A solar trough 2 meters long, 30 centimeters wide and 5 centimeters deep holds 30 liters, so at the carrier’s flow of 0.1 liters per minute each liter spends 5 hours in it, not 6, and only during daylight does it receive any ultraviolet. Run it as a daily batch, filling it at dawn and holding the water for at least the 6 hours of sun that the SODIS pages require, because a trough fed around the clock sends under-exposed water to the tap every morning and evening.
Part of Water Treatment at Scale
A solar continuous-flow system is a shallow, sun-lit trough or channel through which clear water moves slowly by gravity so that each part of it spends hours in sunlight before collection.
Time In The Trough Is Not Time In The Sun
The carrier’s design is a shallow, clear-bottomed trough, 2 m long, 30 cm wide and 5 cm deep, laid over a reflective surface, glass or clear polycarbonate. Water flows slowly through its full length, and “at 0.1 L/min flow rate, water gets roughly 6 hours equivalent UV exposure.” The SODIS pages in this wiki require 6 hours of strong sun, or 2 days when it is overcast (SODIS Method). Three things in the arithmetic need checking.
1. The volume gives 5 hours, not 6. Trough volume = 2 × 0.3 × 0.05 = 0.03 m³ = 30 L. At 0.1 L/min each liter takes 30 ÷ 0.1 = 300 minutes = 5 hours to cross. For 6 hours the flow must be 30 ÷ 360 = 0.083 L/min.
2. Night does not count. Water that enters late in the day is carried out in the dark with only a few hours of sun. Water that is already in the trough at dawn has had no sun at all. Only water that enters after sunrise and leaves before sunset has had a full dose.
3. The window material may block the light. The SODIS pages say most glass blocks UV-A and rule out glass bottles. I recall that many grades of polycarbonate absorb UV-A as well. PET is the material those pages endorse. The carrier’s glass or polycarbonate cover is therefore uncertain; an open, screened trough or a PET cover avoids the question. If the trough has no cover, protect it from dirt, insects and animals.
What You Need
- A shallow trough, no deeper than 10 cm (UV penetration falls beyond that), with a clear bottom or open top
- A reflective surface underneath
- Clear water, passing the read-through-the-bottle test
- An inlet valve and an outlet valve, so the trough can be run as a batch
- A cover or screen against dirt and animals, of a material that passes UV-A
Method: Run It As A Daily Batch
Step 1: At dawn, flush the trough and fill it with clear water.
Step 2: Close both valves and hold the water for 6 hours of strong sun, or longer if clouds appear; two overcast days if the sky is dull.
Step 3: Draw off the contents and store them capped.
Step 4: Refill for a second batch only if enough sun remains for another full 6 hours; usually it does not.
Worked Example: Continuous Flow Against A Batch
Assume S = 8 hours of strong sun per day (an assumed input; SODIS needs at least 6) and R = 6 hours of exposure needed.
- Batch: one trough volume per day = 30 L.
- Continuous, flow set for 6 hours: Q = 30 ÷ 6 = 5 L/h. Only water entering in the first S - R = 8 - 6 = 2 hours of sun leaves with a full dose, so usable output = 5 × 2 = 10 L. A pump or valve left open all day would pass 5 × 24 = 120 L, and 110 L of that would be under-exposed.
- Continuous at the carrier’s 0.1 L/min (6 L/h, 5-hour residence): the entering window is 8 - 5 = 3 hours, so usable output = 6 × 3 = 18 L, but each liter has had 5 hours, not 6.
- With 10 hours of sun: continuous with R = 6 gives 5 × 4 = 20 L against 30 L for the batch.
In general continuous flow gives V × (S - R) ÷ R, which beats the batch value V only if S exceeds 2R, 12 hours of strong sun for R = 6. That never happens, so a trough run as a batch always treats at least as much as the same trough run as a flow. Compare the 100-200 L/day of a bottle rack (SODIS Batch System): the 30 L trough is small by comparison.
Common Mistakes
- Taking the 24-hour flow as the output. Water at night is untreated.
- Counting time in the trough as time in the sun.
- Using any clear cover. Many do not pass UV-A.
- Running turbid water. It shields germs.
- Leaving the trough open to birds and dust.
Safety
A trough that looks like a treatment plant can deliver untreated water
Do not release trough water for drinking unless it has had the full exposure in strong sun, and do not trust it in dense cloud or rain. If you cannot be sure, boil it or chlorinate it. Because sunlight leaves no residual, store the water capped and clean. A trough roof or cover must be strong enough to hold nobody: keep children off it.
What’s Next
- Learn next: System Monitoring — checking whether a sunlight system really works
- Combine with: Chlorination — the backup that works in cloud and gives a residual