Treatment of solid waste

Treatment of solid waste (or waste) is the process of reducing, rendering harmless, or partially resourcing solid waste through physical, chemical, or biological means so that it can be easily transported, utilized, stored, or finally disposed of. Treatment does not completely eliminate the hazards of solid waste, but only partially solves the problem. According to the principle of the treatment method used can be divided into physical treatment, chemical treatment, biological treatment, thermal treatment and curing treatment.

(1) physical treatment

Through concentration or phase changes to change the structure of solid waste, and does not destroy the chemical composition of solid waste, so that it becomes easy to transport, storage, utilization or disposal of the form. Physical treatment of solid waste is usually used as a subsequent treatment and disposal or resource utilization before a must process, commonly used compaction, crushing, sorting, concentration, dehydration and so on.

Compaction is the use of mechanical methods to increase the degree of aggregation of solid waste, increase the weight and reduce the volume of the process. The machinery used to carry out the compaction process of solid waste is called compactor. The crushing process of solid waste is the process of reducing its particle size, making it uniform in texture, reducing porosity and increasing bulkiness. Solid waste sorting refers to the solid waste based on particle size, density, magnetic, electrical, photoelectricity, friction, elasticity and surface wettability, etc., which can be recycled or detrimental to the subsequent treatment and disposal of the components of the process of separation out of the requirements of the process. Solid waste sorting according to its nature can be divided into: screening, gravity sorting, magnetic sorting, electric power sorting, photoelectric sorting, friction and elastic sorting and flotation. Solid waste thickening is a very high water content of waste (such as sludge) in the dewatering before the pretreatment. Its purpose is to remove interstitial water from the sludge and reduce the volume of the sludge. Commonly used thickening methods are gravity thickening, centrifugal thickening and air flotation thickening. For solid wastes with high water content such as sludge, dewatering is necessary to facilitate their subsequent treatment and disposal and resource utilization. Common dewatering treatments are vacuum filtration dewatering, filter press dewatering and centrifugal dewatering. After dewatering, some treatment and resourcing processes require drying of the solid waste.

(2) Chemical treatment

The use of chemical methods to destroy the harmful components of solid waste to achieve harmless, or will be converted into a form suitable for further treatment and disposal, so that the solid waste undergoes a chemical transformation in order to recover materials and energy treatment methods. Such methods are suitable for the treatment of wastes containing a single constituent or containing several chemical constituents of similar characteristics, and include methods such as neutralization, oxidation-reduction, chemical precipitation, and chemical leaching.

Neutralization is based on the principle of acid-base neutralization and the law of conservation of mass, which can adjust the pH of solid waste to an acceptable range or neutral. It is mainly used for acidic and alkaline sludge produced in chemical, metallurgical, electroplating and metal surface treatment industries. Oxidation-reduction is through the oxidation-reduction chemical treatment, the solid waste in the valence of certain toxic components can be changed into non-toxic or low-toxic, and has the chemical stability of the components, in order to subsequent treatment and disposal and resource recovery. Chemical precipitation is industrial wastewater (such as electroplating wastewater, tannery industry in the qualified wastewater, etc.) in the heavy metals are usually used to add chemical reagents such as alkali, complexing agent, etc. to make it produce chemical precipitation and removal. For example, lime, magnesium oxide or caustic soda is used to raise the pH of the solution so that the solubility of the metal ions is reduced and metal hydroxide precipitates are formed. Chemical leaching is the addition of solid waste into a liquid solvent to dissolve one or more useful metals in the waste in the liquid solvent so that a subsequent process can extract the useful metal from the solution. This method is often used in the recovery of useful metals from solid wastes, such as the use of hydrochloric acid to dissolve chromium, copper, nickel, manganese, etc. from solid wastes.

(3) biological treatment

The use of microorganisms to decompose solid waste degradable organic matter, so as to achieve the purpose of harmless or comprehensive utilization, into through the role of some specific microorganisms, so that the nature of the solid waste changes, conducive to the dissolution of harmful components. Compared with chemical treatment, biological treatment is more economical and is now widely used in solid waste treatment. According to its degree of oxygen demand is divided into anaerobic treatment, partially anaerobic treatment and aerobic treatment. Biological treatment methods commonly used in the actual treatment of solid waste are biogas fermentation, composting, and bio-dissolution.

Biogas fermentation is a process in which organic material is decomposed by microorganisms under anaerobic conditions to produce biogas. The solid waste suitable for biogas fermentation is organic solid waste with high organic content, such as human and animal feces, sludge from sewage treatment plants, municipal organic waste, and straw from crops. The biogas produced is a clean energy source, and the methane liquid and digestate can be used as bait and fertilizer, and the digestate can be used as a substrate for the cultivation of edible fungi.

Composting is divided into anaerobic composting and aerobic composting. Aerobic composting is the most common application, is organic waste in aerobic microbial action, degradation, and at the same time make organic matter biostabilization (to the direction of the stabilization of humus conversion) process. Aerobic composting process can produce high temperatures above 55 ℃, the duration of more than 1 week, can kill parasitic eggs and pathogenic bacteria and so on. The raw materials for composting are the same as biogas fermentation, and the products of composting can be used as substrate and organic fertilizer for garden flowers.

Bio-dissolution, common biological leaching (formerly also known as biological leaching or biological leaching treatment), mainly the use of acidophilic bacillus thiobacillus in the oxidation of inorganic substances such as ferrous iron and reducing sulfur in the process of oxidation and lead to a decrease in the pH of the medium, and the activation of the heavy metals in the solid waste, dissolved into the solution, after dewatering can be removed from the heavy metals in the solid waste in the solution of the Heavy metals can be recycled. At present, Nanjing Agricultural University has successfully adopted this method to treat tannery sludge and municipal sludge, because the method also has the conditioning effect on sludge at the same time, which greatly improves the dewatering performance of sludge.

(4)Thermal treatment

The composition and structure of solid waste are destroyed and changed through high temperature in order to achieve the purpose of reduction, harmlessness and resourcefulness at the same time, including incineration, pyrolysis, roasting, sintering and wet oxidation. Incineration is a method of resourcing solid waste by controlled combustion and obtaining energy. Solid waste can be simultaneously reduced, rendered harmless and resourced by incineration. Solid waste incineration is carried out in incinerators, the main types of incinerators are grate, fluidized bed and rotary kiln incinerators and other types. Pyrolysis is most organic compounds with thermal instability, in the absence of oxygen and high temperature conditions occur cracking, the formation of molecular mass of smaller gaseous, liquid and solid products of the process. The resulting gaseous and liquid products can be used as fuel gas and fuel oil. Solid wastes suitable for pyrolysis treatment include waste plastics, waste rubber, municipal waste, agricultural solid waste and sludge.

(5)Curing treatment

A treatment process that uses inert materials (curing substrate) to fix or encapsulate hazardous wastes in order to reduce their harm to the environment, and thus make them safer to transport and dispose of. Suitable for solid waste curing treatment is mainly hazardous waste and radioactive waste, it is often hazardous waste and radioactive waste for safe landfill or shallow (deep) ground burial disposal before the pretreatment. According to the different curing agents used are divided into cement curing, asphalt curing, plastic curing and glass curing.