Page 269 - Materials Chemistry, Second Edition

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240 Waste Management Practices: Municipal, Hazardous, and Industrial
● Continuous — The reactor vessel is fed continuously with digestate material. Fully
degraded material is continuously removed from the bottom of the reactor.

Single step vs. Multi-step

● Single step — All digestion processes occur in one reactor vessel.
● Multi-step — Several reactors operate simultaneously. In some cases the acid-forming
stage is separated from the methane-forming stage. This results in increased efficiency as
the two microbial populations are separated in terms of nutrient needs, growth capacity,
and ability to cope with environmental stress. Some multistage systems also use a pre-
liminary aerobic stage to raise the temperature and increase the degradation of the
organic material. In other systems, the reactors are separated into a mesophilic stage and
a thermophilic stage (Chiang Mai University, 1998).

Co-digestion with animal manure. The organic fraction of the MSW is mixed with animal
manure and the two are co-digested. Such mixing improves the carbon/nitrogen ratio and improves
gas production.
Systems in Use. There are a number of HSAD systems either commercially in use or at the pilot
scale. The Dranco (Dry Anaerobic Composting) system was developed in Gent, Belgium. Feed is
introduced into the top of the reactor. Digested materials are removed from the base of the unit as
feed is added. A portion of the digestate is recycled as an inoculum and the remainder is de-watered,
resulting in a usable compost product. No mixing occurs within the reactor. Solids content ranges
from about 15 to 40%. Retention time in the reactor varies between 15 and 30 days and the operat-
ing temperature ranges between 50 and 58°C. The Dranco system can yield between 100 and 200
3
m of gas per ton of MSW input. The gas content is 55% CH (Six and DeBaere, 1992).
4
The Valorga system, developed in France, combines four mesophilic HSAD reactors. Mixing of
feed within the reactor is carried out by circulation of a portion of the biogas under pressure. The
biogas product has about 55 to 60% CH content. The process operates with a solids content of 25
4
to 50% with residence times ranging between 18 and 25 days.
The BIOCEL process is a mesophilic dry anaerobic batch digestion system (ten Brummeler,
2000). In this process, net energy production is achieved by converting the biogas into heat and
power. The first full-scale plant started up in Lelystad, The Netherlands, in 1997. This plant
processes 50,000 tons of MSW per year. Anaerobic digestion with the BIOCEL process has been
found to inactivate several important groups of plant and animal pathogens. The mechanism that
causes the inactivation is not yet fully understood, but may be related to the relatively high volatile
fatty acid concentration during the first 2 weeks of digestion.
The Kompogas system is a thermophilic digestion system developed in Switzerland. The reac-
tion vessel is a horizontal cylinder where feed is introduced daily, and movement through the
cylinder is accomplished via a horizontal plug-flow. An agitator is installed within the cylinder to
mix the material intermittently. Digested material is removed from the end of the cylinder after
about 20 days. The digestate is dewatered, and some of the press water is either used as an inocu-
lum or is sent to a wastewater treatment facility to produce more biogas (University of
Southampton, 2002).
The Wabio Process was developed by Ecotechnology of Finland (Chhabria, no date). There are
two Wabio Process plants in operation. A plant at Vaasa, Finland, has been in operation since 1991
and a facility has been operating in Bottrop, Germany, since 1995. The Wabio process is a single-
stage process operating in the mesophilic temperature range. At the treatment plant, feed preparation
tanks receive the screened material and a slurry of 15% solids concentration is prepared. The slurry
is then pumped to bioreactors. Digestion occurs at 30°C and the retention time of the material is 15
to 20 days. The process can also be operated in the thermophilic range at 55°C. The supernatant liq-
uid is recirculated to make the slurry. Biogas is stored in the gas holder and a portion is used for