How Low is Too Low for DO? Six Years of Unique Low-DO Operations Experience

The Fresno-Clovis Regional Wastewater Reclamation Facility (RWRF) found it difficult to meet effluent total suspended solids (TSS) and biochemical oxygen demand (BOD) requirements in 2001 due to a tremendous increase in organic loading to the plant. Attempts to minimize nitrification and subsequent clarifier denitrification using mean cell residence times (MCRT) of about 1.5 days exacerbated settling problems. In 2001 the plant operations staff embarked on a radical approach to improve effluent quality: use low dissolved oxygen (DO) concentration in the aeration tanks to minimize clarifier denitrification and to improve sludge settleability. Starting with sludge volume indexes (SVI) in the 400-600 mL/g range, the low-DO approach with higher MCRTs of 4-5 days reduced the SVIs to the 200-300 mL/g range within a month and to the 100-200 mL/g range over time. During operations under low-DO conditions, low-DO filamentous microorganisms were not a problem unless the DO was increased. While DO levels near 0.1 mg/L provided the lowest SVIs, settleability deteriorated as the DO approached 0.3 mg/L. This paper presents the data showing that, even though the hydraulic and organics loads increased at the RWRF, low-DO operation was successful in improving effluent quality over the six-year period from 2001 through 2007.