Food industry wastewater

　　The food industry is highly complex, encompassing production processes such as sugar manufacturing, brewing, meat processing, and dairy processing. The wastewater generated from these operations contains organic matter, exhibits strong biochemical oxygen demand, and carries substantial suspended solids. Wastewater from animal‑based food processing further includes animal excreta, blood, hides and fur, and fats and oils, and may also harbor pathogenic microorganisms; consequently, it has a very high oxygen demand and is far more polluting than the wastewater produced by plant‑based food processing.

　　Characteristics of Food Industry Wastewater

　　The characteristics of wastewater are the comprehensive properties exhibited by a complex system composed of water and various impurities, which are expressed through water quality parameters. These parameters can be categorized into physical, chemical, and biological indicators. Each type of parameter reflects the types and concentrations of impurities in the water, enabling an assessment of water quality and whether it meets discharge standards. In wastewater treatment, determining the degree of treatment, selecting the appropriate process flow, and designing and operating the treatment process all require a thorough understanding of the wastewater’s properties; otherwise, optimal results cannot be achieved. The principal water quality parameters for wastewater are as follows.

　　Physical parameters of food industry wastewater

　　The physical parameters of wastewater primarily include temperature, color, odor, and solids content; among these, colorimetry and total suspended solids are the most commonly measured.

　　1. Color: Wastewater from the food industry often contains organic or inorganic dyes, biological pigments, inorganic salts, organic additives, and other substances that impart color, frequently resulting in highly colored effluents. In water quality analysis, color is quantified using the parameter of “colority.” Typically, the true color—after removing suspended solids—is used as the reference standard. Colorimetric analysis is employed to compare the color intensity of a known‑concentration standard solution with that of an unknown sample, thereby determining the latter’s colority.

　　2. Total Solids: Most of the impurities in wastewater exist as solid substances, which may be present in dissolved or suspended forms. Together, these are referred to as total solids and include organic compounds, inorganic compounds, and various biological materials. In water quality analysis, in addition to determining the total solids content, it is also necessary to measure several other parameters, such as suspended solids, volatile suspended solids, and dissolved solids.

　　(1) Total solids (TS) refer to the residue remaining after a specified volume of water sample is dried at 105–110°C, expressed as a mass.

　　(2) Suspended solids (SS), or suspended matter, are the portion of total solids that remain in suspension and contain both organic and inorganic constituents.

　　(3) Volatile suspended solids (VSS) represent the mass loss of suspended solids upon heating and incineration at 600°C, reflecting the organic fraction of the suspended solids. This organic fraction comprises biodegradable suspended solids (BVSS) and non‑biodegradable suspended solids (NBVSS).

　　(4) Non-volatile suspended solids (NVSS) are the residue remaining after the incineration of suspended solids, also known as ash, and represent the inorganic fraction of the suspended solids.

　　(5) Dissolved solids (DS), or dissolved substances, refer to the portion of total solids that exists in a dissolved state; they are determined by drying and weighing the filtrate of a specified volume of water sample.

　　Selection of Treatment Processes and Application of Chemicals for Food Industry Wastewater and Food Processing Wastewater

　　The food industry uses a wide variety of raw materials and produces numerous types of products, resulting in significant variability in both the volume and quality of its wastewater. However, in general, the main pollutants found in food‑industry wastewater include:

　　(1) Solid substances floating in wastewater, such as vegetable leaves, fruit peels, minced meat, and poultry feathers, etc.

　　(2) Substances suspended in wastewater include oils and fats, proteins, starches, colloidal substances, and others.

　　(3) Acids, bases, salts, sugars, and other substances dissolved in wastewater

　　(4) Mud, sand, and other organic matter entrained in the raw materials

　　(5) Deadly pathogens, etc.

　　Food industry wastewater is generally characterized by high levels of organic matter and suspended solids, a tendency to spoil easily, and typically low toxicity. Its primary environmental impacts include eutrophication of water bodies, which can lead to the death of aquatic organisms and fish, as well as the generation of foul odors from organic sediments at the bottom of water bodies, thereby degrading water quality and polluting the environment.

　　In addition to applying appropriate treatment based on the characteristics of the wastewater, biological treatment is generally recommended for food‑industry effluents. When extremely stringent effluent quality is required or when the wastewater contains a high organic load, two‑stage aeration tanks, two‑stage biofilters, multi‑stage rotating biological contactors, or a combination of two biological treatment units may be employed. Alternatively, an anaerobic–aerobic serial biological treatment system can also be adopted.

　　From the perspective of selecting a polyacrylamide flocculant for dewatering food‑industry wastewater sludge, medium‑cationic polyacrylamide is typically preferred—this approach is similar to that used for treating biochemical sludge from municipal wastewater treatment plants. As we know, municipal wastewater also has a relatively high organic content, with most of it originating from domestic sources such as catering effluent, sewage from human waste, and bathing wastewater. In terms of the complexity of its source composition, it closely resembles food‑industry wastewater.