Mount Carroll Wastewater Treatment Facility
Firm: MSA Professional Services, Inc.
Owner: City of Mount Carroll, Illinois
Description: Original or Innovative Application of New or Existing Techniques: The new Aero-Mod wastewater treatment system in the City of Mount Carroll utilizes some of the newest and most advanced wastewater technology available, one of only a dozen or fewer such systems within the entire state of Illinois. In fact, MSA designed the very first Aero-Mod facility in Illinois in the community of Port Byron, applying that prior experience to this much larger and more complex project in Mount Carroll. Aero-Mod is unique in that it incorporates clarifier and biological nutrient removal processes to treat municipal wastewater. This is done via activated sludge with an aerobic digester which blends microorganisms with the right balance of oxygen to treat effluent. The aerobic component is a welcome replacement for the community’s prior anaerobic digester facility which produced significant odors — a less-than-appealing trait for residents living nearby. The Aero-Mod system also more effectively helps municipalities like Mount Carroll meet National Pollutant Discharge Elimination System (NPDES) regulations and total maximum daily load (TMDL) limits for nutrients such as phosphorus and nitrogen which can adversely impact localized and regional watersheds, lakes and rivers. Mount Carroll’s new facility actually exceeds what is required of them via NPDES permit, as it includes unsolicited special provisions for total nitrogen and phosphorus removal, setting the stage for seamless future compliance should those nutrients be later limited and enforced.
Future Value to the Engineering Profession and Perception by the Public: >Modern engineering practices must evolve with our changing climate, so design for the new Mount Carroll facility needed to assess both prior and potential floodplain activity. Located in a severely flood-prone area and having endured significant 100-year storm events, project engineers knew they needed to go big. In response, the new wastewater treatment facility is located on higher ground, above the 500-year flood elevation as opposed to the conventional FEMA standard of just one foot above the 100-year mark. This project also answers to the needs and wants of the public. MSA held multiple Public Involvement Meetings to share project renderings, discuss alternatives and address questions. Public concerns ranged from aesthetics and odors to the possible devaluation of surrounding property. When the public was invited to an open house and tour of the new facility upon completion, the reviews were resoundingly positive — so much so that residents are now referring to the new complex as Mount Carroll’s Taj Mahal.
Social, Economic and Sustainable Design Considerations: Mount Carroll’s wastewater treatment facility is located adjacent to Carroll Creek, which ultimately empties into the Mississippi River near Savanna — a mere 10 miles away. This means that any treatment plant breach, malfunction or flood-initiated discharge threatens the health of watersheds both local and far-reaching — ultimately, channeling to the Gulf of Mexico. Upgrading and relocating this facility above the 500-year flood elevation is one small solution contributing to positive, wide-ranging repercussions. The facility is energy efficient, low maintenance and less polluting of both water and air, built of pre-cast concrete, with a steel roof and LED lighting throughout. The blowers used to aerate the activated sludge have high efficiency motors and probes that measure oxygen levels in order to regulate blower speed and run time. When oxygen levels are well balanced, the blowers automatically shut off, saving energy and lifecycle hours.
Complexity: Location within a designated floodplain posed a challenge. MSA stormwater engineers conducted a comprehensive study, including using a program called XP-SWMM to construct 2D models to define flood zones and assess potential drainage within proposed new sites. After selecting the right site, extremely poor soil conditions were discovered. So poor, that in order to reach suitable load-bearing soil, the project team needed to excavate nearly 10 vertical feet of soil across the entire 4-acre site. In addition, the soils were quite wet and required massive dewatering, employing 8-10 wells to continuously pump groundwater out of the project area. Due to large volume of poor soils, 25 additional vertical feet of engineered fill was added to achieve the designed elevations and get the Headworks Building foundation sufficiently above the floodplain. Just as construction commenced, COVID-19 struck. Up against an unexpected shortage of materials and labor, with parts needed to make some of the treatment equipment now unavailable, the project team organized and moved forward. They closed the project site for disinfection, added on-site washing and sanitizer stations, routine crew temperature checks, mask requirements, and made other process adjustments to accommodate.
Exceeding Owner Needs: When asked for a candid assessment of the City of Mount Carroll’s prior wastewater treatment facility, engineers report it was “still standing by sheer luck,” with creative MacGyver-style adaptations and a fair amount of J-B Weld keeping it together. The treatment pumps and shafts were breaking, brick façade exterior crumbling, digester barely functioning, and equipment parts — many of them original — so old that replacements simply didn’t exist. As a result, the treatment process was insufficient and there had been multiple non-compliant discharge events in recent years. The community needed a new facility, and it needed to be resilient, easy to use and built to last. Enter: Aero-Mod. Treatment includes influent pumping, screening, grit removal, aeration, clarification and UV disinfection, with a flow equalization tank to allow for temporary storage of any high-flow water events. All sludge produced is handled by aerobic digesters and sludge storage tank, after which a belt press dewaters the sludge into a cake consistency, which can then be repurposed by local farmers as a beneficial fertilizer input. The new system is sophisticated, non-odorous, aesthetically pleasing, and provides the owner with all the amenities needed, plus a four-bay garage and workshop area, shared office/lab, open space area, bathrooms and utility room — all fully ADA accessible, with room for future growth and expansion. It also was designed for easy use, with the ability to collect water samples from inside the Headworks Building rather than going outdoors. Samplers pull water from the treatment process directly into the lab for testing through a custom channel. The controls are fully automated as well, with remote online access, allowing staff to easily create and file electronic reports, ascertain machinery start and run times, track system alarms, and manage equipment maintenance needs based on hours in operation. The project engineering fees were less than projected and the community is ecstatic with their new facility — an $11.4-million investment financed through the Illinois Environmental Protection Agency (IEPA) with 45% principal loan forgiveness, which greatly reduces the burden on the City utility users. The City of Mount Carroll celebrated with an open house and tours of the new facility in June, 2021.
City of Mount Carroll, Illinois