NIRAS VOLUME 2 [2.7] 4
2.7.1 Biosolids Production Sludge forms initially as a 3 to 7% suspension of solids; with each person typically generating about 10-15 L of sludge per week, the total quantity generated each day, week, month, and year is significant. Because of the volume and nature of the material, sludge management is a major factor in the design and operation of all water pollution control plants.
Note: Wastewater solids account for more than half of the total costs in a typical secondary treatment plant.
Wastewater sludge is generated in primary, secondary, and chemical treatment processes.
In primary treatment, the solids that float or settle are removed. The floatable material makes up a portion of the solid waste known as scum. Scum is not normally considered sludge; however, it should be disposed of in an environmentally sound way. The settleable material that collects on the bottom of the clarifler is known as primary sludge. Primary sludge can also be referred to as ran' sludge because it has not undergone decomposition. Raw primary sludge from a typical domestic facility is quite objectionable and has a high percentage of water, two characteristics that make handling difficult.
Solids not removed in the primary clarifier are carried out of the primary unit. These solids are known as colloidal suspended solids. The secondary treatment system (e.g., trickling filter, activated sludge) is designed to change those colloidal solids into settleable solids that can be removed. Once in the settleable form, these solids are removed in the secondary clarifier. The sludge at the bottom of the secondary clarifier is called secondary sludge. Secondary sludges are