Used Water Treatment

Primary Clarifier (covered)

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

Conventional Activated Sludge Process


Aeration Tank

The conventional activated sludge process comprises the aeration tanks (bioreactors) and final clarifiers. The used water is mixed with a culture of micro-organism known as activated sludge in the bioreactor. The micro-organism absorbs and breaks down the organic pollutants in the used water. In order to sustain the biological activities in the bioreactor, a certain level of dissolved oxygen has to be maintained in the used water. This is achieved by blowing air through air diffusers to create fine air bubbles in the aeration tank. The aeration process also helps to mix the used water with the micro-organism to promote the biological 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 activated sludge 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 sludge which settles to the bottom is constantly drawn out. A portion of the drawn sludge is returned back into the aeration tanks to maintain a desired concentration of micro-organism in the tank to sustain the optimal bio-reaction process. The excess activated sludge is sent for further treatment.




Final Clarifier (covered)

Membrane Bioreactor


Membrane Bioreactor Tank (covered)

Since 2006, PUB has been adopting the membrane bioreactor (MBR) technology for water reclamation in Singapore. MBR is a 3-in-1 solution that combines conventional bioreactors, secondary sedimentation tanks and microfiltration/ultrafiltration (MF/UF) in one single step.

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

Thickening Centrifuge

Raw sludge collected from the primary sedimentation tanks and excess activated sludge collected from the secondary treatment process contain a high percentage of water. The water content of the sludge is reduced through a thickening process and the thickened sludge is fed into anaerobic sludge digesters for further treatment.

Egg-shaped Digester

With the digested sludge still relatively wet, disposal is difficult. Using mechanical means such as dewatering centrifuges, water content in the digested sludge is substantially reduced to facilitate handling and final disposal. The dewatered sludge is incinerated and the ash is disposed at Pulau Semakau Landfill.

Dewatering Centrifuge

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 is then sent to biogas engines to generate electricity. The electricity produced is used to offset the energy consumption of the water reclamation plants.

In Changi WRP, dewatered sludge is further dried with the use of sludge dyers for the ease of disposal. The dryers, which are powered by biogas, dries the sludge into granulate particles before incineration.

PUB also treats greasy waste collected from eateries and food industries. Grease traps installed at various locations throughout Singapore pre-treat culinary waste by removing the oil and grease before discharge into sewers. Licensed collectors maintain these grease traps and transport the collected greasy waste to WRP for treatment.