Reusing Wastewater

WRPs transform water availability scenario in Singapore

To meet the rapidly increasing demand for water by 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 wastewater treatment infrastructure was set up to cater to the needs of the country’s population of over 3 million. Thereafter, with the growing population, PUB executed a number of projects that have augmented sewage collection, treatment and recycled wastewater supply. As a result, Singapore today boasts of 100 per cent coverage of the sewerage system, enabling the sustainable use of fresh water and enhanced sanitation conditions.

The development of water reclamation plants (WRPs), a key component of water infrastructure, has completely transformed the water availabil-ity scenario in the country. Developing the water reclamation infrastructure involved not only constructing new facilities, but also changing the perspective 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 various 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 Chuang, Bedok and Seletar – were decommissioned in 2007, 2009 and 2011 respectively. At present, the infrastructure in the wastewater segment comprises three water reclamation plants of about 1,325,000 cubic metre (cum) capacity, one industrial wastewater works plant and NEWater factories.

The first WRP was commissioned at Ulu Pandan in 1961 and has a treatment capacity of 361,000 cum per day. The facility, equipped with a membrane bioreactor, was completely overhauled in 2000 using the compact and covered design system. This design requires less land and is equipped with odour minimisation facilities. As a result, the odour buffer zone around the WRP has been reduced.

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

Another water reclamation facility was set up in 2009 in Changi under Deep Tunnel Sewerage System (DTSS) Phase I. It has a capacity to treat 800,000 cum of used water per day. The treated wastewater from this facility is channelled to the Changi NEWater factory for further treatment while excess water is discharged into the sea through 5 km long deep sea outfall pipes. In addition, a 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 of used water per day. It is equipped with a pressurised microfiltration system and fouling resistant reverse osmosis (RO) membrane technology. Jurong Industrial Water Works treats used water as per 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 and Taus islands. However, at present, treated wastewater from this facility is supplied only to Jurong island.

Technology

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 then transported to wastewater reclamation facilities for treatment.

The treatment process includes four stages – preliminary, primary, secondary and tertiary. The preliminary treatment process involves the removal of debris and sand particles from used water. The used water is first lifted to a higher elevation by pumps and is then passed through automated mechanical screens to remove the debris. After this, grit-settling tanks or vortex grit chambers are used to let the heavier sand materials settle down at the bottom to separate them from the water.

After the preliminary treatment, the used water undergoes 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 particles float up to the surface of the tank. 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 fewer suspended pollutants, is subjected to secondary treatment.

Secondary treatment is done to enhance the quality of wastewater and meet the final discharge standards. At 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 in the used water to achieve a mandatory level of dissolved oxygen. This is done by blowing air through air diffuser domes placed at the bottom of the tanks. The aeration process helps mix the used water with the microorganisms and hasten the bioreaction process. This mixture of micro-organisms and treated used water is then channeled into the final clarifiers.

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

At 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 bac-teria, viruses and protozoan cysts. This water is then treated through RO technology, which uses a semi-permeable membrane that has very small pores and only allows water molecules to pass through. As a result, undesirable contaminants such as bacteria, viruses, heavy metals, nitrate, chloride, sulphate, disinfection by-products, aromatic hydrocarbons and pesticides are removed. At the end of the process, the treated wastewater is subjected to ultraviolet (UV) disinfection, which ensures that all organisms present in the water are disinfected to produce high quality water.

Application

After undergoing a rigorous treatment process, the treated used water is utilised for a number of potable and non-potable purposes. It is mainly used by industrial estates and commercial buildings. Its principal users include wafer fabrication plants, which require higher water quality than that mandated for drinking water. In addition, the treated wastewater is used for replenishing reservoirs mostly during dry periods. In March 2015, 25,000 million gallons per day of treated used water was released into reservoirs to maintain sustainable water levels.

Future plans

PUB is implementing the 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. 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, a 30 km long tunnel and 70 km of link sewers will be laid, along with the construction of a centralised WRP at Tuas. 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. Meanwhile, the phasing out of the existing WRPs at Ulu Pandan and Jurong, along with the development of new advanced facilities, is also being planned.

Conclusion

The water demand by various sectors in the country is expected to grow in the near 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 country’s water needs. With the completion of DTSS Phase II, the share is expected to increase by an additional 25 per cent.

The increasing focus of the authorities on promoting the reuse of treated wastewater has been welcomed by all segments of the economy as a collective initiative. The board has undertaken several programmes and drives to create awareness among the general public regarding freshwater scarcity and the need for consuming treated wastewater. As a result, the per capita water demand in the city-state has declined from 165 litres per day (lpd) to 150 lpd. Given the upcoming projects and continuous rehabilitation of the existing infrastructure, Singapore will soon be able to reuse 100 per cent of its reclaimed water.

 

 

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