When wastewater is treated using various mechanical, biological, and physicochemical methods to remove organic and inorganic pollutants to levels required by the permitting authority, the sludge produced will also vary in quantity and characteristics from one treatment plant to another.
Types of Sludge
Types of Sludge and other solids, such as screenings, grit, and scum, in a wastewater treatment plant, vary according to the type of plant and its method of operation.
Wastewater sludge can be classified generally as primary, secondary (also called biological) and chemical.
Sludge contains settleable solids such as (depending on the source) fecal material, fibers, silt, food wastes, biological flocs, organic chemical compounds and inorganics, including heavy metals and trace minerals.
The sludge is raw sludge when it is not treated biologically or chemically for volatile solids or pathogen reduction.
After treatment, the resulting biosolids can be classed as aerobically digested (mesophilic and thermophilic), anaerobically digested (mesophilic and thermophilic), alkaline stabilized, composted, or thermally dried.
The treated sludge can be only primary, secondary, chemical, or a mixture of any two or three of the sludge.
Primary sludge
Most wastewater treatment plants use the physical process of primary settling to remove settleable solids from raw wastewater.
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 of the total sludge solids.
The total solids concentration in raw primary sludge can vary between 2 and 7%.
Primary sludge may be dewatered more quickly than biological and chemical sludges because it is made up of discrete particles and debris, resulting in a drier cake and better solids collection with low conditioning requirements.
However, primary sludge is highly putrescible and generates an unpleasant odor if it is stored without treatment.
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.
As a result of the bacteria digesting the soluble and insoluble organics in the secondary treatment system, primary settling plants typically create a fairly pure biological sludge.
The sludge will also contain those solids that were not readily removed by primary clarification.
Secondary sludge produced in plants that do not have primary settling may contain particles such as grit and fibers.
Activated sludge and trickling filter sludge typically have solids concentrations of 0.4 to 1.5% and 1 to 4% in dry solids weight, respectively.
Because of the light biological flocs present in biological sludge, it is more difficult to dewater than primary sludge.
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.
Typical use in removing a substance from wastewater is the chemical precipitation of phosphorus.
Lime, alum and “pickle liquors” such as ferrous chloride, ferric chloride, ferrous sulfate and ferric sulfate are among the chemicals used to remove phosphorus.
Some treatment plants add chemicals to the biological process; thus, chemical precipitates are mixed with the biological sludge.
Most plants apply chemicals to secondary effluent and use tertiary clarifiers or tertiary filters to remove the chemical precipitates.
Some chemicals can have unexpected consequences, such as lowering the pH and alkalinity of the effluent, which may necessitate the use of alkaline chemicals to correct.
Other wastewater Residuals
In addition to sludge, three other residuals are removed in the wastewater treatment process: screenings, grit and scum.
Although their volume and weight are substantially smaller than that of sludge, their removal and disposal are critical.
Screenings contain big waste such as rags, plastics, cans, leaves and other similar objects that are normally removed by bar screens.
Screening concentrations range from 4 to 40 mL/m3 (0.5 to 5 ft3/MG) of wastewater.
The higher levels are due to waste from correctional institutions, restaurants and some food-processing sectors.
Some treatment plants return the screenings to the liquid stream after macerating or comminuting.
This is not recommended since reconstituted rags and strings can clog several downstream items of equipment, such as mixers, air diffusers and electrical probes.
Grit is made up of heavy and coarse materials like sand, cinders and other inorganic particles.
It also incorporates organic ingredients including grain, seeds and coffee grinds. Grit can wear down pump impellers and pipework if it is not removed from wastewater.
Grit chambers are commonly used to remove grit. Grit is settled in primary clarifiers with primary sludge in some treatment plants and subsequently separated from the sludge in vortex-type grit separators.
The volume of grit removed from wastewater ranges from 4 to 200 mL/m3 (0.5 to 27 ft3/MG).
The higher quantities are typical of municipalities with combined sewer systems and sewers that contribute to excessive infiltration and inflow.
Scum is a product that is skimmed from clarifiers. Fats, oils, grease and floating debris such as plastic and rubber items make up primary scum.
It can accumulate in pipelines, reducing flow and raising pumping costs, and it can foul probes, flow elements and other waste-stream equipment.
Depending on the sort of secondary treatment utilized, secondary scum is typically floating activated sludge or biofilm.
Typically, the quantity and moisture content of scum is not measured.
Pumping to sludge digesters, concentrating and then incinerating with other residuals, or drying and then landfilling are all options for disposal.
Sludge processing
The purpose of primary and secondary treatment is to remove as many organic solids from the liquid as possible while concentrating solids in a much smaller volume for ease of handling and disposal.
Primary sludge has a typical solids content of 4 – 6%. Sludge processing reduces the solids content of this sludge through biological processes and removes more of the liquid content of it before disposal.
The overall sludge processing investment cost at the typical wastewater treatment plant is about one-third of the total investment in the treatment plant.
However, based on the individual wastewater treatment plant’s processing system, operating expenses in sludge processing typically amount to an even larger portion of the total plant operating costs.
To reduce plant operating costs, it is essential to have a properly designed and efficiently operated sludge processing stage.
The design options for each process will be dependent on the type, size, and location of the wastewater treatment plant, and the solid disposal options available.
The design must be able to handle the amount of sludge produced and converted economically to a product that is environmentally acceptable for disposal.
As with water processing, sludge process methods will be determined by the specific constraints and requirements of the individual wastewater treatment plant.
Our schematic covers the general processing steps found in a typical plant.
The typical sludge processing steps include:
- Sludge Thickening
- Sludge Conditioning
- Dewatering
- Disposal
Sludge Thickening
To optimize the sludge conditioning stage, it is important to maximize the solids content of the materials decanted from the water processing stages.
The waste-activated sludge, scum and primary sludge can be thickened to reduce the liquid content before sludge conditioning.
Due to the varying physical nature and liquid content of these materials, facilities may use different thickening processes and equipment for these three materials.
In some cases, the primary sludge may not even be thickened and will be pumped directly to sludge conditioning.
The intent is to optimize the downstream processing capabilities.
The four most common thickening methods include gravity settling, gravity belt thickening, dissolved air flotation and centrifuge thickening.
The recovered liquid or supernatant from thickening is pumped back into the aeration tank or to the beginning of the water processing stage and is reprocessed.
Sludge conditioning
Sludge conditioning is a key stage in the reduction of solids before disposal.
Based on the size and location of the facility five common methods are typically utilized; chemical treatment, anaerobic digestion stabilization, aerobic digestion stabilization, lagoon storage and heat treatment.
Many facilities will have some type of aerobic or anaerobic digestion stage before de-watering.
The purpose of sludge digestion is to convert bulky odorous sludge into a relatively inert material that can be rapidly dewatered without obnoxious odors.
Thickened waste-activated sludge, scum, and primary sludge are pumped into the digester.
In anaerobic digestion, the digester uses naturally occurring anaerobic microorganisms
to break down organic materials into methane and carbon dioxide gases. The sludge is heated to 37°C (100°F) and agitated continuously in the digester to improve the rate of digestion.
There are two different anaerobic processes, single-stage, and two-stage. Single-stage digesters utilize one digester (tank) to digest the sludge, capture methane gas and store the sludge until it is transferred to the de-watering process
Two-stage anaerobic digestion uses a primary and secondary digester. The primary digester is heated and utilizes mixers to completely agitate the sludge, which maximizes sludge digestion.
The secondary digester is not agitated and is utilized for gravity thickening and storage of the digested sludge.
The secondary digester typically incorporates a floating gas dome for methane gas collection and supernatant is pumped out to increase solids content.
Anaerobic digestion is a biological process that breaks down a significant number of organic solids in the sludge and produces methane gas that is utilized as a fuel for the plant.
Consequently, the volume of final sludge is greatly reduced, which in turn reduces the cost of sludge disposal.
The process also reduces the level of pathogenic microorganisms enabling digested sludge to be classified as biosolids that can be utilized as a soil conditioner or fertilizer.
Sludge can also be stabilized by long-term aeration that biologically destroys volatile solids.
An anaerobic digester is normally operated by continuously feeding raw sludge with intermittent supernatant and digested sludge withdrawals.
The digested sludge is continuously aerated during filling and for the specified digestion period after the tank is full.
Aeration is then discontinued to allow the stabilized solids to settle by gravity.
The supernatant is decanted and returned to the head of the treatment plant, and a portion of the gravity-thickened sludge is removed for de-watering.
The next step for the stabilized sludge is de-watering.
De-watering
De-watering is the final stage before sludge disposal. The goal is to economically remove as much liquid as possible from the sludge or digested sludge before disposal.
The most common method of de-watering utilizes a belt filter press.
The belt filter press has two continuous porous belts that pass over a series of rollers to squeeze water out of the sludge that is compressed between the two belts.
Polymers are typically added to the process to enhance de-watering capabilities. Centrifuges are also used for de-watering, typically, in larger wastewater treatment plants.
Any supernatant that is removed in the de-watering process is returned to the beginning of the treatment plant for reprocessing.
Disposal
Digested sludge that is processed into biosolids can be used to spread on farmland as a soil conditioner or can be further processed as fertilizer.
Sludge can also be incinerated and the remaining ash is disposed of in a landfill.
Economics and environmental regulations will be the primary drivers in what disposal method an individual wastewater treatment plant uses.
References
METCALF & EDDY (1991). Wastewater engineering: treatment, disposal, and reuse.
