Imhoff Tank...

The Imhoff tank was developed to correct the two main defects of the septic tank.

- It prevents the solids once removed from the sewage from again being mixed with it, but still provides for the decomposition of these solids in the same unit.
- It provides an effluent amenable to further treatment.

Contact between the waste stream and the anaerobic digesting sludge is practically eliminated and the holding period in primary settling compartment at the tank is reduced. The Imhoff tank may be either circular or rectangular and is divided into three compartments :

- The upper section or sedimentation compartment.
- The lower section known as the digestion compartment.
- The gas vent and scum section.

It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids at one end of the sedimentation compartment. Periodically reversing the flow will result in an even accumulation of sludge across the bottom of the tank. In operation, all of the wastewater flows through the upper compartment. Solids settle to the bottom of this sloped compartment, slide down and pass through an opening or slot to the digestion compartment. One of the bottom slopes extends at least six inches beyond the slot. This forms a trap to prevent gas or digesting sludge particles in the lower section from entering the waste stream in the upper section. The gas and any rising sludge particles are diverted to the gas vent and scum section.

Imhoff Tank Operation...

There are no mechanical parts in an Imhoff tank. Attention should, however, be given to the following :

- Daily removal of grease, scum and floating solids from the sedimentation compartment.
- Weekly scraping of the sides and sloping bottoms of the sedimentation compartment by a rubber squeegee to remove adhering solids which may decompose.
- Weekly cleaning the slot at the bottom of the sedimentation compartment. This can be done by use of a chain drag.
- Periodic reversal of flow where provided for in the design of the tank.
- Control of the scum in the scum chamber, by breaking it up, hosing with water under pressure, keeping it wet with supernatant from the digestion compartment and removal if the depth approaches two to three feet.
- Removal of sludge should be done before the sludge depth approaches within 18 inches of the slot in the sedimentation compartment. It is better to remove small amounts frequently than large amounts at long intervals. Sludge should be removed at a slow regular rate to avoid coning (i.e. the formation of a channel through the sludge) which would permit partially digested sludge and liquid held in storage above the digested sludge to be withdrawn from the tank. Before winter temperatures are expected, most of the digested sludge except that necessary for seeding (about 20 percent) should be removed to provide space for winter accumulations when digestion is very slow. The height of the sludge in the sludge compartment should be determined at inlet and outlet end of the tank at least once a month.
- After each time that sludge is removed, the sludge pipes should be flushed and drained to prevent sludge from hardening in and clogging the pipes.
- Prevention of "Foaming". Every effort should be made to prevent "foaming" because correction after the condition arises is sometimes difficult. "Foaming" is usually associated with an acid condition of the sludge and in such cases may be prevented or corrected by treatment with lime or sodium bicarbonate to counteract the acidity of the sludge. There are a few simple measures which may, under certain circumstances, remedy or improve the condition.

- The use of hydrated lime or sodium bicarbonate added to the gas vents will usually aid in correction. The pH value of the resulting sludge and lime mixture in the digestion compartment should not exceed 7.6.
- Removing the tank from service where possible for a few days and allowing it to rest will sometimes improve conditions.
- Agitation of the gas vent area with a water hose or paddles will sometimes help.

The Imhoff tank has no mechanical parts and is relatively easy and economical to operate. It provides sedimentation and sludge digestion in one unit and should produce a satisfactory primary effluent with a suspended solids removal of 40 to 60 percent and a BOD reduction of 15 to 35 percent. The two-story design requires a deep over-all tank. Primary tanks with separate digesters have largely replaced the Imhoff tank for large municipal installations. The Imhoff tanks is best suited to small municipalities and large institutions where the tributary population is 5,000 or less, and a greater degree of treatment is not needed.