A Septic tank needs to be strong enough
A well built septic tank will not leak and should be able to withstand earth movements, unfortunately many septic tanks in Indonesia are cracked and leaking.
It is easy to underestimate the forces that a septic tank has to withstand. You will see people building septic tanks using lightweight concrete blocks (batako) with a thin skim of cement on the inside. The slightest bit of ground movement, probably even the settlement resulting from the tank's construction, will be enough to crack it.
We should also bear in mind that when a tank is operating it will be holding a metric ton (1,000 kgs) of liquid for each cubic metre, for an average tank this is likely to be as much as 3 or 4 tons. That is a lot of weight.
Septic tanks should be either made from plastics that will give a little with ground movement or be built from reinforced concrete and designed such that in the event of ground movement the tank will move as a single entity and retain its integrity.
In addition how well your septic tank will work is dependant on how well it is designed and built. There are many people around building septic tanks who do not fully understand how they work. I am often asked about the subject so this week we will get technical I'm afraid, we'll look at a well designed septic tank and explain its design features.
How many chambers?
A septic tank should have at least two chambers, 3 is better but 2 is enough. The second chamber can be around half the size of the first chamber.
In this tank the first chamber is 1.2m x 1.0m x 1.6m deep = 1.92 cubic metres liquid capacity. The second chamber is 0.6m x 1.0m x 1.6m deep = 0.96 cu m capacity = a total capacity of 2.88 cubic metres. Note that 1.6 metres is the depth of the effluent in the tank.
This tank is purposely designed to be quite deep while having a smaller footprint for use on smaller blocks of land.
Septic Tank Construction
When casting the concrete shuttering (plywood formwork) is placed around the steel to form a box shaped mould ready to cast the concrete around the steel. This should be placed to make sure that the walls are 4 inches thick and, when the concrete is poured, that there will be a good thick layer of concrete on both sides of the steel mesh to stop the steel rusting.
There should also be a bed of sand underneath the concrete floor of the tank to allow some movement as the tank settles and moves in the ground.
The 15cm (4 inch) diameter PVC inlet, transfer and outlet pipes for the tank need to be installed before the concrete is poured along with a 7.5cm (3 inch) diameter ventilation pipe to allow gas to flow between the 2 chambers.
The concrete should be correctly mixed (the mix should be designed to suit the sand and other materials being used however if this is not possible then 3 parts aggregate : 2 parts good quality sand : 1 part of cement is good). Too much water should NOT be added (a slump of 7 is good) to give a good hard concrete and, during pouring, the concrete should be vibrated with a vibrator to make sure it is properly compacted with no cavities or air holes. The aggregate must be small enough to fit easily between the steel and the plywood.
This might all sound complicated, in fact it is not difficult and following these straightforward guidelines will make sure you have a well built tank that will not leak or crack. If it all sounds too hard get an engineer to help you while you get yourself a good dose of Yoga, the �difficult flying cow pose� is particularly useful when contemplating the mixing of concrete.
So we now have a concrete box with 2 chambers.
Levels are very important
It is important when designing the septic tank that you pay close attention to levels. The inlet pipe to the tank that carries effluent from your toilet to the tank need to be 15 cms diameter and a slope of at least 2% (a fall of 2cms per meter) all the way from the toilet to the tank.
The tank must be built deep enough to allow for this and should be carefully planned before you start.
The inlet pipe should go straight into the septic tank. Many contractors want to install small manholes (locally called control boxes) to access the inlet pipe just before it enters the tank. These manholes are a common cause of blockages and smells. It is better to have a free flow straight to the tank.
Inlet, Outlet and Transfer pipes
We now install the pipework. Note the positions of the inlet, outlet and transfer pipes inside the tank, the inlet pipe is higher than the outlet pipe and the transfer pipe is a bit lower than both. These relative heights are important.
This tank design has 15cm (4 inch) tee pieces on both the inlet and outlet pipes and on both ends of the transfer pipe between the chambers, these keep the pipes clear of the scum that forms on the surface of the effluent as it is digested by the bacteria. They also help to reduce any turbulence that might be caused as effluent runs into the tank and from one tank to the other. We want as gentle a flow as possible through the tank.
The transfer and outlet pipes have vertical extensions on the tee pieces in the upstream chamber. This is so that liquid entering the pipe is taken from the central level of the tank below any turbulence and high enough to not get blocked by deposits at the bottom of the tank.
Finally an open 7.5cm (3 inch) diameter pipe is installed between the 2 chambers above the water and scum level to relieve any pressure buildup and allow free flow of gases between the 2 chambers.
Manholes and Vent Pipe -
We can now install the top of the tank, once again a 15cm (4inch) thick reinforced concrete slab is used. The top has a 2 inch diameter galvanised steel vent pipe in it. In this design the pipe is over 2 metres high to allow any gases to be freely dispersed in the wind. Remember that when operating properly a septic tank should not smell.
The top also has 2 manholes in it. Normally these will be sealed with grout to make them gas tight. The manholes are large enough to allow someone to enter the tank should maintenance be needed and are positioned so that they allow access to the inlet and outlet pipes to clear blockages.
The manholes also allow access to pump out the tank, a process you may have to carry out say once every five years. Many tanks are built without manhole covers but have 15cm (4 inch) diameter pipes in the top to allow the tank to be pumped out. In these cases it is not possible to get into the tank to repair or unblock it should the need arise.
Note that the manhole covers are designed to seal with horizontal surfaces. You will find in some places that contractors may make the manhole and their covers fit together with a tapered joint. It is very difficult to make tapered joints that fit properly and so they are difficult to seal. Flat surfaces are much better. Also make sure the manhole covers have steel handles so they can be lifted.
Why is my Septic tank full?
Last week I received an interesting question:
"I have a faulty septic tank. The tank is quite large, approx. 5 x 2 x 1.5, but fills rapidly - in the wet it needs to be pumped out often, sometimes every two weeks, the dry considerably slower. Obviously, ground water must be entering, but what to do about it is the question?"
When operating normally a septic tank should not fill up and, as a guideline, should only need pumping out probably once every 5 years if that. This is a very large septic tank (15 cubic metres, most tanks are only a quarter of this size) and should take a while to fill up.
The first question is “has the tank been working properly in the past or has it always needed pumping out regularly”. If it has always needed pumping out then it is likely that there was a fault with the original design or installation.
In the opening question the fact that the tank fills faster in the wet season is a bit of a give away and supports the writer's suspicion of ground water entering the tank. We don't go to the toilet more or have more showers in the wet season do we?
If excess water is getting into the tank it may well be that drainage water has been connected to the tank or somehow surface water is running into it. Septic tanks should be isolated from drainage and should only take waste from toilets, showers, washbasins, washing machines and sinks.
Drainage water entering the tank is a health issue in addition to the problem of the tank filling up when it rains. We need to check if the pipework is connected correctly or if there is a pool of water over the tank after heavy rain?
It may be that the tank has been built in an area of clay (rice fields are lined with clay) and the whole area the tank is in fills up with water that can't drain away. It is important that the area around the tank can drain properly.
It may possibly be that the tank is cracked and ground water is getting in from the surrounding ground. This is highly unlikely unless the tank has been incorrectly situated in clay as above. It is very seldom that the water table is higher than the top of a septic tank and so water would more likely drain out from a cracked tank rather than into it.
If the tank is filling up then it may be that the outlet pipe from the tank is blocked. In other countries there are strict regulations regarding what is known as the “soak field” into which septic tanks drain. Here in Bali most people do not give much consideration to where the semi processed effluent from the septic tank goes to, most people have never heard of a soak field even though most people use septic tanks. This “blind eye” attitude to the discharge pipes of septic tanks is probably the main reason why there are so many bowel problems associated with Bali.
A final possibility is that the flow from the primary chamber to the secondary chamber inside the septic tank is blocked. It could be that the level of sediment in the primary chamber of the tanks has risen above the level of the pipe and blocked it. If the tank has been pumped out this is unlikely but possible.
Calculating Septic Tank Size - How Big?
This whole discussion raises the question of how large a septic tank should be. This is of course dependent on how much waste is being produced and is directly related to our water consumption for “internal” purposes such as showers, toilets, baths, dishwashing, clothes washing and face, hair and teeth cleaning.
Obviously when considering water usage we have to consider lifestyle differences. Some people tend to use a lot of water, some like to bath instead of shower, in hot climates people tend to shower more often. It has also been found that the more affluent we are the more water we waste.
Let us, do it.Step1:- Calculate the liquid capacity of the tank.
Let take @120 liters per user as i have told you to consider above.
= 0.12 X 20 = 2.4 meter cube
[Since, 1000 liters = 1-meter cube of water]
Step2:- Assume the liquid depth of the tank.
Let it be 1.3metersrs.
Therefore, the floor area will become,
=2.4/1.3
=1.85 meter square
[ Volume of tank= Area X height]
Step3:- Assume the length of the septic tank
Let it be 2.5 times the breadth,
Then, L = 2.5 X B
Where B is the breadth of the tank.
Or, L X B = 1.85 meter square
Or, 2.5B X B = 1.85
Or, B = Sqrt(1.85/2.5) = 0.86 meter
Hence, the dimension of the tank will be (2.5 X 0.86 X 1.3) m
I Hope friends, you liked my written article on “Design of septic tank calculation” and remains helpful.
How to design a septic tank?
Here i have listed the pointwise requirement to design a proper septic tank. So, read it carefully.
The capacity of the septic tank should be sufficient enough to hold liquid taken 130 liters to 70 liters per member.
For the best design of the septic tank, the mortar used at the wall of the septic tank should be used of thickness more than 20 cm and concrete used below the septic tank should be of M15 or M20 grade.
The outside, inside faces of the wall and top floor of the septic tank should be plastered with a thickness greater than 12mm of 1:3 or 1:4 grade of mortar. The concrete should be prepared waterproof and all the inside corners of the septic tank should be rounded giving slope toward the center base of the septic tank for better extraction of a slug at the time of removing slug. Also, the floor of the septic tank should be given a little slope (1:15) toward the inlet side.
The width of the septic tank should not be less than 750 mm.
The length should be provided 2 to 4 times the width of the septic tank.
The depth of the septic tank should be kept 1000 mm to 3000 mm (+ 300 mm to 500 mm according to water level. But should not exceed 1800mm (+ 450 mm) for freeboard.
The minimum capacity of the septic tank should be 1 cubic meter.
The detention period of septic tanks should be considered to be 24hrs (mostly).
For inlet pipe, an elbow or T pipe of 100mm diameter should be deep below the liquid at the depth of 250mm to 600mm and for outlet pipe at the depth of 200mm to 500mm.
The inlet baffle wall should be at (1/5) the distance of the length of the wall. The baffle wall should be generally enlarged 150 mm above and 400 mm to 700 mm to scum level.
The roof of the septic tanks should be covered with RCC slab of thickness 75 mm to 100 mm. If circular manholes are made then diameter should be 500mm but if rectangular manholes are made then its dimension should be 600X450 mm for inspection and desludging.
The outlet pipes for ventilation purposes may be of cast iron or asbestos should be kept 2-meter minimum above the ground level and diameter should be of 50 mm to 100 mm. The topper open part of this pipe should be covered with net or cowl to prevent the mosquito from entering through it.
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