SCREENING

Screening is a mechanical-physical process used to remove part of the settleable organic substances contained in slurries.
Located upstream of the actual purification processes, screening makes it possible to remove materials and substances that, because of their size and type, can be harmful to the downstream equipment and impair the efficiency of the successive treatment stages.

The aim of the screening stage is to intercept coarse non-settleable solids (rags, plastic, etc.) and settleable coarse solids (gravel, etc.). The screen is always installed internally in the plant inlet channel, supplied by the sewer terminal header, with a gradient of 1:3. The channel widens by a certain proportion in the area of the screen so that the velocity of the downstream wastewater, taking account of the obstruction constituted by the screen bars, remains close to the velocity upstream of the screen, which is defined as the influent velocity and is typically equal to 0.8 m/sec.

The velocity always depends on the gradient of the channel and on whether or not pumps are installed. The screen crossing velocity must not be excessively low such as to give rise to upstream settling phenomena, or excessively high with the resulting increase in system pressure drops.

Proper operation of the screen (irrespective of the type) is only possible in the absence of back pressure at the outlet.
The choice of screen depends on several factors linked to the type of plant, the environment factors of the place of installation, and the type and size of suspended solids to be intercepted.

Among the various screen types, Definitive Ecology offers a complete range of standard and customized solutions for fine screening.

To ensure perfect operation and long working life of treatment systems with membrane bioreactors (MBR), which have been a feature of municipal and industrial wastewater treatment plants for almost thirty years, adequate pre-treatment is required, with fine screening of the wastewater.

Studies carried out on several MBR plants have shown that with efficient screening systems the plants were operating at full efficiency also after more than 10 years of service.

Inefficient pre-treatment of wastewater in MBR plants can lead to:

1. Accumulation of wastes, hair, fabric, fibres, etc.
2. High risk of sludge accumulation
3. Damage to membranes

The cost of a fine screening system is less than 3% of the total cost of an MBR plant.

Definitive Ecology offers several families of fine screens for MBR plant applications, all having:

• 1 or 2 mm perforated plate screen, according to requirements
• Washing of the screen by means of medium pressure water jets