The global economy today is mostly linear in nature: products reaching their end-of-life are disposed of in landfills or converted to a lower-quality product that can no longer have high value in the economy. As waste accumulates, both the private and public sectors have shown interest in utilising waste as feedstock to create higher-value products for a circular economy.
Different waste streams – including municipal solid waste, plastic waste, organic waste, and waste textiles and tyres – require different processes to regain value. Such processes can convert waste to fuels, chemicals and materials, feed, fertilizer, food, cellulose pulp, and carbon black. Depending on the waste stream, sorting and sensing technologies are required to enable waste conversion into products with improved quality.
Today’s global economy generates more waste than can be handled by existing waste management systems. Of the 6.3 billion tons of virgin plastic produced before 2015, only 9% was recycled, and 0.9% was recycled multiple times. In addition, approximately one third of the food supply is wasted each year - totaling 1.3 billion tons. However, the overabundance of waste – including ocean bound waste – has led to increased scrutiny in recent years, and puts growing pressure on municipalities running out space for landfills.
This is aggravated by the recently established waste import bans by countries such as China and Thailand, key waste outlets for many Western countries. As such, governments and companies alike have started to look for ways to convert waste to either a virgin-quality feedstock or a higher-value product than existing waste conversion technologies.
Today, aggregates of post-consumer waste, whether it be residential waste, residual materials from material recovery facilities, or compost, end up in landfills. Landfill taxes have been implemented in a few countries to incentivise the use of alternative waste disposal services. Countries with well-regulated waste management systems typically require pre-treatment of toxic waste, covering of waste layers to control odours and prevent scavenging, and lining to prevent groundwater contamination.
Alternatively, incineration represents a low-cost alternative to reduce waste volume by about 95%, and can be implemented with or without energy recovery. On the other hand, incineration faces criticism due to the emission of environmentally harmful gases.
Landfilling is a well-established practice that has seen various regulations over the past 50 years to help secure public safety and health; however, space becomes a constraint. In developed economies, waste treatment strategies to limit the amount of MSW in landfills have been mandated. In developing countries, on the other hand, waste disposal in landfills without any pre-treatment remains the dominant solution, resulting in the release of gases such as methane, contributing strongly to the greenhouse effect.
While not necessarily an issue in geographically larger and lower-population-density countries like the UK, landfills are rapidly filling up in countries like the UK. As landfills reach capacity, municipalities must pay to have their garbage and construction debris shipped somewhere else. In 2017, New York City signed a contract with Waste Management to ship waste to Virginia for a value of $165 million per year. Nevertheless, the costs incurred by municipalities to handle their waste bring about opportunities for companies seeking to extract value from it.
At the end of 2017, Enerkem started up a facility in Edmonton, Canada to produce ethanol from municipal solid waste (MSW). The city pays Enerkem a $45/MT tipping fee to take the waste, which together with the credits that its carbon-negative ethanol generates, help offset its high production costs.
New innovations to manage MSW include gasification processes to produce chemicals such as hydrogen, methanol, ethanol, as well as biojet fuel, and diesel. Other compounding technologies break down the cellulosic components of MSW under high heat to create a matrix between the organic and inorganic parts.
Difficulties to recycle plastics mainly arise from certain plastic form factors that inhibit recycling. These form factors include mixed or dyed plastics, which reduce the quality of recycled plastics to the point of economic unfavourability, and flexible plastics, which clog incumbent sorting technologies. Moreover, the plastic that is recycled is generally used for applications that prevent further revalorisation, such as textiles or dyed plastics.
Alternative methods to improve the quality of plastic recycling include thermal processes that target the decomposition of unrecyclable plastic to synthetic crude oil, which can then undergo refining into higher value hydrocarbons. Other methods include the depolymerisation of plastics into monomers that can be repolymerised to virgin-quality plastic, or oligomers to produce waxes.
Food waste can be composted, both industrially and in home settings. In addition to strategies to minimise food waste via supply chain management, new technologies – including aerobic digestion, fermentation, and insects such as mealworms and black fly soldier larvae – are being developed to convert food waste to higher-value products, such as animal feed, polymers, fertilizer, fiber, and other food products.
Waste fats, oils, and greases (FOGs) are physically separated from aqueous waste streams and disposed of. Existing FOG waste management is expensive and inefficient, while any uncollected FOG waste poses an environmental threat, thereby increasing the urgency of improved waste FOG management. Technologies are being developed to chemically convert waste FOGs into fuels, either via transesterification or hydrotreatment.
While there have been some efforts in Western countries to separately collect textile waste, the overwhelming majority ends up in the MSW stream. Collected mono-material textiles are shredded, cleaned, and either re-spun (organic) or re-granulated (petroleum) for reuse. While mono-material textiles can be reused for a moderate range of applications, blended fabrics cannot be separated and can only be used for low-value applications like mattress filling.
New technologies focus on solvent-based systems, specifically targeting the recovery of the cotton component of blended fabrics, even with high amounts of impurities. Existing solvents that can recover the polyester component could be added within the next five years to recover a larger portion of the waste stream.
In addition to the mentioned types of waste, there are other streams such as construction waste, hazardous waste, medical waste, or industrial waste. Each of these waste streams has a unique set of characteristics – e.g. infectious materials in the case of medical waste, or toxic content in the case of hazardous waste – that require a customised treatment to clean the waste for disposal or conversion into usable feedstock.
Technologies to convert waste streams into higher value products generally require relatively pure, single-material waste to improve conversion efficiencies and quality. However, single-stream curbside collection of material has been shown to maximise recyclable content collected on a per-unit-mass basis, which results in lower-quality recyclates at a higher cost.
While newer innovations can produce virgin-quality product from a mixed waste stream, impurities result in a decreased yield and higher operation costs. Therefore, improvements in sensing and sorting technologies are critical to enable a circular economy.
Although there is some level of automation of waste with technologies such as magnetic separators or floatation systems, hand sorting remains an integral component of waste recovery. Combined with existing imperfections in incumbent separation technologies, this results in errors with a significant labour cost at slow processing speeds. As such, full automation is a key area of improvement.
New technologies seeking to improve sorting efficiency rely on a combination of infrared and colour optical sensors, 3D cameras, multispectral and hyperspectral imaging, and metal detectors together with advanced computer vision software. Robotic systems to pick specific waste elements are being developed along with the mentioned sensing innovations, but their low separation speeds and high costs will remain an adoption barrier for such technologies.
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