Food waste disposers can mitigate climate change and reduce costs
For climate change mitigation, food waste disposers are better than composting, waste-to-energy and landfilling. Their wider adoption calls for integrated decision making encompassing solid waste management and wastewater.
Food waste disposer (FWDs), devices invented and adopted as a tool of convenience may now represent a unique new front in the fight against climate change. These devices, commonplace in North America, Australia and New Zealand work by shredding household or commercial food waste into small pieces that pass through a municipal sewer system without difficulty. The shredded food particles are then conveyed by existing wastewater infrastructure to wastewater treatment plants where they can contribute to the generation of biogas via anaerobic digestion. This displaces the need for generation of the same amount of biogas using traditional fossil fuels, thereby averting a net addition of greenhouse gases (GHG) to the atmosphere. The use of anaerobic digesters is more common in the treatment of sewage sludge, as implemented in the U.K., but not as much in the treatment of food waste. In addition to this, food waste can also replace methanol (produced from fossil fuels) and citric acid used in advanced wastewater treatment processes which are generally carbon limited.
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Despite an ample number of studies pointing to the evidence of positive impacts of FWDs, concerns regarding its use still exist, notably in Europe. Scotland for example has passed legislation that bans use of FWDs, stating instead that customers must segregate their waste and make it available curbside for pickup. This makes it especially difficult for the hospitality industry, to which the use of disposer is well suited. The U.S. however has seen larger scale adoption of the technology due to the big sales push it received in the 1950s and 60s. In addition to being just kitchen convenience appliances, FWDs are yet to be widely accepted as a tool for positive environmental impact.
Food waste disposers – is the jury still out?
In the hope of contributing to a process of informed decision making, we organized a panel – Food waste disposers – is the jury still out? – which explored the doubts often cited regarding the use of disposer units, discussed where there may be need for further research, and defined region-specific criteria needed for assessing suitability of FWDs for wider use. Beyond discussion of pros and cons of FWDs the panel also cites communication, policy, legal and technical barriers that would need to be overcome to achieve widespread adoption of the technology, wherever deemed a feasible solution.
Prof. Adam Read, Practice Director for Waste, Ricardo-AEA; Dr. Tim Evans, founder at Tim Evans Environment and Michael Keleman, Manager of Environmental Engineering at InSinkErator provide the expertise on Food Waste Disposers in this panel moderated by Ranjith Annepu, co-founder of be Waste Wise.
Responsibility for waste management
Foremost among concerns regarding the use of FWDs is the fear that responsibility for food waste management could shift from one industry (waste) to another (water) and fear of the impact that this change may have on traditional wastewater infrastructure. Results from multiple studies show that in the case where penetration of the technology is assumed to be low (10-20%), there is no significant difference in characteristics (total flow, BOD and TSS) of the wastewater stream. At higher percentage of penetration, greater than 40%, the characteristics of the wastewater stream change enough that it may require modifications to the plant’s capacity. (1) (2)
In the best case scenario known, soil scientist Tim Evans, cites a unique case study from a town in Sweden which witnessed a rapid shift from no disposers to 50% of households using them. The town sampled the wastewater coming into the treatment works every four weeks to obtain a 24-hour composite sample. From this data it appeared that there was no increase in flow, i.e. no increase in water use, but also no increase in other wastewater properties like BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), nitrogen or phosphorous. There was however a 46% increase in biogas recovered.
In one study conducted at the University of Hanover for a scenario of 100% coverage, the results estimated that at a higher percentage coverage in a city the sewage treatment plant will often require capacity additions. But, that may benefit the WWTP in terms of energy efficiency because of increased energy recovery as biogas can be used to run the plant itself. The study estimated an increase in flow of 3-5%, BOD by 10-25%, TSS by 40-60%, Total Kjeldahl Nitrogen (TKN) of 5-10%, Phosphorus of 7-14% and 90-100% increase in biogas production. (3)
Given the evidence Prof. Read suggests that technologies such as food waste disposers would be most ideal in areas where there is new housing development and industrial development is taking place where the sewerage and wastewater treatment infrastructure can be built more efficiently with the strategy of disposers in mind.
Increased water usage
Critiques often cite increased water usage as a deterrent to the wider adoption of food waste disposers. Michael Keleman estimates that the additional water requirement in the use of a disposer amounts to 1 gallon/capita/day, a figure reiterated by several studies estimate approximately 3 – 4.5 litres/capita/day. (4) (5) This may be a cause for concern in water scarce areas thus rendering the use of FWDs unfeasible, however in most places where water is available in sufficient quantities, the additional 1 gallon consumed may be a desirable tradeoff given other benefits that accumulate.
Though they give the impression of being power hungry devices disposers do not use a lot of power when considering an entire annual cycle. Estimates suggest that additional power consumption of a food waste disposer amounts to 3-4 kWh a year, a figure which has been compared in studies to determine the difference in alternative methods of treatment of food waste. Results of these studies suggest that the net benefits gained in terms of climate change are greater when food waste disposers are used over other options.
Load on wastewater infrastructure
The argument that wastewater pipelines may not be suited to carrying the additional load from FWDs Michael Keleman clarifies is only of relevance in certain circumstances. Food waste disposers shred waste into granules, 95% of which are smaller than 3 mm and 40% of which are smaller than 100 microns. This is waste much smaller than fecal matter (Kegebein et al., 2001) and therefore it can be easily carried by traditional wastewater infrastructure that is able to carry fecal matter. Mattsson et al. (2011 and 2014) from University of Lulea have confirmed this by CCTV survey of sewers – at 180 locations totaling 10km. European wastewater infrastructure though older is not much different in design from wastewater infrastructure in U.S. cities that have adopted disposers, thereby negating infrastructure design as a reason for hesitating to adopt disposers.
The potential for buildup of grease in pipelines increases with the use of disposers and in this case it is acknowledged that to avoid this awareness programs are indispensable. (5)
Criteria to determine suitability of food waste disposers.
A life cycle assessment conducted by InSinkErator in collaboration with CDM Smith and PE International in 2010 looked at the use of a disposer in conjunction with 8 different types of wastewater treatment plants and compared each scenario to landfilling, composting and waste-to-energy. It was found that disposers and wastewater treatment are always lower in global warming potential than any other schemes. If the treatment plant uses the process of anaerobic digestion to create energy and recovered bio-solids are used as fertilizer, then this case represents the lowest primary energy demand as well.
To determine suitability of disposers, it is important to consider area-specific characteristics such as availability of water resources, household practices, condition of the sewerage system and wastewater treatment processes. The specific composition of food waste in a particular region is an important consideration. A region already prone to sewers clogging with oil and grease may not do very well if food waste is added to this load. Cold water added along with the food waste helps the grease congeal and it therefore does not deposit. Care must also be taken to ascertain whether the flow of sewage in a particular region achieves self-cleaning velocity, else it may require additional maintenance on the adoption of disposers. Studies suggest that if these considerations are given adequate importance the adoption of disposers should not affect sewer lines.
Research to evaluate the technology
There are a number of countries that are doing their own studies. There have been studies done from Sydney, Australia (2) to Japan (6) and studies going on in the U.S. as well. Research still remains to be done that will help to ascertain the extent and manner of adoption of this technology.
A part of the analysis of the data collected by Dr. Evans indicated that it was the biofilms on the walls of the sewers were doing some of the treatment of food waste before it reached the WWTP. Research conducted to determine how the microbes in these bio films respond to the wastewater flying past them is another aspect of this technology that needs further exploration.
An additional area of research that could shed light on the future prospects of this technology is the variation in suitability of biosolids that results at the end of the anaerobic digestion process in the presence of chemical waste streams of different compositions that arise from the use of a variety of pharmaceutical products in households nowadays.
Barriers to the adoption of food waste disposers
The technology of food waste disposers faces an uphill challenge against preset notions of what is the ideal manner for treatment of household wet waste. Options other than composting are seen in a bad light. Past brand names given to disposers such as ‘garbage gobblers’ and ‘macerators’ convey a sense of irresponsibility towards the treatment of food waste which has not done the technology any favors.
Tim Evans points out the case of Palo Alto in California where people have been lobbied by waste management companies who want them to buy their composting or digestion systems. Palo Alto however is a city where nearly every house already has a food waste disposer. It was a while before the council realized that disposers are actually a part of the recycling process and that if everybody used a disposer then the need for separate collection wouldn’t exist. The wastewater treatment works were then updated with efficient anaerobic digestion and thermo hydrolysis to form an intelligent integrated system.
Traditional recycling targets
The absence of an agreed upon method of measurement for the impact of food waste disposers is another challenge to communicating its efficacy. Adam Read points out that unlike traditional recycling, the waste that goes down a disposer to an anaerobic digestion facility is not considered as waste diverted from a landfill. An appropriate solution in this case might be calculating the weight of waste added to the wastewater stream and subtract from this the weight of biosolids generated that will end up in a landfill due to it not meeting required standards for application on farms.
Jurisdiction and regulations
Prof. Read also highlights the existence of separate geographical areas under the wastewater treatment utility and a municipal council’s waste collection program as a major regulatory barrier to the wider adoption of food waste disposers. This was the case in the U.K. which led to the abandoning of disposers as a strategy to handle food waste since their early adoption in the 1980s. Sweden in contrast is an example of a country where cities have control over multiple utilities. In cases where decision making does not happen at the local municipality level, the city will face challenges to support strategies such as food waste disposers that are most ideal when put into implementation comprehensively at the city level.
Similar to technologies like gasification and pyrolysis which have been operational for decades in other industries, food waste disposers have also found it hard to gain wider acceptance as a solution within the waste management industry. All it would take to change this Prof. Read claims is a few progressive cities taking the first leap and demonstrating with undisputable numbers that these solutions can be implemented and provide positive environmental impact as well as economic gain. This would lead to other cities in turn asking the same question of their own waste strategies and thereby lead to wider adoption.
A sentiment echoed unanimously by all the panelists is that of the need for cities to think about their waste management issues on an integrated basis. Philadelphia for instance has invested over $ 50 billion dollars in their anaerobic digesters and cogeneration at their wastewater treatment to reduce their solid waste collection and disposal cost by using food waste disposers. The city is also planting rain gardens and green roofs, which stop rain rushing rapidly into sewers and overloading the pipes underground. This represents integrated thinking because it is a solution that fits well alongside their investments in wastewater treatment. By increasing renewable energy generation at the treatment plant this strategy is a win-win both from the environmental and economic perspective.
In the developing world
Besides the developed world, food waste disposers can also be put into use by some in developing countries as well. T. H. Culhane, is one such person who has been able to put this into practice by actually carrying a food waste disposer with him to developing countries where he works and uses it to grind food waste, make bio-gas and use it for cooking purposes.
Among its many advantages, food waste disposers offer a solution to reduce the incidence of disease with lesser food waste lying in the open attracting fewer disease vectors.(1) It also reduces the complexities of having to induce behavior change in citizens to segregate waste at source (7) and simplifies the logistics for waste collection. (4) It is important to observe the contrast in motives for using food waste disposers in the developed world and developing world. However, if this technology is considered at a city level, then the criteria mentioned above should be considered to determine their suitability.
SOURCE (47 minutes)
- Environmental Aspects of Food Waste Disposers. Koning, de. s.l. : Delft University of Technology, 2004.
- CRC. Assessment of Food Disposal Options in Multi-Unit dwellings in Sydney. Sydney : Cooperative Research Centre (CRC) for Waste Management & Pollution Control Limited, 2000.
- Rosenwinkel, K.H. and Winkler, D. Influences of Food Waste Disposers on Sewerage System, Wastewater Treatment and Sludge Digestion. s.l. : Institute for Water Quality and Waste Management (University of Hanover), 2001.
- NYC DEP. The Impact of Food Waste Disposers in Combined Sewer Areas of New York City. s.l. : New York City Department of Environmental Protection, 1997.
- Kegebein, Jorg, Erhard, Hoffmann and Hermann, H. Hahn. Co-Transport and Co-Reuse – An Alternative to Separate Bio-Waste Collection? s.l. : Institute for Municipal Water Treatment, University of Karlsruhe.
- National Institute for Land and Infrastructure Management. Report on Social Experiment of Garbage Grinder Introduction. s.l. : Ministry of Land Infrastructure and Transport, Japan, 2005.
- Karlberg, Tina and Norin, Erik. Food Waste Disposers – Effects on Wastewater Treatment Plants. A Study from the Town of Surahammar. s.l. : VA – FORSK REPORT, 1999.