Activatedsludge plants

Activated sludge plants are suitable for purifying residential waste water and industrial biological effluent.

Activated sludge plants
Activated sludge plants
Activated sludge plants

Process description

Residential discharge treatment includes the following stages:

PRIMARY DECANTATION in an Imhoff tank, to retain floating material and any settleable solids and obtain a partially clarified effluent with pollutant load cut by approx. 30%. Suds is pretreated in a fat condensation tank.

BIOLOGICAL OXIDATION in the oxidation tank, where the sewage comes in contact with microorganisms that, in the presence of oxygene provided by high-efficiency submersible aerators, transform organic matter into stable matter. Pollutant load reduction rate, which highly depends on active sludge concentration in the tank, on the organic load and on retention time, can reach 98% in this stage. In addition to providing top performance and high durability, the ultra-low-noise submersible aerators do not require external cabinets.

FINAL DECANTATION in a hopper chamber, in which sludge descends towards the bottom of the tank and purified water flows, along the overflow channel, into the sampling well, before merging with the receiving water body.

The settled sludge is then recovered and recycled in the oxidation tank.

If water is discharged into the ground, or within a sensitive area, it is recommendable to install a pre-denitrification tank upstream the oxidation tank, so that the denitrifying bacteria that develop in the same can reduce the level of nitrate in waste water, making it suitable for discharge.

Specific pretreatment, according to type, is applied to biological industrial waste purified in activated sludge plants.

The options are designed and built to suit different load standards:

  • single-unit plants, with a single tank (monolitic on request) holding both the oxidation and decantation chamber separated by a party wall
  • parallel-module plants, which can be activated separately, and are hence ideal for systems to be used before the maximum load they are designed for is reached
  • industrial plants including pretreatment and special equipment according to type of waste water to be purified

Custom start-up and running programs provided on request.

Running and maintenance

The plants do not require special maintenance. Adequately sized galvanized manholes, positioned right above the equipment, make running operations easier. However, it is recommendable to periodically monitor activated sludge evolution and the degree of treatment efficiency. The essential operations to be carried out following start-up are:

  1. periodical biological sludge concentration and treatment process check
  2. periodical check of the timer clock controlling blower and recirculation pump operation
  3. periodical removal of excessive sludge and pretreatment chamber cleaning

The plant exploits microorganisms, therefore users must absolutely avoid harmful substances able to hamper the evolution of the same.

Also high temperature, substantial variation in organic load and power outage, with consequent lack of biomass oxygenation, can affect microorganism evolution, and hence the degree of efficiency of the treatment.


Plants for up to 80 PE (except for the 40 PE option) are made up by a single concrete tank con- taining a partition defining two chambers (oxidation and decantation). Sludge is recirculated by means of a timer pump, or of a Venturi tube in smaller plants. As recirculation frequency, and hence flow, can be modulated, the system is more versatile and adjusts to load variations. Ventilation is provided by submersible aerators. In plants for loads from 100 PE on, the decan- tation chamber is housed in a separate tank, while 175 to 400 PE plants (except for the 200 PE option) are constituted of different parallel modules.