Ensuring sustainability in consumption-

Owing to the rapid increase in water demand from industrial and domestic consumers, the Public Utilities Board (PUB), Singapore, has been actively undertaking initiatives aimed at reusing the wastewater generated in the country. The first such initiative was taken in the 1910s when used water treatment infrastructure was set up to cater to the needs of a 3 million-plus population. Thereafter, in sync with the growing population, PUB has executed numerous projects which have augmented sewage collection, treatment and recycled wastewater supply services. As a result, today Singapore boasts of 100 per cent coverage of the sewerage system, which has enabled sustainable use of fresh water along with enhanced sanitation conditions.

Establishment of water reclamation plants (WRPs), one of the major water infrastructure developments, has completely transformed the water availability scenario in the city-state. Developing water reclamation infrastructure involved not only the construction of new facilities, but also changing the perspectives of various players towards the consumption of treated wastewater. By renaming the six sewage treatment plants at Bedok, Jurong, Kim Chuan, Kranji, Seletar and Ulu Pandan as WRPs, PUB highlighted its objective of promoting the use of wastewater for potable and non-potable purposes.

Existing infrastructure

To begin with, Singapore had a total of six WRPs – Bedok, Jurong, Kim Chuan, Kranji, Seletar and Ulu Pandan. However, three WRPs – Kim Chuan, Bedok and Seletar – were decommissioned in 2007, 2009 and 2011 respectively. Now, the existing infrastructure in the wastewater segment comprises four WRPs of about 1,325,000 cubic metre (cum) capacity, one industrial wastewater works and NEWater factories.

The first WRP was commissioned at Ulu Pandan in 1961 and has a treatment capacity of 361,000 cum per day. Equipped with a membrane bioreactor, the facility was completely overhauled using the compact and covered design system in the year 2000. This design not only requires less land but is also equipped with odour minimisation facilities. This has reduced the odour buffer zone around the WRP.

The second WRP was commissioned in 1981 at Jurong to treat industrial effluent and domestic sewage through conventional activated sludge technology utilising mechanical surface aerators. At present, the plant has a capacity of 164,000 cum per day which is planned to be expanded by another 55,000 cum per day under the Phase 4 expansion project. The treated wastewater generated at the facility is supplied to the Jurong Industrial Estate in Jurong East, Jurong West, Jurong West Extension and part of Bukit Batok as well as the Tuas, Pulau Samulun and Tanjong Kling areas.

Another water reclamation facility was set up in Changi under the Deep Tunnel Sewerage System Phase I in 2009. It has a capacity to treat 800,000 cum of used water per day. Upon treatment, the wastewater is channelled to the Changi NEWater factory for further treatment while excess water is discharged into the sea through a 5 km long deep sea outfall pipe.

Besides, one WRP and an industrial waterworks plant have been set up at Kranji and Jurong respectively. The Kranji plant has a capacity to treat 72 million litres per day (lpd) of used water. It has been equipped with a pressurised microfiltration system and fouling resistant reverse osmosis (RO) membrane technology. The Jurong Industrial Water Works treats used water to industrial water standards to curb the consumption of fresh water by industries. Commissioned in 1961, this plant had an initial capacity to treat 45,000 cum of used water per day, which was later expanded to 125,000 cum per day to supply water to industries in Jurong Island and Tuas Island. However, at present, treated used water from this facility is supplied only to Jurong Island.

Treatment process

To reclaim used water from the domestic and non-domestic sectors, an organised system of collection, treatment and supply has been established by the utility. Used water is collected through a network of underground sewers and transported to wastewater reclamation facilities for treatment.

The treatment process consists of four stages – preliminary, primary, secondary and tertiary. The preliminary treatment process involves removal of debris and sand particles from the used water. The used water is first lifted to a higher elevation by pumps and then flows through automated mechanical screens that remove the debris. This is followed by grit settling tanks or vortex grit chambers that settle and remove heavier sand material.

After preliminary treatment, the used water undergoes the first stage of treatment called primary treatment. The used water is transferred into primary clarifiers. In these tanks, solid pollutants settle down at the bottom, whereas light materials like scum and greasy materials float up to the surface of the tank. Thereafter, the settled solids known as primary sludge are collected through the help of scrapers at the bottom of the tanks and removed regularly for treatment. Along with the primary sludge, the floating scum is also collected for further treatment. The top water, which contains lesser suspended pollutants, is subjected to secondary treatment.

The secondary treatment is done to enhance the quality of the wastewater and to meet the final discharge standards. In this stage, the primary treated used water is mixed with a culture of microorganisms known as activated sludge in the aeration tank. The microorganisms present in the activated sludge absorb and break down the organic pollutants in the used water. Further, to maintain the biological activities in the aeration tanks, oxygen from the air is dissolved into the used water to maintain a mandatory level of dissolved oxygen. This is achieved by blowing air through air diffuser domes placed at the bottom of the aeration tanks. The aeration process helps to mix the used water with the microorganisms and accelerate the bioreaction process. This mixture of microorganisms and treated used water is then channelled into the final clarifiers.

At the final clarifiers, the microorganisms settle at the bottom of the tanks. The clear water at the top of the tank is then collected and discharged as final effluent. Upon treatment, the final effluent satisfies the discharge standards of 20 milligrams per litre (mg/l) biochemical oxygen demand and 30 mg/l total suspended solids.

In the final stage of treatment, known as tertiary treatment, the treated used water is passed through membranes to filter out suspended solids, colloidal particles, disease-causing bacteria, viruses and protozoan cysts. This water is then treated through RO technology, which uses a semi-permeable membrane. The semi-permeable membrane has very small pores which allow only water molecules to pass through. Consequently, undesirable contaminants such as bacteria, viruses, heavy metals, nitrate, chloride, sulphate, disinfection by-products, aromatic hydrocarbons, pesticides, etc. are removed by this membrane. Lastly, the treated wastewater is subjected to ultraviolet disinfection that ensures all organisms present in the water are disinfected to produce high quality water.

Reuse of treated used water

After undergoing a rigorous treatment process, the treated used water is utilised for a number of potable and non-potable purposes. The treated wastewater is mainly used by industrial estates and commercial buildings. The principal users include wafer fabrication plants that require water quality higher than that mandated for drinking water.

In addition, the treated used water is used for replenishing reservoirs mostly during dry periods. Recently, in March 2015, 25,000 million gallons per day of treated used water was released into the reservoirs to maintain sustainable water levels.

Future plans

PUB is implementing the Deep Tunnel Sewerage System (DTSS) project to develop a network of sewerage pipelines and WRPs that will provide sewerage facilities for the next 100 years. The project has been divided into two phases, and the development of Phase I was completed in 2008 at an estimated cost of SGD 3.4 billion. Under this phase, a 48 km long deep sewer tunnel running from Kranji to Changi, 5 km long deep sea outfall pipes and 60 km of link sewers were laid. In addition, the Changi WRP was also constructed as part of the project. Phase II, on the other hand, involves extending sewerage facilities to the western part of Singapore. Under this phase, 30 km of south tunnel and 70 km of link sewers will be laid and a centralised WRP at Taus will be constructed. This WRP will be integrated with a NEWater factory to facilitate greater recycling and reuse of used water. In addition, the Tuas WRP is planned to be co-located with NEA’s Integrated Waste Management Facility to tap the potential synergies of used water and solid waste treatment processes. Plans are also afoot to phase out the existing WRPs at Ulu Panda and Jurong and develop new advanced facilities.

Conclusion

The water demand by various sectors of the city-state is expected to grow in the foreseeable future. In such a situation, the initiatives aimed at increasing the utilisation of treated wastewater will greatly support PUB in augmenting its water supply. At present, reclaimed water meets about 30 per cent of the state’s water needs. With the completion of DTSS Phase II, the share is expected to increase by another 25 per cent.

The increased focus on promoting reuse of treated wastewater has been welcomed by all segments of the economy. Several programmes and drives have been conducted by the board to create awareness amongst the general public regarding freshwater scarcity and the need to consume treated wastewater. As a result, the per capita water demand in the city-state has declined from 165 lpd to 150 lpd. Given the continuous rehabilitation of existing infrastructure and the upcoming projects, the city-state will soon be able to reuse 100 per cent of its reclaimed water.