Reverse osmosis: sizing by salt content and output

Reverse osmosis is usually sized on a single figure — how many cubic metres an hour the shop floor needs. That is not enough: two units with the same permeate output can sit in different sections of the catalogue and differ in the number of membranes, in the pump and in how much raw water they draw. What separates them is not the flow rate but the salt content of the water coming in.

Salt content sets the unit range

Reverse osmosis forces water through a semi-permeable membrane. Water molecules get through, dissolved salts mostly do not: they stay in the stream that sweeps along the membrane and leaves to drain. The treated water is called permeate, the salty remainder is the concentrate.

The more salts in the raw water, the higher its own osmotic pressure and the harder the pump has to push before any flow through the membrane starts at all. Hence the four ranges in the catalogue, and that is what sets them apart more than anything else.

Three models of identical output show the difference clearly. AWT RO-1000L (4/4040) — fresh water, 1000 l/h, four membrane vessels, 1.5 kW pump. AWT ROB-1000L (6/4040-1W) — the same thousand litres per hour on brackish water, but with six vessels and a 3 kW pump. AWT ROS-1000L (8/4040) — again 1000 l/h, now on sea water and on eight membrane elements.

The same result, a very different bill for membranes and for electricity. Which is why sizing starts not with the output but with an analysis of the raw water.

Working pressure rises along with salinity: RO DUO runs at up to 1.3 MPa, ROB-1300L up to 2.0 MPa, and ROS-2 and ROS-4 are rated for 60 bar. That pressure is held not by the pump alone but by the whole pipework and the vessels with it, so putting in a sea water unit "just in case" does not work: it is built differently from the ground up.

Selectivity: what still gets through the membrane

The degree of treatment is the share of salts the membrane holds back. For the RO range that is demineralisation up to 99 %, for ROB up to 99.5 %, for ROS up to 99.7 %.

What matters is what the remainder is measured against. The share that slips through the membrane is reckoned from the raw water salt content: at one and the same selectivity, salty water leaves more salt in the permeate than fresh water does. That is exactly why the brackish and sea water ranges have higher selectivity — otherwise the permeate would never meet what the consumer needs.

When one stage is not enough, two are put in series. In the AWT RO DUO range the permeate from the first stage is fed to the second, and the degree of treatment reaches 99.9 %. The models run from DUO-0,3 to DUO-3.0; the scheme is the same for all of them and only the flow differs, so within the range the choice comes down to the output required.

Osmosis does more than cut mineral content. Units of the ROS range take out hardness salts, heavy metals, fluorides, nitrates and ammonium, and with them organic matter, bacteria and viruses. That makes osmosis a universal barrier, but it does not do away with pretreatment: mechanical filtration ahead of the membranes is mandatory, and on commercial models it sits inside the unit itself on a BB20 cartridge.

Permeate recovery and where the concentrate goes

Recovery (also called permeate recovery) is the share of the raw water that goes to the consumer. The rest is discharged to drain together with the retained salts.

It is the most underrated figure in a project. The number to work out is not how much clean water you need but how much water goes in at the inlet:

• ✓AWT RO-1000L in production mode takes 1660 l/h of raw water;
• ✓AWT ROB-1150L gives 1150 l/h of permeate at a raw water flow of 1820 l/h;
• ✓AWT ROB-1300L — 1300 l/h of permeate at a flow of no more than 2000 l/h.

The peak flush flow is worked out separately. In hydraulic flushing mode RO-1000L passes 4000 l/h, ROB-1300L up to 4000 l/h. The supply line and the drain both have to take that flow, or the flush will be incomplete and the membranes will start fouling ahead of time.

The vessel configuration affects flushing too. ROB-1000L comes in a single-element and a double-element version: output, connections and pump are the same on both, while the hydraulic flushing flow is 4000 and 2390 l/h respectively.

The concentrate has to go somewhere. For fresh water this is an ordinary line to the sewer; for brackish and sea water the volume and salinity of the discharge are noticeably higher, and the discharge point is agreed in advance — the salt limit in the effluent is set by the water utility or the local regulator, and in the permit it is specific to each site.

How to read the designation

The designations are built the same way, and they show almost the whole make-up of a unit.

• ✓Range prefix. RO — fresh water, ROB — brackish, ROS — sea, RO DUO — two stages. ROL-250L and ROL-500L are the lightweight versions, with a 0.75 kW pump.
• ✓Output. The letter L means litres per hour: RO-1000L is 1000 l/h. Without it the count is in cubic metres: RO-20/8040 is up to 20 m³/h.
• ✓The number in brackets. The first figure is the number of membrane vessels, the second is the element size. RO-1500L (6/4040) is six vessels with 4040 elements, that is, four-inch ones. The industrial range is built on 8040 elements — eight-inch ones.
• ✓Elements per vessel. A "double" or "triple" version means how many elements sit in one vessel in series: RO-9/8040 has triple vessels, RO-10/8040 has a double version.

Inside a range the designation maps almost one to one onto the pump. In the commercial RO line, models up to RO-1250L come with a 1.5 kW pump, and from RO-1500L the series moves to 2.2 kW. In the ROB range the smaller ROB-350L, ROB-500L and ROB-700L come with a 2.2 kW pump, and models from 1000 l/h with a 3 kW one.

Margin on output

Rated output is a nominal figure under certain conditions, not a guaranteed minimum on any day of the year. Permeate flow through the membrane falls on cold water and tails off as the membranes foul between chemical cleanings. Size with no margin and the unit will stop covering demand in winter, or at the end of a run between cleanings.

The second point is the pattern of draw-off. Osmosis works at a steady flow, while a production floor draws in bursts. So the unit is sized on the average hourly demand over a shift and a permeate storage tank goes in, instead of chasing the peak flow: a tank is cheaper than extra membrane vessels and an extra kilowatt on the pump.

The upper limit of the standard solutions in the RO range is the RO-15/8040: 15 m³/h, triple vessels, a pump up to 11 kW, dry system weight 950 kg. Above that you are into build-to-spec — from RO-16/8040 on to RO-210/8040, where the scheme, the number of vessels and the pipework are chosen for one particular source. That is also where you get continuous conductivity monitoring of the feed and treated water, and remote supervision.

What next

Sizing takes four things: an analysis of the raw water with its total salt content, the permeate output required, the pattern of draw-off over a shift, and the consumer's quality requirements for the treated water. It is worth knowing from the start where the concentrate will go — that sometimes changes the scheme more than the output does.

If the water is fresh, look at standard reverse osmosis units: the range covers from 250 l/h up to industrial assemblies. If the analysis has shown elevated mineralisation, a borehole in a coastal area or a brackish source, your section is osmosis for brackish water. It is not worth starting without an analysis: a mistake in the range is not overpaying for an option, it is a different unit altogether.