March 22 is known as World Water Day ".

Water is the source of life, which breeds all things and is related to human well-being. However, with the rapid development of industrialization and urbanization, water resources are becoming increasingly tight and water pollution problems are becoming more and more serious. In order to make good use of every drop of water, sewage treatment technology is particularly important. The current sewage treatment technology is not only a key link to control water pollution and improve the water environment, but also an important way to promote the recycling of water resources and achieve sustainable development.

So how should we deal with the sewage generated in our daily life? What about the sewage discharge standard? Ma Zongkai, senior engineer of the Water Environment and Urban Construction Branch of Chengdu Survey and Design Institute of China Power Construction, may be able to tell us the answer.

Nitrogen Removal Process of Activated Sludge Process

　　A century-old sewage treatment process-activated sludge process

The core purpose of sewage treatment technology is to remove pollutants from wastewater and make it reach the standard of discharge or reuse. This process involves a variety of physical, chemical and biological methods. According to Ma Zongkai, "at present, most of China's urban sewage treatment plants use activated sludge process to treat sewage."

Data show that this sewage treatment method was born in 1914 in war-torn Europe, and in less than 10 years, the activated sludge process has been applied in many small sewage treatment plants. After the Second World War, the construction of sewage treatment plants in the United States reached its peak, and the activated sludge process developed rapidly and gradually began to dominate. Because the overall cost is moderate, the operating cost and the difficulty is appropriate, the activated sludge process has experienced a hundred years of practice from its production to now, and has always maintained a strong vitality, and has been widely promoted in the world.

The activated sludge method is a continuous mixed culture of sewage and various microbial groups under artificial oxygenation conditions to form activated sludge. These sludge particles have a particle size of about 200 to 1000 μm and are alum-like amorphous flocs, usually with a large surface area. It is brown and slightly acidic in the aeration tank, black when the oxygen supply is insufficient, and gray when the oxygen supply is too much.

Activated sludge process

Although large areas of sludge do not look very beautiful, they play a great role in sewage treatment. Engineers use the biological coagulation, adsorption and oxidation of activated sludge to decompose and remove organic pollutants from sewage. The sludge is then separated from the water, and most of the sludge is returned to the aeration tank, and the excess is discharged from the activated sludge system.

In the development of activated sludge process for more than one hundred years, there are oxidation ditch process, AAO process and CASS process and its improvement process. Ma Zongkai said, "According to our more than ten years of operating experience, except for the problem of unsuitable process selection in some rural sewage treatment stations, the current sewage treatment process can meet most of the sewage treatment needs."

　　"Sewage" after treatment and "sludge"?

A typical activated sludge process consists of an aeration tank, a sedimentation tank, a sludge return system and a residual sludge removal system. The sewage and the returned activated sludge enter the aeration tank together to form a mixed liquor. The compressed air sent from the air compressor station enters the sewage in the form of fine bubbles through the air diffusion device laid at the bottom of the aeration tank. The purpose is to increase the dissolved oxygen content in the sewage and make the mixed liquid in a state of vigorous agitation, so that the activated sludge is suspended. Dissolved oxygen, activated sludge and sewage are mixed with each other and fully contacted, so that the activated sludge reaction can be carried out normally.

After the activated sludge purification effect of mixed liquor into the secondary sedimentation tank, mixed liquor in the suspended activated sludge and other solid matter here precipitation and water separation, after clarification of sewage as treated water discharge system. The sludge after sedimentation and concentration is discharged from the bottom of the sedimentation tank, most of which is returned to the aeration tank as inoculation sludge to ensure the suspended solid concentration and microbial concentration in the aeration tank; the proliferation of microorganisms is discharged from the sediment, known as "excess sludge". In fact, pollutants are largely transferred from the sewage to these residual sludge.

After a series of circulations, the activated sludge method has successfully purified wastewater, but how to deal with the large amount of "sludge" produced by "sewage"? Ma Zongkai said: "At present, the common sewage treatment plant sludge treatment methods in Sichuan Province mainly include sanitary landfill, preparation of biomass fuel, thermal power plant/waste incineration plant co-burning and biodegradation resources and other measures." With the continuous improvement of sludge treatment facilities in recent years, the disposal rate of sludge in urban sewage treatment plants has increased significantly.

Source: Sichuan Provincial Department of Ecology and Environment

　　How is the implementation of the discharge standard for sewage treatment?

According to reports, the domestic sewage treatment in Sichuan Province mainly implements three categories: Min and Tuojiang area quasi-four discharge standards, national first-level A discharge standards and rural sewage discharge standards. Some areas are affected by the environmental capacity of the discharged water body to implement the surface quasi-III discharge standards. Since Sichuan is located in the upper reaches of the Yangtze River, the implemented sewage discharge standards are relatively strict in China, which are much higher than those in the middle and lower reaches of the Yangtze River.

"At this stage, through continuous upgrading and transformation, most of the sewage treatment facilities except township areas have good operating efficiency and can meet the discharge standards." Ma Zongkai said that at present, China's sewage treatment technology level is at the leading level in the world, and the sewage treatment discharge standard is also higher than that of most developed countries in Europe and America.

The latest report released by the Sichuan Provincial Department of Ecology and Environment shows that as of January 2024, the 47 monitoring sections of 47 centralized drinking water sources that have been monitored in 21 cities (prefectures) in Sichuan Province have all reached the standard (Reach or better than Class III standards). According to statistics, the total amount of drinking water taken from drinking water sources is 211.3464 million tons, the standard water quantity is 211.3464 million tons, and the water quality compliance rate is 100.

It is worth noting that some villages and towns are affected by factors such as unstable water quality and quantity, poor management and sewage collection pipe network diseases, and some plants and stations fail to operate at full capacity or stably up to standard.

In December 2023, the Ministry of Ecology and Environment and the Ministry of Agriculture and Rural Affairs jointly issued the "Guiding Opinions on Further Promoting the Treatment of Rural Domestic Sewage", which clearly stated that the supervision of sewage treatment stations of more than 100 tons should be increased, and areas with conditions should be added. Online testing facilities for effluent have created conditions for the full implementation of standardized operation of rural sewage and stable discharge standards.

Ma Zongkai also believes that in the future, the efficiency of sewage treatment in rural areas where sewage treatment is relatively weak will be significantly improved. "We will also improve the sewage treatment plant in terms of low carbon, intelligence and energy saving, such as intelligent operation and maintenance, the introduction of clean energy and the use of more energy-saving and environmentally friendly equipment, so as to further reduce the carbon emission index of the sewage treatment plant."