The biofilter and bioscrubber systems are well presented with details on the operation and on the capacity of both systems. The authors stress the fact that both systems will maintain a good efficiency only if they are well maintain and operated under controlled conditions. The biofilter material need to be changed and disposed properly as it is the case also for the sludge produced by the bioscrubber. Both substrates become charged with nutrients that hav to be disposed according to this charge.