Overview of Food-Industry Wastewater Treatment Technologies
The nature of wastewater is a comprehensive characteristic exhibited by a complex system composed of water and various impurities, represented by water-quality indicators. Water-quality indicators are divided into physical, chemical, and biological indicators, etc. Various water-quality indicators indicate the types and quantities of impurities in water and can judge water-quality merits and whether discharge standards are met. In wastewater treatment, determining the treatment level and selecting the process flow, as well as the design and operation management of the treatment process, all require a full understanding of wastewater nature; otherwise good results cannot be obtained. The main water-quality indicators of wastewater are as follows.
The physical indicators of wastewater mainly include temperature, color, odor, and solid content; the commonly tested ones are color and solid content.
1. Color: Food-industry wastewater often contains organic or inorganic dyes, biological pigments, inorganic salts, organic additives, etc., making the wastewater colored and sometimes deeply colored. In water-quality analysis, the indicator measuring the degree of water color is colority, generally based on the true color after removing suspended solids, using a colorimetric analysis method to compare a known-concentration standard colored solution with an unknown-colority water sample and obtain the result.
2. Solid content: Most of the impurities in wastewater are solid substances, existing in dissolved or suspended form in water; the two together are called total solids, including organic compounds, inorganic compounds, and various organisms. In water-quality analysis, besides measuring total solid content, several indicators such as suspended solids, volatile suspended solids, and dissolved solids are also measured.
(1) Total solids (TS) refers to the residue after a certain amount of water sample is dried at 105-110 degC in water-quality analysis, expressed by weighing.
(2) Suspended solids (SS), or suspend, is the part of total solids in suspended state, containing both organic and inorganic components.
(3) Volatile suspended solids (VSS) is the weight loss of suspended solids after ignition at 600 degC, representing the organic part of suspended solids, part of which is biodegradable suspended solids (BVSS) and part non-biodegradable (NBVSS).
(4) Non-volatile suspended solids (NVSS) is the residual part of suspended solids after ignition, also called ash, representing the inorganic part of suspended solids.
(5) Dissolved solids (DS), or dissolved matter, refers to the part of total solids existing in dissolved state, obtained by filtering a certain amount of water sample and drying and weighing the filtrate.
Selection of Food-Wastewater Treatment Process and Application of Treatment Agents
Food-industry raw materials are wide-ranging and product types numerous, so the volume and quality of discharged wastewater vary greatly. But in general, the main pollutants in food wastewater are:
(1) solid matter floating in the wastewater, such as vegetable leaves, fruit peels, minced meat, poultry feathers, etc.
(2) matter suspended in the wastewater, such as oils and fats, proteins, starch, colloidal substances, etc.
(3) acids, alkalis, salts, sugars, etc. dissolved in the wastewater
(4) mud, sand, and other organic matter entrained in the raw materials
The general characteristic of food-industry wastewater is high content of organic matter and suspended solids, easy to putrefy, and generally not highly toxic. Its harm is mainly eutrophication of water bodies, causing the death of aquatic animals and fish, promoting the production of odor from organic matter deposited at the bottom, deteriorating water quality, and polluting the environment.
From the perspective of sludge dewatering-agent (polyacrylamide) selection for food wastewater, the sludge dewatering agent generally chooses medium-cationic polyacrylamide, which is similar to the treatment of biochemical sludge in domestic-sewage treatment plants. We know that the sewage in domestic-sewage treatment plants also has relatively high organic-matter content, and most sewage comes from our daily-life catering wastewater, fecal wastewater, and bathing wastewater, etc. Analyzed from the complexity of wastewater-source structure, it is very similar to food wastewater. [1]
Besides appropriate treatment according to water-quality characteristics, food-industry wastewater treatment should generally adopt biological treatment. If effluent quality requirements are very high, or the organic-matter content in wastewater is very high, two-stage aeration tanks or two-stage biofilters, or multi-stage rotating biological contactors, or a combination of two biological-treatment devices can be used, or an anaerobic-aerobic series biological-treatment system can be adopted.
