History of mainstream wastewater treatment technology

According to the growth mode of microorganisms, biological methods can be divided into suspension growth method represented by activated sludge method and adhesion growth method represented by biofilm method. At present, the activated sludge method is the most widely used in urban sewage treatment. However, because the traditional activated sludge operation needs to consume a lot of energy, the operation cost is also high, need to innovate. In order to develop the new technology and process of urban sewage treatment with high efficiency and low consumption, a lot of research has been carried out at home and abroad and some achievements have been made.

1. Biological treatment of microorganisms

Traditional sewage biological treatment technology mainly relies on two types of microorganisms, namely heterotrophic aerobic microorganisms and heterotrophic anaerobic microorganisms. In recent decades, scientists and engineers have worked together to conduct in-depth research on microorganisms in sewage biological treatment, and have made many achievements, such as: the different types and characteristics of bacteria and protozoa in activated sludge and their synergies have been studied, which has promoted the development of AB process; The research on nitrifying and denitrifying bacteria, as well as the research on the characteristics of phosphorus accumulating bacteria, promoted the development of A/O process with nitrogen removal function and A/A/O process with nitrogen and phosphorus removal function. The research on the population and characteristics of anaerobic microorganisms, and the discovery that anaerobic microorganisms have the ability to partially degrade macromolecular synthetic organic matter, promoted the development of anaerobic biological treatment technology and the process of treating wastewater containing refractory organic matter with anaerobic/aerobic tandem process; The research on the screening, culture and immobilization of high efficiency bacteria provides an effective way to further improve the efficiency of sewage biological treatment, especially the treatment of difficult biodegradable organic matter.

2. Biological treatment process

The three major elements in biological treatment are microorganisms, oxygen and nutrients. The reactor is a place where microorganisms inhabit and grow, and is the main equipment for microbial degradation and utilization of pollutants in sewage. An efficient reactor should be able to maintain the maximum microbial quantity and its activity, be able to effectively supply oxygen or isolate oxygen, and make full contact between microorganisms, oxygen and organic matter in sewage with good mass transfer conditions. According to its characteristics, the reactor can be roughly divided into the following categories:

(1) Suspension growth (such as activated sludge method) or attached growth (such as biofilm method);

② Push flow or completely mixed type;

(3) Continuous operation (such as traditional activated sludge method) or intermittent operation (such as SBR method).

1. Activated sludge method

Since the activated sludge process was pioneered by Arden and Lockett in 1914, it has been developed and practiced for 104 years, and has been continuously innovated and improved in terms of oxygen supply mode, operating conditions, and reactor form. The earliest traditional activated sludge method belongs to the push flow aeration tank. Because the substrate concentration near the water inlet of the tank is higher than that at the outlet, the initial design did not take into account the change in oxygen demand, resulting in insufficient oxygen in some parts. In order to improve the shortcomings of uneven oxygen supply, in 1936, the uniform aeration method was changed to gradually reduce aeration along the direction of push flow, most of the oxygen in the substrate removal is quite fast at the inlet end, and the effluent end with internal metabolism and decay as the main reaction only needs a small amount of oxygen, which is the traditional activated sludge method standard form - gradually reduced aerated activated sludge method.

A variant of the activated sludge process (stage aeration) appeared in 1942. Stage aeration method is also called multi-point water intake method, the water is divided into several strands, and then several strands of sewage from different points of the aeration tank into the person, so that the oxygen demand is evenly distributed. The idea of re-aerating the sludge before mixing it with raw water has been further developed. In 1951, the contact stabilized activated sludge method appeared, which is another development form of the traditional activated sludge method. In order to avoid the microbial inadaptation caused by the substrate concentration gradient in the push flow aeration tank, the microbial community was kept in a relatively stable state. By the end of the 1950s, a completely mixed activated sludge method appeared, the advantage of this form is to provide an environment conducive to the growth of bacterial floc, not conducive to the growth of filamentous bacteria, sludge settlement and compactness are very good, but due to changes in the matrix gradient make the system susceptible to toxic substances. In order to overcome the shortcomings of several other forms of improvement (large amounts of sludge must be disposed of, and the operational control requirements of the process are strict), delayed aeration has emerged, which has a fairly high degree of stability due to a complete average residence time of cells, but due to economic constraints, it is only used in small facilities with low sewage concentrations. In addition, pure oxygen aeration method and deep well aeration method have also appeared.