1. Measured inlet and outlet water quality
The measured data of effluent quality, impurity removal rate and treatment effect of septic tanks in harmless sanitary toilets are not as complete as those of water supply and drainage treatment facilities. We should choose representative buildings in different places, comprehensively test the influent and effluent quality and treatment effect of septic tanks in different seasons and at different times on the same day, and conduct regular tests according to the changes of domestic sewage components, so as to enrich and improve design data and guide engineering design.
2. About the adoption of standard drawings
Septic tank adopts national standard drawings, which simplifies the design work, speeds up the construction progress, ensures the design quality and facilitates the construction, budget and final accounts. However, the adaptability of national standard drawings has certain limitations after all. In the design of building water supply and drainage, we should attach great importance to the design of septic tanks, try not to set up standard drawings, and make detailed hydraulic calculations like the design of horizontal sedimentation tanks, so that the size of septic tanks can meet the hydraulic requirements of sedimentation as much as possible to ensure the treatment effect. On the other hand, the standard drawing should be constantly improved and supplemented. Under the guidance of national standard drawings, prepare provincial and municipal reference standard drawings, and prepare various local standard drawings according to whether fecal sewage meets other domestic sewage and the sewage flow direction of buildings (houses, public buildings, schools, etc.). ) improve adaptability and ensure therapeutic effect. The minimum pool volume convenient for construction should also be considered.
3. Cleaning cycle
The cleaning cycle of septic tank is related to the temperature, air temperature, building properties, drainage quality and water quantity of fecal sewage. If the designed cleaning cycle is too short, the concentration of fecal liquid in septic tank will be too high, and the gap with the actual cleaning cycle will be too large, which will affect the normal fermentation and sewage treatment effect, and even cause fecal liquid to overflow and affect environmental sanitation. If the design cleaning cycle is too long and the septic tank volume is too large, it will increase the cost. Code for Design of Building Water Supply and Drainage (GBJ 15-88) (hereinafter referred to as the code) requires that the cleaning period is 3 ~ 12 months, which is 3 ~ 9 months extra in actual design, while the acid fermentation period is 3 months and the acid decay period is about 5 months. Practice has proved that three factors should be considered in the cleaning cycle: sewage treatment effect, construction cost and management. In general, the cleaning cycle should not be less than 12 months. Septic tanks are used in the bungalow group in the dormitory area of Shangyuanmen Waterworks, with 200 users. The domestic sewage standard is 30L/d/ person, and the sludge volume is 0.4L/d/ person. The moisture content of fresh sludge entering septic tank is 95%, and the moisture content of fermented sludge is 90%. The design cleaning cycle is 12 months, and the calculated septic tank volume is 2 1m3.
dwell time
Design of Three-format Septic Tank The residence time of septic tank is an important index related to the sewage treatment effect, the volume and cost of septic tank. If the residence time is too short, the sewage treatment effect will be poor. If the residence time is too long, it will increase the volume and cost of septic tanks and make the layout difficult. The determination of residence time should consider both sewage treatment effect and construction cost. Considering the influence of bubbles produced by fermentation on the laminar flow state required for sedimentation, the adverse effect of streamline turning of septic tank on sedimentation, and the influence of instantaneous change of domestic sewage discharge on uniform water inflow, the retention time of septic tank should be left room. The specification requires that the residence time should be 12 ~ 24 hours. Practice has proved that the retention time should not be less than 2 hours, and the retention time of septic tanks in the dormitory area of Yuanmen Water Plant is 24 hours to ensure the sewage treatment effect.
5. Impermeability design
In order to protect the environment and prevent pollution, septic tanks should be designed according to the requirements of hydraulic structures, and the impermeability label should not be too low.