Improvements at an Indiana treatment plant and collections system save operating costs and forestall construction of a new plant.
A simple change in effluent filters at the wastewater treatment plant is saving the Indiana city of Crown Point money, time and labor while improving effluent quality. The city is also taking steps to reduce stormwater flows and inflow and infiltration into its combined sewer system, helping delay the $30 million cost of a new treatment plant.
“Our original tertiary filters had gone beyond their useful life,” says Chris Previs, plant superintendent. “They were 30 years old, and needed to be replaced.” Switching filter technology is projected to save $70,000 a year on operations and maintenance.
New disc filters
Commonwealth Engineering oversaw the replacement of the plant’s five pressure filters with three Hydrotech disc filters (Veolia Water) in summer 2015. The project had a snowball effect in the plant because the reduction in pressure needed for the filtration also made it possible to feed the filters by gravity instead of using a series of pumps. Because, unlike the old filters, the disc filters don’t need backwashing, the flow returning to the headworks from the filters has been reduced by 300,000 gpd.
“We eliminated five 30 hp pumps, took out a 200 hp backwash pump, and removed a 15 hp surface wash pump,” says Previs. “We’re not doing maintenance on those seven pumps. The old filters also had seven valves each that we had to operate during normal flow and backwash, and all of that is gone. So that frees up some time.”
It took significant demolition and concrete work to make the effluent channel about a foot deeper to enable gravity feed, but that ended up being an economical choice. Upgrading to a new pressure filtering system would have cost about $400,000 for pumps and piping around the plant; deepening the channel cost just $50,000.
The new filtration system also reduced electricity usage by 60 percent, earning a $29,640 energy efficiency rebate from Northern Indiana Public Service Company.
The filters also improved treatment performance, helping protect water quality in Lake Michigan. “Our effluent TSS is right around 1 ppm,” says Previs. “We’re allowed 10 ppm, and were able to meet that limit most of the time. But with the filters not working as well as they should have, we were violating our TSS limit during some rainstorms.”
The $4 million project was funded with a low-interest loan from the State Revolving Loan Fund through the Indiana Finance Authority. It also included a new mixer on a digester and a new sludge pump.
Keeping clear water out
Meanwhile, due to a history of combined sewer overflows, Crown Point faced the prospect of separating its storm and sanitary systems and building a new wastewater treatment plant. The current plant averages 3 mgd flow; it has a design capacity of 5.1 mgd and a peak capacity of 8.1 mgd.
The city has completed a number of projects to reduce I&I and stormwater flows. Those include repairing cracked sewer lines, improving stormwater storage ponds, replacing missing and damaged manhole covers, and eliminating illegal culverts that were contributing to runoff entering the sewers.
“We have two basins we can fill with stormwater,” says Previs. “We’re reconfiguring them so we can use both at the same time. We’re also increasing our pumping capacity into them so we can keep flows in the plant down and reduce the amount we put out during storms.”
The basins provide 8 million gallons of storage; stormwater is retained in them for later treatment under normal flow conditions. The final cost of the CSO improvements hasn’t been determined, but the city expects to save about $16 million over the $36 to $40 million cost to separate the sewers and build a new plant.