Biosolids or Sludge? The Semantics of Terminology

Feb. 1, 2021

Breaking down the distinctions between sludge and biosolids

About the author:

Izrail S. Turovskiy, D.Sc., is a wastewater and sludge treatment consultant in Jacksonville, Florida. He has over 45 years experience in the field, including a time where he was the Head of the All-Union Research Institute of Water Supply, Sewage Systems and Hydrotechnical Structures in Moscow, Russia.


Knowing how to "speak the speak" is important in any industry. However, definitions must be clearly defined and used identically. This article will discuss the definitions of such wastewater terms as biosolids, sludge, thickening, conditioning, dewatering and stabilization. It seems that some terms have not been clarified enough between specialists, causing confusion in the terminology.

The term biosolids has appeared lately in some publications instead of sludge. However, these terms are not equipollent. In the handout Information for Contributors the Water Environment Federation (WEF) has adopted a policy of encouraging the use of the word biosolids in place of sludge to promote public acceptance of reused projects.

The term biosolids should be used to connotate the primary organic solid product of treatment that meets U.S. EPA or other applicable criteria for beneficial use. Anything preceding that point in the wastewater process should be called wastewater solids, solids or another acceptable term. For instance, anaerobic digesters treat wastewater solids, and the product (if it meets the applicable criteria) is biosolids. Dewatering units treat wastewater solids before digestion, but biosolids after the appropriate digestion. A composting operation can be fed by undigested wastewater solids or properly digested biosolids and in either case produce compost. Undigested biomass that meets criteria for application to agricultural forest land is called biosolids.

As shown, the same product can be called wastewater solids, solids or biosolids depending on the conditions of its usefulness and whether it meets the applicable criteria. However, if the product won’t be utilized or useful, stabilized enough after digestion or disinfected, it should be renamed wastewater solids.

Adaptation to a product’s particular usefulness or uselessness always will lead to confusion in terminology. For example, Frank R. Spellman in his book, Incinerating Biosolids wrote "Biosolids is the solid, slim solid or liquid residue generated during the treatment of domestic sewage in a wastewater treatment facility. Biosolids includes, but it is not limited to, domestic sewage, scum and solids removed during primary, secondary or advanced treatment processes." The definition of biosolids also includes a material derived from biosolids.

Some authors have used the term residual instead of biosolids or sludge. According to Webster’s Collegiate Dictionary (1993) residual means "a residual product or substance." This definition does not precisely characterize deposited solids received in the process of sewage water purification. It is still unclear how to name the activated sludge–should it be activated sludge, waste activated biosolids or waste activated solids?

In addition, what do you call waste from natural water purification or industrial water purification? The term biosolids relates more to the biological cleaning of sewage water. If this term is not used about a biological process but rather in a physical-chemical cleaning of sewage water, what term should be used–waste solids, solids or biosolids?


Some authors and even the magazine Biosolids use the term biosolids to imply sludge. Sludge is an old-fashioned concept. Almost all publications used this term. Why should people retrain themselves? Moreover, sludge is a more specific and more precise definition of a product that excretes after purification of sewage water than biosolids or wastewater solids. The problem is how do you translate the word biosolids in Russian, French or Finnish? What do you call the same sludge deposited during the biological purification of sewage water?

Waste activated wastewater solids is a complicated concept. Difficulties occur when distinguishing between other kinds and types of sludges. Their classification is further described in Improved Sludge Classification by Two-Phase Anaerobic Digestion. According to Webster’s Collegiate Dictionary, "Sludge is a muddy or slushy mass, deposit or sediment as (a) precipitated solid matter produced by water and sewage treatment processes. . . ."

Using the term biosolids instead of sludge is like a stillborn child. The evidence of this is that many authors continue to use the term sludge. However, biosolids should be used for treated beneficial sludge.

I think that the word sludge is a more definitive, more precise concept related to concentrated pollutants excreted from sewage waters during their purification, than the term biosolids. Biosolids can be used for sludge treatment products that meet the U.S. EPA criteria for beneficial use.


In published literature the term thickening is used to describe the thickening of sludge in thickeners, as well as during the settling of sludge in clarifiers. The Manual of Practice (MOP) #FD-1 of the WPCF describes the difference between clarification and thickening. "Gravity sedimentation is a common method of solid-liquid separation in use at wastewater treatment plants. When the separation is intended to produce a substantially solid free liquid, the process is called clarification; when the primary requirement is the production of concentrated sludge, the process is called thickening."

According to OM-8 the thickeners are not to be used as holding tanks. Gravity thickening requires a different type of tank construction than clarifiers. Gravity thickeners have the slope 75:12, while the conventional circular clarifiers’ slope is 1:12. They also have a different depth and sludge retention time. However, thickening of sludge does not occuronly in gravitational thickeners. For thickening of sludge flotation, centrifuges, belt thickeners and other equipment and methods may be used.

What is thickening? Several authors define sludge thickening as the process of producing concentrated sludge to reduce costs following dewatering and disposal. However, dewatering also is the process of receiving the concentrated sludge. For example, low pressure belt presses are related to the process of dewatering.

Thickening is used prior to the subsequent dewatering processes to reduce the volumetric loading to these units and to increase the efficiency of dewatering equipment.

MOP # OM-8 characterizes the term thickening slightly different. Gravity thickening is frequently employed to reduce sludge volume handled in the dewatered/sludge disposal portion of a wastewater treatment facility. The objectives of gravity thickening are to produce a relatively solids free supernatant and to produce a product sludge that is as thick as possible without producing difficulties in subsequent pumping operations. Gravity thickening is an inexpensive way to maximize sludge concentration and thereby reduce operating costs.

Rotary drums and gravity belt thickeners are mechanical devices that remove free water from slurries and wastewater sludge using gravity. Centrifugation is a process in which centrifugal force (usually about 500 to 3,000 times the force of gravity) is applied to sludge slurry to accelerate solid and liquid fractions separation". Dissolved air flotation thickening can be defined as a solid-liquid separation caused by the introduction of fine air bubbles into the liquid phase.

Therefore, the term thickening can be described as an economically effective process to increase the sludge’s concentration and decrease the sludge’s volume by removing some free water to the extent where the sludge remains in the fluid state.



There is not a precise definition for the term conditioning in published literature. Some authors imply that conditioning refers to the preparation of sludge for dewatering; others apply it to the processes related to digesting, thermal drying or composting. MOP of Practice FD-14 considers such methods of conditioning as inorganic chemical conditioning, organic chemical conditioning, thermal conditioning, elutriating and freeze/thaw. In this manual sludge conditioning refers to the process of improving the solid/liquid separating characteristics of sludge by physical, chemical and biological means. It is a unique process vital to the successful operation of sludge thickening and dewatering systems.

Chemical conditioning is a commonly used method. Thermal conditioning is a method that promotes separation of solids from liquid through release of cell-bound water by elevated temperatures and pressures. By using thermal conditioning, sludge often can be mechanically dewatered without conditioning of chemicals. Freeze/thaw conditioning substantially decreases the consumption of reagents needed for conditioning the sludge that was mechanically dewatered.

MOP # 8 defines sludge conditioning as the chemical or thermal treatment of sludge to improve the efficiency of thickening or dewatering. According to Tchobanoglous, "Sludge is conditioned expressly to improve its dewatering characteristics."

Using these guidelines, conditioning can be expressed as the chemical or physical process that improves dewaterability properties of a sludge during its preparation for dewatering.


Dewatering is considered as the process of removing the water from sludge. Consequently, gravity thickening, law pressure dewatering and partial removal of moisture by centrifuge or belt filter thickening, or some other methods of moisture removal from sludge may be related to this term. Even thermal drying may be considered as the process of moisture evaporation from sludge. While in principle these terms are correct, it is better to call all these terms by their real names that reflect the essence of processes such as thickening, thermal drying, etc.

Frank R. Spellman1 wrote, "The processes used to remove water from biosolids and change their form from a liquid to damp solid are critical to the operation of other downstream processes in wastewater treatment." MOP # OM-8 states that the dewatering process includes either natural methods such as air drying or mechanical methods such as belt press filtration, centrifugation, vacuum filtration and pressure filtration. The term dewatering should not be taken literally, because during this process the water is not completely removed from sludge. The moisture remains within 70 to 80 percent limits.

Therefore, dewatering can be characterized as the process of natural or mechanical removal of water from sludge during which sludge is losing its fluidity, becomes a damp solid and can be transported in bulk.


Stabilization is one of more frequently used methods of sludge treatment (especially if the sludge will be utilized as a fertilizer). All sludge requires some form of treatment, whether stabilization, thickening or dewatering possibly followed by drying and incineration, or a combination of one or more of these processes, before being discharged into the natural environment. MOP FD-9 reviews the following methods of sludge stabilization: aerobic digestion, anaerobic digestion, alkaline stabilization, composting, thermal drying, heat/wet air oxidation, acid (oxidation) disinfection and others. Some scientists think that the process of stabilization is necessary to prevent the decay of sludges’ organic part and to prevent offensive odor dissemination.

However, Tchobanoglous and Burton wrote that sludge is stabilized to reduce pathogens, eliminate offensive odors and inhibit, reduce or eliminate the potential for putrefaction. MOP #8 states that the sludge stabilization processes reduce pathogens in the sludge, thus providing a less odorous product. Two criteria typically used to measure sludge stability include the volatile solids content and pathogen indicator organism reduction. In Standards (Bruce, A. M. 1994; Florida Department 1994) stabilization means the use of a treatment process to render domestic wastewater residuals less odorous or putrescible and to reduce the pathogen content.

When comparing the definition of biosolids "solid product of treatment that meets U.S. EPA criteria for beneficial use" with the definition of stabilization, it seems as if the word stabilization fits better.

Taking into consideration these opinions, stabilization is a combination of processes of sludge treatment the purpose of which is to meet U.S. EPA or other applicable criteria for beneficial use.

Naturally, all of these definitions or formulations can be made more accurate and reasonable additions can be proposed. However, having specific functional meanings of terms will allow specialists to be more precise about the essence of the processes of sludge treatment and eliminate misunderstanding in utilizing terminology.

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