Microalgae assisted coupled nitrification-denitrification in a sponge-based trickling photobioreactor during anaerobic digestate treatment

Abstract

Here, a sponge-based trickling photobioreactor was developed for energy-efficiently nitrogen removal from anaerobic digestates via microalgae-assisted coupled nitrification and denitrification. Soluble total nitrogen removal was significantly increased under light irradiation: 0.5 % removal efficiency in phase I (0 lx), increased to 65.1, 77.3, and 73.1 % in phases II (2000 lx), III (3000 lx), and IV (10,000 lx), respectively. Rate of nitrite oxidation by nitrite oxidizing bacteria increased more prominently than the rate of ammonia oxidation by ammonia oxidizing bacteria, rising from 30.1 to 42.9 mg-N L −1 d −1 from phases I to IV, possibly because of enhanced oxygen availability due to microalgal photosynthesis under high light intensities. A strong negative correlation ( p < 0.001) was observed between effluent NO 2 − concentrations and denitrification rates, suggesting that NO 2 − accumulation inhibits denitrification, particularly in phase I. Improved the nitrite oxidation rate in later phases alleviated NO 2 − buildup, thereby facilitating denitrification and enhancing overall nitrogen removal. Stable operation requires sufficient microalgal photooxygenation for the balanced rates of ammonia and nitrite oxidation. To facilitate broader application, an empirical model was developed to predict the optimal light intensity ranges for effective nitrogen removal in reactors of different diameters. Because treatment performance is governed by average internal light intensity rather than incident intensity, the Lambert–Beer law and definite integration were applied to estimate the light distribution in the reactor. The resulting model provided practical equations to determine the minimum and maximum incident light intensities based on the reactor diameter, outlining a rational framework for the design and implementation of symbiotic photobioreactors.

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