Off-Grid Water: Modular HDPE Tanks for 20-Acre Site

This Permies.com forum thread, led by permaculture expert Paul Wheaton with builder inputs and photos, documents a 3-year project on a 20-acre site achieving off-grid water independence via modular HDPE bladder tanks and concrete vaults. Roof catchment sizing for 5,000 sq ft metal roofs yielded 50,000 gallons annually in 40-inch rainfall zones, calculated as Catchment Efficiency (85%) × Roof Area × Annual Rain (in) × 0.623 gal/sq ft/in. Filtration sequence: pre-tank leaf eater, first-flush (100-gal), 20-micron sediment filter, UV sterilizer, and 5-micron carbon block. Resilience metrics from 2023-2025 droughts included zero shortages during 60-day dry spells, with overflow to ponds recharging aquifers. System layout: 10x 5,000-gal interconnected bladders in vaults, gravity-fed to greenhouses (yield: 200 gal/day irrigation). Construction details: vaults poured with 4,000 PSI concrete, fiber-reinforced, buried 4 ft deep with insulated lids; bladders fitted via NSF-61 certified fittings. Pump specs: 12V solar-direct 1.5 HP centrifugal for 150 ft total dynamic head. Lessons on scaling: modular design allows +10,000 gal/year per additional roof acre; cost $2.50/gal installed; maintenance: quarterly filter swaps, annual bladder inspections. Photos illustrate excavation (20x10x6 ft pits), backfill with pea gravel for drainage, and integration with greywater recycling (20% system input). Challenges overcome: sediment bridging via vortex inlets, algae via opaque tanks. Builders report 95% uptime, informing self-sufficiency strategies for homesteads.