The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Avoid fouling to keep RO systems running smoothly
Previous articles in this series explained principles of how reverse osmosis (RO) systems work and the main factors and components that impact their performance. Once a point-of-use (POU) RO system is operating in the field, there are several troubleshooting tips that can prevent failures due to shortened lifetimes. These failures are usually caused by scaling, biofouling or particles. This article will cover techniques and tools that are used to troubleshoot these failures.
Sights, Smells and Sound
RO elements that display poor performance will have reductions in flow rate or increases in salt passage. Confirming that the low flow or high salt passage can be traced to the RO vessel is the first important indicator that the RO element has been compromised. By removing an element from its housing in a residential system, one can begin to troubleshoot where the actual problem lies. This process will render the element unusable; however, the element is usually not serviceable at this point.
One simple test is to tear open the element and look at the leaves. If the leaves are discolored or have particulates, it is usually a sign of debris, scaling or biofouling. Figure 1 shows how fouling often leaves visible signs on the element, such as the scaling in the photo.
Debris fouling may occur when feedwaters with high levels of particulates are entering the RO element. The usual cause is sediment prefilters that are either not installed or need to be replaced. Common sediment prefilters include PPE string filters, which will pass through sedimentation once the prefilter is overwhelmed. Sometimes, activated carbon filters can foul the RO filter by passing black carbon fines into it. Another cause, if the unit is used on a well, could be changes in the well water caused by seasonal or weather impacts. In any of these cases, checking the sediment prefilter or carbon filter is the likely solution. Particulate fouling occurs at the leading edge of the RO element, so this is the first place to look.
Biofouling, which also occurs at the leading edge of an RO element, usually has a slimy consistency and may be green, brown or yellow in color and have a noxious smell. This occurs because of bugs that are growing on the membrane that feed on the organics in the water. Figure 2 shows a photo of extensive organic and biofouling on the membrane surface.
Usually these organics are removed from the water by the carbon prefilter, so check the carbon prefilter first as it may need to be replaced. A carbon prefilter that has spent its capacity will no longer adsorb the organics and they will pass through to the RO element.
Scaling is the only type of fouling that begins at the concentrate end of the element. It occurs when the calcium carbonate in the water precipitates out onto the membrane. If the end of the RO element is squeezed after removing it from the housing and a crunching feel and sound is sensed, scaling is the likely cause. Decreasing the recovery of the unit is an easy way to reduce the likelihood of scaling.
Besides relying on one’s senses, more advanced tests can be used to confirm which type of fouling has occurred. If a few drops of acid are placed on the membrane and the liquid turns yellow, the most likely foulant is iron, as shown in Figure 3.
If a few drops of acid are placed on the membrane and they bubble, then calcium carbonate scaling is the issue. Figure 4 shows acid that bubbles, indicating calcium carbonate scale. Acid also will often clean the membrane surface when mineral scalants have fouled the element.
When a few caustic drops are placed on organic fouling or biofouling, it can clean some of the membrane surface, as shown in Figure 5.
Solving the problem of membrane scaling may seem tricky because the calcium carbonate level is difficult to monitor; however, there are several solutions OEMs or end-users can consider besides decreasing the recovery. The most common solution is to use a water softener to exchange the calcium in the feedwater with sodium. There are a variety of residential softeners on the market. The disadvantage of a softener is that it needs to be regenerated from time to time with salt.
Another solution is to design a system that has a periodic rinse through the element. This will allow any scaling that has begun to form to dissolve back into the water as it flushes by the membrane surface. The disadvantage of this solution is that it increases the complexity of the RO system and wastes some water.
The third solution is to use polyphosphate prefilters, often combined with granulated activated carbon into combo filters. These act similarly to softeners in that they reduce the likelihood of scale formation. Their disadvantage, however, is that the seedings or gels from these filters can deposit on the membrane and decrease RO performance.
If an acid test confirmed that the element has iron fouling, there are also several solutions to consider. The first is to stop air entrainment from entering the feedwater. If the water source is a well, the casing is a likely area to inspect for leaks. Installing oxidizing filters using agents such as chlorine, ozone or manganic oxides to react with iron and filter it out of the water is one possibility for removing iron. Green sand filters, another option, may remove a large percentage of the iron; however, overdosing may be an issue. Similarly, there are micron prefilters that remove rust and large particles, which may also help remedy the iron problem. Lastly, if pH control is available, keeping the water at a lower pH helps to stabilize the iron in the water.
In order to maintain the overall performance of a POU RO system, it is important to carry out the required preventive maintenance. This includes changing the prefilters regularly as recommended by the manufacturer and checking the unit whenever there are large fluctuations in feedwater quality.