Reducing the Footprint: Decentralised wastewater treatment solutions for congested urban areas

Decentralised wastewater treatment solutions for congested urban areas

Urban wastewater management in the country suffers on two counts – inadequate availability of sewage treatment plants (STPs) and dismal performance of most of the existing plants. Currently, there exists a huge deficit in terms of sewerage treatment infrastructure. Only about 27,000 million litres per day (mld) of treatment capacity has been installed against the generation of 80,000 (mld) of sewage. Further, of the total installed capacity, only 80 per cent is operational and the remaining 20 per cent is lying defunct owing to lack of adequate collection infrastructure. At present, about 65 per cent of the sewage generated is discharged into waterbodies without treatment.

To improve the dismal state of affairs, rapid development of infrastructure comprising STPs, tertiary treatment plants and faecal sludge treatment plants (FSTPs) is required. However, given the huge amount of investments needed for acquisition of large tracts of land for setting up centralised systems, a number of projects remain only on paper and actual implementation does not take place. To address this issue, several urban local bodies (ULBs) have set up decentralised wastewater treatment facilities that use less energy and land, require less by way of operations and maintenance, and have lower construction and operation costs.

Cities such as Ambikapur, Jashpur, Leh and Devanahalli, among others, have taken up pilot projects and subsequently replicated the successful small-scale decentralised systems at a pan-city level for effective wastewater management. To systematically manage faecal sludge generated in Devanahalli, a decentralised FSTP was set up in January 2016. The plant is equipped to treat about 6,000 litres of sludge per day. Despite being a new concept in India, decentralised wastewater systems are being widely accepted as a long-term solution for dealing with issues such as inadequate availability of land, lack of funds with ULBs and a rapid increase in the population in urban and semi-urban areas.

Why decentralised systems?

In contrast to a conventional wastewater treatment system, in a decentralised system, wastewater is collected, treated and reused/disposed of at or near the point of generation. This system offers numerous benefits versus conventional systems as it eliminates the need for using large diameter pipelines for transporting wastewater from one part of the city to another for treatment. Instead, small pipes for collecting small volumes of domestic wastewater are installed, and this effectively reduces the need for a longer sewer network. It also proves to be a cost-effective option particularly in semi-urban settlements and remote areas or hilly terrains where accessibility is a challenge. Further, the system can be customised based on specific requirements and provides the option of selecting different technologies based on input parameters. This is otherwise not possible in a conventional set-up. Last, but not the least, lower power and land requirements and lower conveyance costs are some of the other benefits of a decentralised system.

Striking examples

A number of cities across the country have already made the move towards developing decentralised treatment systems to overcome the issues related to setting up large sewage collection and treatment infrastructure.

For instance, Chhattisgarh has set up decentralised treatment systems across its cities in a phased manner as part of the Mission Nirmal City. In Phase I, pilot projects were taken up in Ambikapur, Rajnandgaon and Jashpur and other 165 ULBs in the state to develop small-scale decentralised FSTPs. These plants were developed using different technologies such as plant-bed technology, hybrid technology and reed-bed technology based on the type of wastewater input.

Ambikapur city lacked a centralised sewerage system and households had latrines connected to individual septic tanks. For undertaking de-slugding, trucks were used, but these were inadequate to meet the requirements of the city. To address this issue, the Ambikapur Municipal Corporation developed a 5 kilolitre per day (kld) FSTP, against a requirement of 50 kld, on a pilot basis in 2017. The plant, spread over 108 square metres, uses the hybrid technology for sludge treatment. In this technology, a step-by-step procedure for primary, secondary and tertiary treatment is followed. For primary treatment, sludge is collected in a bio-digester through an inlet chamber where all biodegradable solids are degraded using an anaerobic biological process. Thereafter, it is passed through the anoxy sump for skimming scum, from where the remaining wastewater is sent to a primary lamella clarifier to eliminate flash mix and flocculation and to reduce oil and grease, which is partially removed and solids are settled. To complete the primary treatment, the remaining matter undergoes ozonisation to remove odour, reduce biochemical oxygen demand and kill bacteria. Thereafter, the treated sludge is sent to the primary aeration tank to provide air for biological treatment using the moving-bed biofilm reactor technology.

For secondary treatment, the residual organic and suspended solids are removed through a second aeration process and by passing the primary treated wastewater through a secondary lamella clarifier. The residual waste (sludge) is then sent to a sludge drying bed for solid and liquid separation. Finally, the separated liquids are treated once again for tertiary use using micro-fine sand filters.

In terms of capital cost requirement, the hybrid technology deployed at the new plant has proved to be very cost effective. The plant was set up at a cost of Rs 2.5 million to serve 145,000 people. As against this, a similar plant based on the conventional anaerobic baffled reactor technology was set up in Odisha at a cost of Rs 35.4 million serving 220,000 households.

A similar FSTP, also based on the hybrid technology, is being developed in Jashpur. Apart from this, an FSTP based on the reed-bed technology has been developed at Rajnandgaon.

Further, the plant-bed technology is being used to develop FSTPs in 165 ULBs in the state. Under this technology, planted drying beds with phytorids are used to enable separation of solids and liquids in the faecal sludge.

In addition, the Indian Ports Association (IPA) is also setting up decentralised STPs at 10 major ports. For instance, the Mumbai Port Trust was earlier transferring its sewage to the STP set up by the Municipal Corporation of Greater Mumbai, but this proved to be a costly exercise. Thus, the IPA developed a decentralised system of 3 mld capacity on its premises to generate reusable wastewater. The treated wastewater being generated is sold by Mumbai port to Bharat Petroleum Corporation Limited for non-potable use at a fixed rate. Similarly, the Jawaharlal Nehru Port Trust has developed a 400 kld STP on its premises and set up an 80 kld STP nearby for wastewater management. Similar STPs have been developed at other ports such as the Deendayal Port Trust (800 kld) and the Chennai Port Trust (130 kld).

The road ahead

Going forward, twofold action is required to improve sewage management in the country – increasing the operational efficiency of existing plants and augmenting treatment capacity. While it may not be feasible to replace all natural treatment-based plants, retrofitting conventional plants with efficient systems will help in improving sewage treatment capacity.

Over the past few years it has been noted that a decentralised approach to sewage treatment is not only a cost-effective option for smaller towns and cities with limited finances but has also proved beneficial for big metro cities that have space constraints and competing land uses.

By 2020, on-site containment facilities will store about 60 per cent of the total urban faecal discharge. However, sewerage connections will be able to cater to only 26 per cent of the discharge. Against this backdrop, the Devanahalli FSTP should be replicated by other ULBs for efficient faecal management. ULBs in Tiruchirappalli and Gulbarga have successfully initiated the adoption of similar innovative and cost-effective faecal management practices. For the future, an integrated national-level policy is required to encourage setting up of decentralised wastewater systems across the country. In addition, ULBs need to be incentivised to set up decentralised systems. However, given the lack of funds available with these civic agencies, private participation will also have to be encouraged.