Examining the differences between POU and POE systems in the NSF/ANSI DWTU Standards
There are obvious differences between point-of-use (POU) and point-of-entry (POE) water treatment products. For example, POE treats water for the whole house, or a majority of the house, while POU treats water for a single tap or multiple taps, but not a majority of the house. For POE systems, pressure drop is a major concern because most of the household plumbing is downstream of the system. In addition, POE systems may be treating water for relatively long periods of uninterrupted flow when washing machines or showers are running. POU systems, however, are typically used for relatively short periods of flow as consumers fill glasses of water, and they have longer stagnation periods when they are not used. POE systems typically have much higher capacities because they are treating many times more water than POU systems.
As a result of these differences, distinctions are made between POU and POE systems within the NSF/ANSI DWTU Standards. This article will explore some of those distinctions relative to the various standards and technologies, describe the requirements and explain the logic behind them.
Application of Standards
Different standards address POU vs. POE differently. A summary of this distinction can be seen in Figure 1. Probably the most notable information in Figure 1 is that POE is excluded from the scope of Standard 58. This means that no whole-house reverse osmosis (RO) sys-tems can be certified to the standard. Modifications to the standard would be required to address POE systems in order to make this happen.
Some of you may be wondering why this is the case. Although POE RO systems have become more common over the last few years, there are some technical issues with respect to POE RO systems that are different from POU systems. Currently, Standard 58 does not address these technical issues. The NSF Joint Committee on Drinking Water Treatment Units recognizes this, and therefore has determined that only POU systems are within the scope of the standard in its current form. Some of these issues are identified below.
Because of the differences in POU and POE systems, the test methods for assessing performance are different as well. Let’s examine some of these scenarios to highlight some of the issues, starting with RO systems.
POU RO systems typically connect under the sink, or maybe in the basement, and include all connections up to and including the dispensing faucet — pre-filters, RO elements, post-filters, tanks, tubing, booster pumps, etc. Most of them produce about 15 to 30 gpd, as daily production rates are measured per NSF/ANSI Standard 58.
Contrast POU with a POE RO system. POE RO systems typically connect downstream of the tee-off for outside taps, and possibly downstream of a water softener. They include pre-filters, RO elements, post-filters, tanks, and booster and re-pressurization pumps. The product water for a POE RO system is fed back into the plumbing system of the home to supply kitchens, bathrooms, laundry and so on. These systems may produce upwards of 1,000 gpd.
These differences in system scope and operation lead to differences in criteria and procedures for evaluation of the systems. For example, RO water can be aggressive in corroding copper plumbing. This is not a concern in a POU system because the manufacturer can supply plastic tubing and appropriate faucets to prevent inappropriate materials from being in contact with RO water. But a POE RO system installed in a home with copper plumbing downstream of the unit could lead to problems with corrosion of the copper pipe, and possibly even copper contamination of the drinking water. The standard would need to address this concern through product literature requirements, such that installers would be made aware that the plumbing downstream of the system must be compatible with the product water from the RO system.
Another example is the test protocol for contaminant reduction under Standard 58. Under the weeklong protocol, testing on days two through four consists of drawing a series of samples equal to 5% of the daily production rate for analysis of contaminant reduction performance. For a typical POU system, this means a sample of about 1 gal. But for a system producing 1,000 gpd, for example, this requires a sample of 50 gal. This may be operationally difficult for testing purposes, and it may also be inappropriate. Depending on system design and operating characteristics, contaminant “creep” through the membrane during the first few gallons of operation could be offset by the large sample size. Testing under this protocol could demonstrate acceptable results, whereas depending on system design and operating characteristics, actual users drawing small volumes could receive water of unacceptable quality.
Standard 42 includes protocols for POU and POE filter systems, and they are different from each other because of the differences in system operating characteristics, designs and capacities. The following is a summary of these differences:
- For contaminant reduction testing, POU systems are operated with on/off cycling. This means that flow of contaminated challenge water is intermittent. This protocol mimics the usage pattern of a POU system. POE systems are operated continu- ously 16 hours per day with an overnight rest period because the capacity may be very high, and testing with cycling would cause the testing to take longer and therefore cost more. This is also appropriate because POE systems may also see longer periods of continuous flow.
- POU systems are tested for contaminant reduction in duplicate. Due to the high capacity of POE systems, in order to help control costs, only single POE systems are tested.
- Very specific laboratory-controlled contaminant-reduction test water is required for POU systems. For POE systems, public water supplies containing the contaminant of interest may be used. This allows flexibility and helps control costs for testing of high flow rate, high-capacity POE systems.
It is interesting to note that Standard 53 does not distinguish between POU and POE systems for contaminant-reduction test protocols. On/off cycling is used, duplicate testing is required, and laboratory controlled test waters are required for all POU and POE systems under Standard 53. The rationale is that the contaminant-reduction claims under the scope of Standard 53 have health effects associated with them. The allowances to help control costs for testing of aesthetic claims on POE systems are not appropriate when systems are being evaluated for health claims.
Also of note is that pressure drop of POE systems may not exceed 15 psig at the rated service flow under standards 42 and 53. Alternatively, if the systems include a flow controller, at least 4 gpm of flow at 30 psig inlet pressure must be achieved through the system. There are no pressure drop requirements for POU systems.
Technology, Usage Dictate Requirements
There are seven NSF/ANSI DWTU Standards, with different standards addressing different technologies (see Figure 1). These technologies have varying capabilities, operating parameters and usage scenarios. The standards have varying requirements and test methodologies to address each technology appropriately.
Much as technologies vary, POU and POE have differences, too. The NSF/ ANSI DWTU Standards take these differences into consideration by distinguishing them, and where appropriate, developing alternate test procedures and requirements to address them.