Characterisation of Distillery Molasses Stillage and Assessment of Water Quality of a River Running through a Sugar Cane Plantation in Southern Zimbabwe

Abstract

The aims of this case study were to characterise partially-treated distillery molasses stillage, determine river pollutant loading and its self purification capacity during the dry season (April to September) in 2012 in southern Zimbabwe. Monthly water and stillage samples were analysed in triplicate for BOD5, DO, NO3-N and PO4-P using standard procedures for water and wastewater analysis. Discharge was estimated using the area-velocity method. River pollutant loading was determined as the product of pollutant concentration and discharge. River self-purification was determined using the upstream and downstream approach. Normality-tested data were subjected to analysis of variance and the least significant difference post-hoc procedure at 95% confidence limit using GENSTAT statistical package. Pearson correlation test was used to determine the strength of associations among parameters. Average monthly values for pond stillage (4 559±9.01 mgBOD5/l; 3 647±2.31 mgTDS/l; 876±2.08 mgNO3-N/l; 729.40±1.15 mgPO4-P/l; 40.35±1.61°C, pH: 4.50±0.06 and 0.46±0.06 mgDO/l) were outside the Zimbabwean permissible limits for safe surface discharge. Average monthly concentrations of parameters upstream differed from those at the first point downstream by between 1.76 and 725.00%, but decreased further downstream (except for DO and pH). Trends of loading rates were: TDS (SP4<SP3=SP1<SP2); BOD5 (SP1<SP4<SP3<SP2); NO3-N (SP1=SP4<SP3<SP2) and PO4-P (SP1=SP3=SP4<SP2). Distillery pond stillage was found to be acidic with high organic matter, nutrients and total dissolved solids for surface discharge. The characteristics of the river upstream were not sufficient for its self-purification within the studied stretch prompting the need for further pre-treatment of stillage. The variation of pollutant concentrations was attributed to uncharacterised watershed and in-stream non-point sources of pollution.