Immobilized microalage as an alternative for wastewater nutrient removal

Abstract

This study is on the use of unicellular microalga Chlorella vulgaris, immobilized by calcium alginate, to treat simulated wastewater and examines the nutrient (ammonium nitrogen (NH4+-N) and ortho-phosphate (PO43-) compounds) removal efficiency under various environmental conditions. The study also aims to determine the optimum operation strategy for reducing nutrients from primarily settled municipal wastewater. To achieve these objectives, five experiments were performed. The first experiment shows that immobilized algal beads could effectively remove nitrogen (N) and phosphorus (P) from synthetic sewage simultaneously. The wastewater NH4+-N decreased from an initial 29 to 0 mg 1-1 within 48 hours of algal treatment, and over 95% PO43--P Was also reduced at the same reaction time under continuous illumination (4000 lux). The bead density experiment reveals that the rate of nutrient removal increased with bead density when the bead density was less than 12 beads ml-1 wastewater. The optimal bead density was found to be 11.68 beads ml-1 wastewater and the retention time of N and P treatment could be shortened to less than 24 hours. The third experiment shows that the optimal operation temperature was around 20°C to 25°C. A significantly higher percentage of N and P reduction and a shorter retention time were recorded when the treatment system was operated within this temperature range. The fourth experiment demonstrates that continuous illumination (i.e. 24 hours light and no dark period) had the best removal efficiency and the treatment performance declined as the illumination duration shortened, in the descending order of 24/0 > 16/8 > 12/12 > 0/24 light/dark hours. Moreover, the ammonium loading experiment shows that the best ammonium removal efficiency was found in wastewater containing 30 mg NH4+-N 1-1, followed by 15 mg NH4+-N 1-1. The nutrient removal efficiencies were significantly decreased in wastewater loaded with 150 and 300 mg NH4+-N 1-1. In conclusion, the immobilized Chlorella vulgaris beads could remove nitrogen and phosphorus from primarily settled wastewater simultaneously. The treatment efficiency was affected by various environmental factors including bead density, temperature, illumination and nutrient loading. Under the optimum operation condition, more than 95% removal of NH4+-N and PO43--P from wastewater could be achieved within 24 hours treatment by illuminated immobilized micro-algal beads at density of 12 beads ml-1 and within a 20°C - 25°C temperature range.

Date of Award	24 Oct 1997
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Fung Yee Nora TAM (Supervisor)