Waste Water

The Wastewater Treatment Plant of North Liberty is featured in the July 2010 of Treatment Operator Magazine. Check out the article: http://www.tpomag.com/ezine/2010/07/12

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 Contactor facility began operation. This facility was designed to process 450,000 gallons per day on a monthly average. 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. The SBR was designed to process 1,160,000 gallons per day and a maximum flow of 1,418,000 gallons per day.

Current Facility:
Current Water Pollution Control Facility as of August 2008

In April 2007, with a population nearing 11,000 and the rapid population growth of the city, the city council approved a new Water Pollution Control Facility expansion. This 8.4 million dollar project consisted of converting the existing SBR process to a Membrane Bio-Reactor (MBR) process. The MBR process is the most advanced wastewater treatment process in the world. This facility is the first MBR treatment plant in Iowa. The following is a brief description on the process control for the plant.

Sewage Pretreatment:
1. Treatment begins with 55 miles of sewer pipe, which transports the sewage to the treatment facility from homes, businesses, and industries throughout the city. Some of the sewage flows to the facility by gravity; the rest is pumped by seven lift stations throughout the city. A detailed preventative maintenance program is set up on all lift stations pumps and motors.

2. Bar Screen Removal – Large inorganic solids such as rags and sticks are removed from the sewage prior to pumping.

3. Grit Removal – Small materials such as sand, gravel and eggshells are removed to prevent pump damage and grit accumulation in the process tanks.

4. Pumping – Once the sewage goes through the Bar Screen and Grit removal systems it has to be pumped up to the process tanks. Three self priming pumps deliver the pretreated sewage to the Membrane Bio-Reactors. These pumps are each capable of pumping 2,000 gallons per minute. They are computer controlled and alternate automatically throughout the week.

Membrane BioReactors:
After the wastewater is pumped from the pretreatment building, it enters the MBR building and flows through another fine screen that is able to remove all inorganic matter including human hair. Influent is then distributed to two activated sludge tanks and flows into an anoxic zone where denitrification occurs. This step is followed by an aerobic zone for nitrification. Mixed liquor from the two biological process tanks is combined and re-distributed to three membrane tanks. Inside each membrane tank is four ZeeWeed ultrafiltration membrane modules containing thousands of individual fibers.

The hollow core ultrafiltration fibers have a 0.04 micron (um) pore diameter which provides a physical barrier to suspended solids, colloidal material, and bacteria in the wastewater. The effluent is drawn through the surface of the hollow fiber membranes from the outside-in using relatively low pressure vacuum that is generated by permeate pumps. The water at this stage is so clean and clear that it meets or exceeds numerous drinking water quality standards. The water passing through the membrane (permeate) goes through ultraviolet disinfection before final discharge to Muddy Creek which flows to the Iowa River.

The new MBR plant went on-line August 12, 2008. The MBR facility is designed to process up to 3,378,000 gallons per day. The plant is designed for a Phase II addition which will increase capacity to 5,270,000 gallons per day. The plant’s state certified laboratory performs a variety of operational tests throughout the day to make critical operational changes as needed. The facility also has a very detailed preventative maintenance program and keeps a large supply of spare parts for repair and maintenance of several million dollars worth of equipment.

Digestion:
The facility has two 575,000 gallon sludge digesters and one 799,000 gallon sludge storage tank. This sludge pumped to these tanks is the biological mass pumped out of the MBR’s as needed to maintain a certain level of microorganisms inside the aeration tanks. 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. A yearly report is sent to the Iowa Department of Natural Resources as required.

Flow Equalization:
This basin has a capacity of 5 million gallons and was completed in 2005. With the new plant online this basin will see very limited use because the new facility can treat the higher flows due to the population growth. Only during heavy rain events will the equalization basin be used as a temporary storage until the peak flows subside. The plant will then be able to return all the stored wastewater in a very short period of time. In addition to improving management of peak flows, equalization basins are effective tools in dealing with rapid population growth.