Wastewater treatment systems consist of many different processes resulting in many kinds of sludge. The following table provides a comprehensive list and characteristics of different types of sludges, defining which are in-scope or out of scope for the applicability of the Wastewater and Sludge Guidelines.
In scope
Type of Sludge | Processes that Generate this Type of Sludge | Additional Process Information | Sludge Characteritics |
|---|---|---|---|
| Anaerobically digested secondary sludge |
| This is a secondary (biological) treatment process involving microorganisms in the absence of oxygen. The digestion step takes place in a sludge digester and is a process used to reduce the organic loading and volume of the sludge. | Dark brown to black in colour and can contain an exceptionally large quantity of gas. When thoroughly digested, they are not offensive, the odour being relatively faint and like that of hot tar, burnt rubber or sealing wax. |
| Aerobically Digested Secondary Sludge |
| Sludge generated via secondary (biological) treatment processes involves microorganisms in the presence of oxygen. Digestion steps take place in a sludge digester and are used to reduce organic load and volume of sludge. | Brown to dark brown and have a flocculent appearance. Odour is not offensive; it is often characterised as musty. Well-digested aerobic sludge dewaters easily. |
| Primary Treatment Sludge |
| Substantial quantities of gas may be given off and the sludge density increased by long residence times in storage. | Sludge from primary settling tanks is usually grey and slimy and, in most cases, has an extremely offensive odour. • Sludge from chemical precipitation with metal salts is usually dark in colour, though its surface may be red if it contains much iron. • Lime sludge is greyish-brown. • While chemical sludge is somewhat slimy, the hydrate of iron or aluminium in it makes it gelatinous. |
| Biomass or secondary treatment and tertiary treatment sludge |
| Secondary / tertiary treatment may be used to remove organic compounds, nitrogen, phosphorus, additional suspended solids, heavy metals, and dissolved solids. Tertiary treatment processes including:
| Generally has a brown flocculent appearance. Tertiary sludge may be light brown in colour and less odorous. • If the colour is dark, the sludge may be approaching a septic condition. • If the colour is lighter than usual, the sludge may have been under aeration with a tendency for the solids to settle slowly. • Sludge in good condition has an inoffensive "earthy odour". • When sludge contains many worms, it may become inoffensive quickly. Trickling-filter sludge digests readily |
| Mechanically dewatered sludge “cake” |
| Sludge dewatering is the separation of solid and liquid phase in order to create a solid called sludge “cake” | Dewatered sludge cake contains moisture in the range of 65% - 85%. This type of sludge may be brown and have an offensive odour. |
| Dried sludge or ash |
| Sludge drying is the process of further reducing moisture content of sludge in order to reduce volume and transportation costs. Onsite incineration of sludge (with or without energy recovery) is considered to be a treatment step, and the ash must be collected and disposed of properly | Sludge drying processes are used to produce sludge with water content of less than 10 to 20 weight percent moisture. |
| Adsorption or filtration system waste | Various filtration processes using media like sand, activated carbon, resins, and similar materials. | Media used to remove organic compounds (including MRSL compounds) or media used to remove metals in wastewater must be changed out periodically. | Appearance can vary depending on the media being used. |
Out of Scope
| Type of Waste | Description of the Type of Waste | Processes that Generate this Type of Waste |
|---|---|---|
| Screening Waste | Screening waste includes all types of organic and inorganic materials large enough to be removed by large mechanical screens (bar racks), lint screens, or other screens. The organic content varies, depending on the nature of the system | Screening waste generated by mechanical pretreatment process (mechanical screening). Screening is done manually or automatically using screens with different mesh sizes. |
| Grit Waste | Grit is usually made up of the heavier inorganic solids (including sand) that settle with relatively high velocities. Depending on the operating conditions, grit may also contain significant amounts of organic matter, especially fats and grease. | Generated by hydro-mechanical process. |
| Scum/Grease Waste | Scum consists of the floatable materials skimmed from the surface of primary and secondary settling tanks and from grit chambers and chlorine contact tanks, if so equipped. Scum may contain grease, vegetable and mineral oils, animal fats, waxes, soaps, paper, plastic materials, grit particles, and similar materials. | Scum is collected by skimming processes. Grease may come from machinery or from kitchen/cafeteria areas and may also be removed by skimming processes. |
| Chemical Cleaning Waste | Machine cleaning chemicals are used for cleaning knitting or weaving machines, dyeing machines, sewing machines, etc. Floor cleaning chemicals are used for cleaning floors. | Any waste generated due to cleaning of chemical containers and drums. |
| Print Paste Waste | Waste generated from print paste residues, screen making, screen cleaning or other activities related to printing. | May be generated in the manufacturing facility or in a separate area. |
| Other waste not specifically listed in the “In scope” section | All other waste generated in the manufacturing facility or the wastewater treatment system that is not comingled with in scope sludge is considered to be out of scope. | Refer to the list of In Scope wastes above. Comingling means the mixing of wastes together prior to disposal. |