2.14.1 What is sewage sludge?

Sewage sludge is defined in this text as the solid matters discharged from an activated sludge wastewater treatment facility as the result of aerobic wastewater treatment. The sewage sludge, which is usually a mixture of settleable matters in sewage and reproduced microorganisms, is high in organic contents that are possibly considered as reusable biomass.

2.14.2 Types and characteristics of sewage sludge

The standard activated sludge process is depicted in Fig. 2.14.1 as a typical aerobic wastewater treatment process.

In the activated sludge process, settleable matters contained in in-coming sewage are removed in the primary settler and withdrawn as raw sludge. The sewage is then introduced to aeration basin(s) where the sewage is mixed with return activated sludge, aerated and vigorously agitated. The mixture of sewage and return activated sludge in the aeration basin is usually called mixed liquor. Most of the organic pollutants are removed from a liquid phase by bio-adsorption and eventually decomposed by microbial assimilation of the activated sludge. The mixed liquor is then introduced to the final settler where suspended solids (activated sludge) are settled and withdrawn as return activated sludge. A portion of return activated sludge is discharged from the facility as excess sludge and its quantity is approximately equivalent to the amount of microbes reproduced in the aeration basin. The sewage sludge is either a mixture of the raw sludge and the excess sludge, or each of the sludges discharged separately.

The discharged sewage sludge is thickened and dewatered, and is subjected to the final landfill disposal or recycled after an intermediate treatment such as incineration or
gasification-melting. Typical treatment/disposal processes for sewage sludge are shown in Fig. 2.14.2.

©Sewage sludge

©Sewage sludge ©Sewage sludge