ro cleaning guidelines

8/10/2019 Ro Cleaning Guidelines

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Cleaning Guidelines EN Aug-07

TechnicalBulletin

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Operating Guidelines CleaningCleaning Pure Water Membrane Elements

Introduction

Regular cleaning of Desal membrane elements isimportant because foulants can build up on mem-brane surfaces, reducing permeate flow and qual-ity. A precipitant deposited on the membrane mayreduce flow, cause permanent chemical damage tothe membrane, and reduce membrane elementlife. Regular removal of foulants, which is more ef-fective than sporadic cleaning, extends membraneelement life and enhances overall system per-formance.

These guidelines address when to clean and whatcleaners or sanitizers to use for each type of Desalmembrane element. The guidelines are based ontechnical information which Osmonics believes tobe accurate and reliable. They are intended forpersons with technical skill to use at their owndiscretion and risk. Because of the conditions ofuse are outside our control, Osmonics does notassume liability for results obtained or damagesincurred through the application of the cleaningsolutions or procedures suggested.

When selecting a cleaner or sanitizer, severalthings must be considered, including the foulant tobe removed, membrane element compatibility, andmembrane type. Cleaning solutions must fall withinpH ranges specified for the membrane element. Inaddition, the cleaner must not contain certainchemical substances incompatible with the mem-brane element, such as certain surface-activeagents and, in some instances, oxidizing agentssuch as chlorine.

Use of cleaning solutions other than those knownto be compatible may reduce membrane life andvoid the Desal membrane element warranty.

Recommended pH ranges for cleaning solutionsare as follows:

Membrane Cleaning pH Range

Cellulose Acetate (CA) 3.0 - 8.0

Polysulfone (PS) 2.0 - 11.5

Polyamide (PA) 2.0 - 11.5

When a cleaning solutions pH approaches the ex-treme ends of the recommended ranges, contacttime with the membrane element and solutiontemperature must be considered and should beminimized whenever practical.

Feedwater composition, seasonal water qualityvariability, system recovery rates, flow rates, oper-ating pressure and feedwater temperatures all af-fect the rate of fouling. As a result, these variablesshould be studied for each installation before de-termining how often to clean the membrane ele-ments. A change in these variables may require achange in the cleaning regimes.

Establishing Baselline OperatingConditions

Permeate flow rates, salt passages for RO mem-brane elements and pressure drop (initial pressureminus final pressure) should be recorded at systemstart-up.


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These records are used to establish a baseline thatwill be used to monitor the system to determinethe proper time to clean.

Salt passage percentages are usually calculatedby determining the total dissolved solids (TDS) orconductivities of the permeate, feed and concen-

trate (brine) streams. Calculating the ratio of TDS orconductivities in the permeate to the average TDSor conductivity between system feed and concen-trate streams will determine salt passage. UsingTDS, the equations are:

When current permeate flow rates are comparedwith start-up rates, it is important to normalize the

data to get a more accurate comparison of systemperformance at different operating parameters.Permeate flow is dramatically affected by feedwa-ter temperature, so the system operator must con-sider the difference in temperature in order togenerate comparable data.

Temperature Correction Factors

Temperature correction factors for all Osmonics Desal reverse osmosis, nanofiltration, ultrafiltration, andmicrofiltration elements. The reference temperature is 77F (25C).

TEMPERATURE F (C) POLYAMIDE MEM-BRANE (PA) POLYSULFONE MEM-BRANE (PS) CELLULOSE ACETATEMEMBRANE (CA)

40 (4) 0.48 0.54 0.55

50 (10) 0.60 0.64 0.66

60 (16) 0.73 0.76 0.77

70 (21) 0.88 0.90 0.90

77 (25) 1.00 1.00 1.00

80 (27) 1.06 1.05 1.04

90 (32) 1.26 1.22 1.20

(Note: Because of special characteristics of the membrane, Duratherm does not follow temperature correction factortrends. Please consult with an Osmonics engineer.)

Please refer to the Operating Guidelines/Terminology section for greater detail.

The operator should also consider other condi-tions in order to generate comparable data. Forexample, changes in the effective pressure (oper-

ating pressure less osmotic pressure and backpressure) also affect the permeate flow rate. Ifthe TDS concentration of the feedwater changesor if system recovery changes, osmotic pressurewill vary and thus change the effective pressurefor RO membrane element applications.

In addition, salt passage percentages for ROmembrane elements are affected by operatingpressure changes just as permeate flow is af-

fected by the effective pressure and feed tem-perature. If the current operating pressure isnoticeably different than the start-up pressure,the current reading should be adjusted and thesystem allowed to reach equilibrium before thereading is compared with the start-up salt pas-sage percentages. For pressure correction fac-tors, consult Osmonics.


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When to Clean

Membrane elements should be cleaned if eitherof the following conditions occur:

1. Membrane elements require cleaning if a 5-

15% drop in permeate flow occurs after the

initial flow stabilization, assuming the Silt Den-sity Index of RO feedwater is less than 5.0 (if

SDI is 5.0 or greater, consult Osmonics for fur-

ther information). In many cases, the operator

may expect some irreversible loss of perme-

ate flow due to system stabilization during the

first 100 hours of initial use. This loss is usually

normal flow loss and does not necessarily in-

dicate a need for cleaning. However, the

amount of lost flow should be carefully moni-

tored in case it is the result of an RO pre-

treatment system malfunction or theexistence of conditions not anticipated during

system design.

2. Cleaning should also be considered when salt

passage from RO membrane elements in-

creases by 30 40%. For example, if an initial

salt passage of 5% increases to 7%, a 40%

increase has occurred. Note - Abrupt and

significant changes in permeate flow or salt

passage can also be attributed to other fac-

tors, such as defective O-rings or flow by-

pass around membrane element concentrate

seals.

The system operator should not make systemcleaning judgements based solely on differentialpressure measurements, as this may result in notcleaning frequently enough. Membrane elementpressure drop increases when foulants or scaleplug the feed spacers between the membrane inspiral-wound membrane elements. When the dif-ferential pressure has increased markedly, a con-

siderable amount of contaminants/sedimentalready has built up in the membrane element.By the time a membrane element reaches thisstage of fouling, it is difficult to restore the mem-brane elements flow and salt rejection perform-ance. To prevent this, cleaning frequencydecisions should be based on changes in perme-ate flow rates or TDS passage.

For systems with membrane elements banked inseries, the circulating and soaking operations canbe done on the whole group of membrane ele-ments or on each bank individually to maintainthe flow rates and pressures recommended inthis section.

During the cleaner circulation process, care mustbe taken not to damage the membrane elementsby exceeding the recommended maximum feedflow rates per membrane element. These maxi-mum feed flow guidelines are found in the Desalmembrane element specifications for each mem-brane element type and vary according to mem-brane element model used.

The systems fluid dynamics must be considered.In many cases, low circulation pressure, typically50% of the normal operating pressure, is recom-

mended. The objective is to have the minimumpressure needed to circulate the cleaner throughthe system. Operation at low pressures with highflow rates will help prevent contaminants fromredepositing on the membrane surface after theyhave been loosened during the soak phase of thecleaning cycle. Higher pressure cleaning is ac-ceptable as long as the fluid dynamics of the sys-tem support it. Consult the system manufacturerfor its recommendation.

Recommended Cleaner Circulationand Soaking Times

Most cleaners should be circulated for 10 - 30minutes, followed by a 10 - 30 minute soakingperiod and then a final 10 minute recirculationprior to flushing to crossflow filtration machine.The cleaner should be thoroughly flushed to drainwith RO quality water. Note - Enzyme cleanersrequire longer residence times to allow for com-plete reaction with the contaminate. Detergentscontaining enzymes should be allowed to recircu-

late and soak for at least 1 - 2 hours before flush-ing.

When contaminant removal is difficult, longer cir-culation and possibly additional soaking timesmay be useful. An additional cleaning cycle withfresh cleaning solution is usually more effective. Afoulant may be composed of different types ofmaterials, making different cleaners and/or multi-


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ple cleaning cycles necessary to increase clean-ing effectiveness.

The circulation flow during cleaning should be inthe same direction as during normal system op-eration. Do not reverse flush from permeate mani-folding through membrane element as damage will

occur.

Cleaning Solution Temperatures

The circulation of a heated cleaning solutionthrough the membrane elements often provesadvantageous because higher temperatures in-crease chemical reactions. Warm solutions oftenstrip scale and/or contaminants faster than ambi-ent temperature solutions. However, maximumcleaning solution temperatures should be keptunder the limits specified for each membrane

element model when using the cleaners cited inthese guidelines. The operator risks damaging themembrane element if cleaning solution tempera-tures go beyond their maximum recommendedcleaning temperature.

Safety Precautions

When using any cleaning chemical, follow ac-cepted safety practices. Read the labels on clean-ing chemical container and refer to yourcrossflow filtration machine operating manual. If

in doubt about handling, safety or disposal pro-cedures, contact the cleaner supplier for detailedinformation before proceeding to prepare or usecleaners.

Cleaning Solution Preparation

All solid cleaning chemicals should be fully dis-solved and well mixed before the cleaning solu-tion is introduced into the system. Use RO qualitywater or filtered, low hardness water (less thanone grain per gallon or 17 milligrams hardness

per liter of water) to prepare cleaning solutions.Acceptability of chlorine or other oxidizing agentsin the water depends on the membrane con-tained in the membrane elements. Reuse ofcleaning solutions is not recommended. Somecleaners have limited shelf life so check the ageof cleaners before using them.

Suggested EquipmentA cleaning solution mixing tank with a cover and atemperature gauge is suggested. Appropriatevalving, sample ports, flow meters, pH monitor,pressure gauges, recirculation pump and car-tridge filter are also recommended. When select-ing cleaning system equipment, the material ofconstruction of the systems components shouldbe chemically and physically compatible with thecleaners and temperatures to be used. A car-tridge filter on the cleaning solution return-to-tankMe or feed line to the crossflow filtration machinewill remove particles dislodged from the mem-brane elements.

Amount of Cleaning Solution Needed

To determine the amount of cleaning solution re-

quired, estimate the hold-up volume of the clean-ing loop piping and membrane element housings.Then add sufficient water to the CIP tank to pre-vent it from emptying when filling the system. Atthe beginning of the cleaning cycle, the processwater in the system should be discharged todrain as it is displaced by the cleaning solution.This process will prevent dilution of the cleaningsolution.

System Sanitizing

To prevent microbial growth in a crossflow filtra-tion system, periodic sanitization with the appro-priate chemical agent may be required. The exactprocedure and chemical used depends on thetype of membrane element and the severity ofthe microbial problem.

System Shutdown Following Cleaning

If the system is to be shut down for 72 hours ormore after a cleaning, a biocidal storage solutionshould be prepared and recirculated into the sys-

tem. (A 0.5% formaldehyde solution is preferred;other biocide or biostat agents are acceptable.Compatibility with the membrane must be deter-mined first.) Note - New (PA) RO membrane ele-ments must be run in a system for at least 8hours before a formaldehyde solution should beused as a preservative or severe flow loss willoccur.


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Refer to the following pages of suggested clean-ers for Desal membrane elements. The listedcleaners are not meant to be all inclusive. If youwish to use different cleaners, consult Osmonicsfor compatibility. Our suggested list of cleanersincludes generic cleaners and a group of proprie-

tary cleaners that have been effective in manyfield applications. Remember that foulants may becomprised of more than one type of contaminant,making multiple cleanings and/or a mixture ofcleaning solutions necessary.

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