Municipal Sludge Treatment Solutions

Municipal sludge is a sediment produced in the process of treating municipal wastewater, which is composed of inorganic particles, colloids, organic substances, bacteria, and water, and its water content, high organic matter content, and unstable properties are easy to rot. The beneficial part of it is that it contains plant-based trace elements such as nitrogen, phosphorus and potassium, but there are also harmful toxic substances such as heavy metals, pathogenic bacteria, parasites and PCBs that are not easily degradable.

By analyzing the composition of primary and residual sludge from urban wastewater plants in China, it was learned that the main components of the wastewater are freshwater compounds with a low percentage of fat and organic matter.

(i) High temperature composting of sludge

Cities are places where the population is relatively concentrated, and industrial and agricultural water and domestic sewage are concentrated and the volume is very large, which will inevitably cause serious damage to the environment if not treated properly. The water content of sludge separated from sewage is very high, and it generally has to undergo treatment such as compression, dewatering and drying, and high temperature killing. The following methods are commonly used for sludge treatment and disposal in China. Composting is actually the best use of bacteria and fungi in the natural environment. This process is the microbiological process that effectively allows these components to control the hard-to-degrade organic matter in sludge and biochemically transform them to stable humus-like substances. Currently, sludge hyperthermia composting is achieved in different ways. There are static, dynamic, aerobic and anaerobic. Static composting is done by piling the sludge on a substrate, which can be a well-ventilated material consisting of small wooden blocks, shredded grass clippings, and underneath the substrate is a wrapped ventilation duct. A blower can be placed in the middle or at both ends of the ventilation duct for air extraction.

This way the upper sludge pile can gradually neutralize harmful substances by the chemistry of oxygen. The treated sludge can be transported directly to farmland and other places and used as fertilizer. And the air blown from the blower, after deodorization, can also be discharged directly. However, the disadvantage is very obvious, namely, it is greatly affected by the weather, and when it comes to rainy days, the ventilation ducts are prone to water ingress, thus causing short circuits. But no matter which of the above composting methods, will go through three processes, namely, medium temperature, high temperature, decomposition stage. Among them, the high-temperature stage volatile odor is heavier, and oxygen consumption is also large. High-temperature composting has particularly high temperature requirements, which must reach above 60°C and continue to compost for more than 15 days. The advantage of high-temperature composting is that the treatment capacity is large, and a large amount of sludge obtained from the treatment of sewage in cities can be treated in this way, but the disadvantage is that it requires a large area of the site, heavy odor, affects the human life in the surrounding area, and costs a lot of money. In addition, the soil is seriously damaged in the place of composting.

(ii) landfill disposal

The most direct and efficient way to deal with the large amount of sludge produced after urban sewage treatment is to landfill, and the landfill mentioned here is not directly buried into the ground, but the surrounding environment, leakage treatment, and anti-permeability should be well thought out. First, we should dewater the sludge, dry it and transport it to a remote area far from the city, where it is far from water sources and where groundwater is scarce and permeability is low. Then we should build a landfill pit and use a high-density polyethylene impermeable layer as the bottom layer of the landfill pit. And consider adding a permeate collection device to prevent contamination to soil and water. The advantage of this method is that it is fast, inexpensive and saves money for the government and other responsible agencies, but the disadvantage is that there are fewer and fewer landfill sites to choose from, while sludge generation is continuous. Therefore, careful consideration needs to be given to the choice, as land resources are limited.

(iii) incineration treatment

The treatment of sludge in municipal wastewater treatment plants should not only be considered safe, cost appropriate and without after-effects, but also efficient and fast. Incineration is a method that can basically meet these requirements at the same time. Incineration treatment can kill the harmful organic substances in the sludge to the maximum extent and has a low impact on the surrounding environment. However, the sludge incineration technology in China is still being explored, and there are still many technical bottlenecks to overcome. For incineration, the sludge needs to be dewatered and dried.

The disadvantages of incineration measures are also relatively obvious, that is, the requirements for equipment are very high, need to build an incineration plant specifically in the vicinity of the sewage treatment plant, and the smoke and dust generated will be dispersed with the wind and will pollute the atmosphere without attention. In addition, made into construction materials is also a good choice, after testing, pollution is not serious, relatively little harmful substances of sludge, can be sterilized after high-temperature treatment, placed into the hollow bricks, used as a building foundation, exposed to the air, the impact is also small.

(iv) Concentration and dewatering technology

Thickening is the process of reducing the water content of sludge by thickening and effectively reducing the volume of sludge. The thickening method is subdivided into two ways: mechanical thickening and gravity thickening. Mechanical thickening has the advantages of saving floor space, low total construction cost, good operating environment and no anaerobic phosphorus in sludge compared to gravity thickening. Considering that biological phosphorus removal is applied in this project, the remaining sludge in the structure should not stay too long in order to prevent secondary phosphorus release from activated sludge under anaerobic conditions. Dewatering refers to the treatment method that separates the adsorbed and capillary water from the sludge and transforms it into semi-solid or solid clay. After dewatering, the sludge water content will be reduced rapidly, and the actual dewatering effect sludge and sediment nature and the efficiency of the dewatering equipment are related.

Further dewatering is called sludge drying, where the moisture content of the dried sludge is less than 1/10. the methods of dewatering, mainly mechanical dewatering method, natural drying method and granulation. The natural drying method and mechanical dewatering method are suitable for treating sludge in sewage. The granulation method is suitable for treating sludge in coagulation state and sedimentation. Because of the long time period required for the natural drying method, the mechanical dewatering method is used more in municipal wastewater treatment plants.

(V) Anaerobic digestion of sludge

Anaerobic digestion of sludge is a common method to reduce and stabilize sludge under anaerobic conditions, where organic matter in sludge that can be degraded by microorganisms is decomposed by parthenogenic anaerobic bacteria into carbon dioxide, methane and water, etc. to produce biogas, which stabilizes the state of sludge. The main advantages of anaerobic digestion technology are that it produces methane that can be used to generate electricity, and it can effectively reduce the concentration of organic matter in sludge and reduce the volume of solid sludge. It improves the dewatering capacity of sludge, makes sludge without significant odor, and reduces the cost of subsequent treatment. High temperature digestion can kill parasite eggs and high pathogen removal rate. There are also disadvantages in practical application such as complicated treatment methods, long anaerobic digestion time and large floor space. The initial investment and operation cost are high, and it is suitable for large scale and sewage treatment plants that treat a large amount of sludge.

(vi) Aerobic fermentation of sludge

Aerobic digestion of sludge, also known as aerobic composting, is an endogenous metabolic process in which thermophilic, aerobic thermophilic bacteria in sludge decompose organic matter and finally form a substance similar to humus soil. By filling the sludge pile with oxygen through aeration, the organisms in the activated sludge oxidize and decompose themselves, converting them into water, carbon dioxide, ammonia, etc. The process of organic degradation generates heat, which creates high temperature in the pile layer and can effectively kill microorganisms such as pathogens and parasitic eggs and stabilize the sludge, thus achieving sludge stabilization, reduction and harmlessness. The aerobic fermentation stage is usually carried out in a single fermentation, sometimes requiring drying and granulation.