Many processors striving to lower operating costs are missing an opportunity for savings in their reverse osmosis (RO) water treatment systems. Proactive steps to optimize both the cleaning frequency and cleaning method can extend the life of the installed membranes. Proper tracking of system performance combined with this optimized cleaning can reduce chemical, electrical and membrane-replacement costs. Additionally, a wise choice when it's time to replace membranes that operate in a fouling-prone environment can pay dividends.
MAINTAIN OPTIMAL OPERATION
Proper maintenance is the key to protect the investment in your current membranes. So, here, we'll look at some guidelines that can help you extend the productive life of the membranes and reduce overall operating costs of the RO plant.
The loss of permeate flow during operation is normal for a membrane system, so the first question is: "When to clean?" The frequency depends on the feed water source, operating parameters such as flux, and pretreatment. Commonly, systems are cleaned two-to-three times/year with well water, three-to-four times/year with city water, and four-to-six times/year with surface water. But it really depends on the specific situation. So, it's important to vigilantly look for signs of fouling. Any of the following observations should trigger a cleaning:
• Normalized permeate flow declines by 10–15%.
• Normalized feed pressure increases by 10–15%.
• Pressure drop rises by 10–15%.
• Normalized salt passage increases by 5–10%.
To make proper judgments, it's essential to normalize the permeate flow, feed pressure and salt passage to a standard reference point. Otherwise, fluctuations in feed temperature, salinity or pressure will either mask or accentuate the trends, leading to inaccurate conclusions about when to clean. Membrane suppliers can help provide software tools to normalize the data.
Foulants usually can be cleaned from the membrane surface with the right cleaning chemicals and good technique. Waiting too long to clean can permanently reduce RO performance (Figure 1).
ACHIEVE EFFECTIVE CLEANING
Before cleaning, it's very important to determine the type and location of the fouling:
• Colloidal and particle fouling (Figure 2) is specific to the first RO stage. (Its feed screen tends to catch these foulants.)
• Scaling (Figure 3) appears in the second stage. (Recovery of product water in this stage boosts the concentration of salts in the remaining water, possibly exceeding the solubility limit of certain salts.)
• Organic and microbiological fouling (Figure 4) can occur in either the first or second stage of the system.
Before starting to clean, find out the cleaning pH and temperature limits set by the membrane manufacturer and make sure the cleaning chemicals are compatible with the membranes.
Clean with alkaline cleaners first and then, if necessary, with acid. High-pH cleaners are more likely to break down fouling layers. Acid may react with organics, silica and biofouling, possibly leading to irreversible performance decline — that's why you should remove these foulants first with an alkaline cleaner.