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Millions going down toilet yearly?
Gravity could cost you $70 a year for treating groundwater
A measuring stick shows a disconnected 54-inch sewer pipe running below a neighborhood park off Woodward Avenue in southwest Manteca that is designed to function by gravity flow. It is filled completely with water. - photo by DENNIS WYATT
It sounded like a good idea back in the 1990s: Build a massive gravity flow line to ferry wastewater from future development south of the Highway 120 Bypass to the treatment plant to reduce pumping costs.

The theory sounded good. No pumping was supposed to mean lower costs in the long run. Now, though, as plans move forward for thousands of homes on land yet to be annexed to the city developers are being told the gravity line will need a pump station. That is in addition to the pump station needed when the line is extended to the plant to lift it into the treatment system.

It gets worse.

The 54-inch pipe segments that have been in place for close to a decade are completely filled with water  Basic physics means the heavier ground water is seeping through the concrete pipe seals. It won’t get better even when homes are finally connected to the pipe. That’s because it is designed to be roughly only half full. Given the pressure of ground water that means the pipe will be filled to 100 percent capacity at all times.

Water tables rise, by the way, when the irrigation season gets underway in earnest next month. Keep in mind the pipes are completely filled right now with water even though this is the time of year the water table is actually at its lowest.

The gravity line means hundreds and possibly thousands of dollars more per home built compared to the costs of a forced main. That, however, isn’t the big cost. The city will be using expensive wastewater treatment plant capacity to treat ground water that is the equivalent of 2,300 phantom homes.

Should the line be hooked to the treatment plant it could cost each sewer customer upwards of $70 a year as their share of treating groundwater that seeps into the gravity line designed to be filled at only roughly 50 percent capacity.

Even if the design was changed, unless it was filled to the max with sewage, ground water would seep into it and translate into higher operating costs at the sewer plant that is in turn spread across all users in the form of higher rates.

It is also expensive to expand capacity at the plant. The current expansion and retrofit is costing in excess of $50 million.

Although sewer treatment is an enterprise account paid by users and not the general fund, it is the most expensive municipal operation.

City leaders in January told staff to rethink everything from sewer service to transportation service levels in a bid to keep the costs of running the city and it enterprise accounts down and reduce the cost of housing.

The reason why putting in the gravity line is well over twice that of a forced main is a matter of geography.

Gravity line construction problems responsible for delay on Woodward Ave.
There is relatively only a minuscule drop from Moffat Boulevard to McKinley Avenue south of the Highway 120 Bypass. That means the pipe has to go down 20 feet or more in soils that are predominately sandy loam. That requires deeper digging and more shoring during construction.

Dewatering is also an expense and timely process. It is why Woodard Avenue between Van Ryn and Atherton Drive was closed so long do to issues with putting in that segment of the gravity sewer line which, by the way, isn’t being used either. Homes now being built in Tesoro are having their sewer pumped into existing lines – forced mains - and sent to the treatment plant.

The gravity flow main will also add $8 million to the cost of the trunk line within the 1,050-acre Austin Road Business Park planned southeast of Tesoro.

The City Council recently instructed staff to attach an annual operating cost on to various projects that are built so everyone is aware of ongoing maintenance costs. The dog park, as an example, may cost around $130,000 to build but it will cost $15,000 a year to maintain.

Developers have long argued that if there is a problem with the gravity line, it will be much more expensive to dig down farther especially since additional shoring would be needed.

Some developers have argued the power costs for forced main pumps could be reduced by employing newer solar technology for power panels at pump stations or partnering with the South San Joaquin Irrigation District to develop a solar farm at the treatment plant site to reduce overall operating costs by generating electricity for the treatment plant.