The screening in a municipal or industrial water treatment plant is the first phase in the treatment process.
It consists in removing from the water any kind of bodies which could clog the piping or demage the other equipment.
The screens are usually placed in the incoming flow channel, but they can be installed even out of the channel, downstream the lifting pump.
The screen can be manual or automatic; coarse or fine.

The screening material retained by the screens of wastewater treatment plants often have a high percentage of water and/or material.
The compactors solve this problem by washing-compacting and dewatering the screened material.

The process for grit removal in a municipal or industrial wastewater treatment plant is usually placed just downstream the screening and before the sewage and wastewater treatment.
A grit removal plant reduces clogging and wearing problems for the equipment, installed downstream.
This treatment, besides reducing the organic load for the subsequent treatment process, reduces the formation of scum.
The grit removal process takes place in tanks called sand traps in which is exploited the force of gravity to remove all the solid particles which have a specific weight greater than that of water.
The sand traps are tanks in which the sewage flows at a speed such as to create the settling of the solids dragged in suspension or for transport of the bottom.

The sedimentation is the process by which the solid substances suspended in water settle on the tank bottom, thanks to the force of gravity only.
The sedimented material is scraped, collected and conveyed out of the sedimentation tank.
There are two types of sedimentation.
Primary sedimentation (before the biological treatment) in which the scraper are accompanied by scraper blades and a system of water collection floating.
In the secondary phase (after the biological treatment) in which the scrapers may be scraping or suction.

The flotation is a process which allows to take to the water surface both the suspended particles which tend to float freely (having a lower density than water) and the ones with a feeble tendency to settle.
On the contrary, they heavy particles settle on the floatation system is possible to achieve a removal of the suspended substances which is generally higher and better than the one obtained with the sedimentation system, and this relatively more quickly.
The flotation is not only used for water clarifying, but also for the thickening of chemical and biological sludge.

Biological treatment allows the treatment of industrial or civil wastewater, in cases where the water contains a fair amount of heavy metals, of surfactants, and of materials in suspension, without the use of chemicals, not generating toxic waste.
With biological processes you want to remove organic and inorganic substances from water that can be assimilated by way of aerobic and or anaerobic by the bacteria and microorganism that are part of the ecosystem.
In the artificial biological processes designed to purify water, operate also microorganisms that can be active in the sense aerobic and anaerobic, resulting determinant their type of activity from the circumstances in which they find.

The mud of a plant or is re-circulated in the tanks, or follows a disposal process called sludge treatment.
The primary objective of sludge treatment is to stabilize the organic substances contained in it, and obtain a reduction of the volume of material to be treated, to realize in the later stages: of treatment a significant lowering of investment costs and operating of facilities for the disposal of sludge.
The processes that characterize the sludge treatment are: thickening, dehydration, drying.

The thickening is the process in which is removed the excess water, reduces the volumes, and homogenized the solid phase of the sludge.
The sludge extracted from the hoppers of sedimentation tanks, are thickened by gravity in a circular basin equipped with scraping system and tines of thickening.
The prongs connected to the arms, turning inside the tank crumbles the piece of mud, favoring the expulsion of water and gas and therefore its sedimentation.
With thickening, the slurry is concentrated to obtain a solid content ranging from 3% to 10%.
With subsequently dehydration, is obtained the further evacuation of the liquid part, obtaining a solid content variable from 18% to 40%.

Dehydration: sludge dewatering serves to reduce their quantity through the almost total elimination of the water that is located between the solid substances.
In this way are created sludge more or less compact and can be handled or disposed of.

Drying: sludge from wastewater treatment are dried to minimize their quantity and in this way save on transportation costs and disposal.
With thickening, the sludge are heated to a maximum temperature of 180°, and they are transformed in a dry product, that retains its organic nature.

FILTRATION
The filtration is one of the most important treatments to which waters are subject, in order to use them.
It consist in the removal of small-sized particles, particularly light and difficult to remove by other type of treatment.

ULTRAFILTRATION
The ultrafiltration plants are based on the principle of the physical separation; substances present in the water, if they are larger than the pores of the membrane type, are stopped, instead, if the size are smaller, pass producing what is called permeate.
This technique uses membranes with pores between 1 and 100 millicrom.