Greenhouse gas emissions and energy production potential from an oil palm wastewater lagoon in southwest Costa Rica

Background/Question/Methods

Palm oil (PO) is the fastest growing agricultural commodity in the world, with diverse uses across market sectors, including biofuels. The rapid expansion of oil palm is a leading driver of deforestation and is replacing farming and husbandry practices pantropically. Policy debates to date have focused primarily on the loss of carbon and biodiversity due to plantation establishment, with less focus on the greenhouse gas (GHG) emissions from palm oil mill effluent (POME). A large volume of POME, which contains high levels of degradable organic matter, is generated during PO extraction and must be treated under regulation. We addressed two questions: 1) How large are GHG fluxes from an oil palm effluent lagoon? 2) What are the tradeoffs between GHG emissions and energy generation potential?

We addressed these questions using a prototypical POME treatment system located in southwest Costa Rica. It was comprised of 7 gravity-driven, serial ponds open to the atmosphere. We measured GHG fluxes in addition to nutrient levels, chemical oxygen demand (COD), redox potential in lagoon water and sediment. Rate calculations were determined from headspace methane, nitrous oxide and carbon dioxide concentrations in bottle incubations at t₀, 2 and 8 hours.

Results/Conclusions

All the ponds were strongly anaerobic and had extremely high COD, which ranged between 3050 and 368,253 mg/l. Methane dominated GHG fluxes and varied tremendously between ponds. Methane fluxes increased dramatically to a high point in pond #2 (2,115 kg/hr), then declined to 0.04 kg/hr before POME discharge to a small stream. The left-skewed distribution indicates that optimal substrate conditions for methanogensis peak midstream in the treatment process. At these rates, the lagoon system may emit up to ~8,700 Mg of carbon as methane per year. In carbon dioxide equivalents, this equals roughly 55 hectares of aboveground biomass carbon stored in primary rainforests of the region. As such, this POME lagoon system is a hotspot for GHG emissions.  However, using BIOWIN (a professional wastewater treatment software package), we show that the climate and biogeochemical conditions are highly suitable for maximizing a mesophilic biogas reactor.  Harnessing methane emissions via such a reactor would greatly reduce the lagoon’s climate impact and likely enhance the local economy.