The compostable organics fraction makes up a significant portion of the weight in municipal solid waste (MSW), frequently 20 to 30 percent and sometimes more. The options for beneficial reuse of this portion are limited, however this material can be used as a feedstock for anaerobic digestion (AD), which will convert the organic materials to biogas, which can be used to generate electricity or be further conditioned to make renewable natural gas (RNG).
During the summer of 2015, Renewable Energy from Waste (REW) magazine and Gershman Brickner & Bratton (GBB) hosted the REW Summer School webinar series. The first in the series of webinars, “Anaerobic Digestion Fundamentals” outlined the fundamentals of this conversion technology in three sections: the biochemistry and technology of AD; the integration of the use of AD into solid waste management systems; and the current marketplace in the U.S. for AD from MSW.
What is AD?
In its strictest terms, AD is the microbial decomposition of organic matter into methane, carbon dioxide, inorganic nutrients and other organic compost in an oxygen-depleted environment and the presence of hydrogen gas. In layman’s terms, little bugs eat the organics and discharge methane and other gasses in air that has almost no oxygen. This is a natural process that takes place in such environments as wetlands, rice fields, and both within animal intestines and manure piles. There are several reactions that occur simultaneously during this process, and each is facilitated by different microorganisms that are intrinsically dependent on each other for the entire process to take place. The reaction steps are as follows:
1. Hydrolysis. This is the first step of the AD process and hydrolytic bacteria ingest the food waste (break down complex molecules) to create soluble sugars, amino acids, glycerol and long-chain carboxylic acid.
2. Fermentation and acetogenesis. These are separate steps although they may occur at the same time. Fermentative bacteria consume the sugars produced during hydrolysis to create ethanol and propionate (an acid), and in turn acetogenic bacteria consume both sugars and the ethanol and propionate to produce an acetate (acetic acid).
3. Methane generation. Methanogenic archeae (think prehistoric bacteria) consume these acetic acids and produce methane (and other) gasses.
These reactions can all occur within an area or container, with the reactions ramping up or declining depending upon the timing and other factors. This is called single-stage AD and is in general what occurs in nature. Some AD systems use what is called a multistage process, in which hydrolysis and acid forming is in a tank separate from the methane production. This is similar to the AD that occurs in the intestines of many animals.
There are two ranges of temperatures in which these reactions take place. From 70 to 104 degrees Fahrenheit, the AD reactions are called mesophilic, and for temperatures ranging from 120 to 150 F the reactions are called thermophilic. Which reaction range works best depends on the feedstock (incoming organics) as well as environmental and other factors. However, for AD systems with MSW or similar feedstocks, thermophilic temperature can increase the rate of the occurring reactions that may result in slightly higher biogas generation and a shorter retention time.
There are other factors that will greatly influence the effectiveness of each of the reactions (or completely stop them). Besides operating temperature, the pH value needs to be within a range of 6.5 to 7.5 or else some of the types of bacteria may not survive. This can be affected by the composition of the incoming feedstock and the loading rate of that feedstock (or in some cases the loading rate of bacteria onto the feedstock). For example, if too much “bacteria food” is available during the hydrolysis stage, the hydrolytic bacteria may multiply too quickly and produce too much acid, dropping the pH and killing off other bacteria, limiting the next reactions.
Another critical parameter for the production of biogas is the retention time of the organic material in the AD reaction tanks. As time progresses, more and more of the available carbon within the feedstock becomes converted to methane, and at a certain point, the overall production of high quality methane gas begins to decrease as the available carbon in the feedstock decreases. For most AD systems processing organics from MSW, a retention time of between 18 and 21 days is standard to reach the optimal methane production.
AD from MSW
Although the AD plants can process different types of organic feedstock, in this article our main focus is the organic fraction of the MSW. There are two main avenues for collecting feedstock from residential and commercial MSW producers: either separated at the source or as an output from a mixed-waste processing facility. Source separated organics (SSO) require a separate container and collection of that container, along with the active participation of the business or household to separately place the organics fraction into the appropriate container. This seems to work best at facilities or businesses that produce a high amount of clean organic food materials such as grocers, restaurants and food processing facilities.
An organics and fines fraction is almost always a byproduct of any mixed-waste processing facility (MWPF), however it is frequently treated as a residue. While not as clean as SSO, it can still be used in an AD system, although its final usage as a compost or soil amendment may be limited. Once the feedstock is collected and delivered to the AD facility the processing can be divided into three main steps:
1. Preprocessing. All of the collected feedstocks require some sort of pre-processing to be prepped for introduction to the AD processing system. The level of preprocessing depends on the type of AD system and the type(s) of feedstock delivered. Depending on the level of automation with the preprocessing, this is the step where recyclables that could be within the stream may be recovered.
2. Anaerobic digestion. This is the automated process of digesting the organic fractions of the feedstock as described previously. The main outputs from this process are biogas and digestate material.
3. Postprocessing. This is where the outputs from the AD processed are further refined (cleaned or composted) to be suitable for use as final marketable products. Biogas can be used for production of combined heat and power, or can be conditioned to a gas pipe quality; the digestate can be dewatered and further processed to a final compost product.
There are several different types of commercial AD processing systems, and each have unique requirements for the pre-processing of the incoming feedstock. The three most common types of AD systems are continuous flow, plug flow, and batch systems, with each requiring that the incoming feedstock be of a certain solids content. Each type is outlined as follows:
1. Continuous flow. These systems can handle a low solids, pumpable (wet) input that is less than 15 percent solids content. The reaction chamber(s) is continuously stirred with the material being mixed together throughout the stage it is in. Recirculated digesting liquid is added to the incoming feedstock that has generally been ground or shredded to achieve the proper consistency.
2. Plug flow. These systems require a high solids but still pumpable input of between 25 to 30 percent solids content. The material is continuously stirred in a reaction chamber but the material moves linearly through the chamber over time so that new material added to the system doesn’t mix with older material further down the length of the chamber. As with continuous flow, a percentage of recirculated digestate liquid is added to the incoming feedstock to achieve the proper solids content.
3. Batch. These systems require a high solids and stackable material generally of greater than 50 percent solids content. The feedstock is placed within a chamber with a sealing door and the digesting liquid is sprayed over the material with the overflow draining to a tank below the chambers. Frequently ground yard waste or biomass is added to the feedstock to achieve the proper stackability.
The preprocessing step for wet digestion tends to be more intensive in order to break down the organic materials into smaller, pumpable sizes and to remove other residue from the stream. Dry digesters can be fed nearly directly into the batch processes, although source-separated feedstocks may need additional dry materials to be stackable. While the organics and fines from a MWPF require large equipment to separate out this stream, the output generally contains enough other inorganics to be placed directly into the batch chamber.
The biogas produced by the AD process generally contains 55 to 60 percent range of all the gasses produced. With minor cleaning to remove sulfur, this can be directly fed to an internal combustion engine that can be used to power a generator to produce electricity. Frequently the residue heat from the engine also can be used to keep the digesting liquids at the correct operating temperature. The methane content of RNG is above 97 percent, so further processing of the AD output gasses are required to achieve this purity level.
While the microorganisms create gasses from some of the material in the feedstock, they can’t convert all of it. The other output from the AD process is a digestate material of remaining organic and inorganic materials. Wet AD processes produce a slurry that generally needs to be dewatered and dried before the material can be used as a compost or soil amendment. Dry AD tends to be a pile of partially digested food and other organics (along with any inorganics) that still need to go through the aerobic composting process
After composting, the dry digestate can be screened to separate by size and also processed by an air classification machine to separate the materials by density and even by optical units to remove plastics and other impurities. Most composts that come from source-separated feedstocks can be used as a fertilizer and soil amendments in many applications, while those that come from MWPFs tend to be relegated to roadside or other nonfood applications, or even as alternative daily cover on landfills. New technologies are continually adapting to better clean the MSW digestates to increase the areas of use for this commodity.
While AD has enjoyed extensive commercialization in Europe, a limited number of large commercial facilities are operating in the U.S. that utilize organics from MSW. The number has certainly been increasing lately due to new mandates to increase diversion and recycling. As well, biogas produced from AD is eligible for D3 and D5 Renewable Identification Numbers (RINs).
Food waste is a significant portion of the MSW composition. Achieving zero-waste or high landfill diversion needs to have a plan to deal with these organics. AD is an efficient and proven method to extract the chemical energy into a useful gas and to also create compost.
Ljupka Arsova is consultant II, and Bradley Kelley is senior project engineer, both of Gershman, Brickner & Bratton Inc. (GBB), Fairfax, Virginia.