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Used Water Conventional Treatment Process

    Used water from both domestic and non-domestic sources is collected through a comprehensive network of underground public sewers. Used water from trade sources and industries are required to meet certain standards before it can be discharged into public sewers. The collected used water is treated at the five water reclamation plants of PUB to render it clean for reuse or discharge into watercourses.

    Preliminary Treatment

    The preliminary treatment process is designed to remove the debris and sandy materials from the used water. Upon arrival at the water reclamation plant, the used water is first lifted up to a higher elevation by pumps. This allows the used water to flow through the various treatment tanks by gravity. The used water then flows through automated mechanical screens to remove the debris. This is followed by grit settling tanks or vortex grit chambers to settled out and remove the heavier sandy materials present in the used water.

    Primary Treatment

     

    The used water which is now free of debris and sandy materials flows through very slowly across large tanks called primary clarifiers. The process allows the solid pollutants in suspension in the used water to settle to the bottom of the tanks. It also allows the light materials like scum, greasy materials to float up to the surface of the tank. The settled solids known as primary sludge is collected through the help of scrapers at the bottom of the tanks and is removed regularly for treatment. The floating scum is collected and combined with the sludge for further treatment. The top water, which contains much less pollutants in suspension, leaves the primary clarifiers for secondary treatment.

    Secondary Treatment

     

    The secondary treatment comprises the aeration tanks which is a bio-reactor and final clarifiers. The used water is mixed with a culture of micro-organism known as activated sludge in the aeration tank. The micro-organism absorbs and breaks down the organic pollutants in the used water. To sustain the biological activities in the aeration tanks, oxygen from the air has to be dissolved into the used water to maintain certain level of dissolved oxygen in the used water. This is achieved by blowing air through air diffuser domes placed at the bottom of the aeration tanks to create fine air bubbles in the aeration tank. In some of the plants, mechanical surface aerators are used to stir up the used water and splash it into the air to help to dissolve the oxygen into the used water. The aeration process also helps to mix the used water with the micro-organism to promote the bio-reaction process. By the time the used water reaches the end of the aeration tanks, most of the pollutants would have been absorbed by the micro-organism. The mixture of micro-organism and the treated water is then channelled into the final clarifiers.

    At the final clarifiers, the micro-organism settles to the bottom of the tanks. The clear supernatant water at the top of the tank is collected and discharged from the tanks as final effluent. The micro-organism which settles to the bottom as sludge is constantly drawn out from the final sedimentation tank. A portion of the sludge is constantly returned back into the aeration tanks to maintain a desired concentration of micro-organism in the aeration tank to sustain the optimal bio-reaction process. The excess activated sludge is sent for further treatment.

     

    Final effluent

    The final effluent meets the discharge standards of 20 mg/l biochemical oxygen demand (BOD) and 30 mg/l total suspended solids (TSS). Part of the final effluent is further treated to industrial water which is supplied to the industries in Jurong Island. The final effluent is also further treated using advanced membrane and reverse osmosis technologies to high grade water called NEWater. The NEWater is supplied to the industries for use in the industrial processes to conserve potable water

    Sludge Thickening

    Raw sludge collected from the primary sedimentation tanks and excess activated sludge from the secondary treatment process contains a high percentage of water. The water content of the sludge is reduced by using dissolved air flotation thickener or centrifuge. The thickened sludge is fed into anaerobic sludge digesters for further treatment.

    Sludge Digestion

     

    In the digesters, another culture of micro-organism thriving in an oxygen-deficient environment breaks down the organic substances in the sludge. The sludge is allowed to remain in the digesters for 20 - 30 days. The digestion process converts the organic matter into biogas which contains 60 - 70% methane. The biogas produced is used as fuel to power dual-fuel engine generators to generate electricity. The electricity is used to provide part of the electrical energy required for the operation of the plant.

    Sludge Dewatering

     

    The digested sludge is still relatively wet, thus making it difficult for disposal. The water content has to be substantially reduced to facilitate handling and final disposal. This is achieved through mechanical means using equipment such as dewatering centrifuges. The dewatered sludge is currently incinerated and the ash is disposed off at Pulau Semakau Landfill.

    Last updated on 2 Mar 2013