Decentralized organics to energy with containerized anaerobic digestion

By John D. Forcier, P.E., Lee Enterprises Consulting, Inc.

Special to The Digest

Most trash and recycled waste materials are being processed on a regional basis to maximize the economy of scale. There have been various attempts to gather and process organic waste materials utilizing similar methods, but the challenges are far from resolved. As we transition to the bans on organic materials from landfills and even more organic materials will need to be processed, other organics reuse solutions will need to be deployed.

Organics Bans and Waste Recycling Laws

There are several States (including Vermont, Massachusetts, Connecticut, Rhode Island and California) and some municipalities (including New York City) that have passed new organic waste bans and recycling laws. Generally, they ban most or all organics (other than wastewater biosolids in most cases) from landfills by the years 2018 to 2020. There are intermediate deadlines that are based on thresholds (with less and less tons per year per producer) that are already in place. There are other intermediate deadlines until the final effective dates. In some States, there are exemptions, based on the distance to organics processing facilities, while in some cases there are no distance exemptions.

Current Organics Reuse Solutions

The two most popular beneficial reuse solutions for organic waste are:

• Composting- To date, most of the reused organic waste has been brought to compost facilities. However, there are significant amounts of land that would be needed for most composting solutions. Also, some of the organic waste products are not well suited for compost facilities. Most importantly, most compost facilities give off significant amounts of greenhouse gases in the form of CO₂.
• Anaerobic Digestion- Many of the current anaerobic digester systems are not well suited for mixed substrates and can’t take full advantage of the energy from the organic feedstocks. Of those that are well suited for organic feedstocks, most of them are larger regional AD systems for economy of scale.

Organics Collection and Trucking

With either composting or anaerobic digestion, most of the current organic reuse solutions have been developed on a regional basis. Since there are many small producers of organic waste spread across each region, substantial trucking is required, resulting in major trucking expenses and in significant greenhouse gases being produced. Also, since there are few organics collection/separation solutions in place, it is difficult to develop regional solutions. With the newer organics bans, there are even more organics with many smaller producers across the region, making regional organics solutions even more difficult to implement.

Anaerobic Digesters- Economy of Scale

For typical agricultural and commercial AD systems, many of them are in the 500 kW to 1 MW range, with the smallest economical systems at 100 kW capacity. Now, with newer containerized AD systems, they can scale down as low as 10 kW for economical decentralized solutions. That size will handle as little as 0.5 tons/day (180 tons/year) of food waste or as few as 50 milking cows, with other standard sizes up to 3 tons/day (1,100 tons/year) of food waste.

Containerized Anaerobic Digester Solutions/Applications

Containerized anaerobic digesters can provide an economical decentralized solution that can take the local facility’s food waste and other organics and convert them to provide most or all of their electrical energy and thermal energy (hot water heating, etc.) needs. By providing a local, decentralized solution, the cost of transportation, diesel consumption and carbon emissions are all reduced. There are many great applications at 10 kW – 100 kW for these containerized AD systems including: colleges, hospitals, hotels, grocery stores, small to medium farms, food processing facilities, etc. A simple dewatering/nutrient capture system can also be provided to produce valuable organic fertilizers for local use and/or sale.

Financial Stability

This provides a base-line industry for the next generation. There is typically a 3 to 5 year payback on this base-load renewable energy system. Typical revenues and savings can include: tax incentives, fixed energy rates from feed-in-tariffs, renewable energy certificates/carbon credits, tipping fees from feedstocks, energy savings/energy revenue from captured heat energy and revenue from sales of organic fertilizers. Also, because the vast majority of this AD system is portable, most of it can be depreciated typically over a 7 year period. For smaller decentralized systems, most or all of the local electrical and thermal energy can be provided to the facility without connecting to the grid. This saves on substantial grid connection fees and minimizes permitting time and costs. For farms, the savings can also include significant savings on animal bedding costs and major savings from reduction in the purchase of chemical fertilizers.

Typical Feedstocks

Some typical feedstocks (substrates) for containerized anaerobic digesters can include animal manures, pre-consumer and post-consumer food waste, food processing waste, certain farm crops, fats, oils and grease (FOG), source-separated-organics (SSO) and other organic byproducts.

Decentralized Organics to Energy Solutions

In general, containerized anaerobic digesters can provide economical decentralized solutions for organic waste materials by providing baseload renewable energy, reducing the carbon footprint, providing organic fertilizer, lowering costs, reducing odors and helping the environment.