Simulation of the suppressive effect of zinc on cyanobacteria in paper mill wastewater

Abstract

Harmful algal blooms (i.e. blue-green algae or cyanobacteria) and their impacts are a recurring and significant issue worldwide. Factors supporting the extensive growth of cyanobacteria include warmer temperatures, sufficient levels of reactive nitrogen and phosphorus, reduced rainfall, long hydraulic retention times within the water body (slow-moving water) and adequate sunlight. Problems associated with the growth of cyanobacteria include formation of unpleasant odours and toxins that have the potential to harm humans and wildlife that come into contact with contaminated water. Aerated stabilization basin (ASB) systems, commonly used by the pulp and paper mill industry for treatment of wastewater, are predominantly light limited due to high colour and consequently, blooms of cyanobacteria have been reported to rarely occur in pulp and paper ASBs. However, with the relatively low colour of paper mill (PML) wastewater (without integrated pulping processes) in ASB treatment with long hydraulic retention times (HRTs), there would be risk of occurrence of cyanobacteria at bloom levels. In 2014, two occurrences of cyanobacterial blooms were observed at a PML site in South Australia and in April 2016, Microcystis aeruginosa occurred at bloom levels (> 106 cells/mL) in an ASB pilot plant (3 tanks of ~200 L each) operated at the PML site to simulate the full scale ASB operation. Following these incidences, it was evident that there is need for improved understanding and modelling of strategies to control cyanobacteria present in PML wastewaters. The aim of the research reported here was to establish models that describe the dynamics of cyanobacteria presence, indicated by phycocyanin, in PML wastewaters when exposed to zinc. This was investigated based on the inhibitory effect of Zn on cyanobacteria. In this research, batch experiments were conducted using a strain of Microcystis aeruginosa (MA338) and Pseudanabaena spp. (naturally occurring) in samples collected from ASB wastewaters. Growth and inhibition of the cyanobacteria were assessed by fluorometric detection of phycocyanin (using an EXO1 sonde with EXO Total Algae PC Smart Sensor), which is a cyanobacterial specific pigment that increases linearly with cyanobacterial biomass increase. Phycocyanin levels were determined for batch experiments where Zn was dosed at between 0.3 and 2.4 mg/L and in controls, for 7 days. At lower concentration (~0.3 mg/L), Zn was found to be supportive for the growth of MA338 based on phycocyanin data. In contrast, Zn ≥ 0.7 mg/L was found to be effective in suppressing the cyanobacteria tested in this study. For test samples (with Zn ≥ of 0.7 mg/L), two functions [exponential peak (EP) and logistic dose response (LDR)] were fitted to % phycocyanin vs time data. From the collected data at initial and on days 1, 3, 4 and 7, the phycocyanin reduction (decay) rates at the various Zn doses (0.7 ≤ Zn ≤ 2.4 mg/L) were determined. The reduction rates (Δ%Phy/Δtime) differ between the two models but trended consistently (EP: 52, 64, 85, 164 and LDR: 85, 249, 188, 1621 for Zn concentrations of 0.7, 1.1, 1.6 and 2.4 mg/L, respectively). The two models broadly described the dynamics of phycocyanin levels in samples of PML wastewaters in response to Zn exposure up to 2.4 mg/L over 7 days. Both models differ in their fittings over the first few days to Zn exposure, where the EP model correctly describes an increase in phycocyanin after initial Zn exposure. However, the maximum level estimated by this approach is speculative of what is the true maximum value, and for this much finer resolution data would be needed over the early stage of batch experiments. Further investigation is needed to determine the Zn concentration with exposure time that would suppress or enhance cyanobacteria growth and the fate of Zn in PML ASB systems. Integrated modelling with phycocyanin monitoring has the potential to better implement strategies for cyanobacterial control, through rapid assessment of the responses and efficiencies of chemicals applied to inhibit cyanobacteria growth.