Recycling and reuse of wastewater will do away with the need of using fresh water. The extent to which the processes involved in wastewater treatment are cost effective and easy to follow will act as an impetus. Hence discussing about the processes involved in wastewater treatment and their relative merits and demerits becomes pertinent.

Like Sedimentation tanks, Septic tanks (Imhoff tanks) can play a major role in the process of removing solids from wastewater.

Designed by Karl Imhoff of Germany, an Imhoff tank is an improved septic tank in which the incoming sewage or influent is not allowed to get mixed up with the sludge

produced. Also, the outgoing sewage or effluent is not allowed to carry with it any large amount of the suspended matter as in the case of a septic tank.

**Construction and operational features**

It is a double chamber tank, the upper chamber is called the sedimentation tank or flowing-through chamber, through which sewage flows at a very low velocity; the lower chamber is the digestion chamber in which anaerobic or septic decomposition occurs.

Solids of the sewage settle to the bottom of the sedimentation chamber through the sloping bottom walls (slope 5 vertical to 4 horizontal). They are made to fall in

the digestion chamber through an entrance slot at the lowest point of the sedimentation chamber. The slot is trapped or overlapped in such a way that the gases generated in the

digestion chamber cannot enter the sedimentation chamber.

A gas vent, also called scum chamber is provided with the digestion chamber to take care of the gases escaping to the surface. The chief gas is methane (CH ) having a

considerable fuel value and may, therefore, be separately collected for use. In order to prevent particles of sludge or scum from penetrating into the sedimentation chamber, the

sludge and scum must be maintained at a distance of at least 45 cm below and above the slots respectively. The free or clear zone is called neutral zone.

The digestion chamber is made up of two or three inverted cones called hoppers with sides sloping (1 : 1) so as to concentrate the sludge at the bottom of the hopper. The

sludge is removed periodically through sludge-pipe, the flow being under a hydrostatic pressure of 1.2 to 1.8 m. All the sludge is not removed, only the lower layers which are

completely decomposed are withdrawn, leaving some sludge to keep the tank seeded with anaerobic bacteria.

To permit uniform distribution of settled solids throughout the length of the digestion chamber, so as to utilize the storage capacity in the greatest measure, arrangements for

reversing the direction of flow through the tanks are commonly made.


Imhoff tanks combine the advantages of both the septic and sedimentation tanks and, as such find use in case of small treatment plants requiring only preliminary treatment. They

have better economy and give good results without skilled attention with minimum problems of sludge disposal.


(i) Greater depth means greater costs and especially where excavation is to be done in quick sands or solids rock, they become uneconomical. (ii) Unsuitable to acidic wastewater exists. (iii) There’s no adequate control over their operation. This makes them unsuitable for use in large treatment plants where separate sludge digestion tanks are preferred.

The above is an excerpt from the free newsletter on “All About Wastewater Treatment”

published by Geostar Publishing & Services LLC.

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