Hello Bibilore Help: A biological solution is not as simple as expressing the biological term. Wastewater must be biological, protected from all chemical pollution, wastewater must not have deteriorated in its biological characteristics, the treatment system must be biological in nature, it must also be liquid output device is recycled immediately otherwise it loses its biological characteristics and becomes a real pollution. Sludge is evidence of the deterioration of the biological characteristics of the wastewater effluent The world produces more than 45 million tons of sludge annually, it is a product that is impossible to treat in a biological way. This is why some incinerate them, bury them in the soil or scatter them in agricultural land under the image of compost when it is a poison for the soil. When the totality of the effluent is with biological characteristic, all the pollutions disappear.