AC20 HOT TOPIC PREVIEW
The Tahoe-Truckee Sanitation Agency’s water reclamation plant in the Martis Valley east of Truckee is using treatment process simulation software to better understand the liquid and solid phases of its process train and prepare more adequately for the future.
“Older, more conventional methods of calculating influent flows and loadings have become less accurate,” says operations manager Michael Peak. “With low-flow toilets and other water conservation programs, the old methods – like using 100 gpd per person – no longer reflect actual influent characteristics,” he says. “Flows may not have gone up, but concentrations have. We needed to look at the actual loadings to the plant.”
Working with Carollo Engineers, the plant is using the BioWin™ process simulator (developed by EnviroSim Associates of Hamilton, Ontario, Canada) to model every process step throughout the plant. An update on the results of the project will be presented by Ron Appleton of Carollo and Michael Peak at the CWEA Annual Conference, March 31-April 3 in Reno.
The Tahoe-Truckee Sanitation Agency serves approximately 29,000 customers in five member districts on the northern and western shores of Lake Tahoe and several communities along the Truckee River corridor to the California-Nevada border. The member districts include North Tahoe Public Utility District, Tahoe City Public Utility District, Alpine Springs County Water District, Squaw Valley Public Service District, and the Truckee Sanitary District (TSD). The TSD also serves the Northstar Community Services District by way of agreement
Collections systems operated by each member district feed the 19-mile-long Truckee River Interceptor – owned and operated by TTSA – which flows by gravity to the water reclamation plant. The interceptor is mostly reinforced concrete pipe with diameters ranging from 24 to 42 inches.
The plant is designed for a peak seven-day average flow between June 21 and September 21 of 9.6 mgd; current peak seven-day average flow is 5.5 mgd for 2019 (max historical is 6.4 mgd). Preliminary treatment includes bar screens for debris removal followed by grit removal. After primary clarification, wastewater is directed to a high-purity oxygen activated sludge system and then to secondary clarifiers. The high-purity oxygen is from purchased liquid oxygen.
Phosphorus is removed from the activated sludge biomass using a Phostrip® process, producing a phosphorus-rich side stream which is subjected to high pH lime treatment. The chemically treated side stream is then blended with the secondary effluent.
A BNR process follows, using submerged biological filters operated in series – one for nitrification, the other for denitrification. The process was the highlight of the treatment plant’s most recent expansion in 2005-2006, replacing an ion exchange process which had been used for ammonia removal.
Pressurized dual media filters polish the effluent, which is then disinfected with chlorine and pumped to a subsurface percolation system adjacent to the plant, upgradient from Martis Creek and the Truckee River.
The plant meets stringent discharge requirements for both total nitrogen and total phosphorus. For the six-month period from May 1 through October 31, the plant has an average limit of 2.0 mgN/L for total nitrogen at Well 31, the farthest downstream well in the percolation field. The permitted annual average limit is 3.0 mgN/L.
The year-round monthly average total phosphorus limit in the treated wastewater is 0.8 mgP/L, with a daily average limit of 1.5 mgP/L.
“We’re an iconic nutrient removal facility,” says Jay Parker, engineering manager. “In the mid-1970s, we were one of the first plants to have stringent effluent limits for both nitrogen and phosphorus.”
Anaerobic digesters stabilize the thickened biological solids. Peak says the addition of the Phostrip process in 1983 represented a ‘major change’ in the plant’s ability to efficiently remove phosphorus with reduced chemical addition. The digested sludge is blended with thickened chemical sludge and dewatered through centrifuges. The remaining chemical sludge is dewatered through a plate and frame filter press. Centrifuge cake is trucked to a Nevada ranch for composting/soil amendment and the dewatered chemical sludge is trucked to a landfill.
The complexity of the water reclamation plant, aging infrastructure, and changing flow conditions all contributed to the decision to evaluate the plant using one of the commercially available wastewater process simulators. Biowin was selected based on TTSA’s familiarity with the software. According to the product’s website, the simulator integrates biological, chemical, and physical processes models, to help utilities design, upgrade, and optimize all types of wastewater treatment plants. Digital graphics and the ability to transfer data to spreadsheets, reports, and performance charts help users get a better handle on their plant processes and understand changes to flow conditions.
“Our plant has performed well,” says Parker. “But some of the original components are over 40 years old. At the same time, our service area is projected to grow in the future, and we anticipate having to deal with changes in influent concentration and flow. “The model will help us predict such things as capacity limitations and process optimization.”
Appleton says the use of the simulator is key to the plant’s master plan and evaluation of unit capacities. With BNR, pure oxygen activated sludge and phosphorus stripping, the plant embodies complex technologies. “Traditional ways of measuring capacities wouldn’t be as accurate as simulation,” he says. “The technology helps us simulate integrated performance of liquid treatment and solids handling facilities and account for nutrient recycles.”
He says simulation results should be available in the next few weeks. The results of the process, as well as examples of the data, will be included in the presentation at the annual conference.
A. Ron Appleton, Jr., Michael Peak, Jay Parker, Andre Gharagoziana, Ricky Gutierreza, Coral Taylor,
Carollo Engineers, Inc.
Tahoe Truckee Sanitation Agency