Newsletter and Technical Publications
Freshwater Management Series No. 1
Biosolids Management: An Environmentally
for Managing Sewage Treatment Plant Sludge
An Introductory Guide To Decision-Makers
Wastewater Treatment: The Municipal Sludge Production
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.
2: Preparing the the land for the incorporation of biosolids on the
soil. (Source: UNU/INWEH)
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
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
collected through tertiary treatment consists mainly of chemicals added
to clean the final effluent, which are reclaimed before discharge, and therefore
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.
2: Wastewater treatment Process.
The Municipal Sludge-to-Biosolids
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.
principal process employed to convert municipal sludge into biosolids is
called stabilisation. Stabilisation accelerates the biodegradation
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
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
the pH level of the solids using lime or other alkaline materials.
Thermal drying and composting can also be used to stabilise biosolids. Full
biosolids is not needed when the primary use is cropland application.
Any potential risk to human health due to exposure is eliminated through
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
in determining the choice of a utilisation disposal option. Put simply,
habitually used the sewer system as a convenient and low-cost way
to discharge hazardous wastes. These contaminants accumulate in the biomass
and can render the material unfit for any beneficial use. The most
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
option when the wastewater is from municipal sources only, or when
fully enforced industrial pre-treatment and discharge control system
is in place.
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.
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
dewatering involves mechanical equipment such as filter presses,
vacuum filters, and centrifuges. Mechanically dewatered solids
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
the point of full compliance with any regulatory requirement.
This method is used
where there is a viable commercial market for the pelletised