The Co-digestion of Wastewater

The Co-digestion of Wastewater

In an effort to reduce food waste and enhance resource recovery, states and cities across the country are enacting legislation to cut down on the amount food and organic waste sent to landfills. Organic waste has significant energy potential, producing on average 3 times that of wastewater biosolids. These recent organics laws are forcing large waste generators like grocery stores and hospitals to donate or send their leftover food to composting sites, animal-feed operations, or anaerobic digestion facilities.  While these laws are restricting large waste generators, they are also opening new opportunities for wastewater treatment facilities to implement co-digestion programs at their facilities. But what’s in it for wastewater facilities to create such programs? While the payback incentives and regulatory frameworks are still in flux, as food bans become more mainstream across the country, facilities should be keeping an eye on this evolving opportunity. 

Could my facility take in food waste?     

Co-digestion is feasible for any wastewater treatment facility already equipped with anaerobic digesters that have excess capacity or those facilities considering the addition of anaerobic processes. Currently four states in New England (Vermont, Rhode Island, Massachusetts, Connecticut), California, and four major cities (New York City, Austin, Seattle, Portland (OR)) have implemented mandatory food bans or organics recycling laws, which means that facilities in those areas have a higher possibility of receiving a consistent stream of organic waste. Though most of the organics bans are concentrated in New England, historically this region has had few facilities that use anaerobic digestion. This gap was part of the reason Woodard & Curran worked with Quantum Biopower on preliminary design, permitting services and operations support to build and start-up one of the first standalone anaerobic food digestors in the country in Southington, CT. 

What’s in it for wastewater facilities?

Wastewater facilities can gain the most direct benefit from co-digestion through improved quality and quantity of digester by-products, namely biogas and biosolids. Food waste tends to have a larger methane production potential than wastewater sludge resulting in an increase in biogas output and more energy-rich biosolids. If facilities are currently harnessing or selling these byproducts, co-digestion could increase profits and/or renewable energy production.

Additionally, wastewater facilities that are in states or near cities where food bans are in place can gain extra income through tipping fees from waste generators. As food waste bans become more mainstream, additional funding sources will likely be available to facilities looking to create a co-digestion program at their facility.

What’s holding co-digestion back? 

While co-digestion of organics and wastewater solids seems like a great idea, it also comes with significant upfront costs and a high amount of uncertainty. One key concern is the quality of the incoming food waste and the impact it could have the facility and the on biosolids. Facilities located near areas affected by food bans may have access to more homogenous waste sources, but it will be nearly impossible to ensure a truly consistent source. In preparation to receive food waste and other imported organic materials, facilities may need to install a pretreatment system to make the waste more homogeneous and remove unwanted material. Currently, there are many pretreatment technologies on the market, but little comparative information on performance and cost. The Water Research Foundation has made it their goal provide utilities with information and tools to make informed decisions about pre-treatment and management practices by 2022, making this challenge more manageable in the near future.

Some of the more practical challenges surrounding co-digestion center around storage of excess food waste and minimization of odor and vector populations, which may require additional or repurposed equipment and can add additional effort for operations and maintenance for facilities to manage.

As the long-term sustainability of our existing landfills continues to decline, especially in areas of significant and growing populations, co-digestion of organics with wastewater solids may become increasingly viable from both an economic as well as environmental and societal perspective. This is a process that existing wastewater facilities and operations should consider when planning for future upgrades or capital improvement projects. 

 

Author

Director of Technical Practices
Water

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