Field Guide for Point-of-Use RO Systems
Impact on permeate water from changes in residential POU RO system components
There are several factors that impact the performance of a residential point-of-use (POU) reverse osmosis (RO) system. The previous article in this series (“Factors that Impact RO Filter Performance,” March 2010) highlighted how changes in the feedwater quality can impact the quality of the permeate water. This article focuses on how changes in the components used in a residential POU RO system impact the permeate water.
Before building an RO system, one must consider what water capacity is needed. The average person needs to consume a minimum of 2 liters of potable water daily. Daily POU RO water consumption can be approximately 1% to 2% of total home water use, or approximately 4 to 8 gal per day (gpd) when factoring in RO water used for cooking, watering indoor plants or tasks such as operating small steam appliances. During cooking and meal times, peak demand use could range from 1 to 2 gal per hour for several hours. This amounts to potentially needing a water production rate of 24 to 48 gpd during peak demand times.
Standard POU elements are typically listed based on a product flow production rate covering 24 hours at a specified feed pressure and feedwater condition. If a storage tank is used, the POU system will produce 30% to 45% less flow than the element rating. This is referred to as the Daily Production Rate (DPR) as described in ANSI/NSF 58. Under the standard, DPR is determined by measuring the time and volume required to fill the tank from empty to the point where the automatic shut-off valve (ASOV) terminates flow, and the time and volume required to fill the tank from the point where the ASOV starts flow to the point where the ASOV terminates flow. It is important to choose elements with a daily production rate high enough to meet the peak demand rate and total daily water consumption of the POU unit.
Furthermore, changes in feedwater pressure and temperature can impact the system performance. Factors such as winter water temperatures or if the water is gravity fed versus using municipal tap water can dramatically influence element options for a system. A low-flow-rated element may be suitable for a small family or when using a booster pressure pump, whereas a high-flow-rated element may be required for large households or water with low feed pressures.
Most tanks consist of a butyl rubber diaphragm with an air chamber on one side and water storage on the other. The air chamber is pumped with air so that when the tap is opened, there is sufficient air pressure to push water out of the tank to the consumer. This air pressure creates back pressure on the membrane, which reduces the net driving pressure (NDP). As the water volume of the tank increases, the back pressure increases, which continues to reduce the NDP, as shown below in Figures 1 and 2.
There are other components of a POU RO system that also impact system performance. As mentioned previously, the ASOV determines when the system stops producing water. These valves operate by opening and closing when the pressure between the permeate water to the tank reaches certain percentages of the inlet feed pressure to the RO element. This, in effect, limits the back pressure on the RO element. ASOVs that are set to open and close when the tank back pressure is higher maximizes the water available for consumption because more water is stored in the tank. However, the element also produces water of a lower quality as the tank fills because it is forced to operate at a lower NDP. ASOVs that are set to open and close when the tank back pressure is lower maximize the RO element performance in terms of flow rate and water quality because it operates at a higher net driving pressure; however, less water is ultimately stored in the tank. Figures 3 (see page 12) and 4 (above right) present these two contrasts for the impact of the ASOV with storage tanks on NDP and salt rejection.
Testing RO systems under NSF/ANSI 58 involves a variety of usage conditions (full tank draws, partial tank draws, stagnation periods, etc.) not only when measuring DPR but also when evaluating contaminant reduction performance. The impact of the operating pressure range of the ASOV on membrane performance influences the DPR measurement and contaminant reduction percentages of the RO system as determined under Standard 58.
Another important characteristic of any POU RO system is the system recovery. The recovery is defined as the percentage of feedwater that is turned into product water with the RO faucet open. Most residential systems operate at recoveries between 20% to 40%. The recovery of a system is primarily controlled through a restrictor line or orifice on the concentrate line, which limits the flow of water going to the drain. These valves are usually made up of long capillary tubes and are rated for the desired concentrate flow rate.
OEMs that install restrictors with lower flow rates force the system to operate at a higher recovery. This has the benefit of producing more water and sending less to drain. However, as Figure 5 (see page 14) shows, both the quality of the water and the flow rate generally decline as the recovery increases for residential elements. These effects can also influence the DPR measurement and contaminant reduction percentages of the RO system as determined under Standard 58.
Furthermore, certain feedwaters are incompatible to be run at high recoveries because of their tendency to scale. Feedwaters that are high in hardness are not good candidates to be run at high recoveries because the calcium will tend to precipitate out of the feedwater onto the membrane surface as the concentration increases. This increase in concentration occurs because permeate water is pushed out as the feedwater travels along the element. Therefore, the hardness concentration is much lower at the feed end of the element versus the concentrate end. This is why scaling failures are always seen at the concentrate end of elements first.
OEMs who are interested in operating systems at higher recoveries to minimize wastewater sent to the drain must first ensure that the feedwaters where the systems are used are not too high in hardness or other unstable mineral scalants. Second, they may want to compensate for the lower flow and rejection caused by the high recovery setting by using another RO element that achieves a nominal flow and rejection. OEMs should check with their RO element supplier to discuss these alternatives.
The next article in this series will focus on how to avoid field failures from reduced lifetimes in RO systems by providing techniques to troubleshoot the root cause.