While a single downpour can deliver thousands of litres of water to a rooftop, in most cities much of it rushes away unused into stormwater drains. In Coimbatore, civic engineers are turning that fleeting resource into a steady supply for non-potable needs through large-capacity rainwater harvesting and reuse systems now being installed across civic offices.
At the Central Zone office in Race Course, the ground-level storage tank with a capacity of 100,000 litres is being built. Rainwater collected from the rooftops will be brought through a network of pipelines to a two-stage filtration system. Stage one is about a removable stainless steel dirt basket that will be regularly cleaned to avoid silt building up. The second stage involves fine filtration to keep the quality of the water in good condition so that the stored supply does not stagnate. A 7 HP pump will pump water twice a day from the underground reservoir to nine overhead tanks of 1,000 litres each to meet the estimated 8,000 litres per day demand for flushing toilets and watering the gardens.
The project follows on the heels of the success of the system commissioned in July at the Corporation’s main office in Town Hall. That installation caters to a 25,000 sq. ft. catchment area, drawing from 40 rooftop points, and stores up to 1.08 lakh litres in an underground tank. According to Corporation Commissioner M. Sivaguru Prabakaran, The system at the corporation’s main office is designed to collect about 1.08 lakh litres of water with a European filtration system… The stored water can be managed for about 10 weeks. The filtration unit, sourced from 3P Technik Filtersysteme GmbH, is engineered for high throughput and low maintenance, a critical factor in sustaining performance during peak rainfall.
Excess water from such tanks is then diverted to nearby groundwater recharge structures, easing pressure on stormwater drains and replenishing aquifers. This twin approach-reuse for day-to-day use and recharge for longer-term security of water-strictly adheres to the principles of sustainable urban water management identified in economic feasibility studies of rainwater harvesting systems. In fact, such a design can potentially reduce municipal tap water use by 60-80% in public buildings with minimal flood risk during monsoon saturation.
Engineering decisions in Coimbatore’s systems reflect learning from large-scale projects around the world. Filtration units whose parts are easily serviced reduce operation and maintenance costs, crucial for long-term viability. Storage sizing matches rainfall patterns and demand cycles; avoiding over-sizing decreases the capital outlay without proportional benefit. Larger tanks or modular expansion will be needed for climates with seasonal variation in rainfall to bridge the dry months, but cost-benefit analysis using metrics such as Payback Period, Net Present Value, and Benefits-Costs Ratio is required to justify such investment.
Beyond conventional tanks, the Corporation is exploring harvesting structures based on eco-bloc units made from recycled polyethylene. These modular units, lined with geotextiles and fitted with stone filters, minimize siltation and are designed for a service life of more than ten years. Already piloted in parks and schools, such systems store runoff for irrigation, with planned installations having capacities of up to 1.5 million litres. The eco-bloc approach ensures rapid deployment and low maintenance, hence suitable for decentralized applications in flood-prone residential areas.
Technical integration is also key. It ensures that these pumps, controls, and backflow prevention devices deliver reliably to the end-use points without compromising municipal supply networks. Flow meters with data logging capacity enable the accurate monitoring of production and consumption for performance optimization over time. Regular inspection schedules, such as weekly for collection surfaces and inlet filters, monthly for pumps and overflow channels, and annual professional testing for backflow preventers, are instituted within the maintenance protocols to maintain efficiency.
As the Corporation goes about replicating these systems in other offices, schools, and parks, the strategy showcases how urban infrastructure can adapt to climate pressures by combining engineered reuse with recharge. In so doing, Coimbatore is setting a precedent not only in reducing its dependence on borewell and municipal water but also for integrated, cost-effective rainwater management in Indian cities.
