Lecture 8
2.4.4. Structures for the biological clearing of sewer water
Biological methods of sewer water clearing are seen in oxidation of the smallest particles of suspension, colloids and dissolved organic substances by microorganisms, which can use many organic and some inorganic contaminations that are contained in sewer water as habitat area. In the process of sewer water biological clearing the part of contamination that are oxidized by microorganisms are used in the processes of biosynthesis (biomass creation), and another part is transformed into the harmless products of oxidation: water, CO4, NO3 and others. Principles of work of modern apparatus and structures of biological clearing of sewer water are based on the methods of uninterrupted microorganisms’ cultivation.
Wide usage of biological methods for clearing is caused by their advantages: ability to remove different organic connections including toxic ones from sewer water; simple hardware equipping; relatively small exploitation expends. To the disadvantages of the method it is necessary to refer high capital expends, necessity of strict keeping the technological mode of clearing, toxic action of organic and inorganic connections on the microorganisms, necessity to dilute sewer water in case of high concentration of admixtures.
Structures intended for biological clearing of sewer water are divided into two groups. To the first group are referred the structures in which clearing of water is conducted in conditions close to natural: fields of irrigation and fields of filtration, structures of underground filtration and bio clearing ponds. In the second group of structures the clearing is carried out in artificially created conditions in biological filters and aerotanks.
Fields of irrigation are specially prepared and planed land plots that are intended for clearing of sewer water and simultaneous usage of these plots for agricultural aims.
Fields of filtration are land plots intended only for clearing sewer water without growing on them agricultural plants.
Essence of the process of sewer water clearing in soil during filtration of sewer water through soil in its top layer the hanged-up and colloid particles, which create densely populated by microorganism layer on the surface of soil particles, linger on. This layer adsorbs by its surface the diluted organic substances that are located in sewer water. Microorganisms use oxygen from atmosphere during the oxidation of contamination and transform this contamination on mineral one. Intensive oxidation takes place in top (0.2 – 0.4m) layers o soil. In deeper layers the oxygen coming is enough limited, and that’s why the processes of oxidation are performed slower there. Ground water at the territories that are used as fields should be located nit higher then 1.5 m from the surface.
Degree of sewer water clearing at the irrigative and filtration fields on average accounts 95-98% by hanged up substances and БПКповне.
Bio clearing ponds are used for clearing or additional clearing of sewer water. Oxidation of organic admixtures takes place under the action of microorganisms. Anaerobic or aerobic ponds are used for clearing. In their turn aerobic ponds can be with natural or enforced aeration. In ponds with natural aeration the treatment of sewer water depends on type and concentration of admixtures and range in terms 7-60 days. Ponds with enforced aeration have considerably less volume and the necessary degree of water clearing in them is reached in 1-3 days.
Bio-clearing ponds have a range of the considerable disadvantages that limit their usage: low oxidation ability; location on the considerable territory; seasonality of work; presence of stagnancy zones; bad management of clearing process; difficulty of structure clearing.
Bio-clearing ponds are mainly used for sewer water clearing, that passed the structures of biological clearing. Additional clearing allows to improve sanitary-hygienic indexes of cleared water, decrease the concentration of contamination, increase the concentration of oxygen diluted in water.
Bio-filters are used for partial or total biological clearing of sewer water with leading БПКповне to 15mg/l. Bio-filter is a structure (fig. 2.33), loaded with filtration material on the surface of which the microorganisms are developed (bio-layer).
