Wastewater treatment in the food industry
Abstract: The content of the food industry is extremely complex, including the production processes of sugar, brewing, meat, dairy processing, etc. The wastewater discharged contains organic matter, has strong oxygen consumption, and a large amount of suspended solids are discharged with the wastewater. The wastewater discharged from animal food processing also contains animal excrement, blood, fur, oil, and may contain bacteria, resulting in high oxygen consumption and much higher pollution than the wastewater discharged from plant-based food processing. Therefore, the wastewater from the food industry has a significant impact on both the environment and humans, and requires certain technologies to treat, purify, and then discharge it。
Keywords: Food industry wastewater treatment methods
1 summary:
The food industry includes many industries such as meat processing, seafood processing, dairy processing, canned food production, brewing, sugar production, etc. Therefore, the food industry has a wide range of raw materials and a wide variety of products, and the amount and quality of wastewater discharged vary greatly. The difference from other industrial wastewater is that food industry wastewater generally does not contain harmful substances such as heavy metals and pesticides. But its BOD, COD, etc. are generally higher. Due to the fact that most of the raw materials used in the food industry are organic substances that can be digested by the human body. Therefore, the biodegradability of food industry wastewater is generally good.
The main pollutants in wastewater include: (1) solid substances floating in the wastewater, such as vegetable leaves, fruit peels, minced meat, poultry feathers, etc; (2) Substances suspended in wastewater include oil, protein, starch, colloidal substances, etc; (3) Dissolved acids, alkalis, salts, sugars, etc. in wastewater: (4) Mud, sand, and other organic matter carried by raw materials; (5) Pathogenic bacteria and toxins, etc. Food wastewater includes wastewater discharged during the processing of alcohol, beer, monosodium glutamate, starch, lactose, citric acid, vegetables, and various soft drinks. If the wastewater is not treated in a timely manner, it will cause great pollution to the environment。
The characteristics of food industry wastewater are high organic matter and suspended solids content, easy to decompose, generally non-toxic, water quality and quantity vary with seasons, and high concentration wastewater is abundant. Its harm mainly lies in eutrophication of water bodies, leading to the death of aquatic animals and fish, causing organic matter deposited at the bottom of the water to produce odors, deteriorating water quality, and polluting the environment. In addition to appropriate pretreatment based on water quality characteristics, biological treatment is generally recommended for the treatment of wastewater in the food industry. If the requirements for effluent water are high or the organic content in wastewater is high, two-stage aeration tanks or two-stage biological filters, or multi-stage biological turntables, or a combination of two biological technologies can be used, or anaerobic aerobic series biological treatment systems can be used.
2 Domestic and international overview
Compared with developed Western countries, China started relatively late in the treatment of food wastewater pollution. Based on the experience of foreign treatment technologies, China has introduced and developed a large number of new technologies for food wastewater treatment through national scientific and technological research projects, and some projects have reached a high level. The production and operation of these new technologies have played a crucial role in alleviating China's severe water pollution situation and improving the water environment.
According to the China Environmental Status Bulletin, surface water pollution remained severe nationwide in 2008. Among the 407 national monitoring sections of the seven major water systems in China, the proportion of sections with water quality of Class I-III, IV-V, and poor V was 49.9%, 26.5%, and 23.6%, respectively. The overall water quality of the seven major water systems was moderately polluted, while the water quality of rivers in Zhejiang and Fujian regions was mildly polluted. The water quality of rivers in the northwest was excellent, while the water quality of rivers in the southwest was good. The problem of eutrophication in lakes (reservoirs) was prominent; The overall water quality in the nearshore waters is mildly polluted. And a large proportion of it is food industry wastewater。
There are a large number of food processing enterprises distributed in China, especially in small and medium-sized towns. The modernization level and production scale of these enterprises are increasing day by day, but the quality of the wastewater generated is poor, and the amount of wastewater is constantly increasing, which poses a serious threat to the environment. The food industry, including the beverage industry, is a major water consumer. Only a small portion of the water consumed is used for food production itself, while the majority is used for washing and cleaning during the food production process. Therefore, these wastewater can be recycled and reused. The pollutants generated by the fermentation industry, which mainly uses grain as the main raw material, are mainly caused by the unused grain. Therefore, the vast majority of pollutants discharged into the water environment are products and by-products with recycling value。
Modern wastewater treatment technologies can be divided into three categories: physical treatment methods, chemical treatment methods, and biochemical treatment methods. Physical processing methods include: precipitation method, filtration method, centrifugal separation method, flotation method; Chemical treatment methods include coagulation, neutralization, oxidation-reduction, electrolysis, extraction, stripping, adsorption, electrodialysis, and reverse osmosis; Biochemical treatment methods: activated sludge method, biofilm method, oxidation pond method, land treatment method. In addition, there are some new technologies, such as enzymatic hydrolysis, the application of efficient degradation bacteria, the application of ultrasonic technology, magnetic separation method, supercritical water oxidation method, electrode biofilm method, etc。
3 Wastewater treatment
3.1 Indicators for measuring the degree and characteristics of wastewater pollution: biological oxygen demand (BOD), chemical oxygen demand (BOD), total suspended solids (TSS), volatile suspended solids (VSS), nutrients, acidity and alkalinity, temperature, toxic substances, etc。
3.2 Physical treatment method: Physical treatment method refers to the treatment method that applies physical actions to change the composition of wastewater. The methods used for treating wastewater in the food industry include screening, removal, adjustment, sedimentation, air flotation, centrifugal separation, filtration, microfiltration, etc. The first five processes are mostly used for pre-treatment or primary treatment, while the last three are mainly used for deep treatment.
3.2.1sifted
Screening is a widely used method in pretreatment. The main function is to separate coarse dispersed suspended solids from wastewater. The equipment used includes grilles and screens. The grille intercepts coarser suspended solids and serves to protect the water pump and subsequent processing equipment.
3.2.2 Removal
Some food industry wastewater contains a large amount of oil and fat, which must be removed before entering the biological treatment process, otherwise it will cause blockages in pipelines, pumps, and some equipment, and also affect the biological treatment process. In addition, removing and recycling fats and oils has significant economic value.
Oil in wastewater can be divided into two types: free floating and emulsified. Usually, the oil separation tank can remove floating grease with a removal rate of over 90%. If there is a regulating tank or sedimentation tank in the processing, it can save investment and land occupation。
3.2.3 Adjustment
For food industry wastewater with significant changes in tax system water volume, setting up regulating tanks to adjust the water quality and quantity of the wastewater。
3.2.4 Precipitation
Precipitation is used to remove inorganic and organic solids from raw wastewater, as well as to separate solid and liquid phases in biological treatment processes. Remove inorganic solids from the original wastewater using a sedimentation tank; Use a primary sedimentation tank to remove organic solids from the original wastewater; Separate the biological and liquid phases in the biological treatment process using a secondary sedimentation tank, which is generally located after the grid and sieve. In order to avoid the sedimentation of organic solids in the wastewater during removal, an aeration sedimentation tank can be used. The use of a primary sedimentation tank can reduce the burden of subsequent work. Divided into horizontal sedimentation tank, vertical sedimentation tank, and radial sedimentation tank according to the direction of water flow in the pool。
3.2.5 Air flotation
Air flotation is mainly used to remove emulsified oil, surface active substances, and other suspended solids from wastewater. There are vacuum type, pressurized gas dissolving type, and plate type. Adding chemical coagulants or coagulants to the wastewater before it enters the air flotation tank can improve the removal rate。
3.3 Chemical treatment method
Chemical treatment refers to the application of chemical principles and chemical reactions to convert pollutants in wastewater into harmless substances, thereby purifying the wastewater. Pollutants change their chemical nature after undergoing chemical treatment, and chemical changes always accompany the treatment process. The methods used for treating food wastewater include neutralization, coagulation, electrolysis, oxidation-reduction, ion exchange, membrane separation, etc.
3.3.1 Coagulation method
Coagulation is the main chemical treatment method and cannot be used alone. It needs to be combined with sedimentation, clarification, or air flotation methods in physical treatment processes to form coagulation sedimentation or coagulation air flotation。
For some wastewater with small colloidal particles or colloidal solutions that are difficult to precipitate, coagulants are added to form large particles that are easy to precipitate and remove them。
3.3.2 Redox
Oxidation reduction is an effective method for converting pollutants in wastewater. Organic and inorganic substances in wastewater are oxidized or reduced into slightly toxic or non-toxic substances through chemical reactions, or converted into forms that are easily separated from water, thus achieving the purpose of treatment。
3.3.3 Ion exchange
Ion exchange is mainly a method of using ion exchangers to exchange and remove harmful ions present in water.
3.4 Biological treatment method
Biochemical treatment is one of the important processes in organic wastewater treatment systems. In the treatment of wastewater in the food industry, biological treatment processes can be divided into good quality, anaerobic, stable pond, land treatment, and various combination processes formed by their combination. Biological treatment is the main secondary treatment process aimed at degrading COD and BOD.
Aerobic biological treatment processes are divided into activated sludge process and membrane process based on the growth form of the microorganisms used. The former includes traditional activated sludge method, stage aeration method, biological adsorption method, complete mixing method, delayed aeration method, oxidation ditch, intermittent activated sludge method, etc. The latter includes biofilters, tower biofilters, biological turntables, activated biofilters, biological contact oxidation methods, aerobic fluidized beds, etc。
The anaerobic biological treatment method is suitable for food industry wastewater, mainly because the wastewater contains high concentrations of organic matter that is easily biodegradable and non-toxic. In addition, anaerobic treatment has low power consumption, the generated biogas can be used as energy, and the amount of residual sludge generated is small. The anaerobic treatment system is fully enclosed, which is conducive to improving environmental hygiene.
4 Conclusion and Prospect
Through the use and implementation of various effective methods mentioned above, the wastewater in the food industry has been greatly improved, and other substances can be obtained from it to produce other raw materials instead of throwing away waste, which can be recycled and reduce unnecessary losses and waste. The harm to water pollution, human health, and other aspects has been greatly reduced, but the equipment and energy consumption used in various treatment methods are a great burden. For rural or small and medium-sized food enterprises, most of them are unable to bear it, and some enterprises are unwilling to invest too much money in wastewater treatment in order to obtain more profits. On the other hand, the country's policies and relevant laws and regulations are not sound and the enforcement is not strong enough, and the country's investment in this area is not significant; Furthermore, some people in food enterprises have low quality and awareness, and do not see the seriousness of wastewater pollution. They only care about their own interests。
In the future, if the country can increase the investment in this area and establish a perfect system and legal system so that these detection methods can be widely used, the waste water treatment situation of the food industry will be greatly improved, the impact on water pollution will be minimal, and the impact on human health will be much reduced. Food enterprises will also obtain other gains from the treatment project, so that resources can be recycled. China's per capita resources are already very small, so the burden in this regard will be reduced。
I believe that if the above detection methods can be popularized, our country will get great benefits, not only in terms of money, but also in terms of the national people.
