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    Water Resources Recovery Facility (WRRF) with AD

    Diversion Potential:
    1,637K Tons

    Economic Value Per Ton:
    $23

    GHGs Reduced:
    728K Tons

    Jobs Created:
    100 Jobs

    Definition

    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

    Overview

    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.

    Challenges

    • Industry concerns exist regarding the conditions under which drain disposal can be a cost-effective and scalable solution. The following concerns reflect the potential for many MSAs to consider a truck-based transportation system for incorporating additional food scraps at a WRRF AD facility
    • Cities vary in ISG policy due to pipe concerns or questions about whether WRRF facilities can handle the material load. Some cities prohibit ISGs in commercial facilities but allow them in restaurants, institutions, and homes. Detroit, for instance, requires commercial ISGs while New York City prohibits ISGs for commercial establishments.
    • Commercial businesses often require grease traps to prevent fats and oils from causing blockages and odor issues within sewer pipes. It is unclear if this is feasible in homes.
    • Benefits of ISGs vary geographically. Colder climates with steeper pipe gradients have sewers that move wastewater to the WRRF at a higher velocity and experience fewer fugitive GHG emissions in the pipe. Warm regions with flatter pipe gradients will experience a higher degree of fugitive emissions and sewer maintenance complications. Areas facing drought may also have concerns that putting additional food waste down the drain will increase net water usage.
    • ISGs are not suitable for transporting all types of household food waste to WRRFs (e.g., animal products).
    • Without proper pretreatment, additional biological loading from food waste can significantly increase the operating cost at a WRRF, from $200 to $300 per ton.
    • Regardless of how waste is transported to the facility, communities often resist the land application of biosolids produced from AD projects due to concerns around negative health effects and contamination from pharmaceuticals and industrial materials. Increasing biosolids output without addressing these concerns will result in biosolids in landfills rather than more beneficial applications such as farming.

    Stakeholder Actions

    • Today, approximately half of residential households are equipped with ISGs in kitchen sinks. Cities with high existing penetration or lower density where collection by trucking is especially expensive can be good targets for expansion.
    • WRRF plant managers need a financial incentive and end-market customer for biosolids to ensure that it is repurposed for beneficial use and not sent to the landfill. Cooperation between relevant organizational stakeholders can foster transparent, reliant markets for biosolids. For example, the National Biosolids Partnership, which is hosted by the Water Environment Federation, promotes safe and best practices for biosolids applications.
    • The capacity of existing WRRF ADs in urban areas to accept additional materials to boost gas production can be explored. Successful WRRF co-digestion projects have demonstrated the potential for this AD approach for food waste in places such as Oakland, Calif.; Tacoma, Wash.; and Milwaukee.

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