Pretreatment Essential to RO Membrane Success

The United States Environmental Protection Agency classifies chlorine as a disinfectant and allows a maximum contaminant level goal (MCLG) of four milligrams per liter (mg/L) in drinking water supplies. The agency does recognize that chlorine’s presence in water can cause digestive discomfort and eye or nose irritation in some consumers. For most residential and medicinal purposes, four mg/L or ppm is unacceptable. Furthermore, the chlorine in public drinking water supplies is damaging to the membranes in a reverse osmosis (RO) system.

Since the membranes are typically the most costly part of a water filtration system, it is best to remove as much chlorine as possible from the water before it even touches an RO membrane.

Carbon: Simple and Inexpensive

This is where carbon pretreatment earns its keep. Although technically not part of the reverse osmosis process, one or more carbon pretreatments to remove harmful chlorine is essential to the life of RO membranes. It is not an exaggeration to state that chlorine will damage an RO membrane.

Pretreatment: No Compromises

To save money and/or space, some users may be tempted to skip the carbon or the sediment removal pretreatment steps when considering a reverse osmosis system for home use. Both steps are not only critical to the process of producing clean water but are also imperative to preserving the life and investment made in the RO membranes.

RO Membrane Considerations for Purified Water

Certain considerations should be kept in mind to achieve high-purity water from a reverse osmosis system. A properly designed and operated RO system is able to produce water with a resistivity greater than 1 megohm and there are several factors that can affect the final product.

Factors that Affect Membrane Rejection

Controlled experiments show that membrane rejection fluctuates depending on the feed’s pH values and total dissolved solids, along with crossflow rates and element recovery levels. Minor feedwater constituents like ammonia and alkalinity are also seen having a great part in achieving high purity permeate, and these elements are seen as important ones that ensure the success of producing high-purity water.

How These Affect Membranes

Feeds having high pH levels contribute to the reduction of the membranes’ rejection, and acid addition that can lower its pH level can help in correcting this condition. Since a high pH level also contain a high concentration of hydroxide ion, membranes cannot reject this.

Feedwater with minimal total dissolved solids also reduces the membrane’s capability to reject ions, and this reduction in ion rejection must be considered when calculating the final permeate quality of a two pass system. The feedwater chemistry, meanwhile, can greatly affect permeate conductivity. Particularly feeds containing dissolved gases pass directly through these membranes, making it necessary for certain changes in the feedwaters’ pH levels to be done. These considerations should be remembered when designing an effective RO system.