February 8, 2025

Plastic recycling processing equipment with washing water.

Three experts at a wastewater treatment system provider cover several issues that plastic reclaimers need to consider when shopping for a system. | Nordroden/Shutterstock

Wastewater treatment for plastics recycling

Recycling of plastics and other materials is becoming increasingly important around the world. The industry is experiencing a growth, driven by laws and regulations that mostly apply at the national level. Consequently, existing recycling companies are planning to expand their capacities, new entrants in the recycling sector are no longer rare and companies from other sectors are discovering the recycling sector. -recycle as another tree.

One part of the recycling process is the use of wash water, which is included in two phases of the recycling process. Recyclers usually wash the incoming material before processing it. When the recycler cuts the plastic into pellets, a second washing process is required. Depending on the type of recycled plastic, the size of the plant or the washing process, different amounts of waste water are produced – up to two cubic meters per metric ton of recycled plastic.

The following substances can be mixed with waste water: fats and proteins from organic pollution, plastic abrasion, salt and sugar from residual liquids, printing inks, washing chemicals, fibers cellulose and adhesives from labels. This has an effect on two of the sum parameters used to evaluate water quality: chemical oxygen demand (COD) and biochemical oxygen demand (BOD).5). While the BOD5 can be as high as 12,000 milligrams per liter, COD usually goes up to 20,000 milligrams per liter. The pH value is equally variable, usually between 6 and 14.

Thomas Leistner

Thomas Leistner

The hot sodium hydroxide washing process is essential for a high wastewater load and the associated effective cleaning. This process is usually used to wash the crushed flakes with washing water at a temperature of at least 80 degrees Celsius (176 F), which, with the addition of caustic soda, frees the flakes from their contaminants. Typical consumption is about 30 liters (7.9 gallons) of caustic soda per metric ton of recycled material. The remaining cellulose fibers swell due to the caustic soda and are therefore removed from the recycled material.

There are many ways to further use the waste produced:

One such option is known as indirect discharge, meaning that the wastewater is treated before being discharged into a public sewer system. The municipal wastewater treatment plant purifies the wastewater and then discharges it into a receiving body that stands or flows into public water. The reuse of waste water in its own cycle is prevented.

Increasing capacity is not always possible. Although some municipal plants are generously designed and can therefore accommodate larger flows of wastewater, most are already at their load limits, so some recycling companies indirect discharge of bulk waste is prohibited.

Another option is direct discharge, where the recycling company releases independently treated wastewater into a receiving body of water, or into the natural environment if the company does not have access to the sewer. The discharge parameters are determined in the region by the relevant authorities or municipalities based on the required purity of the wastewater. In this way, the recycling company can save on operating costs, since the fees for indirect discharge are not applied.

A third way to deal with waste water is to reuse it in the recycling process. Despite the availability of state-of-the-art treatment solutions, 100% reuse of wastewater is rarely achieved. The main reason for this is the concentration of dissolved and non-dissolved substances. Also, part of the water – about 5% – evaporates during the washing process. Therefore, a fresh water supply of at least 10% is required to maintain a smooth washing process.

Wastewater treatment is beneficial for many companies

Sabine Düreth-Joneck

Sabine Düreth-Joneck

The European Commission, as part of its Green Deal initiative, released a set of values ​​for recycling companies that want to throw their waste water into a receiving body of water. It is flexible but binding for recyclers. This gives them an indication of the level at which waste treatment should be carried out. Through laboratory analysis, the relevant parameters of the wastewater can be determined and the treatment solution most suitable for achieving the discharge requirements can be found.

When the recycling company expands its capacities, more waste water is produced. However, if the municipal waste treatment plant is operating at full capacity, domestic waste must be treated. Treated wastewater can be reused in the company’s own cycle or discharged directly. Depending on the type of treatment plant, about 50% to 90% of treated wastewater should be reused.

In-house power generation is a more common topic among recycling companies, and special wastewater treatment processes offer ways to use electricity. Solutions such as oxidation processes or reverse osmosis use electrical energy to produce pure wastewater. With these processes, about 90% can be reused.

It is mandatory to provide fire extinguishing water for recycling companies. It should be changed regularly to prevent the growth of bacteria. A supply of treated recycling waste can help keep storage tanks at the required level.

Things to consider when installing a wastewater treatment plant

Since the wastewater can vary depending on the type of recycling, the size of the company and the processing method, the treatment plant must be adapted precisely to the needs of the customer. A treatment plant manufacturer needs to know many important pieces of information, such as flow rate, waste amounts, discharge parameters or company location. Recyclers should therefore ensure that the supplier requests this information in advance.

After setting the above parameters, plastic reclaimers must find products of the best quality possible. High-quality products are recognized by recycling companies through independent certifications, which can usually be found as downloads on the company’s website. The more certificates available, the better the cooperation between testing institutions and thus the quality of medicinal plants.

Markus Mostegel

Markus Mostegel

Next, reclaimers need to consider what advanced technologies are available to them. Biological treatment processes and dissolved air flotation (DAF) have long been proven solutions for waste treatment, but there is ongoing research and thus further development in this area. Recyclers should also pay attention to newer treatment processes such as oxidation, ultrafiltration or reverse osmosis.

Another important consideration is laboratory analysis. Usually the exact sum parameters, such as COD, BOD5, total suspended solids (TSS), nitrogen, phosphorus, chlorides or sulfates, unknown. Some wastewater treatment plant manufacturers therefore offer analysis in their in-house laboratories. In the laboratory, tests are carried out that provide information about the efficiency and compatibility of precipitants and flocculants. This allows the DAF system to be designed to precisely fit the facility. For this, only a few liters of wastewater sample is required. The recycler will receive the exact amount of their wastewater in written form, usually accompanied by an offer for an appropriate treatment solution based on the lab analysis.

Energy efficiency is another important factor to consider. If a plant consumes very little electricity, it is not only easy on the recycler’s wallet, but also evidence of advanced technology and the use of state-of-the-art treatment processes. Some producers have consumption certificates and thus can prove energy efficiency.

Not every wastewater treatment plant needs to come with a purchase, and rental offers can help get a project off the ground. Plant manufacturers often offer rental deals for six months, one year or two years. These are usually easier to use because they don’t need to be built first. In individual cases, the leased plant is available for purchase if the recycler is satisfied with the treatment results.

Even after the plant is built or leased, recyclers must not forget to repair. A wastewater treatment plant requires care and maintenance at regular intervals. In some cases, an employee of the recycling company is responsible for this. Even if this person receives all the information from the plant manufacturer during the installation, he should be available around the clock, if possible, to address issues that arise during operation.

Finally, plastic reclaimers should look for guarantees. To ensure that recyclers buy a wastewater treatment plant without risk, they should look for long-term guarantees everywhere. In fact, producers offer warranties of up to 15 years on selected parts.

Thomas Leistner, Sabine Düreth-Joneck and Markus Mostegel work for PPU Environmental Technologies, based in Bayreuth, Germany. Leistner is the marketing and sales assistant, Düreth-Joneck is the head of the F&E Department and Mostegel is the technical manager of the PPU. The company supplies more than 40,000 private, municipal and industrial wastewater treatment systems in 50 countries.

The views and opinions expressed are those of the author and do not imply endorsement by Resource Recycling, Inc. [email protected] for consideration.

Leave a Reply

Your email address will not be published. Required fields are marked *