We thank Lindquist and colleagues (Lindquist et al. 2015) for their critical review of our research article on fouling in hollow fiber membrane microfilters used for household water treatment (HWTS) (Murray et al. 2015). We welcome intelligent discourse which furthers efforts to provide microbiologically safe and reliable drinking water to those currently without it.

Although we feel the original article provides clear background information, we would like to re-iterate the conditions under which this study was undertaken, as Lindquist et al. (2015) raised several questions indicating misunderstanding:

Pure Water for the World (PWW) distributed Sawyer PointOne filters (Sawyer Products, Inc.) following their standard training and distribution program, and not as a controlled field trial. To monitor and evaluate the pilot project, PWW conducted household surveys and microbiological and turbidity testing of both source and filtered water at various times post-distribution. This in situ testing identified six filters that demonstrated >99% mean Escherichia coli removal 2 months following distribution and 54% mean E. coli removal 23 months following distribution (Goeb 2013), despite users demonstrating the correct backwashing procedure and self-reporting backwashing with adequate frequency. PWW's concern about reduced performance prompted this laboratory investigation of those six filters.

The critical review identified concerns about the laboratory study methodology with regard to: (1) pre-analysis filter storage conditions, (2) filter cartridge entry, (3) sample size, and (4) article figures. We address each comment below.

PRE-ANALYSIS FILTER STORAGE

We would like to take this opportunity to rectify that we did not include ‘physical placement and ambient conditions’ details for filter transportation. Filters were removed from homes as found, sealed together in one plastic bag, transported to the USA in carry-on luggage, and sent by ground transportation to the University of Maine, where they were analyzed upon receipt.

Microbiological testing and cleaning could not be done in situ for various reasons. The filters were removed from homes due to unexpected poor performance identified through field microbiological testing, but additional laboratory equipment was unavailable. Detailed cleaning instructions (to soak the filter in hot water, backwash, soak in vinegar, and backwash again) were obtained after subsequent discussions with a Sawyer Products technical representative, and as such, could not have been performed in the field as suggested. Also note that after cleaning the filters as above, water with no bacteria or turbidity (sterile) was run through the filters, and that sterile water exited as visually turbid and contained bacteria.

So while there was a remote possibility of cross-contamination within the storage bag and a possibility that existing bacteria from a biofouled membrane could have grown, there was no possibility that the membranes froze. Furthermore, none of these concerns impact the conclusions of our paper, specifically: appropriate cleaning procedures for the Sawyer PointOne filter membrane, and the filter's useful life span. Filters intended for use in development and recreational contexts demand a robust design; adequate literature indicating safe cleaning, storage, and transportation requirements must accompany the product.

FILTER CARTRIGE ENTRY

Both new and used filter cartridges were opened in the same manner, and plastic fragments are visible on both membrane surfaces in article Figure 2 (Murray et al. 2015). If plastic debris contributed to the fouling layer, as suggested: (1) it would potentially appear on scanning electron microscope (SEM) images of both NEW AND USED membrane surfaces, and (2) surface elemental composition would likely identify a higher percentage of carbon in the used membrane. Instead, the fouled membrane exhibited increased silicon, aluminum, iron, lead, potassium, calcium, and magnesium on exterior and interior membrane surfaces.

SAMPLE SIZE

This research investigated filters pre-identified as performing poorly based on microbiological field testing. We freely acknowledge and stated that ‘limitations of this work include that few filters were analyzed’. We make no claim to identify the extent of the problem, and intend these findings to be illustrative of a potential long-term challenge that requires further investigation. We recommend additional research to determine: long-term microbiological performance, impacts of varying water quality, fouling and breakage rates, fouling management recommendations, and lifespan and end-of-life indicators for the PointOne filter.

ARTICLE FIGURES

Regardless of SEM image magnification levels, we believe readers will recognize the marked difference between the new membrane (whose structure can be clearly observed), and used membrane surface (which appears to be covered by a compressed layer of foulants) (Figure 3, Murray et al. 2015). We also believe particles on the interior membrane surface are clearly visible in Figure 4. While no image of burst membrane fibers is included in the article, and we cannot be completely certain, based on visual observation we contend that fibers appeared to have burst prior to filter cartridge opening. Cartridges were carefully cut in the gap between the filter inlet and looped membrane fibers, which are not attached to the cartridge at this end. Broken fibers were observed throughout the bundle, and not solely at looped ends as they potentially would have been from careless cartridge opening.

Finally, while the corresponding author would have gladly provided any references upon request, we regret that some were inaccessible at the time of publication. Reports are now available on the PWW webpage.

We would like to emphasize that we do not refute findings that the Sawyer PointOne filter is capable of removing bacteria and protozoan parasites in laboratory test waters (Hydreion 2005), and has been shown to reduce diarrheal disease among users <5 years over a 3-month follow-up (Lindquist et al. 2014). Membrane filtration has long been used for drinking water treatment, and is a promising HWTS technology if applied in the appropriate setting with an adequate and attainable operation and maintenance regimen.

Microbiological efficacy of new units and short-term diarrheal disease reduction evidence are necessary, but not sufficient, to ensure safe and effective long-term HWTS product use. We share Lindquist et al.'s view that more research is needed on long-term household PointOne filter performance, and, even if the work is manufacturer-funded, thank them for their efforts to contribute to this body of knowledge.

However, it is critical that technological limitations of products be recognized; reversible and irreversible fouling of hollow fiber membranes is well documented (Guo et al. 2012), and fouling management cannot be ignored. Lifespan claims must be defensible by evidence representing realistic usage conditions in the demanding contexts of developing country communities where products are promoted.

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