Wastewater Treatment:. Aerobic wastewater treatment methods is classified according to the type of reservoir where the contaminants are oxidized. The reservoirs can be such structures as absorption fields, ponds, biofilters and aerotanks.
The aerobic oxidation (mineralization) occurs in biological ponds due to the microbes, algae and higher plants. The small depth, no currents, abundance of microalgae saturating water with oxygen and simple organisms feeding on bacteria etc.
Cultivation of higher water plants in such ponds to absorb not only a large part of biogenic elements, but also toxic substances (heavy metals, oil, phenol, nitric compounds, pesticides etc) intensifies the treatment process. Using biological ponds, both household and industrial wastewater can be treated, included mining wastewater.
Absorption fields are special plots of land populated with aerobic microbes which biochemically transform biological contamination into water and carbon dioxide.
Wide use of biological ponds and absorption fields is limited by the seasonal variations, low throughput, as well as the large areas required, along with constant control of ground water level. Artificial reservoirs, such as biofilters and aerotanks do not have these limitations.
Biofilters are special biological reactors loaded with a filtering element, which is covered with a biological film.
Thanks to the biological film, which consists of microorganisms, intensive biological oxidation processes occur. The film plays the chief role in treatment of wastewater.
The contaminated water in biofilters passes through a layer of loaded material (crushed minerals, pieces of plastic, synthetic fabrics etc), covered with biological film. Unsolved contaminants, colloids and organic materials in the water are captured by the biological film and remain in the filter material. The thickness of the biofilm formed by the microorganisms depends on the mean surface area of the material, concentration of organic material and external factors. After die-off, the film is carried out of the reservoir with water.
If the average yearly temperature in the environment does not exceed +3°С, it is recommended to place biofilters indoors with heating, if the average temperature is higher, they can be operated without external heating.
Biofilters are rectangular or round with double bottom: the lower bottom is solid, the upper is perforated. In the course of filtration, microbial film grows on the surfaces of the filter. Air is supplied through the lower part of the filter in the direction opposite to the flow of water.
Processed water goes to a settling tank where particles of the biofilm are deposited. Immobilization of biomass cells facilitates several stages of purification, with specific types of microbes.
Aerotanks are homogeneous bioreactors. They are typically concrete rectangular tanks, 3-6 meters high, equipped with aeration devices and connected to a settling tank. Aerotanks are divided into three or four corridors by screens. The types of these reactors are defined by the method of oxygen supply, the design of the reactor and the volume to material load. Treatment of water in an aerotank occurs when aerated mixture of wastewater and biological sludge pass through the tank.
Biological sludge has a complex structure; it contains many microorganisms, (thread bacteria and nitrification bacteria) and simple organisms (infusoria), with ferments to remove contaminants from wastewater. The treatment process is the continuous fermentation of contaminants. Particles of the sludge, formed by thread bacteria, on the one hand form adsorption skeleton, around which flocсules form, and on the other hand prevent formation of foam and stimulate sedimentation. The simple organisms consume bacteria, clarifying the water.
After treatment in aerotanks, water goes to settling tanks, where bio sludge is sedimented and partially returned to the aerotank.
It should be noted that most biogenic elements required for the development of microorganisms (carbon, oxygen, nitrogen etc), solve and are concentrated in wastewater. When the concentration of one of the elements in insufficient, such as nitrogen, phosphorus, potassium, such element is added to the wastewater in the form of salts.