Azuwater and Azuvoda

Imhoff Tank

Overview

Imhoff tank for small domestic wastewater treatment plant

Category / type: wastewater treatment, sedimentation, clarification.
Application: residential users, isolated residential buildings, chalet.
Population Equivalents (PE) served: from 2 to 200.
Flow: from 0.2 m3/day to 40 m3/day.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).


The Imhoff tank (named by the German engineer Karl Imhoff), is composed by two compartments: an upper sedimentation zone and a lower accumulation and sludge digestion zone. It allows suspended solids reduction from 50 to 70%, COD reduction from 25 to 50% and a good sludge stabilisation depending on the dimensions and climatic conditions.
It is an example of simple, compact and effective system for municipal wastewater pre-treatment. The tank may have a cylindrical or parallelepiped shape; the settling zone, with typical V-shaped walls, is provided with a slit at the bottom, which allows solids to settle in digestion compartment. This system also avoid that gas, created by anaerobic reactions, can go back to the surface, disturbing the settling process.
Gas produced in the digestion chamber causes a few dragging of sludge particles from the bottom to the water surface that create a top layer of scum that must be periodically removed. Sludge is accumulated in the digestion chamber, where it undergoes anaerobic process and becomes partially stabilized.
Liquid fraction stays only a few hours in the tank (between 3.5 and 7 hours); this allows to obtain a fresh (no septic) final effluent. Solids remain for much longer time in the chamber digestion with uniform and controlled conditions that guarantees a good thickening. Biogas production is higher compared to septic tanks and a correct vents system is necessary to avoid bad odours problems.
Effluent can be leakage on the ground (according to local regulations provisions) or may require a secondary treatment (e.g activated sludge, trickling filters, constructed wetlands etc.).

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How It Works

The Imhoff tank, is a two-story septic tank, composed of an upper sedimentation compartment, and a bottom sludge digestion compartment. Vent and accumulation foams zone are placed both to sedimentation tank sides.
Hydraulic detention time is usually lower than septic tanks (from 3 to 7 hours) which allows to maintain aerobic conditions in the effluent.
T-shape pipes or baffles are used to reduce water speed and to prevent light liquids dragging. Settling vessel bottom has a typically slope from 50°up to 60°, and slot between the two compartments must be done to prevent gases rising in settlement unit.
For larger solutions, walls of the digestion sludge compartment reach a slope of about 45° to allow mud to slide towards the centre, where it can be removed more easily. Anaerobic digestion compartment sizing depends mainly on the sludge production regarding served population, sludge stabilization level, cleaning frequency and weather conditions (temperature).
Digestion chamber is generally designed considering a range from 6 to 12 months of sludge storage capacity to allow sufficient anaerobic processes. In cold climates a long detention time must be considered to obtain the same level of sludge stabilisation that consequentially required larger volume.
Since the effluent is well stabilized and nearly odourless, Imhoff tank is a choice as a primary treatment, especially if further processes are in contact with the external environment (e.g. phytoremediation, wetlands or surface water).The Imhoff tank, is a two-story septic tank, composed of an upper sedimentation compartment, and a bottom sludge digestion compartment. Vent and accumulation foams zone are placed both to sedimentation tank sides.
Hydraulic detention time is usually lower than septic tanks (from 3 to 7 hours) which allows to maintain aerobic conditions in the effluent.
T-shape pipes or baffles are used to reduce water speed and to prevent light liquids dragging. Settling vessel bottom has a typically slope from 50°up to 60°, and slot between the two compartments must be done to prevent gases rising in settlement unit.
For larger solutions, walls of the digestion sludge compartment reach a slope of about 45° to allow mud to slide towards the centre, where it can be removed more easily. Anaerobic digestion compartment sizing depends mainly on the sludge production regarding served population, sludge stabilization level, cleaning frequency and weather conditions (temperature).
Digestion chamber is generally designed considering a range from 6 to 12 months of sludge storage capacity to allow sufficient anaerobic processes. In cold climates a long detention time must be considered to obtain the same level of sludge stabilisation that consequentially required larger volume.
Since the effluent is well stabilized and nearly odourless, Imhoff tank is a choice as a primary treatment, especially if further processes are in contact with the external environment (e.g. phytoremediation, wetlands or surface water).The Imhoff tank, is a two-story septic tank, composed of an upper sedimentation compartment, and a bottom sludge digestion compartment. Vent and accumulation foams zone are placed both to sedimentation tank sides.
Hydraulic detention time is usually lower than septic tanks (from 3 to 7 hours) which allows to maintain aerobic conditions in the effluent.
T-shape pipes or baffles are used to reduce water speed and to prevent light liquids dragging. Settling vessel bottom has a typically slope from 50°up to 60°, and slot between the two compartments must be done to prevent gases rising in settlement unit.
For larger solutions, walls of the digestion sludge compartment reach a slope of about 45° to allow mud to slide towards the centre, where it can be removed more easily. Anaerobic digestion compartment sizing depends mainly on the sludge production regarding served population, sludge stabilization level, cleaning frequency and weather conditions (temperature).
Digestion chamber is generally designed considering a range from 6 to 12 months of sludge storage capacity to allow sufficient anaerobic processes. In cold climates a long detention time must be considered to obtain the same level of sludge stabilisation that consequentially required larger volume.
Since the effluent is well stabilized and nearly odourless, Imhoff tank is a choice as a primary treatment, especially if further processes are in contact with the external environment (e.g. phytoremediation, wetlands or surface water).

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Operation and maintenance

Imhoff tank start up need a sludge inoculation coming from a similar plant or a biological activators adding.
Because of biological process, aggressive chemicals should not be conferred in waste.
It is important to verify that digestion gases are properly vented and there are no occlusions in the slot passing between sedimentation and digestion zone.
Cleaning operations must be carried out only by a licensed and authorised contractor; the sludge must be brought in suitable treatment plants.
Imhoff tanks should be checked regularly to ensure they are watertight. During cleaning operation all light substances and coarse materials should be removed, taking care to leave part of the digested sludge on the bottom to accelerate biological reactions recovery. When cleaning is finished, tank must be filled with clean water up to operating level.

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Regulatory framework

A Imhoff tank is a wastewater treatment system and for this reason, small installations up to 50 total population refer to the EN 12566-3 specifications.
The European standard EN 12566-3 “specifies requirements, test methods, the marking and factory production control for packaged and/or site assembled domestic wastewater treatment plants (including guest houses and businesses) used for populations up to 50 inhabitants…”
The manufacturer shall state the nominal organic daily load, expressed in kg of BOD5 or BOD7 per day and the nominal hydraulic daily flow, expressed in cubic meters per day.
Plant’s Nominal Designation is the proper daily organic load value (QN) (3.7).
The small wastewater treatment plant must demonstrate compliance with the performance of efficiency… and to operational data declared by the manufacturer: the declaration must be expressed in percentage.
It is also to be considered that the structural resistance and the hydraulic efficiency must be tested only on a representative model of the product line. Representative model for structural resistance test must be the bigger, supposedly as the one of series with the worst characteristic, while the one for hydraulic efficiency test must be the smaller, presuming the one with the worst efficiency.
The standard describes statics and hydraulics methods to determinate resistance and hydraulic efficiency.
Informations to be included in the test report and in the CE mark are also detailed in ZA.3 annex.
For small domestic wastewater treatment plants up to 50 PT (therefore for activated sludge plants), a Declaration of Performance (DoP), is required, as provided by the EU Regulation. 305/2011. ASP
Some local and national regulations refer to Imhoff specifying possible uses and volumes required.
A Imhoff tank installation is always subject to authorization by institutions.

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Features and benefits

Imhoff tank should be used for medium domestic community. Septic tank is a better solution for small population, especially with mixed wastewater (black and gray).
Imhoff tanks can homogenizes effluent on pollutants peaks presence.
It can be placed in hot and cold climates (temperature of wastewater below 15° C or above 2000 m are supported). Tank, in order to guarantees good efficiency must be tall due to ensure a suitable volume for two compartments one above the other.
Usually and preferably it is installed underground and so the footprint is very small.
For this type of plat, installation and maintenance costs are low.

Advantages (benefits)

  • Sedimentation and digestion into a single unit;
  • resistance polluting peak input;
  • small footprint;
  • fresh effluent (aerobic conditions, not septic, less odour);
  • low operating costs;
  • simple operation and maintenance;
  • no energy consumption related to the operation.

Disadvantages

  • Greater height or depth compared to other solutions;
  • low pathogens reduction;
  • periodic operation by specialized companies;
  • risk of occlusion of slot between two compartment;
  • odour problems if not properly connected to vents;
  • sludge disposal
  • more complex than septic tank.

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Products in category Imhoff Tank

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