GRANGEVILLE – “The main sticker shock is the amount of infiltration and inflow [I&I] in your system,” said David Watkins, J-U-B Engineers, speaking to the Grangeville City Council last Monday, Jan. 6, on their 2019 study findings of the municipal wastewater infrastructure.
How bad? According to Watkins, fixing the existing I&I issue would gain the city the equivalent of servicing another 10 Grangeville-sized cities.
“The average annual flow the treatment plant is designed around, according to the latest permit, is 0.8 MGD [millions of gallons per day], to give you a sense of scale,” Watkins said.
On a total 1,569 connections, consumers during a non-irrigation period, November to April, contribute 0.245 MGD or 155 gallons per day per connection. The peak flow (or diurnal; periods of heightened usage such as in the morning and at the end of the work day) is 0.416 MGD or 265 gallons per day per connection.
“What we measured in 2019 is about 12 MGD,” he said. Based on this and the system’s design for a five-year event, this could potentially generate 20 MGD or 12,750 gallons per day per connection.
“It’s a staggering number,” said councilor Beryl Grant.
For definition, infiltration is seasonal groundwater leaking into the system, which is not associated with a storm, and inflow is stormwater discharged into the system.
“Based on these results, we think it’s a combination of pipe conditions,” Watkins said. “So, probably broken services near ditches that are taking in a lot of water during storm events, probably broken mains in places where there is high ground water, and there’s a good chance a lot of sump pumps and roof drains are tied into the system.”
Watkins conducted a 45-minute council presentation on wastewater system survey results from monitoring last April. This establishes the system’s present situation and options for repairs and upgrades, with the next step developing a capital improvement plan to outline a proposed budget for repairs and upgrades, and a preliminary rate analysis on funding.
This work repeats the process completed last year on the city’s water system, for which the council approved a funding plan for repairs to be implemented during a 20-year period. Once the wastewater system process is complete, the city intends to present the combined project scope and budget to the public.
“The main goals are to address I&I over time, and how to plan which sewer mains to upsize and when,” Watkins said. On the extreme ends, one option would be to upsize pipes to pass all the I&I, and the other to upsize enough pipe to stop inflows and tackle the I&I over time. Within the suggested alternatives, Watkins suggested the city opt in addressing I&I, “which is the root problem,” and tackle projects and improvements that gain the city capacity; a longer fix but less expensive.
“It gives you more flexibility in tackling projects that are critical, and spend your money on I&I reduction, rather than on pipes that won’t be needed once you tackle I&I,” he said. “That’s the best, wisest choice for your money.”
In contrast, according to Watkins, upsizing all lines brings with it increased maintenance – bigger pipes with small flows result in more solids depositing and cleanings needed -- and it signals Idaho Department of Environmental Quality to ensure the city’s wastewater treatment plant is compliant to pass flows during storm events to prevent violating its discharge permit.
In agreement, Mayor Wes Lester said in choosing options to just upsize pipe, “we’re pretty much telling DEQ we’re going to build a new treatment plant that is 20 times as big as it is now.”
How the city would tackle sumps and storm drains tied into the wastewater system – with options of city assistance and ordinance enforcement -- is yet to be determined. Watkins noted that, in the past, cities with septic systems would encourage such tie-ins as the flows kept the system moving.
“But now, that’s gone, and it’s hurting you,” he said, “especially now as you have a mechanical treatment plant. You are paying to treat all that as if it’s wastewater, but it’s stormwater. The best place for it is in the creek.”
Consensus with Watkins and public works director Bob Mager was for addressing I&I, which Mager said, “Every time we put a stick of pipe in the ground we’re reducing that,” and both noted it gains system capacity and reduces the expense load in chemicals and power at the treatment plant.
“Every ounce of water that doesn’t go down there saves the public money, and us,” Mager said.