Low concentration of zeolite to enhance microalgal growth and ammonium removal efficiency in a membrane photobioreactor

The aim of this work was to study the growth and nutrient removal efficiency of a mixed microalgal culture with and without the addition of low concentrations (0.5, 1, and 5 g L^-1 of total liquid volume in the reactor) of natural zeolite. A control test in which only zeolite was added into a similar membrane photobioreactor was also conducted. The addition of 0.5 g L⁻¹ zeolite to a continuously-fed membrane photobioreactor increased the microalgal biomass concentration from 0.50 to 0.90–1.17 g particulate organic carbon per L while the average ammonium removal efficiency increased from 14% to 30%. Upon microscopic inspection, microalgal cells were observed growing on the surface of zeolite particles, which indicates that zeolite can support attached microalgal growth. With higher zeolite doses (1 and 5 g L⁻¹) inside the reactor, however, the breaking apart of added zeolite particles into finer particles dramatically increased solution turbidity, which likely was not beneficial for microalgal growth and ammonium removal due to reduced light penetration. This work shows that low doses of zeolite can be used as microcarriers to enhance microalgal biomass concentration and ammonium removal efficiency, while minimizing zeolite dose would likely reduce the turbidity effects.