Page 43 - Materials Chemistry, Second Edition

24 Life Cycle Assessment of Wastewater Treatment

iii. Gasification → Compressed natural gas
Pipeline quality gas
iv. Pyrolysis → fuel product → process heat

3. Potential energy from falling wastewater or effluent (hydropower)
4. Biofuels and microbial fuel cells

In this chapter, the use of biogas and hydro energy will be addressed using the
real-time operational data of a WWTP in Turkey.

2.2 ANAEROBIC DIGESTION: USE OF BIOGAS ENERGY IN WWTP

In all communities, two vitally important services are the supply of clean drinking
water and efficient wastewater treatment (EPRI, 2013). Wastewater treatment is an
energy-intensive process, since pumping, treating, disinfecting, aerating, and pro-
cessing biosolids consumes a large amount of energy. For example, WWTPs in the
United States consumed approximately 30.2 billion kWh of electricity annually as of
2013 (EPRI, 2013). Public awareness of environmental issues, especially greenhouse
gas (GHG) emissions, decarbonization of energy sources, and energy, efficiency
directed WWTPs toward producing their own energy and hence, net neutral energy
consumption (Bachmann, 2015). Fortunately, the organic matter in raw wastewa-
ter contains 10 times more energy than is necessary to treat it. Also, WWTPs can
produce 100% of the energy that they need to operate (Patricia Sinicropi, 2012).
Anaerobic digestion (AD) is a proven technology used for stabilization of sewage
sludge, which allows the generation of biogas from the same process (Bonnier, 2008;
Lebuhn et al., 2015). In the AD process, the organic contents of sewage sludge are
decomposed by bacteria in the absence of free oxygen to yield the biogas, which
consists mainly of methane and carbon dioxide (Table 2.1).
In fact, AD is a natural process that can occur in lakes, dams, and swamps, and
the ignis fatuus arises due to the presence of biogas (therefore, biogas is also called
marsh gas). Although AD is given as a two-step reaction in some texts (Mountein,
2011), it is a complex process, as follows (Mes et al., 2003):

1. Hydrolysis: Conversion of non-soluble biopolymers (saccharides, proteins,
lipids) to soluble organic compounds
2. Acidogenesis: Conversion of soluble organic compounds to volatile fatty
acids (VFA) and CO
2
3. Acetogenesis: Conversion of VFA to acetate and H 2
4. Methanogenesis: Conversion of acetate and CO plus H to methane gas.
2
2
These processes also consist of several elementary reactions, but the detailed
kinetic analysis of these reactions is outside the scope of this chapter; detailed
knowledge can be found in Al Seadi et al. (2008). Biogas contains 45%–70% meth-
ane and has a calorific value close to lignite (biogas containing 65% methane has a
lower heating value [NCV] of 23.3 MJ m [European Environment Agency, 2005])
−3
and can be