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 pticSystem.jpg

Septic Tank

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

Drainfield or Absorption Field

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 ft 2.

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 1- 2 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. fact/images/744_1.jpg

Recirculating Sand Filter Effluent In

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. 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 SoilImportant FeaturesApplication Rate (gpd/ft2) ISandy textures1.2 IICoarse loamy textures0.7 IIIFine loamy textures with PS structures0.5 IVaClayey textures with 1:1 clay and PS structure 0.3 IVbClayey textures with 2:1 clayUnsuitable (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. /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.

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?