From wastewater to resource

From wastewater to resource – optimising recycling processes for plastic packaging

Plastics recycling – especially the recycling of plastic packaging – is becoming increasingly important for a functioning circular economy. However, in addition to the actual recycling process, one aspect is increasingly coming into focus: the treatment of the wastewater produced. Washing plastics produces large quantities of water that are contaminated with a wide variety of impurities. This wash water must be cleaned efficiently and, if possible, recycled.

Sludge discharge after the recycling process

Contamination of the wash water during plastics recycling

When the plastics are washed, various substances dissolve into the process water, which can lead to considerable contamination. The composition of the wastewater depends heavily on the input material (post-consumer packaging vs. clean production residues, etc.). Typically, the wash water from used plastic packaging contains organic residues such as fats, oils and proteins, food residues as well as sugar, acids or other ingredients from packaging residues. These often significantly increase the biological and chemical oxygen demand (BOD₅/COD) – BOD₅ values of up to 12,000 mg/l and COD of up to 20,000 mg/l are not uncommon. The water also contains plastic abrasion, i.e. tiny plastic particles that detach from the flakes during shredding and washing – microplastics.

Washing chemicals and surfactants also contribute to the pollution. Detergents (alkalis, soaps, degreasers) are added to the wash water to dissolve grease and dirt. These chemicals remain in the water and must be neutralised or removed later. The printing inks removed from films and bottle labels during washing also end up in the wastewater. Printing inks contain pigments and binding agents that cause fine colour particles and dissolved organic substances in the water.

Requirements and technologies for optimised wash water treatment

Optimising wash water treatment in plastics recycling requires a combination of different processes in order to efficiently remove the various contaminants. The following treatment technologies are mainly used:

Mechanical processes: These include coarse and fine separation of solids. For example, drum screens or screw screens are used to remove larger particles, paper shreds and plastic debris from the wastewater.
Chemical-physical processes: Chemical additives and physical effects are used to remove finely dispersed or dissolved impurities. A proven process is dissolved air flotation with upstream precipitation and flocculation treatment. In dissolved air flotation (DAF), the flocs are carried to the surface by very fine air bubbles and discharged as sludge. In this way, suspended solids, oils/greases and a large proportion of the load (COD/BOD5) can be removed.

Containerised wastewater treatment plant at the recycling company ReCover

Biological processes: If the organic load (BOD5/COD) is very high and biodegradable, biological wastewater treatment makes sense. In fixed bed biological reactors (FBBR), microorganisms break down dissolved organic substances. A mechanical-biological wastewater treatment plant in recycling mode can significantly reduce the BOD5. The FBBR is particularly robust against load fluctuations and changing inflow conditions.
Membrane process: Membrane filters are often used to remove the smallest particles and colloidal impurities (right down to microplastics). Ultrafiltration systems can retain particles in the 0.02-0.05 µm range – this includes bacteria, microplastics and even many emulsions.
Oxidation processes: To remove residual substances such as dissolved organic traces, odorous substances or colouring substances, an oxidation process is helpful. Innovative approaches use, for example, electrochemical oxidation with special electrodes to directly destroy dissolved colourants. Such Advanced Oxidation Processes (AOP) can attack and oxidise stubborn organic compounds.

A combination of these processes is often used in several stages to achieve maximum purification performance. The aim is to achieve largely purified water that can either be discharged into the sewage system or – preferably – reused in the process.

Why process water treatment is so important

Effective wastewater treatment in plastics recycling is essential for several reasons: environmental requirements and regulation play a major role. Direct discharge of untreated or inadequately treated wash water is prohibited, as otherwise pollutants and microplastics are released into the water. The authorities set strict limits for discharge quality, which recycling companies must comply with.

In addition to the environmental aspect, there are economic reasons: Fresh water is a precious resource – water charges are rising in many regions – and at the same time the disposal of wastewater (charges for discharge or removal) costs money. Good process water treatment can drastically reduce the need for fresh water and the amount of wastewater.

Another key point is the quality of the recyclate and process stability. If the wash water is repeatedly recirculated but not sufficiently purified, impurities will accumulate. These are reflected in the flakes and ultimately in the recyclate.

Ultimately, good wastewater treatment also contributes to the company’s image and sustainability balance. Recyclers can demonstrate that their process not only recycles material but also water and minimises environmental impact – an advantage over competitors in an increasingly environmentally conscious industry.

From the field: Containerised wastewater treatment plant for the plastics recycler ReCover

Sandwich packaging, coffee packaging, packaging for pet food, and shrink film—these are the specialties of Coveris, a company based in England. The company manufactures packaging and supplies it to various food producers, enabling them to package their products safely and hygienically. Shrink films in particular—plastic films wrapped around drink cans or bottles that shrink tightly around the product under heat—are one of the Group’s core products.

Coveris is committed to sustainability and the circular economy. To this end, the company founded a subsidiary in the east of England that focuses exclusively on recycling plastics: ReCover. The recycling subsidiary has an annual capacity of around 5,000 tonnes of plastic. This quantity comes mainly from the company’s own operations, but other plastics are also purchased for recycling.

Coveris aims for 100 per cent recycling—not only of the plastic but also of the process water produced during recycling. For this reason, Coveris approached PPU Umwelttechnik GmbH to design a wastewater treatment plant capable of achieving this goal.

Existing system reaches its limits

The recycling plant produces 5.7 m³ of process water per hour. Of this, 5,000 litres are used for the rinsing stages, and 700 litres are used for the acid bath in the deinking stage.

The company already had a treatment plant, but it was unable to adequately treat the entire volume of wastewater. The entire 5.7 m³ flowed into a collection tank, where the plant adjusted the pH level and removed solids. This treatment allowed the company to recycle 5 m³ back into the rinsing stages. However, the remaining 0.7 m³ had to be transported off-site for further treatment.

The objective was to treat the entire wastewater volume in such a way that 5 m³ could be returned to the recycling process, while the remaining 0.7 m³ could be discharged directly on-site. The treatment process also needed to be easy to monitor to ensure compliance with prescribed discharge values. Furthermore, the customer required the system to be highly automated, as they had limited staff available to operate it.

The high COD (chemical oxygen demand) of 7,500 mg/l mainly results from the dissolved colours, as well as small suspended plastic particles. To address this, a drum screen is used as the first step to sieve out coarse solids from the wastewater before it enters a buffer tank. This tank is aerated to ensure consistent mixing of the wastewater, preventing solids from settling at the bottom.

From the buffer tank, the wastewater flows through a mixing section where the pH level is adjusted to a neutral value. Precipitants and flocculants are then added to bind the remaining small suspended solids and colour particles into larger flocs.

To ensure the process operates effectively, PPU Umwelttechnik GmbH analysed a wastewater sample in its in-house laboratory. The analysis revealed that with the addition of precipitants and flocculants, the suspended solids could be bound and settled within seconds. As a result, a ClearFox® DAF (dissolved air flotation) system was chosen as the optimal solution.

Efficient cleaning with ClearFox® dissolved air flotation

The ClearFox® DAF can remove up to 99 per cent of suspended solids from the wastewater, significantly reducing COD. The system supersaturates the wastewater with air under pressure. When the wastewater enters the flotation reactor, the pressure drops, and the air bubbles out. These fine air bubbles attach to the flocculated suspended solids, lifting them to the surface. This mixture of air and solids forms a sludge layer at the top of the DAF reactor, which is removed via a cone opening. Meanwhile, a clear water zone forms at the bottom of the reactor and is discharged at regular intervals.

The sludge discharged by the system has a dry matter content of approximately two per cent. This is further processed by a newly developed sludge screw press, introduced by PPU at the beginning of 2024. The screw press increases the dry matter content to around 40 per cent, significantly reducing the amount of sludge waste generated.

Treating more wastewater for less money

Modular wastewater treatment for the Food and Beverage sector