In the environmental protection industry, sludge treatment and disposal is still a relatively backward field. It is also an area in which my country urgently needs to accelerate its upgrading.

With the expansion of urban sewage treatment scale, sludge is also produced as a by-product of sewage treatment. According to the estimation of the average solid content of urban sewage of 0.02% in terms of dry matter, it can produce 31,400 tons of dry sludge per day. Based on the 80% moisture content before sludge dewatering, 157,000 tons of wet sludge are produced every day. Counting 360 days throughout the year, in 2014, 56.52 million tons of wet sludge was produced nationwide.

The sludge has a complex composition, containing pathogenic microorganisms, parasite eggs, toxic and harmful heavy metals, and a large amount of refractory substances. If not handled properly, it is easy to cause secondary pollution to the environment.

But our country's sludge disposal rate is very low. In 2016, the national sludge treatment capacity was about 13 million tons/day, and the national sludge treatment rate was only 33%. About 67% of the sludge was not treated in a harmless manner, posing a serious threat to the ecological environment.

1. Definition of sludge treatment and disposal

sludge handling or sludge treatment: The sludge is treated by a combination of unit processes to achieve the whole process of "reduction, stabilization, and harmlessness".

The current main sludge treatment methods in my country include concentration, conditioning, dehydration, stabilization, and drying.

sludge disposal (sludge disposal): The treated sludge is disposed of in the natural environment (ground, underground, water) or reused, which can achieve long-term stability and a final disposal method that has no adverse effects on the ecological environment.

The main sludge disposal methods in China currently include sanitary landfill, land use, and use of building materials after incineration.

2. Problems in sludge treatment and disposal

1. The sludge treatment rate is extremely low

Early sewage treatment plants even ignored the sludge treatment unit and only carried out sewage treatment, but the sludge was dumped at will in lakes, gullies, and fertile fields. In order to save costs, some sewage plants leave sludge treatment facilities empty and discharge sludge at will. More than 80% of the sludge produced by sewage treatment plants in my country has not been properly treated.

2, the development of heavy water, light sludge, and sludge treatment is lagging behind

In recent years, water treatment in the field of environmental protection has developed rapidly, but sludge treatment has not made much progress compared to a decade ago. The proportion of sludge that has been harmlessly treated is low, and most of the sludge discharged into the environment is still harmful, and even illegal discharge incidents are not uncommon. This is due to the immature treatment thinking of "heavy water over mud".

3. The technical route is mechanically copied

Sludge treatment technology mainly includes sludge thickening and dewatering, aerobic digestion, anaerobic digestion, drying, composting and incineration. The sludge disposal technology mainly includes landfill (including ground, underground and water) and land use.

Some people mistakenly believe that sludge drying and incineration is the current advanced sludge treatment technology, which represents the development direction of sludge treatment technology, so it is promoted without analysis. Individual companies blindly exaggerate their advanced nature for the purpose of making profits, which misleads many people who don't understand.

4. Supervision is difficult

Due to the long-term neglect of sludge treatment and the intermittent discharge of sludge, it is difficult for the relevant government departments to supervise the sludge.

5. The payment mechanism is not perfect

The levy of sludge treatment fees in sewage treatment fees is a general trend, but from the current situation, there is a big resistance to the collection of disposal fees. my country’s current sewage treatment fees are relatively low, and the normal operation of sewage plants cannot be guaranteed. The implementation of adding sludge treatment and disposal fees to sewage treatment fees will increase the economic burden of the expropriated to a certain extent. Therefore, subsidies will be the main source of funds for sludge treatment and disposal for a long period of time in the future.

6. "Resourceization" is not the ultimate goal, but the protection of the ecological environment is the ultimate goal

Misunderstanding that sludge is a resource, emphasizes the resource utilization and economic benefits of sludge treatment and disposal, and takes resource utilization as the primary goal. Individual companies use this misunderstanding to emphasize that individual unit processes can achieve energy recovery and material reuse, split the energy and costs required for other treatment and disposal processes, and mislead the choice of technology and the understanding of sludge resource utilization.

Sludge treatment and disposal should aim at "reduction, stabilization, and harmlessness". "Resource" is not the ultimate goal. The energy and materials in the sludge treatment and disposal process should be used as much as possible to achieve economic benefits. And the effect of saving energy, realizing its resource value.

3. How to solve the sludge problem

1. Identify the responsible body and improve the management system

The sewage treatment plant is the government's implementing agency and cannot independently assume responsibility. The government should increase its investment in sludge disposal and grant financial subsidies and tax incentives. Implement the sludge treatment fee into the sewage treatment fee.

2. Enterprises and governments should regard sludge disposal as a responsibility

If the sludge treatment is improper, the sewage treatment company will assume the primary responsibility. Of course, the premise is that the sewage charge must include the cost of sludge treatment.

3. The choice of technical route needs to be adapted to local conditions

The physical, chemical and biological characteristics of the sludge produced by sewage treatment plants in different regions are obviously different. Therefore, it is recommended that the sludge characteristics, geographical location, environmental conditions, and economic and social development levels should be considered when selecting the process route. , Determine the sludge treatment and disposal methods according to local conditions.

Fourth, the mainstream technology of the future

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

Anaerobic digestion has the following advantages:

1) Improve the efficiency of subsequent processing and reduce the energy consumption of subsequent processing. It is generally believed that anaerobic reaction can achieve sludge reduction and stabilization. Through anaerobic reaction, 40%-60% of organic matter in sludge is removed, and harmful bacteria are reduced. In addition, anaerobic digestion improves the stability of sludge dewatering, allowing subsequent treatments such as incineration to reduce energy consumption by more than 35%.

2) The cost of anaerobic digestion is lower. 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 biogas produced is converted into electric energy to meet 33%-100% of the electricity required by the sewage plant.

However, the application of sludge anaerobic digestion in my country is not smooth. Among the approximately 50 sludge anaerobic digestion facilities built in China, only more than 20 can operate stably. The main reason is the poor quality of sludge and the low level of operation and management of treatment plants in my country. my country's sludge has high sand content, low organic content, poor biodegradability, and indicators such as the stability of digestion equipment and the rate of biogas production generally do not meet foreign standards. In addition, my country lacks an incentive mechanism for the utilization of biogas, the investment cost of equipment is high, and the operation of the system is relatively complicated and difficult to master.

However, pretreatment of sludge by 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. And through the accumulation of project experience, the company has gradually mastered more comprehensive operating skills. Sludge anaerobic digestion technology will be a mainstream direction in the future.

2. Technical route of aerobic fermentation based on land use

Aerobic composting is the process of converting sludge into fertilizer through the fermentation of microorganisms under aerobic conditions. Among them, organic materials are metabolized into carbon dioxide, water and heat.

The advantages of 　　 aerobic composting include:

1) High fermentation efficiency and relatively short stabilization time;

2) Less odor, achieving sterilization;

3) The moisture content can be reduced to 40%;

4) The finished sludge is mainly used to repair saline-alkali land, urban greening, garbage dump coverage, and construction.

5) And derived the earthworm bio-compost to strengthen the composting effect, such as the cooperation between Xingrong Environment and Green Mountain.

The main difficulties of 　　 composting include:

1) Net energy expenditure, ventilation energy consumption accounts for 80%;

2) It is necessary to strengthen research on the reasonable ventilation rate at different stages of aerobic composting operation;

3) The lack of theoretical research on control factors such as C/N has resulted in excessive use of conditioning additives.

The sludge is transformed into humus after fermentation, which can restrict agricultural use, landscaping or improve the soil, so as to realize the high utilization of organic matter and nutrient elements in the sludge, with low equipment investment and convenient operation and management.

But the disadvantages of large area and heavy metal pollution in fermented products make it difficult to develop aerobic fermentation technology in our country.

At present, the sludge aerobic fermentation project can adopt a fast, stable, and intensive design and operation mode, which can achieve a significant reduction in the area; in addition, research shows that the proportion of heavy metals in urban domestic sludge exceeds the standard by about 5%, and the pollution risk is relatively high. Small, should not become the main obstacle to restrict land use of sludge fermentation products.

Therefore, in the "Urban Sewage Treatment Plant Sludge Treatment Technology Guide (Trial)", "aerobic fermentation + land use" is also listed as a recommended technical route. This technology has great application prospects in relatively underdeveloped areas.

3. Sludge drying-incineration technical route

For a long time, Chinese people have misunderstood the sludge drying and incineration process, and generally believe that it is a process with high energy consumption and high carbon emissions. In fact, the energy of sludge incineration in the world can be self-sufficient. From the energy consumption of different processes, the incineration process (~100kW/t) is equivalent to the composting process (>100kW/t).

Incineration realizes treatment and disposal, and the subsequent composting needs to consider energy consumption such as storage and transportation. Moreover, the incineration of organic matter in sludge is carbon neutral. In addition, people also mistakenly believe that sludge incineration has the same characteristics as garbage as a source of dioxin emissions.

The equipment investment of the dry incineration process is relatively large, and the flue gas pollution produced by incineration is serious, and a complete flue gas treatment system needs to be established, which also increases the sludge treatment cost. Therefore, the dry incineration process is generally suitable for areas with tight land use and developed economy.

With the continuous deepening of the understanding of carbon emission reduction and sludge biomass resources, the scope of application of dry incineration processes in foreign countries has begun to decrease. However, at this stage, when the technology of anaerobic digestion and aerobic fermentation of sludge in my country is not yet mature, sludge drying and incineration may increase in a certain period of time, especially the method of co-incineration in industrial kilns.

4. Technical route for high-dry sludge dewatering treatment based on the use of building materials

Everyone’s general understanding of the high-dry sludge dewatering technology still lies in the addition of a large number of chemical agents, resulting in volume reduction and no reduction; and the effect of the agents on subsequent sludge incineration, land use, building materials utilization and other products; it is temporary and emergency Technical route of sludge treatment and disposal, etc.

The high-dry dehydration process currently used does not achieve a reduction effect by adding a large amount of chemicals, and it is not combined with subsequent disposal, which will hinder the development of sludge treatment technology and lead to the phenomenon of bad money driving out good money.

Source: Environmental Protection Water Circle