American Consulting Engineers Council of Vermont

  1997 Engineering Excellence Grand Award

CATEGORY D - WATER RESOURCES

WASTEWATER TREATMENT FACILITY

JOHNSON, VERMONT

BACKGROUND

The final completion of the Village of Johnson’s Wastewater Treatment Facilities (WWTF) is the culmination of five years of planning, design and construction to bring the facility into compliance with 1992 State and Federal water pollution control regulations.  When the original WWTF became operational in 1970, it was one of the first in the State to meet the new “secondary” level of treatment required at the time. Since 1970, the facility has served Johnson well.  However, new State legislation in 1992 required much stricter discharge limits on phosphorous, ammonia and residual chlorine.  At the same time, Johnson realized that it needed to increase the WWTF flow capacity and especially its ability to handle the high seasonal flows associated with storm events. Therefore, in 1992 the Village of Johnson began planning and  engineering studies which led to the upgrade of their WWTF to the become first municipal Sequential Batch Reactor (SBR) facility in Vermont!

TREATMENT PROCESS DESCRIPTION

Raw wastewater enters the WWTF in the Pre-Treatment Area, where it flows through a manual bar screen to remove larger objects such as rags. Flow continues through a 30 foot long channel that allows heavier grit to settle out and the influent flow-rate is measured in a 3" wide partial flume with ultrasonic level measurement. Next a sewage grinder grinds all of the solids into small pieces and the flow continues into the Influent Pump Station wetwell.  From there, the dual-alternating variable speed centrifugal pumps pump the flow into one of the two SBR basins for biological and secondary treatment.  The effluent from the  SBR process continues via gravity to the two ultraviolet (UV) disinfection units and over a compound effluent flow measuring weir. The treated effluent discharges into the Gihon River.

SBR TREATMENT SYSTEM

The SBR treatment system is the core of the Johnson WWTF treatment process. As part of this upgrade, the two 1970 aeration tanks were converted into the two SBR process basins.  Both basins are completely enclosed to minimize the negative impact of the cold winter climate on the treatment process while allowing easy access for operations and maintenance. Each SBR basin performs three sequential treatment cycles as follows:

Fill/Aerate - Settle - Decant

During part of the fill cycle, air is added for mixing and to encourage bacteriological growth.  During the remainder of the fill cycle, no air is added and the resulting anaerobic zone aids in biological phosphorous removal. During the settle cycle, the air remains off and the solids are allowed to settle in a perfect quiescent state.  Finally, during the decant cycle, the clarified wastewater is decanted from the basin by slowly lowering the baffled decant trough.

When the first SBR basin begins the settle cycle, flow is shut off to that basin and is directed to the opposite SBR basin to begin the fill/aerate cycle.  The two SBR basins continue to alternate in timed sequences.

Photo and award plaque