Alternate: Advancing Resource Recovery with Sustainable Food Waste Management

Food waste disposers (garbage disposals) are present in nearly 60% of US homes and are used primarily for hygiene and convenience. However, given the current trends of reducing food waste and loss, diversion of organics from landfills, and resource recovery at wastewater treatment plants, disposers are uniquely positioned to become important tools of sustainability. They assist with food waste reduction by responsibly managing the unavoidable portion of our household kitchen waste and limit the amount sent to landfills. Nine US states have recently adopted regulations to prevent food waste from entering landfills, but cities are looking for more solutions. Sending food waste instead to our wastewater treatment plants through disposers allows for recovering clean water, energy, and nutrients. Although using disposers to address environmental challenges may seem counterintuitive, much information is available to dispel common myths and misconceptions, supporting the rationale for encouraging the use of these common household appliances. According to the USEPA, 15% of all residential food waste is managed via sewers, mainly by using food waste disposers (garbage disposals). However, food waste remains the largest fraction of our municipal solid waste. As communities look for alternative solutions to manage food waste and divert organics from landfills to meet state and local mandates, governmental officials should consider existing research and literature supporting the expanded use of disposers. They provide the least expensive option for managing food waste, result in lower global warming potential than landfilling, and can help reduce overall costs for wastewater treatment plants. An evaluation in five different US cities determined that typical homes with disposers reduce food waste in garbage by 30%, and there is an opportunity to divert more with proper education and outreach. This presentation will provide much background information on existing research and available case studies. Parallel to the growing trend to divert organics from landfills, several wastewater treatment plants across North America are focusing on resource recovery the conversion of waste to clean water, energy, and fertilizer. Plants with anaerobic digesters are particularly equipped for resource recovery, with some plants even utilizing commercial and industrial food waste for codigestion feedstock to augment biogas production in their anaerobic digesters. There is also an opportunity to rely on residential food waste disposers to supply feedstock to augment biogas production. Widely accepted and present in approximately 60% of all the U.S. homes, disposers have traditionally been used for convenience and hygiene because they facilitate immediate elimination of food scraps from the kitchen. Since food waste is largely removed during primary clarification and is transferred to digesters, and produces more biogas than sewage sludge, disposer use increases biogas production. Food waste is also high in carbon relative to the nutrients phosphorus and nitrogen, and because many wastewater treatment plants are carbon deficient, it can also improve nutrient removal efficiency. Unfortunately, disposers are often overlooked and misunderstood as viable tools to promote organics diversion and resource recovery. In 2013, to quantify the energy balance and nutrient removal impacts of residential food waste disposers, Harold Leverenz and George Tchobanoglous utilized BioWin wastewater modeling for a community of 130,000 assuming 10%, 50% and 100% adoption of disposers. Three different wastewater treatment plants were modeled, including a conventional activated sludge plant with nitrification, a Modified Ludzack-Ettinger (MLE) plant, and a Bardenpho plant, all with anaerobic digestion with combined heat and power for using biogas. On a net basis, energy produced is higher than demand for sending food waste to all three treatment plants. In addition, there is a negligible increase in nutrients entering each of the plants from food waste disposers, even at 100% usage, and more importantly, there is moderate improvement in the removal of nutrients. More extensive BioWin modeling of the impacts of food waste on wastewater treatment plant was conducted under the direction of George Nakhla of Western University and later published in 2019. This research confirmed food wastes (FW) are an excellent resource for energy recovery through generation of biogas from anaerobic digestion, and as carbon for nutrient removal. This modeling also investigated the impact of food wastes on operational costs, effluent quality, and energy benefit for CAS, MLE, A2O, Bardenpho, as well as sidestream processes. Simulations were conducted at FW addition of 0, 50, and 100% penetration of food waste disposers for a 10 MGD plant. All scenarios employed chemical P removal using ferric chloride to achieve an effluent total P of 0.5 mg/L. The four processes were simulated for four different operational conditions using MWW and FW characteristics from literature. Total operational costs were calculated for aeration, biosolids disposal, and the chemicals required for dewatering, P removal, and P precipitation in anaerobic digesters (BNR processes only). Net operational costs were calculated by deducting methane energy from total operational costs for CAS and by subtracting methane energy and glycerol cost from the total operational costs for the three BNRs. Glycerol cost was estimated based on the quantity of glycerol COD needed to achieve the same effluent TN for the plants without FW as the plants with FW. Results indicate insignificant impact of food waste on effluent BOD quality. Effluent nitrogen concentrations for CAS with/without FW addition were similar, indicating no impact of FW on nitrogen removal. In contrast, with FW addition TN levels for MLE decreased by up to 8 mg/L for all four cases. A2O and Bardenpho also showed that TN decreased with FW addition by 1-2 mg/L. FW addition enhanced nitrogen removal for BNR processes. FW addition increased biosolids production for the four systems by up to 11%. Biogas generation also increased by 38% with FW 100% addition in all four cases. FW addition increased aeration demand by up to 12%-22%. Iron dosage for chemical P removal slightly increased by 3% for CAS and MLE, contrary to significant drop for A2O and Bardenpho, indicating that FW addition enhanced biological P removal with decreasing chemical dosage. Similarly, FW addition increased chemical uses for dewatering by <11% for all processes. FW addition also decreased external carbon addition for nitrogen removal. The estimated saving for external carbon based on glycerol were up to 3504 - 3822 lb/day for MLE, 355-732 lb/day (A2O), and 527-1170 lb/day (Bardenpho). FW addition increased total operational cost by up to 8-11% (CAS and MLE) and by 3%-4% (A2O) but decreased it by 3%-4% (A2O) and by 5-9% (Bardenpho). With FW supplementation, net operational costs decreased by 4%-8% (CAS), 63%-78% (MLE), 40%-46% (A2O), and 54%-62% (Bardenpho), indicating a substantial net cost saving for BNR processes. The net cost savings were because FW enhanced biogas production and nutrient removal which decreased chemical addition for P removal and external carbon for nitrogen removal. FW addition increased net total energy by 6484-6947 kWh/d (CAS), 6759-6942 kWh/d (MLE), 7153-7258 kWh/d (A2O), and 7159-7246 kWh/d (Bardenpho). FW addition decreased effluent nitrogen for BNR processes and increased biosolids and biogas production. Aeration cost increased with FW addition while chemical cost for P removal decreased for A2O and Bardenpho processes. FW addition decreased net operational costs and net total total energy. The impact of FW positively increased with higher solids removal in primary clarification. Based on this modeling, and the trends to divert organics from landfills and improve resource recovery, municipal officials should consider the potential role of residential food waste disposers.