Results and discussion

Waste degradation in open dump

A weight loss in OFMSW of 88 and 61% of wet weight and TS, respectively was observed during the degradation pro- cess of 30 days. Figure 1 shows the wet weight lost (WWL), total solids (TS), and moisture content (MC) with time in open dumps. During the first week, the WWL reduced to 27% of the original weight, whereas TS reduced to 56% of

Fig. 1 Fraction of moisture and TS remaining in open degradation

the original level. The OFMSW lost moisture content rapidly during the early stages of degradation, thereafter the rate of weight loss slowed down and soon reached < 10%. The mois- ture content was considered to identify the potential degra- dative agents functioning at various stages of waste decom- position. The OFMSW left in the open dumps dried rapidly to levels < 60% moisture or wetness within 6–8 days of expo- sure under dry ambient conditions (summer, April–June). At this stage of dryness in OFMSW, bacterial decomposition is expected to slow down and rapidly cease or reach very low levels. The presence of surface moisture could allow a small degree of fungal decay. Yet, the moisture content is quite adequate for micro and meso-fauna to ingest and enable degradation, and field data suggest that they are the most likely actors that remove the degradable organic material after this initial stage of drying. The important role of micro and meso-fauna in such a decomposition process is common in savanna regions (e.g. Australian Bush) where for the deg- radation of leaf and plant litter, termites constitute 10–30% of the dry matter transformations (Collins 1981; Braithwaite et al. 1988; Ngatia et al. 2014). In this case, where the food material is far more digestible, one may expect a wider range of faunal activity. This study shows that the moisture con- tent of degrading MSW falls rapidly to levels below which micro-organisms are unlikely to function, leaving the rest of the degradative process to be carried out by meso and macro-fauna of this area.

OFMSW was covered and protected by a steel mesh (to avoid other fauna). It continued to undergo rapid initial decomposition. As clear from Fig. 2, 48% of VS in OFMSW was lost during the initial 6 days and a further 14% of VS was lost during 6–30 days of degradation. This suggests an important role for residual micro-fauna. The loss of ash con- tent is indicative of OFMSW’s solute and mineral content emerging out of the dumped waste; most likely as (a) lea- chate or (b) random feeding by meso and micro-fauna. The lowering of ash levels appeared to be linear as indicated

Fig. 2 Pattern change in VS (a–c) and ash (d) content in open degra- dation

by a visual fit (D) achieving 40% over 30 days period. This level of loss is quite high and indicative of ash or minerals in the OFMSW being solubilized and migrating away from the location of the dump. This suggests greater involvement of other agents that carry away the wastes from the point of dumping (Brussaard 1997). Under normal circumstances, if the loss of TS, VS, and moisture occurred through biodeg- radation, only ash would be left behind, wherein the mass of the remaining ash fraction observed would continue to be reasonably constant in weight throughout the degrada- tion period. However, depleting ash fractions suggests a small level of ash is lost through processes of consumption and transport by micro and meso faunal organisms over a longer time frame as compared to the other constituents. The large scatter in the replicates is suggestive that these occur at random.

The extent of leachate produced and that lost through infiltration could not be determined or computed in this design of experiment which would in turn help in partition- ing the ash lost as minerals in the leachate produced and that flowing out, as well as that picked up by micro and meso organisms and carried away from the dump location. In the case of open dumps, of the total VS lost, about 48% occurs in the first 6 days (at 8%/day) which slows down to < 1%/ day over the next 24 days. The reduction of VS mass is an indicator of the overall OFMSW degradation process. VS decline by 48% in first 6 days (B) with simultaneous loss of ash content indicates the occurrence of the acidogenesis (volatile fatty acid, VFA production) at a rapid rate and con- comitant disintegration of particulate matter in OFMSW to produce leachate, and the leachate, in turn, migrates away from the dump. However, this explains only the first stage of TS/VS/ash losses.

Moisture loss reaches to 60% during the first 6 days after open dumping. The rapid initial weight reduction is therefore primarily due to the rapid loss of moisture content in the early stages of degradation and the organic matter decompo- sition. Similarly, in the initial phase of aerobic degradation

microbial growth depends on moisture content (Makan et al. 2013). Increasing moisture content to an optimum level improved organic matter degradation. Yet, from earlier studies with wet biomass and dumped garbage containing high moisture content, it is evident that the packing density achieved, and the high rate of decomposition is conducive to anaerobic conditions (Chanakya et al. 1999). These obser- vations suggest that in the case of open dumped OFMSW, most of it is converted to VFA rapidly at the early stages of decomposition with a tendency to produce some extent of free water that turns to leachate (Chanakya et al. 2009).

The OFMSW dries rapidly during this initial phase. Upon reaching a minimum moisture threshold (60%) for micro- bial survival, most microbial degradative activity (especially anaerobic bacterial degradation or fungal growth) ceases within such thinly spread ‘open dumps’ in and around Ban- galore. This is expected to occur as the level of wetness for bacterial degradation (about 60–70%) and fungal degrada- tion (40–70% along with high humidity) is likely to be lost within 6 days after dumping and conditions are not condu- cive for microbiologically mediated degradation (Ryckeboer et al. 2003).

Effect of soil contact

Figure 3 indicates that treatments with no soil contact (NSC), partial soil contact (PSC), and soil contact (SC) decompose OFMSW (initial TS = 190 g/kg) by 68, 87, and 61% of initial TS in 30 days respectively. This indicated that the category with partial soil contact had more reduction than the other two categories. This trend suggests the retention of moisture for a longer time allows the microorganisms to carry out the degradation process (Fig. 4). An analysis of variance (ANOVA) for the mass of TS in the degraded waste samples across these three categories indicated that the observed dif- ferences were not significant (F = 0.182, p > 0.05). Estimated VS content also showed a non-significant difference between

Fig. 3 Change in TS with categories of soil contact in open dumps

Fig. 4 Loss of MC with categories of soil contact in open dumps

the three categories (F = 0.077, p > 0.05). While the relation- ship of the rate of drying as a function of contact with soil suggests that the bacterial inoculum carried by the wastes was adequate for the initial level of decomposition.

OFMSW (809.6 g/kg), under complete soil contact, showed a total moisture loss of 762.6 g/kg while NSC and PSC showed a moisture loss of 740.4 and 789.8 g/ kg, respectively. These three treatments were placed in the open, and there was no significant difference in the pattern of moisture lost (F = 0.195, p > 0.05). Across the three treatments, wet weight loss is quite rapid up to the point when the moisture content reaches ~ 60%. In all treat- ments, the highest reduction of VS content is achieved until this point is reached. MC and VS show R2 values of 0.796, 0.653, and 0.874 for treatments with no soil contact, partial soil contact, and complete soil contact, respectively (Fig. 5). This highlights that the duration of rapid decomposition time depends upon the rate of loss in moisture content. Upon achieving a wet weight loss, leads

Fig. 5 Relation between MC and VS with categories of soil contact in open dumps

to a slowdown in the rate of organic waste degradation or weight loss in open OFMSW dumps. The slowing of the decomposition rates when OFMSW reaches < 60% mois- ture is indicative of the predominance of bacterially medi- ated decomposition and to be the largest contributor to TS reduction in open dumps. Figure 6 presents the changes in C, H, and N with NSC, PSC, and SC. Carbon mass had reduced from 81.2 to 22.2 g/kg (NSC), 8 (PSC), and 30.96 (SC). Some degree of soil contact (partial, without too much moisture losses) enables a more complete OFMSW degradation. Nitrogen mass had reduced from 2.71 g/kg to

1.13 (NSC), 0.62 (PSC), and 1.17 (SC). This confirms the reduction of waste quantity and also its uptake by the soil micro-flora through soil contact and partial soil contact.

Fig. 6 C, H and N mass changes in the presence of soil contact in open dumps (a–c)

Role of agents (organisms) in waste degradation

Decomposition and its pattern with the involvement of organisms (micro and meso categories) were assessed by placing differently sized barriers between the soil and OFMSW samples. The rapidly drying waste is not conducive to microbial decomposition for a long period (> 6 days in the open) as seen earlier. Soil microorganisms help in the mineralization of carbon, whereas other fauna often helps in the mechanical breakdown of OFMSW components (Brus- saard 1997; Gougoulias et al. 2014). Soil organisms include microorganisms (< 0.1 mm diameter) and meso + micro faunal (passing through 2 mm pore size of nylon mesh), both of which seem to have roles in OFMSW degrada- tion. The microorganisms with meso-fauna communities (meso + micro) and microorganisms singly decompose 74 and 83% of initial TS in 30 days (Fig. 7). Statistical analysis of TS reduction also indicated that the observed difference was non-significant (F = 2.157, p > 0.05). Esti- mated VS shows a loss of 87 and 84% with the involve- ment of micro and meso + micro categories, respectively. Therefore, the quantity of reduction in both categories is almost the same. Figure 8 compares these two categories for VS reduction and also shows a non-significant differ- ence (F = 2.088, p > 0.05). Similarly, degradation with the inclusion of organisms of all sizes did not have any negative

Fig. 7 Loss in TS of OFMSW restrained by diferent sized meshes

Fig. 8 Loss of MC for categories with the involvement of organisms

effect on the reduction in the quantity of organic waste lost. The reason could be that all the categories have microbial activity, which is important for the first part of degrada- tion. The complexity of organisms brought about by using larger mesh sizes has a significant effect on moisture content lost (F = 16.27, p < 0.05) and shows a strong relationship between residual VS and available MC for different types of organisms.

Waste degradation with the involvement of micro and meso + micro categories has been found to lose 91 and 88% of initial MC respectively (of 806 g/kg found in fresh MSW), over 30 days. MC and VS show R2 values of 0.8911 and 0.9325 for micro, and meso + microorganisms, respec- tively (Fig. 9) clearly showing moisture availability to be the single most governing factor in TS/VS lost during open dumps. From these studies, it is obvious that the degrada- tion of VS is related to the moisture content in a favourable range. Larger pore size loses moisture quicker than covered systems, which in turn provide longer workable moisture regimes and a congenial environment for micro-organisms to become the predominant agents of degradation. Figure 10 represents changes in C, N, and H mass with the involve- ment of different size groups of organisms. Contact with soil micro and meso + microorganism categories reduced carbon mass from 80.1 to 10.3 g and 11.3 g of TS in residual waste, respectively. Nitrogen mass reduced from 2.95 to 0.99 g (by microorganisms) and 0.92 g (by meso + microorganisms) amounting to about two-third N losses. This level of N loss is a little higher than N volatilization found in open com- posted systems. The hydrogen mass reduction pattern was similar to that of carbon.

Comparison of pattern, rate kinetics, and carbon content

The decomposition of mixed OFMSW followed a general pattern of exponential decay with R2 = 0.72, a pattern

Fig. 9 Residual VS and MC relation for categories with the involve- ment of organisms

Fig. 10 C, H and N mass changes in the relation to various sizes of organisms (a–c)

typically found for organic matter decomposition in soil and anaerobic digesters (Jenkinson and Ayanaba 1977; Jenkinson 1981; Lopes et al. 2004; Chanakya et al. 2007). However, most deviations from typical exponen- tial fit occurred at the initial point and the intermediate switch over points (Fig. 2, plot b) due to the initial rapid decomposition. It thus needs some modifications to this approach. The first six days VS data (rapid degradation phase) shows a straight line fit with an R2 = 0.955 (plot a, b). The daily degradation levels were VS reduced from 14 to 8 g/kg/day after 6 days of degradation and it fur- ther reduced to approximately 4 g/kg/day. A rapid initial decomposition is due to the availability of moisture and also due to the presence of a fraction of easily decompos- able substances in waste. Whereas after this period decom- position slows down both due to the absence of adequate

moisture and the remaining material in OFMSW is slowly decomposable and recalcitrant.

The decomposition of waste thus occurs in two phases involving a change in the pattern of kinetics (Reddy et al. 1980; Ajwa and Tabatabai 1994; Harmon et al. 2009). Phase I occurs till the 6th day of degradation and after a short tran- sition, it is followed by phase II. Phase I is linear and follows zero-order kinetics, as in this phase the rate of degradation appears independent of waste composition. Around the end of Phase I, the reduction in the moisture content of the waste altered the rate as falling moisture leads to lower involve- ment of micro and meso categories of organisms. Thus, Phase II is exponential and follows first-order kinetics since the rate of degradation appears to depend on concentration. The rate constant of phase I and phase II are 16.6 g/kg/day and 0.024/day, respectively. The process of decomposition found in composting, biomethanation and landfill is also reported to follow first-order kinetics from the initial stage till the degradation of organic matter (Hamoda et al. 1998; Chanakya et al. 1999; Abu Qdais and Asheraideh 2008; Ravikumar 2014; Abu Qdais and Al-Widyan 2016). The rate constant of phase I is highest (24.886 g/kg/day) in degrada- tion by micro category, whereas the rate constant of phase II is highest (0.075/day) in the degradation of meso + micro category. Micro and meso categories involve microorgan- isms in degradation, but MC content varies. At the end of phase I, MC content in micro and meso categories were 56 and 49% of initial MC (806.29 g/kg), whereas, at the end of phase II, MC content in micro and meso categories were 9 and 12% of initial MC (Table 2). The decomposition of carbon varies across phase I and phase II for different cat- egories. A rapid reduction of C occurred in phase I as com- pared to phase II due to available MC leading to favorable microbial decomposition. This confirms and supports the rapid decomposition discussed earlier as well as provides the potential cause for change in the order of kinetics (residual

VS). Phase I rate constants are marginally higher in micro (24.886 g/kg/day) compared to micro + meso (22.757 g/kg/ day) organisms. In the case of micro, mainly bacteria and fungi are involved in degradation. The availability of mois- ture at the end of phase I was 56% and the longer period of availability of moisture stretched the duration of phase I degradation. However, in meso + micro category mois- ture availability reduced earlier to 49%. Phase II follows first-order kinetics for both the categories. The estimated rate constants for micro and meso + micro categories are all within the same range. The rate and extent of waste degrada- tion are related to the loss of moisture or the rate of drying.