China's Sewage Treatment Plants Randomly Increase The Effluent Standards

Last month, the Ministry of Ecology and Environment issued the "Implementation Plan for Urban Black-Odorous Water Treatment".

The "Plan" requires that cities with an existing sewage treatment plant with an influent biochemical oxygen concentration of less than 100mg/L formulate a systematic remediation plan, which should include the transformation of the pipeline network, sewage removal and diversion, industrial wastewater and engineering drainage. 

The Plan names two major problems in the treatment of black - odorous water bodies:

1

Sewage treatment plants randomly increase the effluent standards

Currently, many urban sewage treatment plants implement the Class A discharge standard, the highest standard of the current "Pollutant Discharge Standard for Urban Sewage Treatment Plants". Most likely this happens because some plants want quick results, and other plants follow suit and now require the effluent quality to meet the level IV standard. This means that based on the first-level A standard, the COD concentration is reduced by 60%, and the ammonia nitrogen concentration by 80%, which causes the construction and treatment costs to increase exponentially. Taking the upgrading and renovation project of the Second Sewage Treatment Plant in Shangjie District in Zhengzhou (Henan Province) as an example, after the effluent standard was reduced from the original "surface level III water standard" to "surface level IV water standard", the total investment cost decreased by about 36%. Blindly raising the target has no clear legal support and these extra investments will only be wasted.

2

A new expansion of the sewage treatment plants

The central environmental protection inspection reported many failed sewage collection and treatment cases resulting from large-scale sewage treatment plants with lower influent, meaning that many sewage treatment plants hadn’t been used to their full capacity. For example, in April 2021, the central environmental protection inspection found that a large amount of domestic sewage in Nanchang couldn't be effectively collected. The city's largest sewage treatment plant at Qingshan Lake treats more than 500,000 tons of sewage every day, but the COD concentration of the influent water is too low, even lower than the sewage discharge standard. Clean water flows in and out, and the COD concentration of the influent water has decreased year by year from 2018 to 2020, while the city pays more than 10 million yuan per month for the operation of the sewage treatment plant. Without solving the problem of inefficient or even ineffective operation of sewage treatment plants, engaging in the new plant expansion will undoubtedly result in a huge waste of pollution control funds.

These are the two seemingly contradictory but very common problems in the treatment of black and odorous water bodies: it is not that the discharge concentration of the sewage treatment plant is too high, but the concentration of the influent water is too low; it is not that the treatment capacity of the sewage treatment plant is insufficient but the amount of the collected sewage is too low.

What innovative concepts and requirements are included in the Plan?

1

For the first time, decontamination and diversion are formally proposed

An important reason for the low influent concentration of the current urban sewage treatment plants is that relatively clean water bodies such as river water, groundwater, mountain spring water, and construction precipitation enter the municipal pipeline. To solve this problem, the core is to separate the clean external water from the sewage that should be collected, let the river water, mountain spring water, construction precipitation, etc. go through the river, and let the sewage go through the sewage pipeline. The clean-up and diversion have been formally written into documents at the national level, and it has put forward correct and operational technical measures for some southern cities, which is conducive to helping cities implement scientific policies and promptly improve the influent concentration of urban sewage treatment plants.

2

For the first time, it is proposed that sewage treatment plants shouldn’t unreasonably raise standards

The national standard of Class A in the discharge standard of urban sewage treatment in China is much higher than that of some developed countries. However, in the last couple of years, some river basins and cities have successively issued local sewage treatment effluent discharge standards, and some cities blindly pursue V-type water bodies when the sewage pipe network still doesn’t meet the effluent discharge standards of sewage treatment plants. Improving sewage treatment plants' discharge standards has a certain effect on reducing the total amount of pollutants entering rivers and lakes. However, we must also see that some urban sewage pipe networks have obvious shortcomings, the centralized collection rate of sewage is not high enough, and some sewage has the risk of overflowing into the river. From the perspective of output ratio, it is not the best strategy either. Furthermore, in the last couple of years, some cities have pursued excessively high sewage treatment and discharge standards, resulting in high sewage treatment costs.

3

The Plan proposed that sewage treatment plants should not be built randomly

The construction of sewage treatment plants is necessary to solving urban water pollution. Since the beginning of this century, China's sewage treatment industry has developed rapidly. At present, the total scale of China's sewage treatment plants ranks first in the world, with an annual sewage treatment volume of more than 70 billion cubic meters. The scale of sewage treatment plants is no longer the most critical shortcoming in most cities, nor is it the key cause of black and odorous urban water bodies. However, the influent concentration of some urban sewage treatment plants is low, and in some cities with diverted drainage systems, the treatment scale of sewage treatment plants has even far exceeded the scale of tap water supply. That's because the sewage pipe network's construction, operation, and maintenance are not in place. Therefore, the implementation plan proposes that sewage treatment plants should not be built randomly, especially in cities with low influent concentrations. The aim is to encourage these cities to give priority to the implementation of scientific source control and sewage interception and pipeline network construction.

4

The question of whether the layout of WWTPs should be centralized or decentralized has been discussed for several years now. In the last couple of years, with the development of sewage treatment quality and efficiency improvement, everyone has become more aware that a  layout of sewage treatment plants that is too centralized leads to a large investment in pipeline networks and high operation and maintenance costs. This type of layout is also not conducive to the effective use of reclaimed water. In this context, the Plan proposes to use a combination of distribution and concentration to layout sewage treatment plants, which is of great significance to promoting the improvement of sewage treatment quality and efficiency and the utilization of sewage resources and is also conducive to energy conservation and carbon reduction in sewage collection and treatment. Of course, what has been proposed in the Plan is the "combination of distribution and concentration". In addition, the distributed sewage treatment plant is not equivalent to a small sewage treatment station with a processing capacity of 500 tons/day or 1,000 tons/day. Although there is no strict definition, generally speaking, a distributed sewage treatment plant refers to a sewage treatment plant with a processing capacity of 10,000-50,000 tons per day.

Pipe network maintenance

Maintenance of the drainage network is very important for its normal functioning. In the current management system of drainage facilities, the government is generally responsible for the maintenance of the urban municipal drainage pipe network. Specifically, the government can establish a drainage company and a pipe network company, or entrust a third-party professional organization through bidding and other methods. The drainage pipe network in the community is generally considered to be maintained by the community property. Some communities do not have property management companies yet. For those with property management, due to a lack of professionals and funds, property management companies rarely clean and maintain the drainage pipe network inside the community. After investigations in some cities, it was found that a large number of rainwater and sewage pipeline networks were mixed and misconnected. Promoting the maintenance of the drainage pipe network in the community is almost the only way to solve this problem. Of course, to improve the maintenance of the drainage pipe network in the community, cities still need to make some research into the cost-sharing mechanism.

Reference materials: The Ministry of Ecology and Environment's "In-depth implementation plan for the control of urban black and odorous water bodies", China Environment News "In the treatment of black and odorous water bodies, these two problems seem contradictory but very common."