Posted: 1/10/06
Brownsville to upgrade sewer plant
By David Heiller
Argus News Editor
The City of Brownsville is moving forward with plans to overhaul its sewer treatment plant.
The plant is operating at capacity and is reaching the end of its 20-year life expectancy, mayor Tim Serres explained at the January 4 city council meeting.
ìIt is ponding and basically we want to get ahead of the thing,î he said.
Brownsville has undergone significant growth during the 19 years that the plant has been operating. The facility had about 160 users when it was built in 1987-88, and here are about 230 users now, according to city clerk Mike Moriarty.
The sewer is currently in compliance with the Minnesota Pollution Control Agency, Moriarty added. ìBut itís functioning at capacity and the town is continuing to grow,î he said.
Davy Engineering Co. of La Crosse, WI, presented five options to the city council at its December meeting. The recommended one is to replace the sand in the existing filter.
The present filters have had ponding problems fore more than 10 years, according to the facility plan that Davy Engineering prepared. ìIn other words, wastewater does not percolate down through the media as it should,î the report states. It adds that there is a a ìmore or less permanent water surfaceî on the filter, and there is evidence that wastewater has overflowed the liner.
Workers have tried to improve the percolation with a technique called soil fracturing, which involves driving small diameter probes into the soil, but it didnít solve the ponding problem, the report states
Part of the problem is that the sand media is not the proper gradation. The specified media was too expensive and river sand was used instead, according to the facility plan. The river sand is relatively fine with a low percentage of void spaces and thus has lower hydraulic conductivity, the report states.
Replacing the sand filter has an estimated cost of about $920,600. Moriarty said it would be a better option than putting in a new filter west of the current one because the ìfootprintî of the plant is already in place.
The project would be done in two stages so that half the filter could remain in service while the other half was replaced. Annual operation and maintenance is estimated at $45,700.
At the current plant, waste water is pumped from two 20,000 gallon holding tanks and a dosing wet well to the filter, which is over a excavated area lined with impermeable PVC.
The filter consists of three layers: rock on the bottom; 2-1/2 feet of sand, and a two feet top layer of sand and gravel. Water moves through the top layer in tubes, then is forced down through the sand and rock. The clean water is then discharged to the Mississippi River.
The old sand that will be removed will be stored on site and dried, then taken to an approved sludge disposal site.
The ponded wastewater, which is estimated to be about one million gallons, could be hauled to another facility at a cost of about $40,000, or stored in a temporary lagoon, which would cost $50,000. It also might be possible to scrape the top few inches of sand from part of the filer and have the ponded water filter down through the clean sand. A core sample is needed to see if this will work. The city council approved that expenditure at last weekís meeting.
A larger holding tank will also be installed in addition to the two existing settling tanks. ìThe city has outgrown its holding capacity,î Moriarty said.
There will likely be improvements with the new system. The filter medium is small gravel instead of sand, so there is less clogging and better aeration. Also, the distribution piping will be above ground, so the sun can help evaporate the liquid. The material below can also be tilled to restore its permeability, something that is not possible now.
A public hearing on the plan will be held in February or March. The city will then apply for a revolving loan fund to help pay for the project.
Here is a tentative timeline for the project.
ï March: Facility plans due at MPCA;
ï May: Requests for placement on project priority list due at MPCA;
ï June: Requests for placement on Intended Use Plan (IUP);
ï July: Facility plans approved by MPCA for IUP projects.
ï Fall of 2006: Prepare plans and specifications.
ï 2007: construction of new plant.
The cost of the facility plan from Davy Engineering was $40,000. It was authorized by the city council in December 2004, which is when Davy Engineering started working on the project.
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