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Treatment Process

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Collection System

The wastewater generated by residential, commercial, and industrial sources is collected by a system of sewers and transported to the Gillette Wastewater Treatment Facility. Each individual generates about 80-100 gallons of wastewater per day. Flow through the sewers generally occurs by gravity. However, pump stations are necessary in some areas. The City of Gillette currently operates ten sewage lift stations.  Some subdivisions and other system users operate private lift stations. The wastewater is transported to the treatment facility through three major trunk lines and the lift stations.

The lift stations are monitored through the lift station telemetry system. This system was constructed and put into service in 1991, upgraded in 1996 and again in 2006. The telemetry system monitors each station for various data points including wet well levels, pump status, station operating, and alarm status.

Preliminary Treatment

Preliminary treatment, the first treatment process, consists of the removal of substances that may interfere with the downstream processes or be detrimental to the plant equipment. Materials removed may include rags, plastic, lumber, and grit. The Gillette Wastewater Treatment Facility has two mechanically raked bar screens to remove large objects, rags, and plastic that would cause plugging problems in the downstream lines.

The in-plant pump station (which receives the wastewater from in-plant sources) has a Muffin Monster comminutor to grind up large solids before they enter other wastewater treatment processes. There are two aerated grit basins that remove the grit by sedimentation. Grit is abrasive materials such as sand and gravel that must be removed to minimize wear on the downstream equipment. Materials removed by the bar screens and grit removal system are de-watered, discharged to a dump trailer, and transported to the landfill for disposal.

Primary Treatment

The second step in the treatment process is primary treatment. The wastewater enters two primary clarifiers (sedimentation basins) which remove suspended and floating materials. The primary clarifiers remove about 60% of the Total Suspended Solids and about 30% of the Biochemical Oxygen Demand in the incoming wastewater. Biochemical Oxygen Demand is a measure of the amount of oxygen needed to biochemically degrade the organic matter in the wastewater. The Gillette Wastewater Treatment Facility's primary clarifiers removes approximately two dry tons of solids per day. Primary treatment does not remove colloidal or dissolved solids.

Secondary Treatment

Secondary treatment usually consists of two steps which remove the dissolved and colloidal organic material not removed by the primary treatment. There are various secondary treatment processes in use today. The Gillette Wastewater Treatment Facility uses a conventional activated sludge process. The activated sludge process is a biological process utilizing an active biomass (bacteria and other microorganisms) to neutralize the organic matter in the wastewater.

The Gillette Wastewater Treatment Facility secondary treatment train consists of four aeration basins and two secondary clarifiers.

The active biomass (called return activated sludge) is mixed with the wastewater leaving the primary clarifiers and with air in the aeration basins. The microorganisms, in the presence of oxygen, use the organic matter and nutrients in the wastewater to sustain their life processes, including reproduction. Effluent from the aeration basins contains large quantities of suspended biological solids (called activated sludge) that must be removed prior to discharge. Therefore, the second step of the secondary treatment process is sedimentation in the secondary clarifiers. Most of the solids removed in the secondary clarifiers are returned to the aeration basins as returned activated sludge to maintain the appropriate population of microorganisms needed to assimilate the organic matter entering the aeration basins.

The primary and secondary treatment processes generally remove at least 85% of the total suspended solids and biochemical oxygen demand. The Gillette Wastewater Treatment Facility averages removal of about 94% biochemical oxygen demand and 97% total suspended solids. The secondary process is a very sensitive biological process and can be adversely impacted by the discharge of incompatible or toxic wastes into the sewer system. Precise control of this process is necessary to effectively treat the wastewater.


The final treatment step for the wastewater is disinfection to destroy pathogenic (disease-producing) organisms. Disinfection at the Gillette Wastewater Treatment Facility is accomplished by UV disinfection.

A portion of the treated effluent is purchased by the PP&L Wyodak plant and used in their power plant. The remaining portion of the treated effluent is discharged to Stonepile Creek after the disinfection process.

If all of the treatment units are in service, the process takes about 24 hours at the current average flow of 2.7 million gallons per day. The plant has a design flow capacity with the new upgrades of 5.12 million gallons per day, which is sufficient to serve a population of about 50,000.

Sludge Treatment

The solids removed in the primary clarifiers and the excess biological solids generated in the secondary process must be stabilized prior to disposal. Sludge stabilization or digestion is the biological degradation of the putrescible organic solids. Sludge at the Gillette Wastewater Treatment Facility is digested anaerobically (absence of oxygen). The WAS (waste activated sludge) is thickened in the dissolved air floatation unit prior to anaerobic digestion.

In anaerobic digestion the W.A.S. and primary sludge are mixed and air is excluded. Anaerobic digestion is a very complex process and dependent on many physical and chemical factors. The digestion takes place in two steps and involves two distinct groups of bacteria. However, the two steps occur simultaneously under favorable operating conditions. In the first step, acid-forming bacteria convert the complex organic wastes (proteins, carbohydrates, lipids) into organic fatty acids. In the second step, the methane-forming bacteria convert the organic acids into methane, carbon dioxide and other trace gases. The methane gas produced has a high fuel value and can be burned to produce electricity and heat. The anaerobic process stabilizes about 80-90% of the organic waste.

Biosolids Disposal/Reuse

The digested sludge from the anaerobic process is known as biosolids. The biosolids produced by the Gillette Wastewater Treatment Facility are utilized as a soil conditioner.  The stabilized biosolids contain nitrogen, phosphorus, potassium, and other elements beneficial to plant growth. Stonepile Select Compost Information Sheet [PDF]

Plant Performance

The plant effluent discharges meet National Pollutant Discharge Elimination System permit limitations.

Parameter NPDES Monthly Avg. Discharge Limitation Average GWWTF Effluent (2009)
Carbonaceous Biochemical Oxygen Demand
25 mg/L 2.6 mg/L
E Coli        

May 1st-Sept 30



Oct 1 - April 30th


126 colonies/100ml avg

576 colonies/100ml max


126 colonies/100ml avg

630 colonies/100ml max


Total Suspended Solids 30 mg/L 4 mg/L
Flow Through Plant   3.31 MGD



The Federal Clean Water Act which was first passed in 1972, and its subsequent amendments, establishes national water quality goals. These goals are (1)Eliminating the discharge of pollutants, and (2)Obtaining water quality which provides for recreation in and on the water. The current Clean Water Act requires that each discharger to surface water be in compliance with the national secondary standards on or before July 1, 1988. The actual discharge limitations for any given treatment plant may be stricter than the secondary standards and, therefore, additional treatment may be required. Each plant must have a permit to discharge which specifies the discharge limitations for that plant.

The Gillette Wastewater Treatment Facility began treating wastewater in 1974. The original facility was designed for a population of about 12,000 and an average flow of 1.2 million gallons per day (MGD). The plant provided secondary treatment only. There was no primary treatment or disinfection. With the rapid population growth in the 1970s and early 1980s, the plant was soon hydraulically overloaded. Plans began for the upgrade and expansion of the plant so that the national secondary discharge standards could be met prior to the July 1, 1988 deadline. The expansion was completed in four construction phases over a five-year period at a cost of about $14,000,000. Construction was essentially complete with most of the new and expanded facilities on-line by the end of October 1989.

We started a new plant upgrade construction project in August 2005 that was completed in February 2007. The new upgrades include UV disinfection, mechanical dewatering using a high solids centrifuge, a new collection system/plant maintenance facility, a new plant SCADA system and other process improvements.  This upgrade increased the plant capacity from 3.85 MGD to 5.12 MGD.

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