TECHNICAL PAGES: Professor POU/POE
Whole-house RO
I consume an average of 200 gallons of water a month, all of which I collect from rain, channeling roof ( 1,800 sq. ft.) runoff into a 2,000 gallon, above-ground, swimming pool from two gutters. The initial runoff (wash) from each gutter is diverted into two 50-gallon drums; when the drums are filled, the water diverts to the pool. The pool is covered and I maintain the pool water between 1-4 ppm chlo- rine and initially filter it with the pool filter to remove the bulk of any sediment. I then pump the pool water into a 550-gallon poly storage tank, which I also maintain at 1-4 ppm chlo- rine. I then pump the water through a 5-micron carbon filter into a pressure tank for household distribution. For drinking and cooking, I run the water through an RO unit, recycling the RO wastewater. My goal is to eliminate the RO unit because of the waste produced — about five to seven gal- lons for each RO gallon. I also wish to replace the chlorine disinfection with UV. My tentative solution is to pump the filtered, unchlorinated water from the pool to the poly tank for storage; then pump the tank water through the 5-micron (or other) filter; then through the UV unit. At this point, the water would be restricted to gen- eral household use (cleaning, bathing and toi- let). For drinking and cooking (maximum of three gallons a day), I am considering a ceram- ic or other filtration regime. So my search is to define the most efficient way for me to filter the rain water to eliminate shadowing vis-à-vis UV disinfection, and then re-filter the water for drinking and cooking to enhance the taste and remove whatever poten- tially harmful stuff there might be lingering that was washed off the roof and bypassed the carbon filter. — New Mexico
By necessity, you have a rather complicated system and I’m not sure my response will cover everything suf- ficiently. So consider this food for thought as opposed to a guaranteed solution. Every tank where you presently have chlorine you have constant disinfection. If you use UV, which carries no residual dis- infection, you will have to recirculate the
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Editor’s note: In response to a recent Professor POU/POE column, a reader elaborates and provides additional insight for Water Technology readers.
Letter to the Professor
Residential water dealers and participants,
I was motivated to write this response regarding RO induced copper corrosion
because of my extensive experience and research in this subject.
While I completely agree with David’s answer in his POU/POE column, the growing
trend to install total home RO systems is extremely precarious and deserves further
comment.
First, it is clearly impractical to replace copper pipe with plastic in such installations
(a separate central third-faucet plastic distribution system can be an acceptable option).
Dealers and technicians are often lulled into a false security by passing the RO permeate through some neutralizing media, e.g., calcite and magnesium oxide. This is not
a foolproof long term solution because the chemistry of copper corrosion is far more
complex than pH, alkalinity, LSI and other corrosion factors. Such measures may delay
corrosion and potential leaking but it is far from being a 100 percent safeguard.
Doing a search of the AWWA articles (from 1990 forward) on copper corrosion is a
revelation as this moving target is argued among specialists in the field. You will learn
that the balance of specific cations and anions is one of the critical factors agreed
upon by the experts. Unfortunately, media-neutralized RO water does not generally
meet the desired criteria.
Since the liability is so great if one should be responsible for sporadic leaking of res-
idential plumbing from total-home RO treatment, my suggestion is to do your home-
work, encourage the WQA to start a study group on the issue and increase your insur-
ance coverage.
Regards,
Robert Slovak
should not need mineral reduction because your source is rainwater. As a safety measure, you could consid- er another UV after the UF for added pro- tection. WT
David M. Bauman, CWS-VI, CI, CCO, is technical editor of Water Technology® and a water
treatment consultant in Manitowoc, WI. He can
be reached by e-mail at: dp-bauman@sbcglobal.net.
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water in these tanks. This can be done by having a small UV on each tank with a recirculating pump or having a larger UV to recirculate through a greater segment of your system. Discuss“turnover” time with a UV manufacturer. Then consider replacing the RO with a whole-house UF (ultrafiltration) mem- brane system. This can be installed after your pressure tank. These are available, with certification-testing, to remove bacte- ria and cysts, with a nominalmicron rating of about 0.02. Keep your 5-micron filter ahead of this. A whole-house UF requires only brief backwashing upon excessive pressure loss. A UF membrane is coarser than a RO membrane and as such does not remove dissolved minerals like RO. You
