Wastewater solids are usually referred to as sewage sludge and consist of sludge, screenings, grit, and scum. It is the residual material produced as the by-product of water treatment processes. Ratios differ between wastewater treatment plants, but it is helpful to know general estimates to conduct analysis.

Sludge Quantity: Determining the quantity of sludge during the treatment of wastewater is a must when it comes to sludge processing units and equipment. A general rule-of-thumb measurement for solids produced in a typical wastewater plant is 0.24kg/m3.

Screenings: Screenings refer to relatively large debris such as rags, plastic, cans, leaves, and other similar items removed by bar screens. There are higher quantities if there are restaurants and other food processing industries in the area. Screenings are normally taken to a landfill.

Grit: Grit refers to heavy, course materials like sand, cinders, and other similar inorganic matter. It also includes some organic materials like coffee grounds, corn, and seeds. If not removed from the wastewater, it can damage machinery like pumps. Grit is usually removed in grit chambers and then taken to a landfill.

Scum: Scum is the product skimmed from clarifiers and primarily consists of fats, oils, grease and floating rubber and plastic debris. The quantity and moisture content of scum is not typically measured.

The residue from primary wastewater treatment is known as primary sludge and usually makes up 50% of total sludge solids. Primary sludges are made up of discrete particles and debris that produce a drier cake and give better solids capture. However, primary sludge is more likely to decay and cause a bad odour if stored without treatment. The common approach to estimating primary sludge is to compute the quantity of suspended solids entering the treatment plant and assuming the removal rate. A typical removal rate of 60% is used for estimating purposes.

There is also secondary sludge known as biological sludge, waste activated sludge, or trickling filter humus. It is produced by biological treatment processes like activated sludge and membrane bioreactors in the secondary treatment process. It also includes any non-biodegradable matter that was not removed by primary settling.

Influent Wastewater Characterization

Here we will discuss some typical characteristics of influent wastewater, although the characteristics will vary in different areas. Influent wastewater is untreated wastewater and can be used in agricultural applications and others when there is a shortage of water. The following information has been summarized from a table in the Metcalf & Eddy handbook “Wastewater Engineering Treatment and Reuse.” For further information, please refer to the table in the handbook.

Influent water contains contaminants such as solids (390mg/L), suspended solids (120mg/L) and settleable solids (5mg/L). It also has a biochemical oxygen demand of 110mg/L, a total organic carbon demand at 80mg/L and a chemical oxygen demand at 250mg/L. Also contains nitrogen (20mg/L, phosphorus (4mg/L), chlorides (30mg/L), sulfate (20mg/L), oil and grease (50mg/L) and VOCs (<100mg/L).