Water consumption, wastewater generation and characterization of a slaughterhouse for resource conservation and recovery

APEDA

Agricultural and Processed Food Export Development Authority

BOD

Biochemical Oxygen Demand

COD

Chemical Oxygen Demand

EPA

Environmental Protection Act

ETP

Effluent Treatment Plant

FAO

Food and Agriculture Organization

FS

Floating Solids

FSSAI

Food Standards and Safety Authority of India

GoI

Government of India

HACCP

Hazard Analysis and Critical Control Points

ISO

International Standards Organization

MoEF

Ministry of Environment and Forest

MT

Metric Tonne

O&G

Oil & Grease

SG

Specific Gravity

TDS

Total Dissolved Solids

TKN

Total Kjeldahl Nitrogen

TLWK

Tonne of Live Weight Killed

TP

Total Phosphate

TS

Total Solids

TSS

Total Suspended Solids

TVS

Total Volatile Solids

USEPA

United States Environmental Protection Agency

Slaughterhouses pose significant environmental challenges in water, soil, and land pollution. Global market integration and commercialization significantly increase the volume of animal waste (Haan et al. 1997). Increasing demand for meat products resulting from population growth has resulted in slaughterhouse-related pollution problems.

Slaughterhouse waste includes solids (bones, hooves, rumen, intestinal content, dung, etc.) and liquids (blood, urine, internal body fluids, washing and cleaning waters, etc.). If these wastes are not managed adequately and efficiently, they can harm the environment. Robert (2005) reported that waste disposal has always dominated the slaughtering sector and that, on average, 45% of each live beef animal consists of non-meat substances (bones, skin, teeth, hooves, etc.). A survey in India by Singh et al. (2014) revealed that no special disposal systems are used, the solid waste is either thrown and dumped in open fields, or sold to other parties. It becomes important, therefore, to use such a large amount of waste in an integrated and sustainable manner.

Many physico-chemical characterizations have been made of slaughterhouse industrial wastewater. A dedicated study is missing, however, evaluating the physico-chemical characteristics of wastewaters from individual slaughtering processes/operations. The aim of this study was to identify, characterize, and quantify slaughterhouse wastewater from individual processes/operations, to help in designing and implementing an effective and economical wastewater treatment facility. Identification has also been attempted of those areas where valuable resources can be recovered and water conserved.

In this study, the water consumption pattern in slaughterhouse processes and operations, and areas for improvement in water budgeting and waste minimization were identified on the basis of detailed wastewater characterization, to optimize energy and water consumption, and save resources.

A modern, integrated abattoir and meat manufacturing and export plant was selected for study in Uttar Pradesh, India. The plant operates to international standards and is HACCP, ISO 22000:2005, and 9001:2015 certified. The plant's main offering is fresh, boneless buffalo meat. It is classified as a ‘large slaughterhouse’ since its annual slaughtering capacity exceeds 40,000 cattle or dry live weight per day exceeding 70 tonnes (EPA Act [India] 1986). The plant has consent to slaughter 1,100 buffalos per day by APEDA, Ministry of Commerce, GoI, and consent to operate from the Uttar Pradesh Pollution Control Board. It is also licensed by FSSAI under the Food Safety & Standards Act (2006).

Groundwater, drawn from two wells, is the plant's primary water supply. The two pumps extract 90 and 130 m³/hr, respectively, into an overhead tank. The groundwater's characteristics are presented in Table 1. Electromagnetic digital flow meters installed on the pump heads are used to measure daily and cumulative water consumption. Manual, analog, fin-type water meters are installed elsewhere to measure water consumption in different sections of the plant. Water consumption was measured daily at 08.00 from 1 April to 13 July, 2018 (105 days – no readings were taken from 14 to 18 June, due to holidays). The plant does not operate on Fridays, so the consumption on Fridays relates to plant washing and refrigeration, since all slaughterhouse processes and operations are closed. The water consumption studies were initiated first in order to regulate the water supply, since water consumption has direct influence on wastewater generation and characteristics.

The wastewater from each slaughterhouse process/operation is retained in a holding tank before release to the drains. The holding tanks are equipped with agitators and retain wastewater for between 30 and 60 minutes. Samples from individual slaughterhouse processes/operations were collected from the respective holding tanks in 1-liter plastic containers three times in a day and were then mixed to form a single, representative 3-liter sample. The representative sample was then analyzed for COD, BOD, TSS, TDS, TS, O & G, TKN and TP, following APHA standard procedures (APHA 2005). This exercise was carried out once in a month during August to October 2018.

The plant's processes involve numerous different operations. The wastewater streams generated from the processes are presented in Figure 1.

In lairage, buffalos are received and rested before slaughter, to help them recover from the journey to normal conditions. In order to recover as many solids as possible in dry form, cattle dung and bedding are scraped from delivery vehicles and holding areas, and frequent cleaning is necessary to maintain adequate hygiene. This is done by washing the floor regularly. Table 2 presents the wastewater characteristics from the lairage and subsequent sections. Wastewater from lairage contains fully digested suspended solids introduced from cattle dung excreted while resting.

The SS concentration in lairage wastewater depends on the amount of water used to wash the floor. To minimize the solids load from this section, laborers are encouraged to dry scrape dung as much as possible. Lairage wastewater has very high COD_total – 19,250 to 23,450 mg/L – while COD_soluble is between 554 and 986 mg/L. Lairage wash water and manure are nutrient-rich and can be used as fertilizer (IPPC 2003).

The next process is sticking (locally known as halal point), where cattle are slaughtered by cutting the main blood vessels in the neck, causing the loss of around 40 to 60% of the animal's blood (Wilson 2005). The blood goes to a tank just under a sticking/halal point and is pumped on for further processing or treatment. The sticking point wastewater characteristics are shown in Table 2. Blood is the main organic load contributor in slaughterhouse effluent, which has very high COD of around 375,000 mg/L (Sarairah & Jamrah 2008).

Blood clots naturally and quickly, but several anticoagulants – for example, sodium citrate, heparin, and EDTA – can be used to prevent and/or slow this (Bah et al. 2013). Clots start to form in blood in less than 5 seconds, however, and when thrombin is diluted in the ratio of 1:36, clotting starts in less than 15 seconds (Thomazini-santos et al. 1998). Analysis of blood stream SS was impractical since small, coagulated particles stop on filter paper raising the SS value during analysis, so only TS was determined for the sticking point wastewater. Both COD and BOD were determined, however, to assess the location's organic load contribution. The wastewater COD from this section varies between 31,600 and 121,600 mg/L, and the TKN is high, in the range 2,091 to 3,569 mg/L. The considerable variation in the wastewater COD is thought to occur because the floor of this section is washed continuously with a high-pressure nozzle to remove the blood. The amount of wash water could be controlled by using vipers to remove blood from the floor, and a slight gradient would help maximize blood capture and facilitate cleaning.

Dehiding/hide removal is usually done manually by experienced personnel (USEPA 2002; FAO 2004) and safeguards carcasses from contamination. The process is completed with mechanical pullers' help, removing hide from neck to tail after the initial cut. The excess flesh and fat stuck to the hides are removed manually or by fleshing machine, wherein the hide is pressed between rollers while water is poured on continuously. Wet fleshing results in the loss of around 10 to 15% of the raw weight of the hides/skins (ENVIS 2015).

Fleshing section wastewater contains fine meat and fat particles – see Table 2 for its other principal characteristics. The SS concentration was found to be between 229,310 and 285,450 mg/L, and consists mostly of flesh, meat pieces, and skin fat. It seems very likely that this concentration range is overestimated, because the particle sizes are uneven, which can increase SS values significantly. It was observed that SS in wastewater from the fleshing section were floating. Floating meat pieces (hereinafter floating solids (FS)) occupied 50 to 80 mL per liter of wastewater. The COD_soluble of the wastewater from the hide storage section was 80,560 to 164,800 mg/L. This high value of COD_soluble is because of the washing of blood stuck on hides.

Hides are composed largely of water (65% moisture), the rest being proteins and fats (30 to 35%). Moist conditions favor bacterial proliferation, degrading the hides. To prevent this, the hides are dehydrated using sodium chloride (NaCl), applied in the order of 30 to 45% of the hide's weight (MoEF 2010). On average, 3 kg of NaCl is applied on small buffalo hides and 5 kg on big ones. The characteristics of the saline water from the hides are presented in Table 2. While the wastewater is slightly red from blood stuck to the hide, its COD cannot be determined because of the high chloride concentration as chloride interferes with the COD test (Southway 1981). Instead, the wastewater's organic strength can be considered in terms of the total volatile solids (TVS) concentration, which was in the range 7,350 to 9,940 mg/L. The TDS concentration was between 257,602 and 323,534 mg/L, mainly because of the NaCl applied to the hides.

Evisceration is the process of removing the body's internal organs. Intestinal contents – paunch – which are partially digested, are major pollutants in slaughterhouses (Carawan & Pilkington 1986), and their wet weight is typically between 22 and 31 kg/animal (Carawan et al. 1979). Paunch is removed manually and excess solids stuck on the inner wall are washed out, which generates wastewater (Table 2). As can be seen, paunch wastewater is dominated by partially digested SS, unlike SS from lairage.

After evisceration, the carcass is split in two along the spinal cord. The cut carcasses are rinsed. The wastewater contains pieces of meat and bone, and its COD varies between 4,250 and 5,120 mg/L as it includes blood previously retained in the carcass. The carcasses are normally then kept in chillers at about 5 °C for 24 hours, after which they are sent for cutting and dispatch.

The intestines and tripes (edible lining from the stomach) are washed at nearly 70 °C for between 6 and 15 minutes. Most of the solids from tripe washing are floatable, and 400 to 500 mL/L were found in tripe washing wastewater. The wastewater from intestine washing contained between about 250 and 350 mL/L of FS. It should be possible to recover FS for rendering.

Omasum is the third compartment of the bovine stomach. It is dehydrated by applying NaCl for 15 minutes and then stirring for 15 minutes, after which the water squeezed out and discharged. On average about 0.25 to 0.33 kg of NaCl is applied and about 2 to 3 liters of wastewater is produced per omasum.

Rendering starts with heating to remove moisture and separate the fat from the meat (Meeker & Hamilton 2006), to yield products including edible and inedible tallow, animal protein meal, soap, cosmetics, candles, perfumes, and paint (Swisher 2006). Rendering plant effluent contains oils, nutrients (from protein degradation), and pathogenic microorganisms, and has high oxygen demand (Sindt 2006). Most of the COD in rendering plant wastewater in this study arose from fine meat scraps, and the soluble COD concentration was in the range 1,785 to 2,450 mg/L.

Carcasses are finally deboned and cut according to consumer requirements, before being packed and stored. The holding and dispatch room is maintained at 2 to 4 °C. The wastewater from these sections derives mainly from washing – floors, working tables, packaging instruments, knives, and so on – and personal hygiene, and carries no significant organic load, so it was not characterized.

No similar data to those from this study are available from past ones but some resemblances are apparent – see the bottom row (previous studies) in Table 2. Lairage wastewater, for instance, resembles that from livestock/dairy farms. Its characteristics in the slaughterhouse depend on the water used for floor washing and the amount of dung scraped off before washing. Similarly, Tritt & Schuchardt (1992) reported the COD and BOD of raw bovine blood from the sticking section. In the same section in this study, a lot of water is used continuously to clean blood from the floor, so the COD and BOD values reported here are lower because of the wash-water dilution.

Kannan & Rao (2000) reported that soak liquor TDS in tanneries was 40,000 to 60,000 mg/L, which is less than the TDS of wastewater from the hide storage section in this study. Joseph & Nithya (2009) reported that 300 kg of salt is used to preserve hides in the slaughterhouse industry per 100 m² of leather produced.

For paunch, there is wide variation in the BOD values reported by Mittal (2004) and Tritt & Schuchardt (1992). However, Tritt & Schuchardt (1992) report that paunch contains lignocellulose – for example, in straw, grass, and hay – and, as a result, BOD values are lower than COD. Ziara et al. (2018) report comparable COD and BOD values from carcass washing to those found here. No previous studies on effluent characteristics from intestine and tripe washing, or omasum dehydration were found. Black et al. (2013) report that fleshing generates effluent containing flesh particles in suspension and blood, as found in this study.

Water consumption was studied from 1 April to 13 July 13, 2018. The number of animals slaughtered depends on the availability of buffalos from local livestock farmers, which varies daily. Consumption monitoring was not possible on public holidays and the like, because all slaughtering operations are closed (Figure 2). On days when slaughtering does not occur, water consumption reflects the requirements for plant washing and the refrigeration system. The plant is washed at the end of each day and the average requirement was 180 m³/d – over the nine non-working event days – after allowing for consumption in refrigeration.

Total water consumption and that for some specific processes is shown in Figure 2. The slaughtering processes/operations where water meters are not installed include the slaughter hall (dehiding and carcass cutting), fleshing, intestine and tripe washing, plant washing, deboning, and cutting and packaging, as well as personal hygiene, the administrative building, knife washing and the canteen. The unmeasured flow was calculated by subtracting the measured flow (the processes/operations where water meters are installed) from the total water consumption (the main delivery pipeline to all the processes/operations). The unmeasured flow was nearly 45 to 55% of the total flow into the slaughterhouse.

Some 59,087 buffalos were slaughtered during the study period, and the corresponding water consumption was 65,868,806 L, showing that 1,114 L of water is required on average for one buffalo slaughtered and processed (Figure 3; pecked blue line). Average water consumption per buffalo for individual processes was also determined, and is presented in Table 3. Notably, water consumption per buffalo is highest for paunch washing, followed by refrigeration (the plant operates refrigeration for 300 tonnes of final product, as well as carcass chilling, meat cutting, and the packaging hall).

A factor directly associated with water consumption is the degree of cleanliness required. Cleaning and hygiene practices in meat-related industries are usually stringent, which increases water use (COWI 2000). An increase of 20 to 25% in water consumption due to the HACCP program has been reported (USEPA 2002). Water consumption, in this study, was lower than that reported by either Johns (1995) (1.2 to 1.5 m³/head) or Enterprise Ireland (2009) (1.0 to 3.7 m³/head). Plants that keep monitoring their water use and wastewater flow continuously have been able to reduce water consumption by as much as 50% (Carawan et al. 1979). Employee training and constant monitoring of water use are proven ways of reducing water consumption in a plant (Bailone et al. 2020).

Not all water used in the slaughterhouse is discharged as wastewater. Between 5 and 20% of the water supplied is either retained in by-products – for example, blood meal, tallow, poultry food, and waste – or lost through evaporation and heating/steam (MRC 1995; Enterprise Ireland 2009). Some slaughterhouse operations add to the wastewater. Each buffalo releases nearly 10 to 15 kg of blood at the sticking point. As the specific gravity (SG) of buffalo blood is 1.052 (Chaplin et al. 1970), this represents between 9.5 to 14.3 L/animal. Between 2 and 3 L of water is released when the omasum is dehydrated, assuming that the average wet weight of an omasum is 3 kg and its moisture content falls from 70 to less than 3%. Each hide releases around 13.5 L of water – a hide's average wet weight is 27 kg and its moisture content brought down from 65% to below 30. Average water use and wastewater generation in this study from a slaughterhouse industry are shown in Figure 4.

An attempt has been made to identify areas where water could potentially be reused, and/or useful products with commercial value recovered. The findings are based on water consumption patterns, wastewater characterization, and discussion with the industry, and are presented in Table 4.

Slaughterhouse operations are water intensive, in part because of the stringent food safety and sanitary norms prescribed by national and international organizations. On average, 1,114 L of water is required per buffalo for slaughter and processing, with wastewater generation between about 916 and 1,089 L. Per capita water consumption falls with increasing slaughter numbers per day.

Slaughterhouse wastewater is characterized principally by biodegradable organic matter, but its characteristics vary widely in terms of organic strength, solids content, floating solids, and O&G from different slaughterhouse processes/operations. Wastewaters from the lairage and paunch sections contain fully and partially digested dung solids, respectively. Wastewaters from intestine and tripe cleaning, rendering, and hide fleshing contain FS and O & G. Blood from the sticking point exerts a very high organic load. Wastewaters from the hide storage and omasum dehydration areas contain NaCl.

There is ample scope for recovering resources like NaCl from the hide storage and omasum dehydration sections, dung from the lairage and paunch sections, and animal fats from the fleshing, and intestine and tripe washing sections.

It is usual to treat the wastewater streams from all slaughtering processes/operations together, making it difficult to achieve effluent quality within the permissible discharge limits. If the streams are segregated, however, on the basis of similarity, and managed separately, there will be benefits in terms of resource recovery, waste load reduction, and ease in wastewater treatment.

The help rendered by M/s FEPL, Rampur (U.P), and Barabanki (U.P.) is acknowledged gratefully.

The authors declare no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.