Replacing an Old Friend: A Wastewater Treatment Plant Case Study

Clarinda, Iowa This plant has provided treatment for Clarinda for over 50 years. It is a trickling filter plant, so it has no nutrient removal capacity. In addition, the new NPDES permit requires disinfection. Several old buildings no longer meet the National Electric Code and will be rehabilitated or replaced. Parts of the plant have served since the 1920s or 1930s, so some equipment is far beyond its useful life.

Pretreatment- grit chamber, influent,screen, pumps

There are structural deficiencies in several tanks, including the trickling filter and primary clarifier. The existing pumping systems do not have enough pumping capacity and the treatment systems do not have capacity to consistently meet effluent requirements. Operators cannot monitor the amount of flow going into or out of the equalization basin, which is a problem for reporting purposes. The systems do not have sufficient redundancy, particularly in the pumping and clarifiers.

Existing treatment – primary clarifiers and parallel trickling filters
Existing treatment – final clarifiers, pump station, anaerobic digestion

The pretreatment system includes a 3.6 million gallon (MG) earthen equalization basin, submersible pump station, ¼” step screen, manual screen, and 2.5 MGD grit chamber with self-priming grit pump. The treatment system includes a primary clarifier, trickling filter, biotower, and two final clarifiers. An anaerobic digester and Imhoff tank for sludge treatment are part of the overall system.

Sludge storage and flow EQ


The City hired FOX to determine the best way for their system to adequately serve the community for many years. A facility plan evaluated repair and expansion of the existing system as well as replacing the existing trickling filter with activated sludge, an oxidation ditch, or a sequencing batch reactor (SBR). In addition, the facility plan evaluated both submerged and non-contact ultraviolet (UV) disinfection systems.

The plan includes projected flow and loads for the system as shown in the Tables below.


The existing system is a standard trickling filter with rotary distributors and plastic media; it needed to be expanded to increase the treatment capacity and redundancy. This process will not treat for nutrients, which means that a denitrification filter would be required to meet all treatment needs.

Options – Aero-Mod, Oxidation Ditch, SBR

FOX evaluated several treatment systems. The AeroMod process is an activated sludge system consisting of a selector zone, continuously aerated nitrification tank, intermittently aerated nitrification/denitrification tank, rectangular clarifiers, and digesters in a common wall configuration. The return activated sludge is sent to the selector, which promotes phosphorus removal.

The oxidation ditch system consists of an anaerobic zone, anoxic zone, and aerobic zone in a common wall configuration. Separate circular clarifiers are used for solids separation. The tank configuration allows the mixed liquor to flow from the aerobic to the anoxic zone for nitrogen removal. The return activated sludge is sent to the anerobic zone to promote phosphorus removal.

The SBR system uses time instead of tanks or zones to create the anaerobic, anoxic, and aerobic conditions to meet the treatment objectives for the plant. The aeration and mixing system will be turned off in order to allow the sludge to settle in order to clarify the waste. A decanter is used to discharge the treated water.

(The engineers at FOX were responsible for the first custom-designed, full-plant-scale SBR wastewater treatment facility in the U.S.)


Three UV disinfection systems were evaluated. A traditional UV system uses horizontal bulbs submerged in the water. Inclined systems use angled bulbs submerged in the water. The non-contact UV system is an enclosed unit where the water flows through clear tubes with UV bulbs around them.

(The engineers at FOX designed the first in-channel UV system for wastewater disinfection in Iowa.)


As part of the facility planning process, FOX Engineering used a two-pronged approach for selecting the best technology for this project – a monetary evaluation and a non-monetary evaluation. This approach allows our engineering staff to partner with plant personnel to determine the best option.

The monetary evaluation examines the estimated costs for construction, power, labor, chemicals, sludge hauling, and equipment replacement over a period of twenty years. Each system was ranked based on their total life-cycle costs. For Clarinda, expanding the trickling filter without incorporating nutrient removal was the most cost-effective. If nutrient removal was required, the SBR was the most cost-effective. The UV disinfection systems had similar costs.

The non-monetary evaluation includes rankings for ease of implementation and operation, risks to the effluent quality, safety, impact of future regulations, flexibility and operator control, use of proven technology, and space requirements. While each had strengths and weaknesses, the three biological treatment systems scored the same. The non-contact UV scored highest among the disinfection options.


Final Choice – SBR and non-contact UV

The selected option was a new SBR treatment system with a non-contact UV disinfection system. This will include upgrades to the pretreatment system, replacement of the biological treatment system, installation of a new UV system, and upgrading the digester system.

For pretreatment, new equalization pumps were needed to meet flow requirements. A bypass and return flow meter will be installed with replacement valves to allow more control of the flow to downstream equipment. A new headworks building will house the existing screen and grit pump to save costs as well as a new vortex grit removal system, grit classifier, and screenings wash press.

The biological treatment system will be completely replaced with a four tank SBR system. Fine bubble diffusers will provide aeration, and submersible mixers will provide mixing to drive denitrification and enhance phosphorus removal. Stainless steel decanters will discharge treated waste to the downstream systems.

Treated waste will be sent to a non-contact UV disinfection system. There will be no equalization after the SBR, so the UV will be designed to treat the maximum decant rate. While this increases the cost of the UV, removal of the tank, level sensor, and cover saved cost overall.

The existing anaerobic digester and one of the clarifiers will be converted to aerobic digesters. The existing Imhoff tank will be replaced with a new sludge storage tank with a side-mounted mixer and sludge loadout pump. New pumps will provide sludge wasting and transfer sludge from digester to digester or digester to sludge holding tank.

Design is complete and was bid September 2019.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.