Pilot-Scale High-Rate Algal Pond for Landfill Leachate Treatment and CO2 Biofixation Under Tropical Conditions: A Case Study

Microalgae in high-rate algal pond (HRAP) systems sustain elevated rates of photosynthetic carbon fixation and nutrient assimilation compared with natural aquatic ecosystems, owing to their optimized operating conditions, such as shallow depth, continuous mixing, and high light and nutrient availability. HRAPs offer a low-cost option for wastewater treatment and CO2 sequestration, yet their combined performance under tropical conditions, particularly when treating landfill leachate, remains poorly documented. This case study reports the operation of a single pilot-scale HRAP treating undiluted constructed-wetland effluent receiving landfill leachate in Valle del Cauca, Colombia, monitored over one continuous experimental run to jointly assess water-quality performance, daytime CO2 uptake, and the resident algal community. The pond was operated continuously at 0.14 m3 d−1, and physicochemical variables (pH, dissolved oxygen, conductivity, and redox potential) were monitored three times per week. Carbon fixation was estimated from a daytime inorganic-carbon mass balance (influent and effluent carbon, gas exchange, and respiration) combined with static-chamber measurements. The system effectively removed the soluble organic fraction of the leachate (soluble COD of 22.2%; soluble BOD5 of 58.9%), whereas the apparent increases in total COD, total BOD5, and suspended solids were attributable to in-pond microalgal biomass production. Under tropical sunlight, the pond fixed approximately 33.1 g C d−1 (95% CI 14.8–51.5), acting as a net CO2 sink during daylight hours. Sixteen algal taxa, mainly from the Chlorophyceae and Chlamydophyceae, were identified by morphological keys, several of which have not been previously reported in leachate-treatment systems. As an unreplicated pilot-scale case study without a parallel control, the quantitative estimates should be interpreted as system-specific rather than generalizable; nonetheless, the results provide a field reference for CO2 biofixation and leachate treatment in tropical HRAPs and a basis for future controlled, replicated studies.