Algal odorous risk assessments in pilot-scale drinking water distribution system (DWDS): Risk occurrence, influencing factors and degradation patterns

Abstract

Tap water quality is at risk for odor-producing algae. Besides being hazardous to source water and water treatments, these algae threaten drinking water distribution system (DWDS). Yet little information is available for odorous risks occurring, responding, and transforming in real DWDS. Hence, two algae (Microcystis aeruginosa, MA; Pseudanabaena sp., PS) were selected for pilot studies. Two ways were verified for algal occurrence, including insufficient water treatments and DWDS-biofilm attachment. MA survived moderate disinfectant exposures and restored odorants (β-cyclocitral). Also, MA could be attached to biofilm and its detachment occurred upon flushing (0.60 m/s). These occurrence ways were not seen for PS. Three factors were compared for living algae in DWDS. Flow velocities had minimal impacts on MA but affected PS odorant (2-methylisobropanol, 2-MIB) releases. Releasing rates increased by 115.79% with velocity from 0.05 m/s to 1.50 m/s. Disinfectants were the riskiest, maximizing risks for MA and PS, respectively at 0.30 mg/L NH2Cl and 1.00 mg/L Cl2. Coexisting antibiotics showed selective effects and reduced risks, e.g., tetracycline reduced MA intensity and PS odorant production. For naked odorant transformation, 2-MIB in DWDS had reduced degradation ratios compared to beaker conditions. Overall, odorous risks by MA and PS were different. MA was tough and could resist water treatments and adverse stressors in DWDS, with partial odorants remaining within cells. PS was vulnerable to treatments and DWDS, and the released odorants were not degradable. Hence, customized management is required for various algal odorous problems in DWDS.