Reverse osmosis on warm wastewater: what changes in design?

Why a warm-weather trial can mislead

Permeability varies with temperature. DuPont's FilmTec manuals use correction factors to compare RO/NF operation across temperatures. More permeate on a hot day can reflect a reversible physical response, rather than a membrane improvement. Permeate quality and rejection of target species must also be assessed across the design temperature range.

Do not generalise the thermal limit of one FilmTec family to all membranes or a clean-water flux figure to effluent with COD, fats or scaling salts. The supplier must qualify the proposed element, spacers, seals, housings, pumps and cleaning programme.

• Log feed, permeate, concentrate, temperature, pressure, conductivity and recovery together.
• Compare trends after normalising for temperature and membrane age.
• Test both winter minimum and production maximum when they differ substantially.

Pretreatment changes too

A warm stream may change viscosity, precipitation and biological stability as it cools or sits idle. Characterise hardness, silica, alkalinity, organic matter and solids; check what might precipitate as the RO concentrate forms. RO does not replace equalisation, solids removal or scaling control.

Design for inlet temperature and excursions rather than the average. A heat exchanger may recover useful heat before RO, but adds capital, cleaning and pressure drop. An equalisation tank may cool passively and alter water chemistry; measure the condition actually entering RO.

Acceptance criterion

Set the final water use and analytical limits before piloting. Accept a route only if quality holds across the temperature range, pressure and recovery remain sustainable, and the concentrate has a destination. The economic balance includes pumping and heat management.