1. Biochemical method

Biochemical method refers to the removal of wastewater containing heavy metals by microbial treatment of soluble ions into insoluble compounds. Sulfate bioreduction is a typical biochemical process. The law are under the condition of anaerobic sulfate-reducing bacteria through the alienation of sulfate reduction, sulfate reduction into H2S, the heavy metal ions in waste water solubility can be generated and H2S produced by reaction with low metal sulfide precipitation and has been removed, at the same time - H2SO4 reduction of SO42 - can be transferred to S2 - and make the waste water's pH value increasing. Precipitates due to the small ion product of many heavy metal ion hydroxides. Relevant studies have shown that the removal rate of wastewater containing Cr 6+ with concentration of 30-40mg /L by biochemical method can reach 99.67%-99.97% [11]. Some people also use the anaerobic digestion sludge of livestock feces to treat the heavy metal ions in the acid mine wastewater. The results show that this method can effectively remove the heavy metal in the wastewater. Zhao Xiaohong et al. [12] used Enterobacter desulphurization (SRV) to remove copper ions in electroplating wastewater, and the removal rate reached 99.12% in a solution with copper mass concentration of 246.8 mg/L and pH 4.0.

2. Biological flocculation

Biological flocculation is a kind of sewage removal method which uses microorganism or metabolites produced by microorganism to flocculate and precipitate. Microbial flocculants are a kind of metabolites with flocculating activity produced by microorganisms and secreted to extracellular cells. Generally composed of polysaccharide, protein, DNA, cellulose, glycoprotein, poly amino acid and other polymer materials, molecules contain a variety of functional groups, can make the colloidal suspended substances in water coagulate and precipitate each other. Up to now, there are about a dozen kinds of flocculation for heavy metals. The amino and hydroxyl groups in biological flocculants can form stable chelates with Cu2+, Hg2+, Ag+, Au2+ and other heavy metal ions and precipitate. The application of microbial flocculation to wastewater treatment is safe, convenient, non-toxic, no secondary pollution, good flocculation effect, fast growth, easy to achieve industrialization and so on. In addition, microbes can be genetically engineered, domesticated or constructed to produce strains with specific functions. Therefore, microbial flocculation has a broad application prospect.

3. biological adsorption method

Biosorption is a method to adsorb metal ions dissolved in water by using the chemical structure and composition characteristics of the organism itself, and then remove metal ions in aqueous solution by solid-liquid two-phase separation. Extracellular polymers are used to separate metal ions. Proteins released by some bacteria during the growth process can convert soluble heavy metal ions in the solution to precipitate and remove them. Biosorbents have been widely used because of their wide sources, low price, strong adsorption capacity and easy separation and recovery of heavy metals.

4. Aerobic biological treatment

A method for the decomposition of complex organic matter in wastewater by aerobic microorganisms. Typical organic compounds in domestic sewage are carbohydrates, synthetic detergents, fats, proteins and their decomposition products such as urea, glycine, fatty acids and so on. These organisms can be expressed as COHNS in order of the amount of elements contained in the biological system. All reactions in aerobic biological treatment of wastewater can be expressed in the following two formulae:

Microbial cells +COHNS+O2 -- → more cells +CO2+H2O+NH3

These reactions are accelerated by enzymes in the biological system. According to their catalytic reactions, enzymes can be divided into: REDOX enzyme: catalyzes the REDOX reaction of organic matter in the cell, promotes electron transfer, and makes it combine with oxidation or dehydrogenation. It can be divided into oxidase and reductase. Oxidases activate molecular oxygen and act as hydrogen acceptors to form water or hydrogen peroxide. Reductases include various dehydrogenases that activate hydrogen on the substrate and transfer hydrogen to the reduced substance by coenzymes to oxidize the substrate and reduce it by hydrogen bodies. Hydrolase catalyzes the decomposition of organic matter with water. Hydrolysis is the most basic extracellular reaction, which can decompose complex macromolecule organic matter into small molecules, making it easy to penetrate the cell wall. For example, protein is decomposed into amino acids, fat into fatty acids and glycerol, and complex polysaccharides into monosaccharides. In addition, there are enzymes such as deamination, decarboxylation, phosphorylation and dephosphorylation. Many enzymes can catalyze reactions only in the presence of some special substances called coenzymes and activators. Potassium, calcium, magnesium, zinc, cobalt, manganese, chloride and phosphate ions are indispensable coenzymes or activators in the catalytic reactions of many enzymes. In aerobic biological treatment, organic matter in sewage is oxidized and degraded by microbial enzymes in three stages: in the first stage, large organic molecules are degraded into constituent units -- monosaccharides, amino acids or glycerol and fatty acids. In the second stage, the products of the first stage are partially oxidized to one or more of the following substances: carbon dioxide, water, acetylkyiv A, α -ketoglutaric acid (or α -oxyglutaric acid), or oxaloacetic acid (also oxaloacetic acid). The third stage (the tricarboxylic acid cycle, which is the final stage of organic oxidation) is the oxidation of acetylkyiv A, α -ketoglutaric acid, and oxacetic acid to carbon dioxide and water. Organic matter releases certain energy at each stage of oxidative degradation. At the same time of organic degradation, the synthesis of microbial protoplasm also takes place. Carbohydrates, proteins, and fats are synthesized from the constituent units of the affected substance in the first stage, and the protoplasm of the cell is further synthesized. Synthetic energy is obtained by microorganisms during the oxidation of organic matter.