Heat-Based Water Treatment
Heat is the barrier that works when chlorine and UV don't: turbid water, Cryptosporidium, and — with different settings — raw milk. The engineering problem is not making heat but not wasting it, and the answer is one counterflow heat exchanger.
Pasteurize, don't boil
Boiling is a folk standard, not a microbiological one: holding water at 65–70 °C for six minutes kills bacteria, viruses, and protozoa — the WAPI (water pasteurization indicator) standard used in solar cooking for decades. Heating 25→70 °C costs 188 kJ/L; a rolling boil costs 70.8 Wh/L of electricity on the induction hob. The flow pasteurizer — a coil-pot on the hob with a thermostatic valve that only releases water that has held temperature — runs a counterflow heat exchanger at 80% effectiveness: outgoing hot water preheats incoming feed, and the net cost falls to 12.3 Wh/L. That single exchanger is the difference between "water treatment as a luxury" and treating the family's full 20 L/day for a tenth of the cooking budget (~0.25 kWh/day). Hardware: ~$95 for the coil-pot, exchanger, and WAPI-standard thermostat.
UV-C is the third barrier for clear water: a flow cell at 2 Wh/L, instant, no residual, but blocked by turbidity above ~5 NTU. The operating logic across the three: chlorine always (residual protection in storage), pasteurizer for turbid water and Crypto, UV-C for quick clear-water polish; a rolling boil remains the zero-extra-hardware fallback.
The heat couplings
The unit moves low-grade heat around like a small district system. 1 · PV rear-duct preheat (PVT): modules run 25–30 °C above ambient; a sheet-metal duct behind the array captures that waste heat to preheat the pasteurizer's feed water by ~10 °C — cutting its load about 20% — while the airflow cools the modules a few degrees for a 2–3% PV yield gain. The same warm fill feeds the laundry bay, where enzymes and surfactants work markedly better at 30–40 °C than at tap temperature: warm-wash quality at cold-wash cost. 2 · Counterflow recovery, above. 3 · Retained-heat cooking: the insulated basket finishes simmer dishes off-coil, trimming ~15% of cooking energy. 4 · Electronics waste heat warms the 35 °C incubator pocket used for overnight coliform (and milk hygiene) tests. The optional water-from-air module adds a fifth coupling: a glazed solar-thermal box at 55% efficiency does primary sorbent desorption, with the induction coil topping up through the cartridge's steel susceptor shell.
Milk: the same machine, different settings
Pastoralist milk economies ring Kakuma, and spoilage is a chronic loss for the women who dominate the trade. The flow pasteurizer is a milk pasteurizer at different settings — 63 °C held for 30 minutes (the LTLT standard) in a cleanable, foul-tolerant coil variant, because milk proteins scale where water does not. The cluster fridge is the chiller, the incubator pocket runs the hygiene tests, and the resulting cluster milk bay — pasteurize, chill, test, badge — brings dairy vendors onto the network's map exactly as water vendors join. Newborn care rides the same hardware: warm washing water from the mixing tap, and the WHO-specified 70 °C preparation water where formula is unavoidable.