Environmental Engineering Reference
In-Depth Information
The biocell concept involves the operation of a landfill cell as an anaerobic bioreactor
with leachate recirculation to recover the full energy potential of biomass waste. In a second
stage, the biocell is operated in the aerobic mode to produce stable organic material, that can
be used as low-quality compost or refuse derived fuel (RDF). The input of air and operation
of the cell as an aerobic bioreactor (Stessel and Murphy, 1992) enhances waste decomposition
to a level where it could be mined in a third stage for compost/RDF and space recovery, thus
making the landfill operation sustainable. Instead of operating the “entombed” waste cell as a
“long-term storage facility”, the biocell converts the cell into a waste processing facility. The
biocell is a novel and holistic approach; with energy recovery, landfill gas emission control,
groundwater contamination control, and resource/space recovery as direct benefits. This
approach has the potential to revolutionize management of waste in Canada, and in other
countries, both developed and developing.
B. The Calgary Biocell: Background and Construction Phase
In dry-tomb type sanitary landfills, the preferred landfilling option in the United States,
the underlying principle has been to prevent saturation of the waste in order to reduce the
potential for leachate generation and leaking into the sub-surface and groundwater aquifers.
In a bioreactor landfill, leachate is re-circulated to maintain high waste moisture content. Data
compiled by Rees and Grainger (1982) from lysimeter studies suggest the rate of gas
production increases exponentially as the water content of the waste is increased. At the
Brogborough landfill in the United Kingdom, addition of water to test cells containing 15,000
tonnes of waste was found to increase the rate of gas production by 8 m
3
/tonne/yr than that of
a control cell (Knox and Gronow, 1995).
There are three different types of bioreactor landfills corresponding to the operational
processes involved, aerobic, anaerobic, and hybrid (aerobic-anaerobic) as discussed in
Section 2. The primary difference between the aerobic and anaerobic is that, in anaerobic
bioreactors, a key objective is to enhance the generation of landfill gas, containing methane
and carbon dioxide, under anaerobic conditions, by minimizing oxygen infiltration, whereas,
in aerobic bioreactors, the objective is to maintain aerobic conditions by introducing oxygen
into the waste mass.
The Calgary biocell is a unique facility where the three processes, anaerobic bioreactor,
aerobic bioreactor and mining are sequentially applied in one cell. The Calgary biocell is a
full-scale facility, which covers an area of 100 m x 100 m with a waste footprint of 85 m x 85
m and a maximum height of 18 m. The schematic diagram of the biocell is presented in
Figure 10. The base of the biocell is 50m x 50m, and it extends 10 m below the ground
surface. The shape of the biocell is a pyramidal square frustum increasing sectional area with
side slope of 3H : 1V up to ground level. The bottom liner/leachate collection systems are
used to minimize groundwater contamination and maximize recovery of leachate. The biocell
received 43,000 tonnes of residential (high in organics) and selected commercial wastes, with
the resulting feedstock placed in three lifts of 5-6 m each. Collected leachate is re-circulated
after ensuring the quality is acceptable. A pipe system was added to re-circulate the collected
leachate. Re-circulation of leachate adds the much needed moisture and transfer bacteria from
well-inoculated waste to freshly deposited waste, thus accelerating the establishment of
microbial community. The biocell is instrumented to gather moisture, temperature and
