Water Purification Systems

The quality of RO product water depends on the TDS level in the feed water, as RO membranes reject a percentage of the feed TDS. TDS also affects the minimum feed pressure required, and for every 100 ppm of TDS, 1 psi must be deducted from the source pressure.

It is crucial to know the maximum allowable concentrations of dissolved solids for different pH levels to ensure the proper operation of an RO system. The higher the pH of the water, the less tolerant the membrane is to chemicals and minerals, which can deposit on the surface of the membrane and lead to its failure.

The pressure in the RO process refers to the differential pressure across the membrane, affected by feed pressure, back pressure, and osmotic pressure due to TDS. Every 100 ppm of TDS decreases the feed pressure by 1 psi, requiring a higher feedwater pressure for higher TDS levels. The minimum net pressure for thin film TFC POU membranes is 20 psi, and doubling or halving the net pressure affects the production rate proportionally. The percentage rejection of TDS also decreases as the net pressure falls below the minimum net pressure.

Feedwater temperature also determines RO production rate because of the effect it has on the viscosity of water. The industry standard for determining the production rate is 25EC. As a rule of thumb, RO production falls to approximately half with a drop in temperature of about 20EC. The water temperature must be known to ensure that the maximum operating temperature is not exceeded, or membrane degradation will occur over time.

Maximum operating temperatures for cellulose acetate (CA) and cellulose triacetate (CTA) membranes is 29EC and 38EC for TFC membranes. Membranes should never be allowed to freeze as they will instantly become damaged beyond repair. Water temperature is measured with a standard thermometer after allowing the source to flow for at least one minute.

To ensure the safety of your feedwater, check if it contains disinfectants like chlorine, chloramines, iodine, or ozone. You can do this by contacting your municipality or by using a free- and total-chlorine test kit. Even if you don’t see a residual disinfectant at your installation site, your water may still be disinfected at a central source on a regular schedule.

Thin-Film Composite (TFC) membranes are vulnerable to oxidants, so it’s best to use deoxidising filters like carbon filters before standard TFC membranes. These filters can also remove sediment and prevent “blinding” of the membrane. Chloramines have little effect on TFC membranes, but there’s usually some free chlorine in water supplies disinfected with chloramines.

pH measures how acidic or basic a substance is. The scale goes from 0 to 14, with 7 being neutral. Lower values indicate more acidity, while higher values indicate more alkalinity. Acidity is the ability of water to neutralize bases, while alkalinity is the ability of water to neutralize acids.

Water pH affects the rejection of dissolved solids like arsenic and the presence of minerals like iron, manganese, and hardness, which can cause scaling. The pH is measured using a calibrated pH meter on-site.

Impurities in feedwater can have a significant impact on the effectiveness and longevity of point-of-use (POU) membrane systems. Without proper analysis, impurities such as hardness, iron, manganese, and even algae can foul or scale the membrane, leading to decreased performance and increased maintenance costs.

A reliable water analysis is essential for identifying potential impurities in the feedwater and taking appropriate measures to prevent membrane fouling and scaling. By regularly monitoring and treating impurities in feedwater, POU membrane systems can continue to deliver high-quality water without interruption.