Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 118

that digests various types of organic industrial wastes, source-sorted MSW, and
manures [7]. The biogas produced is used to generate electricity and heat.
Apparently, dry AD is advantageous for these low-moisture feedstocks because
it eliminates the need to dilute the feedstocks to a fluid state and produces a low-
moisture digestate, which is easier to transport and disperse [90]. The DRANCO
technology is a dry AD technology successfully used to convert low-moisture
organic wastes (e.g., OFMSW and crop residues) to methane biogas [21]. The
DRANCO technology requires the feedstock to be shredded and milled first to
reduce particle sizes (<4.0 mm in diameter). A digested sludge or digestate is
then mixed with the feedstock in a 6:1 to 8:1 ratio in a mixing compartment.
The mixture is heated by steam (to 30-40 ◦ C for mesophilic AD or 50-55 ◦ Cfor
thermophilic AD) and then pumped into the digester at the top. The feedstock
descends by gravity while digestate is withdrawn at the bottom. The biogas rises
and exits the digester through the roof of the digester. The retention time in a
DRANCO digester averages 20 days with a pass-through time of 2-4 days. The
DRANCO technology is marketed by the Organic Waste System (OWS) in Belgium
( http://www.ows.be/index.php ). According to OWS, the DRANCO technology has
a number of advantages including high solid digestion, high loading rates (10-
20 kg COD/m 3 of reactor/d), high biogas productivity (100-200 m 3 of biogas/dry
ton of feedstock), small digester volumes, no maintenance or failures inside the
digester, less energy consumption, well controlled external inoculation, and kill-
off of pathogens and seeds. The largest DRANCO digester started operation in
2006 in Vitoria, Spain. This digester has an effective volume of 1,770 m 3 and a
capacity of 120,750 tons/yr of primarily OFMSW. It produces 5,962 tons of biogas,
which can generate 6,000 MWh of electricity, and 12,580 tons of compost per year.
As of this writing, most of the DRANCO digesters in use are located in Europe,
and the capacity of dry AD has exceeded that of wet AD of solid wastes [21].
The ECOCORP ( www.ecocorp.com ), BEKON ( www.bekon-energy.de ), Kompogas
( www.kompogas.com ), and Linde ( http://www.anaerobic-digestion.com ) processes
are emerging dry AD technologies mostly used in Europe for dry AD of solid
biomass wastes.
A new two-staged AD process was evaluated by Parawira et al. [67] in digesting
solid potato wastes under mesophilic and thermophilic conditions. This process uses
a solid leaching bed reactor for hydrolysis and acidification while an UASB reactor
is used for methanogenesis. High loading rates (36 g COD/L/d), high methane yields
(0.49 L/g COD removed), and stable operation were observed under mesophilic
conditions. The utility of this new process remains to be validated for other types of
feedstocks containing significant amounts of lignocellulose.
3.4 Anaerobic Treatment of Organic Wastewaters
Wastewaters generated from food- and beverage-processing industries often have
little SS but high concentrations of soluble organic compounds (up to 50,000 mg/L
of biological oxygen demand, BOD) such as starch, sugars, and proteins. Some
common examples of these high-strength wastewaters come from cheese factories,