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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 1

Biosolids Management: An Environmentally Sound Approach
for Managing Sewage Treatment Plant Sludge

An Introductory Guide To Decision-Makers



Wastewater Treatment: The Municipal Sludge Production Process

photo 1:  Preparing the land for the incorporation of biosolids on the soil
Photo 1: Preparing the land for the incorporation of biosolids on the soil. (Source: Wastewater Technology Centre, Environment Canada)
 
In order to produce a clean effluent that can be safely discharged to watercourses, wastewater treatment plant operations use three or four distinct stages of treatment to remove harmful contaminants. Each of these stages mimics and accelerates processes that occur in nature. Preliminary wastewater treatment extracts coarse solids and grit through screens and other filtering devices. These coarse materials are not incorporated in biosolids. Primary wastewater treatment usually involves gravity sedimentation of screened wastewater to remove settled solids. Half of the solids suspended in wastewater are removed through primary treatment. The residual material from this process is a concentrated suspension called primary sludge, which will undergo further treatment to become biosolids.

Photo 2: Preparing the the land for the incorporation of biosolids on the soil
 
Photo 2: Preparing the the land for the incorporation of biosolids on the soil. (Source: UNU/INWEH)
Secondary wastewater treatment is accomplished through a biological process, which removes biodegradable material. This treatment process uses microorganisms to consume dissolved and suspended organic matter, producing carbon dioxide and other by-products. The organic matter also provides nutrients needed to sustain the communities of microorganisms. As the microorganisms feed, their density increases and they settle to the bottom of processing tanks, separated from the clarified water as a concentrated suspension called secondary sludge, biological sludge, waste activated sludge, or trickling filter humus.

Tertiary or advanced treatment is used when extremely high-quality effluent is required, such as direct discharge to a drinking water source. The solid residual collected through tertiary treatment consists mainly of chemicals added to clean the final effluent, which are reclaimed before discharge, and therefore not incorporated into biosolids.

Combined primary and secondary solids comprise the majority of material used at municipal plants for biosolids production. Careful management throughout the entire treatment process allows plant operators to control the solids content, nutrient value and other constituents of biosolids. Figure 2 provides an illustration of the wastewater treatment process.

Figure 2: Wastewater Treatment Process
Figure 2: Wastewater treatment Process.


The Municipal Sludge-to-Biosolids Treatment Process

There are three important factors to be addressed through further processing before this material can be utilised: (1) pathogen levels, (2) presence of potentially harmful industrial contaminants, and (3) water content.

The principal process employed to convert municipal sludge into biosolids is called stabilisation. Stabilisation accelerates the biodegradation of organic compounds, reduces the microbial population including pathogens, and renders the material microbiologically safe for agricultural use. Biological stabilisation uses aerobic or anaerobic treatment to reduce the organic content of solids through controlled biodegradation. Chemical stabilisation does not reduce the quantity of biodegradable organic matter in solids, but creates process conditions that inhibit microorganisms, thereby slowing the degradation of organic materials and reducing odours. The most common chemical stabilisation procedure is to elevate the pH level of the solids using lime or other alkaline materials. Thermal drying and composting can also be used to stabilise biosolids. Full pasteurisation of biosolids is not needed when the primary use is cropland application. Any potential risk to human health due to exposure is eliminated through proper application procedures and in-situ microbial decomposition.

The presence of contaminants in the sludge or biosolids arising from industrial discharges is a more challenging problem and may be the deciding factor in determining the choice of a utilisation disposal option. Put simply, many industries have habitually used the sewer system as a convenient and low-cost way to discharge hazardous wastes. These contaminants accumulate in the biomass and sludge, and can render the material unfit for any beneficial use. The most common options used for disposal of this contaminated material are landfill or incineration, the cost of which is usually borne by the municipality rather than the hazardous waste generator. Biosolids utilisation is a good, environmentally sustainable option when the wastewater is from municipal sources only, or when a fully enforced industrial pre-treatment and discharge control system is in place. The decision to select an environmentally sustainable approach to biosolids management can be used very effectively to review and correct polluting practices up-stream that should not be taking place.

The final concern is the water content of the product. Primary and secondary sludge generally contain no more than four percent solids, and the storage and transportation costs of this semi-liquid material limit the application to nearby farmland. Processes to remove water from solids, therefore, are common in biosolids production. The simplest method for removing water is gravity thickening, which involves concentration by simple sedimentation. Allowing sufficient time for solids to settle in tanks can increase suspended solids concentration to five or six percent. Thickening can also include flotation processes, gravity drainage belts, perforated rotating drums, and centrifuges. Nothing is added to biosolids during the gravity thickening processes.

Dewatering is another standard method of water removal in biosolids production. Simple dewatering involves containment of wastewater solids in drying beds or lagoons, where gravity, drainage, and evaporation remove moisture. More often, dewatering involves mechanical equipment such as filter presses, vacuum filters, and centrifuges. Mechanically dewatered solids typically contain between 20% and 45% solids. Finally, drying processes can be used to remove even larger volumes of water from biosolids. Thermal drying with direct or indirect dryers followed by pelletisation can remove virtually all water and stabilise biosolids to the point of full compliance with any regulatory requirement. This method is used where there is a viable commercial market for the pelletised product.

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