Demystifying NSF/ANSI standards for water filters (part 3): standard 42

Continuing from my earlier 2 posts (1, 2) on NSF/ANSI standard 42, here's the meat in describing this commonly encountered standard for "drinking water treatment unit" (read water filters).


If you are the lucky few who have a copy of the document, it certainly makes for good bedtime reading i.e. getting you to sleep. Therefore, I will try to be short and sweet in this post.


Even though the title of this standard states "aesthetic effects", it actually covers quite a lot of details in a water filter, and not just how well the filter works in improving the aesthetics of drinking water.


The standard apparently can apply to a "system" (e.g. entire filter unit) or a "component" (a single part of a filter unit). I assume my readers are more interested in evaluating an entire system so I shall focus my discussion on a "system". (This decision is also a conservative measure since the requirements for a component are less stringent than for a system.) Buyers beware: make sure the standard covers the whole filter system you are buying, and not just certain components inside.


To be certified, a filter system must satisfy the five following conditions.

1. System materials in contact with drinking water must not leach chemicals into the water within specific limits. Chemicals to be tested include aluminium, mercury, vinyl chloride, xylenes and many others. Much is described about how testing is done and the chemicals of concern.
   
   In other words, you don't want to buy a filter that adds poisons into your drinking water. With such a filter, you don't need poor filtration effectiveness to harm yourself.
2. Structural integrityThis part talks about pressure testing the materials, design and fabrication quality of the complete system. Basically, your system should not fail (burst, leak) under specific conditions.
3. Minimum performance requirementsThis slightly misleading title actually refers to mundane matters not covered in the other 4 criteria e.g.
1. Minimum service flow rates to be achieved under different configurations
2. Electrical safety and operation complying with National Electrical Code or equivalent.
3. Filter media (e.g. activated carbon) should not migrate visibly (read leak out of the filter).
4. A few other requirements
4. Reduction ClaimsThis is most often mentioned aspect of standard 42, whether by filter suppliers or customers - how the filter reduces the concentration of a certain substance in water.
   
   Buyers beware! This requirement is potentially the one most likely to confuse a consumer on the effectiveness of a filter. To be certified, a filter has to fulfil a reduction claim of ONLY ONE substance. Make sure you know which substance that is. ABC filter certified to reduce the concentration of only chloramines will have unknown effectiveness against chlorine.
   
   Again, please remember that standard 42 is designed to certify a filter for improving aesthetic effects e.g. taste, odour, colour. It is not designed for the removal of adverse health effects e.g. heavy metals, pesticides. You have to look to standard 53 for that.Substances of concern under standard 42 include:
   Bacteriostatic (NOT killing bacteria but keeping bacteria levels more or less the same before and after filtration),
   Chloramine reduction,
   Chlorine reduction,
   Hydrogen sulphide reduction,
   Phenol reduction,
   Particulate reduction (there are 6 classes denoting different particulate size ranges so do check out which class is being certified),
   a few more other parameters e.g. iron, zinc, manganese
   
   Again, we see the usual descriptions of testing methods and influent challenge (i.e. the characteristics of the inflow into the filter)
   
   Here is an example of the reduction requirement for chloramine:
   Average influent (aka inflow) challenge concentration: 3.0 mg/L +/- 10%
   Reduction requirement: 0.5 mg/L (meaning the outflow should have a concentration of 0.5mg/L or less)
5. Instruction and installation
1. Details must be provided for installation, operation and maintenance
2. Some other details
3. And easily as important as the reduction claims, a performance data sheet must be available to potential buyers
   
   Buyers' tip! Scrutinise the performance data sheet. (Some filters not certified under this standard may have it too.) It may look discouragingly technical but it should look similar to the following example. (Influent challenge refers to the test sample going into the filter while product water refers to the outflow from the filter.)
   
   Obviously, if the filtered is certified, its performance should equate or exceed that of the standard.

Sample Performance data sheet reduction claims
Substance	influent challenge concentration	maximum permissible product water concentration
Chloramine	3.0mg/L ± 10%	0.5mg/L
Iron	3-5mg/L	0.3mg/L
foaming agent	5mg/L ± 10%	0.5mg/L

Believe me, I have really tried to simplify NSF/ANSI standard 42 in these 3 posts as much as possible for the layperson. Hopefully, you now have a clearer idea what this standard is all about.


Good luck!

Figure: A few rare filter housings I have seen