Waste water treatment
After being collected via the various sewage systems, the waste water is conveyed via inter-municipal collectors to the treatment plant, where it undergoes thorough treatment. The water has to pass through all stages of the treatment process: from screening to the sedimentation and oxidation tanks all the way to disinfection. Once treated and cleaned, the water is then returned to the environment in complete safety.
All the dirty water which comes from our homes or from factories flows into underground pipes, the sewage system, through which it reaches the treatment plant
Dirty water contains a lot of solid waste, even of large dimensions (wood, rags, cans, bottles, etc.) that must be removed also to prevent damage to the plant equipment.
This is why the dirty water is passed through a first screen (resembling a large iron comb), which stops and separates the larger solids.
To begin the treatment, the dirty water is lifted using pumps and sent to a collection channel. From here, the waste water gradually flows down through the different stages of treatment.
Other finer screens are used to trap solid waste of small dimensions (larger than 3 millimetres). This waste escaped the primary screening, but here it is permanently blocked and separated. A curious fact: in the majority of cases, this waste is formed by cotton buds, cigarette butts and small pieces of plastic.
Dirty water, in addition to some solid waste, also contains sand and soil, oils and fats. These tanks separate this waste.
The sand and the soil sink to the bottom because of their weight and are removed by suction and deposited in special containers. Microbubbles of air are injected to accelerate the flotation of oils and fats, which are "skimmed" off and sent to the collection well.
We have finally reached the heart of the plant. These tanks eliminate the substances that pollute the water. To do this, we use micro-organisms that are already found in this dirty water. In fact, dirty water also contains many good bacteria, which can be used for purification. They feed on the organic substances that pollute the water. The problem is that, in order to “eat,” bacteria need a great deal of energy, in the form of oxygen. This is why we inject air into the tanks, which contains oxygen. The bacteria thus begin to “eat” and eliminate the pollutants. In these oxidation tanks, the bacteria tend to join together to form small brown clusters, known as "activated sludge flocs".
The individual bacteria, which are very light, float in the water, while the heavier “sludge flocs” sinks to the bottom. This is how sedimentation tanks separate clean water from the sludge that contains the pollutants. The heavier sludge that has sunk to the bottom is removed by suction and separated from the water, now purified, which, drawn from the surface, is sent to disinfection.
Before being fed into the canal, the clean water undergoes further treatment to ensure perfect and safe quality. This treatment involves adding disinfectant. After this last treatment, the treatment plant can return the treated water to nature.
How do we treat the sludge that is generated from water treatment? Sewerage sludge is the main residue from the treatment and concentrates the pollutants removed from the waste water as well as all the substances that make the water unusable. Sewage sludge first undergoes a dehydration process to reduce its volume. Once it has been dehydrated, it can either be transformed into agricultural fertilizers or used in waste-to-energy plants, depending on the treatment plant.
The sludge that has been separated in the sedimentation tanks is accumulated and thickened in a large tank (thickener). From here, the sludge is sent to an special dehydrator, in which it is pressed between filter sheets to separate any remaining water and reduce the volume
After removing much of the water it contained originally, the sludge can be sent to the treatment plants and then used as fertilizer.
Find out how how does sewerage work, from the waste water management to sewage collection service
Discover all the municipalities served by our treatment plants and the features of each plant, from Peschiera Borromeo to Pero, Rozzano and Sesto San Giovanni.