Mechanical Screen

Wastewater first goes through the mechanical screens to removed the larger materials such as sticks, rags(paper such as toilet paper), large solids and other debris.  The materials are compacted and the water is returned for treatment.

Grit Chamber

The water leaves the screens and goes into an aerated tank that keeps the organic material suspended and allows heavier materials to sink to the bottom.  The grit is removed and the water continues to the primary clarifiers.

Primary Clarifiers

The primary clarifiers allow the final suspended solids to sink to the bottom and keeping the soluble organic materials to move on to the aeration basins.

Aeration Basins

The aeration basins are filled with micro organisms the break down the soluble organic materials in the water.  The water is filled with oxygen to aerate the basin to remove the ammonia and keep the micro-organism in the water health and alive.

Secondary Clarifiers

The secondary clarifiers remove the the micro-organisms from the wastewater.  The live and active organisms are returned to the aeration basins and the dead are sent to be treated.


The last step for treatment is the water will go through Ultraviolet (UV) light reduce the bacteria levels in the water to lower than naturally found.  The water is then set to Milford Creek and on to the Little Sioux River.


As the wastewater enters the IGLSD WWTF, two mechanical screens remove sticks, rags, large solids and other debris that would be harmful to downstream equipment.  The screenings are deposited in a washer/compactor to remove organic material and are dewatered prior to final disposal.  Following screening, the wastewater flow is measured through a Parshall flume.  An automatic sampler withdraws a sample for testing the influent wastewater.

Downstream of the screening process, grit and sand are removed by two aerated grit removal units.  As the wastewater flows through these units, air is diffused across the lower portion of the tank.  The rising air bubbles allow the lighter organic material to remain in suspension while the heavier grit and sand settle to the lower hopper.  The collected grit is pumped up into a grit washer/dewatering unit. Similar to the screening washer, the grit is washed to remove organics and the material dewatered before being deposited into a dumpster for final disposal at the landfill.

Following screening and grit removal, the wastewater enters a flow splitter where flow is evenly divided to the primary clarifiers.  The three 50-foot diameter clarifiers are used to settle the heavier organics out of the wastewater by gravity.  Any floating material is removed as scum by a skimming arm.  The clarifiers are designed to accommodate ferric chloride addition (to accomplish phosphorus removal) if needed in the future.  A fourth primary clarifier tank was constructed but the clarifier equipment was not installed at this time.  It is currently used for holding waster activated sludge prior to thickening.  This tank will be converted to a primary clarifier when needed in the future.

After primary clarification, the wastewater is split again and evenly distributed to the aeration basins. While in the aeration flow splitter the wastewater is combined  with return activated sludge (RAS) from the secondary clarifiers. The aeration tanks are rectangular with fine bubble diffused aeration equipment at the bottom.  Curtainwall baffles are located at the upstream end of the tank to develop and maintain the optimum environments to grow the beneficial microorganisms ("bugs") that consume the pollutants in the wastewater.  The fine bubble diffused aeration system includes circular diffusers with flexible membrane covers.  Air is blown into the diffuser and out small pores in the membrane to create a very fine bubble.  This design efficiently transfers oxygen to the bugs and also serves to completely mix the aeration tank.

Air is supplied by four large blowers located outside.  The blowers are controlled to maintain a target level of dissolved oxygen in the tanks. Varying the speed of the blower provides only as much air as needed by the system and minimizes energy use. The aeration blowers are enclosed in an insulated housing to reduce noise.

Wastewater passes out of the aeration tanks into a secondary flow splitter where it is equally distributed to the three secondary clarifiers.  The secondary clarifiers are used to settle out the remaining biological solids.  each of the 70-foot diameter secondary clarifiers includes a spiral sludge collector, energy-dissipating inlet, baffling and scum skimming.  Space for another tank remains if future conditions dictate the need for a fourth clarifier.

The IGLSD service population increases dramatically during the summer months due to the influx of visitors to the area. This results in high wastewater loads that the facility may not see during the other times of the year.  Additionally, the facility was designed to handle a peak flow event caused by locally heavy or prolonged rains.  In order to provide more flexibility to treat this peak flow or load, the facility was designed to flow equalization (EQ).  A concrete daily EQ tank holds smaller quantities and an earthen peak EQ basin is sized for overflow from the daily EQ tank during larger events.  All of the influent wastewater goes through the screening and grit removal prior to equalization.  The plant's control system uses a modulating value and flow meter to  "shave" off the excess flow above 7.2 mgd for equalization.  When influent flows subside, submersible pumps return the stored wastewater to the primary clarifiers at a controlled rate.

The facility design also includes an ultraviolet (UV) disinfection process to meet the disinfection requirement anticipated in the facility's new NPDES permit.  Disinfection reduces pathogens such as E.coli bacteria which can be harmful to humans.  The original chlorine contact tank was retrofitted to accommodate UV disinfection.  Two UV modules were installed with space for a future module. Disinfection is anticipated to be required from March 15 through November 15.  The use of UV eliminates risks associated with chlorine storage for chemical disinfection.  The final task is to sample the treated wastewater and document compliance with permit requirements.