The reason I asked about TDS is this:
"Q. How long will the filters last? A. Though we are lab-certified for 15 gallons per filter under the strictest conditions, our customers with tap water near 200 TDS tell us that one filter will treat about 30 gallons of water..." (found on zerowater site)
So, Tap with 400 TDS will run 15 gallons, etc.
Filters are about $38 for a 2 pk from Kohls. http://www.kohls.com/product/prd-897400/zerowater-2-pk-replacement-filters.jsp
IMO, once you pay for a 23 cup Zerowater, and 2 filters, you could have bought an RO unit.