Consulting engineers Gosling Czubak Engineering Sciences designed a new facility to serve the Haring township wastewater treatment plant (WWTP) — which has biochemical oxygen demand (BOD) in excess of 700mg/l and had phosphorus levels as high as 20 mg/l. US$900,000 in grant funds and $1m from private business were obtained from US Department of Agriculture (USDA) Rural Development for the 135,000 gallons/day oxidation ditch treatment facility in the northern Michigan township of Haring.
“We faced the challenge of high-strength waste and high-anticipated sewer bills due to the small user base”, said Mark Hurley, director of engineering at Gosling Czubak.
“Following the decision that Cadillac would no longer take in Haring’s wastewater, we had to get this built. We carried out a full study, asking bidders to hold their price for a year. When we had the finances in place, we went with Lakeside Equipment Corporation.”
There at the plant in the very early stages, working closely with the original construction crews, has been Infrastructure Alternatives (IAI), who provide contract operations, maintenance and management services for numerous treatment facilities across the State of Michigan.
Nathan Filley, IAI lead operator at the Haring WWTP, added, “Lakeside’s E.A. Aerotor Plant, which is suitable for small communities, has the components of a system of a larger scale, with extended aeration and mixing, combined with final clarification. Despite the ongoing challenges we face, it works for the flow and organic loading.
“At most of the municipal treatment plants that we manage, BOD levels are around a standard 150-200mg/l, so the average here at Haring of 700mg/l is more concentrated due to the small number of homes. With such a large amount of retail including Walmart, Home Depot and multiple fast-food outlets, our wastewater intake is unique.”
The Covid-19 lockdown created new problems for the majority of WWTP due to many people having to stay at home. At Haring Township, flows were more than cut in half. When the local fast-food outlets reopened, restrictions meant take-away only, so with such little flow coming through the plant, not enough mixed liquor was going into the clarifier, meaning no blanket at the bottom, upsetting the balance.
The low flow also meant that even when the plant’s rotors ramped down to their minimum speed, the dissolved oxygen (DO) was above its normal level of around 2.0mg/l.
Haring’s treated wastewater is discharged into the trout-rich Clam River. The small population means not much wastewater comes from the use of showers or washing machines, so the Lakeside designed rotors with the capability of providing high oxygen transfer to help with the high strength intake at the plant. Gosling Czubak also helped to manage the seasonal differences in levels that are brought on by the big changes in temperature during the year, according to Filley.
He also said that although the fast-food outlets are required to use grease traps, a fair amount of fats, oils and greases (FOG) still comes into the flow of 75,000 gallons/day. He added, “We usually keep one rotor on 24/7 and bring in the second rotor as a lag when the DO gets to around 0.25mg/l below the setpoint — and shuts off at 0.25mg/l above the setpoint.”
Lakeside’s E.A Aerotor Plant is constructed in a concentric wall design, with the outer loop utilising the closed loop reactor (CLR) process for extended aeration. The inner area incorporates Spiraflo Clarifier for final settling.
Eventually settling on cycles of 7hrs on 1hr off, IAI called in local engineers, West Michigan Instrumentation Systems to adjust control settings and add a timer. The change provided enough denitrification to stabilise pH levels at around 7.1-7.3 for most of the year.