Which water analysis do you need before selecting reverse osmosis?

A conductivity reading is only a starting point

Reverse osmosis is used when a project requires separation of dissolved constituents. Before choosing a membrane or requesting a price, establish what the feed water contains and what quality the application needs. A single conductivity or total dissolved solids value gives useful context, but does not describe the individual substances that influence design.

For example, two sources with similar total dissolved solids can have different scaling risks. A groundwater sample, a municipal supply and treated wastewater should not be treated as interchangeable. Record the source, sampling point, date and whether the sample represents normal, seasonal or unusual conditions.

Build an analysis around the actual risks

Ask the project engineer and laboratory to define a suitable analytical programme. It commonly considers pH, conductivity, temperature, major ions, alkalinity, hardness and silica. Suspended solids, turbidity, iron, manganese, organic matter, oxidants and microbiological conditions can also affect the treatment concept. Additional parameters follow the source and end use.

Keep the original laboratory report with units and methods. If the composition appears incomplete or inconsistent, resolve the gap before calculating recovery or chemical dosing. For a variable source, one convenient sample may not capture the conditions that determine reliable operation.

Give every pretreatment stage a purpose

Pretreatment may need to address particles, precipitating salts, organic or biological fouling and membrane compatibility. Ultrafiltration can be useful for controlling particles, but it does not ordinarily remove dissolved hardness or salinity. A softener and a membrane filtration stage therefore solve different problems.

Common polyamide reverse osmosis membranes can be damaged by oxidants. If the upstream system uses chlorine or another disinfectant, assess compatibility and the necessary controls against the selected membrane manufacturer's instructions. Do not assume that a chemical tolerated by one process is suitable for the next.

Define the product water and operating envelope

An equipment enquiry should include:

• Required permeate quality and the intended use of the water.
• Net daily and peak demand, with storage or operating hours.
• Feed-water temperature range and seasonal variation.
• Available utilities and limits on concentrate management.
• A plan for sampling, monitoring and maintenance.

The permeate may need further conditioning or disinfection depending on its use. Reverse osmosis performance is not a blanket guarantee that every contaminant is absent or that the water is suitable for every application.

A useful design discussion ends with agreed inlet assumptions, an outlet specification and the missing evidence needed to confirm both. This makes supplier comparisons more meaningful and reduces the chance of buying a nominal capacity that depends on conditions the site cannot provide.