Bio-process performance, evaluation of enzyme and non-enzyme mediated composting of vegetable market complex waste

India generates about 157,478 tons of municipal solid waste daily, with less than 20% treated and the remainder landfilled, producing methane and carbon dioxide emissions. Large municipal landfills in Chennai (Kodungaiyur and Perungudi) exemplify the scale and impacts. Aerobic composting is a promising biological method that conditions soil and provides nutrients while reducing greenhouse gas emissions and pathogens. Previous work has highlighted the importance of moisture, temperature, C/N ratio, pH, and aeration for compost stability and maturity, and shown that reactors, microbial inocula, and invertebrates can enhance composting performance. Traditional aerobic/anaerobic composting often requires months; recent aerobic systems have reduced the time to 4–5 weeks. Seed inocula have been reported to increase microbial populations, enzyme production, and reduce odors. However, enzyme activation and bio-kinetic modeling in aerobic composting of VMCW are less explored. This study formulates a pre-cultured seed inoculum and applies it in a compact, aerated, mixed in-vessel composter to accelerate VMCW degradation, targeting a 7–9 day process and analyzing biokinetics.

Prior studies have established key process controls in composting, including moisture films around particles (hygroscopic water) that govern microbial activity, optimal temperature ranges (20–60 °C), and the benefits of forced aeration to control temperature, moisture, and oxygen. Anaerobic and aerobic composting can take several months; advanced aerobic systems reduced this to 4–5 weeks. Additives such as jaggery and curd have been reported to enhance microbial enzymatic activity and accelerate compost maturity. Research has also examined reactor designs, microbial community dynamics, pathogen inactivation, and correlations between volatile solids reduction, energy content, and compost stability. Despite these advances, detailed enzyme activation and kinetic modeling (e.g., Michaelis–Menten-type analyses) for accelerated VMCW composting remain limited, motivating the present work.

Feedstock: VMCW (tomato, carrot, cabbage, radish, ridge gourd, bottle gourd, brinjal, chayote, beetroot) was collected from Tambaram Municipality (Chennai, India) using stratified random sampling to obtain a representative composite sample. Initial composition at source: organics 72%, plastics 18%, paper 4%, others 6%; collected sample: moisture 80.22%, total solids (TS) 19.78–22%, volatile solids (VS) 82.11% of TS, neutral pH ~6.8. Size reduction: Two-stage mechanical shredding to approximately 1.5–2 cc, then 0.5–0.6 cc pieces. Reactor: A custom SS316 in-vessel aerobic composter (45×30×60 cm; L×B×H) with manual stirring lever and continuous aeration (air pump 230 V TID-75-P). Air distributed via a nozzle and perforated diffuser for uniform aeration. Seed inoculum (enzyme) preparation: Pre-cultured seed inoculum prepared by mixing 200 g jaggery and 10 g curd in 1 L water, incubated 5 days at room temperature (static). Rice husk was used as a bulking agent to increase surface area; it was sieved out at the end and could serve as seed compost. Operation: 12 kg (wet) shredded VMCW mixed with inoculum and rice husk at a 16:1 ratio (w/w) and loaded into the reactor. Daily sampling at different depths prior to turning for monitoring pH, TS, VS, volume, and temperature. Aeration and manual mixing maintained. Analytical methods: TS (APHA 2540B), VS (APHA 2540G), fixed solids (APHA 2540E). Moisture by oven-drying at 105 °C overnight. Organic matter from ash after ignition at 550 °C for 2 h. pH measured in 1:10 w/v solid-to-water slurry immediately after sampling. Temperature measured manually at various depths. Elemental CHNS (Euro Vector 3000; ASTM D-5291) on dried samples; oxygen by difference (VS minus C+H+N+S). Calorific energy value (CEV) estimated using Channiwala correlation: CEV (MJ/kg) = 0.3491C + 1.1783H − 0.1034O − 0.0151N − 0.0211A, where A is ash (% dry). XRD (Bruker D8, Cu Kα) on samples dried at 50 °C for 24 h and powdered to assess structural changes. Microbiology: Plate counts on nutrient agar from serial dilutions (10^-2 to 10^-8) of 1 g wet compost taken every 24 h; CFU normalized per g dry TS after 24–48 h incubation. Kinetics: Substrate concentration (S; organic matter) and consumption rate (r) tracked over time. Reaction rates determined from tangents to S vs time curves. Lineweaver–Burk plots (1/r vs 1/S) used to estimate Michaelis–Menten constant (Km) and limiting velocity constant from linear regression.

• Composting time reduction: Pre-cultured jaggery–curd inoculum reduced typical composting period from ~45 days to 9 days.
• Volume and mass/organics reduction: Volume decreased from 0.012 m³ (day 0) to 0.003 m³ (day 9). VS decreased from 82.11±2.83% to 40.82±0.62% of TS (about 42 percentage points), indicating substantial organic degradation. Overall volume reduction reached ~77% by day 9.
• Moisture and temperature: Moisture reduced from 80.22±0.35% to 18.37±0.29% by day 9, with a rapid 64% moisture reduction observed by day 3. Temperatures reached thermophilic range (~54±9.27 °C by day 3) at multiple depths, then declined towards the end as the process matured.
• pH dynamics: Initial pH 6.8±0.08; decreased to ~6.23 on day 2, then increased to ~7.8 by day 9, consistent with acidogenesis followed by ammonification and maturation.
• C/N ratio: Varied with process phase, peaking at 17.39 (day 5) and dropping to 2.35 by day 9, reflecting rapid carbon mineralization and relative nitrogen enrichment. Elemental composition changed from C 53.72% TS, N 3.88% TS to C 24.35% TS, N 10.94% TS by day 9.
• Nutrient enrichment (NPK): Mature compost contained N 0.918% w/w, P 0.5% w/w, K 1.029% w/w, comparable to or exceeding values reported for longer composting periods.
• Energy content: Final CEV ~19,450 kJ/kg dry matter; strong correlation between VS and CEV (R²=0.913).
• Correlations and performance metrics: High correlation between VS reduction and volume reduction (R²=0.862). Estimated energy loss during conversion ~5600 kJ/kg TS. Organic carbon decreased by ~29.4% (consistent with VS reduction of ~40.6%).
• Microbial activity: Maximum microbial count reached 3.47×10^6 CFU/g (dry basis) on day 4; a short lag phase (day 2) followed by rapid exponential growth aligned with thermophilic conditions and process acceleration.
• Kinetics: Michaelis–Menten constant Km ≈ 81.06; limiting velocity reaction rate constant ≈ 0.15 (as reported). Peak reaction rate r ≈ 26.71%/day on day 8, indicating high degradation potential.
• Structural changes (XRD): Diffraction peaks decreased in intensity/number from day 0 to day 9, indicating transformation from crystalline to amorphous phases and mineralization of organic matter, consistent with compost maturation.
• Process controls: Bulking agent (rice husk) alleviated initial high moisture/oxygen limitation, improving aeration and surface area, which enhanced microbial activity and drying.

The study demonstrates that a pre-cultured seed inoculum based on jaggery and curd, combined with effective aeration, mixing, and size reduction, can markedly accelerate aerobic composting of VMCW. The inoculum likely boosted initial microbial populations and enzyme activities, shortening the lag phase and hastening the transition to thermophilic conditions. Temperature and moisture trajectories supported rapid biodegradation: by day 3, thermophilic temperatures coincided with a sharp drop in moisture and strong VS reduction trends. pH dynamics followed expected patterns—initial acidification due to hydrolysis and acidogenesis, followed by a rise linked to protein degradation and ammonia formation, supporting microbial activity in a moderately alkaline environment. The strong correlations between VS reduction and volume loss, and between VS and energy content, indicate effective stabilization and mineralization of organics. Elemental shifts (decreased C, increased relative N) and the low final C/N ratio align with advanced maturity, echoed by XRD evidence of transformation from crystalline to amorphous structures. Nutrient levels (NPK) obtained in only 9 days are comparable to literature values typically achieved over much longer durations, underscoring the efficacy of the inoculum-enhanced process. Kinetic analysis yielded a relatively high Km and notable peak reaction rate, consistent with rapid substrate turnover during the later stages, supporting the conclusion that enzyme-mediated inoculation increases degradation rates under aerobic conditions.

A compact, aerated in-vessel system coupled with a pre-cultured jaggery–curd seed inoculum reduced the composting period for VMCW from ~45 days to 9 days while achieving substantial organic (VS) and volume reductions, thermophilic operation, favorable pH evolution, and nutrient-rich mature compost (NPK ~0.918/0.5/1.029% w/w). Elemental and energy analyses, strong correlations among process indicators, and XRD signatures corroborate maturity and stabilization. Kinetic modeling (Km ~81.06; limiting velocity constant ~0.15; peak r ~26.71%/day) indicates high degradation potential under enzyme-enhanced conditions. Future work could evaluate scale-up and long-term field performance, compare directly with non-inoculated controls, assess pathogen inactivation comprehensively, and optimize aeration and bulking strategies for different VMCW compositions.

• The study used a lab-scale/prototype in-vessel reactor and a relatively small batch (12 kg), which may limit generalizability to full-scale operations.
• Although the title references enzyme and non-enzyme mediated composting, detailed comparative performance data for a non-inoculated control were not presented in the results.
• Pathogen indicators (e.g., coliforms) were identified as important quality parameters, but corresponding data are not reported.
• The process was monitored over a 9-day period; longer-term stability, curing needs, and field agronomic performance were not assessed.
• Kinetic constant nomenclature/units for the limiting velocity parameter are not fully specified, which may affect interpretation.