Recycling food waste through composting and anaerobic digestion has the greatest potential by far to reduce the quantity of food waste going to landfills over the next 15 years relative to food waste reduction and reuse, according to ReFED. However, as more cities and states institute landfill food waste bans and other programs to promote recycling, the demand for centralized organic processing facilities is outpacing the supply. To address the gap, vendors are actively marketing to commercial customers new onsite pre-processing systems, including dehydrators, pulpers, and biodigesters. The systems can save money by reducing or eliminating off-site hauling of food waste and are well suited to facilities short on space and staff time. But the question arises: are the nutrients and energy in food waste really being recycled? The answer depends upon the next stage of processing.
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| Photo credit: By Taz [CC BY 2.0 (http://creativecommons.org/ licenses/by/2.0)], via Wikimedia Commons. |
By organic recycling, we refer to recovering the nutrients and energy in food waste through composting or anaerobic digestion (AD) for subsequent reuse as a soil conditioner and potentially as energy. Composting produces a nutrient-rich soil-amendment through the controlled decomposition of organic materials by microorganisms in the presence of oxygen (aerobic) over a period of weeks to months. In contrast, AD uses microorganisms to decompose organic materials in an oxygen-free (anaerobic) environment over a similar time period. AD produces biogas for use as an energy source, as well as biosolids that can be used as a soil amendment either directly or after composting, depending upon the AD process parameters. Onsite composting and AD systems are beginning to enter the market now, but in this blog we focus on pre-processing systems because they dominate the onsite market and are frequently misunderstood.
Pre-processing systems treat food waste by removing water (through heat or mechanical methods) or liquefying them (through grinding or biodigestion), in preparation for disposal or further recycling. Dehydrators break food down into a dry, sterile, odorless biomass that can be recycled as soil conditioner following an additional stage of composting, plus a liquid disposed down a drain to the sewer system. Among liquefiers, grinders macerate food into a pulp and biodigesters use a proprietary mix of nutrients or organisms in an aerobic environment to break food waste down into a slurry within a 24-hour period. The slurry is typically sent down a drain for disposal through the wastewater treatment system; however, the pulp can be used alternatively as a feedstock in anaerobic digestion or the manufacture of fertilizer.
Currently, the capacity of installed onsite pre-processing systems - including in jurisdictions with landfill bans - appears relatively limited compared to offsite recycling capacity, with grinders and biodigesters being the top choice of commercial generators. By using these systems, the generator disposes of food waste through sewer pipes, instead of by truck. This approach raises various questions. Are the products safe for the pipelines? Does the local treatment plant have the capacity to treat them effectively? Since treatment costs have been transferred from the generator to the municipal ratepayer, what is the true cost of treatment? And finally, only 1 out of 3 wastewater treatment plants recycle organic nutrients in their sludge through AD: the resulting energy product is typically recycled, but are the biosolids applied to land as a soil conditioner, or – as is often the case – do they end up in a landfill?
Currently cities and states vary in their regulatory approaches to the various on-site processing systems. Many municipalities have long-standing bans on commercial garbage disposals that grind and then dispose of waste down the sewer to avoid known risks of sewer clogs and pipe damage from corrosive gases created by mixing human and food waste. For the newer systems, however, Bridget Anderson, Deputy Commissioner of NYC Department of Sanitation, notes that regulators are hampered by the lack of information about the environmental impacts of their biosolid and liquid outputs from independent studies of installed systems.
Biodigesters offer an alternative approach to grinders, but present challenges of their own. One study found that biodigester liquids were “stronger than sewage”, with high levels of nitrates, phosphates, and pathogen indicators that can be costly, and potentially unsafe, to treat. Yet due to the relatively small number of biodigesters installed in the United States to date, few wastewater and plumbing authorities have revised guidelines or regulations to address these installations. At this time, cities and states vary as to whether sending liquefied food down the drain complies with their landfill ban: in Connecticut, it does not comply, whereas in Massachusetts and NYC, if the wastewater utility approves sewer disposal, then it does comply.
Dehydrators may help generators sidestep this problem, since a large portion of the organic material is removed from the waste fluid sent down the drain, in order to produce a biosolid. As a result, the liquid effluent generally poses less stress on wastewater treatment plants, though it nonetheless may contain measurable levels of BOD. However, generators face the issue of what to do with the dehydrator biosolids. Though some vendors have claimed the biosolid can be land applied as compost, it has not undergone the biological process of composting to reduce pathogens and decompose the organic material into a stable substance. Nor has it been tested, as is generally required of commercial compost prior to land application. For example, California regulations identify it as a solid waste, as distinct from compost, that may be suitable as a feedstock for composting, or potentially as an ingredient for animal feed or fertilizer.
An appealing variant among liquefying systems is turnkey foodwaste management systems that explicitly incorporate hauling of the food waste slurries for processing to create recyclable products. The Seattle-based WisErg system produces fertilizers that are certified for organic use; Emerson’s Grind2Energy system transports the slurry to nearby anaerobic digesters, which produce energy and biosolid products that – depending upon the type of digestion system – may be used as soil amendments either directly, or with additional composting. A recent study has shown that between the two options for transporting food waste to anaerobic digesters, hauling by truck yields higher biogas production and lower greenhouse gas emissions relative to sending it through the sewer system.
The bottom line is that, when choosing a food waste management strategy, buyers need to look beyond the onsite systems to understand if the food waste is ultimately recycled. Generator options for managing dehydrator biosolids are: 1) to arrange for further processing to enable land application; or 2) to forego further processing. In the latter case, generators can either apply the biosolids directly to the land , which would risk harmful effects and violate California regulations, or dispose of them through the solid waste system to a landfill or incinerator, which is at the bottom of the EPA food waste hierarchy. Meanwhile liquefied food waste sent down the drain for treatment at a wastewater facility may end up in a landfill – again, at the bottom of the food waste hierarchy - unless the wastewater treatment plant recycles its biosolids for land application through AD and, potentially, further composting.
Chris and Carol work on the Food Waste Initiative at ELI. Other blogs in the ELI Food Waste series can be found here.