Different RO water filter systems handle a different number of purified gallons in a day. Normally this capacity goes from 50 to 150 Gallons per day and so on. You must identify the amount of water your family needs in a day. For Small families, reverse osmosis with 50 Gallons per day capacity is suggested while for larger families, Whole house Ro system with minimum 75-100 GPD is suggested.
In 1977 Cape Coral, Florida became the first municipality in the United States to use the RO process on a large scale with an initial operating capacity of 11.35 million liters (3 million US gal) per day. By 1985, due to the rapid growth in population of Cape Coral, the city had the largest low-pressure reverse-osmosis plant in the world, capable of producing 56.8 million liters (15 million US gal) per day (MGD).[7]
The Zip has a similar footprint and appearance similar to a pod coffee maker, but instead of serving up java, this mighty machine delivers purified water. Pour tap water into the reservoir and the Zip will give you a 0.5 gallon of filtered, pH-balanced water in about 15 minutes. Just keep in mind that you’ll need to empty the tank of purified water before you can add water to the fill-up tank for another round of filtration.
What’s unique about the tankless design of the RCS5T is the fact that each time you fill a glass with water or a pot for cooking, the water is purified on demand. As a result, you may notice that it fills slightly slower and with less water pressure than similar systems, but you’ll know that the water has been freshly filtered and hasn’t been sitting in a storage tank.
Pressure exchanger: using the pressurized concentrate flow, in direct contact or via a piston, to pressurize part of the membrane feed flow to near concentrate flow pressure. A boost pump then raises this pressure by typically 3 bar / 50 psi to the membrane feed pressure. This reduces flow needed from the high-pressure pump by an amount equal to the concentrate flow, typically 60%, and thereby its energy input. These are widely used on larger low-energy systems. They are capable of 3 kWh/m3 or less energy consumption.
This is my second RO-PH90 system. Simply one of the best systems on the market, in my opinion. Uses genuine Dow filmtec reverse osmosis membrane. As anyone familiar with RO knows, filmtec membranes are the gold standard and rank among the elite in rejection rates. This is not your generic RO bought in a hardware store, although some large chains carry it. Input TDS = ~225 ppm, output TDS = ~15-20ppm. Does the job. Have not tested PH yet. Water tastes great as it does with my first system. Change your pre filters once per year or at the recommended %TDS interval and expect this RO membrane to last its full schedule of 3-5 years. This is very important. Incoming water pressure must be at least 50psi in my opinion, for this system to operate as intended. At 75psi, outgoing pressure is like a dream, even with 1/4'' stock tubing. ... full review

Countertop RO water systems are those systems that can be placed easily on the top of the Kitchen Table. These are designed for small families as they produce a small quantity of purified water. Countertop RO system is portable and inexpensive than most of other RO water filters. It is ideal for tenants who may not have permission to make changes in the house they live in.

Some small-scale desalination units use 'beach wells'; they are usually drilled on the seashore in close vicinity to the ocean. These intake facilities are relatively simple to build and the seawater they collect is pretreated via slow filtration through the subsurface sand/seabed formations in the area of source water extraction. Raw seawater collected using beach wells is often of better quality in terms of solids, silt, oil and grease, natural organic contamination and aquatic microorganisms, compared to open seawater intakes. Sometimes, beach intakes may also yield source water of lower salinity.