Conventional septic systems work in an environment without oxygen (anaerobic system),
the natural aging process of that oxygen-free environment promotes the growth of a black sludge called biomat. Over time this growth seals and prevents the system from releasing water into the ground.
Since the biomat survives and thrives without oxygen it also dies and cannot survive when exposed to oxygen (aerobic system). Aerobic treatment devices have been found to restore the original functioning of the soil absorption system in a matter of months.
Summary
While the “biomat” in the gravel-soil interface of a septic system’s soil absorption field is critical for wastewater treatment, biomats that become too thick due to excess organic matter or water can reduce the system’s effectiveness and result in system failure. Normally, there are three options for reducing the BOD load on the soil absorption field and correcting this problem:
- Construct an additional soil absorption field
- Rest the absorption field for an extended period
- installation of an Aero-Stream aeration unit
Wastewater Biological Oxygen Demand in Septic Systems
Brad Lee. Don Jones. and Ron Turco
Purdue University Department of Agronomy and Department of Agricultural and Biological Engineering in Doc HENV-14-W States:
Household wastewater contains a mix of chemicals,
impurities, and other materials. One of the most important
of these materials is organic matter, which is composed
of carbon (C), nitrogen and oxygen. In septic systems, C
comes from the digested and undigested food we eat, as
well as the microorganisms that live in the system.
The amount of biodegradable C found in septic systems
is important, because by measuring it, we can determine
the waste stream’s biochemical oxygen demand (or, BOD).
BOD refers to the amount of oxygen microorganisms
must consume in order to oxidize (that is, break down)
all the organic compounds in a liter of wastewater. This is
important, because a high BOD number means potential
septic system problems for homeowners, while a low BOD
means fewer problems for homeowners.
The Proven Science the Wastewater industry Does not want you to Know About
In 2009 Baylor University conducted a research study retrofitting conventional septic systems to aerobic systems. The results of the Baylor University study and the effects of converting a traditional septic system to an aeration system are nothing short of astonishing.
After 9 months of aeration, the retrofitted septic systems were discharging effluent clean enough to surface discharge the Percent Reduction was as follows.
CBOD5 mg/l 87%
TSS mg/l 96%
- coli col/100ml 98%
Fecal col/1000ml 98%
We discovered in 2002, a proven, perfected the science and hold the patent for the technology that converts an anaerobic system to an aerobic system.
The effluent mean values after 30-days of aeration for Septic Tank AA1, Septic Tank AA2, Type 1b soil, and Type 3 soil compared to the Raw influent during those same 30 days in Table 1. More detailed data is found in Appendix A. The more detailed data in the Appendix show median as well as mean values for CBOD5. The median values may be more applicable because of a few outliers in the data.
Table 1. Mean 30-day values for retrofitted systems after 9 months of aeration.
System Raw AA1 AA2 Soil1b Soil 3
Raw to AA1
Raw to AA2
Parameter Mean % reduction % reduction
CBOD5 mg/l 235 43.5 29.4 NA
TSS mg/l 186 22.6 7.4 NA
TKN mg/l 39 36 36 5 1
NH3-N mg/l 24.5 27.8 30.3 5.1 0.2
NO3 mg/l .16 .11 .11 19.5 28.4
PO4 mg/l 4.39 4.21 3.93 0.66 0.06
TP mg/l 6.04 4.66 4.25 0.46 0.05
- coli col/100ml 2,635,000 391,067 59,199 21 711
Fecal col/1000ml 2,322,500 166,040 41,217 10 174
Temperature C 17.6 15.8 15.7 NA
Ec us 933 1289 950 NA
pH units 7.11 6.97 7.30 NA
DO mg/l NA .74 1.29 NA NA
The addition of aerators in septic tanks AA1 and AA2 produced CBOD5 and TSS values that were quite low and there was significant improvement from AA1 to AA2. The addition of oxygen may have continued to drive the N cycle and appeared to increase NH3–N in the septic tanks but the NH3–N decreased drastically in the soil columns.
However, the total nitrogen as measured by TKN appears to decrease slightly from the raw in the septic tanks then decrease drastically in the soil columns. The NO3 values also appear to decrease in the septic tanks slightly but the differences are not statistically significant. The nitrate values increase significantly in the soil columns and much more so, in the Type 3 soil column.
The TP, PO4, E. coli, and Fecal coliforms all decrease throughout the treatment train. The largest decreases occurred in the soil columns but the aerated septic tanks also decreased each of these parameters. Although the soils decreased the TP and PO4 significantly in this 30-day study, the soil should eventually saturate with respect to its ability to adsorb P.
Soil could accept onsite wastewater at rates two to three orders of magnitude higher than the current design loading rates if a clogging mat at the wastewater infiltration surface was limited or not present. The clogging mat controls system design, loading rate, and life.
Maintaining aerobic conditions at the wastewater infiltration surface could substantially reduce or eliminate clogging. This project is studying soil oxygen supply to the zone of clogging in soil. A model based on a form of Fick’s Law for diffusive transport is being applied to oxygen diffusion to the wastewater infiltration surface. Gas-filled porosity controlled by soil characteristics of texture, structure, consistency, and water content as well as the distance to the supply of oxygen and rate of oxygen consumption controls the flux, F, of oxygen.
If the oxygen consumption rate exceeds the maximum flux of soil oxygen, then the soil infiltration surface will become anaerobic. To maximize the delivery of oxygen, soil components should be shallow, narrow, and have separated infiltration areas. Using models that incorporate system depth, geometry, and oxygen diffusion coefficients in soil, efficient loading rates are estimated. The design of wastewater infiltration surfaces should be based on both oxygen transport and hydraulics. In many cases, oxygen transport will be limiting and therefore the basis for design.
