The construction of underground sewage treatment plants should not blindly follow the trend, but should comprehensively consider the cost, carbon emissions, etc

In recent years, underground sewage treatment plants have been increasingly used in the field of water pollution prevention and control in China. Beijing, Shanghai, Shenzhen, Guangzhou, Chengdu, Hainan, Kunming, Hefei and other cities have built underground sewage treatment plants. At present, the scale of China's construction in this area far exceeds that of other countries in the world combined. According to relevant statistics, at present, there are more than 200 underground sewage treatment plants in operation around the world. In addition to China, the Netherlands, Sweden, Norway, France, Japan, Finland and other countries have built a small number of underground sewage treatment plants. At present, there are nearly 200 underground sewage treatment plants built / under construction, which can be described as "one of a kind" in terms of quantity.

For some urban areas, especially the central urban areas with high population density, land shortage, and difficulty in requisition and demolition, the construction of underground sewage treatment plants according to local conditions can play a certain positive role in saving land resources and forming a more reasonable layout of sewage treatment plants. However, experts told reporters that there are obvious drawbacks in building sewage treatment plants underground, and they must be comprehensively evaluated and selected from the aspects of economy and technology, and should not blindly follow the trend.

So, why are all parts of our country more enthusiastic about building underground sewage treatment plants? In light of China's specific situation, how can we measure the pros and cons of underground sewage treatment plants? This reporter interviewed relevant experts on relevant issues.

The construction of underground sewage treatment plants should not blindly follow the trend

Not long ago, the National Development and Reform Commission, the Ministry of Housing and Urban-Rural Development, and the Ministry of Ecology and Environment jointly issued the "Implementation Opinions on Promoting the Synergy of Sewage Treatment and Pollution Reduction and Carbon Reduction" (hereinafter referred to as the "Implementation Opinions") proposed, "Cities with scarce land resources can build fully underground/semi-underground sewage treatment plants, and encourage the rational use of above-ground space through the construction of parks and greening plants, so as to improve the quality of the regional environment and the carbon sink capacity of the urban ecosystem." ”

An industry expert told reporters: "If a place cannot find a suitable place to build a sewage treatment plant because of the shortage of land resources, it can consider building an underground type." But the question is, are the more than 200 underground sewage treatment plants built or under construction in China all because there is no place to build the underground/semi-underground type? It can be said that the construction of many underground sewage treatment plants in China is not just needed for the local area. ”

He believes that one of the disadvantages of burying huge facilities underground is that they occupy a large underground space, through which the laying of water, gas, electricity and heat related to the city's 'lifelines' cannot pass. Secondly, this increases the cost of investment in construction, operation and maintenance. Under normal circumstances, the investment cost of underground sewage treatment plants (4000-6000 yuan/cubic meter) is 2-3 times that of above-ground sewage treatment plants of the same scale. In addition, the underground needs to be ventilated and lit, which requires more electricity. Moreover, it also brings greater uncertainty. For example, an earthquake may cause cracks and deformation in the buildings and structures of a sewage treatment plant, and further damage to machinery and equipment, pipelines, etc., resulting in sewage leakage and environmental pollution. If it is an above-ground wastewater treatment plant, it can better cope with this situation. However, if it is built underground, it is easier for sewage to leak into deeper environments. Therefore, whether the sewage treatment plant should be built underground should be comprehensively measured from the aspects of economy and technology. In practice, some places do not conduct comprehensive comparisons. Some towns have built underground sewage treatment plants, and it is not excluded that there is a certain comparison mentality.

"The document of the three ministries is more pragmatic about the construction of an all-underground/semi-underground sewage treatment plant. The implication is that if there is a real shortage of land, the construction of an underground/semi-underground sewage treatment plant can be considered, but it is not necessary to build an underground plant, but the form of an underground sewage treatment plant can be used as a solution. For those cities where there is no shortage of land, personal understanding, in principle, is discouraged. Wang Jiazhuo, deputy chief engineer of the China Academy of Urban Planning and Design, dean of the Ecological Municipal Institute of the Chinese Academy of Planning and Design (Beijing) Planning and Design Co., Ltd., and a professor-level senior engineer, said.

Underground sewage treatment plants consume a lot of energy and emit a lot of carbon

Hao Xiaodi and Yu Wenbo from the Sino-Dutch Future Wastewater Treatment R&D Center of Beijing University of Civil Engineering and Architecture conducted a study on a fully underground sewage treatment plant in China and found that its total carbon emissions in the whole life cycle are about 871,000 tons of CO2-eq, which is 36.5% higher than that of the above-ground sewage treatment plant. Among them, the operation stage requires additional power consumption such as ventilation and lighting, and the carbon emission can reach more than 95% of the total. During the operation phase of the underground sewage treatment plant (20 years), the carbon emissions generated by additional ventilation were about 184,000 tons of CO2-eq, accounting for 22% of the total emissions. In addition, the construction and demolition phases of underground wastewater treatment plants require additional construction and building material consumption. It can be seen that underground sewage treatment plants will produce a large amount of carbon emissions in the whole life cycle, which is not in line with the social development direction of carbon emission reduction and carbon neutrality.

At present, the main reason for supporting the construction of underground sewage treatment plants is that they can save land resources and make full use of space. In this regard, Hao Xiaodi told reporters: "The land-saving construction of the underground sewage treatment plant and its landscape on the ground are at the expense of large construction and operating costs. Taking a fully underground sewage treatment plant in China as an example, its comprehensive negative impact in terms of environmental impact, infrastructure investment, and ecological benefits is about 20% higher than that of above-ground treatment. Even if the above-ground portion of an underground wastewater treatment plant is used to create green landscaping, the total negative impact on the environment will not be offset by the ecological benefits generated. In fact, the surface landscape of an underground sewage treatment plant is rarely open to the public, and in many cases, its real function is equivalent to the value of a small piece of green space. ”

Some experts believe that the biggest incentive for some local governments to actively build underground sewage treatment plants is to make people 'out of sight', or to increase the value of surrounding real estate, or even to build commercial real estate on top of underground sewage treatment plants to make profits. This is nothing more than passing on pollution and carbon emissions elsewhere.

How to build an underground sewage treatment plant more economical and environmentally friendly?

If cities with scarce land resources really need to build underground sewage treatment plants, they need to "seek advantages and avoid disadvantages" as much as possible to reduce their construction, operation and management costs and negative impact on the environment.

Hao Xiaodi told reporters: "In some countries such as Europe, the United States, and Japan, where the population density is higher, the urban land is more scarce, and the social economy is more developed, underground sewage treatment plants are not everywhere, and they are almost rare. Moreover, in some cases of underground sewage treatment plants in foreign countries, the reasons for their construction site selection are not blindly following the trend. For example, Helsinki in Finland has built underground sewage treatment plants using natural caves in Oslo, Norway, Rotterdam in the Netherlands using abandoned docks, Marseille in France using abandoned quarries, Stockholm in Sweden using natural caves, and Kanagawa in Japan excavating tunnels in combination with the terrain, thus greatly reducing the amount of work. It can be seen that the construction of underground sewage treatment plants should be adapted to local conditions and should not blindly follow the trend. ”

We don't necessarily copy the practices of other countries, but we also need to give full consideration to the issue of site selection.

"Regarding the necessity of the construction of underground sewage plants, in addition to the above-mentioned land scarcity problem, it is actually necessary to consider the trade-off between the dispersion and concentration of the layout of the sewage treatment plant. The former is more in line with the principle of water ecology to improve the capacity of the water environment, while the latter is easier to industrialize and intensively reduce direct costs. Xue Tao, managing partner of the E20 environmental platform and executive director of the E20 Research Institute, told reporters, "If it is in the main urban area, due to the shortage of land and other reasons, the acceptance of the people will be relatively higher." The construction of underground sewage treatment plants can play a certain beneficial role in forming a better layout of urban sewage treatment plants. In the past, the construction of sewage treatment plants generally followed the principle of gravity flow and was distributed along the river, so they were usually built downstream of the river, often downstream of the city or even the border area, so the area with rich land resources also used the above-ground model more to save costs. However, it is not convenient to use the water discharged by the sewage treatment plant to ecologically replenish the river in the main urban area. If you want to get the water upstream, you also need to spend money to lift the pumping station. This means that a large part of the water discharged by a wastewater treatment plant can only be used in the lower reaches of the river, and the urban area is very limited. In addition, the instantaneous flow of rainwater is concentrated downstream for processing, and the pressure load of the related pipe network will also be very large. From this point of view, it makes sense to disperse some of the sewage treatment plants in the relative upstream of the main urban area. As a result, due to the scarcity of land, the underground model or other environment-friendly models of public construction will be chosen, which is a more optimal choice with a bias to disperse in the plant-grid-river system. Due to the problem of the level of pipeline network construction and the lack of funds in many areas of China, this decentralized orientation that has emerged in recent years has the advantage of adapting measures to local conditions in China. Of course, specific problems should be analyzed on a case-by-case basis and treated differently. ”

In addition, underground sewage treatment plants can play a better thermal insulation effect in winter. Relatively speaking, the northern regions are more convenient to take advantage of this characteristic.

When planning and constructing an underground sewage treatment plant, in addition to the location issue, it is also necessary to consider the use of above-ground space. In this regard, China has a relatively successful practice, mainly combined with the construction of parks. The "Implementation Opinions" also encourage the rational use of the above-ground space of fully underground/semi-underground sewage treatment plants through the construction of park greening activity venues.