Paint waste water treatment

Almost always, where there is an industrial pretreatment, there is also a painting. And where there is a liquid painting there can be a water management system.

Coagulated paint
Coagulated paint

Without an appropriate purification of the painting water, the system, the cabin and the pipes would get dirty and become clogged in a short time, saturated with paint not deactivated. To avoid this, it is necessary to continuously add paint coagulants, which can make it inert to water thus avoiding that it sticks to the surfaces of the entire painting system. There are many coagulants on the market but we can distinguish them into two macro-families, organic and inorganic.

Organic coagulants:  They are represented by polymers capable of binding to paint particles. We find for example polyaluminium chloride or special polyamines. They work particularly on water-based paints, with the creation of light and contained sludges, however they have mild efficacy on the solvent.

Inorganic coagulants:  Aluminum or iron salts, products composed of bentonitic dispersions. In industrial painting, the use of slurry (pastes) composed of refined bentonites often has a high absorbing and denaturing power. They are universal coagulants, manage to neutralize the solvent present in paints and work well on all types of paint. They tend to form voluminous sludge.

After coagulation, flocculation intervenes, or the aggregation of many small clots in a large particle (mud), separable from water. This process is possible through the use of polymeric products called precisely flocculants, introduced in a minimum percentage at the end of the water circuit, allowing the denatured paint particles to re-aggregate in the form of sludge that will tend to surface making it possible to remove it mechanically (racle or filters). The sludge collected in this way can be disposed of while the purified water will continue to be reused in the system.

There are many types of flocculants, they are chosen based on the type of waste to be treated and the coagulant used. Flocculants are polymers that have ionic charges that interact with the particles present in the wastewater. In addition to ionicity, there are other specific characteristics, such as the length and branching of the polymer. The flocculants are found on the market both in hydrocarbon solution and in aqueous solution, or in the form of granules.

To allow an optimal water purification process it is necessary to have the correct initial water parameters. Hard water (at least 30 ° f) is more efficient as the ions already present in the water facilitate the chemistry of the reactions. Too soft waters should therefore be added with salts for hardness. Even the pH must have a value normally between 7.0 and 9.5, otherwise it must be corrected with weak bases or acids.


Bentonitic products are widely distributed and known, they are simple dispersions of special refined bentonites, such as hectorites, which have an excellent affinity with water and are able to bind to the particles creating a sort of suspension network bond . To disperse bentonites in general it takes time and mixing energy, otherwise it is easy to find lumps and sediments. Together with the bentonite chosen, it is necessary to add dispersants capable of maintaining the viscosity of the product for as long as possible, they are products that over time will tend to mix or vice versa to precipitate according to the percentages used. Since they are aqueous products it will also be appropriate to add biocides and / or algaecides in fairly high doses. The pastes or slurry thus obtained will then be pumped into the treatment plants. The color of these products will be similar to the original bentonites, usually ocher-gray, however other colors can be formulated.

The formulations based on cationic polyamines are simpler to prepare, in fact it is sufficient to dilute these raw materials in demineralized water. These products are simpler to dose and manage, but they have a more delicate balance during clarification, i.e. an excess of product could compromise the subsequent flocculation capacity, due to the molecular size and ion balance. The correct dosage, on the other hand, is able to create compact sludges that are poor in water.

Other historical formulations are those based on aluminum sulphate, this salt is able to function in practically any condition and can be used either as it is (in powder form) or in aqueous solutions (max 8%). However, it can rarely be used alone, in fact the clots and the respective sludge obtained will be too bulky and light. It is therefore preferable to use it in powder products together with other elements such as bentonites, lime, zeolites. If you want to obtain a multifunctional product you can also add polyelectrolytes that will also guarantee flocculation without the need to put other products.

Refined bentonite
Refined bentonite