Recently, the long-term overloaded operation of sewage treatment plants in some cities has attracted widespread attention. If a sewage treatment plant operates at full capacity or overload for a long time, it is easy to cause domestic sewage overflow pollution problems. So, what are the reasons behind the long-term overload operation of sewage treatment plants? How to solve the problem of domestic sewage overflow pollution? Around the relevant issues, the reporter interviewed industry experts. Overload operation ≠ excessive discharge. The key is to consider the water quality and whether there is sewage overflow. How to understand "overload operation"? What does' load 'refer to? Firstly, what is' design load '? for instance. If the daily sewage production within the service area of a certain urban domestic sewage treatment plant is estimated to be 100000 tons through methods such as "population x per capita sewage discharge", then the design water volume of the sewage treatment plant is 100000 tons/day, which is the design load. In order to cope with normal fluctuations in water volume, the processing capacity of this factory needs to ensure a certain margin, such as designing structures based on a water volume variation coefficient of 1.3. Qiu Yong, from the Institute of Water Environment Protection at Tsinghua University, told reporters: "Whether the sewage treatment system is operating normally mainly depends on whether the water quality meets the standard and whether the upstream pipeline network overflows. If the effluent can be discharged in compliance with the standard, there is no need to pay too much attention to whether it is operating at overload. Just like sleeping, as long as you have sufficient energy after waking up, there is no need to worry too much about whether you have slept for 6 hours or 8 hours. If the effluent cannot meet the standard or there is a problem of sewage overflow due to overload operation, then the reasons should be analyzed, countermeasures should be found, and improvements should be made quickly." According to reports, there are currently three sewage treatment plants in the urban area of Lijiang City, Yunnan Province, all of which are operating at overload. On sunny days, more than 20000 tons of domestic sewage are discharged directly into the Yanggong River, a first level tributary of the Jinsha River, and the discharge is even greater on rainy days. A sewage treatment plant manager said, "When the hydraulic load rate is greater than 80%, except for a few super large sewage treatment plants, most sewage treatment plants cannot shut down and maintain the water supply under the premise of meeting the effluent standards." This means that some machines may have long-term "sick" operation, and the operational risk is naturally higher. Therefore, long-term overloaded operation will have adverse effects on the stable and compliant discharge of effluent water quality. Long term overloaded operation with excessive emissions and pollution overflow, is the problem still with the network or the factory? Firstly, distinguish whether this situation occurs under the rain and sewage diversion system or the rain and sewage combined system. Qiu Yong analyzed that in the case of "good separation of rainwater and sewage+separation of rainwater and sewage", most of the wastewater entering the sewage treatment plant is high concentration sewage. In this case, if the sewage treatment plant operates at overload for a long time, it indicates that the local domestic sewage treatment capacity is insufficient, and it is necessary to consider expanding the sewage treatment plant. If it is in the situation of "rainwater and sewage diversion+non-standard and inadequate rainwater and sewage diversion", it is also possible for sewage treatment plants to operate at overload. The primary task is to conduct a detailed investigation of problems such as misconnection, mixing, and leakage in the drainage pipe network, and improve the quality of the pipe network. If it is in the situation of "rainwater and sewage merging", especially on rainy days, it is easy to encounter problems of excessive discharge or overflow pollution. Therefore, it is necessary to pay attention to dealing with the impact load of rainwater, including upgrading and renovating the pipeline network, carrying out emergency treatment of sewage in rainy days, building new pipeline network storage tanks, installing rapid purification facilities for overflow pollution in rainy seasons, and building sponge cities. Comprehensive measures should be taken. Secondly, distinguish whether it is "overload of incoming water volume" or "overload of incoming water quality"? Hao Xiaoguang, an expert member of the Shanxi Provincial Urban Drainage Professional Committee, explained to reporters that the main reasons for the overload of water inflow are: firstly, the sewage treatment plant has been built for a long time, and the population within its service area has increased, exceeding the original design capacity; secondly, under the combined rainwater and sewage system, a large amount of rainwater enters the sewage treatment plant; thirdly, the pipeline network of the sewage treatment plant is not properly constructed, and various underground water sources along the way enter the sewage pipeline network; fourthly, some areas use other water sources such as self provided wells, which have not entered the water supply range. During the design process of the sewage treatment plant, it is impossible to calculate this part of the water inflow, resulting in the design inflow being lower than the actual inflow, leading to the phenomenon of overloaded inflow He further pointed out the hazards: overloading the inflow of water can lead to a short hydraulic retention time of sewage in the sewage treatment plant (referring to the average residence time of the sewage to be treated in the reactor), and pollutants cannot be effectively degraded, resulting in excessive discharge. In addition, this will increase the operating load of the equipment, and in severe cases, it may cause equipment damage. Hao Xiaoguang introduced: "Firstly, there are changes in residents' domestic drainage within the service area of sewage treatment plants, such as the rapid development of the catering industry and changes in dietary structure. Changing drainage habits may cause an increase in pollution in the drainage, exceeding the design load. Secondly, population growth may lead to an increase in the total amount of pollutants discharged within the service area of sewage treatment plants. Thirdly, there are situations where small and medium-sized enterprises directly discharge sewage into the municipal pipeline network. Many small and medium-sized enterprises lack sewage treatment facilities or are unable to put them into use and maintain them, resulting in drainage water quality indicators exceeding the enterprise's acceptance standards, causing the inflow water quality of sewage treatment plants to exceed the load. Fourthly, what are the main hazards of this situation? He analyzed, "Firstly, sewage treatment plants design and calculate the treatment structures, activated sludge concentration, aeration air volume, and sludge removal facilities within the plant based on the designed inlet water quality. When the actual operating inlet water concentration far exceeds the original designed inlet water quality concentration, which is a 'water quality overload' operation situation, these design treatment parameters are no longer applicable. The direct result is a decrease in the degradation effect of pollutants in the plant, and the effluent water quality exceeds the standard. Secondly, the wastewater illegally discharged by enterprises may contain a large amount of toxic and harmful substances, which can cause toxic impact on the microorganisms in the activated sludge of the sewage treatment plant, resulting in microbial inhibition or death, and ultimately causing the effluent water quality to exceed the standard
How to view taking the initiative to reduce the discharge of sewage during the rainy season through "peak load operation"? For a long time, "ensuring flood prevention safety" and "avoiding pollution caused by rainwater and sewage discharge" have been a dilemma. Under the premise of ensuring flood prevention safety, various regions are actively exploring ways to minimize the discharge of sewage into the external water environment. The reporter learned that since last year, a certain place has been exploring ways to reduce the initial rainwater and sewage discharged into the Yangtze River by continuously maintaining peak load and efficient operation of sewage treatment plants for a long time. For example, operating for a long time according to its designed maximum check value, usually at 130% of the design load, to accommodate more incoming water. This approach is different from passive long-term overload operation. It is based on sufficient preparation and proactive measures to improve treatment processes, refine operations, and enhance treatment efficiency and capacity, ensuring stable and compliant discharge of effluent. The relevant person in charge introduced: "At first, we felt that if the sewage treatment plant operated super efficiently, the stability of standard discharge would definitely be affected. However, after testing, we found that this can be achieved. Two sewage treatment plants with a processing capacity of one million tons eventually achieved the effect of stable and standard discharge throughout the month, and all operating parameters were good." But this does not mean that every sewage treatment plant can completely control the quality of effluent under peak load conditions. The achievement of such an effect is based on two prerequisites. The person in charge said, "The first condition is that the current sewage treatment concentration is generally low and has not reached the designed pollutant load. The design concentration of COD in the influent of the sewage treatment plant where we are located is basically between 350mg/L and 450mg/L, but the actual daily average is about 200mg/L to 260mg/L. Under the current ultra efficient operating conditions, although the influent water load exceeds the design value, due to the low pollution concentration, the pollutant load has not yet exceeded the design value, so the sewage treatment plant can still achieve stable and standard discharge. The second condition is that the equipment is required to reach almost 100% intact rate. When the sewage treatment plant operates at 130% load, backup equipment will be used, and the equipment backup rate will decrease to some extent. This will reduce the system's ability to resist risks. The sentence is:. This requires all equipment, including backup equipment, to be in good condition, especially the main critical equipment. Under ultra efficient operating conditions, once there is a sudden equipment failure, the high load production state is difficult to sustain. Therefore, if the sewage treatment plant decides to implement ultra efficient operation, necessary technological upgrades must be considered to ensure the necessary equipment backup rate for the full operation of the plant What other effective measures are available to address the problem of overflow? In fact, the overflow pollution during the rainy season is a problem for many countries and regions. How to deal with it, all parties can be described as "the Eight Immortals crossing the sea, each showing their own abilities". Qiu Yong believes that: "Firstly, rainwater and sewage separation should be carried out, and the construction and maintenance of drainage networks should be strengthened. Secondly, scientific and effective technical solutions should be implemented. For example, some sewage treatment plants adopt the construction of new first level enhanced treatment facilities to cope with the increased inflow load during the rainy season. The effluent from the facilities should be mixed with the effluent from the normal process section to meet the discharge standards of urban sewage treatment plants. If the rainwater and sewage separation is not thorough and the sewage network load is very large during rain, a set of 'parallel' overflow sewage treatment facilities can be installed to reduce the total amount of pollution discharged, such as the rapid purification facilities for overflow sewage in the rainy season encouraged by the Ministry of Ecology and Environment. Thirdly, direct expansion can be carried out, such as the construction of new sewage treatment plants or in-situ expansion. Before building new sewage treatment facilities, the drainage network should be finely inspected and. If the problems of misconnection, mixing, and leakage in the drainage network are not properly resolved, the newly built sewage treatment plant will not be able to play its expected role. On site expansion can increase the water treatment capacity of a sewage treatment plant through facility renovation or technological upgrades without adding new plant area, while also meeting discharge standards Hao Xiaoguang also put forward his own views. He said, "The solution to this problem needs to be considered from the following aspects. Firstly, the sewage pipeline network needs to be inspected and rectified, and the old pipeline network needs to be maintained and updated. Secondly, the drainage and sewage management of enterprises within the service area of the sewage treatment plant should be strengthened. Drainage and sewage are managed by the urban management and ecological environment departments respectively, and these two departments need to coordinate and control. In places where conditions permit, the integration of plant network management should be actively promoted, and sewage treatment plants should be involved in pipeline management. In this way, sewage treatment plants will actively do relevant work and increase supervision of drainage enterprises for their own stable operation needs. The third is to promote the informatization construction of the drainage network, online instrument control of key nodes in the drainage network, collection of information content of key nodes, and real-time monitoring of enterprise sewage discharge through the construction of an information platform to avoid overloading water quality entering the sewage treatment plant. Fourthly, sewage treatment plants should monitor the inflow of water throughout the entire period and establish the retention of water samples for each period to provide evidence for future traceability. If the excessive inflow leads to excessive outflow, the sewage treatment plant needs to 'prove its innocence' in order to avoid administrative penalties, and it is very important to retain evidence. The fifth is that urban management departments should establish a water fingerprint database for drainage enterprises in the urban area. When there is excessive inflow of water into the sewage treatment plant, water fingerprint comparison can be carried out in a timely manner to determine the source enterprise of the discharge and ensure the stable operation of the sewage treatment plant
