Page 291 - Materials Chemistry, Second Edition
P. 291
272 Life Cycle Assessment of Wastewater Treatment
microbial communities in catabolic processes (as methanogenesis) and biomass
growth. To enhance the process, it is quite common to promote “artificial” hydro-
lysis that can speed up the methane production rate and increase the biochemical
methane potential by converting non- or low-biodegradable solid substrate
into easily accessible soluble substrate (Appels et al., 2008). More typical pre-
treatments include thermal treatment, mostly at high temperature (>100°C) and
constant or variable pressure (“steam explosion”); mechanical treatment, mainly
by ultrasound; and chemical treatment, such as addition of acid or alkali and wet
oxidation. Novel treatments include microwave irradiation, advanced oxidation
processes at mild temperature, pulsed electric fields, and free nitrous acid addition
(Carlsson et al., 2012; Wang et al., 2017).
AD has been also applied to treat domestic, industrial, and agroforestry waste-
water with high organic composition, such as that coming from the food, beverage,
alcohol distillery, and pulp and paper industries, which accounts for more than 2000
anaerobic reactors installed worldwide (van Lier et al., 2015). Domestic wastewater
has been intensively treated by anaerobic technologies especially in various parts of
the tropical world, notably in Latin America and India (Chernicharo et al., 2015),
as will be further discussed. But most experience is in the application of high-rate
anaerobic reactors to treat industrial wastewater.
Since its appearance in the early 1980s, the upflow anaerobic sludge blanket reac-
tor (UASB) has been widely applied to treat wastewater worldwide (van Lier et al.,
2015). The original concept is described as a compact reactor in which the biomass
concentration is greatly increased due to a natural association of microorganisms,
cations, and inert particulates to form highly compact and structured biogranules,
which are formed as a result of the hydraulic stress imposed by a high upflow veloc-
ity (usually higher than 0.5 m h ) (Zeeman and Lettinga, 1999). As a result, hydrau-
−1
lic retention time (HRT) is decoupled from SRT, and this leads to a high increase
in the volumetric activity inside the reactor (with specific activities typically around
0.3–1.0 kgCOD-CH kgVSS d ), which ultimately leads to obtaining high chemi-
−1
−1
4
cal organic demand (COD) removal efficiencies in treating industrial wastewater
with high organic content by imposing HRT similar or even lower than in typical
activated sludge processes (van Lier et al., 2015). Some variants of the original UASB
concept have appeared so far, including the expanded granular sludge bed (EGSB)
reactor, which applies a much higher upflow rate (around 6–30 m h ), leading to an
−1
almost perfect mix, and the internal circulation (IC) reactor, where the produced bio-
gas is separated from the liquid halfway through the reactor by means of an in-built
gas-liquid-solid separator device and conveyed upward through a pipe to a degasifier
unit or expansion device. Another option is a hybrid UASB configuration upgraded
with an external liquid-solid membrane separation (van Lier et al., 2015). Though
UASB-based high-rate anaerobic reactors have quite high COD removal efficiency
in compact configurations, there are still some constraints that have to be considered,
as the partial dissolution of CH in the effluent (highly inversely correlated with
4
the influent COD concentration), which increases the GHG emission potential; the
potential production of reduced gases, such as H S, which can cause odor problems;
2
and their inherent limitations at low temperature and high sulfate concentrations,
which reduce the biogas production potential considerably (Chernicharo et al., 2015).