EIA Study- Final Report-May 10 - UNDP

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Environmental Impact Assessment Khan Younis Wastewater Treatment Plant May 2010 sludge process are determined by the composite metabolism of the microorganisms in the activated sludge. Air is introduced into the system to satisfy the requirements of the activated sludge and to keep the activated sludge dispersed in the aeration liquor. Oxygen transfer to the activated sludge is accomplished by oxygen absorbed from diffused bubbles of air entrained in the mixed liquor. A fine bubble diffused air system consisting of diffusers that are submerged in the wastewater, header pipes, air mains and the blowers and appurtenances through which the air passes is foreseen. To reduce risks of sludge bulking, a contact zone will be provided at entry of each biological tank, where pre-treated effluent is mixed with recirculation sludge. In each zone, intimate blending of the mixed liquor will be performed by a submersible stirrer that can be adjusted and raised on a beam. The stirring power will be approximately 20 W/m3. Contact zone volume is dimensioned for a retention time of 10 minutes for peak effluent flow + sludge recirculation flow. The biological tanks are of channel-flow type and are composed of an anoxic zone and an aerated zone. Reduction of carbonated pollution and nitrification is performed in the aerated zone. Partial denitrification (Nitrate nitrogen reduction) is also performed in this zone, using sequential aeration. However, denitrification needs to be completed in an anoxic zone in order to achieve the required Nitrate Nitrogen effluent discharge quality. Thus, an anoxic zone is implemented in head of the aerated zone and mixed liquor with high Nitrate concentration is recirculated from the aerated zone to the anoxic zone. Aeration system: Table 4.5 summarizes the corresponding oxygen requirements for biological treatment: Table 4.5: Oxygen requirements for biological wastewater treatment. Oxygen requirements Applied coefficient Phase 1 Phase 2 Oxygen requirements for BOD removal 0.65 9 434 kg O 2 /day 14789 kg O 2 /day Oxygen requirements for endogen respiration (sludge present in aerated zones) 0.07 5909 kg O 2 /day 8864 kg O 2 /day Oxygen requirements for nitrification Oxygen liberation by denitrification Total daily oxygen requirements 4.18 9400 kg O 2 /day 16042 kg O 2 /day 2.8 - 5063 kg O 2 /day - 8667 kg O 2 /day 19681 kg O 2 /day 31029 kg O 2 /day Aeration time 14 h/day 14 h/day Peak coefficient 1.5 1.5 Theoretical peak hourly 2109 kg O 2 /h 3325 kg O 2 /h SOGREAH/UG CONSULTANTS 42
Environmental Impact Assessment Khan Younis Wastewater Treatment Plant May 2010 Oxygen requirements oxygen requirement Applied coefficient Phase 1 Phase 2 Due to the important water depth in the aeration tanks, surface aeration system is not applicable at KY WWTP, despite of their lower costs. The required oxygen will thus be introduced in the aeration tanks as fine bubbles of compressed air. The aeration system will consist of adjustable air diffusers mounted on headers. Air diffusers consist of tubes or domes covered by a perforated rubber membrane that resists microorganisms and various compounds contained in the wastewater. Final sedimentation: Prior to clarification, and to assure correct operating conditions in the clarification stage, a deglazing structure is provided for each clarifier. Its function is to allow the bubbles of gas trapped in the turbulent mixed liquor to escape in the open air and thus sludge sedimentation in the clarifier is not disturbed. Each deglazing structure will be fitted with a surface scraper and floating matter is conveyed to a common floating matter pit from where it is conveyed to sludge treatment by submersible pumps. Deglazing structures are located between each pair of secondary clarifiers. The final step in the reduction of BOD 5 and suspended solids is a separation of activated sludge solids from the mixed liquor. Presence of the large volume of flocculated solids in the mixed liquor requires that special consideration should be given to the design of final sedimentation tank. An important function of the final sedimentation tank is to maintain the wastewater quality produced by the preceding unit processes and to remove and return sludge to the system. Four circular tanks with central feed are foreseen for the Phase 1 with additional four for the Phase 2. For sludge collection, the rotating bridge is used which rotates along the sides of the sedimentation tank. The bridge serves as the support for the sludge - removal system, which consists of a radial. Collected sludge is conveyed to a sludge pit. Since upstream of final sedimentation tanks aeration basins are located, and due to fact that large amount of solids may be lost in the effluent if design criteria are exceeded, effluent overflow rates are based on peak flow conditions. A surface scum and light sludge scraping device will trap floating matter, which is conveyed to the floating matter pit of the deglazing structure. 4.2.3.3. Tertiary Treatment The requirements for treated water quality are below the concentrations achievable with secondary treatment. Tertiary treatment is therefore required, including the following process units: • rapid sand filtration • UV disinfection SOGREAH/UG CONSULTANTS 43
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EIA Study- Final Report-May 10 - UNDP

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