Environmental protection | Wastewater treatment | USBF technology
The wastewater flows from a collecting chamber (or sewage main) through a coarse skimmed screen to Parshall flumes and further to sand traps. The traps are followed by pumping stations conveying the wastewater through the grease & oil separator and fine screens to the split chamber. The flow is further split into four equal streams draining mechanical pre-treated wastewater through distribution channels to the biological reactor for processing.
The advanced USBF system for wastewater treatment plants employs the technology of low loaded biological activation process with nitrification /denitrification and dephosphorization using unified suspended activated sludge. The activated sludge separation from mixed liquor is accomplished by up-flow sludge blanket filtration (USBF). All biological treatment and sludge separation processes are provided within the compact integrated biological reactor (IBR). The IBR contains three consecutive biological zones:
Anaerobic and denitrification zones are mixed by mechanical mixers, in the nitrification zone is a fine-bubble aeration system with very high oxygen transfer efficiency providing the oxygen delivery and mixing. The pressure air for aeration is provided by blowers. The USBF separator is build-in in nitrification zone and provides the outflow of treated water. The separated sludge from USBF separator together with nitrates from nitrification zone is recirculated into the denitrification zone, and the mixed liquor from the end of denitrification zone is recirculated to the anaerobic zone. The wastewater inflows into anaerobic zone where it meets with activated sludge recirculated from the denitrification zone. The phosphorus accumulating organisms in activated sludge take in anaerobic conditions some substances from wastewater and release some accumulated phosphorus. The mixed liquor from anaerobic zone then flows into denitrification zone, where facultative aerobic organisms in activated sludge are taking the oxygen from recirculated nitrates for oxidation and consumption of some substances from wastewater. By this process, nitrates are converted to gaseous nitrogen, which is released to air, and it thus reduces the concentration of total nitrogen in water. The mixed liquor from denitrification zone then flows to nitrification zone, where proceeds the oxidation and consumption of remaining organic substances from wastewater and ammonium is oxidized by nitrification bacteria to nitrates, which are then recirculated to denitrification as described above. The phosphorus accumulating organisms there due to preceding phosphorus release take in presence of oxygen surplus of phosphorus and convert it to deposited polyphosphates, which results in biological dephosphorization.
During biological treatment, the suspended activated sludge is therefore repeatedly exposed to nitrifying, denitrifying and anaerobic conditions. The low sludge loading combined with repeatedly changing oxic, anoxic and anaerobic conditions within the bioreactor internal closed circulation loops and the incorporation of a biological selection action (mechanically pre-treated wastewater first enters anaerobic compartment of the bioreactor), results in the formation of very specific biocenosis in activated sludge. The product is activated sludge with a low sludge volume index of typically less than 100 ml/g for sewage treatment case. The inclusion of de-nitrification within the process loop arguments pH recovery after its decrease due to nitrification, and decreases the total nitrogen content.
The described technology is made possible by the high sludge separation efficiency of the USBF. The process of separation takes place in unique separator. The mixed liquor enters the bottom of the separator and, as it rises, upward velocities decreases until the flocks of cells, formed by agglomeration of sludge particles by adhesion, become stationary and thus form a sludge blanket filtering media. Concentrated stream of flocks from the separator is then re-circulated back into the anoxic zone while the treated effluent is withdrawn on top of the separator.
The excess sludge, which is build up in the process, is constantly removed from the process by sludge pre-thickener. The supernatant water flows back to the activation through an overflow pipe while pre-thickened sludge is pumped to sludge holding tank. This tank is brokenly aerated for keeping the sludge in oxic conditions and prevention of kept phosphorus release, and during the off periods there proceeds further sludge thickening by pumping supernatant water back to activation.
USBF technology principles
First stage: In this stage, the influent was entered the system for primary sedimentation. For this stage, a minimum of 60% decrease in TSS concentration is expected.
Second stage: In this stage, raw influent (after aeration) was entered the special elimination system for organic carbon. Nitrification process could also be accomplished in this stage. The hydraulic retention time can about 2 to 8 h.
Third stage: In this stage, the wastewater was entered the denitrification stage after aeration and nitrification. Nitrate may be converted to nitro-gen gas in this stage.
Fourth stage: In this stage, the wastewater was passed from the separators and was filtered from a sludge blanket.
Fifth stage: In this stage, the pre-settled waste water was passed from the channels which were placed on the separators and then was discharged from the system.
The results in waste water treatment shows the BOD of the final effluent at different HRT – number of hours of blowing air; is lower than 20 mg/l with their removal efficiencies up to 82%. The COD of the final effluent at different HRT is lower than 23 mg/l with their removal efficiencies up to 85%. The results of BOD, COD, TSS, and turbidity of the effluent for different stages of wastewater treatment are shown in graphics. In most cases, the TSS concentration in effluent had been less than 1 mg/l and one of the main reasons was formation of compact sludge clots in the sedimentation separators of the system. This phenomenon reduced the possibility of sludge escape from the system.
BOD and COD supstance removal ratio:
Graph BOD5 biological supstance removal depending on number of hours of blowing air
Graph COD biological supstance removal depending on number of hours of blowing air HRT