The Oxnard Wastewater Treatment Plant (OWTP) is a secondary treatment plant located at the southwest portion of the City on City property north of the OWTP. The OWTP has an average dry weather flow (ADWF) design capacity of 31.7 million gallons per day (MGD) with provisions for an ultimate ADWF design capacity of 39.7 MGD and corresponding ultimate peak weather flow of 75.4 MGD. The Headworks facilities will be designed to meet these increased water capacities.

Raw influent flows from the City’s collection system will be delivered to the Inlet Junction Structure. From there the raw influent will be split among new influent channels each containing a bar screen to remove solids. Screenings removed from the influent flow stream will be lifted by the bar screens and discharged to the conveyors. Each conveyor will discharge to the storage bin located in the Grit/Screenings Building. After passing through the bar screens, the influent flow stream will flow into grit chambers where grit particles will settle to the hoppers. Settled grit will be removed from the hoppers via grit pumps and pumped to the grit separators for dewatering. Dewatered grit will be discharge to the storage bin located in the Grit/Screenings Building. Dewatered grit will be transported to the landfill for disposal. The wastewater will then flow into the influent pump station wet well and will be pumped via influent pumps to the OWTP for full secondary treatment.

The new Headworks Project will maintain the treatment plant capacity while substantially enhancing process reliability, performance, and operability. These improvements are attained by providing process and mechanical equipment redundancy throughout key elements of the new Headworks facility.

Key Features of the Headworks Facility (diagram)

A.  Inlet Junction Structure:  A below grade reinforced concrete Influent Junction Structure, 12 inches by 33 inches, to collect tributary sewer flows.

B.  Influent Channels:  Wastewater will flow from the Influent Junction Structure to the Mechanical Bar Screen Facility by means of influent channels. A total of six channels will be constructed; four of which are 4.5 feet wide by 8.5 feet deep, and the remaining two are 5 feet wide by 8.5 feet deep.

C.  Mechanical Bar Screens:  Four (4) mechanical bar screens and two (2) manual bar screens will be located in the influent channels. The mechanical bar screens will be approximately 4.5 feet wide and 50 feet tall, constructed of 316 Stainless Steel, while the manual bar screens will be approximately 5.5 feet wide and 8.5 feet tall. Two of the four mechanical screens will operate while the additional two will function as standby. The two (2) manual bar screens will be used for bypassing around the mechanical screens, when needed. The screenings collected from the mechanical bar screens will be conveyed to the Grit/Screenings Building for storage in a truck roll-off bin, prior to disposal in a landfill.

D.  Grit Chamber:  A 45.5 foot by 45 foot bifurcated below grade reinforced concrete aerated Grit Chamber with concrete walls that extend 4 feet above grade will be located at the new Headworks. The grit chamber will be divided to allow half of the chamber to be taken out of service for maintenance and will be provided with bypass channels with aeration to allow the entire chamber to be taken out of service. In an aerated grit chamber system air is introduced by means of an electric blower, which provides a spiral roll in the sewage flow, resulting in the most efficient removal of grit particles. Two 540 cubic feet per minute electric, centrifugal blowers will be installed, with one functioning as standby. As wastewater flows into the grit chamber, grit particles gradually settle to hoppers at the bottom of the tank. Grit materials include particles of sand, gravel, other mineral matter, and organics such as coffee grounds, eggshells, fruit rinds, and seeds.

E.  Grit Removal and Dewatering Facilities:  Grit collected at the bottom of the grit chamber in hoppers will be pumped to a grit cyclone and classifier located in the Grit and Screenings Building. This equipment will wash, dewater, and then convey the grit to storage in a truck roll-off bin prior to disposal in a landfill.

F.  Influent Pump Station:  A wet/dry pit reinforced concrete Influent Pump Station will be located at the new Headworks. The pump station will be sized to house six 450 hp motor driven influent pumps. Only three of the six pumps are needed during peak flows. The remaining three pumps function as standby until needed.

G.  Odor Control:  Air within the Inlet Junction Structure, Influent Channels, Grit Chamber, Influent Wet Well, and Grit/Screenings Building will be continuously ventilated to remove the odorous atmosphere and hydrogen sulfide gases. The Influent Junction Structure, Grit Chamber, Influent Channels, and Influent Wet Well will be ventilated at 9 air changes per hour. The Grit/Screenings Building will be ventilated at 18 air changes per hour. The Odorous (hydrogen sulfide, H2S) gases displaced from the subject areas will be transferred to an odor control facility for treatment. The odor control facility being constructed is considered the best technology to date for odor treatment. The facility will consist of two treatment trains with one train as a standby unit. Each treatment will consist of a single stage chemical counter-current packed tower, an exhaust fan, and a recirculation pump. The trains may be operated either in parallel or in series. Sodium hypochlorite and sodium hydroxide will be utilized for the odor reduction reaction within the packed towers. Sodium hypochlorite solution (12.5 percent) will be stored in existing storage tanks in the North Chlorination Facility. Sodium hydroxide solution (25 percent) will be stored in a 1,000 gallon insulated fiberglass tank located at the Odor Control Facility adjacent to the Grit/Screenings Building.

H.  Emergency Standby Generator:  A diesel fueled 1500 KVA emergency generator will be located adjacent to the Headworks to provide power to the critical loads including the influent pumps if the Southern California Edison (SCE) electrical feeder fails. This set will be completely enclosed in a metal housing 25 feet long by 16 feet, 4 inches wide by 15 feet high. The metal housing will then be enclosed in a cinder block or poured concrete open-top structure 31 feet long by 22 feet wide x 15 feet high. A permit to construct and operate will be submitted to the Ventura County Air Pollution Control District (APCD).

I.  Southern California Edison (SCE) Substation:  Electrical power to the new Headworks will be fed from the existing SCE substation at the south end of OWTP. Underground conduit duct banks will feed two 10 feet long by 5 feet wide by 6 feet high switch and fuse units. The units will be completely enclosed and painted SCE green. Two transformers, also completely enclosed, will step down the
16 KV feeder voltage to 480 V.

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