Solid-liquid separation
No matter what system or comprehensive measures are used to treat livestock farm wastewater, solid-liquid separation is first carried out, which is an essential process link
Its importance and significance lie mainly in the following.
The solids content of the wastewater discharged from general farms is very high, up to 160,000mg/L, and the corresponding organic content is also very high. Solid-liquid separation can greatly reduce the pollutant load in the liquid part; secondly, solid-liquid separation can prevent larger solids from entering the subsequent treatment process and prevent blockage and damage to equipment. In addition, solid-liquid separation before anaerobic digestion treatment can also increase the reliability of anaerobic digestion operation, reduce the size of the anaerobic reactor and the required residence time, reduce the investment in the facility and improve the COD removal efficiency.
Solid-liquid separation technologies generally include: sieve filtration, centrifugation
Filtration, floatation and removal of sedimentation, sedimentation, flocculation and other processes. At present, there are mature solid-liquid separation technologies and corresponding equipment, and the main types of equipment are screen type, horizontal centrifuges, filter presses and hydrocyclones, rotary conical sieves and centrifugal disc separators. Mature agents include polymeric aluminium chloride, polyacrylamide, activated carbon and other water treatment agents.
II. Anaerobic digestion
As farm wastewater is a "three high" wastewater with high organic matter concentration, high N and P content and high number of harmful microorganisms. Therefore, anaerobic technology has become an indispensable key technology in the treatment of manure from livestock and poultry farms. For this kind of high concentration organic wastewater, the anaerobic digestion process can effectively remove a large amount of soluble organic matter at a low operating cost, with a COD removal rate of 85% to 90%, and it can kill infectious bacteria, which is beneficial to the epidemic prevention of the farm. If the aerobic process is adopted directly to treat the farming wastewater after solid-liquid separation, although the one-time investment can be saved by 20%, the long-term operation cost will bring a heavy economic burden to the farm because of its large power consumption and the consumption of electric water flow is as much as 10 times of anaerobic treatment.
There are many anaerobic processes used to treat farm waste, among which the following are more commonly used: anaerobic filter (AF), up-flow anaerobic sludge bed (UASB), compound anaerobic reactor (UASB+AF), two-stage anaerobic digestion and up-flow sludge bed reactor (USR), etc. In recent years, anaerobic digestion, i.e. biogas fermentation technology, has been widely used in farm waste treatment, and by the end of 2002 the number of large and medium-sized biogas projects in China's livestock and poultry farms had reached more than 2,000, being one of the countries with a large number of biogas installations. Although, there is no lack of failure in the construction of biogas projects in China, and the success rate of the project construction is only 85%, this technology is not lost as an effective technical solution to the problem of harmless and resourcefulness of livestock manure and sewage. Livestock manure and farm wastewater are valuable resources, which can be harmless through anaerobic digestion, and can also recover biogas and organic fertilizer, so the construction of biogas projects will be a wise choice for the treatment of manure and wastewater from small and medium-sized farms.
III. Aerobic treatment
Aerobic treatment refers to a process that uses aerobic microorganisms to treat farm wastewater. Aerobic biological treatment method can be divided into two categories: natural aerobic treatment and artificial aerobic treatment. Natural aerobic biological treatment method is a method of using natural water bodies and microorganisms in soil to purify wastewater, also known as natural biological treatment method, which mainly has two kinds of water body purification and soil purification. The former mainly includes oxidation ponds (aerobic ponds, parthenogenic ponds, anaerobic ponds) and culture ponds, etc.; the latter mainly includes land treatment (slow percolation, rapid method filtration, ground diffuse flow) and artificial wetlands, etc. Natural biological treatment method not only has low infrastructure costs and low power consumption, the method is difficult to biochemical degradation of organic matter
The removal of nutrients such as nitrogen and phosphorus and bacteria is also higher than conventional secondary treatment, and can partially achieve the effect of tertiary treatment. In addition, under certain conditions, the method can be used in conjunction with wastewater irrigation to achieve resource utilization of wastewater. The disadvantages of the method are mainly the large footprint and the treatment effect is easily affected by the season. However, if the scale of the farm is small and there are abandoned ditches and mudflats nearby for use, this method should be chosen as far as possible to save investment and treatment costs. Artificial aerobic biological treatment is a wastewater treatment method that adopts artificially enhanced oxygen supply to improve the vitality of aerobic microorganisms. The main methods are activated sludge, biofilter, bio-rotary, biological contact oxidation, sequential batch activated sludge (SBR), anaerobic/aerobic (A/O) and oxidation ditch. In terms of treatment effect, contact oxidation and biological carousel are more effective than activated sludge method, although the biofilter is also very effective, but it is easy to clog the filter. The oxidation ditch, SBR and A/O processes are all improved activated sludge methods. SBR method has a high degree of automation and control, and is capable of deep treatment of sewage, but its disadvantage is that the BOD load is small and the one-time investment is large. Although the investment is on the large side, the water treated by this method is easy to meet the standard discharge. Therefore, for those farms with large scale farming, large amount of wastewater generation and strong economic ability, the A/O method can be chosen, while for medium scale farms, aerobic treatment processes such as contact oxidation and biological rotor can be chosen.
