The research team, led by Dayanand Kumbar of the College of Technology and Engineering at MPUAT Udaipur together with P. Shaji James of Kelappaji College of Agricultural Engineering and Technology, Kerala, set out to address a stubborn technical problem. Conventional biogas digesters are poorly suited to high-volume, low-strength liquid wastes like coconut water. These traditional systems demand hydraulic retention times, the length of time liquid spends inside the reactor, of roughly 35 to 55 days. That means enormous digester volumes, high installation costs, and large footprints, all of which push agro-industries toward aerobic treatment systems that consume energy rather than produce it. High-rate anaerobic reactors, by contrast, retain dense microbial populations inside the vessel, allowing retention times to shrink to days or even hours.

The UAHBR tested in this study is a hybrid design that combines two biomass-retention strategies. Wastewater enters at the bottom and flows upward through the reactor. In the lower zone, dense microbial sludge accumulates naturally, while the upper portion is packed with an inert support medium on which biofilms of methane-producing microorganisms can attach and grow. Crucially, the researchers chose a medium that is both cheap and locally abundant: broken coconut shells. The team characterized the shells carefully, measuring a bulk density of 410.95 kilograms per cubic meter, a porosity of 67.56 percent, and a specific surface area of 113.60 square meters per cubic meter, comfortably above the recommended threshold of 100 square meters per cubic meter for biofilm support. The rough, porous surface of the shell fragments encourages microbial attachment, turning an agricultural waste product into a functional component of the energy system itself.

The experimental reactor, fabricated from PVC pipe with an internal diameter of 305 millimeters and a height of 2030 millimeters, held 130 liters of liquid and was installed at the Nila Edible Oil Mill in Kuttippuram. It was seeded with sludge from the existing full-scale UAHBR at Pattambi, filled entirely with effluent, and recirculated for ten days before daily feeding began on the eleventh day. This inoculation strategy proved important: the authors recommend that new systems always start with biomass from a reactor already treating the same or similar effluent, since the microbial community is pre-adapted to the substrate. Once a pseudo-steady state was reached, the researchers systematically shortened the hydraulic retention time from 15 days down to 12, 10, 8, and finally 6 days, tracking how gas production, solids removal, and pH responded to each change.

The results reveal a clear trade-off between how much gas a reactor produces per liter of feed and how much it produces per unit of reactor volume. At the longest retention time of 15 days, the experimental reactor achieved its highest specific biogas production, 225.73 liters per kilogram of total solids added, and a productivity of 8.7 liters of biogas per liter of wastewater treated. Total solids reduction exceeded 80 percent, and biochemical oxygen demand removal reached 84.54 percent at the 12-day retention time, outperforming figures reported for distillery spent wash in earlier studies. As retention times shortened, daily and volumetric gas output rose, peaking at 114 liters per day and 877 liters per cubic meter of reactor volume at the 6-day retention time, but the efficiency of conversion per kilogram of solids declined steadily.

The reason for that decline lies in the chemistry of anaerobic digestion itself. The process unfolds in stages: hydrolysis breaks complex organic molecules apart, acidogenesis converts them into volatile fatty acids, and methanogenic archaea consume those acids to produce methane and carbon dioxide. Coconut water enters the reactor with a pH between roughly 3.2 and 4.9, highly acidic conditions that methanogens tolerate poorly. At longer retention times, the microbial community has time to buffer the incoming acidity, and the effluent pH stabilizes near neutral, between 6.5 and 6.75. When the retention time dropped to 8 days, effluent pH began slipping below 6.3, and at 6 days it fell as low as 5.2, signaling acid accumulation and inhibition of the methane producers. Total solids reduction collapsed from about 80 percent to the 40 to 45 percent range, and biochemical oxygen demand removal fell to just 32 percent. The authors note that beyond a 10-day retention time, performance parameters decline sharply, with the transition from 10 to 6 days showing an exponential rather than linear deterioration.

Perhaps the most striking result came from the full-scale reactor itself. The concrete UAHBR at the Edible Oil Mill in Pattambi, with a total volume of 1.15 cubic meters and the upper 60 percent of its volume packed with coconut shell media, had been operating for more than two years, accumulating a rich biomass inventory. When the researchers standardized its operation, first feeding 60 liters of coconut water daily for a 16.67-day retention time and then reducing to 15 days, the mature system outperformed the experimental unit. At the 15-day retention time it achieved a specific biogas production of up to 354.31 liters per kilogram of total solids added and a productivity of 13.50 liters per liter, with total solids reduction between 79.35 and 81.40 percent. Daily biogas production climbed to roughly 850 liters by the fourth week, all while the influent pH remained below 4, demonstrating that a well-established reactor can neutralize highly acidic feed without chemical pretreatment.

Statistical analysis reinforced the picture. Pearson correlation tests showed strong positive relationships between biogas production and time at the 12-day and 15-day retention times, with correlation coefficients of 0.994 and 0.887 respectively at the 0.01 significance level, while shorter retention times showed negative or no correlation. The researchers also translated the results into practical energy terms. A typical coconut oil mill discharges about 200 liters of waste coconut water per day, which could yield approximately 60 megajoules of energy as biogas containing 60 to 65 percent methane. If burned for thermal applications at 60 percent efficiency, that biogas could replace about 12 kilograms of firewood daily, easing pressure on forests and cutting greenhouse gas emissions. A cost analysis of the system estimated a total installation cost of 310,000 rupees and a payback period of 3.8 years, driven by income from gas, sludge, and treated water.

The study also offers operational guidance for anyone hoping to replicate the system. Start-up should use inoculum from an existing reactor treating similar effluent, begin at a 15-day retention time, and monitor pH closely, neutralizing feed if it drops below pH 4. Loading rates should be changed gradually, since sudden shifts can destabilize the microbial balance. If the goal is to extract maximum energy from every liter of coconut water, retention times of 12 to 15 days are preferable; if maximizing gas output from a smaller reactor is the priority, shorter times around 8 days can be adopted despite lower conversion efficiency. The authors acknowledge remaining challenges, including volatile fatty acid accumulation, long lag phases, and clogging from high solids content, and point to co-digestion, pretreatment technologies, advanced reactor designs, and integrated biorefineries as avenues for improvement, with the nutrient-rich digestate offering potential as a soil fertilizer.

What makes this work notable is its scale and realism. Most anaerobic digestion studies of novel wastewaters remain confined to laboratory beakers, where conditions can be controlled in ways real factories never allow. Here, the reactor ran on genuine mill effluent whose quantity fluctuated with market-driven oil production, and the full-scale system, given time to mature, proved more productive than its carefully managed experimental counterpart. For the millions of farming families across Kerala, Tamil Nadu, Karnataka, and Andhra Pradesh who depend on the coconut palm, and for the kopra and oil mills that process its harvest, the message is that the wastewater they currently discard is not waste at all. It is a feedstock, and with the right biology and the right reactor, it can close the loop between agro-industrial pollution and renewable energy in a genuinely circular economy.

Subject of Research: Biogas production from waste coconut water using a full-scale upflow anaerobic hybrid bioreactor

Article Title: Investigation on biogas production from coconut water using a full-scale upflow anaerobic hybrid bioreactor

Article References: Kumbar, D., James, P. S., Kumar, V. K., Hallad, S. C., & Ramappa, D. (2026). Investigation on biogas production from coconut water using a full-scale upflow anaerobic hybrid bioreactor. Discover Green Chemistry, 1(1), Article 2. https://doi.org/10.1007/s44509-026-00002-6

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00002-6

Keywords: biogas, coconut water, anaerobic digestion, upflow anaerobic hybrid bioreactor, hydraulic retention time, wastewater treatment, renewable energy, circular economy, Kerala, agro-industrial waste, methanogenesis, coconut shell media