Biofilm growth on PVC and HDPE pipes impacts chlorine stability in the recycled water

•A flow-through reactor comprising HDPE & PVC pipes was operated in a full-scale system.•There was no significant difference in biofilm formation on HDPE & PVC pipes.•Biofilm growth reached saturation after 13 weeks of reactor operation.•Biofilm played a significant role in decaying chlorine.•Organic chloramines were the major species of chlorine in the recycled water.