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1. Process Optimization and Biomethane Recovery from Anaerobic Digestion of Agro-Industry Wastes

   https://doi.org/10.3390/en16186484  

   Kaur, Harjinder; Kommalapati, Raghava R (September 2023, Energies)

   Among the sustainable initiatives for renewable energy technologies, anaerobic digestion (AD) is a potential contender to replace fossil fuels. The anaerobic co-digestions of goat manure (GM) with sorghum (SG), cotton gin trash (CGT), and food waste (FW) having different mixing ratios, volumes, temperatures, and additives were optimized in single and two-stage bioreactors. The biochemical methane potential assays (having different mixing ratios of double and triple substrates) were run in 250 mL serum bottles in triplicates. The best-yielding ratio was up-scaled to fabricated 2 L bioreactors. The biodegradability, biomethane recovery, and process efficacy are discussed. The co-digestion of GM with SG in a 70:30 ratio yielded the highest biomethane of 239.3 ± 15.6 mL/gvs, and it was further up-scaled to a two-stage temperature-phased process supplemented with an anaerobic medium and fly ash (FA) in fabricated 2 L bioreactors. This system yielded the highest biomethane of 266.0 mL/gvs, having an anaerobic biodegradability of 67.3% in 70:30 GM:SG co-digestion supplemented with an anaerobic medium. The BMP of the FA-amended treatment may be lower because of its high Ca concentration of 205.74 ± 3.6. The liquid fraction of the effluents can be applied as N and P fertigation. The Ca concentration was found to be 24.3, 25.1, and 6.3 g/kg in GM and GM:SG (TS) and SG solid fractions, respectively, whereas K was found to be 26.6, 10.8, and 7.4 g/kg. The carbon to nitrogen ratio of solid fraction varied between 2.0 and 24.8 for return to the soils to enhance its quality. This study involving feedstock acquisition, characterization, and their anaerobic digestion optimization provides comprehensive information and may assist small farmers operating on-farm anaerobic digesters. 

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2. Optimizing anaerobic co-digestion of goat manure and cotton gin trash using biochemical methane potential (BMP) test and mathematical modeling

   https://doi.org/10.1007/s42452-021-04706-1  

   Kaur, Harjinder; Kommalapati, Raghava R. (August 2021, SN Applied Sciences)

   null (Ed.)

   Abstract Anaerobic co-digestion is widely adopted to enhance process efficacy by balancing the C/N ratio of the feedstock while converting organic wastes to biomethane. Goat manure (GM) and cotton gin trash (CGT) were anaerobically co-digested in triplicate batch bioreactors. The process was optimized and evaluated utilizing mathematical equations. The liquid fraction of the digestate was analyzed for nitrate and phosphate. The co-digestions with 10 and 20% CGT having the C/N ratios of 17.7 and 19.8 yielded the highest and statistically similar 261.4 ± 4.8 and 262.6 ± 4.2 mL/g vs biomethane, respectively. The biodegradability (BD) of GM and CGT was 94.5 ± 2.7 and 37.6 ± 0.8%, respectively. The BD decreased proportionally with an increase in CGT percentage. The co-digestion having 10% CGT yielded 80–90% of biomethane in 26–39 d. The modified Gompertz model-predicted and experimental biomethane values were similar. The highest synergistic effect index of 15.6 ± 4.7% was observed in GM/CGT; 30:70 co-digestion. The concentration of nitrate and phosphate was lower in the liquid fraction of digestate than the feedstocks, indicating that these nutrients stay in the solid fraction. The results provide important insights in agro-waste management, further studies determining the effects of effluent application on plants need to be conducted. 

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3. Biochemical Methane Potential and Kinetic Parameters of Goat Manure at Various Inoculum to Substrate Ratios

   https://doi.org/10.3390/su132212806  

   Kaur, Harjinder; Kommalapati, Raghava R (November 2021, Sustainability)

   Anaerobic digestion is a proven technology for managing manure while harvesting natural gas and digestate as a biofertilizer. The biochemical methane potential (BMP), biodegradability, and kinetic parameters of goat manure (GM) were investigated at different inoculum to substrate ratios (ISRs). The cumulative biomethane yields at the ISRs of 0.0, 0.3, 0.5, 0.8, 1.1, 1.3, and 2.6 were 191.7, 214.3, 214.9, 225.9, 222.1, 222.8, and 229.9 mL gvs−1, respectively. The biomethane yield at all ISRs was significantly higher than control (0 ISR). Above the ISR of 0.0, the biomethane yield was similar among all ISRs. The biodegradability of GM at the ISRs of 0.3, 0.5, 0.8, 1.1, 1.3, and 2.6 varied between 73.3% and 78.7% and was statistically similar. In total, 90% of the yield was observed in 31 and 32 days in control and all other ISRs, respectively. The modified Gompertz equation fitted very well (R2 = 0.99) to the BMP of GM but predicted the lag phase (λ) of 3.2–5.2 days against observed 8–10 days among control and other ISRs. 

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4. Solid-State Anaerobic Digestion of Organic Solid Poultry Waste for Biomethane Production

   https://doi.org/10.3390/bioengineering12070712  

   Fatima, Faryal; Kommalapati, Raghava R (July 2025, Bioengineering)

   This study examines biodegradability (BD) and optimum conditions for the solid-state anaerobic digestion (SS-AD) of organic solid poultry waste (organs, intestines, offal, and unprocessed meat) to maximize biomethane production. Three main parameters, substrate-to-inoculum (S/I) ratio, pH, and temperature, were evaluated for the SS-AD of organic solid poultry waste. pH was evaluated at non-adjusted pH, initially adjusted pH, and controlled pH conditions at a constant S/I ratio of 0.5 and temperature of 35 ± 1 °C. The S/I ratios were examined at (0.3, 0.5, 1, and 2) at a controlled pH of ≈7.9 and temperature of 35 ± 1 °C. The temperature was assessed at mesophilic (35 ± 1 °C) and thermophilic (55 ± 1 °C) conditions with a constant S/I ratio of 0.5 and controlled pH of ≈7.9. The results demonstrate that the highest biomethane production and BD were achieved with a controlled pH of ≈7.9 (689 ± 10 mg/L, 97.5 ± 1.4%). The initially adjusted pH (688 ± 14 mg/L, 97.3 ± 1.9%) and an S/I ratio of 0.3 (685 ± 8 mg/L, 96.8 ± 1.2%) had approximately equivalent outcomes. The thermophilic conditions yielded 78% lower biomethane yield than mesophilic conditions. The challenge of lower biomethane yield under thermophilic conditions will be resolved in future studies by determining the rate-limiting step. These observations highlight that SS-AD is a promising technology for biomethane production from solid organic poultry waste. 

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5. Batch Anaerobic Co-Digestion and Biochemical Methane Potential Analysis of Goat Manure and Food Waste

   https://doi.org/10.3390/en14071952  

   Orangun, Ayobami; Kaur, Harjinder; Kommalapati, Raghava R. (April 2021, Energies)

   null (Ed.)

   The improper management of goat manure from concentrated goat feeding operations and food waste leads to the emission of greenhouse gasses and water pollution in the US. The wastes were collected from the International Goat Research Center and a dining facility at Prairie View A&M University. The biochemical methane potential of these two substrates in mono and co-digestion at varied proportions was determined in triplicates and processes were evaluated using two nonlinear regression models. The experiments were conducted at 36 ± 1 °C with an inoculum to substrate ratio of 2.0. The biomethane was measured by water displacement method (pH 10:30), absorbing carbon dioxide. The cumulative yields in goat manure and food waste mono-digestions were 169.7 and 206.0 mL/gVS, respectively. Among co-digestion, 60% goat manure achieved the highest biomethane yields of 380.5 mL/gVS. The biodegradabilities of 33.5 and 65.7% were observed in goat manure and food waste mono-digestions, while 97.4% were observed in the co-digestion having 60% goat manure. The modified Gompertz model is an excellent fit in simulating the anaerobic digestion of food waste and goat manure substrates. These findings provide useful insights into the co-digestion of these substrates. 

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