Page 58 - Materials Chemistry, Second Edition

Life Cycle Assessment of Municipal Wastewater and Sewage Sludge Treatment 39

steady-state conditions. Hospido et al. (2004) evaluated the influent variability of a
WWTP during two different seasons (humid and dry). However, the outcomes from
the environmental analysis demonstrated that variations within each period turned
out to be more significant than those variations attributed to seasonality. Water dis-
charge and sludge application to agricultural soil were found to be the main contribu-
tors to the environmental performance of a WWTP.
It is evident that the configuration of a WWTP will definitively affect the treat-
ment performance in terms of removal yields of target pollutants, but it will also
have indirect related impacts. It is important to compare the impacts generated from
a WWTP scenario against a non-treatment option.
In general, the most significant environmental impacts in the different LCA stud-
ies on wastewater treatment can be classified within three impact categories: energy
consumption associated with aeration in the secondary treatment, accounting for
global warming; the presence of heavy metals in the sewage sludge, which influ-
ences the toxicity-impact categories; and the remaining content of chemical oxygen
demand (COD), N, and P in the treated effluent, linked to eutrophication. Other
relevant impact categories, but with comparatively less importance, are ozone layer
depletion (especially when including N O emissions) and abiotic depletion (associ-
2
ated with fossil energy and material depletion).

3.4 FOCUS ON SLUDGE MANAGEMENT

Sewage sludge is an unavoidable waste product from the treatment of wastewater,
and its final disposal plays a relevant role in the global impact of the WWTP (Gallego
et al., 2008). According to the figures provided to the European Commission, sludge
production associated with sewage treatment has steadily increased between 1995
and 2008 (6.7–10.13 million tons dry matter [DM]). Moreover, it is estimated
that sludge production will reach about 13 million tons DM in 2020 in the EU27
(European Commission, 2008). This can be attributed mainly to the implementation
of the Urban Waste Water Treatment Directive (91/271/EEC), which means a higher
amount of wastewater treated and, in many cases, an increase in the production of
sludge.
There is an increasing tendency to consider sludge as a resource rather than a
waste material; it can be used for nutrients (phosphorus and nitrogen), and carbon
can be recovered and reused in agriculture (Hospido et al., 2005; Johansson et al.,
2008; Lederer and Rechberger, 2010; and Linderholm et al., 2012). In a life cycle
perspective, agricultural sludge application substitutes for the production and use of
mineral fertilizers, reducing the depletion of virgin resources such as mineral phos-
phorus extracted from phosphate rock (Cordell et al., 2011). However, the difference
in the environmental performance on applying sludge instead of mineral fertilizer
to soil depends on the product quality, especially in terms of heavy metals content.
As an alternative to agricultural sludge application, energy can be recovered
from sewage sludge via anaerobic digestion or incineration (Rulkens, 2008). The
energy generated from biogas may be used internally in the WWTPs, thus reduc-
ing their energy demand from external sources, for example from the electric-
ity grid (Appels et al., 2008). The advantage of incineration is the production of