We received a question from a homeowner in Florida wondering whether he should be worried about how his septic system was operating. The description leads to a couple of different topics that should be discussed relative to this system and to installation practices in general.
Here are the homeowner’s concerns:
“I live in Florida, and because our water table is so high, we have a raised septic mound that uses the gravity-flow system. Our house was built about 22 years ago, and the septic system was put in at the same time. We have never had any problems with our system (even after the recent Hurricane Irma). The only thing I’ve had to do a couple of times is replace some washed-out hydraulic cement at the inlet and outlet pipes at either end of the tank.
“This part is worrying me, and I need to figure something out. When I’ve had the septic system pumped every time since it was new, the water level in both sections of the divided 900-gallon concrete tank has always been about 8-10 inches below the top of the tank. The last time I had the tank pumped about 6 years ago, the technician pumping the tank noticed the bottom of the ‘riser pipe’ at the outlet side of the system was still closed from when the tank was installed about 15 years earlier.
“The technician got a hacksaw and cut off a small section at the bottom end of the riser pipe to open it up because he said the water that goes to the drainfield should flow through the ‘riser pipe’ and not as it apparently has been for the past 17 years by flowing over the top of the ‘baffle.’
“When I had the tank pumped about 14 months ago, I should have asked the technician if there was any way to look behind the baffle that the riser pipe goes behind to see if the top outlet of the riser pipe was also molded closed and the water is still flowing over the top of the baffle and directly into our drainfield.”
MANY POTENTIAL PROBLEMS
As we consider the description, we see indicators of problems with this system, even though they say there are no problems. It is always interesting how many times we go to a site where people have indicated there is no problem only to see — on cursory examination — evidence of continual surfacing of effluent. When asked, the homeowner usually says something like “Well, it really only happens during wet periods or when we do a lot of laundry.”
When they indicate they have had to replace the cement around the inlet and outlet, to us it means that the tank is not watertight and subject to root intrusion and infiltration. To be consistent with a watertight tank, the openings for the piping at the inlet and outlet should be evaluated to see if the openings can be made watertight. Using a wet mortar as he described does not provide an effective seal. The mortar does not bond well to the concrete and will fall apart. He has had to replace this mortar several times already.
In this case, the service provider can try some mastics, concrete grout or a bituminous seal to see if it will last longer. These may help, but they will probably need to be replaced periodically as well. Many tanks constructed now have flexible gaskets and boots cast in place during manufacture. Rubber boot seals are flexible, which means that they will allow some movement during backfilling of the tank and maintain the seal. It is important for the installer to protect the gaskets while setting the tank and installing the piping. If the seal is damaged, it is difficult to repair.
He then mentions that the effluent flows from the tank by gravity to an elevated mound because of the high water table in his area. We would agree that a mound is a very good system to deal with high water table problems. However, in a mound system, effluent needs to be distributed uniformly over the infiltration bed within the mound. There are a couple of reasons for this.
BEWARE OF OVERLOADING
In permeable soils found in most of Florida, if effluent is not evenly distributed, it will overload part of the infiltration bed. This leads to rapid movement through the sand into the underlying permeable soil through saturated flow rather than unsaturated flow. As a result, untreated effluent moves quickly into the near surface groundwater.
If the soils were slowly permeable under the mound, lack of uniform distribution will result in effluent concentrating again in one area. But in this case, effluent moves rapidly through the fill and then laterally down slope at the fill natural soil interface. This results in the effluent surfacing at the toes of the mound dikes. In both cases, only part of the infiltration bed is being used.
The only way to avoid this problem is to employ a pressure distribution network to distribute effluent more uniformly over the infiltration bed. When this is done, effluent moves through the sand fill and underlying soil much slower and under unsaturated conditions, providing the time necessary for treatment.
Pressure distribution requires a pump, which delivers the effluent through a manifold and small-diameter pipes with relatively small holes. In our next column, we will explore why we think this was intended in this case and was not installed, as well as the implications it may have for this system.
