Page 70 - Materials Chemistry, Second Edition

Life Cycle Assessment of Beneficial Reuse of Waste Streams 51

4.2 WASTE STREAMS GENERATED IN MUNICIPAL
WASTEWATER FACILITIES

Before introducing the reuse technologies, it is necessary to first understand the
major waste streams created within municipal wastewater treatment facilities. This
section will describe each of the waste streams, how they are treated, their charac-
teristics, and their ecological impacts.

4.2.1 biosoliDs
Biosolids are all solid streams discharged out of wastewater treatment facilities.
A major source of biosolids is sludge collected from the bottom of primary and sec-
ondary clarifiers in the wastewater treatment plants. As the water content can be as
high as 99.5%, the sludge collected is first sent to the sludge thickening/storage tank
to remove water and increase the solid content. It is then stabilized through chemical
stabilization or anaerobic digestion. Lastly, the sludge is dewatered mechanically
until the water content reaches 70% and then sent out of the plants for final disposal.
Anaerobic digestion can also serve as a facility to recycle energy from biosolids.
When biosolids are sent outside of the plant, they are typically incinerated, land-
filled, or applied to land as fertilizer (Davis and Slaughter, 2006).
When biosolids are incinerated, they emit energy, heat, and harmful contami-
nants. Burning biosolids leaches polychlorinated biphenyls (PCBs), a group of prior-
ity pollutants controlled by the US EPA, into the environment (Rice et al., 2003).
While trace exposure to PCBs is more commonly found, extreme exposure to PCBs
has led to the death of fish, wildlife, flora, and fauna. Some common ailments
reported are cancerous lesions, immune suppression, change in enzyme activity, and
liver and reproductive failure (United States Fish and Wildlife Service, 2015).
When biosolids are sent to a landfill, biogas is created and sometimes collected for
generating energy. Ecological harm may arise via leaching of harmful chemicals into
water/soil or via toxic chemicals released into the atmosphere from decomposition of
the compounds in the biosolids. Leachate from waste streams contains nitrate, met-
als, organics, and/or pathogens, which potentially enter groundwater if the landfill
is incorrectly placed geographically or has damage to the lining (Kajitvichyanukul
et al., 2008). Potential ecologically harmful impacts from landfills also include deg-
radation to air quality, public health, wildlife, and habitats of endangered species
(US EPA, 2003).
Applying biosolids to land as fertilizer also raises many concerns. The pathogen
content of sludge is of high concern, as there have been instances in the past when
poor management has led to harmful effects on municipalities. A tragic instance of
this was seen at Walkerton, Ontario when Escherichia coli 0157:H7 and other patho-
gens contaminated the drinking water after sludge was sprayed on rural land. In addi-
tion to the concern about pathogens, contamination by heavy metals and persistent
organic pollutants (POPs) is also of high concern. Heavy metals such as zinc and cop-
per are micronutrients that facilitate plant growth but can be damaging to the plants
in excessive amounts. Heavy metals are not very mobile, tend to accumulate on the
surface of soils, and cannot be easily removed once applied to land. Of all the heavy