Scenario Analysis of Nutrient Removal from Municipal Wastewater by Microalgal Biofilms

The conventional treatment of municipal wastewater consists of activated sludge processes with a combination of nitrification and denitrification and biological or chemical phosphorus removal. However, other treatment systems are also used, including systems based on microalgae, eukaryotic microorganisms and prokaryotic cyanobacteria that carry out oxygenic photosynthesis [1]. Microalgae have a high affinity for nitrogen (N) and phosphorus (P), illustrated by the low values reported for halfsaturation constants, ranging from 0.56 to 3094 μg N/L, and from 0.001 to 81.9 μg P/L [2-5]. Microalgae can either grow in suspension (phytoplankton) or on substrata (benthic) in biofilms [6]. Microalgal biofilms are attached microbial consortia of phototrophs and chemotrophs entrapped in an exopolymeric matrix, and are omnipresent in aquatic environments [7,8]. Although not given a lot of attention, microalgal biofilms systems could form interesting wastewater treatment systems. A microalgal biofilm system can operate at short hydraulic retention times due to the ability of the biofilm to retain the biomass. It is also expected that, in contrast to suspended microalgal systems, little or no separation of microalgae and water is required before discharging the effluent [9,10]. Furthermore, no mixing is needed in the system, resulting in a lower energy requirement than for suspended systems