Plastic recycling washwater and process water is the water used to wash and clean contaminated and polluted plastic before and during the recycling process. Only clean plastic can be treated to produce a high quality recyclate that can be processed by industry. This wastewater can contain a wide range of contaminants depending on the origin and previous use of the washed and processed plastic. This recycled plastic washwater is purified and reused in the washing process to reduce operating costs for washing plants.
Plastic recycling washwater
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Characteristics of wastewater from the plastic industry
Plastic recycling industry overview
The plastic recycling sector covers a very broad range of plastic types. From post-consumer plastics used for food packaging, to HDPE and PVC plastics use in construction and other applications. The following page gives an overview of the wastewater generated from the recycling of plastics. A washing line is an integral part of any plastic recycling process, and clean water is essential for the process to work.
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The growth of the plastic recycling sector is resulting in a demand for customised and fully integrated closed loop washwater recycling solutions. The range of pollutants present in the wastewater is growing due to new recycling and washing processes being developed such as deinking. This creates a better need to understand the composition of process and washing water from the recycling process.
Process water volumes
Water plays a very important role in the recycling process. From the initial separation of plastics based on density in float and sink tanks, to the conveyance of plastics between different sorting steps. Then in the different washing steps, water is again a critical resource that must be used. Many washing lines may have multiple washing steps. From initial washing to remove coarse substances and pollutants, to washing with ultra-pure water for food grade and medical grade flakes and recycled plastic to be used in plastic films.
A typical washing line can use approximately 25 m3/hour of water. Additional side streams can also exist in a washing line for different water qualities. If you consider the cost to buy 1 m3 of water from the water utility, and the cost to dispose of 1 m3 of polluted washing water into the municipal sewer network, the need to reuse water in a closed loop becomes necessary for cost effective operation of a plastic washing line. With a closed loop reuse system there can be a water loss per day of just 2-5%.
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Regardless of whether the washing line and the plastic recycling line is operating using batch or continuous water flow, a customised solution can be developed to suit site specific requirements of space, water quality and cost.
Process water composition
The composition of plastic recycling washwater can be difficult to categorise. But by looking at the individual uses for plastic this can assist with a better understanding of the pollutants that can be present in plastic recycling process water. Some pollutants are very easy to remove, such as oils and suspended solids. Other pollutants such as glue used for labels and ink can require customised treatment steps to clean the washwater and allow reuse.
The following table shows typical average values obtained from real projects completed by PPU recycling different plastics.
PET is the most widely recycled plastic. PET is transformed into r-PET flakes which have many uses. When considering the pollutants that can arise from the recycling of PET, in particular PET bottles and containers, you must consider the liquid or product they hold.
PET material is primarily used for beverage bottles but also food packaging or bubble wrap. The myriad types of food that can be stored in PET containers pose a challenge, as the contaminant load of each batch of plastic that is recycled can vary significantly depending on the origin and type of food.
PET bottles are used extensively in the beverage sector. Plastic bottles used for beverages can have a medium/low level of pollution as the liquid will not typically adhere to the PET, and bottles are typically empty. But with sugar-based drinks, there can be a high BOD in the water arising from sugar residues. Dairy and milk-based products that are often stored in PET bottles can cause elevated levels of BOD and COD in wastewater from milk residues.
PET bottles can also be used to hold other food products such as sauces or mustards. These sauces can have a very varied composition. Washing of these PET bottles and containers can produce a wastewater with a very high COD and BOD load, in addition to suspended solids and fats/oils/grease. Some ingredients in sauces can also have a very pronounced colour which can affect the wastewater and require colour removal.
PET bottles can also store medicines or pharmaceuticals. If used for storing these substances then there is the possibility for many complex and hard to remove, persistent pollutants to enter the washwater. These include active pharmaceutical ingredients, antibiotics, hormones, etc. In such a situation special process steps are required to ensure these pollutants are removed and oxidised so that future materials washed with the recycled washwater do not cause recontamination or accumulate in the washwater.
HDPE plastics typical have a different use than PET. HDPE is a high-density thermoplastic polymer that can resist tearing and damage. It is therefore often used for outdoor furniture, flooring/decking due to its weather resistance. Recycling of HDPE used in this type of application typically has low pollution loads.
However, this composition also allows HDPE to have a very good chemical resistance. So, HDPE is then used extensively for the storage of chemicals and other corrosive materials. Recycling of containers that hold chemicals and other corrosive substances can result in a very polluted wastewater with a pH outside of the normal range, and a very high COD value. So, when processing HDPE into recycled pellets, special consideration must be given to the wastewater and process water cleaning for reuse.
LDPE is another plastic that is extensively recycled. LDPE film is one such product that recycled LDPE is used to produce. These films include cling films used for food wrapping, and in horticulture. The LDPE that is collected for recycling is generally sourced from pallet wrapping, shrink wrapping, plastic bags or bin liners. So, while LDPE that is recycled may not be heavily polluted, the LDPE recycling process requires washwater that is free of very fine particles. If the washwater is not adequately cleaned, then these particles can find their way into the final recycled films making them unsuitable for use in certain applications.
Polypropylene is a plastic that is used in a very wide range of applications. These range from clothing fibres, sheets used in construction and tank building, food containers, bags, films, and foams. It is also resistant to many chemicals. This means that when recycling polypropylene there is the possibility to have a very wide range of pollutants present in the wastewater due to the multiple uses of the plastic.
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In summary, it is very hard to control what pollutants end up in the process water used for washing plastics to clean them prior to recycling. Plastics are often sorted based on their type rather than on the previous use of the plastic. This means that the variation and concentration of pollutants can vary significantly in the washwater.
A multi-step treatment approach is needed to ensure all pollutants are removed, degraded, and separated. This then allows safe recycling of the washwater in a closed loop. And the best way to make sure you implement the correct solution on your project it to get expert help and advice.
To learn how to choose the correct cleaning steps to allow washwater reuse for your recycling and washing line then please click the button below.
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