Septic Tanks & French Drain Systems
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What do we know about septic tanks and French drains?Let me start off by making it clear that we do no profess to be experts in this area. We have cleaned hundreds of sewer systems - mostly linked to the municipal system, but some connected to septic tanks. We have repaired septic tanks and we have even installed a number of new septic tank systems - both the brick and plastic type. What you are going to read is based on this experience. I have noticed also that if you ask 10 septic tank "experts" a question you are likely to get 15 different answers! Anyway - here is some general information on septic tanks. Okay. If you are reading this then you most likely have a problem with a system that is not working? There are two really common issues with septic tanks and they are often related. The first is that the system appears to be "plugged up" and the toilets are not flushing. Also the tank may be overflowing? Our experience is that the drain field (French drain) is the most common culprit. There may be nothing wrong with the actual septic tank itself but the water is coming in faster than it can soak away. There are a couple of reasons for this. - Raised water table due to high rainfall. - Badly designed drainfield (French Drain) for prevailing soil conditions. - Contaminated (clogged up) drain field due to poor septic tank design or simply old age. - Under specified system because there is simply too much waste water to cope with. The second common problem is that the sewer pipes block. This often because of the very poor flow characteristics into the septic tank and out into the drain field. Again caused by the drain field not working correctly. This is why we are very hesitant to try and clear (unblock) a septic tank system that is "full". There is no flow to clear the obstructions and a problem in a sewer pipe is going to be amplified by the very poor flow. Often the solution is to pump out the tank, clear the obstruction and then fix the drain field. What is a drain field? The most common layout is one or more fairly substantial ditches filled with rock and covered with tin, biddim or other material, with topsoil over the top. A perforated 110mm pipe runs the length of the ditch and water is so discharged into the rocks so that it can soak away. The drainfield is a critical component of the system - this is actually where most of the "purification" takes place by means of aerobic (with oxygen) decomposition. The drainfield must be sized correctly based on the soil percolation level and flow rate. In the case of soils with a particularly high level of clay, you may find that a drain field is not an option. In this case you would have to use an aerobic septic tank. Phone me if you need one of these - I design and build them - 083 458 4836. What is a septic tank? Used in rural areas that are not serviced by municipal sewerage systems, The septic tank is a (suitably sized) storage tank constructed from a variety of materials that stores solid matter and water (sewerage) for long enough to enable biological activity to break down the solid matter. Key element here is time - the break down takes time so the stuff needs to sit there for a while - three days is the time often mentioned. The septic tank is traditionally split into two or more compartments - one leading to the next. The first compartment is where the primary solids are broken down. The second compartment is a settling tank that contains smaller suspended solids that settle out and further decomposition takes place. From the settling tank the water flows out into the drain field. The water that goes into the drain field should be relatively free of solids but still biologically toxic to all creatures!
Single tank systems are common now. All the decomposition occurs in a single tank that is only slightly baffled so as to exclude the surface scum from getting out. These systems are made of round domed plastic tanks with a hatch on the top. There is no settling tank and we think that this may put additional strain on the drain field in terms of fats and suspended solids, but we cannot say this for certain. Effective decomposition would occur if the tank is big enough though - but often it's not. This may lead to a shorter drain field life - we think. Tanks of brick or concrete are still being used/built. These are the traditional systems that have split tanks (compartments). The tanks are split about 2/3 from the bottom so that the bottom solids and surface scum are excluded from the next tank. The inlets and outlets are pretty (100mm difference) much at the same level so as to keep the level within the tank constant. Do septic tanks need a breather pipe? Theoretically yes, the anaerobic decomposition releases gasses - but I have seen many tanks without vents and they appear to work with no problems - least of all any smell. Aerobic (oxygenated) systems used extensively overseas actually pump air into a third chamber that promotes aerobic decomposition. This is more effective and ultimately results in cleaner water coming out of the septic tank.
An Aerobic system - Courtesy of The Septictankman at www.septictankman.com Why split the grey water and black water into two systems? This is a good idea on paper. What happens is that you have two types of water - grey and black. Grey is from the bath water and clothes washers. Black is from the toilets and sinks. Grey is not too nasty, black is lethal in terms of bacteria and other nasties. The grey water and black water sources are piped using two distinct systems. The black water goes into the septic tank and the grey water straight into the drain field. So, what's the benefit? You reduce the flow of water going through the septic tank, allowing longer decomposition times and you "poison" less water by mixing it with raw sewerage. You also keep detergents and other harmful stuff out of the septic tank. The water going into the drain field is less toxic - but it's still toxic because the black toilet water still uses the same drain field. You cannot use it - it's not clean because it's mixed with the black water out of the septic tank. So if you want to reuse your grey water you have to keep it far away from the water coming out of your septic tank. You could pump it into settling tanks and use it from there or you could build a separate shallow drain field that you use for growing plants on. So what I am saying is that unless you are going to actually use the grey water don't bother with two systems. If you are going to split the systems then split them totally. Piping grey water (with solids) directly into the drain field can reduce the life of the drain field - washing powder in particular. The septic tank is not partial to too much fat and oil, but you should not be throwing this down the sink anyway! Washing powder ash should be getting caught in the gully trap. If you dont have one then it may be worth installing a gully or grease trap for all the kitchen/laundry waste anyway. Septic Tank Maintenance. None - if everything is working you shouldn't have to maintain anything. It looks after itself. You do need to pump the sludge out every year or two though! As discussed previously the first problem you are most likely to spot is that the water has stopped flowing and the thing may be overflowing - this is invariably the drainfield that is clogged up. You can empty the thing by pumping it out but this is a short term solution. Long term fix is to create a "fresh" drain field. Make sure you don't get other rubbish going into the tank - towels, pads, plastic etc. If the tank is broken or faulty then it's often cheaper and easier to install a new one. Some thoughts on Septic tank construction. 1. Use a plastic type tank unless you have a good reason to build one yourself. They don't leak and they are fairly quick and easy to install. They should also last a very long time if installed correctly. Aerobic systems are easier to build using bricks. 2. Work out the size that you need - 2500 liters is typical for a large house, but with fairly conservative water usage figures. A modern "up market" house may require up to 3000+ liters. 1700 liters is often considered to be the minimum size. 3600 liters is a popular tank size. 3. In adverse soil conditions consult an expert to determine the size of the drainfield - a percolation test must be performed. 4. Do not get carried away with building big tanks using bricks! I would say a safe limit would be no deeper than 2m but it depends on the soil conditions and the quality of the brickwork, mortar and backfill. Use "Brickforce". 5. Remember that many tanks get destroyed when they are emptied - the force outside the tank collapses it - hence the warning about big tanks made out of bricks! Be somewhat cautious if you are pumping out a plastic tank for the first time - they are known to float out of the ground if there is ground water near the surface! 6. If you need to provide sewerage to multiple housing units - a complex, hotel or lodge - consider the use of multiple septic tanks rather than one big one. When the big one fails you are in big poo. 7. If you need a centralised "big" septic tank, seriously consider building two smaller ones near each other. The one can temporarily stand in for the other during trouble or maintenance. 8. Don't make the drain field too deep! The aerobic layer is only in the first half a metre of soil - that's where aerobic decomposition works best. 9. You had better make sure that you get the necessary approval from the relevant authorities. This may not be easy in some areas! 10. Consider using an Aerobic system. The water out of these can be used for irrigation and there is no contamination of the ground water. The only disadvantage is that it needs a bit of electricity to work - a small pump running for two to four hours per day. Contact me if you need more info on aerobic systems. 11. Consider the use of a dual drainfield that is selectable. Septic Tank Repair As a rule I no longer repair broken tanks. It's often an impossible job and it is almost always better to install another in a slightly different location. The old one must be filled up with earth or stone and buried. If you need additional info on septic tanks have a look at www.septictankman.com - it's more detailed :-) Updated : December 2011. Contact me on info@blockbusterdrain.co.za
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