Environmental Engineering Reference
In-Depth Information
Klink et al. (1982) found that the reactor with leachate recycle had 25-50% higher
methane production as compared to reactors having the same moisture content but without
leachate recirculation.
2. pH buffering
The methanogenic bacteria are sensitive to pH and could be inhibited by acidic
conditions. This understanding has led to adding buffer to the leachate prior to recycling it
back to the bioreactor landfill. Leachate recirculation with a buffering system to control the
pH causes a shorter acidogenic stage compared to leachate recirculation without a buffering
system (Komilis et al., 1999a).
In a study conducted by Warith (2002), the reactor with buffered and nutrients amended
recycled leachate resulted in the greatest reduction of COD concentration over time. Also, a
study by San et al. (2001) found that the highest degree of stabilization occurred in a reactor
with a four-time per week recirculation and pH control by addition of buffer. Ağdağ et al.
(2005) studied the effect of alkalinity addition to leachate recycle on the degradation of MSW
in an anaerobic bioreactor. It was observed that lower COD, VFA concentrations and
BOD
5
/COD ratios were obtained in the bioreactors with alkalinity addition in comparison to
the bioreactor (control) without alkalinity addition.
Lab scale experiment results recommend the addition of buffer to leachate recycle,
especially in the acid generation phase, to maintain the pH at a neutral level. This helps to
establish a methanogenic condition.
3. Sludge addition
The effect of sludge addition on the MSW degradation is covered by Pacey (1989),
Leuschner (1982) and Warith (2002). They concluded that the addition of sewage sludge has
both a positive and a negative effect on the MSW biodegradation and methane generation.
The positive effect of sludge addition occurs if the methanogenic bacteria are already
established or the landfill environment is optimum (pH neutral) for methanogenic bacteria
(Christensen et al., 1992). This positive effect can be attributed to the following factors: 1)
sludge can be a source of nutrients and active methanogenic bacteria, and; 2) sludge increases
the moisture content.
The negative effect of sludge addition to fresh waste is attributed to the acid
accumulation that is associated with it which decreases the pH and inhibits the methanogenic
bacteria (Barlaz et al., 1990).
Rees (1980) and Leuschner (1982) found that the anaerobic digested sewage sludge is an
excellent source of microbial inoculum, whereas the septic tank sludge is a poor one. Also,
Komilis et al. (1999a) concluded that adding anaerobic digested sludge to MSW produces
three times more methane than adding primary sludge.
It appears that the addition of anaerobic digested sludge with buffering enhanced the
biodegradation and increased methane generation. Buffering controls the pH of the landfill
around neutral, allowing the methanogenic bacteria in the anaerobic sludge to acclimatize to
the landfill environment faster than without buffer addition.
The addition of old waste or ashes to new waste could improve biodegradation by
diluting the acids produced during the acidogenic stage, thereby enhancing the methane
formation stage. The percentage of ash should not exceed 10% in weight (Komilis et al.,
1999a).
