Stability of chitosan gel as entrapment matrix of viable Scenedesmus bicellularis cells immobilized on screens for tertiary treatment of wastewater

Abstract

An experiment with immobilized microalgal cells was carried out using high and low viscosity chitosan and konjac flour to enhance the stability of hardened gels during tertiary treatment of wastewaters containing a high concentration of phosphate salts (1 m Na 3PO 4). Mechanical properties as well as compression testing, moisture retention in gels and the uptake of nutrients, such as nitrogen and phosphorus, from artificial wastewater were studied in the immobilized gel state. The disruption due to chemical interactions of pure high and low viscosity chitosan or mixed gels with konjac flour was examined daily by microscopic counting of the total cells released in the medium during seven weeks. The removal of NH + 4-N and PO 3− 4-P was monitored during four nutrient starvation-removal cycles. A complete cycle consisted of starving immobilized cells for 48 h in air saturated at 100% relative humidity, followed by removal of nutrients from wastewater. The immobilization obtained from 2% (m/v) high viscosity chitosan gels showed a more significant chemical stability ( P ≤ 0.05) in medium with 1 m Na 3PO 4 that contained viable cells than low viscosity chitosan or mixed gels. Sodium pyrophosphate was the best chelating agent used for cross-linking and did not affect the diffusion of inorganic nutrients through the hardened gels or the microalgal growth inside the network of chitosan immobilized cells. After the second uptake, the elimination of NH + 4-N and PO 3− 4-P was 100% within 3 h, whereas after the fourth uptake, the nitrogen disappearance was 99% within 2 h and 100% for phosphorus with the same incubation time. After more than one month incubation of chitosan gel beads containing Scenedesmus bicellularis cells that had been soaked daily in a fresh 1 m Na 3PO 4 solution, the stability of hardened gels was not affected and no significant release ( P ≥ 0.05) of entrapped cells was observed in the medium.