Rooftop Rainwater Harvesting and Recharge-Well Design for a 24-Flat Bengaluru Apartment under the BWSSB Mandate

Bengaluru buys water by the tanker every summer while monsoon rain runs off its terraces into storm drains. Since the BWSSB (Amendment) Act, 2009, rainwater harvesting (RWH) is compulsory for new buildings on sites of 1,200 sq ft and more and for older buildings on sites of 2,400 sq ft and more, with a penalty added to the water bill for non-compliance. Many apartments installed a token pipe to satisfy an inspector, so the structures neither store nor recharge the required volume.

This diploma project produces a complete, buildable RWH design for a real-type G+3 apartment of 24 flats (a fictional association, Tunga Residency Owners' Association, Vijayanagar) on a 60 × 80 ft site. The team surveys the roof and paved areas, checks existing downpipes and the underground sump, and uses IMD rainfall normals to estimate annual harvest potential. The design meets BWSSB's capacity rule (20 litres per m² of roof and 10 litres per m² of paved area), routes filtered roof water to the sump for use, and sends overflow and paved-area runoff to a recharge well sized after a falling-head percolation test on site.

Deliverables include AutoCAD drawings (layout, filter section, recharge-well section), a quantity schedule, a BOQ and estimate using the Karnataka PWD Schedule of Rates, and a payback comparison against tanker purchases. The work is written for the DTE Karnataka C-20 Semester-6 Internship / Project.

Abstract

A rooftop rainwater harvesting and groundwater recharge system is designed for a 24-flat apartment in Bengaluru to meet the BWSSB mandate. Catchment areas are measured on site, annual harvest potential is estimated from IMD rainfall normals, and the storage and recharge capacity is set by the BWSSB rule. Filters, pipes, the use of the existing sump and a recharge well are designed, with the well sized from an on-site percolation test. AutoCAD drawings, a BOQ and an estimate using the Karnataka PWD Schedule of Rates are prepared, and payback is compared with tanker water costs.

Introduction

Bengaluru's groundwater levels have fallen in many wards, and borewell failures in dry years push apartments to depend on tankers. Rainwater harvesting both supplies non-potable water and recharges aquifers. The city's law sets a minimum capacity, but there is little guidance for residents on designing a system that actually works, and many installed systems lack first-flush diverters, filters or soak capacity.

Existing practice versus proposed design

• Existing: a downpipe connected directly to the sump without filtration, or a shallow pit that fills with silt within a year.
• Proposed: separate roof and paved-area streams; first-flush diversion and a filter chamber for roof water to the sump; overflow and paved runoff to a desilting chamber and recharge well with a percolation-based design.

Feasibility

• Technical: survey instruments and AutoCAD are available in the polytechnic; the percolation test needs a pit, water drums and a scale.
• Economic: the association's secretary provides drawings and permits a site visit; the design budget is within a typical association's maintenance fund.
• Operational: the 16-week project period allows survey, testing, design and drawing; the team of three splits survey, design and costing.
• Regulatory: the design follows the BWSSB capacity rule and CGWB recharge guidance, so the association can submit it with its compliance application and request withdrawal of the penalty after inspection.

The apartment's association pays a monthly penalty on its water bill because BWSSB inspectors found its existing RWH arrangement inadequate, and it buys tanker water for four to five months every year. The existing arrangement is a single unfiltered downpipe into the sump and a small pit that silted up. The association needs a design that satisfies the BWSSB capacity rule, keeps sump water clean, recharges the remaining runoff into the ground, and can be built by a local contractor from clear drawings and an estimate. The project therefore aims to design a compliant, maintainable rooftop rainwater harvesting and recharge system for a 24-flat apartment, with drawings, BOQ, estimate and payback.

The project covers one G+3 apartment of 24 flats on a 60 × 80 ft site in Bengaluru, including roof and paved areas within the plot. It does not include treating rainwater to drinking-water standards, storm-drain design outside the plot or hydrogeological modelling. Rainfall values are long-term normals; actual yearly rainfall will vary. Construction itself is outside scope.

Approach

Field survey + design + estimation, following the CGWB manual on artificial recharge, IS 15797 (rooftop rainwater harvesting guidelines) and NBC 2016 plumbing provisions, checked against the BWSSB rule.

Step 1 — Survey (weeks 1–3)

• Measure terrace area, headroom and parapet outlets; note roof finish (weathering course or tiles) for the runoff coefficient.
• Measure paved driveway and setback areas; note lawns that need not be connected.
• Record existing downpipes (number, diameter), sump capacity and borewell location.
• Level survey of the ground to set chamber invert levels and the recharge-well position at least 3 m from foundations and away from the septic line.

Step 2 — Hydrology (weeks 3–4)

• Annual harvest potential V = A × R × C, where A = catchment area (m²), R = annual rainfall (m) from IMD normals (Bengaluru is close to 970 mm; confirm the station value), C = runoff coefficient (about 0.8–0.9 for a concrete terrace, 0.6–0.7 for paved areas).
• Monthly distribution from IMD normals to estimate how much the sump can capture in peak months.

Step 3 — BWSSB capacity check

• Required capacity = 20 L × roof area (m²) + 10 L × paved area (m²). For a terrace of about 380 m² and paved area of about 120 m², this is 7,600 + 1,200 = 8,800 L minimum, met by the usable sump volume reserved for rainwater plus the recharge well and chamber volumes.

Step 4 — Component design (weeks 5–8)

• Downpipes: number and diameter checked against roof area and design rainfall intensity per NBC guidance.
• First-flush diverter: discard about the first 1–2 mm of each storm from the roof.
• Filter chamber: graded sand–gravel–charcoal or a mesh filter before the sump.
• Recharge well: concrete-ring well (about 0.9–1.0 m diameter); depth from the percolation test so that the well can absorb the design storm runoff within 24 hours; desilting chamber upstream.

Step 5 — Percolation test (week 6)

Falling-head test in a 0.3 m × 0.3 m × 0.6 m pit after pre-soaking; record fall in water level at fixed intervals; compute infiltration rate (mm/h) and infiltration capacity per m² of well surface.

Step 6 — Drawings and estimate (weeks 9–14)

AutoCAD drawings; quantity calculations; BOQ with Karnataka PWD SR rates; abstract estimate; payback = cost ÷ (annual tanker savings + penalty avoided).

Timeline