Page 73 - Materials Chemistry, Second Edition

54 Life Cycle Assessment of Wastewater Treatment

around 0.3 L CH per kg total suspended solids (TSS) of sludge digested. In some
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cases, there may be trace amounts of nitrogen gas (N ), hydrogen gas (H ), ammo-
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nia (NH ), and/or hydrogen sulfide (H S). Biogas outputs from the digestion can be
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burned to power internal combustion engines or upgraded into biomethane and gener-
ate electricity to be returned to the electrical grid (USA, 2010). When biogas is being
used for energy production, it offsets the energy necessity for fossil fuels, which could
reduce greenhouse gas (GHG) emissions from burning fossil fuels and slow down
global climate change. The digestion process itself consumes heat and electricity for
process mixing and heating, biogas clean up, and digestate dewatering. The energy
created by biogas combustion can cover the energy use for the process operation.
The digestate from the sludge digestion process has to be further dewatered for
final disposal or other reuse. This stream is rich in nitrogen and phosphorus and can
be applied to fertilize land and fields after dewatering, thus bringing environmental
benefits by replacing synthetic fertilizer use, improving soil quality, and sequestrat-
ing carbon in soil. The typical digested sludge contains 1.6–3.0 wt.% of N, 1.5–4.0
wt.% of P O , and 0–3.0 wt.% of K O. When applied to land, the nutrient use effi-
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ciency depends on the compositions of certain nutrients in the digestate. For exam-
ple, nitrogen use efficiency is different for NO , NH , and organic nitrogen, at 100%,
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75%, and 40%, respectively (Metcalf and Eddy, 2003). In addition to the environ-
mental benefits from applying biosolids to land, environmental concerns, which will
be described in Section 4.2.1, have been raised. The digestate can also be combusted
or directly landfilled.
The liquid/supernatant collected from the top of the digester contains high levels
of nutrients. The COD level may be up to 1000 mg L . When recycled back to the
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activated sludge process, the supernatant will increase loading in the aeration tank.
A recent study by Min et al. (2011) proposed using this stream to grow algae, which
are then converted to algal biofuels for vehicles. As the supernatant is created by a
moderate temperature digestion, the nutrients are easier for the algae to absorb and
can promote high yields in algae cultivation. The supernatant recycling offers envi-
ronmental benefits by reducing energy use in the activated sludge process, saving
nutrient use in algae cultivation, and replacing fossil fuel use in transportation. The
total nutrients in supernatant depend on the amount of nutrients contained in biogas
and digestate. The nutrient balance calculation can help to estimate the nutrient con-
tent in supernatant.
Many LCA studies have been conducted for biosolid digestion to evaluate the
environmental impacts of this process (Yoshida et al., 2013; Pradel et al., 2016).
A majority of those studies concluded that compared with incineration, composting,
and landfilling, the scenario of anaerobic digestion combined with agricultural land
application was the most environmentally friendly in terms of emissions and con-
sumption of energy, but heavy metals released from sludge contributed significantly
to human toxicity and ecotoxicity. The study by Gourdet et al. (2017) identified four
key factors that are related to environmental impacts in anaerobic digestion. They
are: (1) the biodegradation rate of volatile solids, (2) the nitrogen mineralization rate
during anaerobic digestion, (3) phosphorus and nitrogen capture rates during the
thickening and dewatering processes, and (4) the consumption of chemicals such as
FeCl in the dewatering process.
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