Plastic Recycling Filtration
Why Recycle Plastics?
The need for cost effective and environmentally-friendly ways of dealing with plastics waste continues to be an important topic for discussion and debate. Plastics make up a huge amount of our solid waste (more than 12% of all disposed solid waste, by some estimates). Plastic also takes an incredibly long time to break down when placed in landfill – as long as 500-1000 years!
While plastic waste is prevalent in landfills, it’s not the only place where plastics cause pollution. According to 4waste.com, 90% of all waste that is floating on the surface of the ocean is plastic, which can be estimated as about 46,000 pieces of plastic per square mile!
Plastic recycling conserves energy and the environment. For every 1 ton of plastic that is recycled, an estimated 7 yards of landfill space is conserved. Also, by recycling plastic, 80% of the total energy that goes into manufacturing new plastic products is conserved.
Unfortunately, not all types of plastic can currently be recycled due to numerous economical and technical reasons. These include certain types such as Styrofoam, trash bags and bubble wrap. However, at the same time there are many types of plastic that can be recycled, including agricultural plastics and plastic bottles and containers.
Agricultural Plastics Recycling
Plastic mulch (polyethylene or PE) has been used in agriculture for over 60 years. Its use has continued to expand because it offers several well-known benefits including earlier harvests, efficient water use and weed control. However, the main disadvantage with PE mulch is that it must be removed and disposed of at the end of each growing season.
Some of the methods growers have been using for disposing of plastic mulch film have become obsolete due to changes in laws and rising disposal costs. For example, many growers dispose of PE mulch by open burning on the farm. However, this is now illegal in many states, as burning PE can release carcinogenic substances and other toxic particles into the air.
In areas where incineration is no longer an option, growers are faced with hauling the plastic sheeting to a landfill and paying the ever-increasing tipping fees. According to a report by biodegradablemulch.org, from 1985 to 2010, national average landfill tipping fees per ton have increased by an average of $1.24 per year, and the number of landfills has decreased from about 20,000 in the 1970s to just over 1,900 in 2010.
Due to these limited options, some growers bury the plastic sheeting on the outskirts of the farm or dump it into rivers, which can cause water and soil contamination. This is where recycling becomes one of the best options for disposal. In some areas in the U.S., great strides have been made in recycling plastic mulch film. These include the Recycling Agricultural Plastics Program (RAPP) at Cornell University and the Florida Agricultural Plastic Recyclers (FLAG). A recent innovation from the University of Tennessee Institute of Agriculture includes the development of biodegradable plastic mulch. Biodegradable plastic mulch has a higher price tag than conventional PE mulches, but can ultimately create a savings from reduced end-of-season activities associated with mulch removal and disposal.
Seedling Trays and Nursery Pots
Gardening enthusiasts enjoy bringing new flowers and vegetable plants home from the garden center, as they can add color or flavor to our landscape. Unfortunately, they can leave a less desirable footprint as well – in the form of a stack of empty plastic nursery pots and trays. This is what joegardener.com refers to as “the darkest side of gardening.”
Most fans of horticulture are also advocates for protecting the environment and prefer not to add this material to the landfill, so piles of plastic trays and pots tend to stack up. In fact, an estimated 350 million pounds of plastic trays and pots are generated in America each year – enough to fill two baseball stadiums!
The good news is that many of these plastic pots and trays can be recycled. How does this process work?
According to joegardener.com, there are three types of plastic commonly used for horticultural products:
- Polystyrene: The lightest of the three types and difficult to mark with a recycle indicator. Most commonly used for seedling trays / cell packs.
- Polypropylene: Usually marked with the recycling indicator of 5.
- Polyethylene: Usually marked with the recycling label of 2.
Sorting by type of recyclable plastic is important because once sorted, the plastics are put through a grinder and turned into small chips. These chips are ultimately melted down to be made into other products. However, different types of plastics melt at different rates, so if a container of chips is a mix of types, they cannot be melted.
Adding to the complications is the fact that the ground-up chips contain residual dirt which must be removed before the recycling process can begin. This is where solid-liquid separation equipment, such as a filter press, can be a huge benefit to this process. The plastic chips, along with residual dirt, are washed, and the resulting wastewater forms a slurry. The slurry is then dewatered through pressure filtration to form a filter cake, which is mostly solids. The filter cake, being mostly dirt, can then be disposed of or returned to a field. Oftentimes, a slant plate clarifier is used as a step before the filter press to remove additional impurities.
Since plastic mulch sheeting also must be rid of dirt, clay and other residuals before recycling can occur, a similar separation process may be utilized in preparation for recycling in that application.
Plastic Bottle and Container Recycling
According to earth911.com, most bottles and jugs are #1 plastic (PET) or #2 plastic (HDPE), which are both accepted by most curbside recycling programs. The type of plastic is identified with a resin ID code on the bottle.
Recycling centers use optical scanners to identify the type of plastic resin, so #1 and #2 plastics can be separated from each other. Bottles are then crushed, baled, and sent to a plastics recycler.
Recyclers then shred the plastic into flakes, which are then washed, rinsed and dried before being melted into pellets. These pellets are then transported to a manufacturer to be made into new plastic containers or other products.
Plastic bottles and containers can create a challenge for recycling, particularly from residential recycling mixes. This is because there are other items in the mix to consider, such as paper from labels and residuals from the product that was originally in the bottle. This could be soap, bleach, oils, or even food products.
However, while these additional items in the mix can make the resulting wastewater more difficult to treat, a clarifier and filter press can still be successfully utilized, especially when precoat is added to the filter press slurry.
EQUIPMENT FOR PLASTIC RECYCLING FILTRATION
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