Phycoremediation strategies for rapid tertiary nutrient removal in a waste stream
Introduction
Algae, including freshwater phytoplankton, have been used for tertiary nutrient removal at aquaculture, agricultural, livestock, and small community water resource recovery facilities worldwide (WRRFs; [19], [37], [41], [52], [55]). Within the U.S., a variety of algal nutrient removal technologies has been developed on a large-scale including: high rate algal ponds (HRAPs), settling ponds, and advanced integrated wastewater pond systems (AIWPS) which are a combination of HRAPs and settling ponds [27], [51], [53], with and without rotating biological contactors (RBCs) [27], [42]. Using algae to remove nutrients from wastewater is an appealing strategy because it is inexpensive, simple in design, sustainable, and can benefit the environment as algae can be used as feedstock for biofuel production [13], [14], [19], [37], [54], [57] or as animal feed or fertilizer [7], [66]. Large-scale phycoremediation strategies work well for municipalities that have large land holdings to accommodate shallow ponds so that algal growth is not light limited and in climates where seasonal fluctuations in light and temperature do not inhibit their growth. However, they may be difficult to implement in WRRFs located in urban settings that have small footprints, and operate at high flow rates under continuous flow conditions with short in-plant hydraulic residence times (HRTs). In this setting light and temperature need to be optimized and “wash out” of algal biomass must be prevented to facilitate efficient nutrient removal. Post-treatment, algae must be easily separated and removed from the treated effluent prior to its discharge, and sufficient algal biomass must be recycled to the reactor [19].
In order to address these issues, phycoremediation technologies using immobilized algae been developed [17], [30], [32], [38], [64], [70], [81]. Basic designs for immobilizing algae include encapsulating algae in beads [17], [70] and onto screens [32], [64] or encapsulating algae in natural polymers such as sodium alginate and κ-carrageenan [38] and synthetic materials such as polyurethane and polyvinyl alcohol (PVA) gels [10], [24], [71], [72], [82]. Various strains of freshwater algae have been immobilized using these techniques including: chlorophytes (Chlorella vulgaris, Chlorella pyrenoidosa, Chlorella sorokiniana, Scenedesmus bicellularis, Scenedesmus dimorphus, Scenedesmus obliquus and Scenedesmus rubesencs), cyanobacteria (Phormidium laminosum, Arthospira platensis, Oscillatoria sp.), diatoms, haptophytes, and mixed algal and bacterial populations ([7], [17], [80] and references therein). While various phycoremediation technologies and bioreactor designs using immobilized algae have been described, none have been designed for use in large WRRF applications (> 1–3 million gallons per day) with short HRTs (< 4–8 h) (see review in [43]).
To date, algal treatment processes have focused solely on removal of dissolved inorganic phosphorus (DIP) as phosphate (PO43 −) and dissolved inorganic nitrogen (DIN), primarily as ammonium (NH4+) [12], [17], [33], [43], [70], [71]. However, WRRFs are already capable of efficiently removing DIN using coupled nitrification/denitrification in suspended growth activated sludge processes, consequently, final treated effluents from plants that employ biological nutrient removal (BNR) contain N primarily in the form of residual nitrate (NO3−) and dissolved organic N (DON; [56]). An advantage of using algae to remove the remaining N from treated effluents (as a tertiary treatment, post-denitrification) is that algae can remove NO3− when present at low concentrations as well as DON. In addition, no supplemental organic carbon additions (e.g. methanol) are required because algae are photoautotrophs. Yet another advantage of algal remediation processes is that algae also remove P during their growth at approximately a 16:1N:P ratio [61] and so it may be possible to relax P removal from the upstream nutrient removal processes. However, there are several challenges that must also be overcome before these technologies can be scaled up. Factors limiting the utility of algae in WRRFs include light limitation of their growth in deep opaque tanks, pH and temperature fluctuations outside their optimal physiological ranges, and separation and/or harvesting of immobilized algae from treated effluent prior to discharge. Here results are reported from bench-scale experiments using encapsulated algae designed to optimize algal growth and nutrient removal efficiency under conditions one might find in a municipal WRRF. These results provide information to advise the scale up of algal reactors and suggest that algal wastewater treatment technologies may be cost-effective and offer a secondary benefit through the production of usable biomass.
Section snippets
Materials and methods
Laboratory experiments were done in an iterative fashion, to ensure both high algal growth rates and maximum nutrient removal efficiencies could be achieved at each step. The first group of experiments was designed to test the efficacy of each mode of algal immobilization and was conducted in batch mode, with no flow of growth media into or out of the bioreactor. Subsequently, experiments were conducted in continuous flow mode and the flow rate of growth media into and out of the bioreactor was
Batch bioreactor experiments
Preliminary batch experiments examining temperature effects on growth and nutrient removal efficiency by alginate-embedded Chlorella determined that growth rates were slightly significantly faster (t-test; p < 0.05) in reactors maintained at the lower temperature averaging 0.43 ± 0.03 d− 1 and 0.35 ± 0.03 d− 1 in the 15 °C and 25 °C reactors, respectively. Complete (100%) removal of NO3− + NO2− was achieved after 4 or 6 days in the 15 and 25 °C reactors, respectively. Up to 90% of the TDN and PO43 − were
Conclusions
At hydraulic retention times of 6.5 h, growth rates up to 1.55 d− 1 and 100% removal of NO3− + NO2− within 24 h were achieved. These growth rates and removal efficiencies fall within ranges reported in the literature from experiments with a variety of encapsulation technologies, algal strains, and bioreactor types (Table 2). Our high removal rates were achieved at HRT characteristics of municipal WRRFs, and while higher N removal efficiencies have been reported, many of these experiments were either
Acknowledgments
The authors would like to thank the Water Environment Research Foundation (WERF-U4R10) and project manager Amit Pramanik for financial support and guidance throughout this project. They would also like to thank the Hampton Roads Sanitation District for providing access to the Virginia Initiative Plant where treated effluent for the experiments was obtained. Work in the laboratory would not have been conducted without the help of Peter Bernhardt (lab manager) and Chris Schweitzer (graduate
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2022, Science of the Total EnvironmentCitation Excerpt :Higher values may occur due to CO2 consumption by microalgae and impair bacterial growth, resulting in less CO2 supporting the growth of microalgae and, subsequently, lower P removal from wastewater. Although optimal temperatures of 28–30 °C have been indicated for the growth of microalgae (Park et al., 2011), significant P removal from waste liquid streams (90%) has been reported also at temperatures as low as 15 °C (Filippino et al., 2015) and 5 °C (38%) (Chatterjee et al., 2019) with cultures of Chlorella and Scenedesmus, respectively. Although promising outcomes have been shown at lab- and pilot-scale, integration of algal and bacterial microorganisms for P removal from wastewater has not been successfully tested at full-scale.