EIA Study- Final Report-May 10 - UNDP

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Environmental Impact Assessment Khan Younis Wastewater Treatment Plant May 2010 Applied to secondary treated wastewater, sand filtration allows 50-80% reduction of suspended solids and 30 to 40% reduction of BOD 5 . The wastewater treated in the previous process units can meet all standards except for pathogens. Very good removal of nematodes eggs is achieved by tertiary sand filtration. However, for other pathogens, an additional treatment step is required in order to achieve full compliance. In case of KY WWTP disinfection by UV radiation is proposed, which does not create any byproducts. However, powerful UV lamps are required. In UV disinfection systems, the UV light is produced by germicidal lamps, which are submerged in an open channel. As the wastewater flow passes the UV lamps, the microorganisms are exposed to a lethal dose of UV energy. UV-rays penetrate through small particles and attack micro organisms at their DNA/RNA-core. This mechanism causes inability to replicate. 4.2.3.4. Sludge Treatment Sludge balance: Sludge treatment works are designed for the following sludge production: • Sludge production – Phase 1................................11612 kg DS/day • Sludge production – Phase 2................................18202 kg DS/day • Excess sludge concentration ................................................8.5 g/l • Excess sludge volume – Phase 1................................ 1314 m3/day • Excess sludge volume – Phase 2................................ 2054 m3/day Gravity thickener: Thickening is the first stage in reducing the volume of the sludge. The thickening process usually corresponds to an increase in the concentration of the sludge collected in the final sedimentation tanks. Gravity thickeners are designed on the basis of hydraulic surface loading and solids loading. In order to minimize operating costs, no chemicals are added for sludge treatment at KY WWTP. Biological excess sludge enters the thickeners from above and thickener feeding is continuously. Thickeners are fitted with a centrally driven rotary mechanism with a diametric bridge. Scrapers positioned directly above the thickener floor transfer the deposited sludge to a central hopper from where it is recovered by gravity flow to the sludge pumping station. A vertically mounted picket fence is also attached to the rotary mechanism and enhances the release of interstitial water and gas contained in the sludge and allows therefore correct sludge settling and thickening conditions in the tank. Sludge drying: SOGREAH/UG CONSULTANTS 44
Environmental Impact Assessment Khan Younis Wastewater Treatment Plant May 2010 Feeder pumps at a constant flow rate to the drying beds feed sludge from thickeners. The sludge will be placed on the drying beds in a 20 to 30 cm layer and allowed to dry following two principles: • Drainage trough the sludge mass and supporting sand: Drainage water is collected in under drainage system and returned to the water treatment line upstream biological treatment. • Evaporation from the surface exposed to the air. The layer of sludge spread is limited to about 30cm thickness, in order to avoid clogging of the top layer of sand. The total drying area is divided into individual beds. Distribution boxes are used to divert the thickened sludge flow into the selected drying bed. Splash plates are placed in the drying beds in front of the thickened sludge outlet in order to prevent erosion of the sand and to spread the sludge over the bed. After drying, sludge is spendable and can be removed from the drying beds by a front end loader. For KY WWTP, moisture content in dried sludge is estimated to about 40% after 15 days. Drainage water collected under the drying beds is conveyed by gravity to a return pumping station, from where it is pumped to the distribution chamber in head of biological treatment. For conditions in KY, it is considered that 80% of sludge moisture loss in drying stage is drained and 20% is evaporated. The recycling flow of drainage water is thus estimated to 279 m3/d in Phase 1 and to 436 m3/d in Phase 2, which represents about 1% of KY WWTP inlet flow and could be neglected in terms of flows and loads. Sludge composting: After drying, the sludge produced in KY WWTP is stabilized and transformed by composting. The specific objectives of sludge stabilization include following actions: • Decompose sludge organics to stabilized humus. • Reduce the mass and volume of sludge. • Obtain a sanitized organic soil improvement agent. • Destroy/control pathogenic organisms. A mixture of dried sludge, fresh support agent and compost product will be composed using a front end loader. The mixture is than disposed on the open air composting area in shape of windrows. During fermentation phase, windrows are regularly returned in order to assure sufficient aeration, which is needed to provide oxygen for the biological oxidation and to allow evacuation of the steam released in the compost mass. After fermentation phase, a ripening and storage phase constitutes the last phase of composting. During this phase, degradation of organic matter is completed and the compost obtains the final agronomic value. The initial moisture content in dewatered sludge and the type of support agent determines the required quantities of support agent. In case of KY WWTP, where dried sludge will SOGREAH/UG CONSULTANTS 45
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EIA Study- Final Report-May 10 - UNDP

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