a process of dewatering primary - treated sewage which includes mixing the sewage with a coagulant or flocculant aid , usually activated polymer . the sewage is then mixed and flocculated at conditions which involve extensive mixing turbulence of the sewage and whereby part of the sewage is recycled so as to be again subjected to such mixing and flocculating . flocks form the solid particles in the sewage . the ph of the sewage is chemically adjusted into the basic ph range or to a higher basic ph . the sewage is applied to a sand bed whereby the flocculated solids in the sewage are separated from the liquid in the sewage , by collecting on the top of the sand bed . the flocculated solids located on the top of the sand bed are air dried . the dried flocculated solids are removed from the top of the sand bed .