Page 71 - Materials Chemistry, Second Edition

52 Life Cycle Assessment of Wastewater Treatment

metals, cadmium and mercury are of greatest concern, as they tend to enter the food
chain and be biomagnified (Reilly, 2001). Lastly, POPs found in sludge, such as poly-
chlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), have the poten-
tial to accumulate in soil, thus negatively impacting flora and fauna (Youcai, 2016).
Given the environmental impacts from conventional biosolid disposal methods,
many researchers have focused on new technologies for beneficial reuse. Sludge con-
tains potential energy of 8000 Btu per dry pound (McCarty et al., 2011), suggest-
ing that it is a good source of alternative energy. The idea of converting biosolids
into green transportation fuels, that is, biodiesel, has increasingly become a topic
of interest. New technologies such as pyrolysis, gasification, and liquefaction have
been tested to convert biosolids into various products such as biodiesel and gasoline.

4.2.2 scuM
Scum is a layer skimmed from the top of the primary settling tank during the wastewater
treatment process. Scum contains cooking oils, animal fats, food wastes, soaps, waxes,
and other impurities produced by restaurants, households, and various other entities.
For many wastewater treatment facilities, scum is combined and treated with activated
sludge in the same anaerobic digestion process. When sludge digestion is applied for
energy recycling, the scum will always float on the top of the digester and form a thick
layer, which will impede the digester’s performance. Therefore, other wastewater treat-
ment facilities will treat scum as a waste product, either incinerating it or sending it to a
landfill, which raises similar environmental problems to those of biosolids.
Studies have shown that dried scum contains approximately 62% fats and oils and
almost 14% organic biosolids (Anderson et al., 2016), which make it attractive as
an alternative energy source. Different attempts have been made to convert scum to
biodiesel (Anderson et al., 2016). A novel process for extracting oil from scum and
successfully converting it to fuel grade biodiesel has been achieved at the University
of Minnesota (Bi et al., 2015). Oil extraction from wastewater scum via acid hydroly-
sis and glycerolysis was studied as the pretreatment method for biodiesel production.
The results showed that free fatty acids (FFA) in scum can be reduced effectively
through glycerolysis kinetics, and zinc-based catalysts increased the reaction rate
significantly (Anderson et al., 2016).

4.2.3 cenTraTe anD oTHer liQuiD WasTe sTreaMs
Centrate is the liquid portion that is formed from the sludge dewatering process in the
wastewater treatment plant. Once extracted, the centrate is typically sent back to the
activated sludge process and combined with untreated wastewater for further treat-
ment. Centrate usually contains high levels of pollutants, which can result in COD
−1
levels up to 5000 mg L . The nitrogen (N) and phosphorus (P) levels can be 30–40
times higher than those of untreated municipal wastewater; see Table 4.1. If centrate
is sent back to the secondary treatment process, it will increase the pollutant levels
of inflow to the aeration tank, consequently reducing the treatment effect, and will
raise the energy use in the treatment. The energy use will be higher, since the elevated
quantities of pollutants present require aeration for a longer time to remove them.