Physicochemical tailoring of POME through mechanical pretreatment: Assessing nutrient uptake and biomass characteristics in mixotrophic Chlorella sp. cultivation

Palm oil mill effluent (POME) is organic, nutrient-rich wastewater which physicochemical properties can severely disrupt terrestrial and aquatic ecosystems when discharged untreated. This study investigates the effect of hydrothermal and ultrasonication pretreatments in enhancing nutrient release from POME for microalgae cultivation as an alternative organic-based medium. Liquid and solid fractions of raw POME were characterized prior to pretreatment. Lyophilized POME sludge comprised 35.86% extractives, while the extractive-free sludge consisted mainly of cellulose (28.3%), hemicellulose (13.8%), and lignin (16.1%) on mass weight basis. POME was pretreated at different range of temperature-time for hydrothermal (140, 180, 200, 220 °C; 60 and 120 min) and intensity-time for probe ultrasonication (50% and 100%; 2 and 10 min). Selected conditions were evaluated for Chlorella sp. cultivation and compared with R and BG-11 synthetic media representing mixotrophic and autotrophic growth. The liquid fraction of raw POME used as cultivation media in this study contained high levels of chemical oxygen demand (COD) (19,690 mg/L), total nitrogen (TN) (323 mg/L), total phosphorus (TP) (346 mg/L), corresponding to a COD:N:P mass ratio of 61:1:1. The highest TN consumption (74.99%) by Chlorella sp. was achieved in sonicated POME, while HT-POME yielded the greatest carbohydrate consumption (83.58%). Meanwhile, TP removal in all POME-based media exceeded 98.89%. Biomass productivity in POME-based media was comparable to R medium, although protein content was lower. Thermal analysis revealed increased structural condensation and thermal stability of residual sludge after pretreatment. Overall, US-pretreated POME supports efficient nutrient consumption, comparable microalgal growth, and offers integrated potential for wastewater treatment and downstream resource valorization.