Applicability of Oxidation Reduction Potential Response on a Full-Scale Intermittently Aerated Suspended Culture System

Abstract

On-line monitoring of oxidation reduction potential (ORP) was conducted at the full-scale aeration basin operated with intermittent aeration schedules and fed with the wastewater generated from fruit fly rearing facility owned by the California Department of Food and Agriculture, Waimanalo, Hawaii. The characteristics of the wastewater were fluctuated, severely. The 90 percentile level of total chemical oxygen demand, total Kjeldahl nitrogen, and total suspended solids for the wastewater are 5,200, 142, and 3,350 mg/L, respectively. Two aeration schedules of 4 h aeration/2 h nonaeration and 3 h aeration/3 h nonaeration were investigated in this study. Typical ORP curve with bending points of β (beginning of denitrification), χ (disappearance of nitrate), and plateau (dORP/dt=0) were observed for both aeration schedules. The α (end of nitrification) value couldn’t detect in the aeration schedule of 3 h aeration/3 h nonaeration because of the ORP fluctuation at the onset of aeration. However, the α value detected without any difficulty for the aeration schedule of 4 h aeration/2 h nonaeration. This is due to the relatively lower gradient of the reduced substance concentration between supernatant and settled sludge layers during the air-off period. Through the ORP monitoring of the full-scale aeration basin treating fruit fly rearing facility wastewater, the aeration time can be reduced from 4 h to 3 h without deterioration of organic and nitrogen removal efficiencies. By reducing the aeration time, 25% of energy cost for aeration can be saved. From the result of real time monitoring of ORP response and water quality analysis of nitrogen, the behaviors of nitrogen forms are closely related to the ORP response. Therefore, the ORP monitoring is a practical and easy tool for the evaluation of the degree of nitrification and denitrification condition in biological wastewater-treatment systems.