Commercial Reverse Osmosis Plants: Sizing, Reject Water and Running Costs

Reverse osmosis works by pushing water under pressure through a semi-permeable membrane that blocks dissolved salts, heavy metals, nitrates and most microbiological contaminants. At a domestic scale — covered separately in our guide to reverse osmosis for the home — a countertop or under-sink unit producing 150–400 litres per day is usually sufficient. At commercial or light-industrial scale the requirements are different: flow rates climb into the thousands of litres per hour, feed-water quality is often far worse, and the system must run reliably without daily human intervention.

This article explains how businesses size a commercial reverse osmosis system, why source water (especially borehole water) changes the design, what happens to the water the membranes reject, and what you should realistically expect to spend running such a plant year after year.

How a commercial RO system is sized

Sizing starts with demand, not with a catalogue number. The core calculation is straightforward: add up every water-consuming process on your site — staff amenities, production lines, kitchen or laundry, cooling towers, irrigation — and find the peak hourly consumption. That figure, in litres per hour, becomes your minimum permeate (product water) output requirement.

From there, recovery rate matters. A typical commercial membrane system recovers 65–80 % of feed water as permeate; the rest exits as reject. If you need 1 000 L/h of product water and your system runs at 75 % recovery, your feed-water supply must deliver at least 1 333 L/h continuously. Under-sizing the feed supply is one of the most common and expensive installation mistakes.

A useful rule of thumb is to size the plant to meet your peak demand at no more than about 80 % of its rated capacity, leaving headroom for membrane ageing, high-demand days and the occasional pre-treatment backwash cycle that temporarily reduces output.

For larger operations — hotels, food processors, light manufacturers, multi-tenant office parks — a full site water audit is worth commissioning before any equipment is specified. The audit often reveals demand peaks and troughs that allow a smaller plant with a buffer tank to replace what initially seemed to require a much larger unit.

Why pre-treatment is non-negotiable, especially on borehole water

Municipal water arrives pre-treated and usually meets SANS 241 (2024 edition) limits before it reaches your meter. Borehole water does not. Depending on the aquifer, it can carry iron, manganese, silica, calcium carbonate, biological load and in some areas arsenic or fluoride at levels that will foul or physically damage RO membranes within weeks without adequate pre-treatment.

A well-designed pre-treatment train for a reverse osmosis system for borehole water typically includes:

• Sediment filtration (often multi-media or cartridge, down to 5 micron) to remove suspended solids
• Iron and manganese removal (greensand or catalytic media) if levels exceed roughly 0.1 mg/L — fouling begins quickly above this
• Antiscalant dosing to prevent calcium carbonate and silica scaling on membranes
• Carbon or UV disinfection if biological contamination is a concern
• Softening or acid dosing where hardness is very high

Skipping or under-specifying pre-treatment to save upfront cost is a false economy. Membrane replacement is one of the largest ongoing expenses in any RO plant (more on that below), and premature fouling can shorten membrane life from a typical three to five years to under twelve months.

If your site relies on borehole supply, request a full chemical and biological water analysis — not just a basic test — before any system is designed. The Water Research Commission (WRC) publishes guidance on appropriate analytical methods for South African groundwater that reputable installers should be following.

Running a business on borehole or municipal supply and need a system sized correctly the first time? Compare 3 free quotes — vetted installers, no obligation.

Reject water: the stream most quotes ignore

Every commercial RO system produces two output streams: permeate (the clean water you want) and reject, also called concentrate or brine. The reject stream carries everything the membrane blocked — salts, hardness, silica, whatever was in your feed water — at two to four times the feed concentration, depending on recovery rate.

This stream is not a minor detail. At a 75 % recovery rate, a plant producing 5 000 L/h of permeate is simultaneously discharging roughly 1 667 L/h of reject. Over a 10-hour operating day that is nearly 17 000 litres that must go somewhere.

Options include:

• Municipal sewer discharge — check with your local authority; trade effluent consents may be required and some municipalities levy volumetric charges
• Evaporation/sump systems — only practical in lower-volume or arid-site applications
• Reuse for non-potable purposes — irrigation of hardy plants, toilet flushing or industrial cooling make-up water, depending on reject quality
• Zero-liquid-discharge systems — cost-intensive but sometimes required in water-scarce or environmentally sensitive locations

For businesses in areas where municipal water is already constrained — and Johannesburg Water’s own 2024/25 audited figures show non-revenue water losses of 44.8 %, reflecting the pressure on the network — sites that can beneficially reuse reject water are in a stronger position both commercially and in terms of future regulatory compliance.

Any quote for a commercial RO installation should include a clear reject water management plan. If it does not, ask for one before signing.

What membranes and power actually cost to run

Running costs for a commercial RO plant fall into three main categories.

Electricity is usually the largest single operating cost. RO systems require high-pressure pumps — typically operating at 5–15 bar depending on feed-water salinity — and pump power scales directly with flow rate and pressure. As a rough guide, commercial brackish-water systems consume somewhere in the range of 0.3–1.0 kWh per cubic metre of permeate produced, though this varies considerably with feed-water total dissolved solids (TDS), system design and whether energy recovery devices are fitted. At South African commercial electricity tariffs, which vary significantly by municipality and Eskom schedule, you should model your specific consumption rather than rely on a generic figure — your installer should provide this.

Membrane replacement is the second major cost. Commercial membranes, when properly pre-treated and operated, typically last three to five years. For a detailed look at what replacement involves and how to extend membrane life, see our article on RO membrane replacement. Multi-element systems with four to eight membranes per vessel can represent a meaningful capital outlay at replacement time; factor this into any lifecycle cost comparison.

Consumables and maintenance — antiscalant chemical, filter cartridges, UV lamp replacement, periodic CIP (clean-in-place) chemicals — add a further recurring cost that is easy to underestimate. A realistic maintenance contract from your installer is worth more than a cheap upfront price.

As a Johannesburg reference point, complete borehole systems (including pump, casing and basic filtration) typically run R40,000–R150,000, with most falling between R60,000 and R100,000. A commercial RO plant sits on top of that infrastructure, and the total installed cost for a business-scale system will depend heavily on flow rate, pre-treatment requirements and civil work. Confirm all figures via formal quotes.

Comparing commercial RO to other treatment technologies

RO is not always the right answer. For businesses where the main concern is turbidity or biological contamination — rather than dissolved salts or chemical contaminants — ultrafiltration (UF) or advanced carbon filtration may deliver adequate water quality at lower capital and operating cost.

RO becomes the preferred choice when:

• Feed-water TDS exceeds roughly 500–1 000 mg/L and product water TDS needs to be below 50 mg/L
• Specific regulated contaminants (nitrates, fluoride, arsenic, certain heavy metals) must be reliably reduced
• A consistent, predictable product water quality is required regardless of feed variation — common in food and beverage production

For a broader view of treatment options relevant to commercial sites, the commercial water solutions overview covers the main technology categories and the questions worth asking before committing to any system. Also worth reading: our breakdown of commercial water filtration costs for a sense of the investment range across different treatment approaches.

Before committing to any commercial water treatment plant, it pays to see how three independent installers approach the same brief. Request your free comparison quotes — no pressure, no obligation.

Questions to ask before signing any commercial RO quote

A good commercial RO proposal should answer every one of these without you having to chase:

Question	Why it matters
What is the design permeate flow rate in L/h, and at what feed TDS?	You need to know whether rated output was tested on your actual water, not distilled water
What is the system recovery rate, and what volume of reject does that produce daily?	Reject disposal has a cost and a compliance dimension
What pre-treatment is included, and is it sized for your feed-water analysis?	Pre-treatment adequacy determines membrane lifespan
What is the estimated power draw in kW and annual kWh?	Allows you to model realistic running costs
What is the membrane replacement interval and estimated cost?	Core lifecycle cost item
Is a maintenance contract available, and what does it include?	Unplanned downtime at a food processor or hotel is expensive
What are the warranty terms on membranes, vessels and high-pressure pumps?	Commercial components vary widely in warranty coverage

Getting clear written answers before you commit protects you from vague verbal assurances that are difficult to enforce once the plant is installed.