To solve the sludge problem, it is the consensus of the industry to “dispose” and decide “treatment”. Drawing on international experience, there are four main paths for the technical development of sludge treatment and disposal in the future:
(1) Anaerobic digestion technology route based on biogas energy recovery and land utilization
It is generally believed that the cost of anaerobic digestion is low, and sludge reduction and stabilization can be achieved. According to the statistics of “China Environment News”, the investment cost of simple anaerobic digestion is about 200,000 to 400,000 yuan/(ton/day). The cost is saved due to the lack of blast aeration. The operating cost of simple anaerobic digestion is about 60- 120 yuan/ton (80% moisture content, excluding concentration and dehydration), and the operating cost of aerobic fermentation is 120-160 yuan/ton.
More than 50% of the sludge in Europe and the United States is treated by anaerobic digestion, and the generated biogas can be converted into electricity to meet 33% to 100% of the electricity required by the sewage plant. However, the application of sludge anaerobic digestion in my country is not smooth. Of the approximately 50 sludge anaerobic digestion facilities built in China, only more than 20 can operate stably. The main reasons are the poor quality of sludge and the low level of operation and management of treatment plants in my country.
In recent years, research and practice have shown that pretreatment of sludge through alkaline hydrolysis, heat treatment, ultrasonic treatment, microwave treatment and other methods can increase the hydrolysis rate of sludge and improve the anaerobic digestion performance of sludge. Sludge anaerobic digestion technology will be a mainstream direction in the future.
(2) Aerobic fermentation technology route based on land use
Aerobic fermentation has high efficiency, relatively short stabilization time, reduced moisture content and sterilization. At the same time, the finished sludge is mainly used to repair saline-alkali land, urban greening, garbage dump coverage, and construction, so as to realize the organic matter in the sludge and Efficient use of nutrient elements, less equipment investment and convenient operation and management.
However, at present, aerobic composting still has problems such as net energy expenditure. At the same time, there is a lack of theoretical research on control factors such as reasonable ventilation and C/N in different stages. Traditional slab or trough composting covers a large area and the fermented products are polluted by heavy metals, making it difficult to develop aerobic fermentation technology in my country.
In the future, the sludge aerobic fermentation project can adopt an efficient, fast, stable, and intensive design and operation mode, which can achieve a significant reduction in the area; research shows that the proportion of heavy metals in urban domestic sludge exceeds the standard by about 5%, which is a pollution risk Smaller. This technology has great application prospects in relatively underdeveloped areas.
(3) Sludge drying-incineration technical route
Sludge drying and incineration are the most harmless, but the equipment investment and operating costs are high, and the flue gas pollution produced by incineration is serious. A complete flue gas treatment system needs to be established, which also increases the cost of sludge treatment. Therefore, the dry incineration process is generally suitable for areas with tight land use and developed economies.
At this stage, when the technology of anaerobic digestion and aerobic fermentation of sludge in our country is not mature, the drying and incineration of sludge may increase in a certain period, especially the method of co-incineration in industrial kilns.
(4) Technical route of sludge drying treatment based on building materials utilization
The high-dry dehydration process can reduce the sludge moisture content to 10%. And most building materials companies have sufficient waste heat and waste heat, such as kiln waste heat and flue gas in brick and tile factories. Using waste heat to dry sludge is a popular research direction in recent years. Treat waste with waste and dry sludge at the lowest cost to achieve the goal of substantial reduction. The dried sludge is mixed into building materials in a certain proportion as raw materials to realize resource utilization.
