1 Characteristics and treatment requirements of sewage in small towns
1.1 Characteristics of sewage in small towns
The characteristics of sewage in small towns are determined by various factors. Due to the small population size and low tap water penetration rate of small towns, the sewage discharge in small towns is generally 3000-30000m3/d. The level of industrial and agricultural development in small towns determines that more than 50% of sewage is domestic sewage, and industrial wastewater is mainly wastewater from agricultural product processing. Therefore, the nitrogen and phosphorus content in the water is high, and there are basically no heavy metals and toxic and harmful substances in the water, and the water quality and water volume of sewage fluctuate greatly. The sewage properties of most small towns are not much different, among which BOD5 is 100-150mg/L, COD is generally 250-300mg/L, and SS is about 200mg/L.
1.2 Requirements for treatment technology of sewage in small towns
The requirements for sewage treatment in small towns are determined by the characteristics of its sewage and the conditions of the small towns themselves. First, due to the large fluctuations in the quality and quantity of sewage in small towns, the scale of sewage treatment plants is also small, and the time-varying coefficient is large, the sewage treatment process in small towns must have a strong ability to resist shock loads. Second, due to the weak economic strength of small towns, the selected sewage treatment process should try to achieve low operating costs and low construction costs, and basically no or little chemicals should be added. At the same time, the sludge output of the process should be as small as possible to reduce secondary pollution and reduce the cost of sludge treatment. Third, small towns lack professional sewage treatment workers, so the treatment process should be simple and easy to operate and maintain.
2 Processes suitable for sewage treatment in small towns
The sewage treatment process in large cities has developed relatively maturely. Compared with large cities, small towns are limited by economic strength and their own regions, and the development of sewage treatment technology is relatively slow. From the above situation, it can be seen that the core points of sewage treatment in small towns are: the process flow is simple to operate and easy to maintain. Now introduce several processes suitable for sewage treatment in small towns, and propose the development trend of sewage treatment processes in small towns.
2.1 Natural sewage purification treatment system
Common natural sewage purification treatment systems include stabilization ponds, land treatment systems, and wetland treatment systems.
2.1.1 Stabilization pond. Stabilization pond, also known as oxidation pond or biological pond, is a natural or artificially constructed sewage purification system with the characteristics of low investment, simple operation and management, and energy saving. Countries around the world began to study stabilization ponds in the early 20th century, and developed rapidly after the 1950s. my country’s research on stabilization ponds began in the late 1950s. So far, stabilization ponds that have been built and put into operation are almost all over the country. Stabilization ponds can be divided into aerobic ponds, facultative ponds, anaerobic ponds, aerated ponds, and deep treatment ponds according to the type of microorganisms in the pond water. Compared with other processes, stabilization ponds have the following advantages: First, the treatment cost is low. The stabilization pond has a simple structure, a short construction period, low treatment energy consumption, convenient operation and maintenance, and low cost, so the sewage treatment cost of the stabilization pond is low. Second, due to the large volume of the stabilization pond, it can withstand the fluctuation of sewage water volume, and has strong adaptability and impact load resistance. It is suitable for the process requirements of sewage treatment in small towns. Third, the stabilization pond can make full use of existing local lakes, ponds, etc. Therefore, it can be adapted to local conditions to achieve the purpose of sewage treatment. Fourth, the sludge output of the stabilization pond is small, thus reducing secondary pollution and lowering the cost of sludge treatment and disposal. Due to the above advantages of oxidation ponds, the oxidation pond process has been widely used. However, oxidation ponds also have some disadvantages and limitations: they occupy a large area, the treatment efficiency is relatively low, they may produce odors and breed mosquitoes and flies, and they are not suitable to be built near residential areas.
2.1.2 Land treatment system. The sewage land treatment system refers to an ecological project that uses the land ecosystem composed of soil, microorganisms and plants such as farmland and forest land to comprehensively purify pollutants. It can realize the resource utilization and harmlessness of sewage while treating urban sewage. At present, the commonly used processes are slow infiltration system, rapid infiltration system, surface flow system, wetland treatment system and underground infiltration system. Among the land treatment systems, the most widely used and most mature one is the artificial wetland treatment system. The five indispensable parts of the artificial wetland system are a permeable matrix, plants that can grow in saturated water and anaerobic matrix, water body, invertebrate or vertebrate animals, and aerobic or anaerobic microbial populations. The land treatment system uses these parts to treat pollutants through physical filtration, physical adsorption and deposition, physical and chemical adsorption, chemical reaction and precipitation, microbial metabolism and biodegradation of organic matter.
The advantages of the sewage land treatment system are: First, the sewage land treatment system can promote the circulation of plant nutrients in sewage, and the useful substances in sewage are reused through the growth of crops. Second, the capital construction cost of the sewage land treatment system is low, and it can make full use of land and depressions. Third, the operation and management of the sewage land treatment system is convenient, and the energy consumption is low. Fourth, the sludge is fully utilized and there is less secondary pollution. The sewage land treatment system has some of the above advantages, but also some disadvantages. If it is not designed properly, it will pollute the soil and groundwater, especially cause heavy metal pollution and organic poison pollution, leading to a decline in the quality of agricultural products. It will also emit odors, breed mosquitoes and flies, and even affect human health.
2.1.3 Wetland treatment system. Similar to the above-mentioned natural purification treatment system, the main advantages of artificial wetlands are simple operation, low investment, and low energy consumption. However, its area is relatively large. Therefore, artificial wetland treatment systems are more suitable for small towns in agricultural areas where land is not too tight.
2.2 Oxidation ditch process
The oxidation ditch is a new type of activated sludge method invented by Dutch engineers in the 1950s based on the delayed aeration activated sludge method. According to the structural characteristics of the oxidation ditch, the inventors and patents, the oxidation ditch can be divided into different types. The common ones are Carrousel oxidation ditch, alternating oxidation ditch, double-ditch oxidation ditch for phosphorus removal and nitrogen removal, three-ditch oxidation ditch, ObraI oxidation ditch and integrated oxidation ditch. On the basis of the traditional oxidation ditch used to remove COD and BOD, the second-generation oxidation ditch also has the function of nitrogen removal and phosphorus removal, which greatly improves the application prospects of the oxidation ditch. The oxidation ditch process has the following advantages: First, due to the small number of structures in the oxidation ditch, the primary sedimentation tank and sludge digestion tank do not need to be built, so the treatment process is simple and the operation and management are convenient. Second, the oxidation ditch is suitable for high-concentration industrial wastewater, can withstand the impact load of water quality and water quantity, and overcomes the disadvantage that high-concentration industrial wastewater inhibits the activity of activated sludge bacteria. Third, when nitrogen removal and phosphorus removal are required, the oxidation ditch has the advantages of reducing operating costs and energy consumption compared to traditional nitrogen removal and phosphorus removal processes. Fourth, the effluent water quality is good and the operation is stable. However, since primary sedimentation tanks and sludge digestion tanks are generally not built, the oxidation ditch process increases the load of the reaction tank, which will increase some energy consumption to a certain extent. At the same time, since the aeration device of the oxidation ditch, such as the surface aerator or the aeration brush and other mechanical parts, need to be regularly maintained, the maintenance workload is large.
2.3SBR process
Sequencing Batch Reactor Activated Sludge Process is referred to as SBR, also known as sequential intermittent activated sludge process. The SBR reaction tank is the core system of the process. Homogenization, primary sedimentation, biodegradation and secondary sedimentation processes all occur in the SBR reaction tank. It achieves effective degradation of organic matter through intermittent operation in operation. As an activated sludge treatment technology, the main advantages of SBR are: First, there are few process treatment equipment, no secondary sedimentation tank and sludge return system, so the operation is simple and easy to manage. Second, it is not troubled by sludge swelling. Third, it has strong resistance to shock loads. Fourth, it can achieve alternating aerobic, anoxic and anaerobic states, and has good effects in nitrogen removal and phosphorus removal. Due to the above characteristics, the SBR system is more suitable for sewage treatment in rural small towns with small water volume, dispersed point sources and intermittent pollutant discharge. But at the same time, the SBR process also has some disadvantages that cannot be ignored. Since the decanting depth is generally 1 to 2m, the head loss of sewage lifting is relatively large. The equipment has strict requirements for automatic control, so the requirements for management personnel are also relatively high. At the same time, since the SBR process does not have a primary sedimentation tank, it is easy to produce scum to a certain extent.
2.4 Biological contact oxidation process
The biological contact oxidation method is a biological treatment process consisting of fillers immersed in sewage and an artificial aeration system. Under aerobic conditions, the sewage repeatedly contacts the biofilm on the surface of the filler to purify the sewage. The advantages of the biological contact oxidation method are: First, the process has strong resistance to shock loads, does not require sludge return equipment, is not affected by sludge expansion, and produces less sludge. Second, its biomass per unit volume is large, so the treatment capacity is relatively high. Third, since the process has fewer equipment, the operation is simple and easy to maintain. However, for small towns, the cost of this process is relatively high, and it is not easy to distribute water and air evenly. Although there are fewer equipment, the structure is relatively complex, which increases the difficulty of design and construction. Therefore, when selecting, it is necessary to consider the actual situation of small towns.
2.5 Anaerobic hydrolysis-high-load biological filter
Anaerobic hydrolysis-high-load biological filter is a treatment process that has been developed in recent years to adapt to the characteristics of sewage treatment in small towns. This process mainly changes the pretreatment process from the traditional primary sedimentation tank to the anaerobic hydrolysis filter, and at the same time, it has made important technical innovations in its process structure based on the traditional high-load biological filter. The modified process has the advantages of high load and high efficiency, and by adopting a new type of plastic module filler with high porosity, high attachment area and high secondary water distribution performance, the filter effluent return system is cancelled, thereby greatly reducing the energy consumption of operation and construction investment costs. As a new process, anaerobic hydrolysis-high load biofilter has the following outstanding advantages: First, compared with the ordinary activated sludge method, the sludge production of this process is greatly reduced, which reduces the cost of sludge treatment and disposal to a certain extent, and also reduces secondary pollution. Second, because the process treatment system integrates primary sedimentation tank, aeration tank, sludge return facilities and oxygen supply facilities, the sewage treatment process is simple and the management and operation are simple. Third, the process has a strong ability to resist shock loads. These advantages determine that anaerobic hydrolysis-high load biofilter can meet the requirements of sewage in small towns in my country.
3 Conclusion
By comparing the above-mentioned sewage treatment processes in small towns, the following conclusions can be drawn: sewage treatment in small towns should start with cost saving and convenient management to select sewage treatment processes to maximize economic, environmental and social benefits. Therefore, different towns should choose appropriate processes according to their own characteristics. For towns with less land use, artificial wetland treatment systems can be selected. Practice has proved that artificial wetland treatment systems not only save investment and have good treatment effects, but also help beautify the ecological environment. It is one of the best solutions for sewage treatment processes in small towns. From the perspective of cost, the oxidation ditch process and SBR process equipment are simple, the infrastructure investment is low and the land occupation is small. Therefore, these processes should be given priority for the sewage treatment in small towns. Judging from the development trend in recent years, the technical requirements for sewage treatment in small towns are low investment, low energy consumption, convenient operation and management, and high efficiency. These have become the inevitable trend of the development of sewage treatment technology in small towns. For example, new processes developed in recent years, such as anaerobic hydrolysis-high-load biological filter, are successful representatives. With the support of technology, the government should continue to increase its efforts, combine the main spirit of the “National Environmental Protection “Twelfth Five-Year Plan”, and effectively strengthen the sewage treatment in small towns, so as to truly do a good job in water supply and sewage treatment.
Suitable for testing the water treatment effect of small town sewage plants, Lianhua Technology’s rapid water quality testing instrument is recommended.
1. COD rapid detection instrument, using rapid digestion spectrophotometry, using potassium dichromate method, 165 degrees digestion sample for 10 minutes and then colorimetric results, this method is simple to operate, does not require professional operators, outputs fast values, and the results are accurate.
2. BOD rapid tester LH-BODK81, using microbial electrode method, is more convenient and faster than traditional BOD5, supports 8 minutes to get results, uses continuous constant current micro-injection method, small sample collection volume, no pretreatment reagents added, zero secondary discharge pollution, suitable for surface water, domestic sewage and industrial wastewater that does not have obvious toxic effects on microorganisms.
3. Portable suspended solids tester LH-P3SS, uses photometry to quickly detect the suspended solids content of samples, direct concentration reading, and uses the self-developed Qinglan series dedicated operating system. The system has the advantages of simple operation, intuitive display, and comprehensive functions. It has independent software patent 2023SR0910797.
Post time: May-27-2025
