Terminology

• Cistern: tank for stored rainwater
• Inlet / calming inlet: entry that introduces water with minimal turbulence so settled sediment stays down
• Floating draw-off: intake that pulls water from near the surface, not the bottom
• Overflow: pipe that carries excess water out when the tank is full
• Make-up supply: refill source for dry periods
• Ballast / tie-down: weight or anchors that keep an empty buried tank from floating
• Ferrocement tank: cement or lime with sand applied over metal mesh
• IBC (intermediate bulk container): reused plastic tote in a metal cage

Standards

• Sizing: storage depends on the gap between rains; frequent rain needs less volume and a dry season needs more; use a monthly water balance
• Standards: ARCSA/ASPE/ANSI 63 and CSA B805/ICC 805 cover rainwater system design; tank-specific code provisions are in the plumbing code reference
• Outlet height: one design guide recommends the outlet about 6 in. above the tank bottom to leave room for sediment
• Water quality: a floating draw-off and a series of tanks, drawn from the last, improve quality
• Mosquito control: exclude female mosquitoes from laying eggs, add larvae-eating fish, or treat chemically

Materials

• Polyethylene: least expensive in 5,000 gal sizes; common above ground; the most cost-effective for small and mid-size tanks
• Fiberglass: highly corrosion resistant
• Concrete (precast or cast-in-place): used for large and underground tanks
• Steel: stands up to burial more easily than polyethylene but can rust and corrode
• Ferrocement: custom tanks built on site
• 55 gal drums and IBCs: low-cost small storage

Detailing

• Foundation: base supports full tank weight; buried tanks in saturated soil need ballast or tie-downs
• Sunlight: keep stored water out of light to limit algae
• Inlet and outlet: calm inlet, outlet above the bottom, no turbulence that stirs sediment
• Overflow: sized to the inflow, screened against pests, discharged away from foundations and neighbors, with no shutoff valve
• Vent: screened; kept separate from sanitary vents
• Access: lockable manhole or service port; confined-space warning where people can enter
• Drain: pump or drain at the low point with a cleanout
• Make-up water: backflow protection and an air gap
• Freeze protection: protect tank and piping where sustained freezing occurs
• Labeling: capacity and nonpotable warning on the tank
• Test: fill to the overflow, hold 24 hours, and verify overflow, drain, and make-up shutoff
• Setbacks: from lot lines, septic tanks, seepage pits, and tree roots as the adopted code requires

Uses

Specific uses

• Residential rainwater harvesting for irrigation and flushing
• Commercial and institutional nonpotable supply
• Fire protection storage where permitted
• Stormwater retention for irrigation reuse
• Agricultural and livestock water

Pros and cons

Pros

• Polyethylene and fiberglass tanks are light, available in many sizes, and quick to install
• Concrete tanks can be buried and carry loads

Cons

• Concrete and underground tanks need excavation and structural design
• Steel can corrode; polyethylene is more flexible and may need backfill care
• Large tanks are heavy when full and need engineered foundations
• Sediment and algae need periodic cleaning

Manufacturing and cost

Construction

• Polyethylene tanks are rotationally molded; fiberglass tanks are filament-wound or hand-laid; concrete tanks are precast in sections or cast in place; steel tanks are welded or bolted

Main manufacturers and suppliers

• Norwesco
• Snyder Industries
• Graf USA
• Rainwater Management Solutions

Trade association: ARCSA, American Rainwater Catchment Systems Association.

Typical costs

• 5,000 gal tank, material only: polyethylene about $2,000 to $4,000; fiberglass about $2,000 to $5,000; concrete or steel about $3,000 to $6,000
• Per gallon, 5,000 gal polyethylene or fiberglass: about $0.40 to $0.80 above ground and $0.80 to $1.60 below ground
• 10,000 gal concrete or polyethylene cistern: about $0.50 to $1.00 per gal
• 1,000 gal installed: polyethylene about $2,000 to $4,000; steel or concrete about $4,000 to $8,000

Sources

• Grafman, L.: To Catch the Rain, section 2.5 storage, https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)/02%3A_Components/2.05%3A_Storage
• Auburn University Extension: rainwater harvesting, https://ssl.acesag.auburn.edu/natural-resources/water-resources/watershed-planning/stormwater-management/documents/Chapter4-6RainwaterHarvestingPRINT.pdf
• Wikipedia: rainwater harvesting, https://en.wikipedia.org/wiki/Rainwater_harvesting
• North Carolina Department of Agriculture: cistern design manual, https://ncagr.gov/soil-water/swcccap-cistern-design-manual/open
• Fixr: rainwater collection system cost, https://fixr.com/costs/rainwater-collection-system
• Image: Rainwater harvesting tank (5981896147).jpg, SuSanA Secretariat, CC BY 2.0, https://commons.wikimedia.org/wiki/File:Rainwater_harvesting_tank_(5981896147).jpg