By Jerome Fife
Years ago, I recognized that there’s often a lack of attention paid to the inevitable physical decline of typical drain fields or leach lines. While these issues may occasionally be acknowledged with nods of agreement, few truly investigate the implications behind those nods. The biological breakdown, or biomat clogging, of drain field soils is a natural consequence of conventional anaerobic septic effluent entering a drain field, and this is something everyone should comprehend. There are other physical factors that can diminish the effectiveness of a drain field — though they are limited and deserve explanation. After introducing the concept of remediation for drain fields over a decade ago, we conducted extensive research to identify potential barriers to our remediation success. Surprisingly, we found that there are not many physical issues that arise. I will outline the most prevalent of these concerns: siltation.
Over time, silt can infiltrate the spaces between drain rocks due to years of rainfall or snowmelt — unless you’re in a dry area like Arizona or the Southwest. When I began my career in construction, I was told that siltation meant the end for a drain field. However, that’s not true, and here’s why. An engineering study conducted by a group in Northern California revealed that when you fill a cubic foot of space with round or spherical materials — regardless of size — approximately 65% of that volume is occupied by the material itself, leaving 35% for air pockets. This includes everything from bowling balls to marbles to sand and silt. What matters is avoiding flat surfaces that would compress together and eliminate air space (so crushed aggregate doesn’t fit into this analysis).
This finding translates into potential liquid storage capacity per cubic foot of drain field as follows: 7.48 gallons per cubic foot minus 65% (4.86 gallons), which gives us 2.62 gallons available for liquid storage per cubic foot. For instance, if you had a 100-foot leach line that is one foot wide and one foot high filled with aggregate (natural or synthetic), you’d have a total liquid storage capacity of 262 gallons. This example is purely illustrative and not meant to reflect any specific drain field. When siltation occurs, 65% of the air space becomes occupied by spherical silt particles. There are two important considerations regarding siltation.
First, it’s important to note that the silt does not compact under normal conditions as one might expect. The aggregate bears the load, transferring it to the ground around the air spaces now filled with silt, meaning there’s no pressure causing compaction. As a result, the silt remains light and permeable, allowing liquids to flow through it. This permeability means there’s still capacity for liquids to leach through the uncompacted silt (liquids free from biomat-producing bacteria).
Second, there’s still air space among the silt particles. Recall our earlier example? The spherical shape of the silt means it occupies only 65% of the air space between aggregates. Thus, we retain 35% of 35% for air space and liquid storage volume. In numbers, if we started with 262 gallons of storage capacity in the air pockets around the aggregates, with silt filling those spaces, we’re left with 35% of 262 gallons — amounting to 92 gallons. Drain fields in typical septic systems have significantly more cubic footage than this example suggests.
Now imagine having 300 linear feet of leach line as described above. This would yield at least 276 gallons of liquid storage capacity — likely sufficient for a family of four for a day’s worth of wastewater. The aggregate remains surrounded by highly permeable silt that facilitates liquid movement through it. Consequently, this hypothetical drain field would still function adequately for disposing of water if it were just rainwater or potable water. However, we’re dealing with septic effluent.
Anaerobic effluent brings along biomat-producing bacteria that can clog both the infiltrative soils and the voids between silt particles, rendering them non-permeable. By removing the biomat from both the infiltrative soil voids and the silt, we can restore functionality to the drain field.
Now let’s talk about a viable solution: the Pirana System. This system effectively removes biomat from both the silt and infiltrative soils by biologically reducing its viscosity (liquefying it), thus restoring permeability. Eventually, the Pirana System consumes the biomat entirely. The effluent leaving a septic tank equipped with a Pirana operates similarly to rainwater or drinking water — free from biomat-producing bacteria or significant organic matter.