The effect of volatile fatty acids (VFAs) on nutrient removal in SBR with biomass adapted to dairy wastewater

Abstract

This study aims to determine the effect of volatile fatty acids on nitrates and orthophosphate removal in a sequencing batch reactor (SBR) with activated sludge biomass adapted to process dairy wastewater. The research also determine whether it is the type of fatty acid applied that is responsible for the effectiveness of denitrification and dephosphatation at varying nitrate:orthophosphate ratios, or whether these processes are additionally affected by the presence of microorganisms that have adapted to the specific carbon composition of the wastewater being treated. At the beginning of an operating cycle SBRs were dosed with VFAs to provide a source of carbon. A comparative analysis was performed of nitrate and orthophosphate removal at initial nitrate concentrations of 1.22, 7.3 and 15.2 mgN_NO3·L⁻¹. Doses of fatty acids were approximately 10.5 mgCOD·mg⁻¹P_PO4. They consisted of acetic, propionic, butyric, isobutyric, valeric, isovaleric and caproic acids. Increases of nitrate concentration from 1.22 to 15.2 mg N_NO3·L⁻¹ were observed to reduce the quantity of removed orthophosphate depending on the fatty acid applied, from 7.2–9.2 mgP_PO4·L⁻¹ to 4.5 – 6.7 mgP_PO4·L⁻¹. Every increase in the removed nitrates by 5.0 mgN_NO3·L⁻¹ was accompanied by a decrease in the removed orthophosphate of around 1 mgP_PO4·L⁻¹. The reactor containing acetic acid was found to remove the highest amount of orthophosphate irrespective of the nitrates concentration. Acids present in significant amount in dairy wastewaters (i.e. acetic, propionic and butyric) were more effective source of carbon in the denitrification process compared to low concentration acids.

Keywords:

• Dairy wastewater
• nitrates
• orthophosphate
• sequencing batch reactor
• denitrification
• biological phosphate removal
• external carbon source
• volatile fatty acids.