Today, more than ever, with the increasing population the demand for drinking water has increased, and as a consequence the unloading. Hence is easy to understand the importance of a water treatment plant, essential to remove dirt from waste water, and their design, in which SMEA Engineering is very careful. Every time we wash our self, we clean the dishes or cook, we pour water domestic pollutants, which are better to be purified before going into the nature, and the same goes for industrial water. Nature conservation begin in our houses.
The path of the liquid waste then starts from the drains, which act as the first collection basin before arriving to the purifier. Once there, the water is subjected to three treatments:
- Chemical-physical (which eliminates the phosphorus and nitrogen, separating the water from the mud).
WATER FILTERS – MECHANICAL TREATMENT
Once arrived at the station, usually the water is pumped or otherwise flow by means of augers from the sewer to the plant level. Usually it is calibrated to a water flow rate double to the one in the so-called dry conditions, taking into account a whole series of components and parameters. For the design and for the proper sizing, is important to consider:
- Calculation of hydraulic loads, taking into account that the water treatment needs could increase over time, for this might be useful to think about possible successive partitions to be added later;
- Organic loads, represent the concentration of organic quantity to be managed per cubic meter, indicated by BOD (biochemical oxygen demand) or COD (chemical oxygen demand);
- Amount of nutrients, referring particularly to nitrogen and phosphorus;
- Analysis of pollutants, in particular metals, oils or detergents;
- Analysis of reaction key components, such as pH, dissolved oxygen, water temperature etc;
At the entrance, immediately in the so-called “bypass”, are verified some fundamental parameters such as the temperature and the pH (this in particular, is of fundamental importance for the biological treatment proper functioning).
After the bypass, the first mechanical purifier for water is the grid: this is constituted by a series of metallic filaments which have the purpose of retaining the coarser parts that pass through it. Periodically, namely when the two levels before and after the grid differ by a certain amount, a comb passes between the casings and cleans the filter from impurities. The scraps are taken automatically to the washing system that operates the separation of organic (that go back to top of the plant) and inorganic material (that is inserted in bags and disposed later).
Then the water continues its journey in the water treatment plant to the sand trap and the oil separator. On one side in fact the sand is not biologically disposable and could ruin the plant, on the other side oil and fats, although they are two organic components, could cause blockages or hinder the bacteria in the absorption of nutrients in the organic phase. To subtract the sand, we takes advantage of the higher specific weight of it with respect to water: in a tub, hot air is injected so that it generates a recirculation such to deposit only the grains in suspension and maintain all other substances; than a blade scrapes it off to the pumps that take the mixture into a tank where the sand is cleaned by any organic residues (listed at the beginning of the plant) while the washed granules are collected and prepared to be disposed.
In parallel to the sand trap, operates the oil separator tank: here oils, fats and suspended material float on the calm surface of the water resulting in the so-called “floatation”, being then mechanically plowed away and taken to the sludge treatment.
The next step is the primary sedimentation tanks. Here the impurities accumulation principle is the same, but having here a very calm liquid conditions, substances deposit on the bottom by decantation and generate the primary sludge, driven than by a shovel to a collection pit.
At the end of this process you can define the mechanical treatment phase concluded, this means that most of the impurities and floating solids have been removed (about 40% of the solid organic load). In the next few steps you will take care of those suspended and dissolved.
INDUSTRIAL AND CIVIL WATER – BIOLOGICAL AND CHEMICAL-PHYSICAL TREATMENT
At this stage, with the water treatment plant you help the phenomenon already present in nature of the water self-purification: everything is based on the principle that by microorganisms such as bacteria is possible to transform dissolved organic substances in solid inorganic substance eliminated mechanically. The bacteria enter the process primarily through human waste, infiltration, plant debris or from the ground. They assimilate sugar and fat, but also nitrogen (such as ammonia, extremely dangerous for the environment if not disposed) and phosphorus, necessary to support them: think that these last two promote algae blooms that reduce the oxygen in water available for other forms of life such as fish; the decomposition of flora and fauna then consumes additional oxygen leading to the seabed desertification. That’s why there are legal limits to be observed in regulations. At this point, it becomes therefore very important that the microorganisms have the ideal conditions for transforming the ammonia nitrogen into nitrogen gas and release it into the atmosphere, and this happens in two steps:
- the nitrification, where nitrogenous substances are transformed into nitrates by bacteria who work in the presence of oxygen. For an optimal process, is necessary a water temperature between 20 and 30 Celsius degrees and with a pH from 7.2 to 8.5;
- denitrification, where the nitrate is converted into nitrogen gas by other bacteria; is a process that, however, is carried out only in the absence of dissolved oxygen, that is why sludge resulting from the process of nitrification are carried in the anoxic tanks (free of dissolved oxygen), where bacteria metabolize the organic substances and produce the necessary enzymes for the nitrate to nitrogen gas conversion.
Removed the nitrogen, phosphorus is instead eliminated through the chemical treatment, with chemical precipitation using aluminum or iron salts which create a combination with phosphate ions and fall in the mud.
After approximately 24 hours, the slurry pass to the final sedimentation tank at low current, where the aforesaid active sludge containing phosphorus are deposited on the bottom and are sucked by the pumps that bring it partly back to the beginning of the biological water treatment phase and partly (the so-called excess sludge, or the organic part of the substance transformed by bacteria in biomass) to the digester. The excess water flows out from the discharge channel, purified to about 99% of the total pollutants.
The mud will then be further treated by the industrial plant, but this process is explored in the appropriate section.