Reutilization of waste oyster shell as filler for filter for drinking water pretreatment: Feasibility and implication

Oyster shell (OS) is a kind of reusable resource that can serve as carbon source, biofilms carrier and basifying agent, suggesting it is an attractive filler option for biofiltration, but studies on its application in drinking water treatment are limited. In this study, one pilot-scale up-flow filter filled with OS media were designed to pretreat surface source water. Filter performance and biological functions were investigated to determine its application scope. The results showed that effluent pH increased and was stable around 7.5 due to the alkalinity provided by OS and its buffering capacity. High and stable removal efficiencies of turbidity (mostly >60%) were achieved. The removal efficiencies of NH₄⁺-N changed in a wide range (mostly <30%). TOC and UV₂₅₄ removal rate was low (<10%). The biofilms formation period took about 45 days. During this period, this filter mainly removed pollutants through adsorption by OS. High-throughput sequencing results showed that functional taxa did not play a key role after adsorption saturation in early operation period. Functional microbial taxa formed on the OS surface after long-term operation and NH₄⁺-N removal rate increased to some extent. Our results suggested that unburned OS filter can be used as rough filter for turbidity removal instead of coagulation and sedimentation process. Preoxidation, calcination of OS, mixed with other filler and are recommended to improve the performance if it would be used for biofiltration. This study provides an insight for the reuse of OS in drinking water treatment.