Can the waste from a sewage treatment plant be used as a fertiliser?
Yes, indeed, water from sewage plants sometimes is pured to have drinkable standards, but due to known reasons, we do not drink it. We could use it for flushing the toilets, washing cars. In Holland they release such water into the rivers, such as Rhine, and reuse it, of course for drinking purpose. No harm done. In Israel, they use it for greening the Negev desert, for irrigation. I don’t know how would you use it a fertilizer. The water is pure and as such can only be used for irrigation, but with addition of fertilizers for fertilizing.
