industrial applications began in the late 1980s, following the introduction of polyamide materials that had a significantly higher permeate flow and salt rejection than calcium acetate technology. Since then, spiral-wound RO membranes have in- creasingly replaced ion exchange for water purification in in- dustrial applications, especially for boiler feedwater.
Drought conditions and rising demand for fresh water will increase the usage of RO to purify water from lower-quality sources. Implementing best practices will maximize the effi- ciency and quality of product water from RO units, as well as re- duce operating costs (chemicals and replacement membranes), the rate of fouling and the frequency of offline cleaning.
Monitoring. Plant operators track key operating parameters
and will conduct an offline cleaning procedure when either the quality or the quantity of the permeate declines. These key operating parameters—pressure drop (differential), permeate flowrate and salt passage—are strongly temperature depen- dent. Measuring the changes in these key operating parameters requires using a complex algorithm to normalize (adjust) the measurements to a common temperature.
Best practice for monitoring. Collect these key parame-
ters and temperatures with online analyzers and use a software package to calculate and track the normalization parameters. Clean the RO when any of these three normalized parameters changes by more than 10%.
Pretreatment capability. RO membranes remove dissolved
contaminants from water. The presence of any insoluble or suspended solids in the feedwater will cause fouling of the membrane surface and increase the frequency of cleaning. Proper pretreatment will optimize the cleaning frequency to once every three months or less. An empirical measurement of small-diameter suspended solids commonly referred to as the silt density index (SDI) is the correct parameter to predict the fouling by a specific water source at the inlet to the RO unit.
Best practice for SDI at the inlet to the RO unit. The
SDI of the inlet water for reliable operation are: SDI < 3— No chemical treatment required; 3 < SDI < 5—Anti-scalant chemicals required; SDI >5—Additional pretreatment such as clarification, media or membrane filtration and sodium zeolite softeners, upstream of the RO unit is required.
‘Turndown’ of feed flowrate. Operators sometimes assume
that the presence of a variable-speed drive on the feed pump means that they can operate the unit at variable flowrates. RO units are co-flow, rather than cross-flow—the direction of water flow is parallel to the membrane surface. Consequent-
ly, operation at flowrates significantly lower than the design rate will dramatically increase fouling rate of the membrane surface. The best option is to install a permeate storage tank with a dead band sized to accommodate the normal variability of treated water demand. The other option is to temporarily idle one or more skids to make a step-change in the permeate production rate. Best practice for turndown of feed flowrate is a
maximum of 10%.
Idling. Operators idle membranes based on short- or long-
term changes in permeate demand. Idled RO membranes are very vulnerable to bacteria growth, especially if the feedwater is from a surface source such as a river or lake. Failure to prop- erly idle RO membranes will result in microbiological fouling, reducing the permeate quality and quantity, and increasing the offline cleaning frequency. Sequence idling is the recommended practice to accommodate the normal variability of permeate demand in an industrial application. Lay-up idling is the rec- ommended practice for seasonal changes, turnaround or more permanent reduction in permeate demand.
Best practice for sequence idling. Surface waters have a
maximum idle time of four hours. Well water has a maximum of one week idle time (unless there is bacterial contamination in the well). In both cases, flush the membrane with permeate prior to returning the RO skid to service after the idling period. Consider periodically feeding the biocide, DBNPA (2,2-dibro- mo-3-nitrilo propionamide), during service to control micro- biological growth.
Best practice for lay-up idling. Conduct an offline clean-
ing procedure, followed by a flush with a preservative such as sodium bisulfite immediately prior to idling. Alternatively, substitute the biocide, such as DBNPA, during the flush if the membranes require robust microbiological control.
Best practices yield benefits. RO has become the standard
technology for purifying water for industrial applications, es- pecially for boiler makeup. Implementing best practices for RO units will reduce the risk of off-spec permeate and potential damage or failure to downstream steam generators.
LORAINE A. HUCHLER is president of MarTech Systems, Inc., a consulting firm that provides technical advisory services to manage risk and optimize energy- and water-related systems including steam, cooling and wastewater in refineries and petrochemical plants. She holds a BS degree in chemical engineering, along with professional engineering licenses in New Jersey and Maryland, and is a certified management consultant.