Sustainability

Sustainability of a waste management system requires satisfaction of a minimum of three bottomline: Economic, Environmental and Social sustainability.   At present, city employs a door to door collection system, where waste is collected directly from households and is not dumped in the street bins as before.  This provides little opportunities for conventional ragpickers to receive the recyclables.  From the primary collection, the collection personnel recover a few of the easily saleable recyclable materials from where it goes to dumpsite without any segregation or treatment process.  At two of the three processing sites, there are frontline segregation units that discard lighter materials and break polythene bags containing domestic wastes.  This separates out plastics, rags and fluff, wet fermentables and also heavy materials such as metals, glass, tyres and stones.  With such a pre-processing the fermentable content rises significantly.  Earlier mentioned composition of MSW shows that it has 72% of fermentable waste, with high moisture content.  This situation is conducive to composting or biomethanation.  When composting of such high moisture feedstock is attempted by conventional windrow based composting process it generates excessive amount of leachates in the rainy season and its fermentation results in malodors due to inadequate supply of air (Chanakya et al., 2007).  So it is important that such wastes are treated rapidly in decentralized units of 5 to 10 t/unit.  At this scale of 500-1000 tpd there are few working technologies for Indian USW for biomethanation.  It is estimated that one ton of wastes requires about Rs.250 for processing by windrow composting (Basavaiah, 2008 per comm.).  As a result a large quantity of wastes are found untreated at these large treatment facilities and it is therefore suggested that, when waste collection is zoned and collected zone-wise, the predominant resident and hotel wastes could be collected separately and treated nearer the site of production by biomethanation within each ward as has been done in the case of Yelahanka trial process with small scale (50 kg) composting.  This firstly avoids the need for transportation and thus saves the transportations costs.  This has the capability of recovering a large extent of plastics and other recyclables making the overall process more sustainable.  The sustainability of such decentralized biomethanation systems is discussed.  Small scale biomethanation plants have been in operation in three towns of Karnataka on a trial basis and that in Siraguppa town has been in operation since 2003.  At this location there are three 0.5 tpd capacity 3-zone fermenters daily fed a total of 1.5-2.5 t of secondary segregated USW of Sirguppa town.  The digested material is then subject to vermi-composting and the recovered vermi-compost is re-used in various town gardens etc. 

Economic sustainability :

The existing system of waste management requires a net input of revenue for continuous operation.  Firstly, there is a need to spend Rs.1000-1500/t for transporting wastes after primary collection to locations where it to be tipped (waste treatment facilities) that are between 40-60 km outside the city.  In addition the waste treatment facility charges Rs.600/t (of landfilled USW) as tipping fee. The tipping fee provided is calculated on the basis that 30% of the wastes will be landfilled and consequently 3.3t of input USW will lead to a cost of Rs.600 as tipping fee.  This may be simplified to be Rs.200/t of USW brought into the waste treatment facility.  This indicates that there is a net input of Rs.1450/t of wastes brought in for treatment at the integrated waste treatment site. There is very little revenue streams arising out of this type of facility and therefore it is considered not economically viable in the long run.

In the proposed decentralized system containing a biomethanation plant and primary segregation and resource recovery system as has been demonstrated in Yelahanka trials (size 5-20 tpd), one ton of USW would result in 60 m³ of biogas whose value is Rs.900 as fuel gas or Rs. (Table 4). A decentralized system on the other hand would not require any transportation costs.  One ton of wastes of the composition indicated earlier has the potential to recover the following at 100% recovery (although 100% recovery is difficult we indicate potential).  In Table 3 we have indicated the potential costs and benefits from a 1 tpd scale decentralized biomethanation + recyclable recovery system.  The results show that decentralized systems, not accounting for land costs, are more profitable and hence higher in the scale of economic sustainability than centralized large waste treatment systems currently practiced.

Environmental sustainability :

Open dumping is conducive to the generation and release of GHGs, such as methane – having 21 times more GHG potential than CO₂ (Morgenstern, 1991). Methane is released when USW is dumped on open grounds with a large quantity of moisture as found in Bangalore USW.  However, in this case not all the part of the fermentables are converted to methane.  A significant part of it suffers aerobic decomposition due to large spaces within and IPCC default values suggest that about 50% is subject to anaerobic digestion and only that fraction contributes to methane generation.  Our estimates indicate that fermentable of Bangalore waste has potential for a maximum of 6.24 kgCH₄/t and using IPCC default this value is estimated to be 24.95 kgCH₄/t. As USW in Bangalore has high moisture content, the IPCC default values need to be corrected for its moisture content to obtain sensible emission data.  The key environmental sustainability gained here is by the fact that 6.24 kg of methane is not emitted from USW and consequently a C-footprint of 6.24 kg is reduced.  Second, when accountable for such a C-footprint per ton USW, there is a cost avoided for methane not emitted.  Third, if these fermentable wastes are used in decentralized manner to generate biogas, 70% of biochemical methane potential (BMP) can be recovered and can be a cheaper source of energy. This avoids the use of an equivalent quantity of fossil fuels in the vicinity.  Fourth, the recycling of various components such as plastics, paper, glass and metal would offset various levels of GHG that are produced in the making of this primary product (not estimated in this paper).  As we head into a climate conscious society, it is imperative that we plan to reduce the potential GHG emissions from waste management.

Social sustainability:

Decentralized waste treatment will provide livelihood to 2 persons /ton in the energy unit (biomethanation plant) as well as another two persons in the waste recycling unit.  When compared to the centralized unit, the decentralized system would employ about 7000 persons daily.  The treatment of wastes near the point of generation returns many value added product locally such as gas for use in domestic and commercial uses in the locality, vermi-compost or compost for local uses, recycled plastics for locally useful products including road laying etc.  It will greatly increase the trade and social responsibility of wastes in the locality.  The exact nature and extent of social sustainability will need to be quantified in a detailed study.

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