Ok. Add nasty chemicals then. I think, even cheap chlorine can do the job. In my case higher temperature was enough. I rented a flat that was staying empty for

Ok. Add nasty chemicals then. I think, even cheap chlorine can do the job. In my case higher temperature was enough. I rented a flat that was staying empty for long time and had these problems. Increasing temperature of the water and letting it run for 5 minutes solved it.