We are in similar, but not quite the same situations. My urination problems occurred about 5 or 6 days of Water Fasting as well, then I stopped it immediately by drinking OJ, then started the Water Fast again and another 6 days later, the same urination problem popped back up. I wrote about it here if you're interested.
What I did was switch water purifiers, and drinking-water sources, as well as incline my futon. It seemed to have helped.
I raised part of my futon so that I sleep on an inclined plane. My head being higher, and my feet being lower. Just a few inches seemed to make a good difference. It's not completely better, but much better than it was initially. I trust in my body's detoxing intelligence and intend to see it through as best I can. What gets me is the heart-nausea blahhhs. Fasting aint easy.