Environmental Engineering Reference
In-Depth Information
4. Temperature control
The direct effect of temperature on bacteria activity could be manipulated to optimize the
decomposition of MSW in the bioreactor landfill. Thus, it is necessary to realize the
temperature constraints on individual microorganisms in order to control the activity of
bacteria and enhance waste stabilization.
Baldwin et al. (1998) investigated the effect of temperature on a large scale using two
different landfills, one located in Florida and the other in Wisconsin. The Florida landfill
(30˚C) had a more rapid decomposition compared to the Wisconsin landfill (22˚C). Kasali et
al. (1989) found that by increasing the temperature of MSW with a 60% w/w moisture
content from 18.7 to 30˚C caused a 2.6 times increase in the methanogenic rate, while a 7.8
times increase when the temperature rose from 18.7 to 40˚C. The methanogenic rate is
inhibited when the temperature is increased to 55˚C. Based on the experimental work, the
optimum temperature for enhancing the MSW biodegradation is in the range of 30-40˚C.
5. Reduced waste particle size
Shredding or reducing the particle size of MSW has several advantages, such as
providing more landfill space and greater MSW stabilization. The arguments for shredding
are: 1) it increases homogeneity and distribution of waste within the landfill, 2) it improves
the contact surface area of the waste, 3) it promotes better contact between the organic matter
and microorganisms (Christensen et al., 1992).
Ham et al. (1982) found that the shredding of waste increases the rate of decomposition
and methane production. Some authors (Buivid et al., 1981) concluded that refuse with 2.5 to
3.5 cm particle sizes produced 32% more methane than refuse with 1 to 1.5 cm particle sizes
in a period of 90 days. This is due to the fact that the smaller particle size increases the rate of
hydrolysis and acid formation which in turn decreases the pH and postpones the production of
methane.
Based on that, if the negative effect of smaller particle sizes in the initial stage of
biodegradation can be controlled (by adding buffer or pre-composting), shredding may
enhance the biodegradation process, since the hydrolysis is the rate-limiting step (Yildiz et al.,
2004; El-Fadel et al., 1996b; Pareek et al., 1999; Naranjo et al., 2004).
Sponza et al. (2005) reported that the shredding of MSW has a positive effect on the rate
of biological degradation in anaerobic bioreactors with leachate recycle. They compared three
types of reactors. The first reactor was loaded with raw waste, the second with shredded
waste, and the third with compacted waste. At the end of the experiments (57 days later), they
found that the reactor with waste shredding had the lowest COD and VFA concentrations and
the highest methane percentage.
6. Cell design, daily cover and compaction of waste
The enhancement of waste biodegradation in the landfill is also affected by the cell
thickness, daily cover and compaction of waste.
The cell thickness has an adverse effect on the biodegradation of waste. Ham et al. (1982)
found that the cell with a 2m deep lift produced higher leachate concentrations and took a
longer time to stabilize than the cell with a 1.2m deep lift. By doubling the cell depth from 1.2
to 2.4 m, the concentration of leachate and the stabilization time are doubled as well.
