Breaking Down Anaerobic Digestion

Adoption of anaerobic digestion (AD),  the process of breaking down organic waste with bacteria without oxygen, has been picking up speed in the U.S. over the past few years as a way to divert waste and create energy. Typically, here are two types of AD—wet and dry, with the wet process primarily used in wastewater treatment plants and food and yard waste being better suited for the dry option.

But there are other factors to take into consideration when determining which AD approach to use, according to Jeffrey Draper, senior vice president of Houston-based Zero Waste Energy LLC (ZWE). There are approaches for differing feedstocks based on the percentage of organic content. This is largely related to whether the area has source-separated organics or whether the organics have been separated out from the waste stream at the processing end.

Additionally, economics and considerations for the highest and best use of the biogas for each specific project will influence how to approach the biogas upgrade—is it being converted into electricity or creating compressed natural gas (CNG)?

Another consideration is the intended use of the digestate. Will it be finished into agriculture quality compost or used onsite for landscaping? These elements together determine the best approach to take for each project.

Because ZWE is a development company that designs, builds and operates integrated solid waste processing facilities to optimize waste diversion and the generation of renewable energy, Waste360 sat down with Draper to discuss his company’s use of these different approaches and the overall benefits and challenges of anaerobic digestion.

Waste360: What determines which approach to anaerobic digestion is utilized for a project?

Jeffrey Draper: Available—feedstockboth quantity and composition—is the primary factor in determining how to approach a project.

Waste360: What are the biggest challenges of anaerobic digestion?

Jeffrey Draper: (The biggest challenge is) confusion in the market about the different approaches and their applications. There are not that many reference facilities for people to see and understand how the process would work for them.

Waste360: How does Zero Waste Energy manage those challenges?

Jeffrey Draper: ZWE now has three facilities operating in the U.S. at different sizes and different types of feedstock. We prefer to educate customers with real examples and show them how it would work for them. We can then engineer a system around their specifics.

Waste360: What are the benefits of anaerobic digestion?

Jeffrey Draper: Dry anaerobic digestion with ZWE’s SMARTFERM technology can provide a fully closed loop system. Its only byproducts are methane, which can be converted into electricity or CNG, and digestate, which can be finished into compost. There is little water input and no sludge or other waste output. Dry anaerobic digestion can help organizations achieve diversion and environmental goals. In fact, CNG generated from dry anaerobic digestion is carbon negative.

Waste360: What are some of the latest projects Zero Waste Energy has launched or is working on?

Jeffrey Draper: We have started on the Napa, Calif., project, which will convert organic waste into renewable natural gas for the waste and recycling collection fleet in the city. Other projects are in the planning phase and will be announced as they progress into construction.

The City of Napa and Napa Recycling and Waste Services LLC are working with ZWE to develop an AD-to-biofuel project that will help the city of Napa reach 75 percent total landfill diversion and increase organic waste recovery from food services businesses by 2020.

The commercial food program will expand to service more than 200 businesses up to five times a week. Approximately 25,000 tons per year of yard and food waste will be processed via the SMARTFERM facility to generate the equivalent of 330,000 diesel gallons (DGE) to fuel the waste collection fleet.

The project began preliminary construction efforts this year with the plant expected to be fully completed in 2017.