Abstract
This paper aims to elucidate the role of extracellular polymeric substances (EPS) in regulating anion and cation concentrations and toxicity towards microorganisms in anaerobic granular sludges adapted to low (0.22 M of Na+) and high salinity (0.87 M of Na+). The ion exchange properties of EPS were studied with a novel approach, where EPS were entangled with an inert binder (PVDF-HFP) to form a membrane and characterized in an electrodialysis cell. With a mixture of NaCl and KCl salts the EPS membrane was shown to act as a cation exchange membrane (CEM) with a current efficiency of ∼80%, meaning that EPS do not behave as ideal CEM. Surprisingly, the membrane had selectivity for transport of K+ compared to Na+ with a separation factor (SK+/Na+) of 1.3. These properties were compared to a layer prepared from a model compound of EPS (alginate) and a commercial CEM. The alginate layer had a similar current efficiency (∼80%.), but even higher SK+/Na+ of 1.9, while the commercial CEM did not show selectivity towards K+ or Na+, but exhibited the highest current efficiency of 92%. The selectivity of EPS and alginate towards K+ transport has interesting potential applications for ion separation from water streams and should be further investigated. The anion repelling and cation binding properties of EPS in hydrated and dehydrated granules were further confirmed with microscopy (SEM-EDX, epifluorescence) and ion chromatography (ICP-OES, IC) techniques. Results of specific methanogenic activity (SMA) tests conducted with 0.22 and 0.87 M Na+ adapted granular sludges and with various monovalent salts suggested that ions which are preferentially transported by EPS are also more toxic towards methanogenic cells.
Highlights
In anaerobic wastewater treatment systems, high monovalent salt concentrations are considered to negatively affect microbial activity, especially of the methanogenic population (Rinzema et al, 1988; Vyrides et al, 2010; Fang et al, 2011; De Vrieze et al, 2016)
It was only recently shown that anaerobic granules adapted to approximately 0.35 M Naþ can increase in size (Gagliano et al, 2017) and can even be formed from dispersed biomass (Sudmalis et al, 2018a) at Naþ concentrations as high as 0.87 M, while maintaining high methanogenic activity for successful high rate anaerobic reactor operation
These results suggest that even at high salinity extracellular polymeric substances (EPS) may have a protective role against monovalent cations such as Naþ by hindering their diffusion into microbial cells (Gagliano et al, 2018)
Summary
In anaerobic wastewater treatment systems, high monovalent salt concentrations are considered to negatively affect microbial activity, especially of the methanogenic population (Rinzema et al, 1988; Vyrides et al, 2010; Fang et al, 2011; De Vrieze et al, 2016). A recent microscopy study of our group has shown that mannose-rich EPS surrounding methanogenic cells in high salinity adapted granular sludge adsorb high concentrations of sodium (Gagliano et al, 2018) These results suggest that even at high salinity EPS may have a protective role against monovalent cations such as Naþ by hindering their diffusion into microbial cells (Gagliano et al, 2018). The negative charge of EPS should prevent or hinder the transport of anions to microbial cells, thereby lowering their toxicity towards the microorganisms present in porous biofilm structures, such as sludge granules. This ability of EPS to partially repel anions has not been experimentally demonstrated. To study if the ions with the highest transport rate would more negatively affect methanogenic activity, the effect of different concentrations of Naþ and Kþ towards specific methanogenic activity (SMA) of 0.87 M and 0.22 M Naþ adapted granular sludge was determined
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