Removal of TP, COD, and NH4+-N in Simulated Slaughtering Wastewater by Two Kinds of Immobilized Microalgal Spheres

The treatment of wastewater using microalgae is regarded as a green and potential technology. However, its engineering application has been largely hindered because of the limitation of microalgae separation and harvesting. Therefore, immobilization technology has been widely used to embed microalgae for wastewater treatment. In this paper, sodium alginate (SA) and polyvinyl alcohol (PVA) as the common immobilized carriers were used to immobilize ankistrodesmus falcatus for simulated slaughtering wastewater (SSW) treatment. The experimental results of the mass transfer and adsorption of immobilized carriers were found to show that the mass transfer of SA-SiO2 gel balls (SS-GB) was better than PVA-SA gel balls (PS-GB) and that the adsorption of PS-GB was better than SS-GB. When immobilizing microalgae with the two kinds of carriers, it was found that SA-SiO2 microalgal spheres (SS-MS) were better than PVA-SA microalgal spheres (PS-MS) for the maintenance of microalgal cell activity and that PS-MS were better than SS-MS for the resistance to biodegradation. This is because the carrier of PS-MS had a thick shell and dense structure, while the carrier of SS-MS had a thin shell and loose structure. The results of SSW treatment by PS-MS and SS-MS were found to show that the total phosphorus (TP) removal rates of PS-MS and SS-MS were 90.31% and 86.60%, respectively. This indicates that the TP removal effect of PS-MS was superior to that of SS-MS. The adsorption kinetics simulation showed that the adsorption of TP onto PS-GB was controlled by chemisorption and that the adsorption of TP onto SS-GB was controlled by physical adsorption. The chemical oxygen demand (COD) and ammonium nitrogen (NH4+-N) removal of PS-MS were 9.30% and 10.70%, respectively, and the COD and NH4+-N removal of SS-MS were 54.60% and 62.08%, respectively. This indicates that the COD and NH4+-N removal effect of SS-MS were superior to PS-MS. This is the result of the combined action of the degradation by microalgal cells and adsorption by the carrier.