Rainwater Harvesting for Factories and Commercial Sites

Rainwater harvesting on an industrial or commercial site is two decisions that often get blurred. The first is how much water the roofs and paved areas can deliver, which is arithmetic with local rainfall data. The second is what to do with it: store it for use, recharge it to the aquifer, or both. That depends on the soil, the water quality, what the site uses water for and the conditions in its approvals. A token pit drawn on the plan to pass a sanction answers neither question.

This guide follows that order: rainfall and catchment, losses and first flush, storage versus recharge, and soil, water quality and local permissions, with a catchment checklist and a decision table.

1. Rainfall and catchment

The volume available is the rainfall multiplied by the catchment area and a runoff coefficient for losses. One millimetre of rain on one square metre is one litre. IS 15797:2008, the BIS guideline for rooftop rainwater harvesting, applies a roof runoff coefficient of 0.8 to 0.95 depending on the roof. As an example, a 1,000 m² roof in a district with 800 mm of annual rainfall receives 800 kL a year, of which roughly 640 to 760 kL runs off before first-flush and overflow losses.

Use the district's rainfall normals from the India Meteorological Department rather than a state average, and look at the monthly pattern as well as the annual total. Much of the year's rain arrives in the monsoon months, often in short heavy spells, so storage fills quickly and then overflows; that timing decides how much water a tank can actually deliver.

The rainwater harvesting calculator runs this arithmetic for your roof area and rainfall, and the water demand calculator shows how much of the site's demand that water could meet.

2. Losses and first flush

• Losses. The runoff coefficient covers splash, absorption and evaporation on the roof. Further losses come from first-flush diversion, overflow when storage is full, and leaks.
• First flush. IS 15797 says the first runoff from a roof should be discarded, because it carries dust, leaves and other debris. Divert a first-flush volume for each roof segment before anything enters storage or recharge; industrial roofs with process fallout need more care than clean roofs.
• Screens and desilting. Mesh at gutters and outlets to stop leaves, then a desilting or filter chamber. IS 15797 calls for silt-free water in recharge structures, and the filtration for stored water should match its end use.
• Conveyance for the cloudburst. Gutters, downtakes and drains are sized for short-duration intensity, not the annual average. NBC 2016 Part 9 Section 2 covers the conveyance of rain and storm water within the drainage design the site needs anyway.
• Metering. A flow meter on the reuse line turns the system from a belief into a monthly number that can be checked against the water bill.

3. Storage versus recharge

Storage delivers a direct saving: every kilolitre used displaces water the site would otherwise buy or pump. Its cost is the tank, and its yield depends on demand: a tank that is drawn down between storms captures far more over a season than one that fills once and overflows. Recharge returns water to the aquifer through pits, trenches, existing dug wells or tube wells, or purpose-built recharge wells, the structures IS 15797 describes. It costs less per kilolitre handled, works only where the soil and the depth to the water table allow, and needs its silt layer cleaned periodically to keep the recharge rate.

The groundwater rules also shape the choice. Under the Central Ground Water Authority's guidelines of 24 September 2020 (amended on 29 March 2023), industries drawing groundwater need a No Objection Certificate from the CGWA or, in states with their own groundwater law, the state authority. The 2020 guidelines require rooftop rainwater harvesting or recharge in the project premises, ask for a harvesting proposal as per the Model Building Bye-Laws with the application, require industries likely to pollute groundwater (such as chemicals, pharmaceuticals, dyes, paints, textiles, tanneries and fertilisers) to store harvested rainwater in surface tanks for use rather than recharge it, and prohibit injecting treated or untreated wastewater into the aquifer.

4. Soil, water quality and local permissions

• Soil and siting. Test percolation and find the water-table depth before choosing recharge. Keep recharge structures clear of foundations, septic tanks, effluent lines, chemical stores and fuel tanks, and make sure no contaminated water can reach them.
• Water quality for the use. Flushing, cooling-tower make-up, landscaping and washing need modest treatment; drinking use needs treatment and testing against IS 10500:2012, the BIS drinking-water specification. Keep harvested-water pipes separate and labelled. Harvested water is not fire water: fire water storage is dedicated to firefighting and sized by the fire design.
• Local permissions. Building bye-laws can require rainwater harvesting at plan sanction or completion, with thresholds that vary by state and city, so read the bye-law that applies to your plot. Check the groundwater NOC and the pollution-board consent for conditions on harvesting, recharge or storm water from process areas.
• The money. Value stored water at the site's marginal cost: the tanker rate where water is bought, pumping and treatment where it comes from a borewell, or the municipal tariff. Count only what the storage can deliver given the rain pattern. The 2020 CGWA guidelines also give existing projects whose recharge structures comply and perform a 50% rebate on groundwater abstraction or restoration charges, subject to verification; confirm the current position, since the guidelines have been amended.

Rainwater systems are designed and installed under plumbing and public health engineering, with the harvesting layout, sizing calculation and compliance drawings covered under rainwater harvesting and recycling. To plan yours, share the water balance, the occupancy or shifts and any wastewater test data through a scope request; more planning guides are in Insights.

Sources and how to verify

• Bureau of Indian Standards, IS 15797:2008, Roof top rainwater harvesting: Guidelines (Ground Water and Related Investigations Sectional Committee, WRD 3): runoff coefficients, first flush, storage and recharge structures.
• Bureau of Indian Standards, Guide for Using National Building Code of India 2016: Part 9 Section 2 (conveyance of rain and storm water) and Part 11 (rainwater harvesting within the approach to sustainability) of NBC 2016.
• Central Ground Water Board, ground water regulation: the CGWA Guidelines to regulate and control ground water extraction in India (Gazette notification S.O. 3289(E), 24 September 2020) and the amendment of 29 March 2023; state groundwater authorities issue NOCs in states with their own law.
• India Meteorological Department, district rainfall statistics.
• Bureau of Indian Standards, IS 10500:2012, Drinking Water: Specification (second revision).

Confirm locally: the bye-law that applies to your plot, the conditions in your groundwater NOC and pollution-board consent, the current text of the CGWA guidelines, and the site's hydrogeology before building recharge structures.