At the December Council meeting Joe Zierden, the Sewer
Department Manager, discussed with the council managing the sewer system in a
different way in the future. Our
treatment system is designed around a series of ponds that hold the influent
stream of wastewater. The ponds treat
the wastewater and eventually, when fully treated, the water is irrigation discharged
into the forest.
In managing the system we have two thresholds to worry about. One is the irrigation discharge amount. This amount is set by the MPCA at 72 million
gallons. In the non-winter months we can
discharge effluent up to that amount in the forest irrigation fields. The other threshold is the winter storage
capacity of the ponds themselves.
Because we can’t discharge in the winter, the ponds need to be able to
hold all the winter flow from the sewer system.
This is currently set at 30.7 million gallons. This is the more critical factor in managing
our system.
With the high precipitation amounts we have seen in the past
couple of years the pond capacity has been challenged to some extent. Generally evaporation and precipitation are
about equal but not so in the recent past.
In reviewing the situation some fairly simple things are thought could
be done to improve capacity. Currently
we draw down the ponds in the summer to a three foot level for winter storage. With some changes in piping we hope to be
able to draw them down to a two foot level before winter sets in. This would pick up 4.34 million gallons of
additional storage. Using the aeration
ponds to the fullest capacity will also increase storage but may be a bit
trickier. The aeration pumps would need
to be set at a different level to manage the drawdown of pond depths. If this can be managed effectively this would
give us an additional 5.1 million gallons of storage. Making these changes will increase total capacity
to 40.14 million gallons or about 30% more.
These changes may not be required if precipitation trends
return to normal. However in making
these changes, regardless of weather, it will delay the expansion of the
treatment system. It gets down to when
the system needs to be upgraded, in the next five years or after a decade. Delaying large capital expenses gives us more
time to deal with them. Thanks to Joe Zierden
for taking another look at what can be done.
