7 July 2026

Imhoff tanks in concrete and polyethylene: technical analysis and selection criteria in specifications

The primary treatment of domestic and civil wastewater within settlements isolated from the sewage network requires a preliminary assessment beyond simple hydraulic sizing. While the internal configuration of the Imhoff tank—based on the separation between the upper sedimentation compartment and the lower anaerobic digestion compartment—responds to precise purification standards set by Legislative Decree 152/06, the choice of structural material determines the long-term effectiveness of the system.

In drafting executive projects and managing construction sites, the comparison between prefabricated reinforced concrete structures and high-density polyethylene (HDPE) systems is not limited to a question of supply costs. It involves geological dynamics, logistical constraints, and chemical resistance to fermentation gases.

The Reinforced Concrete Product

Prefabricated concrete stands out for its high mechanical resistance and significant specific weight. These characteristics make it suitable for stable environments and grounds not subject to subsidence. However, the rigidity of the material also represents its main point of vulnerability: in the presence of backfill, clayey soils, or areas subject to micro-settlements, structural tensions can generate linear cracking. In the case of modular tanks made up of superimposed rings, the critical point shifts to the joints, where the action of frost or differential ground movements risks compromising the original hydraulic seal.

Polyethylene Imhoff Tanks

Polyethylene tanks, produced by rotational molding, feature a monoblock structure, free of welds or structural junction points. The absence of discontinuities eliminates the risk of localized leaks.

From a mechanical point of view, furthermore, polyethylene is an elastic material: it does not counteract soil stresses with rigidity, but rather accommodates micro-movements and vibrations (for example, near roads or industrial areas) by flexing without reaching the point of breaking or cracking.

Chemical Resistance to Fluids and System Life Cycles

The cold digestion process that occurs within the lower compartment of the Imhoff tank generates a chemically aggressive environment. Anaerobic fermentation produces biogas containing hydrogen sulfide (H2S) which, upon contact with internal moisture, can oxidize into sulfuric acid.

• Interaction with concrete: If the internal surface of the concrete is not protected by epoxy paints or special coatings, the porosity of the material undergoes acid corrosion over time. This degrades the cementitious matrix, eventually exposing the internal metal reinforcement.
• Interaction with polyethylene: HDPE is a chemically inert, high-density polymer. It does not react with fatty acids, surfactants in domestic detergents, or gases deriving from sludge stabilization, maintaining its structural characteristics and impermeability unaltered for the entire life cycle of the building.

The regulatory framework and regional variations: discharge compliance

In addition to the physical and mechanical assessments related to the material, the choice and installation of an Imhoff tank must comply with the legislative landscape. While Legislative Decree 152/2006 (Consolidated Environmental Act) defines the general principles for off-grid wastewater discharges and primary treatment stages at a national level, the application authority and detailed criteria are delegated to individual Regions and local authorities.

This regulatory decentralization imposes different requirements depending on the territory where the construction site is located, focusing on three critical aspects for the project:

• The obligation of a purification chain: In many regions (such as Lombardy and Emilia-Romagna), the Imhoff tank alone is not sufficient for discharge into the soil. The law requires integrating it with secondary treatments, such as percolating filters or constructed wetlands (phytodepuration).
• Minimum capacities and distances: Local regulations (as in Tuscany or Veneto) often require larger volumes per Population Equivalent (PE) compared to the old national guidelines. Distance constraints also remain strict, potentially reaching up to 30 meters from drinking water wells.
• Geological discharge constraints: The authorization request almost always requires a geological report. If the soil is clayey or poorly permeable, the designer must opt for watertight systems and guided downstream dispersions.

Beyond local specificities, the universal prerequisite remains the obligation to install products with the CE marking according to the UNI EN 12566-1 standard, a parameter that certifies the structural compliance and purification efficiency of the tank to the control bodies during the testing phase.

Di Camillo Srl supports designers and construction companies in overcoming bureaucratic and technical hurdles. We provide all the necessary documentation, certifications, and technical data sheets to properly process authorization procedures in compliance with current regional regulations.

Do you need to include an Imhoff tank in your next specification or need support for calculating Population Equivalents (PE)?

Contact our technicians for targeted advice on sizing and technical installation specifications for your next project.