During the process of promoting the construction of socialist modernization in our country, environmental protection has become a basic national policy. Urban pollution prevention and treatment and ecological environment protection have been increasingly valued, and the construction of centralized urban sewage treatment facilities is an important way to treat sewage. By the end of 1998, 266 sewage treatment plants had been built and put into operation in our country, with a daily sewage treatment capacity of 11.36 million tons, sewage treatment volume of 292.745 billion tons, and a centralized urban sewage treatment rate of 16.2%. Nowadays, most sewage plants use the activated sludge method to treat sewage. In view of its disadvantages such as large land occupation, high investment, and high energy consumption, several latest sewage treatment technologies have been developed domestically and internationally, which are worthy of our reference and application. Below is a brief introduction:
1 Aerated Biofilter Method
The aerated biofilter method uses a new type of granular filter material with a biological film grown on its surface. The sewage flows downward through the filter material, and aeration is provided at the bottom of the pool to make the organic matter in the wastewater aerobic stable. It can use the treated effluent for backwashing to discharge the proliferated activated sludge. This technology has the following advantages:
1.1 Small tank volume and land occupation
Because of its high volumetric load, it can reach 3-8kgBOD5/m3/d, which is 4-10 times that of conventional secondary biological treatment. Its tank volume and land occupation are only 1/10 to 1/5 of conventional secondary biological treatment.
1.2 High-quality treated effluent
When the volumetric load is 6kgBOD5/m3/d, its effluent SS and BOD5 can be maintained below 20mg/L, with high removal rate, greatly meeting domestic environmental protection discharge standards and can be used for reclaimed water treatment.
1.3 Simplified sewage treatment process
This technology can eliminate the need for a secondary sedimentation tank and a sludge return pump room, simplifying the treatment process, reducing land occupation, and drastically reducing infrastructure funds and operating costs. Nowadays, this sewage treatment technology has been widely applied in developed countries such as Europe, America, and Japan, while it is still a novelty in our country. The 120,000-ton treatment plant under construction in Dalian adopts this technology and has achieved good social and economic benefits.
2 Upflow Anaerobic Sludge Blanket Reactor
The structure of this reactor consists of three zones: upper, middle, and lower. The lower part is the sludge bed zone, the middle part is the suspended sludge zone, and the upper part is the three-phase separation zone of gas, solid, and liquid. Wastewater first enters from the bottom of the reactor and flows upward through the sludge bed zone to come into contact with a large number of anaerobic bacteria, where the organic matter is decomposed into biogas. The wastewater then flows upward through the suspended sludge layer, allowing the remaining organic matter to continue being decomposed. Finally, the mixed liquid containing biogas, sludge, and liquid flows upward through the three-phase separator located in the upper part for gas, solid, and liquid separation. The biogas is separated in the gas chamber and discharged through a pipe, while the sludge is separated in the measurement area of the three-phase separator and returned to the sludge bed zone, maintaining sufficient biomass in the reactor. The treated supernatant is discharged through the outlet channel at the top of the reactor. The greatest advantage of this technology is that it internally cultivates anaerobic granular sludge with high methane activity and good settling performance, producing a large amount of biogas, making it an energy-producing wastewater treatment device. There is no mechanical stirring or packing inside the reactor, the structure is relatively simple, operation management is convenient, and no energy consumption is required. Moreover, due to the long generation period of anaerobic bacteria, very little bacterial cell mass is synthesized during the degradation of organic matter, resulting in very little sludge production, which can reduce sludge treatment costs.
Practice has proven that this method can be used to treat various types of organic wastewater, and the recovered biogas can be used for power generation and civilian purposes, providing significant economic benefits.
3 Internal Circulation Anaerobic Reactor
The basic structure of this reactor consists of two upflow anaerobic sludge reactors connected in series, stacked one above the other. Wastewater enters from the bottom of the lower upflow anaerobic sludge reactor and mixes uniformly with highly active anaerobic granular sludge. Most of the organic matter is converted into biogas here. The biogas produced is collected by the lower upflow anaerobic sludge reactor and rises along a specially designed riser pipe. At the same time, the mixed liquor is lifted from the lower upflow reactor to the gas-liquid separator located at the top of the internal circulation reactor. The separated biogas is discharged through the outlet pipe at the top, while the separated sludge-water mixture returns to the bottom of the lower upflow reactor through a return pipe and mixes thoroughly with the granular sludge and influent at the bottom. The result of the internal circulation is that the lower upflow reactor has a high biomass, long sludge retention time, and high upflow velocity, ensuring that the granular sludge in the reaction zone reaches a fully fluidized state, significantly enhancing the ability of the lower upflow anaerobic sludge reactor to remove organic matter.
After being treated by the lower upflow reactor, the wastewater automatically enters the upper upflow reactor for further treatment, where the remaining organic matter can be further degraded. The biogas produced is collected by the upper upflow reactor, and the sludge-water mixture in the reactor undergoes solid-liquid separation in the sedimentation zone. The treated supernatant is discharged through the outlet pipe, while the settled sludge automatically returns to the reaction zone of the upper upflow reactor. At this point, the wastewater has completed the entire treatment process.
The internal circulation anaerobic reactor uses the biogas generated itself as a driving force to achieve internal circulation of the mixed liquor at the bottom, giving the wastewater enhanced pretreatment. Subsequently, the upper reactor continues to treat the wastewater, enabling the effluent to meet the expected treatment requirements. The main advantages of this reactor are: high organic loading rate, short hydraulic retention time, high height-to-diameter ratio, small land occupation, low capital investment, stable effluent quality, and strong load-bearing capacity.
The above mentioned technologies have already been widely used in various sewage treatments in developed countries such as Europe and America, including domestic sewage, urban sewage, and industrial wastewater treatment. Practice has proven that these technologies are efficient and low-cost sewage treatment technologies, with great potential for development and application value in our country. The application of these advanced technologies will surely promote the development of our country's environmental protection cause and contribute to the construction of socialist modernization in our country.