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1. Hybrid Ion Exchange and Algae for High Strength Sidestream Wastewater Treatment

   Wang, M.; Payne, K.; Tong, S.; Ergas, S.J. (June 2017, WEF 2017 Nutrient Symposium)

   A hybrid ion-exchange and algae photosynthesis (HAPIX) process was used for treatment of side stream centrate from an anaerobic digester treating waste activated sludge. Although the high NH4+ -N concentration of the centrate (~1180 mg/L) inhibited the algae growth in unamended controls, addition of 150 g/L of zeolite reduced the ammonia toxicity due to its ion exchange capacity. NH4+-N was reduced from 1,180 mg/L to 107 mg/L within 24 hours by ion exchange. Na+ was the major cation exchanged with NH4+. The addition of algae further reduced the NH4+-N concentration to 10.5 mg/L after 8 days of operation. Zeolite that was saturated with NH4+ can be bioregenerated by the algae growth so that the zeolite can adsorb more NH4+ in the wastewater. The mathematical model that combined ion-exchange and algal photosynthesis processes predicted the aqueous NH4 + -N concentration well. The HAPIX process is feasible to treat high NH4+-N side stream wastewater. 

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2. Resource recovery from anaerobic digestion effluent by a hybrid algal photosynthesis and ion-exchange process

   Wang, M.; Payne, K.; Tong, S.; Ergas, S. (May 2018, IWA Leading Edge Technology Conference on Water and Wastewater Treatment)

   A hybrid algal photosynthesis and ion exchange (HAPIX) process was developed that uses natural zeolite (chabazite) and wild type algae to treat high ammonium strength wastewater. In the HAPIX process, ammonium ions are temporarily adsorbed from the liquid, which reduces the free ammonia concentration below the inhibitory level for algal growth. The slow release of adsorbed NH4+ subsequently supports the continuous growth of algae. In this study, a HAPIX reactor reduced NH4+-N concentrations in centrate from an anaerobic digester from 1,180 mg/L to below 10 mg/L without dilution. Chabazite doses of 60 g/L produced more biomass, with higher protein and starch contents, than doses of 150 g/L and 250 g/L. Approximately 67-70% of fatty acids in the biomass harvested from HAPIX reactors were unsaturated. A mathematical framework that couples a homogeneous surface diffusion model with a co-limitation algal kinetic growth model reasonably predicted the biomass production and NH4+-N concentrations in the HAPIX reactors. The HAPIX process has the potential to serve a two-fold purpose of high NH4+-N strength wastewater treatment and agricultural or commercial biopolymer production. 

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3. Hybrid algal photosynthesis and ion-exchange (HAPIX) process for anaerobic digester centrate treatment

   Wang, M.; Payne, K.; Tong, S.; Zalivina, N.; Ergas, S.J. (March 2017, 1st IWA Conference on Algal Technologies for Wastewater Treatment)

   Algae-based wastewater treatment systems have the potential to reduce the energy cost of wastewater treatment processes by utilizing solar energy for biomass growth and nutrient removal. NH4+-N concentrations as high as 200- 300 mg/L are known to inhibit algae growth. Many research studies on the treatment of centrate after anaerobic digestion have been published recently. However, in these studies the centrate was diluted for the growth of algae due to the high NH4+-N concentrations, which are toxic to algae. Alternative solutions are necessary to treat high NH4+-N strength wastewater without addition of freshwater. Zeolites are natural hydrated aluminosilicate minerals that have been used to reduce ammonium inhibition on microorganisms due to their high affinity for ammonium ions. It is possible to use the ion-exchange (IX) capacity of zeolite to reduce the toxicity of ammonia to algae. Importantly, the zeolite, which becomes saturated with ammonium, can be reused as a slow release fertilizer. The objectives of this research were to evaluate the impact of zeolite dosage on the nutrient removal efficiency for high strength wastewater and develop mathematical models to predict the performance of hybrid IX and algae growth systems with varying doses of zeolite. 

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4. Photodegradation of Landfill Leachate-Induced Dissolved Organic Nitrogen and Its Impact on Algal Growth in Estuaries

   Ahmed, Md Ashik; Deaton, Dawn; Taylor, Kayla; Bowen, Christian; Cheshire, Jack; Paerl, Hans; Zhang, Lifeng; Zhao, Renzun (May 2025, AEESP 2025 Conference)

   Abstract The treatment of landfill leachate and sewage is crucial for mitigating the environmental impacts of dissolved organic nitrogen (DON) effluent in aquatic ecosystems. This study used three sequencing batch reactors (SBRs) to treat sewage mixed with landfill leachates of varying organic carbon content. While the SBRs significantly removed dissolved inorganic nitrogen (DIN), the effluents were enriched with landfill leachate-induced DON. These landfill leachate-induced DON effluents (R1, R2, and R3) were then photodegraded under simulated summer sunlight conditions based on Greensboro, NC, USA weather data. The study utilized visible light (400–780 nm, 9340 μW/cm²), UVA (365 nm, 1442 μW/cm²), UVB (285 nm, 76 μW/cm²), UVC (254 nm, 315 μW/cm²), and dark controls. Effluents were mixed with Neuse River Estuary water, serving as a natural algal source, and exposed for 90 days under these light conditions. Samples were analyzed every 10 days for DON degradation and algal growth, with molecular changes assessed using FTICR-MS, FTIR, and EEM-PARAFAC. Results showed substantial DON degradation across all light treatments, with UVA achieving the highest reduction (up to 99.07%), followed by UVC (88.85%), visible light (86.19%), and UVB (75.11%), while no degradation occurred under dark conditions. Initial DON levels of 2.69–2.7 mg/L were reduced to as low as 0.025 mg/L under UVA in R3 effluent. UVC treatment led to increased NO3-N concentrations due to the oxidation of DON to NH4-N and its subsequent conversion to NO3-N, reaching 2.66, 2.59, and 2.63 mg/L in R1, R2, and R3, respectively. UVC inhibited algal growth, resulting in no NH4-N uptake and subsequent oxidation to stable, elevated NO3-N levels in the samples. Algal growth responses varied by light treatment, with visible light and UVB promoting the highest algae growth, minimal algae growth observed under UVA, and no growth under UVC or dark conditions. These findings demonstrate the evidence of rDON degradation during the long-term retention in the receiving water bodies and potential impact on the algal growth. 

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5. Low-Ammonium Environment Increases the Nutrient Exchange between Diatom–Diazotroph Association Cells and Facilitates Photosynthesis and N2 Fixation—a Mechanistic Modeling Analysis

   https://doi.org/10.3390/cells11182911  

   Gao, Meng; Armin, Gabrielle; Inomura, Keisuke (September 2022, Cells)

   Diatom–diazotroph associations (DDAs) are one of the most important symbiotic dinitrogen (N2) fixing groups in the oligotrophic ocean. Despite their capability to fix N2, ammonium (NH4+) remains a key nitrogen (N) source for DDAs, and the effect of NH4+ on their metabolism remains elusive. Here, we developed a coarse-grained, cellular model of the DDA with NH4+ uptake and quantified how the level of extracellular NH4+ influences metabolism and nutrient exchange within the symbiosis. The model shows that, under a fixed growth rate, an increased NH4+ concentration may lower the required level of N2 fixation and photosynthesis, and decrease carbon (C) and N exchange. A low-NH4+ environment leads to more C and N in nutrient exchange and more fixed N2 to support a higher growth rate. With higher growth rates, nutrient exchange and metabolism increased. Our study shows a strong effect of NH4+ on metabolic processes within DDAs, and thus highlights the importance of in situ measurement of NH4+ concentrations. 

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