Developments in wastewater treatment in the meat processing industry: A review

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ELSEVIER 0960-8524(95)00140-9 Bioresource Technology 54 (1995) 203-216 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0960-8524/96 $9.50 + 0.00 DEVELOPMENTS IN WASTEWATER TREATMENT IN THE MEAT PROCESSING INDUSTRY: A REVIEW M. R. Johns Department of Chemical Engineering, The University of Queensland, Queensland 4072, Australia (Received 24 August 1995; accepted 30 August 1995) Abstract A review of progress in the treatment of wastewater from slaughterhouses is presented. Significant progress in issues such as nutrient removal and high-rate anae- robic treatment are highlighted. Nevertheless, few data concerning waste minimisation and source reduction in slaughterhouses, which offers the most cost-effective route to waste management in the industry, exist. The information presented enables a better understanding of the problems encountered with the effluent from the industry and common pitfalls with its treatment. Key words: Slaughterhouse wastewater, wastewater treatment, minimisation, nutrients. NOTATION AC AF AFB AS AS-BNR BNR BOD BPR (T)COD DAF DO HRT N O&O TP RO SBR SVI TDS TF TKN TSS UASB VSS Anaerobic contact process Anaerobic filter Anaerobic fluidised-bed reactor Activated sludge Activated sludge-biological removal process Biological nutrient removal Biological oxygen demand (mg/l) Biological phosphorus removal (Total) chemical oxygen demand (mg/1) Dissolved air flotation Dissolved oxygen Hydraulic retention time Nitrogen concentration Oil and grease concentration Total phosphorus concentration Reverse osmosis Sequencing batch reactor Sludge volume index Total dissolved solids concentration Trickling filter Total Kjeldahl nitrogen concentration Total suspended solids concentration Upflow anaerobic sludge-blanket reactor Volatile suspended solids concentration nutrient 203 INTRODUCTION The meat processing industry is large, common to many countries and generates large volumes of wastewater, which requires considerable treatment if its release to the environment is to be sustainable. Although a number of reviews have been published on the topic (Reczey, 1984; Cimino, 1987), the latest in English was that of Bull et al. (1982). In the intervening period there have been substantial fundamental and technical advances in generic pol- lution control issues, such as nutrient removal, high-rate anaerobic technology development and water reuse. This review attempts to identify the latest trends in waste management and treatment technology in the meat process industry and to dis- cuss the design and performance data available in the scientific public domain. Characterisation of wastewater from slaughterhouses and rendering plants The concentrations of pollutants in various waste- water streams from slaughterhouses or rendering plants are summarised in Table 1. Despite the sub- stantial variation in the results, these data provide a useful guideline for wastewater-treatment-system design and act as a starting point for waste mini- misation. Whereas the combined slaughterhouse wastewater can vary substantially with respect to COD and TP content, the nitrogen content is usually more invariable. Tritt and Schuchardt (1992) present the most recent and comprehensive summary of the physi- cochemical characteristics of many different waste streams from German slaughterhouses, but care is needed in using these figures. German slaughter- houses have no rendering plant, since this processing must, by law, be performed in a separate off-site facility (Tritt & Schuchardt, 1992). The data of Ste- bor et al. (1990) are derived from a very large, integrated US beef slaughterhouse and show excel- lent similarities with Australian data. For sheep-processing plants, the presence of a fellmong- 204 M. R. Johns Table 1. Reported analyses of important wastewater sources in slaughterhouses Parameter 1 (m~) 2 3 4 5 6 7 (95%limits) (mean; range) Cooker Blood condensate (5,8) (8) BOD 710-4633 490-650 TCOD 1925-11 118 1500-2200 SCOD 780-10 090 O&G 50-100 TSS 1011-1916 TKN 110-240 120-180 NH4 - N Tot P (PO43-) 13-22 12-20 VFA SO4- S Alkalinity 2105 1600-3000 1000-3500 (453) 5113 4200-8500 1400-5000 (1036) 1100-1600 897 100-200 (376) 1774 1300-3400 (357) 248 114-148 250-700 (44.7) 65-87 200-300 22.1 (4.5) 20-30 (1.5-2.5) 80-120 175-400 350-800 1900; 530-4 700 6000 2400-6000 150-200 000 375000 640; 220-2100 6000 110-260 620 DM 18-20% 115; 40-230 550 430-740 16,500 30; 3-70 150 430-740 3500 15; 6-34 50 Developments in wastewater treatment in meat processing 205 & Russell, 1988), since the condensed rendering vapours are the major pollution load. These are high in BOD and nitrogen concentrations, largely as ammonia, but poor in phosphorus, in contrast to slaughterhouse wastewater (Frose & Kayser, 1985). The ammonia nitrogen concentration may vary signi- ficantly between winter and summer, due to the more extensive degradation of raw material in sum- mer, which boosts levels of ammonia nitrogen. WASTE MINIMISATION AND CLEANER TECHNOLOGY A focus of modem waste management practice is the minimisation of process inputs. Slaughterhouses generate large wastewater volumes and are fre- quently inefficient users of water. Water consumption for slaughterhouses is reported by several authors (Table 4). Typical water consump- tion varies considerably. The typical minimum water usage figures worldwide appear to be 1.3-2.5 m3/ beast for beef slaughtering plants, assuming an average live weight of 0.5 tonne/beast in the US and Germany. Most recent Australian slaughterhouses have been designed on the basis of 1.5 m3/head water use, but actual industry use is considerably more. Figures for sheep are not quoted. Typically, the wastewater generated comprises 80% of the fresh water input. No breakdowns of wastewater flow from specific process operations are published, but the major wastewater flows come from rendering, paunch processing and stockyard areas of the slaughterhouse. Considerable effort in the last decade has been directed at plant automa- tion. It is not clear whether the incorporation of these technologies into meat processing will reduce water consumption. Schultheisz and Karpati (1984) summarise tech- nology improvements relating to water use and waste minimisation in the US up to 1977. The trend is clearly towards the choice of processes which reduce waste quantity and strength, particularly improved blood recovery, dry paunch dumping and regulated water use. Unfortunately, more recent sur- veys are not available. The emergence of low-temperature edible-rendering systems has been especially constrained by the very strong wastewater generated (Cooper & Russell, 1988). Many schemes are already implemented in slaugh- terhouses for the reuse of water for non-potable uses within the plant. Most of these involve no treat- ment of the water prior to its reuse. Kane & Osantowski (1981) described a pilot-plant study in which treated wastewater was passed through chemi- cal flocculation/clarification and dual media filtration prior to further upgrading by either ion exchange, reverse osmosis (RO) or electrodialysis. All the latter three processes achieved greater than 90% reduction of TDS, however, the use of RO was the most economic, with estimated capital and oper- ating costs for a 3.8 M1/day (1 MGD) system being at most 40 and 68%, respectively, of those of the other two processes. Furthermore, it required only half the space. It is not known whether the system has been installed at full scale. Consideration and demonstration of the technical feasibility of the reuse of municipal wastewater as potable water has occurred in South Africa (Kfir & Slabbert, 1991) and several US cities (Rogers & Lauer, 1991). The safety and public health issues involved have been widely discussed, but generally pipe-to-pipe potable reuse is not favoured and most schemes in use involve a natural intermediate step (i.e. underground aquifer, reservoir). There are no reports of technology development or trials for the rehabilitation of treated wastewater from slaughter- houses for potable reuse. New or even improved ideas for the recovery of valuable byproducts from slaughterhouse or render- ing plant wastewaters have received little attention, except for high-quality water, protein, biogas and struvite fertiliser. This may reflect the fact that the industry is, in fact, highly efficient in achieving product recovery prior to wastewater discharge. PRIMARY TREATMENT Screening, settling and dissolved air flotation (DAF) remain widely used for the removal of suspended solids and fats, oils and greases from slaughterhouse Table 4. Water consumption and effluent volume in slaughterhouses Reference Nation Water use (m3/head) (m3/tonne live weight) Schultheisz & Karpati (1984) Schultheisz & Karpati (1984) Stebor et al. (1990) Hopwood (1977) Jorgensen (1979) Schultheisz & Karpati (1984) Tritt & Schuchardt (1992) Metzner & Temper (1990) US slaughterhouses 2.1-8.3 4-2-16.7 US meat packing 3.2-14.6 6.3-29.2 US Beef slaughterhouse (265/h) 1-3 a 2.6 a UK slaughterhouses < 2.5-7-5 < 5-15 Europe 2"5-5 5-10 Hungary 1-1.9 2-3-8 Germany 0.4-3.1 a 0-8-6.2 a Germany (rendering) only - - 1-25 ~ a _ Effluent production. 206 M. R. Johns wastewater. Earlier studies of processes such as elec- tro flotation or ion exchange (Bull et al., 1982) to replace DAF systems have not proceeded further, suggesting inherent technical difficulties or unfavourable economics of these systems in practice. The late 1970s saw the introduction of large dis- solved air flotation (DAF) units fitted with chemical precipitation into Europe, New Zealand and the US for protein recovery from wastewater. These pro- cesses gave 75-80% BOD5 reduction (Hopwood, 1977) and had the additional advantage of removing large quantities of nitrogen and phosphorus. Most systems, however, had considerable operating prob- lems, including long retention times and low surface-overflow rate, which led to solids settling, large volumes of putrefactive and bulky sludge (typi- cally 0.8-1.2 kg dry sludge/kg BOD5 removed), sensitivity of the system to flow variation and diffi- culties in sludge dewatering (Stebor et al., 1990). Furthermore, the protein product could not be readily sold and most have been shut down (Camp- bell, personal communication; Dague et al., 1990). The installation of new systems has not been reported in recent literature. Several studies have investigated ways of enhanc- ing DAF operation on slaughterhouse wastewater. Travers and Lovett (1985) reported that optimum fat removal from slaughterhouse wastewater using DAF treatment without chemical addition is obtained at pH 4.0-4.5 (Table 5). However, the pH of slaughterhouse wastewater is rarely adjusted. Sub- stantially improved recoveries of fat and solids can be obtained using chemical addition (Green et al., 1981), which is more commonly practised in the US than Australia (Table 5). Large quantities of unstable sludge are obtained, however. Steiner and Gec (1992) report that the introduc- tion of hydrogen peroxide into grease traps and DAF units can significantly enhance performance for O&G removal, largely through the nucleation of very fine oxygen bubbles. It appears that where chemicals are added to DAFs, the use of hydrogen peroxide may benefit the process. In contrast, Tra- vers and Lovett (1985) found the use of carbon dioxide and carbon dioxide/nitrogen gas mixtures instead of air for DAF systems treating slaughter- house wastewater gave little improvement in removal efficiencies, probably because of the extra turbulence found in the separating vessel using CO2 and larger bubble sizes, which depend on the gas used. An interesting technology recently tested in Aus- tralia for fat and protein removal has been foam flotation, which has historically been used in the mineral industry (Jameson flotation cell). Bremmell et al. (1994) describe the use of foam flotation to reduce BOD by 70% and fats by 6-fold in dairy wastewater. The wastewater is chemically floccu- lated, followed by foam flotation using an induced flotation cell. The technology has not been applied to slaughterhouse effluent, but looks promising. ISSUES IN OVERALL WASTEWATER SYSTEM DESIGN Post-primary treatment of slaughterhouse waste- water depends on the location. In Europe, most slaughterhouses and rendering plants deliver their wastewater to municipal systems after preliminary primary treatment (Sayed et al., 1987). Where this is not feasible, some plants have installed activated- sludge plants with biological nutrient removal. In the US, the situation is unclear, although there seems to have been a trend towards activated-sludge plants. In the majority of Australian slaughterhouses, waste- water has passed through anaerobic treatment ponds then into aerobic-pond systems. In a few, direct aerobic treatment (i.e. activated sludge) has been used. The goal of the former has been to achieve BOD reduction, whereas the goal of the latter has often also included nitrification. In contrast, New Zealand slaughterhouses often irrigate primary- treated or anaerobic-treated wastewater directly to land. TRENDS IN ANAEROBIC SYSTEMS Anaerobic systems are well suited to the treatment of slaughterhouse wastewater. They achieve a high degree of BOD removal at a significantly lower cost than comparable aerobic systems and generate a smaller quantity of highly stabilised, and more easily dewatered, sludge. Furthermore, the methane-rich gas which is generated can be captured for use as a fuel. In most countries, the anaerobic pond has been used to achieve a high reduction in BOD, oil and Table 5. DAF performance on slaughterhouse wastewater Treatment Mean removal efficiency (%) Reference COD TSS O&G Air 40 60 90 Travers & Lovett (1985) Acid, pH 4-4.5 71 78 93 Travers & Lovett (1985) DAF and chem. 32-92 (BOD) 70-97 89-98 Karpati & Szabo (1984) Developments in wastewater treatment in meat processing 207 grease and suspended solids concentrations from the primary-treated slaughterhouse wastewater prior to subsequent aerobic treatment. Unfortunately, the propensity for odour generation from anaerobic ponds has threatened their continued use in many areas. Consequently, new developments in anaerobic technology during the last 15 years are of consider- able interest. These can be divided into two groups: (i) covered anaerobic ponds, (ii) high rate anaerobic systems. Covered anaerobic ponds In the past decade, synthetic floating covers on anaerobic ponds have been installed worldwide to trap odour and biogas (Dague et al., 1990; Safley & Westerman, 1992). These systems occupy a niche between existing anaerobic ponds on the one hand and the more sophisticated high-rate anaerobic sys- tems on the other. Compared to traditional, uncovered anaerobic ponds, covered ponds offer the performance of the former, but reduce odour release and permit the capture of the methane-rich gas. The main advantage of this technology com- pared to traditional digesters is the lower capital costs . Dague et al. (1990) describe the successful instal- lation and operation of a very large covered anaerobic pond for a US pork processing company. The design HRT and BOD5 loading were 8.8 days and 0.33 kg/m3.day, respectively. In practice, average retention times were higher (12-14 days) and BOD loadings lower (0.1 kg/m3.day), but mean BOD5 removals of 85-90% were consistently obtained and biogas production averaged 0.51 m 3 methane/kg BOD5 removed. The system paid for itself within 2 years through the use of the biogas in a natural-gas- fired boiler. Considerable, but variable, levels of H2S (mean, 843 ppm) may be generated from anaerobic ponds and this toxic and corrosive gas must be removed, for example by iron sponge filters, before use (Kayhanian & Hills, 1988). Recently, a slaugh- terhouse in Australia has installed two covered anaerobic ponds, which appear to operate success- fully. The gas produced is flared. Safley and Westerman (1988) and Safley and Westerman (1992) reviewed design data for normal- and low-temperature anaerobic digestion of animal manures. A study of the use of covered anaerobic Table 6. Reported high-rate anaerobic lagoons for treating animal manure in the US has suggested that these lagoons require high BOD loadings to generate economic quantities of biogas. Safley and Westerman (1988) reported that biogas production on lightly loaded (< 0"06 kgVS/m 3) anae- robic ponds generated only low quantities of biogas (up to 0.5 m3/m2.day or 0.23 m3/m3.day). Gas pro- duction is highly sensitive to pond temperature (Safley & Westerman, 1992), although methanoge- nesis occurs at temperatures as low as 4C (Stevens & Schulte, 1979). The higher temperature of slaugh- terhouse wastewater should eliminate this problem. High-rate anaerobic technology The testing of high-rate anaerobic systems has been one of the most active areas of research concerning slaughterhouse-waste treatment during the last dec- ade. The most common systems installed at large scale are the anaerobic contact (AC), upflow anae- robic sludge blanket (UASB) and anaerobic filter (AF) processes. The 1970s saw the use of low-rate anaerobic digesters to treat slaughterhouse wastewater. These processes were essentially mixed digesters with a BOD loading of between 0.2-4 kg/m3.day and have generally proven uneconomic, due to their required size (Borja et al., 1993), and relatively few have been installed (Bull et al., 1982). Since this time, a variety of new high-rate anaerobic technologies have been developed to replace the anaerobic pond. Typically, these are characterised as having higher BOD or COD loadings (typically 5-40 kg COD/m3/day) than low-rate systems or anaerobic ponds. This permits a hydraulic retention time in the order of hours rather than days. The gas generated by the anaerobic activity is methane-rich, but in most cases H2S is also generated at concentrations from 0.2-0-7% (Festino & Aubart, 1986; Metzner & Temper, 1990) from slaughterhouse wastewater and may need removal. Steiner (1987) summarises known large, high-rate anaerobic systems installed in slaughterhouse waste- water-treatment systems (Table 6). The current status of these units is unknown and there are few published data for them. In comparison to their popularity for treating wastewater from many agro-processing industries (i.e. brewing, potato processing, etc.), the applica- tion of high-rate anaerobic systems to systems at slaughterhouses (Steiner, 1987) Place Year System type Volume (m 3) Albert Lea, USA 1959 AC 2,670 Leeds, UK 1968-1980 UASB type 1,240 Den Haag, NL 1983-85 UASB 630 Genk, Belgium since 1983 UASB (2 stage) -- Castres, France since 1980 AC 45 Bylderup, DK since 1984 AF 200 208 M. R. Johns slaughterhouse wastewater has encountered signi- ficant problems. These include (Stebor et al., 1990): (i) The high fat, oil and grease concentrations in the wastewater causing severe problems, due to their insolubility, which slows the rate of degradation, and its tendency to form scums and coat surfaces. High suspended solids con- centrations in the feed adversely affects UASB, fluidised-bed and fixed-media anaero- bic processes. Recent studies have shown that the form of pollutants (i.e. suspended, colloi- dal or soluble) in the influent wastewater greatly affects the performance of high-rate anaerobic systems (Sayed & de Zeeuw, 1988). (ii) The BOD concentration in the feed is rela- tively low for successful operation of high-rate anaerobic processes, which operate better at BOD5 concentrations of 10 000 mg/1 or more. This requires high hydraulic throughput. The results from various high-rate anaerobic pro- cesses are summarised in Table 7. Most are the results of research performed at relatively small scale (often only a few litres). Anaerobic contact (AC) reactor systems have proven relatively popular for use in slaughterhouses, since they tend to circumvent the floating-scum problem found in many high-rate anaerobic systems. Hopwood (1977) reports that many of the UK plants subsequently shut down, due to problems with float- ing sludge, but that US plants were more successful. At the large AC plant servicing a large integrated beef US slaughterhouse, removal efficiencies of more than 84% COD, 93% BOD5 and 75% TSS have been routinely achieved (Stebor et al., 1990). A 3000 litre AC reactor was tested at a French slaugh- terhouse to treat selected wastewater streams, with some success at a BOD5 loading of 5 kg/m3.day, but few data are given regarding performance (Bohm, 1986). A more recent development has been the anaero- bic sequencing batch reactor, in which flocculent microbial floes are claimed to achieve treatment of swine wastes without the need for complete mixing of the AC process (Dague & Pidaparti, 1992). The process is untried at large scale. Application of UASB technology to slaughter- house wastewater appears to have been less successful. A large UASB was installed at a slaugh- terhouse by Den Haag (The Netherlands) from 1983-1985, but ran as a flocculent system, since granules could not be produced, and this signifi- cantly lowered the rate of BOD removal which could be obtained. High fat concentrations led to the loss of sludge. Other UASB units have been installed at a Belgian slaughterhouse (Steiner, 1987) and a large New Zealand sheep slaughterhouse (Campbell, personal communication), but operating data are unavailable. Sayed and de Zeeuw (1988) reported successful operation of a single-stage, flocculent UASB (10 1) at loadings of up to 5 kg COD/m3.day of slaughter- house wastewater at 30C. The nature of the COD in the wastewater appears to have a significant influ- ence on operation and performance of UASBs treating slaughterhouse wastewater. Higher through- put (11 kgCOD/m3.day) was obtained using a granular-sludge UASB reactor (Sayed et al., 1987). This is the highest reported loading for slaughter- house wastewater to date. Hansen and West (1992) report that lab-scale experiments on various frac- tions of rendering-plant wastewater using a UASB gave relatively poor results: long HRTs (10-15 days), low COD loadings (0.3-1 kg/m3.day) and poor COD removal (72-87%). However, it is likely that the reactor was not operating under optimal conditions. The key operating issues for UASBs appear to be the adequate removal of suspended solids and fat from the incoming wastewater to prevent their accu- mulation in the reactor with subsequent complete loss of active sludge from the reactor (Sayed & de Zeeuw, 1988) and obtaining and maintaining a granular sludge. Large scale AFs have been installed in Europe, although little data concerning their performance is Table 7. Reported results for high-rate anaerobic systems Type Scale (m 3) Temp (C) COD load Percent CODf CH4 Reference (kg/m3.d) removal yield UASB (floe) 10 30 5 80-89 4 a UASB 33 20 7 85 2.8 a (gran) 30 11 85 5.1 AF 5 2.5 80-85 ND AF (loop) 82 36 8 75-80 0"32 m 3b AF 21 37 2"3 85 0"33 m 3a AFB 1 35 5 75 Small AC -- 33 1.7 95 -- AC 11 120 35 3-0 92.6 0-24 c Sayed & de Zeeuw (1988) Sayed et al. (1987) Tritt (1992) Metzner & Temper (1990) Festino & Aubart (1986) Toldra et al. (1987) Hopwood (1977) Stebor et al. (1990) akg CH4-COD/m3.day. bkg CHa/kg COD removed. CmJ biogas/kg COD added. Developments in wastewater treatment in meat processing 209 available (Verrier, 1986; Steiner, 1987). In one case, effluent BOD concentrations of under 500 mg/1, at 33C and a COD loading of 4.9 kg/m3.day, were attained (Steiner, 1987). Most authors report COD removals of 80-85% at COD loadings of 2-3 kg/ m3.day, with a high methane content (72-85%) in the gas (Andersen & Schmidt, 1985; Festino & Aubart, 1986; Tritt, 1992). Higher COD loadings appear to lead to poorer performance (Festino & Aubart, 1986; Tritt, 1992). The latter author reported biomass yields of 0.03-0.15 kg VSS/kg COD removed for small (5 1) units treating slaugh- terhouse wastewater. Tritt (1992) reports a very high nitrogen content in the wastewater (NHa-N of 700-1400 mg/l), which may partly account for the low degradation rates. Effective pretreatment of the wastewater to remove FOG and suspended solids is also important for AFs. Andersen and Schmidt (1985) found high grease concentrations caused unstable operation of a pilot-scale AF treating beef slaughterhouse waste- water. High-rate anaerobic technology has also been applied to the treatment of rendering wastewater. An anaerobic fixed-bed loop reactor (AFL) compris- ing a downflow system through a bed of PVC pipes with a recycle ratio of about 3-4 was used with a HRT of 27 h (Metzner & Temper, 1990). High COD removal and gas generation was obtained. Other systems, including anaerobic fluidised bed (AFB) and hybrid anaerobic reactors exist, but there appear to be no reports of large-scale units being used in the meat industry. Small-scale reactors have been tested satisfactorily on slaughterhouse waste- water (Toldra et al., 1987; Borja et al., 1993). In summary, the key requirements for the success- ful adoption of high-rate anaerobic systems to treat slaughterhouse wastewater are effective pretreat- ment to remove fats and suspended solids and dampen fluctuations in flow, relatively low COD loadings (2-11 kg/m3.day) and a temperature between 30 and 37C. AEROBIC BIOLOGICAL-TREATMENT SYSTEMS Developments in wastewater ponds In most countries, ponds remain the main form of aerobic biological treatment for the removal of COD from slaughterhouse wastewater, although a wider variety of secondary biological systems has also been used to some extent, including trickling filters and activated-sludge systems. There have been few exciting developments in the use of classical pond types (facultative; aerated) to treat slaughter- house wastewater in the last decade (Sachon, 1984). Some slaughterhouses treat hide-curing wastewater in evaporative ponds. Tanji et al. (1992) studied Californian evaporation ponds and found that heavy-metal concentrations in salt evaporates were well below hazardous levels and that, for agricultural drainage liquids, the main salts crystallised were relatively non-toxic sulphate, chloride and carbonate salts of sodium, magnesium and calcium. In contrast, it was found that concentrations of toxic heavy met- als exceeded hazardous levels in the concentrated liquor of ponds, especially when conditions of almost complete evaporation were approached. Care is needed in the disposal of such liquid. Three new concepts for ponds have emerged in the last decade, but none have been applied to slaughterhouse wastewater. High-rate algal ponds involve an initial pond in which incoming wastewater enters a deep anaerobic pond section, after which it flows into an aerobic section. An aerobic environ- ment is maintained by recirculation of oxygen-rich water from a second pond containing very high algal concentrations. Israeli experience (domestic waste- water) with these systems has been positive (Shelef & Azov, 1987). A second concept involves the use of facultative ponds with fast-growing, floating macrophytes (duck- weed, water hyacinths, etc.) grown over about 70% of the pond area. These systems achieve similar (Kawai et al., 1987), or slightly improved (Orth et al., 1987), BOD removals to facultative ponds and improved nutrient removal (Kawai et al., 1987). The high ammonium levels (100-200 mg/l) in anaerob- ically treated slaughterhouse wastewater may be toxic to some macrophyte systems (de Casabianca- Chassany et al., 1992). These authors suggest that low ammonium concentrations (i.e. 25 mg/1 NHa-N) are optimal for longer-term performance. In trials on domestic wastewater by various groups world- wide, a number of operational problems have appeared, which include plagues of mosquitos, the difficulty of the disposal of plant material (up to 200 kg dry weight/ha.day), a strongly reducing effluent, which necessitates subsequent aerobic treatment, and the generation of high concentrations of anaero- bic gases (i.e. H2S), due to TSS settling, which cause odour problems (Kawai et al., 1987). Consequently, such systems cannot be recommended as the prime form of nutrient removal for slaughterhouse treat- ment systems, but might be suitable for polishing or run-off situations. In the third system, the active biomass in the pond is increased by using attached growth on floating media. Shin and Polprasert (1987) reported that an attached media volume comprising 10% of total pond volume was optimal and achieved 80% COD removal in 5 days at a loading of 100 kg COD/ ha.day. Nutrient removal was increased due to uptake into the attached films, but faecal coliform die-off was similar to normal ponds. Constructed wetlands The advantages of constructed wetlands systems include low operational cost, their 'green appeal', 210 M. R. Johns simplicity and low energy requirements. For these reasons, there has been a great deal of interest in these systems. Studies using constructed wetlands to treat domestic sewage have shown variable perform- ance, with a major study showing BOD removal of 51-95%, total nitrogen removal of 10-88% (mean, 30%) and total phosphorus of 11-94% (Brix, 1987). The variability appears to be due to the various soil substrata and plants used, and climatic variation. Submerged-flow systems have demonstrated a tend- ency to lose performance within 2-3 years due to clogging (Boutin, 1987). Little has been published on the application of constructed wetlands to meat industry wastewater. Nevertheless, they are being installed in increasing numbers, mainly as a polishing technology prior to final release of wastewater to the environment. In this role, they are probably superior to maturation ponds. Constructed wetlands treating slaughterhouse effluent achieve poor nitrogen removal when the influent nitrogen is present as ammonium nitrogen (Van Oostrum & Cooper, 1990), which is commonly the case when a slaughterhouse treatment system comprises facultative, or COD-overloaded, aerobic pond systems. Furthermore, the high ammonium-ion nitrogen levels found in slaughterhouse wastewater (i.e. 70-250 mg/l) may be toxic to wetland plants (de Casabianca-Chassany et al., 1992). Improved nitrogen removal was reported by Van Oostrum and Russell (1994) when nitrified slaugh- terhouse wastewater was fed to a surface-flow constructed wetland, provided that sufficient carbon was available in either the influent and/or from decaying plant litter. Nitrogen removal rates of up to 9.5 gN/m2/day were obtained, largely due to micro- bial denitrification. No reports of phosphorus removal from a slaugh- terhouse wastewater have been published. This will largely be dependent on the soil/stratum. Activated sludge systems Numerous activated-sludge plants were put into slaughterhouse and rendering-plant wastewater treatment systems in the US in the late 1970s/early 1980s to achieve BOD removal and the conversion of ammonia by biological nitrification to nitrate, typically with better than 95% ammonia removal (Green et al., 1981; Witmayer et al., 1985), provided that the dissolved oxygen concentration was above 2"0 mg/1 and the temperature above 10C (Weber & Hull, 1979). Since the requirement for nitrification increases aeration costs by 50% over that for COD removal only, a better strategy is to retrofit these plants to achieve full nitrogen removal. In this way, substantial reduction in aeration requirement is obtained - - a safe estimate is that aeration costs of such plants are only 10% higher than for COD removal only (Randall et al., 1992). These systems are discussed further in the next section. Design criteria for slaughterhouse wastewater are widely published (Hopwood, 1977; Heddle, 1979; Lovett et al., 1984; Travers & Lovett, 1984). Most systems are of the extended-aeration type to mini- mise sludge production. French design guidelines (Sachon, 1984) are typical of those used for the industry, although BOD loadings up to 3 kg/m3.day have been used successfully (Table 8). Since proteins are less readily biodegradable than simple mole- cules, like glucose and acetate, the HRTs should be longer than for municipal wastewater plants. A sludge age of 5-20 days is recommended for com- pletely mixed AS plants treating slaughterhouse wastewater. Activated-sludge systems treating slaughterhouse wastewater have been reported to produce light, poor settling, floc (Hopwood, 1977). This has been found to be due to a combination of the high fat content of the influent and a low dissolved oxygen concentration (DO) in the AS reactor (Travers & Lovett, 1984). At low DO (Developments in wastewater treatment in meat processing 211 The high salinity of hide-curing-brine wastewater may present a problem in its disposal both to the environment and to biological wastewater-treatment systems (Witmayer et al., 1985). Sodium concentra- tions of greater than 1500 mg/l in the influent caused poorly settling sludge and poor effluent quality. Lab-scale studies, however, found that sludge could acclimatise to sodium levels as high as 7000 mg/l without deleterious effects. This has also been noted by others (Kincannon & Gaudy, 1968; Burnett, 1974). Nevertheless, the performance of activated-sludge systems is severely disrupted by shocks of high salinity influent, with the results including poor flocculation and settling of sludge and inhibition of nitrification (Ludzack & Noran, 1964; Kincannon & Gaudy, 1968; Burnett, 1974) particularly at low MLSS concentration. Shock loads of salt are typically caused by dumping of brine-tank contents. Trickling filters High-rate trickling filters have been used successfully (particularly in Europe) as roughing filters to achieve preliminary removal of BOD from rendering plant (Frose & Kayser, 1985) and slaughterhouse wastewaters (Hopwood, 1977; Moodie & Greenfield, 1978) subsequent to further treatment. Their advan- tage over other treatment systems is their low space and energy requirement. Trickling filter applications in general wastewater treatment in the US have been recently reviewed by Parker et al. (1990). French design guidelines (Sachon, 1984) for high- rate, roughing, trickling filter treatment of slaughterhouse wastewater are given in Table 9. These compare well with those of Hopwood (1977), who reported BOD5 removals of 75% or more. Good fat removal is necessary to prevent it coating the surfaces of the unit. This technology has appar- ently not been widely adopted by American or Australian slaughterhouses, although its suitability has been demonstrated. Other aerobic systems Rotating biological contractors (RBC) have been applied to wastewater from slaughterhouses, but their performance appears inadequate (Bull et al., 1982; Blanc et al., 1984) compared to activated- sludge or high-rate trickling-filter systems. Li et al. (1987) reported the successful laboratory-scale trial of aerobic fluidised beds (AFB) to treat pig slaugh- terhouse wastewater in Taiwan. At a BOD loading of 20 kg/m3.day, BOD, fat and ammonia nitrogen removal were > 90, > 70 and > 70%, respectively. High recirculation ratios were needed to ensure a residence time in the reactor of greater than 30 min and pure oxygen was used to aerate the reactor. The AFB offers potentially effective, but expensive, treatment where space is a critical issue, but it remains untried at large scale. NUTRIENT REMOVAL TECHNOLOGIES AND ISSUES Slaughterhouse treatment systems have commonly nitrified wastewater to lower ammonium levels prior to discharge. Where wastewater ponds have formed the treatment system, however, little nutrient removal tends to occur. The discharge of high loads of nitrogen and phosphorus in slaughterhouse waste- water into sensitive water-bodies or onto permeable soils has emerged as a major problem for the industry worldwide and the removal of these nutri- ents from slaughterhouse and rendering wastewaters has become an important issue. Little information has been published on source reduction of nutrients from slaughterhouses. Many technologies, both physicochemical and bio- logical, have been developed to remove nutrients from wastewater. The underlying theme of them all, compared to existing wastewater-treatment systems built to remove carbonaceous pollutants, is the greater complexity and much greater cost, both operational and capital. The choice of technology to install has tended to depend on historical prefer- ences based largely on geography and regulatory pressure, the form of disposal and the availability of space for the additional treatment plant. Europe is well advanced in addressing the problem of nutrient removal from slaughterhouse and rendering-plant wastewater or from animal slurries, although, in many cases, this is performed by municipal waste- water-treatment systems. By contrast, there is little published from North America and only a minority of slaughterhouses have installed nutrient-removal capacity in Australasia, largely because land irriga- tion of effluent is widely practised. Nevertheless, high nitrogen loads lead to the requirement for large irrigation areas and there is often some benefit in reducing loads prior to irrigation. Table 9. Recommended design criteria for high-rate roughing TF treating slaughterhouse wastewater Criterion Recommended value Hydraulic surface loading > 1.5 m3/m2.h Volumetric BOD5 loading 2-4 kg/m3.day Recycle ratio 3-4 Height of packing > 4 m 212 M. R. Johns There is relatively little information available con- cerning the design and performance of nutrient-removal technologies in treatment of slaughterhouse wastewater. Physicochemical nutrient removal Proven physicochemical methods for nitrogen removal from wastewater include ammonia stripping and breakpoint chlorination, but in most cases bio- logical removal is preferred. Ammonia stripping is relatively inexpensive and has been recently adopted to remove ammonia from rendering-plant waste- water using an aerated pond with lime addition (Kaszas et aL, 1992). However, in general it is uneconomic given the large wastewater volumes of slaughterhouses, the high buffering capacity of the wastewater and the possibility of also stripping offensive odours (Anon, 1987). Breakpoint chlorination has been used as a standby system by slaughterhouses in the US to remove ammonia-nitrogen, if biological nitrification failed to achieve discharge standards (Witmayer et al., 1985). However, regulatory agencies are growing increasingly concerned about the formation of triha- lomethanes and other chlorinated organics during the process (Kaszas et al., 1992). Chemical precipitation of phosphorous permits very low levels (Developments in wastewater treatment in meat processing 213 by fluctuating hydraulic or organic load, but that it was sensitive to pH variations, due to alkalinity con- sumption due to nitrification. Willers et al. (1993) reported that slurries of animal manures (< 2% dry solids) had been treated from 1976 until 1991 in SBR systems in The Netherlands to achieve BODs, TKN, NHa-N and PO4 removals of better than 95%, the last by the addition of lime. Incomplete deni- trification was encountered during cold winters, when the BOD/N ratio of the aerated manure was too low. The popularity of the SBR is due to its single-tank design and the ease of its automation. A number of issues need clarification for AS- BNR treatment of slaughterhouse and rendering wastewater: (i) It is not plain which process variant best suits slaughterhouse operation. For example, the Dutch have used both sequencing-batch and continuous plants for nutrient removal in ani- mal manure slurries (Willers et al., 1993). (ii) It would appear that relatively little is known about denitrification of slaughterhouse waste- water (Frose & Kayser, 1985). Biological denitrification requires the availability of readily degradable carbon. Therefore, prior anaerobic removal of COD from slaughter- house wastewater (in anaerobic ponds or high-rate systems), which is common in the meat industry, must be carefully balanced against this requirement if successful deni- trification is to be obtained without the highly expensive use of external carbon sources. These are probably unnecessary for slaugh- terhouse wastewater. Metzner and Temper (1990) describe the use of an anaerobic filter which preceded a BNR plant which achieved nitrogen removal. Carbon for denitrification was provided by a 14% volume raw effluent bypass of the anaerobic reactor. (iii) Biological phosphorus removal from slaugh- terhouse wastewater has not received much attention. This requires investigation, since in most cases phosphorus concentrations in slaughterhouse wastewater are significantly higher than in domestic wastewater. High levels (> 200 mg/l) of volatile fatty acids, especially acetic acid, enhance BPR (Loetter & Murphy, 1988). Unlike domestic sewage, which is usually limited in this material, slaughterhouse treatment systems can readily supply a sidestream from an acidogenic anae- robic pond or reactor. (iv) There is increasing concern that under cer- tain conditions, a large proportion of nitrogen oxides (N20, NO) rather than nitro- gen gas (N2) are generated by microbial action. Nitrogen oxides contribute to green- house warming and ozone destruction (Banin, 1986). This has become a focus of attention and the outcome may have a signi- ficant impact on how AS-BNR systems are operated. Biological nutrient removal-trickling filter systems The application of newer high-rate nitrifying trick- ling filter (NTF) systems to slaughterhouse or rendering-plant systems has not been reported. A major consideration is that the process does not achieve denitrification, but only ammonia removal by conversion to nitrate. A number of denitrifying filter systems have emerged in recent years to solve this problem. Parker et al. (1990) reported high rates of nitrification of domestic wastewater (about 3 gN/ m2.day at 20C) using new, cross-flow packing media. Volumetric nitrification rates (0.2-0-4 gN/ m3.day at 20C) were similar to those of other processes (activated sludge, submerged filter), but costs were claimed to be lower. USE OF SPECIALIST CULTURES The concept of adding specialist microbial cultures to waste-treatment systems originally arose to deal with emergency situations, such as the shock loading of treatment systems or spills of certain chemicals (Yu & Hung, 1992). Subsequently, it has been extended to continuous addition to improve day-to- day performance of plants and is common practice in many slaughterhouse treatment systems in Aus- tralia. Scientific evidence suggests that only in transient situations (e.g. start-up; shock situations), does bio- augmentation appear worthwhile. Yu and Hung (1992) conducted an extensive and well-designed study to assess whether bioaugmentation improved the performance of lab-scale activated-sludge units treating a variety of wastewaters. No significant dif- ference was observed, despite the use of a culture with seven different bacteria. Similar results were obtained in studies with laboratory anaerobic digest- ers (Koe & Ang, 1992) and bench-scale reactors to simulate a sequencing-batch activated-sludge process degrading 3-chlorobenzoate (3-CB) with a specialist 3-CB-degrading bacterium (Wilderer et al., 1991). These papers provide evidence that bioaugmentation is not required in well-designed systems on an on- going basis. In slaughterhouse wastewater, however, there would seem no requirement for bioaugmenta- tion, since the effluent is readily biodegradable and a wide variety of microorganisms are naturally avail- able. LAND IRRIGATION In many countries, land irrigation remains one of the best disposal routes for slaughterhouse waste- water. The required land area is usually determined by the nitrogen loading, provided that there are rea- 214 M.R. Johns sonable evapo-transpiration rates and good phosphorus adsorption by soil occurs. As nitrate, nitrogen is readily mobile in soils and may therefore leach into groundwater if loading rates are too high, leading to contamination. This is widespread in Europe and has occurred in several places in Aus- tralia (Bowmer & Laut, 1992) and New Zealand (Keeley &Quin, 1979). Grazing of irrigated land has been found to increase the rate of nitrate leaching, since animals recycle 90% of ingested nitrogen to soils (Cooper & Russell, 1988). In highly permeable soils, phosphorus contamination of groundwater has also been observed (Bowmer & Laut, 1992). Nitrogen is removed from soil by plant uptake and microbial denitrification, which requires anoxic con- ditions, a source of readily degradable carbon and nitrogen in the form of nitrate. Denitrification removed only 2% of applied nitrogen (1000 kg N/ha/ year loading) from cropped land (Russell & Cooper, 1987). However, the contribution of denitrification to nitrogen removal depends on the concentration, and probably the type, of COD present in the irriga- tion water. Total nitrogen (N20+N2) emission rates from soils irrigated with anaerobically treated waste- water at very high nitrogen-loadings (about 1000 kg/ha.year) were only one-third those of land irri- gated with primary-treated wastewater (Russell et al., 1993), therefore increasing the danger of nitrate leaching. Presumably, denitrification would be even further reduced in aerobically treated wastewater, in which COD levels are little higher than 1-200 mg/1. Russell et al. (1993) reported that the rate of N20 release from pasture and forest irrigated with efflu- ent rose markedly immediately following irrigation, but fell back to base emission rates within 24 h of irrigation ceasing. Emission was affected by the type of wastewater and temperature. Both N20 emission and denitrification were minimal at temperatures below 12C (Russell et al., 1993). A further concern for the management of slaugh- terhouse-wastewater irrigation systems is human exposure to potentially pathogen-contaminated aerosols. Sprinkler irrigation systems lead to the aerosolisation of 0.1-1% of the sprayed water. Shuval et al. (1984) found that irrigation workers exposed to aerosol-forming (sprinkler) irrigation were three times more likely to be carrying anti- bodies to Legionella pneumophila, the causative agent of Legionnaires disease, than the normal population, regardless of whether the water was clean or treated-wastewater. 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