Slaughterhouse and meat processing wastewater treatment

• 1. Meat processing industry and slaughterhouses.
• 2. Wastewater production in a slaughterhouse and meat processing plant
• 3. Composition of wastewater from slaughterhouses and meat processors.
• 4. Technologies for slaughterhouse wastewater treatment
  • 4.1. Pretreatment
  • 4.2 Physical-chemical treatment
    • 4.2.1. Coagulation-flocculation
    • 4.2.2. Dissolved air flotation
  • 4.3 Biological treatment
    • 4.3.1. Anaerobic Treatment
    • 4.3.2. Aerobic treatment
    • 4.3.3. Artificial wetlands
    • 4.3.4 FBR Process
    • 4.3.5. Fixed bed biological system                                                       
• 5. Post-treatment effluents
• 6. Reuse of wastewater in a slaughterhouse.
  • 6.1 Advanced filtration treatment
  • 6.2 Additional disinfection
  • 6.3 Advanced oxidation processes
  • 6.4 Nutrient treatment
• 7. Performance of technologies
• 8. Conclusions

1. Meat processing industry and slaughterhouses

There are different types of meat industries depending on the animals they process (poultry, pork, beef, etc.), as well as the products they produce (fresh, precooked, cured, cooked, etc.). As a general rule, these types of industries separate the slaughterhouse areas from those of processed products, since they are different processes, as are the discharges that are generated.

One of the types of industries that produce the most biodegradable effluents is slaughterhouses and meat processors. This is due to the fact that most of the pollutants produced during the different phases and operations of these processes are of animal biological origin (organic remains, blood, fat, hair, skin, bones, etc.).

Although these are biodegradable effluents, this does not mean that the treatments to be applied are simple, due to the high loads of organic matter they contain; in addition, very demanding hygienic measures must be maintained, in accordance with the food standards specified in current legislation. These cleaning and disinfection processes usually use, in addition to steam, chemical products such as surfactants and biocides that tend to negatively affect biological processes, which must be taken into account when selecting and managing them appropriately.

2. Wastewater production in a slaughterhouse and meat processing plant

Wastewater is generated mainly due to the activities of slaughtering animals and cleaning equipment and work areas. These are some of the activities in which wastewater is produced:

• Stabling: When the animals to be slaughtered are received, they are placed in stables, where fecal waste and other remains are produced (in greater quantities in the case of poultry).

• Slaughter of animals: During the slaughter process, water is used to wash animals. This water may contain traces of blood, fat and other biological materials.
• Washing of equipment and work areas: After slaughter and butchering of animals, regular cleaning of equipment and utensils is required to maintain hygienic standards. This involves the use of water to rinse utensils, work tables, floors and other areas.
• Processing of animalby-products: Slaughterhouses may process animal by-products such as viscera, bones, fats and other tissues. During these operations, wastewater may be generated due to the washing and cleaning of these materials.
• Treatment of skins: In some slaughterhouses, animal skins may undergo tanning and treatment processes for subsequent use in the leather industry. These processes involve the use of water and chemicals, which can result in wastewater containing pollutants. In these cases, it is preferable to separate these effluents from the rest and treat them specifically.

There are different options for the disposal of slaughterhouse wastewater:

• Direct discharge to the environment.
• Discharge to a municipal wastewater treatment plant.
• Reuse of treated water in individual areas.

Each option requires different treatment standards, with direct discharge and reuse requiring the most stringent standards, as industrial or municipal wastewater treatment plants have effluent treatment lines.        

As a guideline value for the flow of water, an average consumption of 5 liters of water for each kg of weight of the animal to be slaughtered is estimated. In the case of poultry, this figure is higher (5-10 l/kg).

4. Technologies for slaughterhouse wastewater treatment

Wastewater treatment plants for slaughterhouses and meat processors include a pretreatment stage, primary, secondary and tertiary treatments, as well as additional treatments in case you want to reuse treated water and recover valuable resources. 

4.1. Pretreatment

The most commonly employed pretreatment technologies include:

Screening and homogenization.

A necessary step in the pretreatment of slaughterhouse wastewater is mechanical screening, which removes oversized impurities to prevent them from entering the treatment plant. Without screening, the system can become clogged and electromechanical equipment can break down, which can lead to operational problems and a short-term deterioration in process performance.

Usually, before the homogenization basin, whose capacity will depend on the heterogeneity of flow and loads of the discharges, there is a coarse separation grate for sizes of approximately 30 mm. These grids are usually made of bars and usually have a cleaning system consisting of a scraper that drags the separated solids to an attached hopper or container. Next, there is a fine screen with a 3 mm span, which is usually of the ladder type, or a filtering screen. In this case, it is important to bear in mind that the solid remains together with the fats can easily cause clogging, so it is convenient to incorporate a good cleaning system, if possible, automatic. In the case of poultry slaughterhouses, it is important to consider the presence of feathers, which present a potential added problem in this pretreatment process. Specific sieves are commercialized for these uses.

It is common to make a bypass of the screening system, equipped with a manual rake grid, in order to carry out repair or maintenance operations on the main line.                  

Inside the homogenization basin, agitation and aeration systems are arranged according to its dimensions and geometry, in order to improve the mixing of effluents and avoid the formation of dead spots and anoxic zones that would produce the decomposition of the organic matter deposited, with the consequent problems of fermentation and bad odors. It is also common to carry out a first neutralization with acid or soda, for which reagent dosing equipment controlled by a pH meter is used.

4.2 Physical-chemical treatment

The spill contains a significant amount of colloidal and dissolved organic material of low density. This is the case of fats, tissue debris and blood. One of the most practical primary treatment methods for slaughterhouse wastewater is dissolved air flotation (DAF). Dissolved Air Flotation (DAF)which significantly reduces these pollutants.

4.2.1. Coagulation-flocculation

But prior to flotation, a coagulation / flocculation treatment must be carried out. In the coagulation process, the colloidal particles present in slaughterhouse wastewater are grouped into larger particles called coagulates. The colloidal particles in slaughterhouse wastewater have a predominant negative charge, which makes them stable and resistant to aggregation. For this reason, coagulants with positively charged ions are added to destabilize the colloidal particles, thus facilitating the separation process. The most widely used coagulants are inorganic metal-based coagulants, such as aluminum sulfate, aluminum hydrochloride, ferric chloride, ferric sulfate and aluminum polychloride. Depending on the coagulant selected, a pH adjustment will be required for them to work under adequate conditions; thus, for example, PAC (aluminum polychloride), besides being of low toxicity, can work well in a wide pH spectrum, and collaborates in the formation of spongy flocs that can be easily separated by flotation.

The coagulation chamber should have a contact time of about 5 minutes and use fast stirrers for mixing.

After coagulation, the flocculation process is carried out by dosing a long-chain polymer that collects the clots present in the effluent, forming larger lumps called flocs. Depending on the type of flocculant used (cationic/anionic/non-ionic) a further pH adjustment may be necessary. It is important to take care in the selection of these reagents thinking about the recovery of the separated solids for the commercialization of by-products.

In the flocculation process, slow agitations must be carried out, and with suitable propellers, so as not to break the floccules formed. The contact time is about 10 minutes.

4.2.2. Dissolved air flotation

The wastewater containing the formed flocs enters the flotation equipment for contaminant separation. The basic principle of dissolved air flotation involves the introduction of micro air bubbles in the water that adhere to the floccules, which reduce their bulk density, thus causing them to float. In this way, fats, oils and suspended solids move to the surface of the DAF system creating a layer of sludge, which is continuously removed by the scraper blades. The flotation system operates automatically and adapts to the flow of the slaughterhouse wastewater.

Dissolved air flotation offers high performance for slaughterhouse and meat processing wastewater, removing suspended substances from wastewater with a high efficiency of up to 99%.

Based on our extensive experience in the purification of slaughterhouse wastewater, we have optimized our flotation plants and made certain adjustments for this special type of wastewater. Thus, we achieved excellent purification results:

Parameter	COD	BOD	TSS	Grease	Oil	Phosphorus	Blood
Maximum efficiency	85%	85%	95%	99%	98%	95%	90%

The sludge obtained from the DAF system has a high concentration (3 - 5%), which makes it possible to dispense with a sludge thickener and, frequently, also with complementary chemical products for its subsequent mechanical or thermal drying process.

4.3 Biological treatment

Primary treatment and physicochemical processes are usually not sufficient to bring wastewater within discharge limits, so secondary treatment is necessary to remove soluble organic compounds, and those remaining from primary treatment.

Biological treatment of slaughterhouse wastewater is a natural process that does not require the use of chemicals, except for some nutrients in certain cases.

Slaughterhouse wastewater has a high biodegradable organic content, which can be broken down with the growth of microorganisms and the controlled addition of oxygen. A well-optimized biological treatment can achieve organic matter removal efficiencies in excess of 90%.

Classification of traditional biological treatment systems

Broadly speaking, biological treatments can be classified into different types of aerobic and anaerobic processes, with the additional alternative of artificial wetlands.                 

Here are some general considerations about each of these systems:

4.3.1. Anaerobic Treatment

Anaerobic digestion is the most widely used biological process for this type of effluent, due to its effectiveness in the treatment of highly concentrated biodegradable industrial effluents. The organic compounds are degraded by anaerobic bacteria in the absence of oxygen, producing a gas mixture of CO2 and CH₄ (Biogas).

Anaerobic systems have the advantage of low sludge production, low energy consumption with the possibility of resource recovery (energy from biogas), and high COD removal.

Typical anaerobic processes for the treatment of meat processing effluents can be: anaerobic reactor with partitions, anaerobic digester, anaerobic filter, anaerobic lagoon, septic tanks and upflow anaerobic sludge reactor (UASB).

There are other anaerobic process technologies consisting of acidification column and biological reactor that are arranged in height (Pakes process, EGSB, ECSB, etc.), which have high yields and significantly reduce the space required for their implementation.

The gases released contain a high proportion of methane (_CH4), which is stored in a pressurized balloon (gasometer) and used as fuel for a biogas boiler. In this way, it is possible to recover part of the energy used for the purification plant's consumption.                                                                                                     

Despite its high performance, anaerobic treatment usually does not reach the required discharge limits, due to the high concentration of organic matter in slaughterhouse wastewater, and hardly reduces the NTK of the effluent. Therefore, an additional aerobic biological treatment step is recommended to remove the remaining organic compounds until the permitted limits are reached.

4.3.2. Aerobic treatment

There are many advantages to using aerobic processes for wastewater treatment, including low odor production, rapid biological growth rate, and the ability to adjust quickly to changes in temperature and load.                                            

Aerobic processes are commonly used for the removal of medium level biodegradable organic matter (up to approximately 3 - 4 g/l COD).

In slaughterhouses and meat processing industries, they are not usually used as a direct treatment after the physical-chemical process for two reasons:

• The high operating costs due to the high oxygen consumption required
• The important production of sludge, as it is directly related to the load of pollutants to be eliminated.

In addition, the size of the installations is much larger due to the high organic content and the contact time required. It is therefore appropriate to install them after anaerobic treatment.           

An aerobic biological process consists of a sludge concentration, recirculation and drainage system, which is completed with a secondary settling tank, from where the activated sludge is recirculated to the aeration chamber in order to maintain a stable MSS content.

Depending on the sludge load produced, a previous sludge digestion will be carried out, which can be aerobic or anaerobic. The excess sludge is sent to a thickener and from there to drying by means of mechanical systems (belt filters, filter presses, centrifugal decanters , etc.) or thermal drying.                               

A DAF flotation system can replace the secondary clarifier, especially when the nature of the sludge allows it (light sludge), as in the case of effluents from slaughterhouses and meat processing plants. In this way, the treatment line is simplified (the thickener can be eliminated) and a better quality clarified product is obtained.

During the last few years, MBR technology is gaining ground as a complement to secondary treatment, allowing to maintain high MSS loads in the aerobic biological reactor, thus reducing the volume of the oxidation zones.The MBR technology also acts as a tertiary treatment, since its ultrafiltration membranes reach a high level of filtration efficiency, which allows to maintain high MSS loads in the aerobic biological reactor, thus reducing the volume of the oxidation zones. MBR technology also acts as a tertiary treatment, since its ultrafiltration membranes achieve a degree of filtration that allows the reuse of water in low-demand processes (washing, cleaning, irrigation, etc.).                                                         

There are several aerobic systems used for the treatment of slaughterhouse water, such as aerobic activated sludge systems, rotating biological disks and sequential batch reactors (SBR).

4.3.3. Artificial wetlands

Artificially constructed wetlands mimic the degradation mechanisms of natural wetlands for water decontamination, integrating biological and physicochemical processes through the interaction of vegetation, soil, microorganisms and atmosphere for adsorption, biodegradation, filtration, photooxidation and sedimentation of organic compounds and nutrients.

Constructed wetlands have shown efficiency in the elimination of organic compounds and nutrients for different vegetation, with maximum eliminations of 99%, 97%, 85% and 78% for BOD, COD, SSV and NTK, respectively. These are simple methods with low operating and maintenance costs and a low impact on the environment, but they require large spaces for their implementation.

Examples of biological reactors commonly scaled up in the treatment of wastewater from slaughterhouses and meat processing plants:

4.3.4. FBR Process

The FBR process ('flotation bio-reactor') combines three technologies:

• A biological reactor where biological treatment is carried out in the presence of oxygen, which may be preceded by anaerobic and/or anoxic phases.
• A DAF clarification system that provides perfectly clarified water that can be poured.
• To ensure that subsequent separation processes deliver the expected results, it is necessary to dose a polyelectrolyte with flocculating effect into a flocculation system. Optionally, a coagulant can be added to reduce values such as turbidity and phosphorus precipitation.

It is a novel system that uses dissolved air flotation as secondary clarification to improve the performance of traditional systems. The SIGMA BIODAF - FBR (flotation biological reactor) system uses the DAF system to separate activated sludge from water and achieve very high biomass concentrations in the reaction medium, up to 9000 mg/L MLSSV.

It allows to treat flows from 50 to 10000 m3/day and biomass concentrations in the reactor from 2000 to 9000 mg/L MLSSV.

Thanks to our FBR bioflotation systems, sludge with a dry solids content 3 to 4 times higher than any conventional system is achieved.                                                     

The combination of these technologies results in perfectly clarified water that can be discharged in compliance with discharge regulations. On the other hand, a sludge is produced that is partly recirculated to the biological reactor to maintain a stable biomass concentration and partly extracted as a purge.

4.3.5. Fixed bed biological system

Slaughterhouse wastewater is often characterized by a high N (nitrogen) to COD (chemical oxygen demand) ratio after pretreatment. Therefore, biological treatment processes with nitrification and denitrification are particularly suitable. Additional agents for denitrification are generally not required, provided that no prior degradation of carbon compounds takes place.

A cascade fixed bed biological reactor is a reliable and robust process that can handle overloads and underloads without problems. It is a system successfully tested in meat processing plants, providing outstanding results for the removal of COD, BOD5, phosphorus, nitrogen and ammonia from slaughterhouse wastewater.

The purification standards that have been achieved in various projects in the meat processing industry are as follows:

Parameter	COD	BOD	Ammoniacal nitrogen	N	Phosphorus
Value (mg/L)	<100	<10	<5	<3	<1

5. Post-treatment of effluents

The effluents treated by the biological process, even if they reach the limits established by the applicable regulations, must also be free of pathogenic germs, so they must be sterilized before being disposed of.                                                              

The traditional system consists of an oxidation with _Cl2 or NaOCl, giving it sufficient contact time (about 15 minutes) in a pit that usually has a labyrinth geometry (trocadero).                                                  

 At present, the trend is to use other sterilization systems, such as medium and high charge UV radiation equipment, ozonizers, or the dosing of other oxidizing agents that affect the environment as little as possible (_ClO2, Br salts, etc.).

The treated effluents can also be reused for some undemanding processes of the plant itself (flushing, cleaning, etc.), and for this purpose they must be conditioned with other complementary purification processes.

6. Reuse of wastewater in a slaughterhouse.

The consumption of mains water varies considerably in this sector, but in any case, a typical slaughterhouse generates a large amount of wastewater from the slaughtering process and cleaning of the facilities. For this reason, water reuse and the recovery of valuable by-products from meat processing effluents are a growing trend in the agri-food sector.

More and more companies are demanding treatment processes that allow both the reuse of water and the recovery of nutrients and fertilizers, as well as the production and exploitation of biogas.

Treated wastewater from a slaughterhouse can be reused for various purposes, depending on the quality of the treated water and specific requirements. Some of the possible uses include:

• Agricultural irrigation: Treated water can be used for irrigation of agricultural crops, as long as it meets the quality standards and regulatory requirements established for the protection of human and environmental health.
• Industrial use: In certain cases, treated water can be used in industrial processes that do not require potable water, such as equipment cooling, industrial cleaning or manufacturing activities that do not involve direct contact with food or people.
• Internal process water: Treated water can be reused within the slaughterhouse for cleaning, equipment washing, toilet flushing or other non-potable uses that do not require potable quality water.

It is important to note that the reuse of treated water must comply with local requirements and regulations, and be managed responsibly to ensure the protection of human health and the environment. Constant monitoring of treated water quality and proper design of the treatment system are required to ensure that the quality standards required for reuse are met.

Additional technology and more advanced treatment is generally required to ensure that treated wastewater meets the standards necessary for reuse. Some common processes that can be applied to improve water quality and enable reuse include:

6.1 Advanced treatment for filtration and salt separation

The use of finer filtration technologies, such as membrane filtration (microfiltration or ultrafiltration), allows the removal of suspended particles, organic matter and some chemical contaminants present in the treated wastewater.                                                                             

The processes of separation of salts and molecules with semi-permeable membranes are an alternative for the treatment of meat processing effluents in which the treated water is intended to be reused. There are different membrane purification processes, such as nanofiltration (NF) and reverse osmosis (RO), which offer separation efficiencies of more than 90% in the removal of salts, organic matter and pathogens.                                                                          

One aspect to be taken into account in membrane processes is membrane fouling during wastewater treatment, since proper pretreatment, maintenance and replacement protocol is essential to ensure the efficiency of the process.

6.2 Additional disinfection

In addition to the disinfection processes used in conventional treatment, such as chlorination or UV radiation, more stringent disinfection methods, such as membrane disinfection, advanced chlorination or irradiation with high-intensity ultraviolet light, may be necessary to ensure effective elimination of pathogens and microorganisms.

6.3 Advanced oxidation processes

Some persistent chemical contaminants in wastewater (refractory COD) may require advanced oxidation processes such as ozonation or hydrogen peroxide oxidation for removal.

Advanced oxidation processes are a complementary treatment option after primary and secondary treatment of slaughterhouse wastewater, showing excellent efficiency for organic matter degradation and water reuse. There are various processes including gamma radiation, ozonation, ultrasound technology (UST), UV/H2O2, UV/O₃ and photocatalysis, among others, which stand out for their high reaction rate and very low treatment time. Sometimes, refractory organic matter molecules are oxidized, in order to break their bonds and facilitate their subsequent biodegradation, as a less costly alternative to total oxidation.                                     

Disinfection is another benefit of advanced oxidation processes, which can inactivate pathogens without the addition of additional chemicals, thus avoiding the formation of hazardous by-products (THM, chlorine derivatives, etc.).                                                          

Photocatalysis using photo-Fenton-based processes, as well as UV/H2O2 photooxidation, are the most commonly used advanced oxidation processes for the treatment of slaughterhouse wastewater. Although these processes are often costly if applied alone, removal efficiencies above 90% can be achieved for secondary effluents of slaughterhouse wastewater in terms of TOC (total organic carbon) and COD (chemical oxygen demand) as a post-treatment method.

6.4 Nutrient treatment

Depending on reused water quality requirements and local regulations, it may be necessary to implement nutrient removal processes, such as denitrification and phosphorus removal, to prevent eutrophication and protect receiving water quality. 

It is important to note that the design and selection of additional treatment technologies depends on factors such as:

• The initial quality of the wastewater.
• Quality standards for reuse.
• Local requirements.
• The specific needs of the slaughterhouse.

7. Performanceof technologies

The following table includes the most commonly used technologies for the treatment of wastewater generated in slaughterhouses and other meat processing plants.      

Special attention is given to the removal of organic compounds and excess nutrients, in terms of general parameters such as BOD, COD, TOC, total nitrogen (TN) and total phosphorus (TP). As can be seen, treatment efficiencies vary widely and depend on factors such as wastewater characteristics, treatment time, or waste concentration, among others.

Abbreviation	Concept	Abbreviation	Concept
UST	Ultrasound	EC	Electrocoagulation
AF	Flotation	AL	Lime slaked
ABR	Anaerobic reactor with deflector	SBR	Sequential bio reactor
AS	Surface aeration	AD	Anaerobic digestion
AOP	Advanced oxidation	CW	Artificial wetlands

Technology	HRT (h)	BODIN (mg/L)	CODIN (mg/L)	TOCIN (mg/L)	TNIN (mg/L)	BODREM (mg/L)	CODREM (mg/L)	TOCREM (mg/L)	TNREM (mg/L)
EC	1	1123	2171	-	148	-	85	-	-
ABR-AS-AOP	41	4635	-	1200	841	100	-	100	82
SBR	96	-	6580	-	3321	-	81	-	95
AF	46	-	15800	-	-	-	60	-	-
AL	48	5088	9216	-	343	73	59	-	-
SBR	12	-	356	-	175	-	-	-	91
EC	1	1950	3337	-	-	-	78	-	-
SBR	161	4240	6057	1436	576	-	98	-	98
AF	24	-	88	-	-	-	80	-	90
AD	2640	-	18600	-	5200	-	-	-	66
MF	48	-	480	183	115	-	91	45	45
UST-AF-UF	144	-	3000	-	-	-	96	-	-
CW	28	-	468	-	61	-	60	-	46
UF-RO	160	-	7970	-	-	-	80	-	-
AD-AOP	723	658	1494	513	181	98	98	97	31
ABR-AS-AOP	55	1635	2000	1200	841	100	99	100	85
ABR-AS-AOP	10	1831	2043	1691	866	100	99	100	90

8. Conclusions

Meat processing industries and slaughterhouses produce discharges highly contaminated with biodegradable materials, which provide them with a high COD. This COD, by its nature, is highly biodegradable (high proportion of BOD) and is therefore easily separable by biological purification processes. Due to its high solids content, a treatment line consisting basically of the following stages is required:

HOMOGENIZATION AND ROUGHING

• Roughing and screening units
• Homogenization basin with pH adjustment

PRIMARY TREATMENT

• Primary physical-chemical treatment system consisting of coagulation and flocculation chambers and dissolved air flotation (DAF) equipment.

SECONDARY TREATMENT

• Anaerobic biological purification unit
• Aerobic biological purification unit
• DAF dissolved air flotation unit for separation and recirculation of activated sludge to the aerobic biological oxidation zone (replaces the secondary settling tank).
• Disinfection of treated effluent

TERTIARY TREATMENT (Recovery of effluents)

• Replacement of Secondary Decantation or DAF flotation with MBR
• Membrane effluent recovery system (mF/UF/NF/OI).
• Disinfection

SLUDGE TREATMENT

• Anaerobic / aerobic sludge digestion
• Digestate treatment.
• Mechanical/thermal sludge drying

The current trend is:

• Use DAF flotation systems instead of decanters.
• Advanced oxidation treatments, instead of or in combination with biological treatments.

Use of filtration membranes at different levels (UF/NF/OI), depending on the requirements established for the reuse of treated effluents and for the recovery of waste (sludge).