Wash water is the most significant waste product in the plastics recycling sector. Up to 10 cubic metres of wash water are produced per tonne of recycled plastic. The treatment and reuse of wash water is therefore crucial to save both costs and resources in the recycling company. The most efficient technology for this is a combination of several process steps. Depending on the desired degree of reuse, a single process step is not sufficient.
A treatment system refinances itself quickly
If no wash water treatment takes place in the plastics recycling sector, the company’s costs increase enormously. These are, on the one hand, the costs for the regular fresh water supply, and on the other hand, the costs for the discharge into the sewage system. The wash water is usually so heavily polluted that heavy pollution charges are incurred. A treatment plant for the wash water reduces these costs to a minimum – despite operating expenses.
It is crucial to evaluate each wash water individually. Depending on the type of recycled plastic, the residues that adhere to the plastic and the final recycled product, different contaminants are present in the wastewater. These are mainly COD and BOD. These are mainly composed of dissolved and undissolved substances. The amount of wash water produced, and the desired degree of treatment are also decisive factors in planning the plant correctly. PPU Umwelttechnik GmbH offers in-house laboratory analyses for this purpose. A complete analysis of the wash water is completed here within one week and includes a comprehensive test report.
The plant design
Based on the laboratory results, we design the appropriate treatment plant for your requirements. A first treatment step is a simple physical process such as a vibrating screen, belt filter or decanter to separate solids and suspended matter. In most cases, an initial reduction of COD and BOD takes place. These partly adhere to the solids and suspended matter that the filtration processes remove.
Depending on how much sediment is present, it is worthwhile to use a lamella clarifier. In this process, lamellae transport the settleable matter downwards with the help of simple physics. In certain applications, polymers achieve better binding and faster settling. The settled material can then be removed via a sludge discharge.
Dissolved air flotation
The first step after pre-treatment is dissolved air flotation. This is a chemical-physical process. It is designed to remove undissolved suspended matter from the wash water. These include oils, greases, lubricants and microplastics. The first step in dissolved air flotation is a chemical reaction. Flocculants are used to bind the suspended solids in the wash water into larger conglomerates. The type of flocculant and the quantity are adapted to the individual wash water. The water then enters the flotation reactor.
On its way into the reactor, a partial flow of the wash water is supersaturated with air by pressure. If this partial flow also reaches the flotation reactor, the pressure is released so that the air is released from the wash water again. This creates ultra-fine micro-bubbles that rise at a speed of around 4 metres per hour. This enables them to float the bound suspended matter upwards. This process produces a sludge that concentrates in the upper part of the reactor and can be easily discharged through a cone opening. Depending on the contamination of the wastewater, dissolved air flotation achieves a degree of purification between 85 and 99 percent.
Flotation is one of the best known and most popular processes for treating wash water in the plastics recycling industry. Wash water often consists of undissolved substances, which is why dissolved air flotation is particularly efficient. However, reuse is not yet possible because there are also dissolved impurities in the wash water. These are, for example, food residues such as sugar as well as fertilisers or drug residues.
Since the wash water is almost free of suspended matter after flotation, a filtration process is suitable afterwards. Ultrafiltration and reverse osmosis are increasingly used in wash water treatment. These processes use membranes that allow substances to pass through or are retained at the molecular level. Since dissolved air flotation has already been able to remove a large part of the suspended matter, the risk of clogging is lower, and the filtration media need to be replaced less frequently.
Depending on which filtration process is used, a final treatment of the wash water is conceivable to be able to use it completely in the cycle. The best solution for this is an oxidation process. PPU Umwelttechnik GmbH has developed the ClearFox® DiOx – a process that uses boron-doped diamond electrodes for oxidation. These electrodes are present in the wash water as anode and cathode. With the addition of electrical voltage, they convert the contaminants in the wash water into the strongly oxidising OH radical. This oxidises remaining impurities such as heavy metals.
The first advantage of the oxidation process is the conservation of resources. Since only electrical energy is used in this solution, there is no need for the regular addition of treatment chemicals. If the recycling company has its own electricity production, it is worth using the energy generated to operate the ClearFox® DiOx. The second advantage is the reliable removal of even substances that are difficult to oxidise. These include paint particles, which dissolve into the wash water after the deinking process. After this step, the wash water can be reintroduced into the washing cycle without hesitation. Due to various parameters such as evaporation or the water content that remains in the sludge, a fresh water addition of about three to five percent is necessary.
Alternative treatment methods
One treatment process for wash water that is becoming increasingly important in the plastics recycling sector is biological treatment. This reliably breaks down organic compounds. Especially due to the increasing number of plastic milk bottles and other plastic products for daily use, the organic load of the wash water is rising.
To reliably break down this organic load, the ClearFox® FBBR process is worthwhile. This is a submerged fixed bed on which the microorganisms that naturally occur in the wash water can settle and grow. These microorganisms respire the biological contaminants in the wash water and convert them into an activated sludge. This settles on the bottom and supports further biological treatment. If the sludge load is too high, it passes via an automatic discharge into a lamella clarifier, which physically separates the sludge from the clear water.
Through combination to the desired treatment result
Each process is capable of treating wash water on its own. However, it is not possible to achieve a degree of purification that is suitable for reuse of the wash water in the washing cycle or for discharge. Only the combination of the process steps makes this degree of purification possible.
In stand-alone processes, the treated wash water is suitable either for discharge into the public sewer system or for reuse in the wet granulator, as the requirements for the wash water are often not high there. However, if the wash water is to be used in floating and sinking separation, a higher degree of purification is necessary. Here, there is a risk that the degree of contamination will accumulate during regular reuse, which is why the combination of treatment steps is necessary. A high degree of purification is also necessary for the rinsing process, for which an upstream membrane process is suitable.
The correct solution for your wash water
PPU Umwelttechnik GmbH has focused on looking at each wash water individually. For this reason, a treatment plant is created for each customer that is designed for their wash water and their purification standard. In addition, each treatment plant is designed to be as compact and efficient as possible, so that it can be set up either directly in the company or as a container solution on the company premises.
Do you have questions about your individual wash water treatment? Do not hesitate to contact us!