What Is An Imhoff Tank? Design Considerations Of Imhoff Tank

What Is An Imhoff Tank?

The Imhoff tank is a chamber designed for the treatment of sewage. It was named after the German engineer Karl Imhoff who lived from 1876 to 1965. The tank is comprised of two separate chambers: the upper chamber where the sewage undergoes clarification through settling and sedimentation, and the lower chamber where the extracted sludge undergoes anaerobic digestion.

The two chambers are not connected, with only the clearer liquid sewage flowing through the upper chamber and a slower flow of sludge moving to the lower chamber. The lower chamber requires separate biogas vents and pipes for the removal of the digested sludge after around 6 to 9 months.

The Imhoff tank serves as a two-story septic tank, maintaining its simplicity while resolving many of the limitations of traditional septic tanks. However, it is being replaced by plain sedimentation tanks with mechanical methods for the collection of sludge that is then moved to separate digestion tanks.

This setup leads to improved sedimentation results, better temperature control during digestion, and faster and more complete digestion of the sludge.

The Imhoff cone test is used to measure the volume of settleable solids in water, wastewater, and stormwater. The test uses a cone, with or without a stopcock, and the volume of solids is recorded after a specific time period using the graduated markings at the bottom of the cone.

Design Considerations Of Imhoff Tank

The Imhoff tank is a wastewater treatment system, often built with reinforced concrete underground, but can also be above ground for easier sludge removal.

Small prefabricated options are also available. The hydraulic retention time is typically 2-4 hours to maintain aerobic conditions for further treatment. T-shaped pipes or baffles are used to reduce velocity and prevent scum from escaping.

The settling compartment typically has a slope of 1.25 to 1.75 vertical to 1 horizontal, with a slot opening of 150-300 mm wide. The sludge digestion compartment has a wall inclination of 45° or more to allow for easy removal.

The digestion chamber’s size depends on factors such as sludge production, targeted degree of stabilization, and temperature, and is designed for 4-12 months of sludge storage. In colder climates, a larger volume is needed for a longer retention time.

Desludging requires a pipe and pump or access for vacuum trucks and mobile pumps. A bar screen or grit chamber is recommended before the Imhoff tank to prevent coarse material interference.

How Does Imhoff Tank Work?

The Imhoff Basin is a two-story wastewater purification system that consists of a sedimentation chamber at the top and a sludge digestion chamber at the bottom. The water pressure retention time is shorter than in traditional septic tanks, allowing for aerobic conditions in the wastewater.

T-shaped pipes or baffles are used to regulate the flow of water and prevent light liquids from escaping. The settling tank has an inclination of 50-60°, with a slot created between the two chambers to prevent gas buildup.

The sludge digestion chamber has a 45° inclination to facilitate easy removal of sludge. The design of the digestion chamber takes into account the population it serves, sludge stabilization needs, frequency of cleaning, and weather conditions.

The chamber is designed to store sludge for 6-12 months to allow for a complete anaerobic process. Wastewater processed in the Imhoff Basin is stable, nearly odorless, and a good option for pre-treatment, especially if further processes are involved, such as phytoremediation, wetlands, and surface water.

Operation And Maintenance Of Imhoff Tank Work

Starting up an Imhoff tank requires sludge inoculation from a similar plant or the addition of biological activators.

Aggressive chemicals should not be introduced in the waste during the biological process. Ensure proper ventilation of digestion gases and clear any blockages in the passage between the sedimentation and digestion zone.

Cleaning should only be performed by licensed contractors and the sludge should be taken to appropriate treatment plants. Regular checks should be conducted to maintain the tank’s watertightness.

During cleaning, remove light substances and coarse materials, but leave some digested sludge on the bottom to enhance biological reaction recovery. Fill the tank with clean water to the operating level after cleaning is completed.

Benefits Of Imhoff Tank

Imhoff tanks offer several benefits: they combine sedimentation & digestion into one unit, resist peak pollution, have a small footprint, produce fresh effluent, have low operating costs, are easy to operate & maintain, and consume no energy.

Disadvantages Of Imhoff Tank

The disadvantages of this solution include greater height/depth, low pathogen reduction, periodic operation by specialists, risk of occlusion, odour problems, sludge disposal, and more complexity compared to a septic tank.