Making & Dosing Chlorine
Chlorine is the only water barrier that keeps protecting after the bucket is filled — and the unit makes it from table salt for 0.009 Wh per liter. Making it is the easy half; this chapter is mostly about the harder, better-studied half: where the dose goes in.
Salt in, hypochlorite out
Brine electrolysis — the WATA class of devices — passes current through salt water across MMO-titanium electrodes and produces sodium hypochlorite at roughly 4.5 Wh and 4 g of salt per gram of active chlorine. The electrodes last years; the only consumable is salt. Dosing drinking water at 2 mg/L therefore costs 0.009 Wh/L — energetically free against every other barrier on the unit. Chlorine's two real limits: it fails against Cryptosporidium, and it loses potency in turbid water — which is why the pasteurizer (chapter 4) exists as the second barrier.
- Household cellBatch electrolysis, ~$90; the family's 20 L/day needs 40 mg of chlorine — 0.2 Wh/day; one weekly batch covers drinking water plus surface disinfectant
- Camp scale10,000 people × 15 L/day at 2 mg/L = 300 g Cl/day — 1.35 kWh and 1.2 kg salt: one cluster kitchen's spare afternoon capacity (~$2,600 of cells, $220/yr salt)
- Outbreak modeThe same cell surges to produce 0.2–0.5% surface-disinfection solution for cholera response
- SafetyHydrogen off-gas venting on the cell; hard physical separation from acids; DPD residual test strips close the loop
- Chemistry synergyChlorine pre-oxidizes As(III)→As(V), sharpening the iron-oxide arsenic bed (chapter 6)
Where the chlorine goes in
A decade of field evidence says the design decision that determines whether water actually gets treated is where the dose is introduced. Manual household chlorination — a bottle of hypochlorite and a measuring cap — works in supervised trials and then collapses in sustained use, because it demands a measured daily action from every household. The work of Amy Pickering's group (now at UC Berkeley) showed the alternative: move the dose upstream, into the water's path, so treatment happens without anyone doing anything.
In urban Dhaka, Bangladesh, her team ran a double-blind cluster-randomized trial across 100 shared water points fitted with passive in-line dosers that chlorinate automatically, without electricity, as water flows to the tap. Households collected water exactly as they always had. Treated taps carried a detectable free-chlorine residual 83% of the time (mean 0.37 ppm) against 0% at control taps, and child diarrhea fell by nearly a quarter. In Kisumu County, Kenya, the group field-tested an electricity-free Venturi doser at commercial water kiosks: flowing water pulls liquid chlorine into itself through the Venturi effect — no moving parts, 97% dosing consistency, 98% of all water sold carrying a residual and 86% inside the 0.2–1.2 mg/L target band. Just as telling: kiosk owners paid for it — over two-thirds bought the device outright by the end of the six-month lease. Chlorination succeeds as ambient infrastructure and fails as a daily chore.
The SolarHearth dosing chain
The electrolysis cell never asks a household to measure anything: its hypochlorite fills a small reservoir feeding an in-line Venturi doser on the unit's treated-water tap, so every liter drawn is dosed on its way into the storage container. At the cluster kiosk the configuration is exactly Kisumu's — a Venturi at the point of collection — with one upgrade: the on-site cell replaces the purchased-hypochlorite supply chain, so the consumable is table salt. The dose is tuned to the 0.2–1.2 mg/L residual band — enough to protect stored water through the next day, below the taste threshold that drives quiet abandonment — verified by DPD strips, and preserved by a narrow-mouth, tapped storage container (wide-mouth buckets are the classic recontamination pathway). Manual cap-dosing remains only as the fallback mode.
Chlorine as a service: cluster cells wholesale hypochlorite refills for vendor Venturi dosers across the settlement, replacing purchased bleach and tablet supply chains with camp-made chlorine at table-salt prices — the recruitment economics that bring existing water vendors onto the network's quality-and-price map rather than positioning them as competitors.