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.

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 & 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.

Disinfection

The final stage of treatment includes UV (ultraviolet) disinfection before discharge into the St. Vrain River.

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 - free atmosphere and a constant temperature of approximately 98 - 100 degrees) are maintained for the growth of microorganisms that consume the organic material, converting it to water and stable solids. A byproduct of this process is the formation of digester gas 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.