Materials and methods

Composition of mixed MSW

The composition of waste from domestic sources in and around Bangalore, capital city of Karnataka, India, is pre- dominantly of easily degradable components such as fruit waste, vegetable waste, and food waste (Chanakya and Swamy 2011). For the degradation pattern analysis, the mixed MSW was considered to consist of food waste (10%), vegetable waste (38%), and fruit waste (52%) comparable to the segregated composition of OFMSW based on the previous experimental trials. The quantity of each of the components of the OFMSW sample is presented in Table 1. These biomass feedstocks were collected within and around the Indian Institute of Science (IISc) campus in Bangalore, India. Campus boundary is enclosed within 13.01°–13.02° latitude and 77.55°–77.57° longitude. Individual compo- nents OFMSW were collected fresh (within 6–8 h of gen- eration) to synthesize the primary waste sample.

Open degradation of OFMSW with soil contact

OFMSW was shredded to 10 mm fragments and mixed in the above proportion from which replicates of 500 g each (fresh weight basis) were made. The waste quantity was selected based on the trial run. These samples were spread on a base layer of nylon mesh occupying a ground area of 35 × 35 cm of different pore sizes namely 0.1 and 2 mm (to maintain contact with soil and the organisms) as well as on

an impervious low-density polyethylene (LDPE) sheet. This facilitated recovery and analysis of decomposing samples at different periods of decomposition. These experiments were carried out in the field to take advantage of the pre- vailing environmental conditions resembling open dumps. This experimental area was secured to prevent the mixing of samples due to the interference of stray animals. A total of 74 samples (3 treatments, 12 time intervals, 2 replicates with 2 samples representing day 0) were used to record extent of dry weight loss (e.g. 0, 1, 2, 3, 4, 6, 8, 10, 12, 15, 18, 22 and 30 days; time intervals chosen based on a trial run). The three treatments were, (a) open to the sky but with no contact to the soil below by laying out the OFMSW on impervious LDPE sheets, (b) open to the sky with OFMSW placed on a 0.1 mm mesh allowing access to soil micro-organisms but not meso-fauna emerging from soil and (c) laid out on soil separated by a 2 mm mesh permitting both soil contact for micro-organisms as well as meso-fauna. This layout aided in understanding the role of soil as a source of microbial, micro, and meso faunal inoculum that is necessary for the degradation process (Swift et al. 1979; Bradford et al. 2002; Paris et al. 2008). Residual waste samples in the three treat- ments were recovered at the end of each chosen period of decomposition, their wet and dry weights were estimated. From this, loss in weight over the intervening period was determined. It indicated the role of soil contact with/without access to fauna in hastening and improving degradation of OFMSW placed in open dumps.

Role of agents in waste degradation under only ‘soil contact’ conditions

An attempt is made to identify the agents involved in OFMSW degradation emerging only from the ground or soil contact in this experiment. The waste composition, number of samples, and monitoring intervals were based on the previous experimental trials. A total of 50 waste samples (2 treatments, 12 time intervals, 2 replicates with 2 samples representing day 0) were deployed to represent various decomposition periods. The two treatments pro- vided insights on the sizes of organisms involved in the degradation. These were classified as micro (< 0.1 mm) and meso + micro (2 mm). The micro involves the organ- isms < 0.1 mm of body size and meso + micro involve all the organisms of the body size < 2 mm. Sealed mesh bags of the two chosen mesh sizes aided in allowing the organisms to interact with waste and assimilate it (Bradford et al. 2002; Paris et al. 2008). This grouping of micro and meso + micro is based on the body sizes of organisms that are typically involved in the decomposition of organic matter in the food web (Swift et al. 1979). A sample of 500 g OFMSW was placed in nylon mesh pouches (mesh size 0.1 and 2 mm) and tied with a nylon thread. These nylon pouches were

covered from the top with a strong steel mesh of pore size 5 mm to prevent interference by rodents and birds. This method helped in identifying the role of various soil-borne organisms in the degradation process while excluding larger organisms.

Monitoring of waste degradation and analysis

Residual organic waste samples collected on the respective sampling days, were weighed and blended to the homo- geneous sample. Homogenized samples were then oven- dried at 105 °C to determine the total solids (TS) and later oven-dried samples were powdered fine using a laboratory grinder. APHA (American Public Health Association) (2005) methods were used for analysis. Duplicate samples of 2 g each were kept at 550 °C for 2 h to determine volatile solids (VS) and ash content. VS loss was expressed both in mass terms as g VS and as the percentage of initial VS lost at each time interval. A 1 g dry sample of degraded waste was used to analyze the carbon, hydrogen, and nitrogen using CHN analyzer (LECO elemental analyzer).

Analysis of degradation pattern and rate kinetics

VS (g) of residual material remaining in open dumps was plotted against time to obtain insights of the underlying deg- radation process. The best fits (R2) were determined. All sta- tistical analyses were done using PAST statistical software. The VS data were analyzed to explain the rate of degradation of organic waste. The overall decomposition cannot normally be described by simple equations, as different fractions of organic wastes do not necessarily degrade at the same rate. Two broad classes have been generally accepted to represent the degradation process of individual components based on the decomposition rates namely, the ‘rapidly degradable’ with a higher rate constant and the ‘slowly degradable’ with a low decay constant (Chanakya et al. 1999). Thus, in this study, rate kinetics was determined considering the pattern of organic waste decomposition in two phases depending on the relative content of rapid and slow to degrade components in the waste (Reddy et al. 1980; Ajwa and Tabatabai 1994).