1. Sludge treatment and disposal technology
From the current international sludge treatment and disposal projects, common sludge treatment methods include aerobic fermentation (composting), anaerobic digestion, drying, and incineration. The disposal methods of sludge include land use, landfill, and comprehensive utilization. Due to different national conditions, the processing methods and technologies adopted by each country are also different.
1.1 Aerobic fermentation
Aerobic fermentation technology for sludge is a new biological treatment technology that utilizes microorganisms in sludge for fermentation. In practical applications, it can achieve harmless, reduced, and resourceful effects, and has the characteristics of economy, practicality, no need for external energy, and no secondary pollution. At present, domestic and foreign researchers have conducted extensive research on condition control, heavy metal control, nitrogen retention technology, and technical processes in the composting process, and have achieved many valuable results [2-7]. After decades of development, aerobic fermentation technology for sludge has made great progress, but there are still some bottlenecks in technical theory and process, such as the need for a large amount of auxiliary materials, difficulty in controlling odors, and the existence of health and safety risks for humans and animals. Aerobic fermentation technology still has great potential for improvement.
1.2 Anaerobic digestion
Anaerobic digestion of sludge refers to the process in which facultative and anaerobic bacteria decompose biodegradable organic matter in sludge into stable substances such as carbon dioxide, methane, and water under anaerobic conditions, while reducing sludge volume, removing odors, killing parasite eggs, and recycling the biogas produced during the digestion process. Anaerobic digestion of sludge is currently the most widely used method for sludge stabilization and resource utilization internationally, due to its efficient energy recovery and low environmental impact. Many scholars internationally have been committed to the research of anaerobic digestion technology [8], which has been widely applied and developed. Overall, the anaerobic digestion technology of sludge has not made breakthrough progress in China. The key technologies and equipment mainly rely on imports, with relatively high investment and poor operational efficiency. There are obstacles in the utilization of biogas, which together constitute the limiting factors for the promotion and application of this technology in China.
1.3 Drying incineration
Sludge incineration [9] refers to the process of heating sludge under conditions of excessive air supply, and then oxidizing, pyrolyzing, and completely destroying organic matter and pathogens at high temperatures (850-1100 ℃). There are various types of incineration devices, and currently the most commonly used ones include vertical multi-stage incinerators, rotary incinerators, fluidized incinerators, etc. In order to achieve energy-saving goals, it is necessary to first dry the sludge and significantly reduce its moisture content before incineration. Therefore, current sludge incineration projects generally adopt a combined treatment process of drying and incineration.
1.4 Land use
Land use refers to the direct or indirect use of sludge (after aerobic fermentation or anaerobic digestion) for farmland, vegetable fields, orchards, lawns, greening, and soil improvement, or the use of sludge that meets certain standards as cover soil for landfills. In recent years, the United States, Canada, and some EU countries have encouraged the use of land use technology to directly or aerobically ferment sludge that meets sludge requirements for greening, land restoration, and other purposes. The research mainly focuses on the stabilization and harmless land use methods of sludge, the fertilizer efficiency of sludge, and its value in increasing crop yields. Relevant studies have also been conducted on the potential impact of sludge on soil quality, plants, and pollution control [10-14].
1.5 Ocean dumping
The operation of ocean dumping is simple and the treatment cost is relatively low for coastal cities. However, with the strengthening of ecological awareness, people are increasingly concerned about the possible impact of sludge ocean dumping on the marine ecological environment. The United States banned the dumping of sludge into the ocean in 1988, and since the end of 1998, the European Community Urban Wastewater Treatment Act (91/271/EC) has prohibited its member states from dumping sludge into the ocean. The Chinese government accepted three international agreements in early 1994, promising not to dispose of industrial waste and sewage sludge at sea from February 20, 1994.
1.6 Sanitary landfill
Sludge sanitary landfill began in the 1960s, with simple landfill operation, low cost, and strong adaptability. Sludge can be buried separately or together with other solid waste (such as urban garbage). But there are some problems [15]: due to the high water content of sludge and the fact that the leachate is high concentration organic wastewater, it must be collected and treated to prevent secondary pollution; The difficulty of compacting machinery in landfill sites has increased; The sanitary condition of the landfill site is poor.
Current situation of sludge treatment at home and abroad
2.1 Current Status of Sludge Treatment and Disposal Abroad
The treatment and disposal of urban sewage sludge in foreign countries has a history of nearly 100 years. Whether it is for effective utilization or landfill disposal, the purpose of sludge treatment is the same as that of other waste treatment, which is to reduce, stabilize, harmless and resource. To achieve this goal, it is necessary to form a sludge treatment and disposal system through the organic combination of various machinery and processing structures. The basic steps of sludge disposal are concentration, dehydration, drying, incineration, etc., all of which can play a role in concentrating sludge. Under normal circumstances, the process flow of sludge treatment systems in foreign cities generally includes the following four categories [16]: (1) original sludge → concentration → dewatering → disposal of dewatering filter cake; (2) Raw sludge → concentration → dehydration → incineration → disposal of ash content; (3) Raw sludge → concentration → digestion → dehydration → disposal of dehydrated filter cake; (4) Raw sludge → concentration → digestion → dehydration → incineration → disposal of ash content. According to statistics from 318 sewage treatment plants in Japan [17], sludge treatment method (1) accounts for 34%; Method (2) accounts for 8.8%; Method (3) accounts for 26%; Method (4) accounts for 5.7%. The main method for the final disposal of sludge in Japan is incineration, which accounts for about 55% of the total sludge disposal. According to the estimation of the US Environmental Protection Agency, since the government issued the Water Purification Regulations in 1972, the amount of sludge has increased rapidly year by year, reaching 8.2 million tons in 2010. In the UK, according to data [18,19], the annual production of sludge generated from sewage treatment is 1.107 million tons of dry sludge. The main method of final disposal of sludge in the UK is agriculture (46.6%), followed by sludge discharge into the sea (33.5%). With the increasingly serious environmental problems, the European Community stipulated in the agreement that the deadline for discharging sewage sludge into the sea was December 31, 1998, which means that 33.5% of the sludge in the UK will be diverted to land disposal. At present, the proportion of sludge used for landfilling in the UK is relatively small, accounting for only 8% of the sludge treatment capacity. In the long run, the development direction of sludge disposal in the UK is to reuse sludge treated by anaerobic digestion, chemical or thermal treatment, and long-term storage in farmland. At present, the commonly used sludge disposal methods worldwide include agricultural use, landfill, sea disposal, incineration, etc. The United States and the United Kingdom mainly rely on agriculture, Western Europe mainly relies on sludge landfill, Japan mainly relies on incineration, and Australia mainly relies on sludge landfill and sea disposal. EU countries conducted a comprehensive analysis of the development trend of sludge disposal [20]. Due to factors such as available land area, treatment costs, increasingly strict environmental standards, and the popularization of resource recovery policies, as well as the significant changes in sludge properties in the next 10-20 years, the proportion of sludge disposal methods adopted by EU countries in 2005 was as follows: recycling accounted for 45%, incineration accounted for 38%, and landfill accounted for 17%.
2.2 Current situation of domestic sludge treatment and disposal
Some small and medium-sized cities in China have basically not built sewage treatment facilities, and even in large and medium-sized cities with sewage treatment plants, more than 90% of their sludge treatment facilities are not matched. More than 70% of the sewage treatment plants that have been built directly use untreated sludge for agricultural purposes. Even in sewage treatment plants with digestion tanks, the digested sludge is only slightly dehydrated before being directly used for agricultural purposes, making it difficult to meet the hygiene standards for sludge use. The sludge disposal technology is relatively backward compared to developed countries. From the perspective of urban sewage treatment plants already in operation in China, the sludge treatment process includes four main processes: sludge concentration, stabilization, dewatering, and final disposal. At present, China has begun to use sewage treatment plant sludge for land landfill and urban greening, and to use sludge as a substrate to produce compound fertilizers for agriculture and other purposes. But the overall situation is still mainly in the form of land use of sludge, using sludge for agriculture. Due to the insufficient attention paid to the control of physical and chemical indicators such as pathogens, heavy metals, toxic organic compounds, and sensory indicators such as odor in sludge management in China, further disposal and utilization of sludge are limited. The proportion of domestic sludge disposal technologies is as follows: agricultural utilization accounts for 44.83%, land landfill accounts for 31.03%, non sludge disposal accounts for 13.79%, greening accounts for 3.45%, incineration accounts for 3.45%, and mixed landfill with garbage accounts for 3.45%. 13.79% of domestic sludge has not been disposed of, which will pose a huge threat to the environment. The foul odor emitted by sludge is seriously polluted, pathogenic bacteria pose a potential threat to human health, and heavy metals and toxic and harmful organic compounds contaminate surface and groundwater systems. The reasons for this phenomenon include: due to the late start of sludge treatment and disposal in China, many cities have not included sludge disposal sites in their overall urban planning, resulting in many sewage treatment plants finding it difficult to find suitable sludge disposal methods and disposal sites; The foundation of sludge utilization in our country is weak, and people's understanding of sludge utilization is seriously insufficient. There is a lack of attention to the final disposal of sludge, which has left hidden dangers for the final disposal of some harmful sludge; The utilization rate of sludge is not very high, and there is still a portion of sewage treatment plant sludge that is only stored and transported by the environmental sanitation department to the suburbs for direct stacking. The arbitrary stacking of sludge can easily cause secondary pollution and waste of sludge resources. Therefore, the current problem facing our country is to develop sludge disposal technology as soon as possible to solve the growing sewage sludge.
3 Development Trends of Sludge Treatment and Disposal Technologies
In recent years, some emerging technologies have emerged, such as plasma treatment technology for sludge, which is gradually being applied to the treatment of urban organic waste. Sweden, the United States, Germany, Japan and other countries have built plasma treatment plants of certain scale, and have also developed in China in recent years [21]. The newly developed ultrasonic sludge treatment technology has not been widely used due to the efficiency and energy consumption of sound energy utilization, but it has broad prospects for joint use with other sludge treatment processes. Multiple technologies for the utilization of sludge as building materials have been relatively mature in advanced countries around the world, among which technologies such as building bricks, lightweight materials and cement materials have begun large-scale production and application or are planning large-scale production and reuse in countries such as Japan and Germany. Other treatment and disposal methods for sludge, such as sludge modification to produce adsorbents, activated carbon, use as binders, sludge oiling, and degradation of chlorinated compounds, have been studied to some extent, but are still in the exploratory research stage. After decades of development, developed countries such as Europe, America, and Japan have formed relatively complete sludge treatment and disposal technology routes [22], and the application of related equipment has also become mature. Relevant laws, regulations, and standards have been relatively complete. In recent years, Japan has made strategic adjustments to the technology route of sludge treatment and disposal, gradually shifting towards the resource utilization of sludge, and the incineration ash content of sludge is also used for the production of building materials. In summary, the overall approach to sludge treatment and disposal in developed countries such as Europe, America, and Japan is the resource utilization of sludge, with land use as the main method and encouraged direction for sludge disposal. Therefore, resource utilization technologies such as anaerobic digestion, aerobic fermentation, land use, and building materials manufacturing will be the research focus of sludge treatment and disposal internationally. While ensuring the harmless treatment of sludge, achieving the maximum utilization of sludge has become a trend in the development of the international sludge treatment and disposal field.
4 Conclusion
At present, the sludge generated in China is about 48% 28% is for land use and landfill 48%, incineration 3 45%, 13 79% have not been properly disposed of, and the overall situation is mainly in the form of land use, with the majority used for agriculture [23]. There is still a large amount of sludge that has not been properly disposed of, which may pose potential hazards to the environment. Combined with the basic national conditions of China with a large population and relatively scarce resources and energy, the sludge reuse technology is very valuable for development. It can be seen that the resource utilization and energy utilization of sludge will be an important development direction for the domestic sludge industry in the future.
