Rapid infiltration treatment of wastewater

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Abstract: The Rapid Infiltration System (RI system) is a type of land-based wastewater treatment system. Traditional RI systems occupy a large area and have a low hydraulic load; the highest daily hydraulic load is only 0.03 m. This is because traditional RI systems rely primarily on natural sandy soil for infiltration, and the uneven soil conditions in such areas prevent an increase in the hydraulic load. To this end, China University of Geosciences (Beijing) has been devoted to research on Constructed Rapid Infiltration Systems in recent years. To date, it has successfully moved from experimental studies to practical engineering applications, with the technology first being introduced and utilized in the southern regions of China. No research reports on this technology exist abroad, making it the first of its kind to be developed in China. Keywords: wastewater; artificial rapid infiltration treatment; CRI system. The Rapid Infiltration System (abbreviated as RI system) is a type of land-based wastewater treatment system. Traditional RI systems occupy a large area and have a low hydraulic load; the highest daily hydraulic load is only 0.03 m. This is because traditional RI systems rely primarily on natural sandy soil for infiltration, and the uneven soil conditions in such areas prevent an increase in the hydraulic load. To this end, China University of Geosciences (Beijing) has been devoted to research on Constructed Rapid Infiltration Systems in recent years. To date, it has successfully moved from experimental research to practical engineering applications, with the technology first being introduced and utilized in the southern regions of China. No research reports on this technology exist abroad, making it the first of its kind to be developed in China. The infiltration tank in the CRI system is filled with artificially graded natural river sand, to which a certain amount of special filler is added to ensure both a high hydraulic load and compliance with the requirements for wastewater treatment. The CRI system is an ecological treatment method that utilizes a combination of filtration, adsorption, and microbial degradation, achieved through artificial media and special fillers in rapid infiltration tanks, to break down and remove organic substances from wastewater, thereby purifying the water quality. It is suitable for the resource utilization of river wastewater as well as the treatment of domestic wastewater. The CRI system not only boasts advantages such as simple operation, easy operation and management, low energy consumption, low investment, and low operation and maintenance costs, but also features a high hydraulic load and good water quality. 1. CRI system process flow: The main function of the pre-sedimentation tank is to reduce the SS in wastewater, thereby increasing the percolation rate in the percolation tank and preventing blockages. As wastewater passes through the infiltration basin, comprehensive physical, chemical, and biological reactions take place to remove pollutants; among these, it is primarily biochemical reactions that enable the removal of organic pollutants through biodegradation. The function of the underground water collection system is to collect and purify water, which is then stored in a clean water tank for reuse. The core of the rapid infiltration method is the rapid infiltration tank, which is a system consisting of at least two filters filled with sand and gravel in a certain thickness. It operates in a wet-dry cycle, utilizing aerobic, anaerobic, and facultative microorganisms within the filters to degrade pollutants. During the drying period, the infiltration ponds are mostly in an aerobic environment, while they are in an anaerobic environment during the flooding period; therefore, aerobic and anaerobic conditions alternate frequently within the infiltration ponds, which facilitates microorganisms to carry out comprehensive treatment to remove organic matter. Regarding nitrogen removal, ammonification and nitrification occur during drying, while denitrification takes place during flooding; nitrogen is removed through these transformation processes ； Suspended solids are removed through filtration ； Heavy metals are removed through adsorption and precipitation ； Phosphorus is removed through adsorption and the formation of calcium hydroxyposphate precipitates with special fillers in the infiltration tank ； Pathogens are removed through filtration, adsorption, drying, radiation, and phagocytosis ； Organic substances are removed through processes such as volatilization, biological, and chemical degradation. 2. Performance of the CRI system: Small-scale field tests conducted in Zhuozhou, Hebei, have shown that the daily hydraulic load of the CRI system can exceed 2 m; the quality of the treated water meets or exceeds the standards for secondary treatment, with CODcr levels generally below 50 mg/L, and at times even below 20 mg/L. BOD5 levels are generally also below 20 mg/L ； Field tests conducted by Shenzhen Sankezhu and Dongguan Huaxing Electrical Appliance Co., Ltd., as well as practical applications in real projects, have shown that for river wastewater, the daily hydraulic load can be above 1.5 m3/(m2•d); for domestic wastewater, this load can be above 1 m3/(m2•d). The quality of the treated water meets or exceeds the standards for secondary treatment, with CODcr levels generally below 40 mg/L, and sometimes even below 20 mg/L. BOD5 levels are generally below 10 mg/L ； Research on the water from the Maozhou River shows that under a hydraulic load of 1.5 m/d, the CRI system achieves average removal rates of 89.51%, 77.82%, 85.33%, 98.28%, and 60.19% for SS, CODcr, BOD5, NH3-N, and TP respectively. The average concentrations of SS, CODcr, BOD5, NH3-N, and TP in the treated water are 2.5 mg/L, 15.7 mg/L, 2.89 mg/L, 0.32 mg/L, and 0.86 mg/L respectively. The quality of the treated water meets the Class III standards set by the Comprehensive Wastewater Discharge Standards (GB8978-1996), the first-stage discharge standards for urban secondary wastewater treatment plants, the quality standards for domestic non-potable water (CJ25.1-89), and the quality standards for recycled water to be used in landscape waters (GB3838-2002); thus it can be used as a source of drinking water. 3. Advantages of the CRI system: (1) Low construction cost and even lower operating expenses. The largest investment in the construction cost of a CRI system is for the filler, which is mainly river sand. Generally, the construction cost per ton of water treatment is around 800 to 1,000 RMB ； If the wastewater can flow by gravity without the need for pumping, the operating cost is less than 0.2 RMB per ton. (2) It has strong resistance to shock loads and good system stability. A CRI system with a volume of 1 m3 can handle more than 2 tons of river wastewater; its treatment efficiency is 6 times that of conventional artificial wetland systems. The COD load can range from 100 to 900 mg/L, and the system still operates stably under such conditions. (3) Emergency treatment and advanced treatment can be combined effectively, resulting in good water quality without wasting investment. In the CRI system, different water volumes can be treated by adjusting the hydraulic load; variations within a certain range of hydraulic load have little impact on the quality of the treated water. The magnitude of the hydraulic load is related to the grading of the filter media selected; therefore, by choosing filter media with different gradations, it is possible to adjust the hydraulic load and achieve various treatment outcomes. For advanced treatment, by reducing the hydraulic load, the effluent quality is better than that achieved through secondary treatment; it also offers excellent phosphorus removal capabilities and some nitrogen removal function as well. Advanced treatment can be achieved simply by partially replacing the filter media, without any need to modify other facilities, thus avoiding investment waste and enabling a combination of emergency treatment and advanced treatment. (4) It does not cause secondary pollution, and no treatment is required for the sludge. The CRI system does not need to use any chemicals; it treats wastewater through biochemical processes, thereby avoiding secondary pollution ； The sludge is digested by bacteria in the filler, resulting in no sludge formation. There is also no need to backwash the system; this is mainly done using special filter media. (5) The land area required is relatively small. The optimal depth of the filter layer in a CRI system is around 2 meters; 1 cubic meter of this filter medium can handle more than 2 cubic meters of wastewater. To treat 100,000 cubic meters of wastewater, approximately 50,000 square meters of land are needed, which is **less than that required by traditional artificial wetlands, and comparable to the land area required by conventional secondary wastewater treatment processes. 4. Prospects for the application of CRI systems: China is a country with limited per capita water resources; the ecological environment of surface water bodies has deteriorated severely, and many regions and cities suffer from severe water shortages. Severe water pollution and water scarcity in our country have become prominent problems that hinder economic development, damage the ecological environment, and affect people’s lives and health. Therefore, it is essential to adopt a combination of decentralized and centralized wastewater treatment technologies tailored to local conditions, in order to make wastewater treatment harmless and enable its resource utilization. In recent years, our country has **invested heavily in addressing river pollution. Taking Shenzhen as an example, 7 billion yuan has been allocated to clean up seven rivers within its jurisdiction, and an additional 2.5 billion yuan will be invested in the special cleanup of the Pearl River basin. Plans such as water pollution control in the Three Gorges Reservoir area, the planning of Beijing’s water systems, water environment improvement in Shanghai, and comprehensive water environment management for the South-to-North Water Diversion Project are all under implementation. Thanks to its cost-effective treatment advantages, the CRI system holds broad application prospects in the field of river water pollution control. Furthermore, the issue of treating domestic wastewater at the village and town level in our country has also become an important aspect of environmental protection there. With a large rural population and relatively weak economic capabilities, wastewater treatment technologies that require low construction costs and are easy to manage constitute the main approach for treating wastewater in villages and towns. The CRI system fits well with the specific conditions related to wastewater treatment in these areas, and it is bound to play an important role in this field. Therefore, CRI technology offers significant economic, environmental, and social benefits in the recycling of river wastewater and the treatment of municipal domestic wastewater. This post was last edited by johncom on 2009-3-20 09:22.]