In the face of overflow in the rainy season, is it feasible for sewage treatment standards to be "different standards for rain and shine"?

Recently, many places in the mainland have suffered heavy rainfall. Ma Xueyuan, chief forecaster of the Central Meteorological Observatory, said in an interview with this newspaper: "The temperature in Jiangnan, South China and other places is significantly higher than that of the same period in normal years, and the cumulative precipitation in many places has exceeded the historical record in April. ”

Ma Jianhong, director of the Shanghai Municipal Emergency Management Bureau, which is located in the rainy area of the south of the Yangtze River, also said on the 25th that the weather in Shanghai may be abnormal this year, with more rainfall on rainy days, and it is necessary to prevent the impact of extreme weather.

The long duration and intensity of rainfall will bring great pressure and challenges to sewage treatment. Gao Hongjie, a researcher at the Chinese Academy of Environmental Sciences, told reporters: "At this stage, the problem of overflow pollution from the urban confluence system has gradually become prominent, and how to accurately and effectively control urban overflow pollution has become a 'bull's nose' problem to be solved in the 'post-black and odorous era'." ”

The State is paying more and more attention to this aspect. Not long ago, the Ministry of Housing and Urban-Rural Development, the Ministry of Ecology and Environment and other five departments issued the "Notice on Strengthening the Construction, Operation and Maintenance of Urban Domestic Sewage Pipe Network", specifically mentioning "promoting the reduction of the total amount of overflow pollution in the rainy season".

So, what are the characteristics of overflow pollution in the rainy season? How can we effectively reduce the total amount of overflow pollution in the rainy season? Although some localities have tried to "shine or rain with different standards," are there any other feasible ways to deal with regulatory problems? This reporter interviewed relevant experts.

High concentration of overflow sewage on rainy days,

May be higher than the average influent concentration of the wastewater treatment plant

In the process of water pollution control, sewage treatment enterprises in the front line can clearly feel the seriousness of the overflow pollution problem.

Mao Jianhua, vice president of Beijing Enterprises Water Group Co., Ltd. and general manager of Beijing Water Future Technology Co., Ltd., gave reporters an example: "Taking the treatment of a lake basin in a city we participated in as an example, before the treatment, direct sewage discharge accounted for 86% of the pollution load and 14% of rainwater runoff pollution. After treatment, the pollution load was reduced by 76.9%, and the effect was remarkable. However, 97% of the remaining pollution load is overflow pollution from the combined system, which is mainly due to the imperfect drainage network and misconnection. ”

Gao Hongjie said: "The overflow pollution caused by rainfall has two main characteristics: first, the high concentration of pollutants. The concentration of chemical oxygen demand and suspended solids can reach more than 100 mg/L, and even higher than the average influent concentration of urban sewage treatment plants. Second, the frequency of overflow pollution is high. According to the literature, overflows occur in some cities when the rainfall exceeds 10 mm in a single time, especially in southern cities, and the number of overflows can account for 70%-80% of the annual rainfall. ”

Is it feasible to enforce different emission standards on rainy and sunny days?

In order to promote the reduction of the total amount of overflow pollution in the rainy season, the "Notice on Strengthening the Construction, Operation and Maintenance of Urban Domestic Sewage Pipe Network" issued not long ago encourages the construction of rapid purification facilities for overflow pollution in the rainy season for the first time, giving full play to its characteristics of rapid and timely treatment of sewage, and clarifying that all localities can formulate emission control requirements in combination with actual conditions. This avoids the fact that "one-size-fits-all" emission standards limit the widespread use of such equipment.

However, industry insiders told reporters that the use of such facilities is only an auxiliary means, and it is fundamentally necessary to expand the capacity of sewage treatment plants or build reservoirs to improve sewage collection and treatment capacity. However, due to the constraints of various factors such as capital and land, the construction or expansion of sewage treatment plants is not a one-time thing. In this case, could we consider lowering the discharge standards of the wastewater treatment plant on rainy days?"The freed up capacity can be used to treat more wastewater. ”

"Under the current discharge standards, it should be feasible to increase the amount of water treated by a sewage treatment plant by 20 to 30 percent," Mao said. If the discharge standard is lowered a little, the amount of water treated may be doubled, thereby mitigating overflow pollution during the rainy season. Of course, the implementation of differentiated sewage treatment and discharge standards will add a certain degree of difficulty to monitoring and supervision. For example, some sewage treatment plants may use rain as an excuse to discharge sewage beyond the standard. So, how much rain can lower the emission standard, and how much can the emission standard be lowered? ”

It is reported that at present, some local standards in the mainland have made useful attempts in "different standards for weather and rain".

For example, in 2020, the city of Kunming, Yunnan Province, issued the "Discharge Limits for Major Water Pollutants from Urban Sewage Treatment Plants" (DB5301/T 43-2020), which proposes "zoning and grading" implementation requirements for five major water pollutants, including five-day biochemical oxygen demand, chemical oxygen demand, ammonia nitrogen, total nitrogen, and total phosphorus. This standard innovatively proposes an emission standard for overflow pollution control in rainy days.

Chen Guang, executive director and general manager of Shanghai Chengtou Sewage Treatment Co., Ltd., told reporters: "The industry's call for 'different standards for rain and shine' is getting higher and higher, and moderately reducing the pollutant discharge standards of treatment facilities on rainy days is conducive to giving full play to the environmental benefits of existing facilities to a certain extent." ”

However, the difficulty of supervision has been repeatedly emphasized by many experts. Experts believe that if the implementation of "rain and shine standards" is not only difficult to supervise, but also may cause the sewage that should be treated to the first-class A standard to be discharged according to the relaxed standard, which is a high risk.

The fundamental solution is the diversion of rain and sewage.

When it is difficult to implement, try "in-situ capacity expansion"

Finding solutions to tough problems is a top priority. Wang Haiyan, a researcher at the Chinese Academy of Environmental Sciences, said: "At present, the most fundamental solution is the diversion of rain and sewage. When it is difficult to implement the diversion of rain and sewage, the amount of rainwater entering the confluence pipe network and the amount of pollutants in the rainwater should be reduced through sponge city construction and road cleaning, the receiving capacity of storage facilities and terminal sewage treatment facilities should be improved, and large particles and garbage and other floating objects should be controlled through overflow port transformation, and the treatment capacity of sewage treatment plants should be comprehensively improved through the use of rapid treatment facilities, new reconstruction and expansion or in-situ transformation, so as to reduce the discharge of overflow pollutants. To this end, the next step should be to strengthen the system construction, strengthen the source emission reduction, control according to local conditions, and improve regulatory measures. ”

In particular, she noted that "in-situ capacity expansion" provides a viable solution for overflow pollution control in a combined system. This approach means increasing the capacity of the wastewater treatment plant without the need to reduce discharge standards, without the need to construct new buildings.

Taking the Chengdong and Gulin sewage treatment plants in Luzhou, Sichuan Province, which is close to the Yangtze River, as an example, the overload of the sewage treatment plant is as high as 40% during the flood season, and the operating pressure is huge, so it is urgent to solve the problem of overflow pollution control of the combined system. On the basis of the original process, there is no need to add buildings, and the rapid separation of slurry and water is realized by adding nano flocculant, the treatment load of the secondary sedimentation tank is increased, and the purpose of expanding the treatment scale is finally achieved, so that the treatment scale of the sewage treatment plant in the rainy season is at least 50% higher than the design value, and the effluent can stably reach the first-class A standard.