November 7, 2001 -- A US $10.2-million expansion completed recently at the Portage, Indiana wastewater treatment plant - including 30 energy-efficient submersible pumps from ITT Industries' Flygt unit - will support the community's growth while eliminating sanitary sewer overflows (SSOs) that had been occurring during heavy storm events.
Much of the plant's original infrastructure was preserved during the project which nearly doubled the treatment capacity and added off-line storage, a 560-bulb UV disinfection system and SCADA controls.
Portage, Indiana, with a population of about 31,000, is about 40 miles southeast of Chicago and within a mile of Lake Michigan. The plant supports a 150-mile long grid of sanitary sewers with 41 lift stations, 18" to 48" diameter trunk lines and 6" to 15" laterals. The system has approximately 12,000 residential connections and only a nominal industrial load.
Although the sanitary sewers are separate from the stormwater drainage system, a number of factors -- high water table, infiltration and sump pump connections in older subdivisions -- would combine to overwhelm the plant's wet weather peak capacity during some storm events. When this occurred, bypasses would flow downline into Lake Michigan. Although the expansion was already initiated, the EPA had notified the City that further bypasses would be prohibited after June 1, 1998.
The plant improvements increased the dry weather capacity from 3.5 million gallons per day (MGD) to 4.95 MGD, and wet weather peak management to 18 MGD. Because the 30" diameter pipe from the oxidation ditch to the new mixed-liquor station was too small to handle the increased capacity, the plant was converted from gravity to a forced-flow operation by installing a series of 27 ITT Flygt submersible pumps. Only ten of the energy-efficient pumps, which are equipped with variable frequency drives, run during a dry weather mode. The 2-HP to 65-HP units now maintain the desired flow velocity and agitation sought from the altered operating scheme.
During construction, three screw pumps at the headworks were also replaced with three additional, 45-HP ITT Flygt submersible pumps. The original screw pumps had failed repeatedly in previous years. In fact, repair costs reached USD 25,000 in l997, alone, when the decision was finally made to replace, rather than retrofit the obsolete units. As much as $100,000 had been spent on the pumps during the previous five years.
Stormwater diversion at the updated facility is now fast and efficient. It begins when two magnetic meters monitoring the flow of influent sense it will exceed the plant's treatment capacity.
The SCADA then brings on line one or more of three, 60-HP pumps that divert the stream into the 165' x 85' x 16' surface storage facility. After the flow subsides, the raw sewage diverted into this 1.5 MG concrete tank is pumped back into the plant's treatment process at a controlled rate. The labour-saving flushing system, only the second to be installed in a US plant up until that time, is then activated manually after the off-line storage is drained.
The wet weather flow management provisions now prevent plant bypasses but also the accompanying risks of washouts of the sludge blanket in the final clarifiers and the storage increases to settings between 1,000 to 10,000 GPM, the computer starts up the second and third pumps to manage the increasing flow. The reverse occurs while process biomass. Plans call for possibly adding another 1.5 MG off-line storage facility adjacent to the present holding vessel.
