Wastewater Treatment

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The City of Sioux City is served by a municipally-owned, 28.73 million gallon per day wastewater plant located at 3100 South Lewis Blvd. near the intersection of Interstate Highway 29 and U.S. Route 20.

Originally constructed in 1961, the Sioux City Regional Wastewater Treatment Plant serves five communities from three different states, including:

  • Sioux City, IA
  • Sergeant Bluff, IA
  • South Sioux City, NE
  • North Sioux City, SD
  • Dakota Dunes, SD

Wastewater Treatment Plant Process

Preliminary Treatment

Raw wastewater enters the WWTP through the Headworks Building and passes through bar screens where large solids are removed. The wastewater then flows to a vortex grit system for removal of small inorganic solids. Solids removed from the bar screens and the grit system are dewatered prior to disposal at the local landfill.

Primary Treatment

Following preliminary treatment, the wastewater flows to the primary clarifiers where heavy organic solids and grease are removed. The clarifiers are equipped with continuous chain flight collectors and primary sludge pumps. The heavy settled primary sludge and the skimmed grease is pumped to the primary digesters for further treatment.

Secondary Treatment

Primary treated wastewater is pumped from the Primary Effluent Pump Station to six aeration basins. These basins are designed to provide nitrification and denitrification using the Modified Ludzak-Ettinger (MLE) Process. Each basin includes four passes which operate in series. Pass 1 is the anoxic volume for denitrification. The first pass is divided into three biological selector zones followed by a fourth anoxic zone. Submersible mixers provide mixing in Pass 1. The other three passes have fine bubble aeration for oxygen and mixing. An internal recycle pump, located at the end of the fourth pass, returns nitrate rich mixed liquor (MLSS) to selector Zone No. 1 where it is mixed with primary treated wastewater and return activated sludge (RAS).

MLSS from the aeration basins is directed to five 115-foot diameter final clarifiers for settling. The solids settle to the bottom and are collected for return to the RAS wet well. The RAS pumps remove solids from the RAS wet well and return them to the aeration basins to maintain the MLSS concentration required for biological treatment. Excess solids are pumped to the solids handling facilities (GBTs) by the waste activated sludge (WAS) pumps. The clarified effluent flows by gravity to the chlorine contact basin.

Chlorine Contact Basin

Before leaving the WWTP, flash mixers blend the effluent with the sodium hypochlorite to provide disinfection. Sodium bisulfite is then added to dechlorinate the water prior to discharge to the Missouri River.

Solids Processing

Primary sludge and scum from the primary and secondary clarifiers is pumped directly into the anaerobic digestion process. WAS, thickened by polymer addition and gravity belt thickeners, is also pumped to the anaerobic digesters.

Two sets of four anaerobic digesters, East Complex and West Complex, are used to stabilize solids from the primary and secondary treatment processes. Each digester complex incorporates two primary and two secondary digesters to digest and stabilize the sludge. The end product is a Class ‘B’ sludge, per EPA 503 regulations, which can be disposed of within a landfill or land applied in accordance with 503 regulations.

Digested sludge is pumped to belt filter presses (BFP) for further dewatering and stored for hauling off site to either a landfill or used in land application.

Methane gas produced by the anaerobic digestion process is used within the WWTP as fuel for building heat and to heat the primary digesters.


A major priority at the Sioux City Wastewater Treatment Plant continues to be the safety of the employees, facilities, and the general public. Monthly Safety Training (including hands-on safety exercises) and Safety Committee Meetings are used to discuss and address any new or outstanding safety concerns, safety awareness and safe work ethics practiced by each of the employees. Each employee is required to participate in all monthly safety courses and/or be current with all required safety certifications.

The safety committee is made up of members from various departments, including departmental personnel, supervisors and managers. At the meetings, the committee discusses any new or outstanding safety concerns. By hosting an open forum for employees to express safety concerns, it not only gives the employees an opportunity to discuss their concerns, it also allows employees the opportunity to contribute to the resolution and awareness of any hazards that may exist.

In addition to domestic wastewater, the Sioux City WWTP receives flow from approximately 20 industries throughout the region.

Following treatment, as authorized by Iowa Department of Natural Resources issued NPDES permit, treated wastewater is discharged into the Missouri River.

For more information on the Wastewater Treatment Plant, view The History of the Wastewater Treatment Plant.