Sludge Treatment and Disposal Technology (6)

26. What Is China’s Policy on Sludge Disposal?

(1) Sludge generated from urban sewage treatment should be stabilized by anaerobic, aerobic and composting methods. It can also be properly disposed of by sanitary landfill.

(2) Sludge generated by secondary sewage treatment facilities with a daily processing capacity of more than 100,000 cubic meters should be treated by anaerobic digestion technology, and the generated biogas should be comprehensively utilized. Sludge generated by sewage treatment facilities with a daily processing capacity of less than 100,000 cubic meters can be composted and comprehensively utilized. Sludge generated by sewage treatment facilities using technologies such as delayed aeration oxidation ditch method and SBR method must be stabilized. Sludge generated by sewage treatment facilities using physical and chemical primary enhanced treatment must be properly treated and disposed of.

(3) Sludge that has been treated and meets the requirements of stability and harmlessness can be used in farmland; sludge that cannot be used in farmland should be disposed of by sanitary landfill in accordance with relevant standards and requirements.

27. What Are the Main Trends in Sludge Treatment Internationally?

Different countries have formulated different policies according to their geographical environment and land resources.

Due to its extremely small land area, Japan adopts a strategy of incineration. This strategy of incineration at any cost is affordable for its economic development.

The European Community interprets sludge as a resource, so its guidance requires countries to encourage agricultural use, limit landfill, and encourage all forms of energy use from the perspective of resource utilization. Different countries have different policies based on their own geographical conditions. Germany mostly adopts drying and semi-drying before incineration, and uses a large amount of forest garbage as supplementary energy. Italy has extremely important tourism resources, restricts incineration, and encourages reduction, so most of the sludge is landfilled or used as building materials after drying. France and Spain are actively promoting policies of sludge drying and reduction and encouraging agricultural use.

In short, the necessity of sludge reduction is the main economic factor, and drying is the main technical means and trend.

28. What Are the Difficulties in Solving the Sludge Problem in China at Present?

The sludge problem in China has recently become prominent. The main difficulties are:

(1) Legislation: Legislation on sludge is obviously lagging behind, and there is no real sense of urgency for sludge producers.

(2) Valuation of disposal resources. Since the owners and managers of sewage treatment plants and landfills are the same entity (state-owned), they cannot reflect market rules.

(3) The "one-size-fits-all" environmental protection policy (such as prohibiting the use of coal, regardless of whether emissions meet standards) has led to a sharp increase in processing costs, making the processing costs of drying projects extremely expensive.

29. What Are the Methods for Sludge Reduction Treatment?

The most effective method to reduce the sludge volume is thermal drying, which can reduce the moisture content from 70% to less than 10%. In fact, according to the requirements of final disposal, the final solid content of sludge can vary between 60% and 95%. At this time, the reduction of sludge exceeds 60%, which is more reasonable in terms of cost and safety.

The cheapest method to reduce the sludge volume is composting, which uses the heat generated by the fermentation of the sludge's own organic matter for evaporation, and can reduce the moisture content from more than 70% to less than 30%. However, due to the huge land occupation and difficulty in odor treatment, it is significantly less efficient than the former, and it is difficult to apply in large cities.

The most thorough method to reduce the sludge volume is incineration, which reduces the volume to less than 15% of the original wet sludge volume. But this is also the most expensive treatment option, and generally must be considered in parallel with thermal drying.

30. Why Are the Resources for Sludge Disposal Limited?

Resources for sludge disposal mainly refer to landfills and agricultural land. Other treatment methods that help reduce the sludge volume and assist in achieving disposal goals can also be regarded as resources, including incinerators, drying plants, composting plants, etc.

For any country, using land around cities for landfill is considered a loss of resources. Sludge is obviously not suitable for long-distance transportation to uninhabited desert areas. In densely populated areas, the economic value of permanent abandonment of land resources has been prominently exposed. Some urban landfills have begun to refuse sludge for various reasons, which is a signal. Sludge may occupy more landfill resources than urban solid waste, this is mainly because of its high water content, which will reduce the utilization efficiency of landfills.