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Wastewater Solids

Updated: Oct 12, 2023

It can be helpful to quickly estimate solids production in wastewater. Certainly, each wastewater treatment plant and its collection system, with variable ratios of domestic, commercial, and industrial flow contributions, is going to be unique regarding its specific wastewater characterization. But for those situations where you don’t have plant operating data you can use more general estimates to begin an analysis.

Municipal plant sludge dewatering operation.
Andritz Belt Filter Presses

The information provided below is quoted from an excellent, highly recommended, textbook called “Wastewater Sludge Processing” written by Izrail S. Turovskiy and P.K. Mathai. This text is loaded with informative data, tables, graphs, and diagrams. If you click on the book cover graphic you can download a PDF file showing the table of contents.

Sludge Quantity

Determining the quantity of sludge produced in the treatment of wastewater is required for the sizing of sludge processing units and equipment such as sludge pumps, storage tanks, thickeners, digesters, and incinerators. Generally, solids production rates range between 0.2 and 0.3 kg/m3 (0.8 to 1.2 dry tons/MG) of wastewater treated. In the absence of historic or plant-specific data, a rule-of-thumb approximation for solids produced in a typical wastewater treatment plant is 0.24 kg/m3 (1 dry ton/MG) of wastewater treated (WEF, 1998).


Screenings include relatively large debris, such as rags, plastics, cans, leaves, and similar items that are typically removed by bar screens. Quantities of screenings vary from 4 to 40 mL/m3 (0.5 to 5 ft3/MG) of wastewater. The higher quantities are attributable to wastes from correctional institutions, restaurants, and some food-processing industries. Screenings are normally hauled to a landfill.


Grit consists of heavy and coarse materials, such as sand, cinders, and similar inorganic matter. It also contains organic materials, such as corn, seeds, and coffee grinds. If not removed from wastewater, grit can wear out pump impellers and piping. Grit is typically removed in grit chambers. In some treatment plants, grit is settled in primary clarifiers along with primary sludge and then separated from sludge in vortex-type grit separators. The volume of grit removed varies from 4 to 200 mL/m3 (0.5 to 27 ft3/MG) of wastewater. The higher quantities are typical of municipalities with combined sewer systems and sewers that contribute excessive infiltration and inflow. Grit is almost always landfilled.


Scum is the product that is skimmed from clarifiers. Primary scum consists of fats, oils, grease, and floating debris such as plastic and rubber products. Secondary scum tends to be mostly floating activated sludge or biofilm, depending on the type of secondary treatment used. The quantity and moisture content of scum typically are not measured.

Primary Sludge

In a typical plant with primary settling and a conventional activated sludge secondary treatment process, the dry weight of the primary sludge solids is about 50% of that for the total sludge solids. The total solids concentration in raw primary sludge can vary between 2 and 7%. Compared to biological and chemical sludges, primary sludge can be dewatered rapidly because it is comprised of discrete particles and debris and will produce a drier cake and give better solids capture with low conditioning requirements. However, primary sludge is highly putrescible and generates an unpleasant odor if it is stored without treatment.

Primary sludge solids production can vary typically from 0.1 to 0.3 kg/m3 (800 to 2500 lb/MG) of wastewater. A rule-of-thumb approximation is 0.05 kg/capita (0.12 lb/capita) per day of primary sludge solids production. The most common approach in estimating primary sludge production is by computing the quantity of suspended solids entering the treatment plant and assuming a removal rate. The removal rate is usually in the range of 50 to 65%. A removal rate of 60% is commonly used for estimating purposes, provided the effects of industrial contribution are minimal and no major sidestreams from the sludge processing units are discharged to the primary clarifier influent.

The typical BOD removal rate is 50% of suspended solids removed.

Secondary Sludge

Secondary sludge, also known as biological sludge, is produced by biological treatment processes such as activated sludge, membrane bioreactors, trickling filters, and rotating biological contactors. Plants with primary settling normally produced a fairly pure biological sludge as a result of the bacteria consuming the soluble and insoluble organics in the secondary treatment system. The sludge will also contain those solids that were not readily removed by primary clarification. Secondary sludge generated in plants that lack primary settling may contain debris such as grit and fibers. Activated sludge and trickling filter sludge generally contain solids concentrations of 0.4 to 1.5% and 1 to 4%, respectively, in dry solids weight. Biological sludge is more difficult to dewater than primary sludge because of the light biological flocs inherent in biological sludge.

Secondary sludge is the biomass produced by a secondary treatment process, such as activated sludge with its numerous variations, such as extended aeration, oxidation ditch, complete mix, plug flow, step feed, sequencing batch reactor (SBR), and membrane bioreactor (MBR); and attached growth systems, such as trickling filters and rotating biological contactors. Secondary sludge also includes non-biodegradable inorganic particulate matter not removed by primary settling. The solids become incorporated into the biomass. Since the amount of organic loading to the secondary treatment process is the most important factor in the production of biological solids, the rate of removal of biological or chemical oxygen demand (BOD or COD) in primary settling is very important. As mentioned earlier, BOD removal is approximately 50% of the suspended solids removal in primary clarification.

Chemical Sludge

Chemicals are used widely in wastewater treatment, especially in industrial wastewater treatment, to precipitate and remove hard-to-remove substances, and in some instances, to improve suspended solids removal. In all such instances, chemical sludges are formed. A typical use in removing a substance from wastewater is the chemical precipitation of phosphorus. The chemicals used for phosphorus removal include lime, alum, and “pickle liquors” such as ferrous chloride, ferric chloride, ferrous sulfate, and ferric sulfate.


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