The actual test data for the effluent quality of septic tanks, the removal rate of impurities, and the treatment effectiveness are not as comprehensive as those for water and wastewater treatment facilities. It is necessary to select representative types of buildings in various regions, and conduct comprehensive tests on the water quality at the input and output of septic tanks, as well as the treatment effectiveness, at different times of the day across various seasons.
Regular testing of the composition changes in domestic wastewater to enrich and perfect design data, guiding engineering design.
Cleaning cycle
The septic tank cleaning cycle is related to the temperature of fecal wastewater, ambient temperature, building characteristics, and the quality and volume of drainage water. If the designed cleaning cycle is too short, the concentration of fecal sludge in the septic tank will be excessively high, resulting in a significant discrepancy from the actual cleaning cycle. This can affect normal fermentation and wastewater treatment effectiveness, and even lead to overflow of fecal sludge, impacting the environment.
Sanitation. If the cleaning cycle is too long, the septic tank volume will be excessive, increasing the cost. The "Code for Design of Water Supply and Drainage of Buildings" (GBJ15-88) (hereinafter referred to as the "Code") requires a cleaning cycle of 3 to 12 months, with most actual designs opting for 3 to 9 months, and the acid fermentation period during the acidic fermentation stage is 3 months.
In the month, the acid reduction period is approximately 5 months. Practical proof shows that the determination of the cleaning cycle should take into account three factors: the treatment effect of wastewater, construction cost, and management. Generally, the cleaning cycle should not be less than 12 months. The Shangyuanmen Waterworks dormitory area has a common septic tank, serving 200 people, with standard domestic wastewater.
Each person generates 30L/day, sludge volume per person is 0.4L/day, the fresh sludge moisture content in the septic tank is set at 95%, and the moisture content of the sludge after fermentation is set at 90%. The design cleaning cycle is selected as 12 months, with a calculated septic tank volume of 21 m³.
Dwell time
The detention time in septic tanks is a crucial indicator related to the effectiveness of wastewater treatment and the volume and cost of the septic tanks. If the detention time is too short, the wastewater treatment effectiveness is poor; if it's too long, it increases the volume and cost of the septic tanks, and also makes the arrangement difficult. Determining the detention time should take into account both the effectiveness of wastewater treatment and the construction cost.
Price considerations should take into account the impact of fermentation-generated bubbles on the laminar flow state required for sedimentation, the adverse effects of numerous streamline turns in septic tanks on sedimentation, and the significant instantaneous changes in domestic wastewater discharge affecting the uniformity of inflow. There should be ample leeway for the retention time in septic tanks. "The Specification" requires: the retention time should be...
Pump interval: 12 to 24 hours. Practical evidence shows that the retention time should not be less than 12 hours to ensure effective wastewater treatment. The septic tank in the dormitory area of the Shangyuanmen Waterworks has a retention time of 24 hours.
Mud and water mixture
The traditional septic tank has a history of over a century. Its technical approach involves the contact between wastewater and sludge. The deposited sludge undergoes digestion and degradation, producing biogas, carbon dioxide, and other digestion gases. The buoyant action of the digestion gases causes disturbance in the sludge, which facilitates the mixing of sludge with
A more thorough mixture of microbial communities aids in digestion and degradation. However, as the digestate gases rise, the bottom sludge is carried up, releasing bubbles. After the bubbles escape, the sludge re-sinks, thus re-polluting the wastewater.
In the technological mode where septic tank wastewater and sludge come into contact and mix, the sedimentation and effluent water quality of the septic tank are affected. To improve the sedimentation effect and effluent water quality, it is necessary to extend the wastewater retention time, which is generally 12-24 hours.
Three-phase separation
The Three-Phase Separation Septic Tank Technology is an improvement on the traditional septic tank, retaining the benefits of the mixed sludge and water in the septic tank, and adding a technology for the separation of "wastewater, sludge, and digestion gas" into three phases. It includes a three-phase separation device at the outlet of the septic tank, separating the sludge and digestion gas from the wastewater treatment process.
Prevent the interference of air flotation on wastewater treatment. The sedimentation tank at the outlet end adheres to the technical standards of horizontal sedimentation ponds, with a wastewater sedimentation time of no more than 2 hours.
Technical Comparison
The septic tank's capacity is composed of wastewater volume and sludge volume. In a three-phase separation septic tank, the wastewater retention time is 4-6 hours, compared to 12-24 hours in a mud-water mixed septic tank. By reducing the wastewater retention time, the effective volume is saved, which can store more wastewater.
Clay.
