MPCA Design Guidance for Large Subsurface Wastewater ...

To arrive at the most successful system design, the designer must consider the following factors:
• waste strength treatment devices
• nutrient treatment devices
• fecal reduction devices
• soil dispersal system
o
o
o
sizing
geometry
dosing frequency
• natural soil treatment capabilities
• dilution
• operation and maintenance requirements
11. Septic tank sizing and design considerations (complete Attachment 11)
Now that the drainfield size and shape have been determined and the location and treatment
effectiveness have been proven, the designer is ready to design the rest of the subsurface wastewater
system.
The first step in this process involves appropriate sizing of septic tanks. LSTS designers have the option
of designing either individual septic tanks or community septic tank systems. Thus, there will either a
small (e.g. 1,500-gallon) septic tank at each house or a series of large septic tanks at one central location.
Having a tank at each house and then pumping the septic tank effluent to a common treatment system is
referred to as a STEP system (septic tank effluent pump) system. To size individual septic tanks for a
STEP system, refer to Minnesota Rule 7080.0130. For the sizing of community septic tank systems,
follow the sizing protocol below.
If a STEP system is utilized, agreements have to be made as to whose responsibility it is to maintain and
pump the tanks (either the individual home owner or the wastewater operator for the treatment system).
There is a third option that could be considered: instead of installing septic tanks in which solids settle
out and accumulate, the designer may consider installing a wastewater clarifier with a continuous solidsremoval
system. Because capacity for solids accumulation is no longer needed with a clarifier, one can
often build smaller primary treatment units than if septic tanks were to be installed. Note, however, that
clarifiers have mechanical moving parts. This may offset any savings. Also, one has to decide whether
the wastewater system should look more like a mechanical wastewater plant or a large-scale septic
system. Either system works, but it is the designer’s decision as to what will work best for the situation
at hand.
Septic tank sizing determination
Septic tanks are designed for the removal of suspended solids, carbonaceous biochemical oxygen
demand (CBOD), heavy metals, nutrients, and some pathogenic organisms. For normal, domesticstrength
wastewater, properly designed septic tanks can provide discharge concentrations of 220 mg/L
or less CBOD, 65 mg/L or less total suspended solids, and 30 mg/L or less fats, oil and grease. Note that
the main design concern for septic tank sizing should be to assure the effluent exiting the last septic tank
and entering the secondary-treatment device (if applicable) is within the design CBOD and TSS loading
rates for that secondary treatment unit. If no secondary treatment unit is employed and the soil treatment
unit is instead the secondary treatment unit, the soil treatment unit needs to be sized to accommodate the
CBOD/TSS loading exiting the last septic tank.
Septic tank sizing is based on the peak design flows which ensures an adequate retention time during
high-flow conditions. Additional volume above the minimum required volume will provide additional
solids-settling capacity and will minimize downstream impacts from variable flows.
Design Guidance for Large Subsurface
Minnesota Pollution Control Agency
Wastewater Treatment Systems April 2010
21