Wastewater sludge disposal issue
In accordance with the technical standards for septic tanks, the sludge removal time for septic tanks is generally 90 days, 180 days, or 360 days. [3] The removed sludge can be used as organic fertilizer or disposed of in a harmless manner. However, with the widespread use of chemical fertilizers and societal development, the use of fecal fertilizer has become less common, and with this...
At the same time, our country's sludge disposal industry is seriously lagging behind. The harmless treatment of fecal sludge is insufficient. With a population of 1.36 billion, China produces over 100 million tons of fecal sludge annually. According to statistics data, only 8.014 million tons of fecal sludge were treated harmlessly in 2012. The large-scale removal and transfer of fecal sludge has caused...
Pollution. Therefore, if the problem of non-hazardous disposal of septic tank sludge cannot be resolved, the sludge will become a source of water pollution. The septic tank issue is an environmental problem related to the effectiveness of water pollution control. [2] [3]
Septic tank blockage issue
Sewage tanks accumulate sludge at the bottom, with floating sludge on the surface and suspended sludge and wastewater in between. The tank is divided into two or three compartments by partition walls, with overflow openings located in the middle of each partition wall. If the tank is not cleaned in a timely manner, when the sludge settles...
Excess sludge accumulates above the overflow opening, which can block the opening and lead to a septic tank blockage. Septic tank blockages severely disrupt normal daily life for users and are urgent issues in the application of septic tanks.
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Replaced traditional brick-and-mortar septic tanks and reinforced concrete septic tanks, which could contaminate groundwater quality and affect the safety of surrounding buildings due to leakage and poor operational conditions. This product utilizes the gravitational flow of water, requiring no external power or operational costs, thereby saving energy and facilitating management. It offers excellent social, environmental, and economic benefits.
Benefits.
The actual test data for the effluent quality of septic tanks, the removal rate of impurities, and the treatment effect are not as comprehensive as those for water and wastewater treatment facilities. Representative types of buildings should be selected in various regions, and the water quality in and out of septic tanks, as well as the treatment effects, should be tested at different times of the day across various seasons. Regular tests should be conducted based on the changes in the composition of domestic wastewater to enrich and perfect the design data, guiding engineering design.
Cleaning Cycle
The cleaning cycle of septic tanks is related to the temperature of fecal wastewater, ambient temperature, nature of the building, and the quality and volume of drainage water. If the designed cleaning cycle is too short, the concentration of fecal slurry in the septic tank will be excessively high, significantly differing from the actual cleaning cycle, which can affect normal fermentation and wastewater treatment, and even lead to overflow of the slurry, impacting the environment.
Sanitation. If the cleaning cycle design is too long, the septic tank volume will be excessively large, 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, while the acid fermentation period in the acidic fermentation stage is 3 months.
The acid reduction period lasts for about 5 months. Practical evidence shows that when determining the cleaning cycle, one should consider three factors: the wastewater treatment effect, construction cost, and management. Generally, the cleaning cycle should not be less than 12 months. The Shangyuamen Waterworks dormitory area has a common septic tank, serving 200 people, with standard domestic wastewater.
Each person generates 30L/day of sludge, with an individual sludge volume of 0.4L/day. The fresh sludge moisture content entering the septic tank is set at 95%, and after fermentation, the sludge moisture content is reduced to 90%. The design cleaning cycle is selected as 12 months, resulting in 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 effect is poor; if it's too long, it increases the volume and cost of the septic tanks, and also makes the layout difficult. Determining the detention time should take into account both the wastewater treatment effectiveness and the construction cost.
Price considerations include the impact of foaming bubbles on the laminar flow required for sedimentation, the negative effects of numerous turns in the septic tank on sedimentation, the significant instantaneous changes in domestic wastewater discharge affecting the uniformity of influent flow, and the retention time in the septic tank should allow for leeway. "The Standard" requires: retention time
Retrieval 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.
A septic tank is a primary transitional treatment structure that utilizes sedimentation and anaerobic fermentation to remove suspended organic matter from domestic wastewater. It contains a high concentration of faeces, paper debris, pathogens, and other suspended solids, with a concentration ranging from 100 to 350 mg/L. The concentration of organic matter, CODCR, is between 100 and 400 mg/L, with the concentration of suspended organic matter, BOD5, ranging from 50 to 200 mg/L. After 12 to 24 hours of sedimentation in the septic tank, up to 50% to 60% of the suspended solids can be removed. The sludge that settles is decomposed by anaerobic fermentation for over 3 months, converting organic matter into stable inorganic matter and transforming raw sludge into mature sludge, thereby altering the sludge structure and reducing its moisture content. The sludge is periodically removed and disposed of, either buried or used as fertilizer. Requirements: The calculated volume of the sedimentation and decomposition sections of the septic tank should be determined according to Article 4.8.4 to 4.8.7 of the "Code for Design of Building Water Supply and Drainage" (GB50015-2003). The recommended retention time of wastewater in the septic tank should be between 12 to 36 hours. For wastewater treatment systems without sludge disposal, the volume of the septic tank should also include the storage volume for sludge.
