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Abstract:
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Solid waste generation is a natural consequence of human activity and
is increasing along with population growth, urbanization and industrialization.
Improper disposal of the huge amount of solid waste seriously affects the
environment and contributes to climate change by the release of greenhouse
gases. Practicing anaerobic digestion (AD) for the organic fraction of
municipal solid waste (OFMSW) can reduce emissions to environment and
thereby alleviate the environmental problems together with production of
biogas, an energy source, and digestate, a soil amendment. The amenability
of substrate for biogasification varies from substrate to substrate and
different environmental and operating conditions such as pH, temperature,
type and quality of substrate, mixing, retention time etc. Therefore, the
purpose of this research work is to develop feasible semi-dry anaerobic
digestion process for the treatment of OFMSW from Kerala, India for
potential energy recovery and sustainable waste management. This study
was carried out in three phases in order to reach the research purpose.
In the first phase, batch study of anaerobic digestion of OFMSW was
carried out for 100 days at 32°C (mesophilic digestion) for varying substrate
concentrations. The aim of this study was to obtain the optimal conditions
for biogas production using response surface methodology (RSM). The
parameters studied were initial pH, substrate concentration and total organic
carbon (TOC). The experimental results showed that the linear model terms
of initial pH and substrate concentration and the quadratic model terms of
the substrate concentration and TOC had significant individual effect
(p < 0.05) on biogas yield. However, there was no interactive effect between
these variables (p > 0.05). The optimum conditions for maximizing the
biogas yield were a substrate concentration of 99 g/l, an initial pH of 6.5 and
TOC of 20.32 g/l. AD of OFMSW with optimized substrate concentration of
99 g/l [Total Solid (TS)-10.5%] is a semi-dry digestion system .Under the
optimized condition, the maximum biogas yield was 53.4 L/kg VS (volatile
solid)..
In the second phase, semi-dry anaerobic digestion of organic solid
wastes was conducted for 45 days in a lab-scale batch experiment for
substrate concentration of 100 g/l (TS-11.2%) for investigating the start-up
performances under thermophilic condition (50°C). The performance of the
reactor was evaluated by measuring the daily biogas production and
calculating the degradation of total solids and the total volatile solids. The
biogas yield at the end of the digestion was 52.9 L/kg VS for the substrate
concentration of 100 g/l. About 66.7% of volatile solid degradation was
obtained during the digestion. A first order model based on the availability
of substrate as the limiting factor was used to perform the kinetic studies of
batch anaerobic digestion system. The value of reaction rate constant, k,
obtained was 0.0249 day-1.
A laboratory bench scale reactor with a capacity of 36.8 litres was
designed and fabricated to carry out the continuous anaerobic digestion of
OFMSW in the third phase. The purpose of this study was to evaluate the
performance of the digester at total solid concentration of 12% (semi-dry)
under mesophlic condition (32°C). The digester was operated with different
organic loading rates (OLRs) and constant retention time. The performance
of the reactor was evaluated using parameters such as pH, volatile fatty
acid (VFA), alkalinity, chemical oxygen demand (COD), TOC and
ammonia-N as well as biogas yield. During the reactor’s start-up period, the
process is stable and there is no inhibition occurred and the average biogas
production was 14.7 L/day. The reactor was fed in continuous mode with
different OLRs (3.1,4.2 and 5.65 kg VS/m3/d) at constant retention time of
30 days. The highest volatile solid degradation of 65.9%, with specific
biogas production of 368 L/kg VS fed was achieved with OLR of 3.1 kg
VS/m3/d.
Modelling and simulation of anaerobic digestion of OFMSW in
continuous operation is done using adapted Anaerobic Digestion Model
No 1 (ADM1).The proposed model, which has 34 dynamic state variables,
considers both biochemical and physicochemical processes and contains
several inhibition factors including three gas components. The number of
processes considered is 28. The model is implemented in Matlab® version
7.11.0.584(R2010b). The model based on adapted ADM1 was tested to
simulate the behaviour of a bioreactor for the mesophilic anaerobic
digestion of OFMSW at OLR of 3.1 kg VS/m3/d. ADM1 showed acceptable
simulating results. |