HomeMy WebLinkAbout1981-10-19 Memo, 1285 Brown Septic System Pg1CTO: John R. Gerhardson, Public Works Coordinator
FROM: Michael P. Gaffron, Septic System Inspector
DATE: October 19, 1981
SUBJECT: Septic System, Police Building 1"5- _Ct) gcvc-« 4
As you are aware, the septic system at the police station has been
backing up on occasion this oast year. At your request I have
reviewed the situation and wish to make the following comments:
1. ESTIMATED WATER USAGE
2 toilets, 2 sinks, 1 urinal
Assume 3 flushes per day per employee, at 4 gallons
per flush = 12 gallons/day/employee. Number of
employees - Mel, Jack, Secretary, average 3 officers
Frank, Jim, Dale, Steve, John = 8 employees (day only)
plus 3 x 3 shifts = 9 police = 17 employees
12 gallons/day x 17 employees = 204 gallons per day
PLUS Daily Public Use - Assume 200 gallons per day to
be safe. Total is - 400 gallons estimated water use.
2. SOIL CAPABILITIES
According to the Soil Survey of Hennepin County, the
soil type on the site is Anoka loamy fine sand. I
made a confirmation boring on Wednesday, October 14,
1981 and found essentially a loamy sand down to the
5' depth where the boring terminated. Mottling,
indicating a seasonal zone of soil saturation, occured
at about this 5' depth, so trenches should be no more
than 24" deep to maintain the 3' required separation
distance. The loamy sand on this site would be
expected to have an average percolation rate of faster
than 15 minutes per inch (although no testing was done).
3. SYSTEM DESIGN AND SIZING
Based on the Orono On -Site Code, a septic tank serving
an establishment other than a single family residence
must provide a detention time of 36 hours (1.5 days);
therefore the minimum tank size should be 400 gpd x 1.5 days
= 600 gallons. The code also specifies a minimum tank
size of 1000 gallons. I would recommend a 2 -compartment
1500 gallon precast tank be used, divided 1000-500, since
space is severely limited for this site. The size of
drainfield required is based on percolation rate and
water use. With a 6 to 15 mpi perc rate, 1.27 sf of
bottom area per gallon per day is required. 1.27 x 400
= 508.00 500 sf.
Police Building
Septic System
Page 2
4. SITE LIMITATIONS AND VARIANCES TO CODE NEEDED
Two major limitations present a situation on this site
which severely limits the potential for use as drainfield.
First, the entire west side of the site is at the bottom
of a; -.slope, so that spring runoff and heavy rains will
tend to accumulate in the area currently used for drainfield.
Second, the proximity of the property lines and buildings
leaves very little area which can be used for drainfield
without variances to the code. Specifically, the minimum
setback from property lines to drainfield is 10'. The
recommended minimum setback of drainfield from hardcover,
buildings, etc. is 20'; 10' is a minimum that should not
be compromised. Adhering to these 10' minimums, only
300 sf of drainfiela (in a bed rather than trenches) can
be constructed west of the police building. The only
way to gain the additional square -footage would be to
run a line around the police building to the lawn area
east of the building. The elevations might be such that
a pump would become necessary to keep the trenches from
getting too deep. Also the proximity to paved walkways
and parking area might cause freezing problems in any
system to the east.
The utility lines between the police building and the
shops would have to be moved to accommodate any drainfield
in that area.
Another option is to reroute the indoor wasteline towards
the front lawn area towards the east, and construct the system.
RECOMMENDATIONS:
1. Recognize that a system that will meet all requirements
of the code (based on soil capability and water use
assumptions) cannot be installed west of the police
building. Furthermore, rerouting everything to the
front of the building will disrupt existing trees
and walkways, as well as necessitates plumbing changes
in the basement. The bottom line is that you can
probably get by with a minimal system for current use,
but any expansion of use might create a need to use
the front yard for drainfield.
2. $ee attached sketch) Eliminate existing septic tank.
Replace with a 1500 gallon 2 -compartment precast tank.
Add a bed drainfield of approximately 370 3f as per
sketch. Add gutters to both buildings to eliminate
roof runoff from the drainfield area. Re-route
underground utility lines between buildings.
3. As an added comment, note that no good area exists for
drainfield for the Council Chambers other than the front
yard of the police building. Use of this area would require
a pump station should the Council Chambers ever have toilets.
The entire Council Chamber/Shops/Police Building complex
really does not have the open space availabe for adequate
and conforming septic systems to serve those units.
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