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Water and Wastewater Plant Directory
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Longmont, Colorado, USA
Longmont Wastewater Treatment Plant
501 E. First Ave.
Longmont, Colorado, 80501

Contact Information:
Contact:  John Anderson
Email:  john.anderson@ci.longmont.co.us

Plant Operation: Municipal

Processing: Wastewater

Web site:

Plant/Process Description:
The wastewater treatment plant is currently rated to treat up to 17 million gallons per day (MGD). It incorporates primary, secondary and tertiary treatment processes to remove organic and inorganic constituents found in wastewater. Treated plant effluent is discharged to the St. Vrain Creek at an average daily flow of about 8.5 MGD.

The Influent "Headworks" - Raw Sewage Enters the Plant

When the raw wastewater enters the plant influent, debris that is larger than a quarter of an inch is removed by bar screens. This debris passes through grinders and then augers that squeeze the water out of the debris before it is discharged to a conveyor that deposits the dry material in a dumpster. The remaining debris is hauled to a local landfill for disposal.

Grit (sand, eggshells, etc.) and other heavy particles that could damage equipment or cause other problems in the plant are removed in the aerated grit chambers. The grit is "washed" and separated from the water before being hauled to the local landfill.

Primary Clarifiers - More Solids are Removed

The wastewater is then routed to a flow distribution structure, with excess flow diverted to a flow equalization basin so that large and rapid changes in flow to the downstream treatment processes are eliminated. This basin allows the plant to operate at a nearly constant flow rate, which improves the performance of the various treatment processes.

Flow is directed to the primary clarifiers, which remove settleable solids (approximately 50-65 percent of the suspended matter) under quiescent conditions. The solid material that settles to the bottom of the clarifiers is called primary sludge and is pumped to the gravity thickeners for further treatment.

Activated Sludge and Nitrifying Trickling Filters

Flow from the primary clarifiers is pumped onto a biological treatment process called "activated sludge". The first step of this process is the aeration basins. Here, large blowers are used to add air to the water as it passes through the basins, hence the term "aeration basins". Oxygen is added to promote the growth of microorganisms that break down organic material and improve the settling characteristics of the remaining solids.

Wastewater flows from the aeration basins to two final clarifiers. In these tanks, the wastewater is again under quiescent conditions that allow the suspended solids to settle to the bottom of the tanks. These solids are then pumped back to the aeration basins where the microorganisms are reintroduced to a food source. Plant staff monitors the quantity of microorganisms in the activated sludge treatment process daily. Excessive mixed liquor is removed or "wasted" to the Dissolved Air Flotation Thickener (DAFT).

The water that passes over the weirs of the final clarifiers is routed to the Nitrifying Trickling Filters (NTFs). Specific bacteria grow on the media of these filters to convert ammonia in the wastewater stream to nitrate. This process is utilized in order to meet the stringent effluent limitations for ammonia concentrations discharged to the receiving stream. Some of the clarifier effluent is bypassed around the NTFs, based on the level of ammonia removal required at any particular time of the year. Pumping less flow to the NTFs also reduces the power costs at the plant. Staff also control the amount of NTF effluent that is recycled through the filters in order to maintain a minimum filter wetting rate.


The final stage of treatment is disinfection which destroys any remaining microorganisms in the treated water before discharge into the St. Vrain Creek. A strong chlorine solution is mixed with the plant flow to disinfect the wastewater. After chlorine is added the flow passes through a chlorine contact chamber to give the chlorine time to destroy the microorganisms in the plant flow. Because chlorine is toxic and can have an adverse impact on the receiving stream, sulfur dioxide is added at the end of the chlorine contact tank to remove any residual chlorine from the flow before it is discharged.

Solids Handling

Solids settled in the primary clarifiers are pumped to the gravity thickeners for concentration. The sludge settles to the bottom of the thickener, similar to a clarifier. Thickening of the sludge reduces the hydraulic loading to the digesters or belt presses and reduces the cost of operating these units. The water that flows over the weirs of the gravity thickeners is routed back to the plant headworks for treatment along with the incoming wastewater.

The thickened primary sludge may be dewatered by the belt filter press. The sludge entering the press contains about 5% solids. The press squeezes the water out of the sludge via a series of rollers. The dewatered sludge or "sludge cake" contains about 25% solids. The water removed from the sludge is routed back to the plant headworks.

Thickened sludge from the thickener may also be pumped to the anaerobic digesters. The digesters are tanks where optimal conditions (oxygen - freeatmosphere and a constant temperature of approximately 98 - 100 degrees) are maintained for the growth of microorganisms that consume theorganic material, converting it to water and stable solids.

A byproduct of this process is the formation of digestergas containing carbon dioxide and methane. The digester gas is either burned in a digester gas flare, or used as a fuel source for the boilers. The boilers are used to heat water, which in turn is used to heat the sludge in the digester to the proper temperature. Digested sludge is then pumped to the biosolids storage tank for storage prior to land application at agronomic rates.

City of Longmont
Web site:  http://www.ci.longmont.co.us/
Operating Company:
City of Longmont
Web site:  http://www.ci.longmont.co.us/
From I-25, take the HWY 119 exit for Longmont. Head west towards town. At the first red light in town, HWY 119 becomes the Ken Pratt Extension. Continue straight to the second red light and make a right on Main Street (AKA HWY 287). Continue north on Main and make a right on 1st Avenue. Continue east on 1st Avenue which bends north and becomes Martin Street. Just past the bend, but before the railroad tracks, make a right onto the utility road that parallels the tracks. The treatment plant is located on the right at the end of this utility road.

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Disclaimer:  Water and Wastewater.com provides the information on this page for reference purposes only. We do not certify the accuracy of the information provided by third-party individuals for this listing.

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