16 April 2024

Differences between Imhoff tanks and septic tanks

There are two technologies for wastewater treatment: Imhoff tanks and septic tanks. Whether you are building a civil wastewater treatment plant in areas isolated from sewerage networks, or a pre-treatment plant, both systems separate solid waste from liquid waste, supporting the self-purifying capacity of the soil, but with differences in terms of use and effectiveness.

BOD5: what it is, and how it determines the effectiveness of pit purification

The “Biochemical Oxygen Demand” (BOD5) is a parameter used to evaluate the amount of biodegradable organic matter present in a water sample. In this sense, when used to evaluate the water leaving a treatment plant, it provides an indication (albeit indirect) of their degree of pollution.

To analyze BOD5, the amount of oxygen needed by aerobic microorganisms is measured in the 5 days of oxidation of organic matter, in the dark and at a temperature of 20° C. A low BOD5 value indicates a lower presence of organic matter in the aqueous solution, and therefore a greater purification capacity of the system. On the contrary, a higher value is a symptom of a greater need for oxygen to complete the decomposition process, and therefore a higher concentration of organic substances due to potential water pollution.

Septic tank clarification process

Septic tanks treat wastewater in a process called primary clarification. When wastewater enters the pit, it naturally separates into different layers: liquids accumulate at the top, where they are discharged by the dispersal system, while heavier solids end up at the bottom, where they undergo bacterial fermentation.

The gases produced by fermentation then carry part of the sludge to the surface, which can be easily removed. In this way it is possible to remove up to 50% of suspended solids, and reduce the BOD5 maximum by 30%. But to further improve these results, multiple septic tanks can be connected through T-baffles.

Wastewater treatment process with Imhoff pits

The Imhoff tanks, or Imhoff pits, allow for a more complete decantation of organic substances, and therefore a greater reduction of pollutants, due to their very structure. The tanks have two compartments, one for sedimentation and one for digestion, into which the suspended sludge can pass thanks to the hydraulic connection.

For this reason, Imhoff tanks ensure a reduction in BOD5 of over 30%, but also in COD and suspended solids by 50%. This means that they can be used in the pre-treatment of domestic and civil wastewater from homes, schools, restaurants, factories and offices before being conveyed to sewer systems or secondary treatment plants.

Imhoff tank or septic tank? Selection criteria

The choice between installing an Imhoff tank or a septic tank therefore depends on specific needs, as well as environmental conditions. Imhoff pits are recommended for creating autonomous purification systems, to be integrated within a treatment plant for direct discharge into the ground, without connections to the sewage system.

Septic tanks, on the other hand, are more suitable in cases of limited natural slope, and for this reason they are used in urban centers mainly to block foams and solids. Furthermore, the same results as Imhoff tanks can also be obtained with septic tanks, if connected in series: in this case, however, they require more space than an Imhoff tank equipped with grease trap and percolating filter.

Whatever your purification needs, however, and therefore whether you are looking for an Imhoff tank or a septic tank, turn to Di Camillo’s experience: we specialize in the production and construction of tanks, cisterns and systems for the recovery of waste water and rain. With over 50 years of experience, we will be able to find and recommend the best solution for your needs and treatment systems.