Lurch, 1990a, b, c; Niemann, 1990).

The overwhelming part of the rumen content is material containing lignocellulose (such as hay, straw, grass, etc.) and the digestive ferments present in the rumen. Untreated rumen content must be classed as epidemiologically dubious. The rumens, even after the slaughter of healthy cattle, have been found to contain somewhat rare salmonella types as well as bacteria, viruses and parasites (e.g. worms) in numbers that are alarm- ing from the epidemiological point of view (Jochemczyk, 1986; Zimmermann & Eggersgliil3, 1986). In addition a number of foreign bodies are to be found in the rumen contents. These have been introduced into the rumen for veterinary reasons or have got into the rumenal contents by feeding or during the slaughter process and they can lead to considerable disturbances or damage to plant during further processing. They include, among other things, sand, stones, rope, thread, magnets, metal cartridges and flesh fragments. Investigations into the heavy metal components of rumen content showed that these were usually below the permitted level (EggersgliJl3 & Zimmer- mann, 1986) and thus far below the limits set under the fertilizer and sludge regulations.

Because of the smaller weights, pig stomach contents are not as a rule collected and disposed of separately but, where the stomachs are opened, washed down with the tripery wastewater. Where there is a larger number of slaughtered pigs the amount of waste is worth collecting and disposing of in the tripery itself.

Screenings Some of the solid residue and waste materials, such as flesh and skin fragments, rumen, stomach and intestinal contents, bristles, hair and horn, bone and hoof fragments, get into the wastewater flow. The amount and composition of the solids retained in the screening plant depends on the process, the rake fineness or screen aperture and the sequence of processes in the total flow pattern. In Table 1 only a few data can be given on these materials, since at present there are no results of systematic investigations.

Flotation tailings and grease trap residues If there is a tripery attached to the slaughterhouse, then because of the high proportion of fat in these parts of the wastewater flow a grease trap and/or flotation should be installed. While only the fats in undissolved form can be separated with the grease trap, flotation additionally eliminates suspended material and colloidally dissolved protein substances. Thus flotation produces improved grease removal. The flotation tailings occurring, in relation to the solid substance content, are characterized by a relatively high protein and fat content.

In the flotation tailings, because of the high protein and water content, there is a rapid growth of saprophytic microbes. The residues of grease removal and the fat-rich flotation tailings are sub- ject to rapid decay, so that as a result of the putre- faction process organic acids are formed, which make processing difficult and lead to higher production costs. The processing of residues of grease traps and flotation plants into, among other things, industrial greases in special processing enterprises (Anon., 1990e) or, as far as separate processing lines are installed, in carcass-disposal plants (Lurch, 1990d) is already practised. Repro- cessing these wastes into fodder is not possible (Anon., 1981, 1987).

Pressed solids and press water from the separation of rumen and stomach contents Independently of the subsequent disposal stage (Fig. 1) dehydration of the rumen and stomach contents is necessary. Whereas practical experience has been gathered in the separation of rumen contents (Zimmermann & Eggersgliil3, 1986), dehydration of pig stomach contents is at present not carried out in slaughterhouses. The data on substances given in Table 1 for rumen contents are independent of the given press, in particular the pressing principle, and of screen aperture size and the pressing pressure.

The pressing causes an uneven distribution of the solid components in the pressed materials and the press water (liquid phase). While the nutrient and mineral concentrations in the solid phase decrease as the dry matter (DM) increases, the organic dry matter (ODM) in relation to the dry matter is of the same order as in untreated rumen content. The press water is a grey-green colloidal suspension with low viscosity. In its consistency it corresponds approximately to semi-liquid pig manure and must be classed as organically highly contaminated (Table 1). The organic contamination increases as the pressure increases, since

Materials flow and possibilities of treating liquid and solid wastes 241

besides the free water more liquid is pressed out of the food residues (Zimmermann & EggersgliiB, 1986).

The nitrogen in the press water is present for the most part in an organically bound form. According to Zimmermann and EggersgliiB (1986) it can be assumed that the ammonium, nitrate and nitrite in the press water are negligible. Sedi- mentation experiments in accordance with the Standard DIN 38409 have shown that phase separation takes place very quickly (15 min) and completely. The proportions of settlable substances are independent of the DM content at between 25 and 100% by volume.

WASTE PROCESSING

Combinations of processes, and principal chains, for the treatment and disposal of liquid and solid residues and wastes Figure l shows., besides the flow of primary materials from the slaughtering process and the secondary materials from the waste and wastewater treatment phases, possible principal chains resulting from the combination of the individual processes. The borderline between pretreatment of the liquid and solid wastes on the slaughterhouse site and external purification or treatment is rather fluid, depending on local conditions. Physical processes used for wastewater treatment, such as coarse material removal, grease removal and flotation, or the presses used for dehydrating rumen contents, or aerobic wastewater treatments will not be dealt: with further here. These processes have been used successfully for years in slaughterhouses, so that experience has been gained on dimensioning and operational safety iZimmermann & Eggersglfil3, 1986; ATV Seminar, 1990).

On the disposal phase of anaerobic pretreatment of slaughterhouse waste and wastewater it must he noted that the principle of not mixing solid and liquid wastes unnecessarily before further treatment also applies to liquid and solid slaughterhouse wastes.

The role of the carcass-disposal plants and of the special processors in the framework of an ecologically desirable concept of disposal for the materials occurring in slaughterhouse wastes, byproducts and confiscates is in general desirable, because the wastes can then, after sterilization (3 bars, 133°C, 20 min), be processed into useful products (such as carcass meal, animal fat, blood

meal, bonemeal, etc.) (Neumann, 1986a), thus ful- tilling the utilization regulations laid down by law (Anon., 1986). A disturbance of the link between slaughterhouses and carcass-disposal plants fixed by the law should be avoided if possible, because removing the residues and wastes to the carcass- disposal plants is the most inexpensive solution for the slaughterhouses. Ecologically harmful disposal practices no longer admissible under the latest legislation (e.g. depositing waste on dumps or on agricultural land) will not be considered further.

Anaerobic wastewater purification The choice of a better alternative -- to pay wastewater charges or to (partially) purify wastewater -- has to be faced anew by enterprises of the meat industry every time existing regulations are amended. The use of anaerobic techniques presents itself as a solution since on the one hand slaughterhouse liquid wastes, on account of their composition and contamination concentration, are very well suited to anaerobic pretreatment and on the other hand it is possible to reduce the highest contamination found in such wastes.

The advantages associated with anaerobic treatment of slaughterhouse wastewaters include:

-- considerable reduction of the concentration of impurities in the water;

-- low excess sludge production; -- biologically stable excess sludge; -- no odour emission; -- production of energy-rich gas that can be

used in slaughtering as a substitute for con- ventional primary energy.

Although these advantages have been brought out and documented in publications (Martin, 1990), findings in this field have been applied on a large scale in industry only in the United States and Europe (Tritt, 1990). Whereas in earlier years preference was given to the use of the activated- sludge process, in the further course of development processes with an improved biomass (fixed-bed reactors) came to the fore. The associated higher space-time yield contributes con- siderably to the economic viability of such plants. The steps in treatment suggested in Fig. 1 prior to the anaerobic stage are not mutually conditioning in every case, but the combination of these steps must be coordinated with a view to the anaerobic process used and its purification output.

Experiments with only screened (screen aperture 2mm), homogenized slaughterhouse


wastewater in laboratory and pilot-scale fixed-bed reactors (V= 5.5 litre or 2.8 m 3) have shown that with COD loading rates between 1 and 4 kg/m 3 d a COD removal efficiency of 73-95% can be achieved. A two-stage process in the treatment of the wastewater seems not to be useful. Preacidifi- cation does not lead to the desired conditioning of the substrate since, due to the dynamics of the buffer system in the wastewater, as a result of the formation of bicarbonates and the mineralization of the nitrogen, and the changes in the pH value dependent on these processes, only a low acid production takes place and no acid mixture emerges that would be useful for the methane stage.

The desired extent of purification in a pre- liminary anaerobic stage depends on the subsequent aerobic purification stage. According to the aim of the particular recipient, parameters of the effluent quality (carbon degradation, nitrifica- tion, denitrification) and the appropriate C:N ratio (ATV Report, 1987) must be guaranteed in the outflow of the anaerobic reactor. This means that an anaerobic plant must not be designed for the full purification output, with regard to the BOD 5 or COD, in every case but must rather be adapted to the requirements of the subsequent aerobic stage. If necessary, part of the wastewater flow should be led directly to the aerobic stage. During the above pilot-scale experiments the C O D : N ratio in the reactor outflow was between 4-1 : 1 (LR, cOD ~ 2 kg/m 3 d; HRT ~- 4 d) and 6.7 : 1 (LR.cOD = 4 kg/m 3 d; H R T = 2 d).

Anaerobic treatment of solid materials Removal of the solids-rich wastes from slaughter and of residues from wastewater treatment has always been a problem for slaughterhouses with regard to disposal costs. As a rule these partly dehydrated substrates have been taken to domestic refuse-disposal sites. However, the costs for this kind of disposal are between 35 and 130 deutschmarks per tonne, although in comparison with other methods it has been the least expensive solution. Other possibilities of disposal were not known or were not sufficiently developed for large-scale use. For instance, the costs for the treatment of the flotation tailings in the digester of a municipal sewage plant are between 80 and 120 DM/t, while the costs for the disposal of fat and flotation tailings using a special processing plant can be up to 250 DM/t. However, a problem here is to find a suitable sewage plant or specialist enterprise.

One consequence of the latest legislation and regulations for slaughterhouses is that in future they may not deposit such wastes on sites, since they cannot show that they are impossible to utilize. Treatment as 'special waste' would no doubt be too expensive for the slaughterhouses. Experience gained in practice shows that up to 600 DM/m 3 must be spent for burning the wastes or treating them as special wastes (Lurch, 1990b).

Biomethanization of solid residues such as rumen and stomach contents and screenings and of solids-rich substrates such as rumen press water, flotation tailings, grease trap residues and the excrement and urine mixture from the sheds can compete very well with the disposal methods mentioned above, on account of the energy poten- tial of such wastes. It does presuppose, however, that a suitable reactor system can be provided. The control of the formation of a floating scum and the guarantee of safe treatment of substances regarding foreign bodies in the substrate are car- dinal problems in this connection. Whereas there is already some experience of the anaerobic pretreatment of slaughterhouse wastewater in large- scale reactors, so far only one industrial-scale plant for the methanization of rumen contents is known, in Greinsfurt (Austria)(N6bl, 1990). This, however, has had to be closed and reconstructed because of technical problems. The first large- scale plant in Germany for the methanization of rumen press water and flotation tailings is an aerobic reactor being built on the site of the slaughterhouse in Hamburg (Tritt, 1990). The results and experience gained with these two reactors are not yet known and will presumably be forthcoming. The first results of a research and development project on the methanization of fresh rumen contents in a completely stirred tank reactor (CSTR) (V= 25m 3) on a pilot scale have already been obtained. The dry matter content (DM) in the reactor is kept at 70-80 g/litre. The gas productivity with a retention time of 39d can be put at 1"4 m3/m3d. The methane content of the gas is about 60% by volume. Further experiments with batch reactors (V= 3 litre) to find out the anaerobic degradation behaviour, the substrate- specific methane yields, the maximum degrad- ability and the substrate decay rates of flotation tailings, grease trap residues, rumen press water and screenings have shown suitability in principle and possible energy gains. Control of the fluid- mechanical properties of the substrates used in a pilot-scale or large-scale reactor has yet to be demonstrated.

Materials flow and possibilities of treating liquid and solid wastes 243

In Fig. 1, besides the substances suitable for anaerobic pretreatment, the possible combinations of processes involved are also shown. Dependent on the subsequent stages of treatment (such as composting the solid materials) and/or the substrates used and their mixing ratio, separation of the solid materials is to be carried out before or after the anaerobic treatment. For the dehydration of fresh or also anaerobically pretreated rumen contents reciprocating or screw presses have turned out to be successful in various slaughterhouses or in experiments (Zimmermann & Eggersgliil3, 1986). Since neither a mesophilic (Jochemczyk, 1986) nor a continuous-thermophilic Marchaim, 1988) process in an anaerobic reactor leads to a positive decontamination of the substrate, the starting material must subsequently be given hygienic treatment.

Apart from a thermophilic batch process and the well-known physical and chemical processes (B6hm, 1989), composting in this connection is a suitable process. One interesting combination is a prior biomethanization accompanied by mechanical phase separation (e.g. pressing) and subsequent composting. The advantages of mass and volume reduction involved in anaerobic treatment, apart from obtaining energy, can be com- bined with obtaining valuable materials in the form of marketable compost material. Over and above this, anaerobic pretreatment leads to a minimization of expenditure on capturing and treating the outlet air to remove the odours given off as a result of the open handling of wastes and composting.

Against the background of the present costs mentioned above for the disposal of solid slaughterhouse wastes, economic viability of an anaerobic plant in connection with composting could easily be achieved, even without marketing the composted material.

Composting On the basis of the new legal requirements on slaughterhouse wastes mentioned above, composting comes to the fore as an integrated, or even sole, disposal method. The fact that rumen contents are composted by only nine slaughterhouses in the 11 Federal States of former Western Germany (figures from 1986) (Zimmermann, 1986) can be ascribed both to lack of knowledge of the compostability of this material and to the more convenient and, until 1991 in some cases, cheaper possibility of disposal by dumping. With the exception of the dung occurring in the sheds and during vehicle washing no material from a

slaughterhouse fulfils a priori the necessary conditions for optimum composting. Rumen and stomach contents and screenings contain veg- etable structural materials but at the same time have high water contents. Flotation tailings and fat from the grease trap contain no structural substances. Composting is only possible through the appropriate steps in processing, such as mechanical phase separation or mixing moisture sorb- ing and structural components with these liquid or pasty sludges (see Fig. 1 ).

It has been shown in experiments that both fresh and anaerobically pretreated rumen contents, after mechanical dehydration to a dry matter content >- 20%, can be composted without additives with a bed depth of 1 m. With greater bed depths the dry matter content should be at least 22%. Pig stomach content can be mechanic- ally dehydrated and composted just like rumen content. Where stack composting is carried out, a reaction time of approximately 6 weeks should be set. Anaerobic pretreatment of these substances reduces the reaction time by about a third and can remove the very unpleasant odours issuing (especially from pig stomach contents).

By the use of strongly dehydrating machines (DM>35%), such as screw presses, a water- absorbing material can be produced from rumen and stomach contents. To the dehydrated substrates larger amounts of undehydrated flotation tailings and/or fat from the grease trap can then be added. Experiments with dehydrated rumen contents (DM=37-6%) and flotation tailings (DM = 8-8%) have shown that after about 6 or 8 weeks there is a finished compost. Because of the heating of this mixture to over 70°C during corn- posting decontamination is guaranteed. Related to the fresh weight of the substrates to be corn- posted, rumen contents (DM = 12°/,)) and flotation tailings (DM = 8-8%) can be mixed in the ratio of 5.9 rumen content: 1 flotation tailings (w/w). However, so far there has been no verification of these experiments on a large scale.

Composting can be carried out either in stacks (see above) or in special composting tanks (bio- reactors), possibly with final maturing in stacks (Schuchardt et al., 1988). Composting in stacks requires less investment and running costs than in reactors. The costs for stack composting can be put at between 40 and 80 deutschmarks per tonne of starting material. By composting in reactors, the process can be controlled better with regard to the exchange of respiration gases and temperature conditions than it can be in a stack. Thus the starting materials can be decontaminated, and the


odorous and ammonia- laden outlet air can be captured and treated. The condit ions for year- round compost ing of slaughterhouse wastes in stacks are a cover and a wind shield to the side of the compost ing material. To achieve decontamination during stack compost ing it is necessary to use intensive-turning machines and to turn at least twice a week during the high-temperature (over 50°C) phase (Schuchardt, 1990).

SUMMARY

The rising costs of wastewater and waste removal in the wake of current legislation are forcing the slaughterhouses to rethink their present concepts for waste disposal and to find economical solu- tions.

The solid and liquid wastes and residues of a slaughterhouse, on account of their composit ion, are ideally suited to biological processing and disposal. Whereas in the past the emphasis was on mechanical and aerobic t reatment of the wastewater and dumping of the refuse, what is required now is processes whose performance is up to the very much changed ecological and also economic requirements. As is already the case for other organically highly-contaminated wastewater, anaerobic techniques are an efficient, and at the same time operationally safe, purification process for treating slaughterhouse wastewater. While experience has already been gained in anaerobic wastewater t reatment in large-scale plants there is only one plant for the biomethanizat ion of cattle rumen contents, in Austria. Experience with this plant is still awaited.

Especially for the treatment of solids-rich slaughtering residues or of wastewater, if the fluid- mechanical propert ies are taken into account in planning the concept and scale of the reactor system, anaerobic technology as part of the overall concept can be an extremely economic solution.

Composting, because of the advantages involved, will in future play a bigger part in the treatment and processing of slaughterhouse wastes. After suitable processing and mixing, composting is possible for most of the residues and waste materials. Depending on the amount of plant structural fibres available in the given mixtures, compost ing can be the sole me thod of disposal for rumen and s tomach contents, screenings, fat and the flotation tailings. The combination of prior biomethanizat ion of solids-rich substrates

and subsequent compost ing of the solids can, depending on local conditions and by using the process-specific advantages, be the ecologically most sensible, and economically most interesting, solution.

A C K N O W L E D G E M E N T

The work was carried out as part of the R & D project 'Biological treatment of slaughterhouse waste' suppor ted by the German Federal Ministry of Research and Technology in the f ramework of the scientific cooperat ion between Germany and Indonesia in the field of biotechnology.

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