The Imhoff biological tank is generally used as a primary treatment plant for domestic and/or assimilated wastewater in small or medium-sized purification plants.
The main function of the Imhoff biological tank is to separate coarse materials and to carry out a first phase of treatment of black water, that is, wastewater coming from sanitary services (toilets), both from homes and all types of residential construction, as reported by Legislative Decree 152/06 and subsequent amendments, as well as by Regional Regulations.
The Imhoff biological tank consists of a conical upper chamber where the sedimentation of settleable material occurs and the initial clarification of the effluent, which flows through the opening at the bottom into the underlying chamber where anaerobic digestion takes place.
In the lower chamber, anaerobic bacteria feed on and digest the organic part of the wastewater, allowing the removal of the partially treated liquid.
The treatment of black wastewater (toilets) from users treated by Imhoff tanks helps to safeguard hygiene needs without implementing costly and complicated purification works.
The Imhoff biological tank offers the advantage of having in a single chamber the compartments designated respectively for primary sedimentation and sludge digestion.
Imhoff biological tanks can be constructed from concrete, plastic, or fiberglass and must be sized based on the number of equivalent inhabitants that discharge into them.
According to the indications of Legislative Decree 152/06, the final discharge points can be the same as those of the septic biological tank.
For complete purification of wastewater, appropriate secondary treatments such as phytodepuration systems, aerobic percolating filters, and/or anaerobic percolating filters, activated sludge treatment plants, and/or systems regulated by Legislative Decree 152/06, Environmental Code part III on Environmental Protection, such as sub-irrigation and/or drainage trenches, must be installed after the Imhoff tank.
Indeed, these methods of wastewater treatment, in addition to allowing the disposal of the effluent, guarantee further reduction of the organic load through the purifying capacities (mechanical, chemical, and biological) of the treatment plants or the final discharge (soil, surface waters, lake environment, etc.).
Each Imhoff biological tank must undergo periodic maintenance consisting of its emptying and cleaning. This is carried out, as with grease separators, by specialized companies authorized to transport domestic waste and to deliver it to public treatment plants and/or landfills.
Imhoff biological tanks can be part of condominium services, and the cost of their maintenance is shared among those who benefit from them according to criteria established by law or condominium regulations.
Good Usage Practices
The Imhoff biological tank should, as a good practice, be installed as close as possible to the discharge points of so-called black waters. This will ensure that the conveying pipes to either the public sewer or the treatment plant remain clean and not clogged by pollutants contained in the wastewater.
Moreover, in the presence of discharge difficulties, following the Imhoff biological tanks, we will need minimum slopes to allow the wastewater, now devoid of most of the solid part, to flow without the need to install lifting systems.
It is always advisable to size the Imhoff biological tank with at least 0.25 mc overall per equivalent inhabitant, even though the regulations state a minimum of 0.04 mc for the sedimentation compartment and 0.10 mc for the digestion compartment, and to use materials that remain waterproof over time (high-density polyethylene, fiberglass, and/or waterproof concrete) to avoid infiltration phenomena near the foundations of buildings.
It is reminded that rainwater must not be discharged into Imhoff biological tanks under any circumstances.
Fields of Application
The Imhoff biological tank is particularly useful in users with separate discharges (black water – gray water) of domestic and/or assimilated type, of small and medium size discharging outside the sewer system, as its particular internal configuration allows for adequate sedimentation of the sedimentable content present in black waters.
It is advisable not to allow gray waters, even after the grease separator, to flow into the sedimentation cone of the Imhoff biological tank, as this could create internal congestion phenomena and impede the regular outflow of the waters, causing blockages that obstruct the discharge.
Regulatory References
The Imhoff tank is indicated as a pre-treatment plant element by Legislative Decree 152/06 and reported in various Regional regulations, to be used together with other systems necessary to reduce the pollutant load of wastewater and to return it to the environment as purified as possible.
IMPORTANT
The European Regulation No. 305/11 has stipulated that this type of plant must also be certified according to the CE Directive EN 12566:1 for both structural and functional parameters.
Links:
- Legislative Decree 152/06
- CEE Directive No. 91/271 of 21/05/91
- Law 319 10/05/76
- Resolution 04/02/77
- Regional Protection Plans
The Ministerial resolution of 04/02/77 indicates the dimensional parameters of Imhoff biological tanks, providing for sedimentation volumes between 0.040 mc and 0.050 mc per equivalent inhabitant with a volume not less than 250/300 lt; for the digestion phase, however, the resolution provides for a sizing of 100/200 lt per equivalent inhabitant.
Maintenance
To optimize the functioning of this plant element, periodic maintenance is necessary for the removal of the sediment that settles inside the dedicated compartment.
Moreover, the poor functioning of the pre-treatment system can cause anomalies to any wastewater treatment system installed downstream of it, such as a phytodepuration, a percolating filter, an activated sludge treatment plant, or others.
Periodic maintenance operations must be carried out by vacuum truck companies that will then have to dispose of the waste either at the landfill or at the public treatment plant.