Rainwater Harvesting for London Victorian Terraces: Water Butts to Underground Tanks

SIMPLE ABOVE-GROUND WATER BUTTS: THE MOST STRAIGHTFORWARD FORM OF RAINWATER HARVESTING FOR A LONDON VICTORIAN TERRACE IS ONE OR MORE ABOVE-GROUND WATER BUTTS CONNECTED TO THE DOWNPIPE DRAINAGE SYSTEM: - DIVERTER FITTING: A RAINWATER DIVERTER (ALSO CALLED A WATER BUTT KIT OR DOWNPIPE DIVERTER) IS FITTED IN THE DOWNPIPE — IT DIVERTS THE FLOW FROM THE DOWNPIPE INTO THE WATER BUTT WHEN THE BUTT IS NOT FULL, AND ALLOWS EXCESS TO CONTINUE DOWN THE DOWNPIPE TO THE DRAIN WHEN THE BUTT IS FULL. COMMON DIVERTERS: GARLAND (BLUE MARBLE) FLAT-BACK WATER BUTT 100L (FITS INTO A CORNER), GRAF PLATIN SLIM 300L, PROSPERO WATER BUTT 200L. DIVERTER FITTINGS: PURCHASED WITH THE BUTT OR SEPARATELY (WHITEFURZE STANDARD DIVERTER, GARDENA ROOF COLLECTOR). - FIRST-FLUSH DIVERTER: THE FIRST RAINWATER TO RUN OFF A ROOF CARRIES THE MOST CONTAMINATION (DUST, BIRD DROPPINGS, MOSS PARTICLES, ALGAE). A FIRST-FLUSH DIVERTER DIVERTS AND DISCARDS THE FIRST VOLUME OF RUNOFF (APPROXIMATELY 1 LITRE PER 10M² OF ROOF) BEFORE ALLOWING THE CLEANER SUBSEQUENT FLOW TO ENTER THE STORAGE VESSEL. A FIRST-FLUSH DIVERTER IS PARTICULARLY VALUABLE WHERE HARVESTED WATER IS INTENDED FOR ANY CONTACT USE (WATERING VEGETABLES, FILLING A PADDLING POOL) OR FOR WC FLUSHING WHERE SYSTEM HYGIENE IS IMPORTANT. - WATER BUTT USES: GARDEN WATERING (HOSE ATTACHED TO THE BUTT TAP) AND OUTDOOR WASHING (CAR, PATIO). NOT SUITABLE FOR DRINKING, COOKING, OR DIRECT BODY CONTACT WITHOUT FURTHER TREATMENT. - LIMITATIONS: A TYPICAL 200L WATER BUTT FILLS FROM A HEAVY LONDON SHOWER IN MINUTES AND THEN SPILLS DIRECTLY TO DRAIN — IT CAPTURES A SMALL PROPORTION OF THE ANNUAL CATCHMENT POTENTIAL. IN A TERRACE WITH A SMALL REAR GARDEN, SPACE FOR MULTIPLE LARGE BUTTS IS OFTEN LIMITED.

CONNECTING TO CAST IRON DOWNPIPES: IN A LONDON VICTORIAN TERRACE WITH ORIGINAL CAST IRON DOWNPIPES, FITTING A WATER BUTT DIVERTER REQUIRES: - IDENTIFYING THE DOWNPIPE SIZE (65MM OR 75MM ROUND CAST IRON — THE MOST COMMON) - PURCHASING A CAST-IRON-COMPATIBLE DIVERTER (SOME UNIVERSAL DIVERTERS FIT; OTHERS ARE SIZED FOR 68MM UPVC ONLY — CHECK THE FITTING BEFORE PURCHASE) - ALTERNATIVELY: FIT A SHORT SECTION OF ADAPTOR COUPLING (CAST IRON TO UPVC PUSH-FIT ADAPTOR — AVAILABLE FROM CAST IRON RAINWATER GOODS SUPPLIERS) IN THE DOWNPIPE AT THE POINT OF DIVERSION, THEN FIT THE STANDARD UPVC DIVERTER

UNDERGROUND RAINWATER HARVESTING TANKS: FOR A LONDON VICTORIAN TERRACE WHERE ABOVE-GROUND SPACE IS LIMITED AND THE HOMEOWNER WANTS TO HARVEST RAINWATER FOR WC FLUSHING (WHICH ACCOUNTS FOR APPROXIMATELY 30-35% OF HOUSEHOLD WATER CONSUMPTION — 50 LITRES PER PERSON PER DAY IN AN AVERAGE LONDON HOUSEHOLD), AN UNDERGROUND RAINWATER HARVESTING TANK IS THE APPROPRIATE SOLUTION: - TANK SIZES: 1,500L (ENTRY LEVEL), 3,000L, 5,000L (COMMON SIZE FOR A FAMILY HOME WITH WC FLUSHING AND GARDEN USE), 7,500L. FOR A LONDON VICTORIAN TERRACE WITH A 55M² ROOF CATCHMENT AND 600MM ANNUAL RAINFALL, A 3,000-5,000L TANK PROVIDES APPROXIMATELY 3-6 WEEKS OF STORAGE AT TYPICAL WC FLUSHING DEMAND — CAPTURING A SUBSTANTIAL PROPORTION OF SPRING AND AUTUMN RAINFALL. - TANK MATERIALS: POLYETHYLENE (COEXTRUSION HDPE — THE MOST COMMON AND COST-EFFECTIVE FOR DOMESTIC UNDERGROUND TANKS: VAREM, AQUAPOINT, RAINHARVESTING SYSTEMS LTD); GLASS-FIBRE REINFORCED PLASTIC (GRP — STRONGER BUT MORE EXPENSIVE, USED FOR LARGER COMMERCIAL INSTALLATIONS OR WHERE GROUND CONDITIONS REQUIRE HIGHER STRUCTURAL INTEGRITY); CONCRETE (RARE FOR NEW INSTALLATIONS — HIGH WEIGHT, DIFFICULT INSTALLATION). - FILTRATION: RAINWATER MUST BE FILTERED BEFORE STORAGE — A CALTECH VORTEX FILTER OR WISY VORTEX FINE FILTER FITTED BEFORE THE TANK INLET REMOVES LEAVES, DEBRIS, AND SUSPENDED SOLIDS (FILTER MESH: 0.28MM). WITHOUT PRE-FILTRATION, SEDIMENT ACCUMULATES IN THE TANK AND BLOCKS PUMP INTAKES. - PUMP AND DISTRIBUTION (WC FLUSHING SYSTEM): A SUBMERSIBLE PUMP (GRUNDFOS SB SUBMERSIBLE PUMP, FLOATRONICS PUMP) IN THE TANK DELIVERS HARVESTED RAINWATER TO THE WC CISTERNS THROUGH A SEPARATE PIPEWORK CIRCUIT (COLOURED GREY OR MARKED "RAINWATER — NOT FOR DRINKING" TO DISTINGUISH FROM THE POTABLE MAINS SUPPLY). A MAINS BACKUP (AUTOMATIC TOP-UP VALVE — FILLS THE TANK FROM THE MAINS WHEN THE RAINWATER LEVEL IS LOW) ENSURES CONTINUITY OF WC FLUSHING EVEN IN DRY PERIODS. THE BACKUP SUPPLY MUST BE DESIGNED WITH AN AIR GAP (TYPE AA OR AB AIR GAP) TO PREVENT BACKFLOW CONTAMINATION OF THE MAINS SUPPLY — THIS IS A WATER REGULATIONS REQUIREMENT.

LONDON VICTORIAN TERRACE PRACTICALITY ISSUES: - CLAY GROUND: MUCH OF LONDON IS ON LONDON CLAY — INSTALLING AN UNDERGROUND TANK IN CLAY REQUIRES EITHER (A) CONCRETE SURROUND (TO PREVENT FLOTATION WHEN THE TANK IS EMPTY AND THE WATER TABLE RISES AFTER RAIN) OR (B) SPECIFIC ENGINEER APPROVAL OF TANK BALLASTING. MOST DOMESTIC HDPE UNDERGROUND TANKS REQUIRE A CONCRETE HAUNCH AND COLLAR IN CLAY GROUND. - PLANNING: UNDERGROUND RAINWATER HARVESTING TANKS (BELOW GROUND) ARE GENERALLY PERMITTED DEVELOPMENT IN A RESIDENTIAL GARDEN — BUT CHECK WITH THE LOCAL PLANNING AUTHORITY IF THE PROPERTY IS IN A CONSERVATION AREA OR IF THE INSTALLATION INVOLVES SIGNIFICANT EXCAVATION CLOSE TO PARTY WALLS. - INSTALLATION: THE TYPICAL INSTALLATION INVOLVES: EXCAVATION OF A PIT (TYPICALLY 2.5-3.5M DEEP × 2.0-2.5M WIDE FOR A 3,000-5,000L TANK), SAND BEDDING, TANK PLACEMENT BY MINI-DIGGER + CRANE LIFT, CONCRETE HAUNCH, INLET FILTER CONNECTION FROM DOWNPIPE, PUMP AND CONTROLS INSTALLATION, SEPARATE DISTRIBUTION PIPEWORK TO WC CISTERNS, MAINS BACKUP FLOAT VALVE WITH AIR GAP.

COSTS (SUPPLY AND FIT, LONDON 2025-2026): WATER BUTT (200L WITH DIVERTER — SUPPLY AND SELF-FIT): £40-100 WATER BUTT WITH PROFESSIONAL INSTALLATION: £150-300 UNDERGROUND TANK (3,000L — SUPPLY, EXCAVATION, AND FIT): £3,000-6,000 UNDERGROUND TANK (5,000L — SUPPLY, EXCAVATION, AND FIT): £4,500-8,000 WC FLUSHING PUMP AND CONTROLS (ADD TO TANK COST): £500-1,200 MAINS BACKUP TOP-UP VALVE WITH AIR GAP: £200-500