Septic tank design should be very good and most importantly accurate. This ensures its effectiveness and most importantly its durability.
A septic tank is a chamber which helps manage and control wastewater. This is as a result of bioactivities inside the tank which convert solid waste to liquid. Most septic tanks just separate the solid and liquid waste material. The wastewater will collect at the slung heap up at the bottom. As a result, the tanks will require emptying from time to time.
Basically, a septic tank design should ensure that: the inlet and outlet that are well placed, allow the sludge to settle at the bottom while the scum collects at the surface. In addition, it should have enough space is in between to facilitate the free flow of waste.
Factors to Consider When Designing a Septic Tank
For a septic tank design to fulfil these requirements and fully effective, it is important to consider the following during design:
Amount of Sewage Flow
The amount of sewage flow highly depends on the number of users. The tank should be able to hold the total waste for all users during the detention period. If the design does not account for all users, the tank will block more often and require emptying more frequently. This, on the other hand, increases the cost for maintenance.
Sludge holding Period
The septic tank design should account for the sludge holding period. The minimum sewage detention period in a septic tank is 24 hours. However, due to the total volume requirement for sludge and scum accumulation, a septic tank is designed for a detention period of not less than 48 hours.
The septic tank capacity depends on the sewage flow and detention period. In addition, space for sludge and scum accumulation before emptying is also considered. On average, the capacity should be 1 cubic meter.
Dimensions of Septic Tank
Septic tanks are usually rectangular in shape with a minimum width of 0.75 m and a length of 2 to 4 times the width. The minimum depth is 1 m. The size is different for housing colonies, hostels and boarding schools.
Inlet and Outlets
A septic tank has an inlet on one side and the outlet on the opposite ends of the tank. However, they are at different levels so as to prevent short-circuiting. The best pipes to use are elbow or T pipe which have a 100 mm diameter. Most importantly, they should submerge into a depth of about 200 to 500 mm below the liquid level.
Sludge Withdrawal and Disposal
Emptying a septic tank happens once or twice a year. Individual home tanks can lust for 2 years. Note that disinfection agent like phenyl can kill the bacteria in the tank. As a result, do not use flush chemicals down the toilet. In addition, non-biodegradable waste material like condoms and sanitary pads cannot decompose when inside the tank. As a result, Do not throw them into the toilet because they may lead to blocking or fill the tank sooner.
They provide a way out for the foul smell. The pipe should extend up to about 2 meters. At the top of the ventilation pipe, there should be a suitable mosquito proof wire mesh cover.
If you fail to account for any of the above, the septic tank will fail to meet the expectation on effectiveness. It most certainly will not last for long.
It is best to construct a single compartment for small capacity septic tanks. Larger capacities are the best fit for a two-compartment tank. The wall separating the two is about two-thirds the length away from the inlet. It, therefore, perform better than the single compartment tank. The two compartments interconnect above the sludge storage using pipes or square openings.
A concrete or plastic tank is the best to use. The floor and side walls are made using concrete. In addition, they are plastered to render a smooth surface. The floor has a slight slope of 10 per cent towards the sludge outlet. As a result, slung moves out more easily. The top cover of a septic tank is watertight and very thick. However, a manhole of relative size is created. This allows inspection and emptying of the tanks.
The septic tank effluent should be well and carefully disposed of away so as to prevent any nuisance or danger to public health. This is because it has a foul smell and contains a substantial portion of dissolved organic matter and pathogenic organisms. The disposing of the effluent mostly occur in two ways which are as follows:
Land disposal methods
In this method, an exhauster truck fetches the wastewater from the septic tank using hydraulic power. The track then carries it to land set aside for disposing and treating of the wastewater.
Soil absorption methods
In this method, the septic tank effluent is open to freely percolate into the surrounding soil. The effectiveness of this method depends greatly on the soil porosity characteristic. Moreover, some other factors like the level of the water table, the presence of vegetation, the current climate conditions, ventilation and concentration of suspended solids in the wastewater also influence the reliability of these methods.
Soil absorption takes place in two ways that are Soak pits or seepage pits and use of Dispersion trenches. Both methods are useful in all porous soils where the percolation rate is less than 25 minutes per cent meter.
Dispersion trenches have a percolation rate of between 12 to 25 minutes per cent meter. As a result, it is more preferred. However, in areas where the water table is high, it is best to place the dispersion trenches partly or fully above ground level and most importantly in a mound.
A septic tank design has to be well really though out so that it will never crack and pollute the groundwater. In addition, they should be able to hold the weight of the wastewater. In addition, it is odourless, More importantly, they should be able to stand any kind of earth movement. A septic tank design should be good and accurate. As a result, it can last up to 50 years of course with good use and maintenance.
At BlueFlame Energy Solutions, we discourage the use of traditional septic tanks because they require constant emptying using commercial methods. We advise the use of biodigesters that are capable of treating the wastewater and getting rid of the resultant water. You can read about our trademark B-Zone BioDigester System here.