Nutrient Transformations in Algae Raceway Ponds Fed Municipal Wastewater

Abstract

Microalgae can be useful in wastewater treatment processes by releasing dissolved oxygen into the water and assimilating nutrients. After treatment, the resulting algal-bacterial biomass can be harvested and used as a biofuel feedstock while the water is discharged or reused off-site. However, to sustainably produce nationally significant quantities of algae biofuel, the water and nutrients used in cultivation would need to be recycled on site for additional algae cultivation. In the present thesis research, several pilot and lab scale experiments were conducted on algae wastewater treatment and recycling of water and nutrients in wastewater-based algae cultivation. The main topics were (1) the treatment of primary clarifier effluent by algae raceways and the resulting transformations of nitrogen and phosphorus compounds, and (2) the potential of algae cell disruption techniques to increase nutrient solubilization from algal-bacterial biomass to support cultivation of additional algae from biomass. The pilot-scale experiments were conducted using nine 33-m 2 , 0.3-m deep, paddle wheel-mixed raceway ponds. These ponds were setup in triplicates and fed municipal wastewater (primary clarifier effluent) to grow algae polycultures. Three experiments conducted using these raceways are reported herein: a comparison of grab and 24-hour composite samples of influent, a study of the effect of hydraulic residence times (HRT) on nitrogen and phosphorus transformations, and a nitrogen and phosphorus depletion study with water recycle. v The grab and 24-hour composite comparison samples were collected weekly during the latter half of 2014. Dissolved reactive phosphorus (DRP) and total ammonia nitrogen (TAN) concentrations of the two sample types were compared and linear correlation equations were made to allow an approximate conversion of grab concentrations to 24-hour concentrations. The results from the sample types, for both DRP and TAN, were on average within 10% of each other. A comparison was made of data from parallel raceway ponds operated at different HRTs, and linear correlations were determined for soluble nitrogen and phosphorus removals. From the 1.5-day HRT to the 6-day HRT the soluble nitrogen removal ranged from 23% to 61% and the DRP removal ranged from 17% to 53%. The experiment on nutrient depletion (removal to low concentrations) was conducted in October, 2015. In this experiment, raceway effluent was passed through settling tanks for algae harvest, with the supernatant returned to the raceway. Soluble nitrogen removal was linear with a rate of 1.5 mg-N/L-day (14 days required to decrease from 24 mg-N/L to 2.6 mg-N/L). DRP removal was also linear with a rate of 0.18 mg-P/L-day. For algae biofuel production, the rate and ultimate extent of nutrient resolubilization from biodegradation of biomass is important to estimating the amount of supplemental fertilizer to be provided, which is a factor in both cost and environmental sustainability. Mechanical or thermal cell disruption methods were evaluated for their...

Production and Harvest of Microalgae in Wastewater Raceways with Resource Recycling

“…In this study, soluble nitrogen and phosphorus were never diminished to the point of limit algal growth rates, based on typical Monod half-saturation constants for green algae (Fulton 2009). Therefore, insolation was assumed to be the main growth limiting factor (Reiff, 2015 andKraetsch, 2015), and all the ponds received equal levels of solar radiation.…”