Question:

I see a higher failure rate for new systems compared to systems built 20-30 years ago. Most systems now are the newer technology. New houses in my area use more gallons per day than older households would. Some with open ends to allow air into soil can recover and work a few more years. I disagree on your March article about piping the whole system. How could you vent both ends if piping didn’t carry the air through the system?

Answer:

The higher failure rate is an observation you have likely made as you do your job as an onsite sewage treatment professional. Can one conclude the “old” systems are better than the “newer” technology? I’m sure many would take exception to such a conclusion.

I understand manufacturers of many of the newer types of effluent distribution systems have convinced people who issue onsite system permits that systems can be smaller in area than the “old” rock-filled trenches. There has been some analysis supporting these claims.

To me the basic problem is the code enforcers who have not required the installation of any devices to measure the actual sewage flow into these systems. Such information would quickly tell if the daily water use rate was too high or if the soil treatment area was too small.

We conducted some studies at the University of Minnesota to measure sewage flow rates from typical residences. The general finding was, as I recall, households were using only 50 to 60 percent of the daily sewage flow for which the soil absorption system was sized. So the rock filled trench system could have been smaller in area for the actual sewage flow and it would not have failed. I am not suggesting this would have been good policy.

PROPER SIZING A KEY

The point I am making is the “newer” types of effluent distribution may not be as effective as claimed. But they have been working, for the most part, because the actual sewage flow from the household has been less than the amount used in sizing the system.

Perhaps many of the reduced area “newer” systems are almost full to capacity, but still working adequately. The use of extra water or failure to clean the septic tank regularly may result in system overload and failure.

Granted, this is a supposition on my part, but I am of the opinion this is what is happening. With proper technology and proper system sizing, one would assume the newer systems should be lasting longer than systems installed with the older technology. It is interesting you have observed the opposite is happening.

Your observation that new houses use more water than older houses is interesting. What information do you have about this? Do you have water meter readings?

Studies measuring sewage flow from households some years ago found about 40 percent of the total daily sewage flow was from the flush toilet. Those toilets were discharging about 5 gallons per flush. Toilets installed today discharge 1.6 gallons per flush.

For an older household using 300 gallons per day, 40 percent or 120 gallons would have come from the flush toilet. At 5 gallons per flush, this would be 24 flushes. At 1.6 gallons per flush, 24 flushes of the newer toilet would discharge only 38.4 gallons, a daily reduction of 81.6 gallons. I fail to understand why the newer houses should be discharging more sewage each day, unless the occupants are wasting water.

WHY VENT A TRENCH?

Venting the trench is another worthless practice that seems to persist in the onsite industry. To those advocating vents I ask: What is the purpose of attempting to vent a trench? Why do some states have a regulation in their onsite code preventing the trench vent from being too close to a residence? What causes the movement of air or gases through the vent system?

I will answer those questions as I understand them. The idea behind venting is to keep the trench aerobic or with a surplus of oxygen. However, septic tank effluent is anaerobic and no amount of air flowing through the so-called distribution pipe can provide oxygen to the liquid effluent below. The need for oxygen is in the soil under the trench bottom and along the trench sidewalls. The amount of oxygen present in the soil depends on the soil texture and depth of the trench. The oxygen in the soil air is needed by the aerobic bacteria in the soil to break down the biomat and prevent it from getting thicker and thicker and restricting flow into the soil.

When the methane gas generated in the septic tank builds up pressure, this gas, along with mercaptans, which cause odor, comes out of the vent at the end of the trench. To prevent a problem with odor in the home, the onsite code establishes a distance the vent pipe must be separated from the building. When odor is being discharged from the vent pipe there certainly is no air entering the vent pipe.

The movement of air, or gases, through the vent pipe is caused by a difference in pressure. If there is no difference in pressure between the “inlet” and “outlet” of the so-called venting system, there will be no movement of either gas or atmospheric air through the system. Wind blowing from a certain direction may cause air to move into the vent pipe at the end of the trench. Wind blowing from the opposite direction may cause air or gas to move out of the vent pipe at the end of the trench.

CONCLUSION

In my opinion, the vent pipe at the end of the trench serves no practical purpose for the operation of an onsite sewage treatment system. And the vent pipes certainly do not add to the beauty of a landscaped lawn.

However, a fringe benefit may be they do serve to identify the location of the end of the trenches and remind the homeowner an onsite sewage treatment system is present and needs periodic maintenance.