In this article we will discuss about the design of Imhoff tanks, explained with suitable diagram.
Imhoff tanks are the improvements over the septic tanks. These tanks derive the name ‘ Imhoff tank’ after its inventor Karl Imhoff of Germany. Imhoff tanks are two-storeyed tanks which have large settling tanks and below it are sludge digestion chambers. Fig. 18.6 shows the essentials of an Imhoff tank. Number of interconnected hoppers is provided below the settling tanks.
First the sewage enters the upper sedimentation tank whose bottom has sharp inclinations. The solids are allowed to settle in the upper tank from where they slip in the lower hoppers through the slots. In the hoppers the settled solids remain stored for a long period – about 30-45 days. During this period, they are acted upon by the bacteria and are converted into stable solids, organic acids and gases.
The gases are allowed to escape in the atmosphere. The stabilized solids are taken out by means of a sludge pipe under hydrostatic pressure. The flow of solids in the lower hoppers is regulated by means of a triangular beam. When one hopper is filled up with the solids its top is closed by means of he triangular beam and in that hopper digestion starts.
The effluent of Imhoff tank is similar to the primary settling tank. The organic matters are digested in the lower compartments. The digested sludge has black colour and has no odour. The moisture contents of this sludge is 90-95%, therefore it can easily flow in the pipes. This moisture can be removed by passing it through sand beds and sun-drying.
These tanks are simple in operation and the process is automatic, uniform and continuous. The sludge can be easily removed under hydrostatic pressure, therefore no pumping is required. The disadvantage of these tanks being more depth (8- 10 m), on operational control and the fouling of the atmosphere due to the developed gases, which are allowed to escape in the atmosphere.
The slope of the septum side walls of the lower portion should have steep slope more than 55° to the horizontal to prevent adhesion of solids to them.
ADVERTISEMENTS:
The design of the upper flow tank is done on the basis of the plain sedimentation tank with 1.5 hour to 2.5 hour detention period. The depth of the upper flow chamber is kept about 2.5 m. The minimum total depth of the tank is kept 5.0 m. The design of the sludge storage capacity is done on the basis of 0.05 to 0.02 cu.m per capita.
The surface overflow rate is kept less than 30,000 litres/sq.m/day. The gas vent area is provided between 20 to 25% of the total plan area. If the sufficient gas vent space is not provided, very bad condition of foaming will develop.
Minimum slope of 30° is provided to the hopper bottom for easy removal of sludge through the sludge outlet. The vertical distance between the upper sludge level and slot which is known as neutral zone is kept more than 45 cm.
These tanks require daily attention for obtaining high efficiency and for preventing unsightly conditions and objectionable odours. The scum collected at the surface of the liquid should be removed daily with some type of skimmer.
ADVERTISEMENTS:
If care is not taken the solids may adhere to the septum walls and become septic and lightened by gas, break off and come out with the effluent. It is better to seed the tank with new well-digested sludge from other tank. The long handled squeegee run down in the lower tank from the upper through the slot.