Waste Treatment – Septic Systems
Lab #7
Objectives
 Know how a conventional septic system
works.
 Be able to describe three alternative systems
that could be used to dispose of household
waste.
 Know what soil conditions are needed for
proper functioning of conventional and
alternative waste disposal systems.
Background Info
 Every day the average U.S. family of four
generates approximately 200 gallons of
sewage effluent through use of toilets,
showers, bathtubs, sinks, washing machines,
and dishwashers.
 This sewage is 95% water and 5% solid
material.
Disposing of the Material
 In houses connected to city sewer systems, disposal
of household sewage is simple; the waste water
simply flows out of the houses, and down into the
city sewer pipe.
 However, in N. C., about 50% of the houses
(approximately 1.5 million) are not connected to
city a sewer, and wastes from these homes must be
disposed of in other ways, usually on the
homeowner's property.
Septic Systems
 The easiest way to dispose of most
household's daily sewage is to let it seep into
the soil surrounding the home.
 To do this, most of the solid materials are
first removed from the effluent, and then the
wastewater is spread over the soil and
allowed to seep in.
Soil’s Role in the Process
 The soil's job is to filter out any remaining
solids, and to kill the germs in the effluent.
 The purified water flows downward through
the soil to the water table and should be fit to
drink once it reaches the water table.
Schematic of Septic System
http://www.cet.nau.edu/Projects/WDP/resources/Conventional_Overview/ConventionalSe
pticSystem.jpg
Septic Tank
http://www.co.thurston.wa.us/health/ehoss/images/drainfield.jpg
Septic Tank
 The septic tank is a settling tank where
sewage from the house is stored temporarily
so that solid materials settle to the bottom of
the tank to form sludge, while grease floats
to the top to form a scum layer.
 The scum and sludge must be periodically
removed from the septic tank by simply
pumping them out.
Septic Tank
http://rps.uvi.edu/CES/septicx.jpg
Drainfield or Absorption Field
http://www.co.thurston.wa.us/health/ehoss/images/drainfield.jpg
Drainfield
 The absorption field is frequently a network
of drain lines (pipes with holes in them) that
carry wastewater from the septic tank and
apply it to the soil.
 The size of the absorption field varies from
site to site, but most are small enough to fit
in a backyard and cover and area between
400 and 800 ft2.
Suitable Soils
 Although effluent can be applied to the surface of
any soil, not all soils are able to absorb and purify
wastewater.
 Soils that can be used for waste disposal are
permeable, deep, well drained (not waterlogged)
and are not on a steep slope.
 In N. C., state law requires that all soils pass an
inspection before a septic system is installed on any
property.
Alternative Septic Systems
 If a soil is not suitable for a conventional
septic system, several alternative systems
have been developed to overcome specific
soil problems.
 These alternative systems include the low
pressure pipe system, the mound system, and
the recirculating sand filter.
Low Pressure Pipe System
 The Low Pressure Pipe system (LPP)
includes a small pump, which may turn on 12 times a day, to pump a large amount of
effluent to the drainfield.
 This allows the soil to absorb and filter the
effluent effectively.
Mound System
 Soils with shallow depth over a limiting layer or
soils with high percolation rates may be suitable for
a mound system.
 In a mound system,specially selected sand is placed
on top of the natural soil to help treat and dispose of
septic tank effluent.
http://ohioline.osu.edu/aexfact/images/744_1.jpg
Recirculating Sand Filter
Effluent In
http://www.nesc.wvu.edu/nsfc/pdf/eti/RSF_tech.pdf
How do we determine if our
soils are suitable?
 The ability of a soil to properly absorb and treat
sewage waste will depend on several soil
properties.
 Each County Health Department should be
consulted for specific criteria used in a given
county.
 Remember that these are the rules that state whether
or not you can build a soil drainfield on your land.
General Order of Events
 Determine the types of soils on the lot.
 Find out what type of septic system, if any, will
work on those soils.
 Obtain a septic system improvement permit (IP)
from your local health department.
 Obtain a cost estimate for installing the kind of
septic system you need. Ask about the operating
requirements and maintenance costs for the system.
Reviewing the Site
 Walk over each lot and look for indications
of soil problems or site limitations that could
affect the performance of a septic system.
 Look for steep slopes, streams nearby, wet
soils, wetlands, bedrock protruding, etc.
Reviewing the Soils
 If you do not observe any obvious site problems,
you may wish to identify the soil types.
 The county soil survey report shows which soils are
most likely to occur on the land you are
investigating.
 You can use this report to help screen land parcels
and focus your efforts upon those lots that have the
most desirable soil and site characteristics.
 Do not substitute it for an on-site evaluation of soil
and site conditions.
Unsuitable Soils
 Unfortunately, not all soils can
absorb wastewater or purify it.
 Septic systems that are
installed in unsuitable soils
usually malfunction by leaking
raw, untreated sewage to the
soil surface or a roadside ditch,
or by contaminating the
groundwater.
 The sewage may contain
deadly bacteria and viruses.
http://www.ncdc.gov.uk/media/image
s/d/l/WaterPollution_640.JPG
Are the soils suitable?
 In order to determine if a soil is suitable, a
basic morphological description including
texture, structure, color and clay type is
required.
 We must first complete a description of the
soils and then follow the guidelines provided
by NC.
Soil Suitability in NC –
Landscape Position
 >30% Slope, Floodplain or Depressional
Area = Unsuitable (US)
 15-30% Slope = Provisionally Suitable (PS)
 <15% Slope = Suitable (S)
Soil Suitability in NC –
Textural Class
 Group I: Sand & Loamy Sand = S
 Group II: Sandy Loam & Loam = S
 Group III: Sandy Clay Loam, Silt Loam,
Silty Clay Loam & Clay Loam = PS
 Group IVa: 1:1 Clay (Kaolinite) = PS
 Group Ivb: 2:1 Clay (Montmorillonite or
Vermiculite) = US
Soil Suitability in NC –
Structure
 All Groups are S, except:
 Groups III & IV with blocky structure = PS
 Groups III & IV with platy structure = US
 Groups II. III & IV with massive structure = US
Soil Suitability in NC –
Organic Soils
 All organic soils are unsuitable (US).
Soil Suitability in NC –
Drainage
 To determine the suitability, we need to look
for depth to seasonal high water table
(chroma <2 indicates reducing conditions).
 > 4 ft = S
 3-4 ft = PS
 <3 ft = US
Soil Suitability in NC –
Restrictive Horizons
 To determine suitability, we must locate any
restrictive layers (clay pans, etc.) or the C
horizons. If we have either of these, use the
following:
 > 4 ft = S
 3-4 ft = PS
 <3 ft = US
Determining Loading Rates
 Now we must determine how much effluent
we can load onto the drainfield so that the
system works properly.
Loading Rates
Soil
Important Features
Application Rate
(gpd/ft2)
I
Sandy textures
1.2
II
Coarse loamy textures
0.7
III
Fine loamy textures with PS structures
0.5
IVa
Clayey textures with 1:1 clay and PS
structure
0.3
IVb
Clayey textures with 2:1 clay
Unsuitable (US)
How big must the drainfield
be?
 The loading rates and the size of the house
are used to determining how large the soil
drain field must be.
 The rule of thumb is 120 gpd of effluent for
each bedroom in the house.
 For example; a 3-bedroom house is expected
to produce 3 X 120 gpd effluent or 360 gpd
of effluent.
Requirements for a
Conventional System
 <30% Slope
 >3 ft to seasonal high water table, bedrock,
saprolite or restrictive horizon
 Suitable (S) or Provisionally Suitable (PS)
texture and structure to 3 ft.
Requirements for Low Pressure
Pipe Systems
 Soil depth > 2 ft to seasonal high water table,
bedrock, saprolite or restrictive horizon
 S or PS texture and structure to 3 ft.
http://www.ces.ncsu.edu/plymouth/septic2
/FAST-Complete1.gif
Requirements for Mound
Systems
 < 10% Slope
 Soil depth >1 ft to seasonal high water table,
bedrock, saprolite or restrictive horizon
 S or PS texture and structure to 1 ft.
http://www.co.kenosha.wi.us/plandev/images/mound01.jpg
Our Turn
 For this lab, we will review soil profiles and
determine whether or not the soil would be
suitable for a conventional septic system.
 If it is not, we will determine if an alternative
system could be used.
 Please see the Lab #7 Help file for any help
on the lab.
Questions?