Abstract:
The dry batch digesters established at Dickovita biogas plant in Wattala, Sri Lanka, treat
pre-sorted Municipal Solid Waste (MSW) anaerobically to recover both energy and material
in an environmentally friendly manner. Efficient biogas production has been achieved by
the plant by adding three cow dung (inoculum) layers. Since a continuous supply of cow
dung is not available, switching to an alternative inoculum source is a priority requirement
for the continuation of the process. Hence, a laboratory scale study was planned to
determine the impact of replacing the top cow dung layer by previously produced one month
old sludge by the plant, on biogas flow rate. Dry batch type digesters designate treatment
1, 2 and control were set up using empty 20 L bottles, each consisting of 3 replicates. The
experimental setup was comparable to the field operations conducted at the Dickovita
biogas plant. The contents in each digester from bottom to top were bottom inoculum layer,
first organic waste layer, middle inoculum layer, second organic waste layer and top
inoculum layer respectively and the volume of water in each digester was 50% of the
volume of the digester. The top inoculum layer of treatment 1, 2 and control consisted of
cow dung, previously produced one month old sludge and organic waste respectively. Daily
biogas flow rate, sludge temperature, sludge pH and internal and external air temperatures
were recorded. The significant highest and lowest bio gas flow rates were recorded from
treatment-2 and control respectively on day 04. One way ANOVA followed by Tukeys pair
wise comparison tests confirmed that there were no significant differences (P≥ 0.05)
between the mean biogas flow rates generated from treatment-1 and treatment-2. The
mean internal air temperature and the mean internal sludge temperature of digesters varied
between 28.2 °C – 33.9 °C and 27.1 °C – 31.2 °C respectively. Hence, the digesters
showed a mesophilic type digestion. The study recommends that the previously produced
one month old sludge by the plant could be used as the top inoculum layer to produce
biogas without having an adverse impact on biogas flow rate, under mesophilic conditions.
Keywords: Biogas, cow dung, dry batch type digesters, inoculum layer, sludge
