The aerobic acivated sludge process has been in use for many years, and it provides a lost cost cleanup of waste streams. Materials that are harder to break down, such as P.C.B. and plastics are usually taken care of in an anaerobic process. The waste sludge is taken care of this way. The anaerobic process has a much slower degradation time. The main microorganisms in action are methanbacterium. For more information go to anaerobic sludge process.