Industrial flotators for wastewater treatment: types, device, principle of operation

Today, environmental issues are at the head of the table at virtually every meeting of the World Governments. It's no secret that ecology has become a new religion of the 21st century. 2017 is declared the year of environmental protection in Russia, and therefore environmental education is one of the tasks for this year.

Why do I need to clean the water?

Of the total stock of the World Ocean, only 3% is fresh water, of which 68% are glaciers (not suitable for drinking), 30% are underground sources (often contaminated by soils) and only 2% are land-based sources of water supply. From the global picture of the world it is clear that the availability of clean fresh water is not just a necessity, but sometimes a luxury.

Sewage generated during the economic activities of enterprises, contain a large number of pollutants in concentrations exceeding permissible and regulatory. As a rule, we are talking about heavy metals (iron, nickel, copper, lead, mercury, cadmium, etc.), oil products, suspended solids, aluminum, surfactants (synthetic surface-active substances, for the layman it's all that foams). These substances, getting into water bodies, disrupt the normal functioning of aquatic biogeocenoses, poison the soil, provoke the growth of blue-green algae, toxic to animals. These pollutants are also toxic to humans.

A large number of pollutants also form from human economic activity in residential apartment buildings and private houses. Basically, it is synthetic surfactant and organic waste, but also metal salts fall into the sewage system.

What is a waste water treatment flotator?

A flotator is a device designed to remove finely dispersed impurities from water by a physicochemical method. Relatively speaking, this is the mechanism of one of the main processing modules in the technology and technology of wastewater treatment. It is on the flotator that the main release of dissolved substances and the purification of water to the normative level.

Industrial flotators can be designed for large factories, as well as for car washes, differing in size and material.

The main task of the flotator is to isolate and dislodge the pollutants dissolved in it, transferring them into an insoluble form. In this case, air is supplied to the device to increase the cleaning effect.

Principle of operation of the flotator for wastewater treatment

The principle of operation of the flotator is based on the transmission of air bubbles through the medium to be purified to form a foam. This foam is called flotoslam, which is removed and taken to special devices for dehydration. In order for bubbles to capture and carry with them pollution, it is necessary to pre-add special substances - coagulants and flocculants. These substances have a high adhesiveness, that is, they help pollutants stick together with each other and with air bubbles, forming so-called flocculi.

The bubble, passing from the nozzle or spray nozzle of the dispenser upward, captures sticky contaminants with it. This process is carried out until the water reaches the desired cleaning effect.

The complexity of the process is to accurately adjust the dose of the coagulant and flocculant so that the adhesion strength is high enough to adhere to the bubble, but the flakes formed are not too heavy so as not to damage the air bubble.

Scheme including a flotator for wastewater treatment

The technology that assumes the flotator as the main processing module always includes a reagent farm and an air bubble device. The reagent farm is a reagent tank (coagulants, flocculants, alkali to adjust the pH) and a reactor for mixing the reagent with water.

As a device for creating air bubbles, as a rule, a saturator is used, which is a chamber for mixing air with water in order to create a water-air mixture. Further this mixture is sent to the flotator. The saturation device is equipped with a powerful pump for air injection.

The flotator is never used alone, it is always included in the general scheme of water purification. The complete scheme, as a rule, consists of the stages of preliminary settling, physicochemical treatment (a flotator or coagulator) and subsequent mechanical cleaning on filters.

In other words, the flotator can not provide all the cleaning, it is only a separate node, which requires pre-processing and subsequent. If a sand or other coarse impurities enters the flotator, the device will break down. Also this device can not provide disinfection and complete cleaning of oil products. Therefore, after it is necessary ultraviolet installation and sorption (or mechanical) filters.

The schematic diagram is based on the flotation process. Flotation is the treatment of sewage with air bubbles to extract soluble and emulsified substances. Water enters the main processing module. There, in the pressure (or non-pressure) mode, a pre-prepared reagent is fed in the reactor. Air bubbles are also fed to the flotator using a saturation device. In the flotator for water treatment, sewage treatment with reagents and air bubbles occurs, most of the flocculation floats in the form of a flotation sludge. The floating flotation vessel is removed from the surface of the water by a scraper conveyor into the slurry tank.

This sludge is very unstable to mechanical tilting, so from the surface of the water it is collected neatly in order not to break the foam.

Flotator device

The flotator is an open container of steel or plastic, equipped with a scraper mechanism for collection of the flotation sludge and having a conical shape from below. The flotator implies the presence in it of nozzles for the supply of a water-air mixture from the saturator, for the discharge of the flotation sludge and emergency emptying, for the supply of waste water and for the removal of purified water. Installation of the flotator, as a rule, is located on the maintenance site for convenience.

Types of Flotators

Flotators for wastewater treatment differ by the way the water saturation with bubbles and the nature of the bubbles. The most common methods are mechanical, pressure and electroflotation. Pressure flotation implies the presence of a saturation chamber and a pumping group. In addition, reagents are often used in this method. Electro flotation does not need a reagent farm and a saturator, since it is based on the dissolution of electrodes in water.

Mechanical flotation

Mechanical (or impeller) involves the presence of a stirrer, which, at high speed, breaks air bubbles in the water. This kind of water treatment is suitable for water, prone to foaming and saturated gases. In the mechanical method, reagents can not be used, since the turbulent flows created by the stirrer simply break down flakes of contaminants. At the moment, mechanical flotation is not common, as it rarely provides a sufficient cleaning effect.

Typically, this segment of cleaning includes flotators for wastewater treatment from oil products.

Pressure Flotation

In this case, flotators for wastewater treatment are equipped with a saturation device and a reagent farm. The saturator is a chamber in which air is injected at a pressure above atmospheric pressure. The medium, prepared in a saturator, is called a water-air mixture. This is the most common type of flotation and is most commonly used. The purification process occurs due to preliminary treatment of water with a reagent (coagulant or flocculant) and subsequent treatment with a head of water-air mixture. Each gas bubble attaches dirt to itself, since it has a great attraction force due to the interface between the phases (water-air). Preliminary preparation of water with a reagent improves the purification, since it creates flocculi (micelles), which also possess a certain attractive force. The main part of the water is discharged through the branch pipe of purified water for further purification or for discharge. From above, a special scraper device removes flotation muds - contaminations carried away with air bubbles upward in a concentrated form.

The main advantage of pressure flotation is a wide range of applications. The disadvantages include the presence of additional devices (reagent economy, saturator, pumps), which take up a lot of space, and they need to be automated (for example, the selection of a dose of reagents). The determination of the amount of reagent plays a large role, since a small dose will result in insufficient purification (not all small dissolved particles will enlarge), and a large dose can cause the bubbles to not withstand the weight of the flakes and break down, which will also reduce the cleaning effect.

Electroflotator

This type of flotator for wastewater treatment is laconic and user-friendly. The essence of the method is the electrolysis of the liquid to be purified and the release of gases from the electrodes. The process of electrolysis takes place in the flotator: hydrogen is produced on the cathode, oxygen is placed on the anode. When soluble electrodes are used (for example, aluminum or iron), water is additionally saturated with metal ions with a high degree of oxidation, which plays the role of reagents to create contaminant flocculation. This process helps to isolate and dispose of even more pollutants from the water. Since the space of the flotator is not large, in such conditions there is a good adhesion of flakes and air pockets, which provides the highest cleaning effect.

The main advantage of such a device is the absence of reagent farming and other cumbersome devices, with a high degree of water purification. The drawbacks include high energy consumption and the need for equipment for tapping hydrogen.

Nozzle flotation

In this case, special nozzles are used to introduce air into the process water, which is fed to the flotator, where it is broken into a two-phase mixture. The advantage of this method will be less wear and tear on the parts of the installation, and therefore a longer service life.

Reagent farm

In some flotation methods, the following reagents are used to improve the cleaning effect:

• Reagents for pH adjustment are acids and alkalis, which are added to water to ensure normal working conditions of the coagulant and flocculant;
• Coagulants - reagents that promote flocculation and are salts of iron and aluminum;
• Flocculants are reagents that create larger and more stable flocculi (flocculi) and are polyacrylamide compounds.

The main disadvantages of having a reagent water treatment method are the need for the presence of personnel, as well as the areas that must be allocated to tanks and reactors. It is also very important to choose the correct dose of reagents, which is possible only empirically.