Fig.2.33. Bio-filter:
1-containing walls; 2-bottom with holes; 3-full bottom; 4-filtrating loading; 5-water supplying; 6-separating device; 7-air supplying; 8-hydraulic water-gate; 9-water-passing tray; 10-dosing device with flushing valve.
During passing of sewer water through loading of bio-filter the contamination are adsorbed by bio-layer and are oxidized by microorganisms with presence of air oxygen. There is a constant growing of dying off microorganisms inside the bio-filter. Cleared water with parts of dyed off bio-layer comes to the secondary settling tank. The bacteria are the main microorganisms that oxidize the contamination of sewer water. Besides this, micro flora and micro fauna of bio-layer are represented by different molds, worms, insects, etc. Crushed rocks, gravel, expanded clay, slag, ceramic elements, asbestos-cement elements, plastic elements, metal gauze; synthetic layers are used for bio-filter loading. The height of loading is usually assumed to be 1.5-4m but it can be considerably bigger.
Bio-filters with natural and enforced aeration are considered. By carrying capacity bio-filters are divided into drop ones (with small capacity – up to 1000m3/day) and highly loaded (up to 50000m3/day).
Reliable work of bio-filters can be achieved only during uniform irrigation of its surface by sewer water. Irrigation is carried out by special separating devices of moving or immovable type (pipes with holes, special sprinklers, reactive irrigators).
Degree of sewer water clearing mainly depends on content of sewer water, height of loading, temperature of sewer water, special air amount and hydraulic loading.
In aero tanks, like in bio-filters bio-chemical oxidation of organic substances of sewer water by microorganisms takes place. Aero tanks are the structures in which the water mixed with active mule is constantly flowing. Active mule is a mixture of aerobic microorganisms which are able to adsorb and oxidize contaminated sewer water. Quality of active mule depends on type and amount of organic contamination, presence of toxic admixtures, completeness of previous settling, duration and intensity of aeration, loading on the active mule.
Qualitative active mule is able to settle quickly and considerably. Such ability is evaluated by mule index which shows the volume of active mule in cubic centimeters (millimeters) after settling during 30 min. in relation to 1g of mule dry substance. Mule index at normal state of active mule for city sewer water doesn’t exceed 130 mc3/g. If its value is more than settling of mule mixture in secondary settling tanks is right and there is a considerable loss of mule.
Oxygen should continuously come to the aero tank for providing on normal life activity of microorganisms. Mixture of sewer water with active mule continuously aerated by systems of aeration for this (fig.2.34). Systems provide the supplying and division of oxygen or air in aero tank and support the active mule in hanged up state for better contact of mule with sewer water contamination.
