Regional Physicochemical Characterization and Spatial Heterogeneity of Palm Oil Mill Effluent (POME) Across Six States in Southern Nigeria

The palm oil industry in Nigeria generates significant quantities of Palm Oil Mill Effluent (POME), a highly polluting wastewater. This study evaluates the physicochemical properties, spatial heterogeneity and environmental compliance of POME from 12 small-scale mills across six southern Nigerian states (Abia, Akwa Ibom, Delta, Ekiti, Imo, Osun). Samples were analyzed for 13 parameters: pH, temperature, electrical conductivity (EC), total dissolved solids (TDS), total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic nitrogen (TON), nitrite, nitrate, and phosphate, using American Public Health Association (APHA) methods. The findings revealed universal acidity (pH 3.03–3.89), uniform temperatures (27–29 oC), and elevated nutrient levels, with Imo and Delta showing highest TON (up to 58 mg/L), nitrite (up to 33 mg/L), and EC (>1700 µS/cm). Ekiti and Osun exhibited the lowest pollution. Shapiro-Wilk tests confirmed non-normality for all parameters. Intra-state comparisons via Mann-Whitney U tests indicated no significant differences (p > .05) between sites, though large effect sizes (r ≈ 0.80) suggested practical variations driven by mill-specific practices. Spatial analysis using Moran's I demonstrated weak autocorrelation (predominantly negative, p > .05 except for pH at p = .0107), with distance-decay correlations non-significant, refuting geographic dependence of effluent quality. Pollution index maps highlighted heterogeneous hotspots without regional clustering. Findings underscore that discharge of untreated POME poses substantial risks of eutrophication and soil degradation in receiving environments, highlighting the urgent need for treatment system reform in southern Nigeria's small-scale palm oil sector.