The suitability of the microalgal metabolism for the recovery of nutrients from wastewater: the implications of solids residence time and diel cycles

Abstract

The development of technologies for resource recovery from wastewater is an important part of modernizing our water infrastructure to become an integral part of a clean economy. Wastewater is a continuous source of renewable chemical and thermal energy as well as nutrient resources like nitrogen and phosphorus. In particular, nutrient recovery technologies can act a source of sustainable fertilizer, allowing for the harvest and reuse of limited mineral phosphorus resources. Microalgae show great promise to remove both nitrogen and phosphorus from wastewater, reducing aqueous nutrients to undetectable levels and recovering organic nutrients, making them a candidate for advanced nutrient removal processes targeting effluent levels not currently achievable with conventional biological nutrient removal processes. This thesis discusses the relevant body of work regarding the potential of phototrophic microalgal systems to become an integral part of a water resource recovery process, and the implications of solids residence time (SRT) and diel cycles on microalgal metabolism influencing wastewater treatment. Experimental results showed that SRT dictated intracellular nitrogen to phosphorus ratio of microalgal biomass, while 24-hour (i.e., continuous) removal of N and P was achieved, and levels of stored carbohydrates increased and decreased in phase with light intensity. Carbohydrate mobilization suggests that microalgal communities will maintain growth through periods of darkness to prioritize the recovery of nutrients. The relationship between SRT and nitrogen to phosphorus ratio suggests a governance of nutrient recovery ratio by the biological requirements of growth rate in agreement with previous theoretical considerations of performance at the high SRTs used in this study. Metabolic flexibility, in particular carbon dynamics and predictable nitrogen and phosphorus recovery, makes microalgae a promising organism for the development of reliable, financially viable biological nutrient recovery systems.