Page 290 - Materials Chemistry, Second Edition

Life Cycle Analysis of Anaerobic Digestion of Wastewater Treatment Plants 271

But other emerging applications of AD have been proposed recently to completely
envelope AD into the global circular economy, as is the case with the potential
resource recovery from wastewater.
Another direct application of AD besides biogas production is to use the digestate
(which is the remnant residue after the digestion) to extract the commodities that
remain in this line (Verstraete et al., 2009). Traditional applications of digestate are
the use of its solid fraction as a natural biofertilizer and more recently, the use of
the liquid fraction containing the majority of the N and P released on hydrolysis to
recover these nutrients as struvite (NH MgPO x6H O), a mineral which contains an
4
2
4
equilibrated amount of N and P but requires, in most cases, the addition of external
Mg salts (Vaneeckhaute et al., 2017). Other options to reclaim N are ammonia
2+
stripping and electro-dialysis (Batstone and Virdis, 2014). These processes are not
directly related to the structure of AD metabolic pathways and do not require con-
trol and optimization but just enhance hydrolysis. However, new research lines have
emerged recently that are focused on the specialization of AD processes’ metabo-
lism to produce high value-added products based on the recovery of carbon (sin-
gle-cell proteins, bioplastics, organic acids …) or metals from wastewater (Puyol
et al., 2017b).

13.1.2 convenTional applicaTion of aD
Biological wastewater treatment entails the transformation of soluble contamination
(essentially C, N, and P) into particulate biomass. The disposal of sewage waste
sludge represents a major problem in wastewater treatment plants (WWTPs), costing
around 50% of the total expenses of the plant. AD is the most used method for sta-
bilizing the sludge, also considerably reducing its volume by between 20% and 70%
(Appels et al., 2008). Typical operations of waste sludge include thickening before
feeding into the digester. Then, the gas line (biogas) is stored before direct use or
upgraded to obtain biomethane. The supernatant (liquid phase) is commonly sub-
mitted to nitrification-denitrification to remove excess ammonium and is recycled
into the main line. The residual sludge (solid phase) is dewatered and further dried
before combustion or land application as biofertilizer. Characteristic configurations
of AD reactors include continuous stirred-tank reactor (CSTR), high-rate CSTR with
active mixing, and two-stage hydrolysis+methanogenesis. Most applied temperature
is in the mesophilic range (30–37°C), though thermophilic processes (50–65°C) are
becoming an interesting option to enhance the hydrolysis step in low-biodegradable
feedstocks, considerably reducing the reactor’s volume (De la Rubia et al., 2012).
Another option is two-stages thermophilic+mesophilic (TPAD), where the thermo-
philic stage improves the hydrolysis through imposing very short solid retention
times (SRT, around 2–4 d), whereas the mesophilic reactor produces most of the
biogas (Wang et al., 2017). However, new applications of AD for low-biodegradable
feedstocks have promoted the need to pre-treat the biomass to enhance the biogas
production.
Hydrolysis is the limiting step in the AD process treating solid feedstocks, as
some microorganisms must release specialized enzymes into the medium (hydro-
lytic enzymes) to solubilize the solid organics, which are further used by other