RURAL WASTE TO ENERGY & SWACHH BHARAT
The issue of solid waste management, till now, was not so
much of a problem as per capita consumption of materials and hence, per capita
generation of solid waste was much lower among rural populace as compared to
the urban. Moreover, the rural population was also diffused and most of the
solid waste generated was essentially organic in nature (food waste and agri
residue), which was either composted or used as fodder. There was very less
usage of packaged materials and hence, very little packaging waste generation.
Situation
is however fast changing in rural, more so, in rural-urban areas, where use of
packaged materials is growing and the waste composition is also becoming
complex. This can no longer be handled by simple processing techniques like
composting but requires more robust and locally available waste processing
options to deal with the solid waste without polluting the environment.
As
of now, it is estimated that rural India is generating liquid waste (grey water)
of the order of 1,500 to 1,800 crore litres and solid waste
(organic/recyclable) 3 to 4 lakh metric tones per day respectively. The
production of energy from such waste, especially solid waste, will depend upon
the specific treatment processes, seasonal variations etc. Important physical
parameters are the size of the constituents and the density and moisture
content of the waste. The important chemical parameters are the fixed carbon
content, calorific value, VS (volatile solid) contents, Carbon to Nitrogen
ratio (C:N ratio), toxicity and inert
fraction present in waste.
In addition
to solid and liquid waste, the rural areas also generate agri-residues and
forestry biomass waste, which can be utilized for energy generation. A study
carried out by The Energy and Resources Institute (TERI) on availability of
biomass for energy generation in selected districts reveals that there is
surplus biomass available in rural areas which can be effectively used for
energy generation. For example, the survey estimated that there is 54,600 and
70,000 tones of annual surplus biomass from agricultural and industrial sources
in Pune and Thane districts of Maharashtra.
Technology option for energy generation from waste
in rural areas
Different
waste processing technology options, which are commercially viable in Indian
conditions and can be used at the scale from 50 kg/day to 1.5-2.0 tonnes/day,
can be deployed in rural areas include
1. Anaerobic
(without air) digestion or biomethanation process and using it for
a)Thermal application (e.g. cooking)
b) Power generation
c) The residue of the process can be stabilised
and used as manure
2. Processing
waste into refuse derived fuel (RDF) and using it for
a)Thermal application (e.g. cooking mid-day
meals in schools)
b) Power generation by burning RDF
3. Use
of waste in gasifiers to convert it into producer gas and use it for power
generation
These technology options are describes below
Anaerobic digestion
Solid
and liquid wastes consist of both organic and inorganic constituents, and the
degradation of the organic constituents can take place in the presence or
absence of oxygen (air). When microbial degradation of organics takes place in
the absence of air, the process is known as ‘anaerobic digestion’ or
‘biomethanation’. This results in the production of biogas, which contains
methane, carbon dioxide and traces of other gases.
Anaerobic
digestion occurs naturally in swamps, waterlogged soils and rice fields,
deepwater bodies and in the digestive systems of animals. Anaerobic processes
can take place in a reactor such as digester vessel, covered lagoon or landfill
in order to recover the methane gas (as biogas), which can be used for power
generation. Waste, which contains particulate organic material (waste sludge,
food waste, vegetable waste, etc.), must first be solubilised by the action of
extra cellular enzymes that are produced by the hydrolytic bacteria. The
solubilisation of particulate material is relatively slow and takes time. The
process can be sometimes hastened by chemical solubilisation or leaching.
After
solubilisation, in next phase, wastes containing soluble organics will require
short retention times for achieving high treatment efficiency to produce
methane rich biogas using acidogenic and methanogenic bacteria.
The typical
food waste orvegetable waste having moisture content of 45 per cent or more, is
an ideal waste to be processed anaerobically as the process requires easily
biodegradable waste with high moisture content. Typical composition of biogas
can range from 45 to 65 per cent methane which can be used both for thermal as
well as electrical applications. The leftover residue, as stated earlier, can
be used as manure.
Refuse Derived Fuel (RDF)
Refuse
Derived Fuel (RDF) commonly refers to solid waste that has been mechanically
processed to produce a storable, transportable, and more homogeneous fuel for
combustion. The typical processes involved in producing waste to RDF pellets
include following steps:
a. l Drying to reduce moisture
content
b.l Size reduction
c. l Screening
d.l Pneumatic separation for the
removal of noncombustible
e.materials
f. l Mixing with additives
g. l Pelletisation
The
waste is pre-processed to remove incombustible materials, thus increasing the
calorific value of the fuel. The incombustible materials are removed using
various mechanical methods for example, ferrous metals are remove during
magnetic separators; glass, grit, and sand is removed through screening. Some
systems utilize air classifiers, trommel screens or
rotary drums to further refine the waste. To reduce the cost of producing RDF,
one can also look at producing waste fluff instead of palletisation which adds
to the cost, but pellets can be stored for longer time and can be transported
easily. Good candidate wastes for producing RDF include agri residues, hard
woody waste, waste plastics, paper, rags, leather and such organic waste which
are not easily biodegradable and have low inherent moisture content (typically
in the range of 10%).
RDF can be co-fired
with fossil fuels in existing industrial (e.g. boilers in sugar mills or paper
and pulp industry which are often located in rural areas) or utility boiler, or
it can be used as the sole or primary fuel in specially designed “dedicated”
boiler for producing either heat or power or both. Co-firing of RDF has the
obvious advantage of capital cost savings since a new boiler is not required.
However, RDF as the primary fuel burning in a dedicated boiler has become more
common, since the dedicated boiler can be designed to accommodate some of the
characteristics of RDF that can otherwise cause operating problems in existing
boilers designed for conventional fuels
The processed RDF
can also be used as fuelin cooking mid-day meals for the schools. Biomass based
briquettes or pellets are already being used as fuel in such scheme in suitably
designed stoves. Use of this fuel comes out to be cheaper than using LPG and
thus reduces the cost of cooking. The gross calorific value of RDF with binder
is in the range of 4000-4500 Kcal/Kg and without binder 3000-3700 Kcal/Kg.
If power generation
through use of RDF is considered as an option in rural settings, itis suggested
to use “hub and spoke” model wherein RDF processing is in semi-mechanised
manner in decentralised locations, preferably close to the source of waste.
This would not only ensure that the cost of processing is less but also
transportation cost of waste is minimized and power generated in this manner
can be easily transported and stored.
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