Forms of Wastewater Solids
Wastewater solids come in many forms. Solid pollutants in the water body mainly in the form of suspended matter, colloidal matter and dissolved solids exist.
- Suspended matter: Suspended solids are particulate materials that remain dispersed in wastewater and can include sand, silt, organic particles, microorganisms, fibers, and biological flocs. In laboratory water-quality analysis, suspended and dissolved fractions are commonly distinguished by filtration using a specified filter.
- Colloid: Colloids are very small particles that remain dispersed because they do not readily settle under gravity. They occupy an intermediate size range between larger suspended particles and truly dissolved substances. Coagulation and flocculation are commonly used to destabilize colloids and form larger flocs for subsequent separation.
- Dissolved solids: Dissolved solids include salts, minerals, ions, and low-molecular-weight compounds that pass through the specified laboratory filter. They cannot normally be removed by conventional sedimentation alone and may require processes such as precipitation, ion exchange, membrane separation, or biological conversion, depending on their composition.
Different types of wastewater solids in water treatment have their own characteristics and targeted treatment methods. Some conventional data on solids pollutants are listed below.
Sludge volume
Sizing sludge treatment units and equipment (e.g. sludge pumps, tanks, thickeners, digesters, filter presses or screw presses and incinerators) requires determining the amount of sludge generated during the wastewater treatment process. Typically, the solids generation rate for treated wastewater ranges from 0.2 to 0.4 kg/m3.
Screenings
The screen includes relatively large debris such as rags, plastics, cans, branches, etc. Screening volumes vary from 5 to 50 mL/m3 of wastewater. Screened residue is typically transported to landfills.
Grit
Gravel consists of heavy and coarse materials, such as sand and cinder. It also contains organic materials, e.g. corn, seeds. Grit can wear out pump impellers and pipes if not removed from the wastewater. Grit is usually removed in grit settling tanks. In some treatment plants, the grit settles with the primary sludge in a primary clarifier and is then separated from the sludge in a grit separator. The amount of grit removed varies from 3 to 210 mL/m3 of wastewater.
Scum
Scum is the product of skimming from the clarifier. Primary scum consists of fats, oils, greases and floating materials such as plastics and rubber products. Secondary scum tends to be primarily floating activated sludge or biofilm, depending on the type of secondary treatment used. In some cases, dissolved air flotation systems are used instead of clarifiers.
Primary sludge
In water treatment plants with primary settling and conventional activated sludge secondary treatment processes, the dry weight of primary sludge solids is approximately 45% of the total sludge solids. The total solids concentration in raw primary sludge can vary between 3% and 8%. Compared to biological and chemical sludge, primary sludge can be dewatered quickly and will produce a drier cake and provide better solids capture with lower conditioning (added polymer) requirements.
Primary sludge production is best estimated from the influent TSS load and the expected removal efficiency of the primary clarifier. For preliminary municipal wastewater calculations, reported primary-sludge dry-solids production is commonly on the order of approximately 0.1–0.3 kg DS per m³ of wastewater, but site-specific wastewater data should always take priority.
A properly designed and operated primary clarifier typically removes approximately 50–65% of influent suspended solids and about 25–40% of influent BOD₅. These are separate removal efficiencies and should be calculated independently.
Primary sludge is generally easier to thicken and dewater than waste activated sludge because of its larger and denser particulate structure.
Secondary sludge
Secondary sludge, also known as biological sludge, is produced by biological treatment processes such as activated sludge, membrane bioreactors, trickling filter tanks, and rotating biological contactors. Plants using primary sedimentation typically produce a fairly pure biological sludge as bacteria consume soluble and insoluble organic matter from the secondary treatment system. Activated sludge and trickling sludge typically contain 0.3% to 1.4% and 1.5% to 4.5% solids concentration by weight of dry solids.
Secondary sludge is the biomass produced by secondary treatment processes such as activated sludge and its many variants such as extended aeration, oxidation ditches, piston flow, step feed, sequencing batch reactors (SBR) and membrane bioreactors (MBR). Secondary sludge also includes non-biodegradable inorganic particulate matter that is not removed by primary settling. Since the amount of organic loading during secondary treatment is the most important factor in biosolids production, the removal rate of biological or chemical oxygen demand (BOD or COD) from primary settling is very important.
Chemical sludge
Chemical sludge is produced when treatment chemicals react with contaminants or suspended material and form precipitates or additional flocs.
A common example is chemical phosphorus removal. Chemicals used for this purpose include lime and metal salts such as aluminum sulfate (alum) and ferric chloride. Ferrous salts may also be used under appropriate process conditions.
These chemicals react with phosphate and other wastewater constituents to form insoluble solids that are subsequently removed by clarification, flotation, filtration, or other solid-liquid separation processes.
Chemical dosing increases the amount of sludge generated, so the additional chemical solids should be included when sizing sludge thickening, dewatering, and disposal systems.
Summary
Wastewater contains various solid materials that require effective removal for proper treatment. These solids include suspended solids, colloids, and dissolved solids, each differing in size and composition. Managing sludge volume is essential for designing treatment systems, as different types of solids contribute to sludge generation.
KUOSI offers sludge dewatering machines, sludge dryers, sludge conveyors, dosing systems, aeration blowers, disinfection systems, wastewater screens, scrapers, and grit removal equipment. Please contact us for further information.