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Odours are controlled early in the procedure 
Sewage travels slowly through primary sedimentation tanks (there are seven of them) and suspended organic matter settles to the bottom, where a slow moving scraper drags the raw sludge to the hopper. The sludge is pumped to digesters for separate treatment. Floating scum is skimmed off with water sprays and is pumped into the digesters. This primary treatment removes 30 per cent of the organic pollution load and up to 60% of the suspended solids. Then the effluent is pumped to the top of two trickling filtering towers, which are 8 metres high and 53m in diameter. There, a biological slime is produced containing micro- organisms that feed on the dissolved organic pollution as liquid trickles over a plastic grid structure down through the towers. Portions of the slime are constantly breaking away and are washed into secondary sedimentation tanks. This part of the system will be improved under the upgrading. Currently submerged pipes take the liquid to oxidation ponds. At this stage about 80% of the remaining organic pollution load, and up to 30% of the remaining suspended solids, have gone. Clear liquid remains in the ponds for three weeks, mainly to kill bacteria by sunlight’s ultra- violet disinfection. Filter pumps in the engine room pump wastewater over the filters. pic: “Scum collector” lights on the control panel (in the control room) tell staff members whether or not every part of the complex process is working well. |
