Anaerobic Degradation Pattern of Urban Solid Waste Components

RESULTS AND DISCUSSION

Composition of waste substrate:

The TS content ranged from 14 to 29% except for onion peels (>90%) and watermelon (<10%; Table 2). Onion, hyacinth beans, peas, sweet lime and orange have a high concentration of VS (>90%; Fig 1), so these can be good substrates for biogas production. Cauliflower had the maximum content of ash indicating the presence of inorganic constituents which does not lead to gas production (Figure 1).

Biological methane production assay:

In this study BMP assay has been defined as a measure of substrate decomposability under typical anaerobic biomethanation conditions and also to estimate the gas production from different category of waste substrate under near ideal conditions of TS, inoculum and physicochemical conditions.

Figure 1: VS and Ash constituents of waste substrate

Table 3: First order rate constants for decomposition of waste substrate

The result of BMP assay at a 0.5% concentration of feedstock is presented in Fig 2. The biogas production ranged from 205 to 616 ml/g of TS. Orange and Sweet lime showed low gas production levels. Citrus fruit peels have a tendency for rapid volatile fatty acid (VFA) formation which interrupts methanogenesis. Pea shells, flat bean (Vicia sp), cauliflower, onion and radish showed a high gas production whereas banana peel showed a moderate gas production level (≈465 ml/g TS). Mixed fruit, mixed vegetable and mixed fruit+vegetable had moderate gas production of 404, 407 and 446 ml/g TS, respectively

Figure 2: Biological methane potential (BMP) measured during various stages of BMP assay for three replicates (A,
B, C) at 0.5% TS concentration of the 13 feedstocks (5 vegetables, 5 fruits, 1 mixed fruit, 1 mixed veg and 1 mixed
fruits and veg) studied

The gas composition measurements showed the presence of >50% of methane from most of the waste substrates. This indicates that the rate of acidogenesis and methanogenesis are well balanced. Cauliflower, peas, watermelon and flat beans showed a good methane content (>60%) and it is an indication of absence of VFA induced suppression of methanogenesis. Whereas onion, orange and sweet lime showed poor methane yield of 11%, 22% and 20%, respectively, due to accumulation of VFA and subsequent methanogenic inhibition.

Degradation pattern and rate kinetics:

The biogas evolution data revealed that anaerobic decomposition of different organic wastes occurred in two to three phases. The results obtained are shown in Figure 2. Among the feedstocks only banana peel and pea shells showed two phase degradation whereas other feedstocks showed three phase decomposition.
The extent of organic carbon evolved in phase 1 is expected to emerge from the easily decomposable fractions of waste - pectin, starches, etc., whereas phase 2 represents breakdown of the slowly decomposing and/or resistant fractions. Stationary phase is considered as phase 3. Among the ten feedstocks, banana peel, cauliflower leaves and citrus peels had similar kinds of decomposition patterns, involving a very rapid initial decay. In most cases the phases 1 and 2 representing decomposition of pectins and structural materials were merged as phase-I and stationary phase (phase 3) is considered as phase-II. Usually, phase-II occurred between 20 to 30 days of incubation. Flat bean and banana show a single degradation over a 20 to 25 days period. Decomposition of only sweet lime and orange waste occurred with a second phase after 8-12 days of incubation.
BMP of waste degradation was assumed to follow the first-order rate of decay, as solid waste degradation in composting and landfill also follows first order kinetics (Hamoda et al., 1998; Qdais et al., 2008). The first-order rate constant was determined using gas yield on either 1 st day for different categories, whereas 3rd day is considered for sweet lime and watermelon as gas production has started from 3rd day of incubation. For all kinetic constants, the linear correlation of data was assessed by the coefficient of determination (R2). Decomposition rate constants k1 and k2 corresponding to phase-I and II were determined for various waste substrate (Table 3). Among the ten feedstock, flat bean showed a most rapid initial decay with k1 and k2 value of 0.419 and 0.009 day-1.respectively, whereas onion peels showed a very slow initial decay with k1 value of 0.127 day-1, probably due to high sulphur content (H2S). Thus kinetics constants can be used to know the time required to oxidize the readily and slowly degradable fraction of organic waste substrates.


Abstract	Introduction	Materials and methods	Results and Discussion	Conclusion	References	PDF