Getting Ready for Winter Construction in Atlantic, Iowa

10 Atlantic Update.pdf

The construction phase of this project continues to be on schedule!

Most of the new cast-in-place concrete structures have been poured.  The general contractor, Hawkins Construction, has done a great job of scheduling the formwork and rebar setter crews ahead of the regularly scheduled concrete pours. 

The SBR structure was constructed in 21 separate concrete pours.  A frequent visitor to the site was the concrete pumping truck, as seen in the first photo (left, this page).  In this photo, the final third of the main SBR base slab was being poured.

By October, the remainder of the exterior SBR tank walls were completed.  The temporary dike that protected the excavation from flooding was excavated and the soil materials were used as backfill up against the exterior walls of the SBR tanks. 

The second photo (left, this page) shows the SBR site as viewed from the east at the end of October.

 The masonry subcontractor, Jim McLaughlin Construction of Manning, Iowa, began work in September and made quick progress on the masonry walls of the Preliminary Treatment Building.  By the end of October, the masonry on this building was completed and the roof was installed – ready for winter.

Hawkins began pouring and hand forming the grout infill mix inside the raw wastewater wet well.  (Photo at left)

The essential feature of this wet well design is that it is self-cleaning.  The steep sides direct solids into the pump intake channel so that they will be pumped to the next step in treatment.  A wet well without these sloped sides would allow solids to collect in the corners, which requires operator entry to clean out. This wet well does not even have an access ladder since the operators should never have to enter the wet well.  The submerisble non-clog sewage pumps in this wet well are raised by a motorized hoist when they require maintenance or repair.

The photo to the left shows the shoring and forming effort in progress for the cast-in-place concrete top to the SBR Flow Equalization Tank.  Previous SBR designs did not cover the flow equalization tank and it was found that algae grows in the effluent and can foul the UV equipment during the warm months.  To control algae, the flow equalization tank following the SBR is covered.  During design FOX estimated that a concrete cover was the most cost effective and durable solution for covering this tank. 

In the photo to the left, a grout topping was applied and leveled the SBR tank floors.  The purpose of the grout was to accomplish a gentle slope to the floor to assist operators when cleaning out a tank drained of its wastewater.  Hoses  are used for washdowns and a gentle slope helps to eliminate pools of water and sludge from remaining.  The rectangular box-outs at the bottom of the far wall allows wastewater to pass from the pre-react zone of the SBR to the main reactor tank.  This feature, coupled with the relatively high length to width ratio of the reactor, enables these SBRs to operate with continuous influent flow and maintain the essential part of the SBR cycles (fill, react, settle, decant, idle).

The photo also shows shoring and form work needed to pour the elevated walkway, that runs along the long side of the tank. 

 The photos to the left show the two-story tall automatically cleaned screen. The screen is designed to remove all particles ¼-inch and larger from the raw wastewater immediately after it enters the Preliminary Treatment Building.

The screen catches the particles in the screen (lower picture) and then a rake periodically removes the solids from the screen and transports the solids to the upper level and dumps into the screenings washer (behind the screen in the upper photo).

The screenings washer uses treatment plant effluent to wash the organics out of the solids and return them to the wastewater to be treated.  The washer also dewaters or “wrings out” the solids and pushes them into a dumpster for landfill disposal. 

The washer is a new feature for Atlantic.  The existing plant only had a dewatering screw, which did not wash the smelly organics out of screenings and grit.  This problem required the staff to spend an excessive amount of labor and fuel for removing the smelly solids from the headworks dumpster and haul to the landfill themselves. 

At the landfill, staff was required to empty the solids at a special location, separate from the main dumping location. With the new treatment works, staff will no longer haul screenings and grit, since solids will be clean enough to be hauled by the local solid waste hauling company. 

This allows staff to work on tasks such as inspecting and maintening sewers and pump stations – a direct benefit to the citizens of Atlantic. 

The new outfall has been installed and final grading completed (photo at left). As you can see, the location where the treated water enters the river has not changed the main channel of the river nor its natural bank.  A tree was removed from this part of bank, which was unavoidable, but the slope will be protected to prevent washouts should flooding occur prior to the bank becoming vegetated again.  There are still large, mature trees on either side of the outfall.

Inside one of the SBR Tanks, installation of the aeration equipment, which consists of PVC pipe headers, PVC air supply distribution piping, and the fine bubble diffuser assemblies (seen at left), has begun. The rest of this equipment will be installed and protected inside the empty tanks over the winter.  

 The SBR decanter equipment has arrived on site and installation of this equipment has started. These stainless steel assemblies are dunked below the surface of the wastewater in the SBR during the decant phase, which follows a settling period. 

One can see at the top of the photo that there is a collection trough on each decanter that allows clarified effluent to flow into the decanter assembly. The vertical tubes transport the effluent to a header which is the horizontal pipe in the photo.  This header is piped to the SBR Flow Equalization Tank.  These decanters are scheduled to be installed before winter.

The buried air supply piping was installed below the Blower Building and the main floor slab was poured.  As seen in the photo on the left, the masonry subcontractor has started work on constructing the walls of the Blower Building, which will house the blowers that supply air to the SBR tanks and most of the electrical power gear for the process equipment on the west side of Sunnyside Lane.

The photo above shows the Preliminary Treatment Building from the west, or the existing plant’s main entrance.  The new concrete pavement driveway has been poured and this site is almost ready for winter.  Temporary wood doors and heating will be added to the building to keep it above freezing and allow work inside to continue.

For those who are only recently following this project, the city of Atlantic entered into a contract with Hawkins Construction, of Omaha, in early December 2010.  Work began onsite in March 2011 and the plant is scheduled to be operational by November 2012.  The new Preliminary Treatment facilities consist of an open channel that contains a fine screen and stacked tray grit removal.  The preliminary treatment effluent is followed by raw wastewater self-cleaning wet well that houses the process and storm water pumps.  A 7 MG Flow Equalization Basin works in concert with these pumps during storm events that bring 15 times the normal flow rates. Four SBRs provide secondary treatment, and the final treatment process is UV disinfection.  Between the SBRs and UV channel is a covered equalization tank that optimizes UV dosing and power consumption.  The solids treatment processes include aerobic sludge holding tanks and reed beds.  The reed bed is a natural treatment process that will use less energy than the plant’s existing anaerobic solids digestion process or a comparable aerobic solids digestion process. 
 
Contact:
FOX Engineering Associates, Inc.
Rob Baker, P.E.
800.433.3469
www.foxeng.com