Small Medium Sized Septic Tanks Design

Modern Septic Tanks are usually prefab using fiberglass while other material such as polyethylene (HDPE) has also been used in the construction of the tank depending on cost and design considerations. It is usually used in small wastewater treatment systems with the source coming from individual residences and it serve as a primary treatment option with the alternative being the Imhoff tank. Selecting the suitable septic tank is very important especially on the water-tightness aspect and it is the first step towards setting up your mini treatment facilities.

Design of a small and medium sized septic tank is very much dependent on unit loading factors and the expected incoming wastewater constituent concentrations. Depending on whether it will serve individual residence or scaled up to attend to the needs of small clusters of communities, typically interior baffles have to be incorporated and built-in to create compartments and divide the tank into partitions so that it can play effective roles towards waste removal. The size of the tank should be calculated based on in feed so that it can have retention time of at least 24 hours with 36 hours being the ideal figure. In order to prevent accidental discharge and carry over of untreated solids, usually an effluent filter has to be put in place and it can be installed without causing your budget and cost allocation to run off.

Maintenance of a septic tank can be minimal depending on how you construct and work on the design of the tank. Most common problem usually involves clogging of the system with large objects through the incoming feed line and thus before passing into the center of the vault, a screen filter with large surface area is definitely needed. Wastewater also contains other materials and settleable solids and in time these will form a think sludge cake layer collecting at the bottom of the tank. For those incoming wastewater that also contain oil and grease, these will float to the top and accumulation of scum will reduce the oxygen content going to the compartment that will prevent anaerobic breakdown of the waste. Thus usually the best approach involves emptying out the tank by pumping out the sludge layer accumulated at the bottom and removal of the floating scum as well.

septic tank design
For better understanding, let’s analyze the drawing of a typical small two-compartment septic tanks provided above to find out how design itself can help to prevent solids from escaping out of the system. As you can see, instead of using a typical straight pipe, there are actually tee pipes (T-shaped) used for the inlet and outlet line. The tee, will prevent floating scum from escaping because the wastewater before going to the discharge is actually drawn from below the water line. The same thing applies as well for transfer of wastewater between compartment 1 and compartment 2. The interconnecting pipe joint is actually placed in the middle of the tank and this will allow only water to pass through while heavy sludge solid will settle and collected at the bottom in compartment 1.

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