Imhoff Tank

The Imhoff tank (also known as Emscherbrunnen or Emscher Tank), which works similar to a communal septic tank, is a robust and effective settler that causes a suspended solids reduction of 50 to 70%, COD reduction of 25 to 50%, and leads to potentially good sludge stabilisation – depending on the design and conditions. It is a compact and efficient system for pre-treatment of municipal wastewater.

The settling compartment has a circular or rectangular shape with V-shaped walls and a slot at the bottom, allowing solids to settle into the digestion compartment, while preventing foul gas from rising up and disturbing the settling process. Gas produced in the digestion chamber rises into the gas vents at the edge of the reactor. It transports sludge particles to the water surface, creating a scum layer. The sludge accumulates in the sludge digestion chamber, and is compacted and  partially stabilised through anaerobic digestion. The liquid fraction remains only some hours in the tank, while the solids remain for several years in the digestion chamber.

There is more biogas production than in septic tanks, but unfortunately the organic load in communal wastewater is usually not high enough for the economical collection and usage (or flaring) of biogas (HOFFMANN et al. 2011). The pre-treated wastewater from the Imhoff tank requires a secondary treatment (e.g. leach field, soak pits, horizontal flow, vertical flow or free-surface constructed wetlands).

Also the faecal sludge needs to be correctly disposed and further treated (e.g. small or large scale composting, settling - thickening ponds or drying beds). If the sludge is composted either directly or after drying, it can be used as fertiliser to improve the soil quality (see also use of compost or fertiliser from sludge).

The Imhoff tank is designed with three compartments (see picture below):

1. Upper compartment for sedimentation
2. Lower section for sludge digestion
3. Gas vent and scum section

Scum and gas vent chambers are located at the sides of the tank. It can be an open or covered tank (WSP 2007). A covered tank avoids infiltration of precipitation and pollutants. However, it is always preferred to cover the tank to prevent contact with people and animals (see also water sanitation and health or health risk management). An outlet for desludging can be added.

The Imhoff tank is usually built underground with reinforced concrete. It can, however, also be built above ground, which makes sludge removal easier due to gravity, although still requiring pumping up of the influent. Small prefabricated Imhoff tanks are also available on the market. Hydraulic retention time is usually not more than 2 to 4 h to preserve an aerobic effluent for further treatment or discharge (find other treatment technologies here and also under http://ecompendium.sswm.info/sanitation-technologies). T-shaped pipes or baffles are used at the inlet and the outlet to reduce velocity and prevent scum from leaving the system.

The total water depth in the tank from the bottom to the water surface may reach 7 to 9.5 m. The bottom of the settling compartment is typically sloped 1.25 to 1.75 vertical to 1 horizontal and the slot opening can be 150 to 300 mm wide. The walls of the sludge digestion compartment should have an inclination of 45° or more. This allows the sludge to slide down to the centre where it can be removed. Dimensioning of the anaerobic digestion compartment depends mainly on sludge production per population equivalent, on the targeted degree of sludge stabilisation (linked to the desludging frequency) and the temperature. The digestion chamber is usually designed for 4 to 12 months sludge storage capacity to allow for sufficient anaerobic digestion.

In colder climates longer sludge retention time and, therefore, a greater volume is needed. For desludging, a pipe and pump have to be installed or access provided for vacuum trucks and mobile pumps (see also motorised emptying and transport). A bar screen or grit chamber (see pre-treatment technologies) is recommended before the Imhoff tank to prevent coarse material from disturbing the system.

As the effluent is almost odourless, it is a good option for primary treatment if subsequent treatment takes place, e.g., in open ponds, constructed wetlands (see also vertical flow constructed wetland, horizontal flow constructed wetland or free-water surface constructed wetland) or trickling filters. Greenhouse gases produced in low quantities may, however, generate odours locally. Pathogen removal is low and all outputs should be treated before they are used e.g. for irrigation or as fertiliser. Although the removal of pathogens is not high, the entire tank is usually below the surface so users do not come in contact with any of the wastewater . Adequate protective clothing is necessary for workers who may get in contact with the effluent, scum or sludge. A vacuum truck should be used to empty the sludge, if there is no pumped outlet installed. Users should not attempt to empty the pit themselves except with a manual technology like the gulper (see also motorised emptying and human-powered emptying). Users should be careful when opening the tank because noxious and flammable gases may be released.

Construction costs are slightly higher than the costs for a septic tank (SANIMAS 2005). Also the costs for desludging (motorised or manual) must be considered. Moreover, it is a pre-treatment facility and in many cases connected to further treatment installations (e.g. leach field, soak pits, horizontal flow, vertical flow or free-surface constructed wetlands).

Operation and maintenance (click here for further information on operation and maintenance) are possible at low cost, if trained personnel are in charge. Flow paths have to be kept open and cleaned out weekly, while scum in the settling compartment and the gas vents has to be removed daily if necessary. Regular cleaning of the sides of the settling chamber and slot by rake or squeegee is very important. Another activity, which should be done twice a month is to reversing the flow (backwash) of water to even up the solids in the digestion chamber (WSP 2007; SANIMAS 2005; SASSE 1998). Stabilised sludge from the bottom of the digestion compartment should be removed according to the design. The sludge can be removed by pumping or hydraulic pressure pipes right from the bottom or by a vacuum truck (see also motorised emptying). A minimum clearance of 50 cm between the sludge blanket and the slot of the settling chamber has to be ensured at all times.