Inlet Works

Primary treatment begins with two inlet works that receive flow from three influent sewers. Influent sewers J.O.-A and J.O.-B enter Inlet Works No. 1 and J.O.-D enters Inlet Works No. 2.

Inlet Works No. 1 receives approximately 70 percent of the total plant flow and Inlet Works No. 2 receives the remaining 30 percent. Initially, six bar screens for Inlet Works No. 1 and three bar screens for Inlet Works No. 2 remove solids by capturing large debris through bars spaced approximately one inch apart. Captured debris is continuously removed from each bar screen by five equally spaced rakes. The rakes travel up the inclined face of the bars, thereby, removing debris that is then deposited into a trough. The trough delivers the screenings to one of two dewatering compactors. Water removed in the compactors is returned to the treatment process upstream of the bar screens. The dewatered debris is disposed of in a landfill. Wastewater effluent from the bar screens is directed to one of six grit chambers using inlet sewage pumps. Inlet Works No. 1 contains five pumps and Inlet Works No. 2 has four pumps.

Grit Chambers

The grit chambers remove heavy inorganic material consisting of small pebbles, sand, eggshells, and other debris that settle quickly. The material must be removed to prevent excessive abrasion of pumps and sedimentation tank sludge flights. Removal is also necessary to save digester volume as the debris tends to occupy space without contributing to the breakdown of solids in the digester. Grit chambers function similar to sedimentation tanks, whereby, the influent velocity is reduced to permit solids deposition. The velocity through the chambers is controlled to prevent deposition of the lighter organic solids. In addition, the chambers are aerated to aid in the suspension of lighter organic material.

At the Warren Facility, air is added using one of two air compressors housed at the air compressor stations. Grit slurry (grit and water) is pumped from the tanks and dewatered with the use of cyclones and classifiers. The water is returned to the inlet of the grit chambers and the dewatered grit is disposed of at a landfill.

Sedimentation Tanks

From the grit chambers, wastewater is directed to the sedimentation tanks. Sedimentation is a simple and effective means to remove settleable and floatable solids. The slow fluid velocity allows lighter, suspended organic material to settle and floating material to rise. These solids are removed, resulting in a clarified flow stream.

The Warren Facility has 52 primary sedimentation tanks arranged into three sedimentation tank batteries. Settleable solids are separated from the raw wastewater by gravity. Wastewater enters each tank through three inlet gates with diffusers. Wastewater distribution is regulated by these inlet gates. Wastewater flow is reduced from roughly 3 feet per second to 3 feet per minute. The decrease in velocity permits suspended solids to settle in the tank. Solids that settle form a blanket that is pushed to a sludge hopper at the influent end of the tank. From there, the sludge is directed through draw-off lines and pumped to raw sludge transfer stations before transfer to anaerobic digesters. Floatable solids, consisting of oils and grease, are pushed to the effluent end of the tank where they are pulled into a skimmings trough. From the skimmings trough, floatable solids are conveyed to one of four skimmings wet wells. Skimmings from the wetwells is directed to a skimmings concentration tank. Ultimately, concentrated skimmings are directed to anaerobic digesters for stabilization.

Anaerobic Digesters

Anaerobic digestion of the sludge reduces the quantity of pathogens, offensive odors, and solids for disposal. In addition, it helps to stabilize the solids for dewatering and creates methane as a by-product that can be burned to produce steam for heating purposes and electricity to power the Warren Facility.

Raw sludge from two raw sludge transfer stations, one for the north digesters and a second for the south digesters, is fed to 24 circular anaerobic digesters. Ferrous chloride is added to the raw sludge at the suction side of the raw sludge pumps to reduce hydrogen sulfide levels in the digester gas. This is accomplished using two ferrous chloride stations with a combined storage capacity of 100,000 gallons. In addition, thickened waste activated sludge from secondary treatment is directed from the waste activated sludge dissolved air flotation facility to the circular digesters. Each circular digester has a volume of approximately 500,000 cubic feet. The digesters are heated to 96 degrees Fahrenheit to provide an ideal environment for anaerobic bacteria. Heating is accomplished using steam produced from the combustion of digester gas. Steam is injected directly into the digesters and mixed using compressed, recirculated digester gas from the digester headspace. This mixing also helps to distribute the sludge feed and prevents settling and short-circuiting.

Digested sludge overflows out of the digesters into runoff piping and is collected in digested sludge pump stations. Digested sludge can be transferred through several piping routes between the digested sludge pump stations and is ultimately pumped to Solids Processing for dewatering.