Delivering waste by truck or through existing sink disposal pipes to a municipal water resource recovery facility (WRRF), where it is treated with anaerobic digestion; the remaining biosolids can be applied to land for beneficial reuse
There are over 1,200 AD facilities installed at WRRFs today, typically in the 20% of WRRF facilities in large MSAs that treat 80% of U.S. wastewater. Electricity costs are typically the largest operational expense of WRRFs, and the recent trend of accepting municipal food scraps is a way to boost gas production of existing AD facilities.60 Expansion of existing facilities is the most cost-effective option, but some WRRFs may build new AD facilities designed from the start to digest both food scraps and municipal waste sent down the drain. Today, only 55% of WRRF ADs recover the biosolids for beneficial reuse versus landfilling. Given that WRRFs are generally operated by the municipality, most WRRF AD projects are publicly financed and operated and therefore developed based on the net public benefit.
Food scraps can go to WRRFs in one of two ways: by truck or down the drain by pipe. Many factors specific to local communities and infrastructure will influence the benefits and costs of each delivery method. The Roadmap modeled the expansion of WRRF AD systems using assumptions of a drain-and-pipe-based system, which will eliminate collection trucks and routes. However, there are advantages to consider with truck-based collection and delivery. Truck-based collection systems help avert unintended impacts of food scraps to pipes, such as blockages, and eliminate the high energy demands and costs associated with primary treatment at the WRRF. Once delivered, food may be injected directly into a digester at the WRRF, preferably into a dedicated unit which can keep material separate from sewage and maximize the end market material value. Some research indicates the greatest environmental value may come from truck delivery.
The case for drain disposal focuses on in-sink grinders (ISGs) in larger urban areas equipped with modern water treatment plants and in sewer systems with capacity to handle extra waste. ISGs have been utilized in commercial settings, and the Roadmap incorporates the potential to expand their use by residential users — also known as garbage disposals or by the brand name InSinkErator — under the right conditions. Proponents of drain disposal promote its convenience, which can increase participation rates, reduce the need to purchase and operate a truck fleet, and eliminate storage odors. For it to be economically attractive to send food scraps down the drain, WRRFs must use primary treatment to remove a high fraction of carbon, the most energy-intensive component of wastewater treatment. Otherwise, the high energy cost to treat the additional organic content in the waste can run into the hundreds of dollars per ton treated, outweighing any gains from the AD. In addition, a drain-based approach will only be effective at large-scale WRRFs in big cities that utilize advanced energy-efficient processes to further reduce the cost of treating the organic material.
A series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen resulting in two end products: biogas and digestate. There are many different AD technologies, including wet and dry versions, the latter being generally better suited for food waste mixed with yard waste.
Composting at small-scale at institutions or businesses with heat and mechanical power to compost relatively quickly (less than one month versus more than two months for windrow composting)
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