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Building Principles: Septic Tanks — One of Camp's Dirtiest Little Secrets
Septic tanks are one of the most frequent topics during our “Ask an Engineer” sessions at conferences. They seem to be an ongoing point of contention between directors and the facilities staff — and often, we are asked to provide the “final” answers. As much as we’d like to be the tiebreakers, the best we can offer is to be the peacemakers. The following summary provides you with some of the most frequently asked questions, along with the answers, about subsurface sewage disposal systems.
How often do we need to pumpour septic tank?
As a general rule of thumb, our answer has been every two years for systems that serve both boys and girls, as well as for those systems that serve girls only. Boys’ systems can probably go three to four years. And this depends, of course, on whether the tank itself is properly sized.
OK, then. How big shouldyour septic tank be?
Generally, the design of a new septic tank installation requires that the tank be sized for at least 1.5 times the average daily flow received. The idea here is that the solids, grit, and heavier particles in the waste are able to settle out of solution. As you would expect, particles suspended (not dissolved) in the solution can fall to the bottom only when they are provided a space where the energy in the fluid itself can drop below a certain level. The energy in the fluid is greatest at the inlet side of the tank, and it decreases the further away from the inlet you get. This settling can be mechanically assisted by one or more baffles or walls inside the tank. These help contain the energy of the turbulent solution to the “in” side of the tank. In this way, clarified effluent (waste liquid) is decanted from the outlet side of the tank.
At many older installations, though, there is only one, single-chamber tank. Most often, it is very undersized, as well. In these cases, adding a second tank downstream of the existing one is a solution to consider. By doing so, additional storage volume can be inserted (to equal the 1.5 times the daily flow), letting the first tank behave like the baffled quiescent zone in a single tank.
Wonderful information, but you still haven’t told me how big the tank should be.
It’s a function of the amount of water used at that particular facility. We frequently recommend that operations install and read water meters at locations that are connected to sewage treatment facilities of any sort.
Water meters? I thought that we were talking about sewage.
Meter installation and reading is an important practice for several reasons related to your sewage disposal system. First, since your current sewage disposal system has to treat all the water that comes into the building, it makes sense to know how much water is passing through the system.
Next, no system lasts forever. When the time comes for a replacement, your designer must size the system in accordance with current regulations and will not be allowed to duplicate what’s already there. Without site-specific, current, and detailed data, the designer will be required to use standard estimates of water usage to determine the size of the new components. To help insure that the system is not undersized, these standards are usually very conservative (i.e., “high”) estimates of what water usage could be at a similar facility. They do not take into account any water conserving fixtures you may have in place, and so you may end up installing a system, which is much larger than what you really require.
As an aside, it’s important to mention that the “reading” part of the earlier statement is every bit as important as the “installing” part. Most regulations allow the designer to use one year of continuously collected meter data in place of the standard design figures. If you are a seasonal operation, you will probably be able to use one full camping season instead of twelve months, but only if the data is collected from the time that the water is turned on until the time that the water is turned off.
If I have a meter at my well,do I need another one?
The meter at your well is a great thing. However, your system probably has leaks. In older distribution systems, you could be losing as much as 50 percent of the water produced to the ground around the pipes. In addition, it’s likely that your water distribution network has common, non-treated uses for water such as drinking fountains strapped to trees, which drain to the ground, and vehicle wash areas. Without “point-of-use” meters, there is no way for your designer to justify not using the standard sewage production estimates. In fact, it is possible that because of leaks and such, the meter at your well will indicate that your system has to be bigger than the standard design figures require.
Let’s get back to the tank. What about things that float and don’t settle out?
Anything that floats in the tank should be kept in the turbulent side of the tank — again, by the baffles inside. As you can see from the figure below, the baffle extends to the top of the tank, and the liquid passes through holes below the water line. In this way, only partially clarified liquid — devoid of “floaters” — is permitted to pass to the secondary settling area or “quiescent zone.” Notice also that the outlet has a pipe that extends below the normal surface level of the liquid. This helps to ensure that only clarified effluent is allowed to pass.
At this point, we need to return to the pumping frequency question. I have been redressed more than once for recommending that systems that serve mixed gender populations should be pumped more frequently than systems which serve boys and men exclusively. My assertion is based on the question at hand. Feminine hygiene products and hair conditioner are two of the most notable septic tank floaters. The two products together create a dense, floating mat. Unless they are removed often, not only will the tank tend to back up the incoming sewage lines, that mat will force other incoming solids into the other side of the tank. This defeats the baffle altogether, allowing the solids or grease to get out into the septic field or mound, leading to a very expensive failure.
Why should we need to pump?We feed our system a little packetof powder and it cleans the system out for us.
This is probably the biggest fallacy we encounter. Most of the “magic” powders contain dried enzymes similar to those found naturally in the human digestive tract. With every flush, scores of the digestive microbes are delivered to the system — so many, in fact, that the large majority of them die of starvation. To add more is a literal case of flushing money down the toilet. In addition, many of these powders contain other components, which vigorously give off gases — agitating the tank. This brings previously settled solids back into suspension. There is no documentation or studies of any sort whatsoever which indicate that tank additives perform as advertised. Save your money, and pump on a regular schedule instead.
Any closing thoughts on the care and feeding of our septic tank?
Yes, watch the show! When your contract hauler comes, make sure that he’s supervised closely. Septic tanks are often old and weak, and their exact underground location and orientation is often a mystery. More than one hauler has sunk a truck while pumping a tank. An extra pair of eyes can help spot this before it becomes a real mess involving tow trucks and emergency repair permits. Also, the digestive processes associated with a septic tank create several different gases, which may overwhelm the person who first opens the tank.
At least once, your staff should take depth measurements before and after pumping. When done, the hauler should present a slip that indicates the number of gallons removed. Combining that volume with the measurements enables you to easily calculate the actual usable volume of the tank, and then determine whether you have enough storage.
Don’t allow pumping through a small pipe (usually four inches in diameter) called the “inspection port.” The hose from the truck should be about four inches in diameter, and the tank opening should be about two inches. The contents of the inlet side should be drained through a large manhole-sized opening. This allows the operator to use a long, paddle-shaped device to agitate the contents, mixing the floaters and grease with the solids in the bottom. You’re paying for the number of gallons removed, and you want to be sure that the maximum volume of solids is suspended in the liquid taken.
One last point about supervising the pumping operation is that it provides your staff an opportunity to inspect the covers and the visible interior of the tank for deterioration and to insure that the cover is replaced properly.
And, fill ‘er up! Refill the tank with clear water after pumping. Really. Remember that the baffle confines both solids and floaters to the inlet side of the tank. The most intensive water use is with hair conditioner being a principal component of the floaters. If the tank has been drained below the holes in the baffle, the oil will flow to the downstream side of the baffle until the level in the tank is higher than the top of the holes.
Finally, depending on the type and age of the pumped tank — a tank which has had most of the water removed has also had most of its weight removed. If the groundwater level is up, it is possible that your septic tank could pop right out of the ground like a fishing bobber, ripping up all of the connected lines with it! It happens, even with concrete tanks.
With proper care, your on-site sewage disposal system will last and serve your operation for many years.
Rick Stryker is a professional engineer with Camp Facilities Consulting providing study, design, permitting, and construction consultation services to the camp and conference center community. Camp personnel may contact him at 570-296-2765 or by e-mail at firstname.lastname@example.org.
Originally published in the 2003 January/February issue of Camping Magazine.