Sludge is a byproduct produced during the sewage treatment process of a sewage treatment plant. It is an aggregate formed by suspended matter, microorganisms, organic matter adsorbed by microorganisms, and products of microbial metabolic activities in sewage. It may also contain a large amount of heavy metals, pathogens, and viruses.

Although sludge contains many kinds of toxic substances, it also contains a variety of usable elements, such as various inorganic components: silicon, aluminum, iron, calcium, etc. If it is not properly treated and disposed of, it will not only cause secondary pollution to the environment, but also a serious waste of resources.

Sludge treatment and disposal is a process of reducing, stabilizing, harmless, and resource-based sludge by concentrating, conditioning, dehydrating, stabilizing, drying, or incinerating sludge.

The following briefly introduces several common sludge disposal methods, most of which are also the current sludge disposal methods of ecological companies.

I. Utilization of building materials

1. Ceramic granules made from sewage sludge

Ceramic granules are a kind of artificial stone particles with light ceramic texture. They are made of clay, mudstone, various shales, coal gangue, fly ash and other main raw materials. They are processed into granules or granulated into balls, and then burned to form artificial light aggregates. They have the characteristics of low density, high strength, good thermal insulation, heat insulation and seismic resistance. They are widely used in building materials, gardening, refractory insulation materials, chemical industry and other fields.

Sludge can replace part of clay, mix with clay in a certain proportion, and after granulation by a granulator, it is sent to the rotary kiln. At high temperature, the granules melt to produce suitable viscosity and surface tension. At the same time, chemical reactions occur inside to release gas. The gas acts on the molten liquid phase to produce pores and expand. After cooling, the surface is a dense and hard glaze layer, and the inside is a closed porous structure.

2 Cement kiln blending

The main chemical components of sludge and cement raw materials are the same. The "waste heat drying + cement kiln blending" process is used to treat sludge. Sludge can be used as an auxiliary fuel for cement clinker calcination, and can also partially replace clay raw materials to reduce the damage of cement production to arable land. By using cement kiln blending to treat urban sludge, the organic and inorganic components in the sludge can be fully utilized. The organic components will generate heat when calcined in the cement kiln, and the inorganic components will eventually be converted into cement products.

3 Sludge brick making

Bricks are made of clay, shale, coal gangue or fly ash as raw materials, which are formed and baked at high temperature and used for building load-bearing and non-load-bearing walls.

The chemical composition of sludge is similar to that of clay. The dried sludge and clay are mixed with water in a certain proportion, and then pressed into brick modules, aged at room temperature and fired into bricks. During the firing process, the organic matter in the sludge will also burn to generate heat, which can save coal. Heavy metals in sludge are formed into stable solid solutions after high-temperature roasting, which will not pollute the environment again.

II. Anaerobic digestion + drying/incineration

Sludge anaerobic digestion refers to the digestion technology in which facultative bacteria and anaerobic bacteria decompose biodegradable organic matter in sludge into CH4, CO2, H20 and H2S under anaerobic conditions. 30% to 50% of organic matter in waste can be removed and stabilized.

Sludge anaerobic digestion can decompose sludge organic matter and produce a large amount of high calorific value biogas. The digestate after anaerobic digestion is deeply dehydrated by a plate and frame filter press to reduce the amount of sludge, stabilize it, and make it a resource.

III. Composting

Composting is generally divided into aerobic composting and anaerobic composting. Aerobic composting uses aerobic bacteria and oxygen to ferment sludge at high temperature, completely decomposing the harmful organic matter in it and transforming the organic matter from an unstable state into stable humus. The compost product does not contain pathogens, weed seeds, and is odorless. It is an excellent soil conditioner and organic fertilizer.