Sewage treatment plants, once listed as new pollution sources, have become a hot topic of environmental protection recently. Behind it, stricter environmental regulation has led to a shift in the entire wastewater treatment industry from construction to operation and more refined management. Sewage treatment plants have undergone qualitative changes in both the treatment of incoming water quality and the improvement of their own processes.

Regarding the highly concerned issue of excessive inflow of wastewater from wastewater treatment plants, the "Three Year Action Plan for Improving the Quality and Efficiency of Urban Wastewater Treatment" recently issued by three ministries and commissions once again clarifies the biochemical oxygen demand (BOD) value of inflow: "The concentration is lower than 100 mg/L." The Action Plan also requires that all localities adjust the charging standards for wastewater treatment fees as soon as possible, In principle, it is necessary to compensate for the normal operating costs of sewage treatment and sludge treatment and disposal facilities and make reasonable profits; We need to increase the collection rate of self-provided water and sewage treatment fees.

For a long time, wastewater treatment plants, as the last pass in urban water treatment, have been plagued by "pollution causing" incidents due to the excessive quality of incoming water. According to the "Announcement on Major Pollutant Emissions Exceeding the Standard and Punishments of National Key Monitoring Enterprises in 2017" previously released by the Ministry of Ecology and Environment, more than 100 sewage treatment plants ranked prominently among the key national controlled enterprises that exceeded the standard that year, accounting for nearly half of all types of enterprises.

Pollution control=impossible to complete the task?

In the inherent cognition, sewage treatment plants are originally used to treat sewage. If we also need to restrict the inflow of water from the upstream, it would be quite an inversion of the cart before the horse. After sorting out some cases in recent years, it is found that the effluent from sewage treatment plants exceeds the standard, which is closely related to the substandard discharge or illegal discharge of upstream pollution sources.

When the quality of incoming water exceeds the standard or the amount of incoming water far exceeds the previously designed treatment scale, or even when the process originally targeted at treating domestic sewage encounters industrial wastewater, overloaded sewage treatment plants move from "pollution control" to "pollution causing". An industry veteran pointed out that the relatively backward process routes, high energy consumption, and complex quality of urban domestic sewage further exacerbate the embarrassing situation of sewage treatment plants. According to public data over the past seven years, three major pollutants, including ammonia nitrogen, total phosphorus, and fecal coliform bacteria, have long plagued wastewater treatment plants.

As early as the beginning of the construction of a sewage treatment plant, a series of parameters, including treatment scale, inlet water quality, and outlet water quality, have been formed. Moreover, for various pollution sources such as domestic sewage and industrial wastewater, sewage treatment plants will also tailor their specific implementation paths. Specifically, in terms of domestic sewage, sewage treatment plants generally sign agreements with the local government to regulate the quality of incoming water.

However, during the actual operation of a sewage treatment plant, it is not so simple, and the most difficult task is "the impossible task". For example, if a sewage treatment plant is operated under BOT or commission, it should accept specific sewage. The practical problem is that industrial sewage and municipal sewage are often mixed together. In addition, some urban sewage treatment plants have corresponding biochemical treatment concentration coefficients according to the commissioned operation agreement at that time, and a large number of industrial wastewater inflow exceeds the standard, which is still far higher than the specified coefficient.

Increasing cost war

Why are wastewater treatment plants so sensitive to excessive inflow? Environmental supervision is becoming stricter, and enterprises cannot discharge the wastewater without authorization. After intercepting the wastewater and receiving it, the pressure on wastewater treatment has all reached the sewage plant. "As a senior industry person said frankly," How can a manufacturing enterprise not completely control its raw materials, while sewage treatment plants become what they come from and produce? ".

But in fact, unlike ordinary sewage enterprises, sewage treatment plants also have a public service function: providing sewage treatment to the public. This has also determined that up to now, some sewage treatment plants in China are still in a state of near "losing money and making a profit". In recent years, when the inflow of water exceeds the standard, there are also cases where sewage plants bear high fines.

From the perspective of the cost composition of sewage treatment plants, the most important cost comes from energy consumption. As electricity is now the main energy source in cities, the cost of energy consumption is ultimately reflected through electricity bills. With the increasingly stringent effluent standards for urban sewage treatment plants, chemicals have also become an important part of the cost of sewage plants. Even in some sewage plants, the cost of chemicals is far higher than the cost of energy consumption. Most of the reagents used in sewage treatment plants are polyacrylamide used by sludge dehydrators or iron trichloride and calcium oxide used for dehydration by sludge plate and frame filter presses.

Correspondingly, the composition of incoming water has become increasingly complex and diverse, and the cost of sewage treatment plants has skyrocketed. It is true that biological phosphorus removal in conventional wastewater treatment processes is generally difficult to meet the water demand. If the heavy task of removing total phosphorus is to be achieved, the operating costs will increase. The same is true for ammonia nitrogen. At the same time, due to the special working environment in which the equipment in the sewage treatment plant is in direct contact with sewage, the original equipment is constantly aging and damaged, which also increases the expenditure on maintenance costs of the sewage treatment plant, and increases the operating costs.

Where is the way out

Where are the effective endorsements that sewage treatment plants can rely on? Although there are no clear provisions in environmental protection laws and regulations on the exemption of excessive inflow of water resulting in excessive outflow of water, in the view of many industry insiders, the "obvious misconduct principle" stipulated in the Administrative Reconsideration Law and the Administrative Procedure Law can also be applied to achieve fair law enforcement. Wang Hongchen, Vice President of the School of Environment at Renmin University of China, believes that whether or not a guaranteed minimum water quantity is set should be negotiated in the contract between both parties, standing at the level of game between both parties.

Currently, another more common view is gradually being recognized by the industry: whether industrial enterprises discharging industrial wastewater into municipal wastewater treatment plants or wastewater treatment plants receiving wastewater, it is necessary to strengthen the ability to self certify innocence. As mentioned earlier, the issue of "where does the money come from" should focus on environmental externalities, such as expanding the range of users and beneficiaries of sewage treatment, and conducting corresponding premium recovery.

However, in any case, "As long as the sewage treatment plant accepts sewage, it must comply with the legal requirements to discharge it up to the standard." As for the exemption reason for excessive inflow of water, the Secretary for Water Ecology and Environment, Zhang Bo, has publicly made it clear. At the same time, Article 50 of the Law on the Prevention and Control of Water Pollution stipulates that "the operating unit of urban sewage centralized treatment facilities shall be responsible for the quality of the effluent from the urban sewage centralized treatment facilities."