Waste water flowing continuously or intermittently when you are NOT making water can be several things. The main things are the ASOV failing or the filters (especially the pre-filters) being clogged.. The ASOV can be easily replaced if need be. Filters will need to be ordered. Occasionally, the flow restrictor will fail. Though this normally doesn't happen, we will replace it if need be. Steps to correct: 1. Sometimes a small particle gets stuck inside the ASOV; you can clear this by thumping the ASOV up and down while making water. 2. Another problem (mainly for newer units) might be low feed water pressure. Remember, the RO membrane needs at least 40-50 psi (ideally 65 PSI) in order to operate properly. Anything below 40PSI will slow (or stop) water production while increasing waste water flow. Either add a booster pump or do not rely on the ASOV and just turn off supply water when not making water. 3. Another problem is if the tap supply PSI is TOO high as in 70+ PSI, this can overpower the ASOV and cause it to turn on and off intermittently allowing small amounts of waste water to escape. If you do have 70+ PSI, either get a Pressure reducing valve (which we sell)  or do not rely on the ASOV and just turn off supply water when not making water. 4. To verify that the ASOV and/or flow restrictor needs to be replaced, do these tests: Remove all the pre-filters and the membrane (and the DI filter if applicable), turn on the unit to make water, now turn on and off the fast flush quickly for 15 times. Now turn off the pure water output, open & close the fast flush rapidly for 5 times, now leave fast flush valve closed, after a minute or two the drain water should stop; if not the ASOV valve is bad. If this test does work, then your unit is fine. You either need new prefilters, new membrane or you have insufficient/excess water pressure.