McMinnville Wastewater Treatment Plant

The 5.6 MGD US Filter/Envirex Orbal plant at McMinnville, OR, which started up in 1996, has both the City and the consultant, CH2M Hill, giving it good reviews.  Designed for an effluent phosphorus concentration of 0.07 mg/l and an effluent ammonia of 0.5 mg/l, the Orbal has achieved all of its target goals (and more) even while handling unexpected loads that were several times higher than the design load.  Effluent phosphorus out of the secondary clarifiers following the Orbal is consistently in the range of 0.2 – 0.5 mg/l range without chemical addition, while the effluent phosphorus following the reactor clarifiers and filters is below 0.05 mg/l with only 35 mg/l of alum being added (less than 1/4 of the design projections). Effluent ammonia levels are consistently below 0.1 mg/l while nitrates are also exceedingly low – averaging about 2 mg/l during December without any internal recycle or methanol additions.  What makes these figures outstanding is the relatively short SRT (solids retention time) used with the Orbal.  Simultaneous nitrification-denitrification is occurring with only a 6 – 8 day SRT and with the more than 80% of the total reactor volume maintained under anoxic (near zero DO) conditions. Does keeping a large portion of the reactor volume near zero DO result in poor settling sludges and high SVIs?  No, the opposite is true.  Typically, the summertime SVI at McMinnville is near 50 and the mixed liquor is almost completely void of filaments. This is CH2M Hill’s first Orbal plant in the Pacific Northwest. With the outstanding performance of the Orbal in so many key areas,  we look forward to more Orbal plants in the near future. The plant also features Continuous Backwash Filters for final effluent polishing to ensure the low phosphorus levels are met at all times. The McMinnville facility is also home of the first Fuchs ATAD (Autothermal thermophylic aerobic digestion) for production of Class A biosolids in the Pacific Northwest.  The system has been on-line since February, 1996 and has been providing Class A  biosolids from the start.