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GRAVITY SEWER LINE

City: Water infiltration not an issue

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POSTED April 2, 2009 4:56 a.m.

To access City Manager Steve Pinkerton’s blog go to www.ci.manteca.ca.us/blog/ 

Question: Are millions going down the toilet?

Answer: Absolutely not!

Sunday’s article in the Bulletin talked a lot about ground water and pipes that we use to collect sewer and the potential cost to residents. The bottom line is that we don’t believe that water infiltration is an issue, but in an absolute worst-case scenario it might cost the typical homeowner around 10 cents per month - more likely the cost is about 1/1000th of that - so about 1/8th of a cent per year per household.
Some of the information presented in the article might be misunderstood, so we would like to present some information about groundwater and sewer collection. In an attempt to clarify some of the issues, our staff wastewater experts have provided answers to a series of questions that might have been created by the article. In the future we’ll also be addressing some of the other issues brought up in the article such as gravity flow versus force main and alternate energy applications at the sewer plant.

Q1 – Why is the 54” line referenced by the Bulletin full of water?
A – The line was constructed over a decade ago, and over that time groundwater has filled the line from a leak in a manhole. This line does not convey sewage at this time. It is part of a future pipe system that will be completed as development occurs.

Q2
– Would this water reduce capacity at the wastewater treatment plant.
A – No, before this line could be put in service, the leak would have to be repaired.

Q3
– How much seepage or infiltration should be expected from a pipe installed below groundwater?
A – Industry standards have been developed for this situation, and the amount of infiltration is dependent on the pipe length, diameter, and how deep the pipe is in groundwater. If the City completed all 10-plus miles of the gravity system as proposed, the worst-case infiltration would be about 70,000 gallons per day, and cost each home owner $1.50 per year. Actual infiltration is expected to be much lower (see Q4)

Q4 – Are there any other unused lines that have remained dry?

A – The City installed another segment of large diameter (54” diameter) trunk three years ago, and recent inspection revealed three inches of water that had accumulated in the pipe. That is a total of 2.8 gallons/foot of pipe in three years. If the entire proposed 10-plus miles of gravity collection system experienced this rate of infiltration or seepage, it would equate to an average of 43,200 gallons per year or 120 gallons per day of groundwater being added to the treatment plant. That is the equivalent of about half of a typical home.

Q5 – Are there any other leaks that are allowing groundwater to go to the plant?
A – There will always be some minor leakage, more so in the local lines that collect sewage from the individual houses and businesses than in the trunk lines that carry the sewage to the treatment plant. The most common source of leakage is at the point where a residential lateral connects to the local sewer line. Next most common problem area is at manholes where lines intersect.

Q6 – What is the City doing to prevent groundwater from entering the sewer system?

A
– The City does regular maintenance and monitoring of the entire collection system to insure that infiltration or seepage is eliminated. In February, City crews cleaned and inspected over 40,000 feet of City sewer lines.

Q7 – If groundwater did seep into sewer lines, how would it affect the treatment plant capacity?

A
– Treatment capacity is the plant’s ability to treat dry weather flows and wet weather flows. Dry weather flow consists of wastewater that is generated on a day-to-day basis from homes and businesses. Dry weather flow is made up of both hydraulic (water) loading and organic (human waste) loading. Dry weather flow is also a very consistent flow in that its amount and strength changes slowly over time. Wet weather flow is generated from stormwater and groundwater that finds its way into the gravity collection system through leaks and cracks in pipes and manholes. Wet weather flow is made up entirely of hydraulic loading, and it is a very inconsistent flow in that the amount of wet weather flow changes with the rainy season and as groundwater levels rise and fall.

Both flows – dry weather and wet weather – were accounted for during design of the Manteca treatment plant. Pumps and structures were sized to handle the hydraulic loading from both dry and wet weather flows. However, the biological systems designed to remove and treat the organic loads were sized to accommodate only the dry weather flows since this is the only source of organic loading. The biological treatment systems include facilities such as aerators, filters, digesters, and disinfection.

So, the answer to the question is that stormwater and groundwater infiltration have already been accounted for in the plant’s treatment capacity. Incidentally, designing for dry and wet weather flows is a standard engineering practice, and virtually every treatment plant in the country was designed this way.

Q8
– If groundwater or stormwater infiltration were reduced, can the “freed up” capacity be used for new homes and businesses?

A – Reducing a wet weather flow, such as groundwater infiltration, would free up hydraulic capacity at the plant. However, if a dry weather flow was inserted in its place, both the hydraulic loading and organic loading would increase at the plant. The plant could handle the hydraulic loading, but not the increase in organic loading. So, to accommodate additional homes or businesses that take the place of groundwater infiltration, the City would need to expand the plant’s biological treatment systems. These are typically the most expensive parts to maintain and operate.

Finally, a brief note about groundwater. Shallow groundwater is present in most of the area currently in the Manteca City limits, and future growth areas. The groundwater tends to be higher at lower elevations and near permanent water bodies. Groundwater levels fluctuate depending upon a number of factors including geology or soils type, precipitation, irrigation, and drainage or pumping. High or shallow groundwater is common in much of the world’s population centers which are typically located adjacent to water bodies. Fortunately engineers and geologists are able to identify, monitor, and compensate for the presence of groundwater in almost every situation, and have done so for hundreds of years. This capability has allowed us to build large buildings, tunnels, sewers, and other infrastructure below groundwater levels. Certainly the City of Manteca can construct a few miles of pipe below groundwater and still provide reliable cost effective service for all the existing and future rate payers.

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