Introduction |
Cities in South India have demarcated pockets such as residential, educational, commercial etc. areas created and protected by city regulations that prevent very drastic changes in urban land use and changes occur only slowly. This being the case it is possible to collect solid wastes of specific composition within these demarcated areas of the city based on the type of major land usage. Many South Indian cities are characterized by having a high degree of fermentable component in municipal solid wastes (MSW) from residential areas. The fermentable components of MSW consist predominantly vegetable and fruit wastes that can range between 65 and 90% (Rajabapaiah 1988; TIDE 2000; Aus-AID 2002; Hui et al. 2006; Ramachandra 2006). As and when all these cities begin source segregation into fermentables, recyclables and that to be inertized, as specified by new laws (MSW handling rules 2000), it is expected that a significant component of fermentable fraction will be produced and needs to be picked up on a daily basis.
Today the collection and transportation costs of MSW in such cities range between Rs. 500–1,500 (US$12–36) per ton and is often accompanied by uncertainties in frequency and quality of collection and transport (Yedla and Parikh 2002). These have emerged as a source of concern. In response to such uncertainties in daily collection, there have been a large number of decentralized individual and community based collection, processing and reuse efforts (Yedla and Parikh 2002). In such a case the type of treatment option varies according to two inherent factors namely the daily MSW production size and its typical composition. Individual house based treatment units of the organic fraction of MSW use various types of aerobic or anaerobic composting (Chanakya and Jagadish 1997), vermicomposting and insect larvae based composting processes (black soldier fly larva). Amongst these only aerobic composting and vermicomposting techniques have been tried in South Indian cities.
Earlier efforts show that in source segregated solid wastes (henceforth called organic fraction of municipal solid wastes, OFMSW) coupled to door-to-door collection systems are reasonably efficient and have nearly 97–98% OFMSW and only a small content of nonfermentable material (Chanakya and Jagadish 1997). Such segregation and collection efficiency provides potential for this OFMSW to be directly composted aerobically, aerobically composted followed by vermicomposting or anaerobically composted to biogas and compost. As the daily per capita OFMSW production varies between 0.2–0.5 kg in Karnataka state, depending upon lifestyles in the cities (indicated by city population (KUIDFC 2003; CST 2005). Thus, due to the small size of the total fermentable MSW generated at the household scale only composting and vermicomposting seem feasible at the household level. Many commercial and non-commercial devices for household composting and vermicomposting have been tried in Bangalore (Aus-AID 2002). Most of these have run into difficulties either due to excessive malodours due to the generation of large volumes of leachates and organic acids in case of household composters and worm predation or worm migration in household vermicomposting devices (Chanakya et al. 2006; TIDE 2000, 2003; Rajabapaiah 1995).
Area or community scale options indicated above have been more successful in Bangalore and in various places in India. Area or zone wise collection has been shown to simplify collection systems and enables collection of MSW of similar composition (Sathishkumar et al. 2001). Leaf litter and garden wastes, vegetable and fruit wastes, domestic and kitchen wastes, etc. thus are manually carted and treated at scales between 0.050 and 0.250 tpd scale (TIDE 2003). Such efforts have survived long periods of operation with much fewer technical problems. The influence of using such feedstocks on the underlying processes has been poorly documented with regards to technical process parameters, yields, costs and operational difficulties. In this study we have carried out micro-composting of some of the typical feedstocks of zone-wise collected OFMSW and studied the process as typically carried out in the field and attempted to determine the process parameters and yields. Zone wise handling has been considered before (Daskalopoulos et al. 1997). Street sweepings and leaf litter generation in residential localities of the city form a significant year round component of MSW (Sathishkumar et al. 2001). The potential for distributed biogas plants for collected leaf litter component of OFMSW was studied using a plug flow bioreactor concept (Chanakya and Jagadish 1997; Chanakya and Moletta 2005) as a representative of decentralized operation with simple machinery (Garcia et al. 2005). The biological methane potential (BMP) of these individual feedstocks needs to be determined to indicate their suitability for anaerobic digestion in distributed biogas plants.
Composting is the biological oxidative decomposition of organic constituents in wastes under controlled conditions. It thus requires special conditions, temperature, moisture, aeration, pH and C/N ratio, to enable optimum biological activity in the different stages of the process (Barrington et al. 2002, 2003). The main products of aerobic composting are CO2, H2O, mineral ions and stabilized organic matter, often called humus. The process is accomplished through different phases (Bari and Koenig 2001) i.e. initial phase during which readily degradable components are decomposed. During a thermophilic phase cellulosic materials are oxidatively degraded rapidly by microbes. This is followed by maturation and stabilisation phases. The process may also be discussed in terms of two well-defined phases mineralisation and humification (Sharma et al. 1997). The degradation is followed by intensive microbial activities producing heat. This high temperature is important to kill pathogenic organisms sometimes present even in MSW components. After this, when the assimilable organic fraction is exhausted, most cells undergo decay by auto-oxidation. This transformation occurs in the second phase and leads to the formation more stable humic substances. The humification leads to tri-dimensional polymers also that provide energy for future microbial activities (Sharma et al. 1997). While composting of agro-residues has been in existence for long, composting of municipal (urban solid wastes is of recent origin, more so when attempts are being made to carry this out at micro-scale (up to 10 kg/day). Many locally evolved processes have been optimized over time (Gotaas 1956). A wellknown method is the Bangalore method adapted to ensure >90% nitrogen conservation in N-starved semi arid soils. An important modification has been the frequent turning of compost pile to maintain aerobic conditions. This provided rapid degradation and shortened composting time. None of these early process addressed MSW. de Bertoldi et al. (1983) provides a detailed review of transformation processes of the organic fraction of solid urban waste into compost including processes used when the waste is mixed with sewage sludge. The Vuilafvoer Maatschappij (VAM) process adapted the Indore process to municipal refuse at a large scale (Slater et al. 2001) and has been in operation in Bangalore since 1978 (SANDEC 2002). Zurbrugg et al. (2005) developed a low-cost technique using the “Indonesian windrow technique” for MSW. For micro-composting attempted in this study we try to determine whether there are adequate thermophilic stages, adequate stabilization and physico-chemical changes. Finally we characterize the predominant components of the OFMSW for their suitability to micro-composting or small anaerobic digestion which are considered as ideal decentralized process options.
