Home>City>Wastewater Water Pollution Control Facility History of Treatment, Pretreatment, SBRs, Digestion, Flow Equalization, Dump Station, Expansion, Plant Staff

PLANT EXPANSION! Our water pollution control facility is now undergoing a multi-million dollar expansion.
Click here for construction photos.

The History of North Liberty's Wastewater Treatment
The City of North Liberty was incorporated November 10, 1913. The population was approximately 190 and out houses were the primary means of sewage disposal.

The population dipped to 150 in 1930 and doubled to 300 by 1940. Septic systems were installed mid-century. These systems broke down sewage with naturally occurring bacteria.

In 1960 the population was 334, and the city initiated a study to determine the feasibility of city-wide sewage treatment. In 1965 a letter written to the Department of Health, Education, and Welfare stated, "Existing septic tanks and soil absorption systems are grossly inadequate because of clay and gumbo soil conditions. Sewage is discharged into street ditches creating a local nuisance and health hazard. Final disposal is into Muddy Creek which discharges into the Iowa River less than a mile upstream from the Iowa City city limits."

In 1967, with a population of 782, the City of North Liberty installed a collection system to transport sewage to a Trickling Filter facility designed to process 125,000 gallons per day. By 1978 the facility was processing 290,000 gallons per day and a larger facility was designed.

In 1981, with a population of 2,046, a Rotating Biological Contractor facility began operation. This facility was designed to process 450,000 gallons per day on a monthly average and 1,125,000 gallons per day maximum flow. Sewage flowed through turning plastic rolls on which a biological mass grew. This mass used the sewage as food and cleaned the liquid for transfer to Muddy Creek.

In 1998, with a population nearing 5,000, a new treatment facility was designed to handle the increasing sewage flow. On November 23, 1998 the new facility became operational. A Sequential Batch Reactor or SBR process was the treatment method chosen. Treatment begins with 35 miles of pipe, which transports the sewage to the treatment facility from homes, businesses, and industries.

Sewage Pretreatment
Fine Screen Removal - Large inorganic solids are removed from the sewage prior to pumping.

Grit removal - Small materials such as sand, gravel, and eggshells are removed to prevent pump damage and accumulation in the process tanks.

Pumping - Some of the sewage flows to the facility by gravity. The rest is pumped by seven lift stations throughout the city. Once the sewage reaches the pretreatment building it has to be pumped or lifted up to the process tanks. Three self-priming pumps deliver the pretreated sewage to the sequential batch reactors. These pumps are each capable of pumping 2,000 gallons per minute. The pumps are computer-controlled and alternate automatically throughout the week.

Sequential Batch Reactors - SBRs
Two 575,000 gallon SBRs process the raw sewage. The SBRs clean the sewage through a series of processes.

1. Mix Fill - a large mixer combines incoming raw sewage with the biological mass in the SBR.
2. React Fill - the mixer continues to stir the contents. Aeration adds oxygen from a series of blowers. The oxygen brings the biological mass from a dormant state to a food-consuming active state.
3. React Cycle - during this phase the raw sewage flow ceases and is transferred to the second SBR. The biological mass now breaks down the remaining sewage and consumes it as food.
4. Settle - the mixer and blower shut down and the biological mass begins to settle. The biological mass continues to use oxygen and goes dormant when all the oxygen is consumed.
5. Decant - After settling occurs, a clear liquid is left to be discharged to the disinfection system.
6. Sludge Wasting - At the end of the process cycle a percentage of the biological mass is pumped to the digesters from the SBRs. This keeps the amount of biological mass under control.
7. Disinfection - This is achieved by the flow of clear liquid through banks of ultraviolet lights. This disinfection system was the first of its type to be put on line in the Western Hemisphere. Ultra-violet disinfection poses no threat to the receiving stream environment.
8. Discharge - the completely processed liquid is then transported to Muddy Creek via a 24-inch diameter effluent pipe.

The SBR facility is designed to process 1,160,000 gallons per day average monthly flow. It has a maximum process capacity of 2,270,000 gallons per day.

The SBR treatment process is computer controlled. Operating staff enters the cycle, blower, and mixer times. Equipment maintenance is performed by staff, according to times established in the equipment manuals, and more frequently depending on circumstances related to this particular operation. The plant's state certified laboratory performs a variety of operational tests throughout the day. These results are used to make critical operational changes as needed.

Digestion
The facility has two 800,000-gallon sludge digesters. This sludge is the biological mass pumped out of the SBRs at the end of their process cycles. The sludge is land-applied as fertilizer several times per year, as needed, by the City. Samples are sent to the State Hygienic Laboratory for analysis. The sludge must meet parameters for the application set by the Environmental Protection Agency.

Flow Equalization
The most recent expansion is the addition of an equalization basin. This basin has a capacity of 5 million gallons and was completed in 2005. In general, flow equalization provides temporary storage for flows in excess of plant capacity. These flows are diverted to the basin until peak flows subside then returned to the facility for treatment.

In addition to improving management of peak flows, equalization basins are effective tools in dealing with rapid population growth.

Future Plant Expansion
With a population nearing 11,000 and the rapid population growth rate, the current plant is at capacity both organically and hydraulically. The City Council has approved a new treatment addition converting the existing SBR process to a membrane bioreactor (MBR) process. The MBR process is the most advanced wastwater treatment process in the world. This facility will be the only MBR treatment plant in the state of Iowa. The new plant will be constructed in two phases. Phase I will be designed for a population of 14,000 and started construction in April 2007 and will be operational in fall 2008. Phase II will be designed for a population of 22,000.

Plant expansion is now underway - please click here for construction photos.

Plant Staff
Dave Ramsey - Wastewater Treatment Plant Manager. Wastewater Treatment Grade IV, Water Treatment Grade III, Water Distribution Grade IV

Kevin Stensland - Asst. Plant Superintendent. Wastewater Treatment Grade III, Water Treatment Grade II, Water Distribution Grade III

Mark Farrier - Plant Operator II. Wastewater Treatment Grade II, Water Treatment Grade II, Water Distribution Grade II

Contact the plant at (319) 626-5738. A plant tour can be given to any interested citizens of North Liberty by calling the plant and arranging the tour day and time in advance.
 

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