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The treatment facility's original tank where chlorination removes bacteria from the wastewater

Mariposa Utility District Upgrades Wastewater Treatment Facility

MARIPOSA — There’s a popular misconception about the capacity of Mariposa’s wastewater treatment plant on Miller Road. Reportedly a number of Mariposa County residents don’t believe the Mariposa Public Utility District’s (MPUD) decades-old sewage management system could provide service to potential new motels or hotels and multi-family housing units. That perception is false.

In fact, upon completion of the current retrofit and upgrade, MPUD officials say the wastewater treatment facility could easily handle three times as much capacity as it now processes.

The permitted capacity for the treatment facility is 610,000 gallons per day. According to general manager Mark Rowney, once the current project is complete, the plant could treat 440,000 gallons per day of wastewater, as it was originally designed.

With the addition of two new tank basins (300,000 anoxic basin and one 25′ diameter clarifier) the facility will be able to treat 610,000 gallons per day.

Sewage flows through this single pipe into the MPUD wastewater treatment plant on Miller Road next to Mariposa Creek on Highway 49 South

Currently the waste water treatment facility treats 180,000 gallons per day. The wastewater is delivered through a single sewage line under Miller Road.

The facility treats sewage generated from MPUD’s wastewater collection system for the township of Mariposa ranging from the Cal Fire station on Highway 49 North, east on Highway 140 to Williams Road, through the middle of town, and extending to the plant that borders Highway 49 South.

The service area covers approximately 873 acres.

MPUD was established in 1947.

In 2010, MPUD conducted a feasibility study to determine what upgrades were need to comply with its state and federal (EPA) wastewater treatment permit.

New permit requirements and limits were established in 2007.

The current project began construction in Sept. of 2018 and is contracted to finish BY the end of March this year.

In October of 2018 the MPUD board of directors released a statement on the project: “The Mariposa Public Utility District (MPUD) Wastewater Treatment Facility (WWTF), constructed in 1983, requires significant unit process modifications to comply with final effluent water quality limitations specified in the State and Federal discharge permit for the facility. MPUD has been working on Facilities Plan, environmental documentation, design and funding applications since 2008. Project schedules were interrupted by the Friends of Mariposa Creek vs. MPUD lawsuit filed in 2014. The proposed project features include: nitrate reduction, filtration, temperature control, ultraviolet light disinfection (to replace existing chlorination systems), replacement and modernization of electrical and control systems.”

The facility’s electrical network dated back to 1983. The facility, which was constructed in 1983, will continue to operate through the biological process.

The goal of the current upgrade is to facilitate the removal of nitrates and other turbidity an extended aeration process and replace the disinfection system currently using chlorine gas with an ultraviolet light disinfection system.

One of two gigantic, concrete aeration ditches at the MPUD wastewater treatment plant

The utility district is licensed and permitted to return the treated wastewater to surface water, or to be exact, to Mariposa Creek. The lawsuit was spawned by a belief that the plant’s chlorination process left residuals in the treated water that flowed into Mariposa Creek affecting the creek water downstream.

Rowney says, “The MPUD facility is in compliance with interim limits for the chlorination byproduct set by the State and MPUD has been working on a project (the current project) to meet final limits for the chlorination byproducts. Bacteriologically, the water downstream from this plant will be cleaner than the upstream flow of the creek.”

Provost and Pritchard Consulting Group of Fresno provided the engineering on the project.

The construction contract was awarded to Clark Brothers, Inc. from Fresno.

The new process, which Rowney describes as “speeding up Mother Nature,” incorporates a bacteriological function ultimately finishing with filtration ultra-violet light.

A single sewage pipe delivers an average of 180,000 gallons a day to the wastewater plant. The volume ebbs and flows with collection during different hours of the day.

The raw sewage dumps into two gigantic, circular concrete oxidation “ditches.” There the fluid is aerated which feeds oxygen to the resident bacteria and creates new bacteria (bugs) that feeds on the solid waste. The wastewater enters the ditches as ammonia, and the oxidation turns it into nitrates, which the bugs feed on.

Treated wastewater is returned to surface water in Mariposa Creek

From the ditches, the waste is moved to a 13-foot deep clarification tank where sludge settles to the bottom. The bacteriological process speeds up there, diminishing the solids.

Out of the clarification tank the sewage flows to a chlorination tank, where is stays for a specified period to time to treat the bugs (bacteria). The chlorination process actually takes place in the original concrete treatment structure that was built in 1957.

Rowney said that the original plant’s process was outdated before the facility was paid for.

A special cooling tower is also part of the bacterial treatment process as MPUD’s permit doesn’t allow the treated water to increase creek water temperature by more than five degrees.

From there the wastewater flows through a complex filter structure that contains four chambers consisting of a combination of sand, gravel and anthrasite coal.

The final stage of wastewater treatment is completed when it flows through chambers that are illuminated by powerful UV lights. The bulbs don’t actually touch the water, thereby avoiding increasing the water temperature before it flows to the creek.

The treated water from the UV disinfection system flows into a 14-foot holding tank before it is sent to the creek, or used at the facility.

According to Rowney, the type of filter and the UV light process is a first application in treating wastewater in California. “This process is actually more complex than what takes place in a water treatment plant for drinking water,” he added.

The construction cost will reach around $10 million.

Six million in grant funding was provided by the Clean Water State Revolving Fund through an agreement with the State Water Resources Control Board.

California’s Clean Water State Revolving Fund is capitalized through a variety of funding sources, including grants from the United States Environmental Protection Agency and state bond proceeds. There is also a $3 million loan secured by MPUD from the State Water Resources Control Board, and $900,000 the utility district had on hand. The 1.7 percent loan is paid back annually at around $138,000.

MPUD has approximately 730 customers in its service area, both commercial and residential.

When construction concludes, the entire facility will be fenced and multiple security cameras are being installed throughout the area. “We haven’t had any vandalism so far through this construction process,” Rowney said.

MPUD also is permitted to operate the town’s sewage collection systems. Most of the utility district’s collection pipes are made from vitrified clay, which Rowney says if installed properly, are nearly indestructible.

There are also some cast iron pipes, which since 1970 have been being replaced by PVC (plastic) pipes. “Some collection pipes will have to be replace, but we’ll do those as we go,” Rowney explained.

The original plant was built in 1957 and upgraded in 1983. “I was 21 years old when I started working here, and 24 when the current treatment plant was built,” Rowney said.

The board of directors for MPUD includes William Bondshu, Frank Mock, Mike Cleary, Dana Finney and Larry Enrico.

For more information about MPUD, visit www.mariposampud.org.

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