New rural domestic sewage treatment project

Overview

1.1 Project Overview
Project Name: Domestic sewage treatment design plan
Project size: 100t/d
Design unit: Shandong Jixu Environmental Engineering Co., Ltd.

1.2 Village conditions

1.2.1 Status of drainage
According to the data provided by Party A, the resident population of the community is about 237, according to the daily average water consumption of 400L. The average daily displacement is about 100 tons.

1.2.2 Necessity of construction of sewage treatment plant
The construction project of village and town sewage treatment plant is an important part of the comprehensive improvement project of water pollution. It is one of the signs of improving the infrastructure of villages and towns and measuring the modernization of the town. It not only reflects the economic strength, social development and population quality of the villages and towns, but also It can enhance the attractiveness of domestic and foreign capital as the environment improves. The improvement of the sewage treatment system is closely related to the economic development of the region. The economic development and the beautiful environment are the fundamental guarantee for sustainable development. Therefore, it is very necessary to construct a sewage treatment project. The social, environmental and economic benefits generated cannot be measured by money, and it is a great cause for the benefit of future generations. With the active regional economy, the development of cities and towns will continue to climb to a new level. Industrial enterprises are bound to increase. The population, especially the foreign population, will continue to grow for a long period of time. If not treated as soon as possible, it will inevitably lead to pollution. The further aggravation caused the regional water quality to deteriorate further. The people's living environment was dirty, chaotic and poor, and the ecological environment of the town was damaged. Eventually, the environmental pollution problem became an important factor restricting economic development.

2.1.1 Sewage treatment station treatment of sewage water
According to the relevant information on the sewage provided, the amount of water treated by the sewage treatment station is: Qd=100 m3/d.

2.1.2 sewage treatment station sewage quality indicators
According to our company's experience in urban sewage treatment, first determine the list of sewage inflow water quality indicators of the town sewage treatment station as follows:

2.1.3 Sewage treatment discharge standards
The project implements the first-class standard A of the “Emission Standards for Pollutants in Urban Sewage Treatment Plants” (GB18918-2002). The wastewater should meet the following criteria after treatment:
Sewage discharge control standard

Process plan selection principle
Since the construction and operation of sewage treatment is not only costly, but also subject to various factors, the optimization of the treatment process plan is the most important to ensure the operation performance and cost reduction of the treatment plant. Therefore, it is necessary to The general principle, starting from the concept of overall optimization, combined with the design scale, sewage water quality characteristics and local actual conditions and requirements, select a practical and economical treatment process plan, after a comprehensive comparison, the best overall process plan and implementation are selected. the way.

Small town sewage treatment process design should meet the following principles:

1 According to the composition and concentration of influent water, select the economical and effective small town sewage and sludge treatment process to ensure that the effluent can meet the water quality requirements for reuse or discharge, and the sludge can be safely used and disposed of;

2 The treatment process must be considered together with the wastewater treatment process and the sludge treatment process.

3 Comprehensive consideration of the scale of the sewage treatment plant, local climate, geology, topography, personnel quality, economic level and other factors.
In the process plan determination of this sewage treatment plant, in addition to following the above principles, the following principles will be followed:

1) The technology is mature, the treatment effect is stable, and the effluent water quality meets the specified emission requirements.
2) Infrastructure investment and operating costs are low, and as much as possible with as little investment as possible.
3) Convenient operation and management, flexible operation, and adjustment of operation mode and process parameters according to different influent water quality and effluent water quality requirements, and maximize the processing capacity of the treatment device and the treatment structure.
4) The technology and equipment of the selected process are advanced, reliable and mature.
5) It is convenient to realize reasonable automatic control of the process, improve management level, and reduce labor intensity and labor cost.

2.4.2 Design of the process plan

According to the above analysis of sewage water quality, this project requires higher requirements for BOD5, CODCr, SS, animal and vegetable oil removal rate. The sewage treatment process designed by this scheme selects the mature treatment process with strong adaptability, flexible regulation, low energy consumption, low input, small land occupation and convenient operation and management considering the sewage volume and sewage water quality as well as the local economic conditions and management level. The characteristics of the various processes are discussed below in order to select a practical solution.

1) BOD5/CODCr ratio
The BOD5/CODCr value of wastewater is the easiest and most common method for determining the biodegradability of wastewater. It is generally considered that BOD5/CODCr>0.45 is biodegradable, BOD5/CODCr<0.3 is more difficult to biochemical, and BOD5/CODCr<0.25 is not biochemical.
Analyze the influent water quality of the village sewage treatment plant, BOD5=200mg/L, CODCr=350mg/L, BOD5/CODCr=200/350=0.57, its biodegradability belongs to better type of urban sewage, so this project is suitable for adopting biological The treatment process is processed.

2) BOD5/TN (ie C/N) ratio
The C/N ratio is an important indicator for determining whether or not nitrogen can be effectively removed. Theoretically, C/N ≥ 2.86 can be used for denitrification, but it is generally considered that C/N ≥ 3.50 can be effectively denitrified.
The influent water quality determined by analysis, C/N=200/25=8, meets the requirements of biological nitrogen removal.

3) BOD5/TP ratio
This indicator is the main indicator for identifying biological phosphorus removal. The ratio of BOD5/TP is an important index to determine whether the phosphorus removal effect can be achieved. It is generally considered that the value is greater than 20, and the larger the ratio, the more obvious the biological phosphorus removal effect.
The influent water quality of this project, BOD5/TP=200/4=50, meets the conditions for adopting the biological phosphorus removal process.
In summary, the influent water quality of the scheme is not only suitable for the secondary biochemical treatment process, but also suitable for the biological nitrogen and phosphorus removal process.
In recent years, the commonly used biological nitrogen and phosphorus removal (secondary enhanced biochemical treatment) mature processes are: A / O method, A / A / O method, AB method, SBR and so on.

1), A / O (anaerobic / aerobic) method

The A/O (Anaerobic/Oxic) process (with nitrification), the anaerobic/aerobic process, is the simplest enhanced biological phosphorus removal process consisting of an anaerobic zone and an anoxic zone. The process flow is shown in Figure 2-1.

Figure 2-1 A/O process flow diagram
The reflux activated sludge is returned to the anaerobic zone, and the polyphosphate bacteria in the sludge are depressurized under anaerobic conditions to release phosphate in the body, generating energy for absorbing rapidly degrading organic matter and converting it into PHB (polymerization). Beta hydroxybutyrate) is stored. Then the mixed solution enters the aerobic zone, and the polyphosphate bacteria decomposes the PHB stored in the body under aerobic conditions to generate energy, which is used for cell synthesis and phosphorus absorption, forms a high concentration of phosphorus-containing sludge, and is discharged out of the system along with the excess sludge. Thereby achieving the purpose of biological phosphorus removal.
Under conditions of sufficient sludge age, BOD5 is also degraded in the aerobic tank, and the nitrification reaction is also completed.
Since the reflux activated sludge is returned to the anaerobic zone, the system also has a nitrogen removal function in the aerobic zone according to the nitrification design, and the denitrification efficiency depends on the activated sludge reflux ratio.
The A/O process has the following disadvantages when it is nitrated:
In order to avoid the effect of the anaerobic system on the denitrification system, the sludge return flow needs to be controlled, so the denitrification efficiency is limited. That is to say, the main function of the process is to remove phosphorus.
Because of the nitrification reaction, the sludge age of the system is longer than that of the non-nitrification A/O process, so that the phosphorus removal efficiency

2） A/A/O

The A/A/O method is an anaerobic/anoxic/aerobic activated sludge process. The structure is to add an anoxic zone after the anaerobic zone of the A/O process and before the aerobic zone. The aerobic zone has a nitrification function, and the mixed solution in the aerobic zone is returned to the anoxic zone for denitrification. Denitrification. In the process of flowing through three different functional zones, the sewage removes organic matter, nitrogen and phosphorus from the sewage under the action of different microbial flora, achieving the purpose of simultaneous biological phosphorus removal and biological nitrogen removal. The process is shown in Figure 2-2.
Figure 2-2 A/A/O process flow diagram
In the system, the process is the simplest process of dephosphorization and denitrification. Under the conditions of anaerobic, anoxic and aerobic alternate operation, the growth of filamentous bacteria can be inhibited, and the sludge expansion can be overcome, so that the SVI value is generally less than 100. It is beneficial to the separation of mud and water, and only a mixer is provided in the anaerobic and anoxic sections. Because the three regions of anaerobic, anoxic and aerobic are strictly separated, it is beneficial to the reproduction and growth of different microbial flora, and the effect of nitrogen and phosphorus removal is good. At present, the law widely uses domestic large and medium-sized sewage treatment plants at home and abroad.
The disadvantage of the A2/O method is mainly in the process itself. Denitrification needs to maintain a low sludge load in order to fully carry out nitrification and achieve a higher denitrification rate, while biological phosphorus removal needs to maintain a higher sludge load and obtain a larger excess sludge volume in order to achieve better In addition to the phosphorus removal effect, it is necessary to take necessary measures and further optimization in the design, so that the two can be organically combined to achieve the best phosphorus and nitrogen removal effect.
The disadvantage of the A2/O series is that the process is complicated. A separate secondary sinking tank and blower room must be installed. Usually, the front primary sinking tank is required. The floor space is large and the system head loss is high. At the same time, special reflow equipment and reflow structure are required. The management level is relatively high. Despite this, the process is relatively mature and reliable, and the treatment effect is stable. It is generally suitable for large sewage plants with high operational management level.
3), AB method
The AB method is a biosorption-degradation two-stage activated sludge process. The A-stage load is high, the aeration time is short, about 0.5 h, and the sludge load is as high as 2-6 kg BOD5/kg MLSS·d; A/A/O design) The sludge load is low, 0.1-0.3 kg BOD5/kg MLSS·d; this method has a certain removal rate for organic matter, nitrogen and phosphorus. It is suitable for sewage with high influent concentration (usually required to enter BOD5≥250mg/L), high degree of treatment, and large changes in water quality and quantity.
4), SBR method
When water is introduced into the same container, anaerobic (no aeration) and oxygen deficiency are formed, and then the water is stopped, aeration and oxygenation are started, the process of nitrogen and phosphorus removal is completed, and precipitation is carried out in the same container, and cesium is added. The water is discharged and a program is completed. This method is different from the continuous system that divides by space. It does not require returning sludge, and there is no special anaerobic, anoxic, or aerobic zone. Instead, it is stirred and aerated in the same container at different intervals. , precipitation, formation of anaerobic, anoxic, aerobic process.

The characteristics of the SBR process are as follows

1. Flexible operation, the time of each stage can be adjusted according to the change of water quantity and water quantity, or the process can be adjusted or increased or decreased according to needs to ensure that the effluent water quality meets the requirements.
2. Similar to the characteristics of static settlement, the separation of mud and water is not disturbed, and the effluent SS (suspended matter) is low and stable.
3. The biological reaction and sedimentation are all completed in one structure, saving land occupation and low cost.
4. The ability to withstand water and water impact load is strong.
5. The sludge has good sedimentation performance and is not easy to cause sludge expansion.
6. The removal of organic matter and nitrogen is good.
The anaerobic tank of this method has higher oxidation-reduction potential, poor phosphorus removal effect, low total volume utilization rate, generally less than 50%, and is suitable for occasions with small sewage volume.
Compared with other treatment processes, SBR can adopt blast aeration or mechanical aeration. Since the system is always in aeration and non-aeration conditions, the facility is also in a state of stop and open phase, so the energy saving effect is good. The biological reaction is completed in a pool, no need to set up a secondary settling tank, the effective water depth is deeper, and the corresponding floor space is less.
In the sewage treatment process, aerobic biochemical treatment is the main operating unit that determines the efficiency of removing organic pollutants. This program plans to use the SBR biochemical treatment process, which has the fastest development of wastewater treatment at home and abroad.

2.5 treatment process
The selection of sewage treatment process requires advanced technology, stable operation, simple operation, low operating cost and low investment. According to the characteristics of the sewage, combined with the company's many years of industrial wastewater, domestic sewage treatment engineering experience.

2.5.1 Description of sewage treatment process
Domestic sewage first flows through the mechanical grille, removing large debris and floating objects through the mechanical grille to avoid clogging of subsequent pipelines and equipment. The sewage flows into the septic tank through the mechanical grille. After the sewage stays in the septic tank for a period of time, it flows into the regulating tank to regulate the water quality and quantity. After the sewage stays in the regulating tank for a period of time, the sewage pump is lifted by the sewage pump. In the buried equipment, the sewage first enters the integrated A pool, and the A pool is the anoxic tank. The denitrifying bacteria in the anoxic tank use the organic matter in the raw water and the nitrate and nitrite in the reflux supernatant. The denitrification reaction is carried out, and the nitrate and nitrite are denitrified to form nitrogen gas, and are removed to achieve the purpose of denitrification.
The effluent from the anoxic tank flows into the biological contact oxidation tank and suspends the high-efficiency three-dimensional elastic filler, and is aerated by the blower to maintain the dissolved oxygen concentration in the tank of 2~4mg/l. There are mainly aerobic microorganisms and autotrophic bacteria (nitrifying bacteria) in the contact oxidation pond. Aerobic microorganisms can decompose organic matter into CO2 and H2O, and greatly degrade the pollution load of BOD5, CODCr and SS in wastewater.

The contact oxidation tank is operated by pushing flow to increase the removal rate of organic matter. The biological contact oxidation pond effluent enters the secondary settling tank, and the separated biofilm sludge is separated from the clean water to achieve the purpose of discharging the discharged water. The secondary sedimentation tank is set to a vertical flow type, and the sludge in the secondary sedimentation tank is driven into the sludge concentration tank by the gas stripping device to carry out sludge concentration.

The supernatant liquid in the secondary sedimentation tank flows into the disinfection tank by itself, and the disinfectant is added to the disinfection tank through the chlorine dioxide generator to achieve the function of sterilization and disinfection, and the sewage discharged into the above process can reach the standard discharge.
Advanced treatment: The water in the disinfection tank is lifted by the sewage pump into the quartz sand and activated carbon filter for filtration and adsorption. The filtered water flows into the reuse pool and returns to the water in the pool for reuse or as a quartz sand filter and activated carbon. The backwash water of the filter.

2.5.3 Characteristics of the sewage treatment plan designed by this design
The different environmental conditions of hypoxia and aerobic and the organic combination of different kinds of microbial flora can simultaneously have the functions of removing organic matter and removing nitrogen and phosphorus.
In the absence of oxygen-aerobic operation, filamentous bacteria do not multiply. SVI is generally less than 100 and does not cause sludge bulking.
The integrated buried equipment is compact in structure, small in floor space, simple in operation, and can be buried in the upper layer of the ground for greening without affecting the surrounding environment.