Diurnal patterns and removal of selected elements in two Norwegian wastewater treatment plants with significant industrial loading

Wastewater treatment plants (WWTP) that collect industrial and household discharges can be a significant source of pollutants to the environment. In this study, we investigated influent patterns and removal of selected elements in two full-scale WWTPs, Ladehammeren (LARA) and Høvringen (HØRA) in Trondheim, Norway. Both WWTPs have significant industrial loading contributions (up to 40% in LARA). The WWTP employ preliminary and primary treatment steps, including chemically aided flocculation, and discharge directly into Trondheimsfjord. In a 7 day sampling campaign, 24 h composite samples of influent and effluent wastewater as well as sludge samples were taken to determine influent concentrations and removal of selected elements including Al, P, S, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba, Gd and Pd. To study release patterns and gain information about potential sources such as industrial discharges, we determined and modelled diurnal variations of elemental concentrations in 8 h composite samples covering morning, evening and night discharges.
Element concentrations in 24 h composite influent samples were highest for S>P and Al and lowest for CdElement concentrations in 8 h composite samples mostly followed general diurnal discharge patterns, with higher concentrations in mornings and evenings and lower concentrations at night. Concentrations of several elements correlated well with total suspended solid concentrations (TSS), with the strongest correlations observed for P, Fe and Al (R2>0.8 in LARA, R2>0.7 HØRA). Correlations of TSS with Cu, Cr, As, Cd, and Pb were more pronounced in HØRA than in LARA, which can potentially be attributed to the higher industrial loading contributions in LARA. Enrichment factors were high for P>Cu>Zn>Cd>As, and were above 10 for Cr and Ni in biosolids, indicating anthropogenic sources for these elements. Several elements also occurred as nano- and micron-sized particles.