How to treat slaughterhouse wastewater

Slaughterhouse wastewatermainly contains high concentrations of nitrogen-containing organic compounds, suspended solids, dissolved solids, grease, and protein, including blood, grease, meat scraps, undigested food residues, hair, feces, and sediment. It has a high concentration of suspended solids, a displeasing reddish-brown color, and an offensive bloody odor. It may also contain various bacteria harmful to human health. Due to its large volume and strong pollution potential, its treatment has received widespread attention in China. Some advanced treatment processes have been applied in slaughterhouses and have achieved good environmental benefits. For such easily degradable organic wastewater, biological treatment is one of the most economical and effective methods. Therefore, biological methods or physicochemical-biological combined processes with biological treatment as the main component are used domestically and internationally to treat slaughterhouse wastewater. Currently, commonly used methods include:

1. Chemical Methods

(1) Alkaline hydrolysis and enzymatic hydrolysis

This method uses alkaline substances or enzymes for hydrolysis to reduce fat globules in wastewater and is often used as a pretreatment for slaughterhouse wastewater. Lime, NaOH, lipase, bacterial enzymes, etc., are commonly used. Lime is economical and practical but produces a large amount of waste residue. When NaOH is used for pretreatment, controlling the NaOH concentration within the range of 150-300 mg/L can reduce the average fat globule size to 73% ± 7% of the initial fat globule size (Din) before treatment. Pretreatment with pancreatic lipase is most effective. Pancreatic lipase PL-250 can reduce the maximum fat globule size to 60% ± 3% of the fat globules in the wastewater before treatment, and pancreatic lipase is more suitable for hydrolyzing bovine fat. Bacterial enzyme treatment requires a larger dosage to achieve a significant hydrolysis effect. However, alkaline hydrolysis of slaughterhouse wastewater can cause fluctuations in wastewater pH, making it difficult to control and hindering the normal operation of subsequent processes such as biological oxidation.

(2) Coagulation Treatment

For a long time, biological methods have been primarily used to treat slaughterhouse wastewater, but their treatment effectiveness at low temperatures is extremely difficult to meet requirements. To solve this problem, researchers have explored new avenues for chemical coagulation and flocculation treatment of slaughterhouse wastewater. By adding chemical agents of a certain concentration, various particles in the wastewater are precipitated, colloids are destabilized, and some soluble pollutants can also be removed to a certain extent, reducing the pollution load in a very short period. Its advantages include: simple process, easy to implement, quick results; short reaction time, small footprint of structures; wide availability of coagulant raw materials, low cost; treatment effect is less affected by temperature; adaptable to fluctuations in water volume and quality, can deodorize simultaneously, and suitable for different treatment scales.

Commonly used coagulants include aluminum salts, iron salts, etc. Among them, polyferric sulfate has a good effect in treating slaughterhouse wastewater. To reduce the amount of aluminum salt used, polyaluminum chloride (PAC) and polyethylene can also be mixed as coagulants. In the synthesis of polyferric sulfate, aluminum salts in any proportion and a certain proportion of silicates, as well as a small amount of polyacrylamide, are added to form a new coagulant CPFA-CS. This composite inorganic polymer coagulant has a wide pH and temperature applicable range. When used as a coagulant to treat slaughterhouse wastewater, the removal rates of COD and color can reach over 75% and 95% respectively. A single coagulation treatment can meet or approach the comprehensive wastewater discharge standard. A significant problem with simple coagulation treatment is that the blood water produced during the slaughtering process is difficult to remove, and it also produces a large amount of sludge and waste residue. Therefore, if the slaughterhouse wastewater is pre-treated appropriately before using coagulants, and then treated with a composite coagulant of ferrous sulfate and calcium oxide, the COD concentration in the effluent can be reduced to 197.4 mg/L, achieving good treatment effects. This method is simple, efficient, and has good environmental benefits. However, this method is only suitable for treating wastewater with a COD concentration less than 1000 mg/L. Coagulation method for wastewater treatment has low cost and good treatment effect at low temperatures. This method is mostly used to treat low-concentration wastewater, or as a pretreatment for high-concentration wastewater to reduce the load of subsequent biological treatment.

A slaughterhouse in Yingkou uses polyferric sulfate (PFS) in combination with a coagulant aid (MZ). The COD removal rate is over 88%, and the decolorization rate is over 90%. The effluent indicators meet the national emission standards.

2 Physical Methods

The mechanism of flotation is to provide a sufficient number of micro-bubbles to the wastewater. These highly dispersed micro-bubbles act as carriers to attach suspended solids in the wastewater, making their density less than water and causing them to float to the surface for solid-liquid separation. It can be used for the separation of solid-solid, solid-liquid, liquid-liquid, and even ions in solutes in water. Flotation, as an efficient and rapid solid-liquid separation technology, was first applied in the mineral processing industry. In 1905, a US patent published the dissolved gas flotation technology, and in 1907, H. Norris invented the jet dissolved gas flotation technology. Due to the invention of these technologies, dissolved gas flotation has been widely applied, not only for domestic drinking water treatment and industrial water treatment but also for the treatment of various industrial wastewaters and urban domestic sewage from oil refining, chemical industry, papermaking, slaughtering, textile, printing and dyeing, steel, food, and pharmaceutical industries. Since the 1970s, this technology has received considerable attention from scholars at home and abroad in the field of water treatment and has developed rapidly. It is currently widely used in water supply, especially for the purification of low-temperature, low-turbidity, and algae-rich water bodies, as well as for the treatment of urban sewage and industrial wastewater. According to the different ways bubbles are generated, flotation can be divided into electrolytic (coagulation) flotation, air diffusion flotation, and dissolved gas flotation. Among these, partially recycled pressurized dissolved gas flotation is the most commonly used process in water treatment, and in some aspects, it can serve as a new technology to replace sedimentation. Nevertheless, the design and optimal operation of flotation plants still rely on pilot tests and experience, thus further research is needed on flotation mechanisms, equipment, and process combinations for flotation technology. The treatment of water supply, wastewater, urban domestic sewage, and sludge through different flotation agents, flotation flocculation process parameters, and bubble-floc co-coagulation is the direction for further research in flotation.

3 Combined Processes

To achieve better treatment effects while reducing treatment costs, slaughterhouse wastewater treatment often employs a combination of multiple methods. Several typical combined processes are described below:

(1) Pressurized Biological Contact Oxidation - Coagulation and Sedimentation Combined Process

This process is suitable for treating medium-concentration slaughterhouse wastewater. After pressurized biological contact oxidation, the dissolved oxygen and degradation rate of organic matter in the wastewater are increased. Subsequent coagulation and sedimentation can achieve the secondary discharge standards of existing enterprises. This process is highly efficient for treating medium-concentration wastewater, but it is costly to operate and difficult to maintain and manage.

(2) Two-stage Upflow Anaerobic Sludge Blanket (UASB) and Dissolved Air Flotation-Upflow Anaerobic Sludge Blanket (DAF-UASB) methods

This process is an improved version of the single UASB method and is suitable for treating slaughterhouse wastewater containing high concentrations of suspended solids, fat globules, and grease.

(3) Oxidation Pond Process

The treatment process is as follows: Wastewater → Pretreatment → Anaerobic Pond → Facultative Pond → Aerobic Pond → Effluent.

The operating results of this process indicate that oxidation ponds can withstand significant fluctuations in hydraulic and organic loads. The effluent quality can be reused in slaughterhouses.

(4) UASB + SBR Process

This process uses UASB technology for anaerobic treatment and SBR (Sequencing Batch Reactor) technology for aerobic treatment, making it an ideal equipment for high and low concentration wastewater treatment projects. This process has good treatment effects on carbon source organic matter, is flexible in operation, convenient to operate, and has denitrification function. The process is mature, reliable, and stable to meet standards. The initial investment and daily operating costs are low. The entire process flow is simple and smooth, easy to operate, and has broad promotion value.

(5) Hydrolysis acidification-bio-adsorption regeneration-contact oxidation process. This process is particularly suitable for treating wastewater with high concentrations and significant variations in water quality and quantity.

(6) CAF Vortex Air Flotation - SBR Method

This process uses mechanical gratings to remove most solid pollutants, preventing large solid particles from affecting air flotation and aeration processes. This significantly reduces the treatment load of subsequent processes. Mechanical filtration is then used to ensure stable effluent quality that meets standards, saving on operation and maintenance costs. Wastewater treated with this process achieves a COD removal rate of 80-90%.

(7) Upflow Anaerobic Sludge Blanket Filter (UASBAF) - Sequencing Batch Reactor (SBR) Process

This process is suitable for treating wastewater with significant water quality fluctuations and high protein content. It features a simple process flow, resistance to shock loads, easy operation and management, low engineering costs, and low operating expenses. It is suitable for wastewater treatment projects in small-scale slaughterhouses of meat processing plants.

(8) Research on SBBR Treatment of Slaughterhouse Wastewater

Li Weiguang et al. conducted experimental research on treating slaughterhouse wastewater using a Sequencing Batch Biofilm Reactor (SBBR). Slaughterhouse wastewater is first passed through a screen to remove coarse suspended solids and then undergoes oil removal by static settling. Subsequently, it is treated by a Sequencing Batch Biofilm Reactor (SBBR) to further remove organic matter, followed by filtration to remove color and fine suspended solids. The results showed removal rates for various pollutants: COD at 97%, BOD5 at 99%, oil at 52%, TKN at 92%, and SS at 82.4%. The final effluent quality met the national Class II discharge standard. Fang Qian et al. conducted a comparative experiment on SBBR and SBR for treating slaughterhouse wastewater. The experimental results indicated that under the same operating conditions, the biofilm system achieved better treatment effects than the activated sludge system. When the same pollutant removal rate was achieved, the biofilm system was easier to operate and manage, and it overcame some of the problems associated with the activated sludge system. Zhao Ling studied the removal process and effectiveness of organic matter and nitrogen using a composite SBR system with YDT elastic stereoscopic packing as a biological carrier. The results showed that under aerobic conditions, denitrification occurred, meaning simultaneous nitrification and denitrification. Under experimental conditions, when the dissolved oxygen was 3.0-5.0 mg/L, the total nitrogen removal rate reached 84%, and the COD removal rate reached 95%.