In the environmental protection industry, sludge treatment and disposal remains a relatively underdeveloped area, and it is one in which our country urgently needs to accelerate its modernization.

With the expansion of urban sewage treatment capacity, sludge is generated as a by-product of the treatment process. Based on an estimated average dry-solids content of 0.02% in urban sewage, daily production amounts to 31,400 tonnes of dry sludge. Considering that the moisture content of raw sludge prior to dewatering is 80%, this translates into 157,000 tonnes of wet sludge per day. Over a 360-day year, total wet-sludge production nationwide reached 56.52 million tonnes in 2014.

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.

However, China’s sludge disposal rate remains very low. In 2016, the national sludge treatment capacity was approximately 13 million tonnes per day, yet the national sludge treatment rate was only 33%. This means that about 67% of sludge was not treated in an environmentally safe 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: The treated sludge is disposed of in the natural environment (on land, underground, or in water) or reused, thereby achieving long-term stability and ensuring that the final disposal method has no adverse impacts 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

In the early days, sewage treatment plants often neglected sludge treatment altogether and focused solely on wastewater treatment, resulting in sludge being indiscriminately dumped in lakes, gullies, and fertile farmland. To cut costs, some plants leave their sludge treatment facilities underutilized or operate them negligently, leading to uncontrolled sludge discharge. As a result, more than 80% of the sludge generated by sewage treatment plants in China is currently not 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 technologies primarily encompass sludge thickening and dewatering, aerobic digestion, anaerobic digestion, drying, composting, and incineration. Sludge disposal technologies mainly include landfilling (including surface, underground, and aquatic landfills) and land application.

Some people mistakenly believe that sludge drying and incineration is the current state-of-the-art sludge treatment technology and represents the future direction of sludge treatment, leading to its uncritical promotion. Certain companies, in pursuit of profit, blindly overstate its advanced features, thereby misleading many who lack a thorough understanding of the subject.

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 inclusion of sludge treatment fees within sewage treatment charges is a general trend; however, at present, there is significant resistance to the collection of such disposal fees. Currently, sewage treatment fees in China are relatively low, which makes it difficult to ensure the normal operation of wastewater treatment plants. The introduction of sludge treatment and disposal fees as an additional component of sewage treatment charges will, to some extent, increase the financial burden on fee payers. Consequently, subsidies will remain the primary source of funding for sludge treatment and disposal for the foreseeable future.

6. “Resourceization” is not the ultimate goal; the ultimate goal is the protection of the ecological environment.

The misconception that sludge is a resource emphasizes the resource utilization and economic benefits of sludge treatment and disposal, positioning resource recovery as the primary objective. Some companies exploit this misunderstanding by claiming that individual unit processes can achieve energy recovery and material reuse, thereby isolating the energy and cost requirements of other treatment and disposal stages and misleading both technology selection and the broader understanding of sludge resource utilization.

Sludge treatment and disposal should aim at “reduction, stabilization, and harmlessness.” “Resource recovery” is not the ultimate goal. The energy and materials contained in the sludge treatment and disposal process should be utilized to the greatest extent possible to achieve economic benefits and to realize energy savings and resource value.

3. How to solve the sludge problem

1. Identify the responsible body and improve the management system

The sewage treatment plant serves as the government’s implementing agency and is therefore unable to assume responsibility on its own. The government should increase its investment in sludge disposal and provide financial subsidies and tax incentives. Furthermore, sludge treatment fees should be incorporated into the overall sewage treatment fee.

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

If sludge treatment is handled improperly, the wastewater treatment company shall bear primary responsibility. Of course, this assumes that the wastewater charge includes the cost of sludge treatment.

3. The selection of the technical approach must be tailored to local conditions.

The physical, chemical, and biological characteristics of sludge produced by wastewater treatment plants vary significantly across different regions. Therefore, it is recommended that sludge characteristics, geographic location, environmental conditions, and levels of economic and social development be taken into account when selecting the process route, and that sludge treatment and disposal methods be determined on a case-by-case basis 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 pathway based on biogas energy recovery and land utilization

Anaerobic digestion has the following advantages:

1) Enhance the efficiency of downstream processing and reduce its energy consumption. It is widely recognized that anaerobic treatment can achieve sludge reduction and stabilization. During anaerobic digestion, 40%–60% of the organic matter in the sludge is removed, and the population of pathogenic bacteria is significantly reduced. Furthermore, anaerobic digestion improves the dewaterability of the sludge, enabling subsequent treatments such as incineration to achieve energy savings of 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 sludge in Europe and the United States is treated via anaerobic digestion, with the resulting biogas converted into electricity to meet 33%–100% of the wastewater treatment plant’s electricity needs.

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) Reduced 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) Furthermore, earthworm bio-compost was developed to enhance the composting process, as exemplified by the collaboration 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.

After fermentation, the sludge is converted into humus, which can be used for agricultural purposes, landscaping, or soil improvement, thereby achieving high utilization of organic matter and nutrient elements in the sludge while requiring low capital investment and offering convenient operation and management.

However, the large-scale production and heavy metal contamination of fermented products pose significant challenges, making it difficult to advance aerobic fermentation technology in China.

Currently, the aerobic fermentation project for sludge can adopt a fast, stable, and intensive design and operational mode, which can significantly reduce the required land area. In addition, research indicates that the heavy-metal content in urban domestic sludge exceeds regulatory standards by approximately 5%, posing a relatively high pollution risk. However, this factor should not become the primary obstacle restricting the land-use application of sludge fermentation products.

Therefore, the “Urban Sewage Treatment Plant Sludge Treatment Technology Guide (Trial)” also lists “aerobic fermentation + land use” as a recommended technical route. This technology holds significant potential for application in relatively underdeveloped regions.

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 achieves both treatment and disposal, while subsequent composting requires consideration of energy consumption associated with storage and transportation. Moreover, the incineration of organic matter in sludge is carbon-neutral. In addition, there is a common misconception that sludge incineration shares the same characteristics as municipal waste incineration as a source of dioxin emissions.

The dry incineration process requires a relatively large capital investment, generates significant flue-gas pollution, and necessitates the installation of a comprehensive flue-gas treatment system, thereby increasing sludge treatment costs. Consequently, dry incineration is generally best suited for regions with limited land availability and well-developed economies.

As understanding of carbon emission reduction and sludge biomass resources continues to deepen, the scope of application for dry incineration processes in foreign countries has begun to shrink. However, at the present stage—when anaerobic digestion and aerobic fermentation technologies for sludge in China are still not fully mature—sludge drying and incineration may see increased adoption over a certain period, particularly the co-incineration approach in industrial kilns.

4. Technical Route for High-Dry Sludge Dewatering Treatment Based on the Use of Building Materials

The prevailing public perception of high-solids sludge dewatering technology remains that it relies heavily on the addition of large quantities of chemical agents, resulting in volume reduction without actual resource recovery; moreover, the impact of these chemicals on downstream applications such as sludge incineration, land application, and utilization in construction materials is often overlooked. As a result, this approach is viewed as a temporary, emergency-based solution for sludge treatment and disposal.

The high-solids dewatering process currently in use does not achieve reduction through the addition of large quantities of chemicals, nor is it integrated with subsequent disposal, which will impede the advancement of sludge treatment technology and give rise to the “bad money drives out good” phenomenon.

Source: Environmental Protection Water Circle