The invention discloses a method for water purification by 
biological decontamination. The method includes: firstly, 
sludge stirring and separation, to be more specific, adopting mobile 
sludge exposure equipment for re-deposition of 
sludge at bottoms of treated rivers and lakes; secondly, establishment of a 
microorganism survival 
carrier system, to be more specific, adding carbon sources and 
microorganism bacterial beds into the treated rivers and lakes to provide a 
reproduction, 
parasitism and growth environment for dominant bacterial communities; thirdly, culture and feeding of the dominant bacterial communities, to be more specific, extracting microorganisms with a water purification function from the treated rivers and lakes to form the dominant bacterial communities, and feeding into polluted water 
engineering sections; fourthly, installation of 
water aeration equipment; finally, promotion of a water self-purification function, to be more specific, allowing the dominant bacterial communities to quickly reproduce under the 
aeration action so as to realize in-situ 
nitrification and degradation of organics in sludge and water and degradation and conversion of 
ammonia, 
nitrogen and 
phosphorus pollutants. The method has advantages that in-situ 
nitrification and degradation of sludge can be realized, effectiveness in treatment, low investment, 
short duration and convenience in maintenance are realized, 
water treatment effects can be kept for a long time, and the problem of a vicious circle of treatment and 
pollution is solved thoroughly.