Have you noticed? Since the country first proposed the treatment of black and odorous water in rural areas four years ago, many regions have obvious Cognitive inertia in the treatment process, that is, "borrowing" the urban black and odorous water.
Industry insiders have stated that "if the treatment of rural black and odorous water bodies is simply applied to urban governance models and blindly" advanced ", it will ultimately lead to low investment efficiency and difficult to sustain the treatment effect.
Therefore, in February, the Ministry of Ecology and Environment and the Ministry of Finance clearly stated in the "Notice on Pilot Work of Rural Black and Odor Water Treatment" that pilot cities should carry out treatment according to local conditions. Based on the causes of pollution, pilot projects will be carried out according to local conditions, including rural types, natural environment, and economic development level.
01
《Analysis Report on the Current Situation of Rural Sewage Treatment》
Treatment scale, process technology, and water quality indicators
1. Rural sewage collection and treatment tend to focus on small-scale centralized treatment
When designing sewage treatment schemes, the treatment scale should be considered as the first major factor and also a necessary screening condition for process technology selection. Researchers conducted statistical analysis on the scale of sewage treatment in over 200 engineering cases, and the results showed that:
The treatment scale of rural domestic sewage has nearly 30% of engineering cases ranging from 20 to 50m3/d, accounting for the largest proportion; Cases with processing scales of 50-100m3/d and 100-300m3/d respectively account for 23% and 20% of the total; The number of cases with a processing scale of less than 20m3/d is relatively small, accounting for 18% of the total; The scale greater than 500m3/d is more inclined towards the sewage treatment capacity of townships, so it accounts for a relatively small proportion.
It is not difficult to find that the amount of rural sewage is small and the water volume fluctuates greatly, and the collection and treatment of rural sewage is more inclined towards small-scale centralized treatment.
2. The application of rural sewage treatment technology shows polarization
At present, the commonly used treatment technologies for rural sewage can be divided into two categories:
One is ecological treatment, including Constructed wetland, land treatment and stabilization pond.
The other is biological treatment, including A/O, A2/O, SBR, oxidation ditch and other Activated sludge, biofilm processes such as biofilter, biological contact oxidation, and combined membrane bioreactor and other technologies.
It should be noted here that due to the generally low effluent requirements for rural sewage treatment and the advantages of terrain and terrain in rural areas, ecological treatment with less economic investment can be prioritized. However, considering the limited ecological treatment capacity, when the inflow load is high, a combination technology combining biology and ecology is usually used.
In order to explore and select the most suitable technology for rural sewage treatment, the researchers summarized the distribution range of technologies used in different water volume ranges in more than 200 rural sewage treatment cases surveyed and counted, and the results showed:
The application of rural sewage treatment technology presents a phenomenon of polarization. The rural sewage treatment scale has the highest number of cases in the range of 20-300m3/d, and artificial wetlands and biofilm technology are the most widely used in these cases.
Among them, in the treatment scale of 20~300m3/d stage of Constructed wetland technology, with the increase of the treatment scale, the application cases of the technology are gradually rising. This shows that the Constructed wetland process is a common technology in rural sewage treatment at present.
3. Rural sewage treatment mainly considers the degradation of organic matter, reduction of pollution level, and emission reduction of total pollutants
In order to explore the current situation of rural sewage treatment indicator assessment, researchers conducted statistics on various indicators in nearly 200 cases and reached the following conclusions:
COD, NH3-N, and TP are the most evaluated water quality indicators. The assessment rates for COD and NH3-N are 94.42% and 91.19% respectively, both exceeding 90%; The assessment rate of TP is 83.94%; SS is second only to the above three items in the water quality assessment of rural sewage, with BOD and TN assessment rates below 70%, with TN assessment rate only being 50%.
From this, it can be seen that on the one hand, rural sewage treatment considers reducing the total amount of pollutants and preventing the generation of black and odorous water bodies; On the other hand, it is mainly considered to degrade organic matter, reduce pollution and prevent Eutrophication of water body.
02
Why cannot urban thinking be applied to rural sewage treatment?
Characteristics and difficulties of rural sewage treatment
As is well known, there are significant differences between rural and urban areas in terms of economic development level, production and lifestyle, population agglomeration, and many other aspects, which determine the significant differences in the causes and treatment of rural and urban sewage.
1. Insufficient investment in operation and maintenance funds
At present, the investment and operation and maintenance funds for rural domestic sewage treatment mainly come from the public funds of the village committee.
Although environmental protection and local government departments may provide certain financial support, the funding investment is often relatively insufficient and there is no fixed source of funding, making it difficult to achieve sustained investment, resulting in a significant funding gap in the operation and maintenance management of rural domestic sewage treatment.
Of course, in addition to funding issues, other operational and maintenance management issues after the completion of rural sewage treatment facilities are also frequent.
2. Unclear main body of operation and maintenance management
The operation and maintenance management entities of rural domestic sewage treatment are both numerous and complex, including multiple government departments including township governments, enterprise units including construction units and operation and maintenance units, as well as local village committees.
The division of labor among various units and departments is not clear, the boundaries are not clearly defined, and the power and obligation are not unified, making it difficult to effectively unify management. When encountering difficulties, various departments and units are prone to shirking and shirking, which is not conducive to timely and effective problem-solving and brings inconvenience to operation and maintenance management.
At present, the idle phenomenon of sewage treatment facilities in most villages and towns is relatively serious, forming the so-called "solar engineering".
After being built, it has been idle for a long time, without effective management by professional personnel, nor has a long-term operation and maintenance mechanism been established.
3. Large fluctuations in treatment water volume and quality
Rural sewage mainly consists of shower water, kitchen wastewater, self provided well water, and fecal sewage. The properties of most village sewage are not significantly different, and the water bodies contain a certain amount of nitrogen and phosphorus. There are basically no heavy metals or other toxic and harmful substances, and the water quality fluctuates greatly with good biodegradability.
With the uneven discharge of domestic sewage, the amount of water changes greatly, and the coefficient of change of water quality also increases accordingly. The daily coefficient of change is generally between 3.0 and 5.0, and even exceeds 10.0. The sewage discharge in summer is higher than that in other seasons, mainly due to the increase in washing frequency and other water consumption of farmers in summer.
In addition, there is a phenomenon of enterprises mistakenly connecting and mixing industrial wastewater, resulting in significant changes in the pH value of the incoming water and difficulty in meeting the effluent standards.
These issues have brought significant difficulties to the effective operation of rural sewage projects.
4. Incomplete management supervision and assessment mechanism
Generally speaking, the assessment mechanism for rural domestic sewage treatment work is a hierarchical assessment, but the focus of the assessment is not on the management mechanism, but on the construction of facilities, with a focus on assessing the completion of the target tasks of rural domestic sewage treatment points. However, the assessment content for the overall project operation management has not yet been covered.
At the same time, due to the scattered layout of villages and the large number of sewage treatment facilities in the area, it brings great difficulties to actual supervision. However, due to high costs, complex technology, and insufficient manpower and financial resources, online monitoring is difficult to fully implement.
5. Lack of sustained and professional operation and maintenance
After the completion of sewage treatment facilities or Septic tank in rural areas, the idea of "once and for all" follows.
But the older the year, the more severe the aging of the equipment, with some experiencing significant leakage of sewage, and some residents' sewage pipelines not connected to sewage treatment facilities.
In addition, some areas even experience situations where residents are entrusted with management and there is no one to take care of them, resulting in a long-term lack of corresponding professional technical guidance and inability to complete the daily operation of highly specialized sewage treatment equipment. As a result, the rural domestic sewage treatment system has not been fully utilized and has not fully realized its due value.
After all, the difficulty in rural sewage treatment is not in terms of technology, but in terms of "acclimatization" under the urban model.
03
How can rural sewage treatment be tailored to local conditions?
Based on local environment and combined with climate and terrain
Faced with the distinct characteristics of urban sewage, the key to rural sewage treatment is the organic combination of treatment technology and the actual rural environment.
At present, the commonly used technologies for rural sewage treatment include biogas digester, Activated sludge, Constructed wetland, land treatment, oxidation pond, biofilm method, and some combined processes, such as activated sludge membrane bioreactor, activated sludge Constructed wetland, biofilm Constructed wetland, etc.
In practical engineering cases, in order to achieve effective sewage removal, it is usually necessary to combine technologies based on local environmental characteristics, combined with factors such as climate and terrain, to achieve effective pollutant removal.
1. Shanghai City, Jiangsu Province, Zhejiang Province, Anhui Province, Fujian Province, Jiangxi Province, Shandong Province
East China has a well-developed water system, with high temperature and rain in summer, mild and little rain in winter, and a remarkable monsoon climate.
Through the analysis of engineering cases in East China, it is found that Activated sludge process and its combination process, biofilm process and its combination process are widely used, accounting for more than 75% of the total cases; The utilization rate of Constructed wetland and its combination process was slightly lower, only 9 cases.
As the Yangtze River Delta region represented by Jiangsu, Zhejiang and Shanghai has developed rural economy, large population density, tight land resources and serious water quality shortage, it is necessary to select Aerobic organism treatment technology with good treatment effect and small floor area or adopt a combination of biological and ecological methods.
2. Beijing, Tianjin, Hebei, Shanxi, Inner Mongolia
The environmental characteristics of North China are different from those of East China. The precipitation in this region is not sufficient, and the lack of water resources leads to frequent drought and flood disasters.
In terms of terrain. North China covers not only the low mountains and hills in eastern Liaoning and eastern Shandong, the Huang Huai Hai Plain and the lower reaches of the Liaohe River in the middle, but also the loess plateau in the west and the mountains in northern Hebei. Therefore, when sewage treatment technology is selected in North China, topographic conditions are considered as the main influencing factor.
For example, in mountainous areas with tight land and resources and large terrain drop, Anaerobic organism filter treatment technology can be considered.
For example, the northwest of Beijing is mainly mountainous, while the southeast is plain. Moreover, the economic situation of rural areas in Beijing is good. Therefore, MBR technology and a combination of MBR and land treatment, as well as a combination of activated sludge and land treatment, can be considered.
3. Henan Province, Hubei Province, Hunan Province, Guangdong Province, Guangxi, Hainan Province
The terrain conditions in the central southern region are complex, and compared to plains, the coverage area of mountainous and hilly areas is larger. However, mountainous and hilly areas are often important water sources, and sewage discharge is also limited.
In the engineering cases in the central and southern regions, the combined process of Activated sludge and Constructed wetland, Constructed wetland, land treatment and biofilm treatment and Constructed wetland are widely used, and some facilities adopt the processes of stabilization pond and membrane biological treatment.
Generally, the rural areas to the south of Qinling Huaihe River are built according to the water, and can rely on the existing pond resources to build ecological processes such as Constructed wetland; The area north of the Qinling Huaihe River can adopt a combination of biological and ecological treatment technologies such as biofilm treatment and activated sludge treatment.
4. Chongqing, Sichuan, Guizhou, Yunnan, Tibet Autonomous Region
The terrain of the southwest region is mainly hilly, mountainous, and plateau, similar to that of the central and southern regions. The rural population is large, and ethnic minorities gather. The rural leisure industry derived from tourism has increased the speed of sewage discharge, so the southwest is an area with relatively weak rural sewage control technology.
For example, the terrain of Chongqing is mainly hilly. Therefore, when selecting the process, we can consider single household Septic tank and biogas digester, small multi household anaerobic flower platform wetland and anaerobic drop oxidation, medium-sized multi household anaerobic ecological purification ditch and anaerobic Constructed wetland, and large multi household anaerobic Constructed wetland and anaerobic aerobic integrated reactor.
Taking Yunnan as an example, because its sewage treatment facilities are relatively backward and there are many poverty-stricken areas, local rural sewage treatment takes full advantage of local resources, such as oxidation ponds, Constructed wetland, land treatment and biological treatment.
5. Shaanxi Province, Gansu Province, Qinghai Province, Ningxia, Xinjiang
The northwest region is mainly composed of plateaus, basins, and mountains, with higher terrain hindering moist airflow, resulting in sparse overall rainfall and arid climate.
Due to the geographical boundary between the north and south being the Qinling Mountains, although the southern Shaanxi region south of the Qinling Mountains is the same as northern Shaanxi and Guanzhong as the northwest region, there are significant differences in climate.
The geographical location of Geography of Northwest Territories is relatively remote, the residents' awareness of environmental protection is weak, rural sewage treatment facilities are lacking, many sewage is directly discharged without treatment, and regional freshwater resources are in crisis. Based on the climate and sewage characteristics of the northwest region, biochemical treatment technology, ecological purification technology, and biological+biochemical treatment technology are considered suitable for local sewage treatment.
6. Liaoning Province, Jilin Province, Heilongjiang Province
Northeast China has Greater Khingan, Lesser Khingan and other mountains, as well as plains and peninsulas. The terrain is relatively complex. In such high cold areas, the summer is hot and rainy, while the winter is cold and dry.
Due to the low winter temperature and relatively backward economy, technologies with high investment and operating costs such as MBR are not suitable for Northeast China; However, Constructed wetland, land treatment and other processes with low economic input will affect their treatment effect due to the low winter climate.
It is worth mentioning that Harbin Institute of Technology once found that when the main material of Constructed wetland is zeolite, it can ensure that the system still has good removal capacity for total nitrogen and total phosphorus at low temperature. Therefore, selecting plants suitable for low-temperature growth and suitable fillers to effectively improve the removal efficiency of pollutants is a key factor in the current technological selection in Northeast China.
