One would think that with a population of just 1,200 people, wastewater treatment in the Michigan community of Baldwin would be pretty straightforward, but this village has taken a yo-yo of a journey for the past two decades, treating 75,000 gpd to 200,000 gpd, and back down again, and at various levels in between. For treatment plant operators, that has presented quite a challenge, and even now it appears the flow could go back all the way down again to 75,000 gpd.

The flow variability led to a significant buildup of grit in a force main feeding septage to the treatment plant. The facility team and its engineers solved the problem with a headworks upgrade that included a new screen and grit removal system.

Change of plans

Rural Baldwin attracts plenty of visitors for its world-class trout fishing and canoeing on the Pere Marquette River, protected by the Wild and Scenic Rivers Act. But the big differentials in population have come from the North Lake Correctional Facility, which at capacity has had up to 1,600 inmates and 400 staff. Under new Biden administration, it is expected to close — again — but time will tell.

In the late 1990s, the village brought in the Fishbeck engineering and construction firm to design the original 75,000 gpd sequencing batch reactor plant. But construction was well underway when everything had to change with the building of a prison, observes Jim Truxton, village president.

“At the time, it was a unique opportunity for the state to house the facility, and we couldn’t be blind to the fact that there would be significant federal grants towards upscaling our wastewater treatment plant as a solid investment for the future of Baldwin,” he says.

New revenue source

“Over the years, I doubt anyone could have foreseen the on/off, open/closed saga, but then, opinions and policies can change when new administrations are elected. During a long period of closure for North Lake, for around seven and half years, our treatment plant was running at only 30% capacity, so to generate some much-needed revenue, we took in septage.”

Septage delivered by trucks went into a primary pump station and was sent through a nearly three-mile-long force main that travels under an abandoned railroad and wetlands area before discharging at the treatment plant.

In early 2018, Fishbeck completed the design and bidding for an $8 million plant expansion, including an upgrade of the primary pump station, and proceeded with construction. Previously, there was too little flow to move the grit, which accumulated in the force main.

Dealing with grit

For the new headworks, Fishbeck and the village chose a Raptor Micro Strainer screen with a quarter-inch perforated plate screenings basket and a SpiraGrit vortex grit system, both made by Lakeside Equipment.

Dave Conklin, senior engineer with Fishbeck, states, “We had previously specified Lakeside headworks equipment based on its competitive pricing and performance. We also had excellent experience with the manufacturer’s representative, Dubois Cooper. Both businesses were highly involved in the startup of equipment and any troubleshooting. They also organized good-quality training and kept us up to date.”

The real fun began at Baldwin in March 2019 when, as part of the upgrade, new and larger pumps were installed to help move the legacy of grit, which was estimated to occupy two-thirds of the force main.

John Barthels, division director with Infrastructure Alternatives, which manages 130 wastewater treatment plants, observes, “Getting all this accumulated grit out of the line resulted in a huge amount of material for the Lakeside equipment to deal with, probably six to ten times the normal amount.”

The equipment proved robust in handling the material. “There was an amazing amount of grit,” Truxton says. “For the first six months of the installation, a 10-cubic-yard container was filled to the brim every week. Now after that initial wave of grit, the whole thing works just fine. All that is required is some periodic cleaning and basic preventive maintenance.”

Handling variable flows

Over a wide range of daily flow rates, the SpiraGrit system captures grit particles in a flat-floor circular upper chamber. Rotating paddles maintain the flow velocity in the chamber; grit is removed from the storage hopper by a self-priming pump to a Raptor dry grit washer.

Organics are separated from the grit particles by introduction of upwardly directed wash water and a grit stirrer assembly. The organics are automatically recycled back to the treatment process. The cleaned grit is removed via a grit dewatering screw and discharged into a dump container.

In the headworks channel, the Raptor Screen captures debris, and a brush on the central screw cleans the screenings basket and conveys solids up to the compaction zone and dewatering chamber. After the compaction zone, the dewatered material enters a discharge chute that directs it into a dump container at a lower level. The screen and grit equipment are constructed from Type 316 stainless steel for corrosion resistance.

The long haul

“One way or another, it has been quite a journey. With or without North Lake Correctional Facility, we now have a very professionally managed wastewater treatment plant with equipment that delivers excellent performance for the long-term. Plus, we have a team of highly skilled and conscientious professionals behind us all the way,” Truxton says.

“With our groundwater discharge and close proximity to a precious waterway such as the Pere Marquette, we rightly have to be careful with our treatment processes.”