By Matthew Wirth

There are stories about the membrane that lasted eight years. Manufacturers sing the praises of minimal reject to product outputs. While the stories are often true, there is cause and effect to everything. The one side giveth and the other taketh away. Small systems, for lack of sophisticated flushing and cleaning mechanisms, foul more readily than commercial and industrial (C&I) membranes.

This discussion concentrates on small systems. Membrane applications for commercial and industrial systems function under far more scrutiny and are outside this discussion. Industrial and commercial systems often utilized membrane flushing, clean-in-place (CIP) stations and anti-fouling chemical feed. These system functions keep the far more expensive membranes in C&I application operating in spite of the fouling conditions affecting smaller systems.

Several base principles factor greatly into membrane life, product-water quality and rejection rates. By understanding these principles, one can begin to make better choices, explain to customers their options for membrane operation and maintenance and prescribe best practices for servicing, sizing and installation.

Operation review
The RO process uses pressure to force water containing inorganic mineral salts, organics, suspended colloidal particles and other disassociated species through a semi-permeable membrane. The osmotic pressure exerted by the flow of the feed water through the semi-permeable RO membrane separates the feed water solute with its high solids concentration and the RO product-water with a low concentration of solids. In layman’s terms, the RO process retains foreign matter on the feed-water side of the membrane, allowing very clean water to diffuse through to the product-water side.

Feed line or booster-pump pressure must provide enough kinetic energy to overcome the trans-membrane (osmotic) pressure requirement and allow water molecules to pass through the membrane. Porous backing material allows rinse water to rush aggressively across the membrane surface in a “washing” action. A throttling valve restricts the concentrate wash water to hold pressure against the membrane. (Figure 1).

Physics of RO
There are several unavoidable truths about filtration/separation affecting every membrane application from microfiltration to ultrafiltration to RO. The pores of the membrane must be small enough to remove the intended impurity. While RO is not actually a filtering mechanism, RO membranes do have pores and they do plug. The same fouling that affect other membrane filtering mechanisms also affect an RO membrane.

In RO, the membrane allows water molecules to pass while retaining particulates and mineral salts on the feed-water side of the membrane. If particulates, organics and/or minerals dead-end into the membrane pores, the process slows or stops. The pores become plugged (Figure 4).

This is why RO utilizes cross-flow filtration. The flow across a membrane surface, also referred to as tangential, keeps water and foreign bodies moving. This flowing action helps prevent the formation of a cake layer on the membrane surface.

The result of a cake layer building on the membrane surface reduces water production and increases the osmotic pressure requirement for passing water through the membrane.

Cross-flow (washing flow in Figure 5) stratifies foreign matter along the membrane surface so water freely permeates through the membrane. It becomes a balancing act to keep the flow to the drain high enough to prevent material from caking and penetrating into the membrane, but not so high as to be wasteful with the feed-water. Additionally, the flow to drain reduces pressure on the membrane thus reducing permeate flow or flux. Add to this the practice of recirculating a portion of the waste stream back across the membrane (increasing the concentration of solids on the feed water side of the membrane) and the fact that the reverse osmosis process attracts and draws particles into the membrane pores, one quickly sees that membranes function in a hostile environment.

Concentration ratios
It becomes imperative that designers and technicians follow manufacturer recommended permeate-to-concentrate ratios. Water chemistry plays a huge role in calculating these ratios. Without a comprehensive water analysis, feed water temperature and pressure the manufacture will find it difficult to offer quality information on operation.

Water is one of a very few substances that can exist as a gas, a liquid, or a solid. Water refers only to its liquid form. It is also ice and steam. The viscosity of water changes as it heats and cools. The warmer water is, the less viscous it is until it reaches 212°F (100 °C) where it begins to boil and becomes a gas. At 32°F (0 °C), it freezes and becomes a solid. In between, it ranges in viscosity. The ‘thicker’ water is, the more difficult it becomes to push it through a membrane.

RO membrane construction limits their resistance to temperature. High temperatures, approximately 104 °F (40 °C), will damage a membrane – membrane warranties offer a high temperature limit.

In addition, water temperature effects membrane output and changes the concentrate to permeate ratio. Water temperature affect a mineral’s solubility and organic growth. Because many smaller systems do not have adjustable concentrate flow restrictors, one can only increase pressure to increase output in colder water (Figure 7).

As an example, steam flows through cloth quite readily, while slush does not. Compromising the cross-flow may adversely affect membrane function. Poor or inadequate washing of small system membranes leads to system failure.

Selection decisions
There are many types of membranes, including some for cold water, high TDS and standard operating conditions. There are even nano membranes and high flow membranes. No membrane, however, can escape the law of physics relative to osmotic pressure.

Membranes require pressure to drive water molecules through the membrane pores. The work required to overcome the osmotic pressure loss across a membrane is the ‘trans-membrane pressure’ (TMP.) TMP varies with the external conditions such as temperature, TDS levels/feed water quality, provided pressure and concentrate flows. System designs must consider these physical conditions during sizing and planning.

If the cross-flow is too slow, there is a risk of plugging the pores. In contrast, if the rejection rate is lower, pressure against the membrane is greater; therefore, permeating flow is initially greater. While this appears to be beneficial to water conservation and output, this enhances the cake-layer development and membranes will likely foul at an accelerated rate.

Water softeners and pretreatment filtration can help with chlorine degradation and scaling. There are chemical pretreatment regiments utilizing acidifiers, anti-scalants, disperants, etc., to lessen the effects of reduced production caused by fouling. But with many of the smaller systems, this is impractical.

It often becomes an exercise in economics and conservation, deciding to decrease the concentrate stream in order to reduce rejection and increase permeate flow. If water supplies are limited, then it may pay to replace membranes frequently to save water or apply an aggressive chemical pretreatment.

If concentrate goes to a gray-water system or backing waterways, it may pay to increase concentrate flow and lengthen membrane life. Chemicals used to prolong membrane life have an operation and maintenance cost.

Do these costs outweigh the costs of new membranes or membrane cleaning? It quickly becomes evident that there are no simple answers.

Membrane life expectancy
No one can forecast the operational life of a membrane across the entire range of water supplies and pressure conditions of the system. The best predictions are only guesses.

Expect that a small-system membrane will fail sooner on higher TDS water where a high volume of colloidal particles and scaling minerals are in close proximity to the membrane surface. This condition likely requires higher cross- flows.

Membranes in cleaner water will provide better permeate flows with less rejection, but may see more organic fouling from extended in-service runs. In colder water, one might decrease the cross-flow to increase permeate production. But this is a risky practice.

Increasing feed-water temperature has its own risks. Polymer membranes become more compressible in warmer water and the resulting compaction can prematurely foul the membrane.

The best practice is to schedule system check-ups and monitor changes in permeate to concentrate rates. In this way, one can track the history of the system and schedule filter and membrane change-outs on a timely basis.

Water-borne constituent
A number of water borne constituents effect reverse osmosis membranes such as:

• Chlorine/chloramines
• Calcium carbonate scale
• Sulfate scale of calcium, barium or strontium
• Metal oxides (iron, manganese, copper, nickel, aluminum, etc.)
• Polymerized silica scale
• Inorganic colloidal deposits
• Mixed inorganic/organic colloidal deposits
• NOM material (natural organic matter)
• Man-made organic material (e.g. anti-scalant/dispersants, cationic polyelectrolyte)
• Biological (bacterial bio-slime, algae, mold or fungi)

Always check with the membrane manufacturer for water test criteria and provide current, accurate water test results before installing an RO system. Membranes vary in materials and operational conditions. No one knows better than the manufacturer how their membranes functions under specific conditions.

Vernacular
RO has its own vernacular and a review of the terms will help better explain the activity.

TDS: Total dissolved solids in the water.

Trans membrane pressure: TMP = {(feed pressure + *retentate pressure) / 2} – permeate pressure (Novasep, 2009). It is the minimum pressure needed to push water molecules past the membrane. This will vary with changes in pressure, membrane fouling, temperature and TDS (Retentate) levels.

Retentate: That which is retained. These retained solids do not pass through the membrane and flow to the drain. Retentate is the TDS contained in the concentrate/reject stream and on the feed-water side of the membrane.

Concentrate: Concentrate or reject is RO system wastewater. This is water-containing colloids, salts, organics and other disassociated species removed from feed-water.

Cross-flow filtration: In cross-flow (tangential) filtration, water flows along the surface of the membrane. Driven by pressure, some water passes through the membrane. Tangent means ‘touching,’ Thus cross-flow is touching the membrane. (Figure 2)

RO treated water: Reverse osmosis product-water has several names. The most common is permeate (water that permeated the membrane). Other terms for RO water are product-water filtrate.

Flux: Is the rate of flow through an RO membrane.

Dead-end flow filtration: The dead-end flow process is similar to and includes that of a cartridge filter where there is only a feed flow and filtrate flow (no concentrate flow). In dead-end filtration, an immobile cake layer of matter builds on the surface of the filter medium.

Cake layer: This is a layer of filtered particulate and scale
build-up on a filtering surface that accumulates over time and blinds filter surfaces. In some cases, cake layers assist in filtration and in other cases, it blocks the filtration process. The result of organics dead-ending onto a membrane and the resulting cake layer (Figure 3) can be damaging. (Johnson, Culkin, & Monroe, 2003).

References

1. Johnson, Culkin, Monroe, (2003). Kinetics of Mineral Scale Membrane Fouling, technical article, New Logic Research, Incorporated, Emeryville, CA, retrieved from www.vsep.com.

2. Novasep for the Life Sciences, (2009). Definition of Transmembrane Pressure, retrieved from www.novasep.com/misc/glossary.asp?defId=216&lookfor=&search=T.

About the author
Matthew Wirth is Regional Sales Manager/Dealer Development for Hellenbrand, Inc. He has spent 29 years as a water professional since earning his BA from Concordia University in St. Paul, MN and engineering training from the South Dakota School of Mines & Technology in Rapid City, SD. For additional information, contact Wirth by phone (608) 849-3050, fax (608) 849-7389 or email [email protected].