Nutrient removal from an anaerobic membrane bioreactor effluent using microalgaestudy and modeling of the process

Abstract

Anaerobic membrane bioreactors for urban wastewater treatment present interesting advantages when compared with aerobic treatments, such as less sludge production, lower energy demand and biogas generation. However, the generated effluent cannot generally be discharged without further ammonium and phosphate elimination. This thesis studies the removal of these inorganic nutrients by means of microalgae cultivation. The main objective of this work is therefore to obtain an autochthonous microalgal culture and to investigate its ability to grow on an already existing anaerobic effluent, as well as to research the extent to which ammonium and phosphate can be removed. Moreover, this thesis aims at providing the kinetic expressions which reproduce the main processes involved, in order to provide the basis for process simulation and design. Microalgae were isolated from a local wastewater treatment plant and their ability to grow on the anaerobic effluent ¿while successfully removing ammonium and phosphate¿ was demonstrated. An excellent water quality was obtained with a semicontinuous cultivation mode under constant illumination. The Scenedesmus and Chlorococcum genus proliferated more efficiently and thus became predominant in the culture. Results also showed that phosphorus was the limiting nutrient in the anaerobic effluent to be treated. The influence of phosphorus limitation on ammonium and phosphate removal, as well as the influence of temperature in ammonium removal, were then studied under laboratory conditions. Kinetic expressions which reproduce the observed effects were proposed and validated, taking also into account the effect of light intensity. Additionally, a Scenedesmus-dominated culture was grown under varying light and temperature in an outdoor flat-plate photobioreactor, with constant monitoring of light intensity, temperature and ammonium concentration. Acceptable results were obtained in the reproduction of the experimental data, albeit with less accuracy than under laboratory conditions. The work here presented demonstrates the feasibility of coupling a microalgal cultivation system to an anaerobic membrane bioreactor for urban wastewater treatment. The basic factors affecting microalgal nutrient removal are researched, and mathematical models are provided which reproduce these effects. This Ph.D. thesis is enclosed in a national research project funded by the Spanish Ministry of Economy and Competitiveness entitled ¿Estudio experimental de la recuperación como biogás de la energía de la materia orgánica y nutrientes del agua residual, acoplando un AnBRM y un cultivo de microalgas¿ (MINECO project CTM2011-28595-C02-01/02). This research was also supported by the Spanish Ministry of Education, Culture and Sport via a pre doctoral FPU fellowship to the author (AP2009-4903).