Under-sink reverse osmosis, often called RO, is one of the most effective ways to reduce many dissolved contaminants at a single kitchen tap. It also does three things that a brochure may not stress: it sends some water down the drain as it works, it keeps a tank of treated water, and it asks for filter changes on a schedule. Read those three parts as closely as the performance claims before you sign.
Large public projects can help you keep perspective. The San Francisco Public Utilities Commission has awarded the $1 billion Southeast Water Pollution Control Plant nutrient reduction project to a joint venture of PCL Construction Inc. and Jacobs Project Management, as reported by Water Finance and Management. In a press release, PCL said the project will advance a "major investment" in the city's wastewater system. That is a plant upgrade, not a change to what comes out of your faucet. Your decision about a home unit starts with your own water report and the specifications for the exact model you are considering.
Ask about the drain ratio first
An RO membrane uses water pressure to push water through a very fine barrier. Only part of the incoming water becomes drinking water. The rest carries the rejected material to the drain. Sellers may describe this as a drain ratio, or as the amount of wastewater per gallon of product water. Ask for that number for the specific model, and ask whether it assumes ideal pressure. A figure measured under conditions your home does not have is not a useful number.
Also check how the drain line connects. Some units tie into an existing sink drain, while others need a separate connection. Confirm that the drain line will not be pinched or kinked under the cabinet, because a restricted drain can cause backpressure and slow the unit down.
Check the tank and the faucet
Most under-sink systems store treated water in a small tank, so a glass of filtered water is ready without waiting for the membrane to make it. Ask about tank size and whether the unit includes a way to check the tank's air pressure. An undersized tank will leave you waiting when you fill a pitcher, a kettle, or a pot for pasta. Ask, too, whether the system uses a dedicated faucet, which needs its own mounting hole, or shares a sink faucet hole that you may already have.
Read the filter schedule like a bill
Count the stages. A typical under-sink unit has one or more pre-filters for sediment and carbon, a membrane, and a post-filter. Each part has its own service interval and price. Ask for the replacement cost of every cartridge, the expected change interval for your water, and whether the company sells cartridges directly or through a service plan. Your water report matters here. Higher sediment or chlorine levels will generally shorten the life of the pre-filters, so a schedule written for ideal water may be too optimistic for your house.
Add up one year of replacements and write that figure down before you compare models. A unit with a lower purchase price and expensive cartridges can cost more over five years than a pricier unit with cheaper parts. Compare annual costs the same way you would compare two phone plans.
Put the service promises in writing
Ask the installer who handles leaks, membrane failures, and warranty claims, and how long that coverage lasts. Ask whether the company's own staff does the install or whether a subcontractor does. Get the warranty terms in writing, and keep the receipt, the install sheet, and the date of your first filter change in one folder. If a problem shows up in two years, that folder will answer most of the questions.
Installers such as Reverse Osmosis STL can answer these questions in writing before you commit. A clear, specific answer about drain ratio, filter pricing, and warranty terms is a good sign. A vague answer is a reason to ask again or to compare another quote.
A short checklist before you buy
Bring your most recent water test to the conversation. Ask for the drain ratio for your pressure, the tank size, the faucet type, and the cost of every cartridge over one year. Confirm the drain connection and the warranty owner. If the answers fit your kitchen and your budget, you have a fair basis for a decision. If they do not, the unit is not the right one for your home, no matter how well it is advertised.