Basic concepts of sewage treatment

Most municipal sewage treatment plants have adopted the activated sludge process. The principle of biological treatment is to complete the decomposition of organic matter and the synthesis of biological organisms through biological action, especially the role of microorganisms, and transform organic pollutants into harmless gas products (CO2), liquid products (water) and solid products rich in organic matter (microbial communities or biological sludge); The excess biological sludge is separated by solid and liquid in the sedimentation tank and removed from the purified sewage.

In the process of sewage biochemical treatment, the factors affecting microbial activity can be divided into two categories: substrate and environment:

First, the matrix includes nutrients, such as organic compounds dominated by carbon elements, namely nutrients such as carbon source substances, nitrogen sources, phosphorus sources, and trace elements such as iron, zinc, manganese; In addition, it also includes some toxic and harmful chemicals such as phenols, benzene and other compounds, as well as some heavy metal ions such as copper, cadmium, lead ions and so on.

Second, the main environmental impact factors are:

(1) Temperature.

The influence of temperature on microorganisms is very extensive, although some types of bacteria are also active in high temperature environments (50 ° C ~ 70 ° C) and low temperature environments (-5 ~ 0 ° C), the most suitable temperature range for the growth of most microorganisms in sewage treatment is 20-30 ° C. In the appropriate temperature range, the physiological activity of the microorganism is vigorous, and its activity is enhanced with the increase of temperature, and the treatment effect is better. Beyond this range, the activity of microorganisms becomes worse, and the biological reaction process will be affected. In general, the maximum and minimum limits for controlling the reaction process are 35 ° C and 10 ° C respectively.

(2)PH value.

The most suitable PH range for microorganisms in activated sludge system is 6.5-8.5, and the acidic or alkaline environment is not conducive to the survival and growth of microorganisms. In serious cases, the sludge floc will be destroyed, the bacteria colloidal group will disintegrated, and the treatment effect will deteriorate sharply.

(3) Dissolved oxygen.

For aerobic biological reactions, it is important to maintain a certain concentration of dissolved oxygen in the mixture. When the dissolved oxygen in the environment is higher than 0.3mg/l, both facultative and aerobic bacteria perform aerobic respiration. When the dissolved oxygen is less than 0.2-0.3mg/l and close to zero, the facultative bacteria are transferred to anaerobic respiration, most of the aerobic bacteria basically stop breathing, and some aerobic bacteria (most of them are filamentous bacteria) may also grow well, which often leads to sludge swelling after dominating the system.

In general, the dissolved oxygen at the outlet of the aeration tank is appropriate to maintain about 2mg/l, and too high will increase energy consumption, which is economically uneconomical. Among all the influencing factors, matrix factors and PH value are determined by the influent water quality, and the control of these factors mainly depends on daily monitoring and the strict implementation of relevant regulations and regulations.

For the general municipal sewage, most of these factors will not constitute too much impact, and the parameters can be basically maintained within the appropriate range. The change of temperature is related to the climate, and it is difficult to control the temperature for the 10,000-ton municipal sewage treatment plant, especially when the activated sludge process is used, and it is not very feasible economically and engineering.

Therefore, it is generally through the appropriate selection of design parameters to meet the processing requirements of different temperature changes in order to achieve the processing target.

Therefore, the main goal of process control falls on the activated sludge itself and the environmental factors that can be changed by regulatory means, and the main task of control is to take appropriate measures to overcome the influence of external factors on the activated sludge system, so that it can continue to play a stable role. The key to the process control of biological reaction system is the selection of control objects or control parameters, which is closely related to the processing technology or processing target.

As mentioned above, dissolved oxygen is a very important indicator parameter in the type and process of biological reaction, which can intuitively and quickly reflect the operating status of the entire system, convenient operation and management, installation and maintenance of instruments and meters are also relatively simple, which is also the reason why the newly built sewage treatment plants in China have basically realized on-site and online monitoring of dissolved oxygen in the past decade.

Tertiary treatment is the deep treatment of water, and the current sewage treatment plants in China are not put into practical application. It will be the secondary treatment of water denitrification, dephosphorization treatment, with activated carbon adsorption or reverse osmosis method to remove the remaining pollutants in the water, and ozone or chlorine disinfection to kill bacteria and viruses, and then the treated water into the waterway, as flushing toilets, spraying streets, watering green belt, industrial water, fire prevention and other water sources.