A nursery outside Coatesville called in June. They irrigate from a well that runs low in August, they have 2,400 square feet of greenhouse roof shedding straight onto gravel, and they wanted to know whether we could do something about that. We could.
This is the build log, including the part where we plumbed it wrong.
Sizing
A roof sheds about 0.62 gallons per square foot per inch of rainfall before losses. So 2,400 square feet yields roughly 1,490 gallons per inch of rain. Chester County averages around 46 inches a year, which is nominally 68,000 gallons off that one roof.
The temptation is to size for the annual figure and build a reservoir the size of a swimming pool. That is the wrong instinct. What matters is the storm and the draw-down rate: how much falls in one event, and how fast you use it before the next one.
| Storm | Yield | 3 totes (825 gal) | 6 totes (1,650 gal) |
|---|---|---|---|
| 0.25 in | 372 gal | 45% full | 23% full |
| 0.5 in | 745 gal | 90% full | 45% full |
| 1.0 in | 1,490 gal | Full, 665 gal overflow | Full, minus 160 gal |
| 2.0 in | 2,980 gal | Full, heavy overflow | Full, heavy overflow |
Their summer draw is roughly 120 gallons a day for propagation benches. Three containers is seven days of supply and fills from any ordinary storm. Six would fill less often relative to its size and cost twice as much. Three was right, and the overflow goes to a rain garden they were planning anyway.
The build
- Three used 275-gallon containers with black UV-stabilised bottles, documented non-toxic prior contents — in this case two had held food-grade glycerine and one had held vinegar.
- A welded steel stand, 26 inches, carrying all three side by side. Rated for 7,500 lb, which is three full containers plus a generous margin.
- Downpipe modification with a 6-gallon first-flush diverter on the primary gutter run.
- Stainless mesh at the inlet, at each vent and across the overflow bulkhead.
- Bottom-to-bottom 2-inch manifold with isolation valves at each container.
- Single 2-inch camlock takeoff to their existing irrigation pump, with a strainer.
- Overflow bulkhead near the top of tank three, piped to the rain garden.
What we got wrong first
The first plumbing attempt connected the containers at the top — inlet into tank one, overflow from one into two, two into three. It is the obvious arrangement and it is wrong for two reasons.
First, the tanks do not equalise. Tank one fills completely before tank two receives anything, so during a small storm you fill a third of your capacity and the rest sits empty. Second, and worse, you can only draw usable head from the tank the takeoff is on. As tank one empties, tanks two and three cannot feed it because water does not flow uphill through a top connection.
We repiped it bottom-to-bottom on day two. Now all three fill and draw as a single 825-gallon vessel, head pressure is consistent to the last hundred gallons, and isolation valves mean any one container can be taken out for cleaning without dropping the bank.
Cost
| Item | Cost |
|---|---|
| 3 × used 275 gal, black bottle, documented prior | $465 |
| Welded steel stand, 26 in, 3-bay | $610 |
| First-flush diverter, 6 gal | $78 |
| Mesh, bulkheads, fittings, isolation valves | $184 |
| 2-inch manifold and camlock takeoff | $142 |
| Overflow pipe run to rain garden | $95 |
| Labour and delivery | $266 |
| Total | $1,840 |
For comparison, a purpose-built 800-gallon poly cistern with comparable plumbing was quoted at $4,100 without the stand. The reused version is 45% of that, and it is repairable component by component rather than being a single moulded object that is finished when it cracks.
One year on
They report it has covered their propagation irrigation from June through mid-September both seasons, which is exactly the window when the well struggles. The first-flush diverter needs emptying after each storm, which is thirty seconds and the only maintenance anyone has performed.
The mesh has been the unsung component. Their neighbour built a similar system without screening the overflow and spent a summer breeding mosquitoes in it. Ours has a fine stainless screen on every opening including the vents, checked each spring, and there has been nothing living in it at all.