Hulsen, T., Hsieh, K., & Bastone, D. J. (2019). Saline wastewater treatment with purple phototrophic bacteria. Water Research 160: 259-267.
The processes responsible for eliminating nutrients and pathogens from wastewater vary based on what materials can be found in the wastewater, itself. One material of particular interest in wastewater treatment is salt. Salt ends up in wastewater from various industrial sources, such as via the production and use of road salts and the use of seawater for irrigation and household sanitation purposes. Such processes have resulted in wastewater with high concentrations of dissolved salts in both residential onsite wastewater treatment systems and large-scale wastewater treatment plants.
High concentrations of salt can inhibit wastewater treatment processes. More specifically, the sodium chloride (NaCl, aka table salt) found in saline wastewater impedes the anaerobic decomposition of organic matter, an essential component of wastewater treatment. Biological materials such as urine, feces, and cell materials make up the high concentrations of organic matter that can be found in wastewater. The first step involved in most wastewater treatment methods is to send organic-rich wastewater into an environment with little to no oxygen present (anaerobic), where microorganisms can work to anaerobically break down the organic matter, thus preventing that organic matter from leaving the wastewater treatment system and polluting surrounding water bodies. In addition to NaCl, seawater also contains high concentrations of sulfates. When seawater is introduced to the wastewater stream, bacteria convert the sulfates present into sulfides, which can also inhibit many of the nutrient removal pathways required in wastewater treatment, such as nitrogen removal. The challenges involved with the incorporation of salt and seawater into wastewater calls for unique management strategies. Because microorganisms are largely responsible for making the magic of wastewater treatment happen, researchers often turn to them in attempts to better understand/improve wastewater treatment overall. While many microorganisms cannot tolerate high concentrations of salt, there are some that can, such as purple phototrophic bacteria.
In this study, researchers from the University of Queensland (Australia) explored the option of relying on a group of salt-tolerant microorganisms called purple phototrophic bacteria (PPB), which are capable of simultaneously removing organic matter and nutrients in the presence of light. In addition to their ability to tolerate salty environments, PPB can also typically use high sulfide concentrations to their advantage; they can utilize sulfides as an energy source, effectively removing sulfides from the wastewater completely. PPB also work to lower sulfide concentrations in saline wastewater by outcompeting the microorganisms responsible for converting sulfates into sulfides.
Researchers evaluated PPB’s capacity for treatment of two types of saline wastewater: wastewater with high concentrations of NaCl, and marine wastewater (specifically sourced from a seawater environment). The researchers evaluated treatment performance, behavior of the microbial community, and PPB’s ability to prevent sulfide production in saline wastewater. In order to simulate the mechanisms utilized at many wastewater treatment plants, the researchers created their own small-scale version of an anaerobic photo membrane bioreactor (essentially, a rectangular chamber with a membrane for microbial growth – the chamber was exposed to continuous light and amended to replicate the anaerobic/organic-rich environment of a typical large-scale bioreactor). Domestic wastewater collected from an external source was amended with NaCl (to replicate saline wastewater) or a mixture of constituents that make up seawater (primarily Red Sea Salt, sulfates, and potassium; to replicate marine wastewater).
Study Findings and Implications
Overall the removal rates of organic matter, total nitrogen, and total phosphorous were in line with rates observed in freshwater wastewater treatment systems, suggesting that neither high salt concentrations nor PPB inhibit nutrient removal processes. On the contrary, high salt concentrations that likely allowed PPB to outcompete less salt-tolerant microorganisms suggest that PPB were the primary organisms responsible for encouraging/facilitating this nutrient removal. Results confirmed that PPB species remained the dominant microbial species over the entire course of the study. Finally, researchers found that PPB were able to outcompete the microorganisms that convert sulfates to sulfides, which benefits wastewater treatment.
Results suggest that PPB are a feasible option for treating wastewater with high salt and even high sulfate concentrations. As we continue to rely on salt for industrial purposes, innovative wastewater treatment options such as microbial enrichment could serve as viable solutions.