Whole-house RO vs under-sink RO

Under-sink RO covers the kitchen cold tap and, if plumbed for it, the fridge and ice maker. Everything else in the house — showers, baths, laundry, outside taps — receives untreated water. If the concern is drinking and cooking, that is complete coverage. If the concern is bathing in the water, it is not.

Whole-house RO treats everything downstream of the point of entry. That matters where the objection is to contact with the water rather than to ingesting it, or where a contaminant is volatile and enters air during a shower. It also means every gallon used for the lawn or the washing machine has passed through a membrane, which is where the running cost argument starts to bite.

This is the real dividing line. A residential under-sink membrane produces roughly 50 to 100 gallons per day and buffers it in a 2 to 4 gallon pressurised tank — enough for a household's drinking and cooking without noticing.

A house uses far more than that, in bursts. Two showers running is a demand no membrane can meet in real time, so whole-house systems produce into a large atmospheric tank — commonly several hundred gallons — and then a booster pump repressurises from the tank into the house. That adds a pump, a control system, tank space in a basement or garage, and a sanitation consideration, since stored treated water with no disinfectant residual needs managing (usually with UV on the outlet).

Under-sink systems are the cheapest meaningful treatment in the house: a modest equipment cost, a straightforward installation if there is a drain connection and room in the cabinet, and annual filter changes plus a membrane every few years. Most of the lifetime cost is consumables.

Whole-house RO is an order of magnitude more involved. Multiple membranes sized to household daily volume, pretreatment (sediment, carbon and often softening to protect the membranes from scale), the storage tank, the repressurisation pump, UV, and the electrical and plumbing work to tie it together. It also raises the drain-water question from a footnote to a live one, because the ratio now applies to every gallon the house uses rather than to the few you drink.

Anyone quoting whole-house RO should be able to state the daily production figure, the recovery ratio and the pretreatment it depends on. If those three numbers are not in the proposal, the proposal is incomplete.

Start from the contaminant and the exposure route. A nitrate or lead concern is an ingestion problem, and an under-sink unit at the tap you drink from resolves it. A high-TDS or brackish supply that leaves residue on every surface and makes soap behave badly is a whole-house problem — but so is hardness, and softening addresses hardness at a fraction of the cost, so establish which one you actually have first.

On a municipal New Jersey supply, hardness and the reported panel are published per system; the system pages will usually tell you whether there is a case for membranes at all. The mechanics of the membrane itself are in reverse osmosis explained.