What are the three types of anaerobic digesters?

Types of Anaerobic Digesters

Types of Anaerobic Digesters

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All anaerobic digesters are designed to perform the same basic task &#; break down organic matter through anaerobic digestion to produce biogas, a mixture of gases created when waste decomposes, which serves as a renewable energy source. However, different types of digesters achieve that objective in different ways. Ultimately, digesters can feature subtle differences in construction and material handling techniques that affect the way they function.

The sections below will go over the distinguishing characteristics of anaerobic digesters, some specific types of anaerobic digesters and their common uses. Keep reading to learn more about the kinds of anaerobic digesters available and how to maintain them for optimal performance.

Common Differences Among Anaerobic Digesters

These systems are constructed for numerous reasons and are distinct in many different ways. This section describes some of the main differences among types of anaerobic digesters. The main differences include temperature, feedstock variation, flow and moisture content.

Operating Temperature

An anaerobic digester is built to run at particular target temperature ranges because different types of anaerobic microbes thrive in different temperature zones. Typically, the temperature range for mesophilic anaerobic digestion is between 86 and 100 degrees Fahrenheit, while the range for thermophilic anaerobic digestion is between 122 and 140 degrees Fahrenheit.

Usually, thermophilic anaerobic digestion is necessary for greater pathogen kill. The higher temperature range produces Class A Biosolids, which can be legally used as fertilizer on farms or commercial vegetable gardens and sold for use as compost or fertilizer for home gardens. While thermophilic digesters take less time to process feedstocks, they often come with higher costs and may be challenging to operate.

On the other hand, mesophilic anaerobic digesters are generally easier to operate and maintain. However, a mesophilic digester does not produce sufficient pathogen kill to generate Class A Biosolids.

Feedstock Variation

Different anaerobic digesters are designed to process various types of feedstock. Some digesters are even designed to process multiple feedstocks, which is known as co-digestion. Some feedstocks may also need or benefit from pre-processing measures before digestion, such as screening, blending or thermal conditioning.

Batch or Continuous Flow

For batch digesters, every feedstock is loaded at one time. After loading, digestion occurs within a set period of time before the digester gets emptied and reloaded manually. For a continuous flow digester, the feedstocks are constantly fed into the digester, and the digested material gets continuously removed after digestion.

Wet or Dry

The feedstocks' moisture content classifies it as wet or dry. Known as a low solids anaerobic digestion system, wet digesters are more common than dry and usually process feedstock with less than 15% solids content. These types of feedstocks are most often in slurry form and able to be pumped.

A dry digester, also referred to as a high solids anaerobic digestion system, typically processes feedstock with a solids content higher than 15%. These kinds of feedstocks are commonly described as stackable.

What Types of Anaerobic Digesters Exist?

Now that you know how anaerobic digesters can vary, it's time to discuss the specific types of anaerobic digesters available. In general, there are three main types of anaerobic digesters &#; passive systems, low rate systems and high rate systems. Within these broad categories, you'll find different types of anaerobic digesters useful for different purposes.

Below, you'll learn more about the variations of passive systems, low rate systems and high rate systems.

1. Passive Systems

These systems refer to when biogas recovery is added to an existing treatment component. The most common digester in a passive system is a covered lagoon. This system capitalizes on the low-maintenance nature of a lagoon to help capture biogas under an impermeable cover. Most covered lagoons follow a two-cell system, enabling the lagoon to serve as a storage space and a treatment system.

In a two-cell covered lagoon, the first cell is covered, while the second cell is left uncovered. The level of liquid in the first cell remains steady to promote manure breakdown and the liquid level in the second cell fluctuates to create storage. Because covered lagoons are not heated, temperatures follow seasonal patterns.

It's possible to store sludge in covered lagoons for up to 20 years, meaning methane-forming microorganisms can also stay in covered lagoons for up to 20 years. In addition, much of the fertilizer nutrients, such as phosphorus, can also get trapped in a covered lagoon for decades. This tendency to retain substances makes regular cleanings critical for covered lagoons.

Only professionals can perform a covered lagoon cleaning. Ideally, anaerobic digester cleaning services can clean the storage portion of the covered lagoon without removing its contents, halting production or endangering anyone's safety. Bristola has developed an innovative solution by using sonar robotic vacuums to clean the covered lagoon without shutting down the system.

2. Low Rate Systems

An anaerobic digester qualifies as part of a low rate system when the manure flowing through the digester serves as the main source of methane-forming microorganisms. These low rate systems can feature different types of anaerobic digesters, which are discussed in greater detail below.

Complete Mix Digesters

Essentially, a complete mix digester is a tank that heats manure and mixes it with an active mass of microorganisms. Any incoming liquid displaces the digester's current volume, allowing an equal amount of liquid to flow out of the digester. Along with the displaced liquid, methane-forming microorganisms flow out of the digester.

Here, liquids remain in the complete mix digester for about 20-30 days, which maintains biogas production effectively. However, retention times may be shorter for thermophilic systems. Throughout the retention period, the digester can be either continuously or intermittently mixed, meaning the tank gets stirred during feeding and occasionally in between feeding times.

In some cases, the process occurs in more than one tank. For example, acid formers sometimes break down manure in one tank, followed by methane formers converting organic acids to biogas in another tank. Generally, complete mix digesters operate best when manure contains 3-6% solids. As lower solids levels correlate with greater volume, a larger digester must retain the microbes longer.

Plug Flow Digesters

The concept of a plug flow digester is similar to a complete mix digester in that manure flows into the digester and displaces the digester volume, causing an equal amount of material to flow out of the digester. However, the manure within a plug flow digester includes contents thick enough to prevent particles from settling at the bottom.

Minimal mixing occurs with a plug flow digester, making the manure move through the digester as a plug. These digesters are best for manure with solids as high as 20%, meaning extra material may need to be added to manure designated for a plug flow digester. An increase in biodegradable material results in more biogas.

The recommended retention time for plug flow digesters is about 15-20 days. Like all anaerobic digesters, plug flow digesters should also be cleaned routinely to keep sediment from building up and interfering with the quality of the liquids. Bristola's liquid tank cleaning services provide a feasible way for organizations to have their digesters cleaned without halting operations.

3. High Rate Systems

Alternatively, high rate systems confine methane-forming microorganisms within the anaerobic digester to boost efficiency. More specifically, the following anaerobic digesters are commonly associated with high rate systems.

Anaerobic Contact Digesters

Known as contact stabilization digesters, anaerobic contact digester systems have to do with solids recycling. The digestion time can be decreased by returning some of the active organisms to the anaerobic digester. A plug flow system can achieve that advantage by pumping some of the effluent toward the front of the digester. On the other hand, complete mix digesters involve solids settling in an external clarifier before the microbe-rich slurry gets recycled back into the digester.

Fixed Film Digesters

At its most basic, a fixed film digester is a column filled with media, such as small rings of plastic or wood chips, that methane-forming microorganisms grow on. In this case, manure liquids pass through this media, coating the media in a slimy growth known as biofilm. This type of digester is also called an anaerobic filter or attached growth digester.

The retention time for fixed film digesters is relatively short &#; sometimes less than five days. However, manure solids can occasionally plug the media, requiring professional attention before returning to its usual processing. For this reason, this type of system needs regular anaerobic digester cleaning to remain efficient and effective.

Suspended Media Digesters

This type of digester involves suspending microbes in a continuous upward flow of liquid. The flow can be modified to let smaller particles wash out while keeping larger particles within the digester. Then, microorganisms create biofilms around the large particles and methane formers remain in the digester.

Certain suspended media digester designs include an artificial media like sand for microbes to produce more biofilm. These are referred to as fluidized bed digesters. Along with incorporating fluidized bed digesters, effluent is sometimes recycled as a way to maintain a steady upward flow.Sequencing Batch Reactor Digester.

Sequencing Batch Reactor Digester

This type of anaerobic digester is a specific version of an intermittently mixed digester that's well-suited for extremely dilute manures. In an anaerobic sequencing batch reactor (ASBR) digester, methane-forming microorganisms get stored via settling solids and decanting liquid. This process includes four phases.

During the fill stage, the ASBR digester is fed. If filled with enough active microbes, an ASBR digester may produce biogas with completely soluble organic liquids. Next, the microbes and manure get mixed together in the react phase. This phase is quickly followed by the settle stage, which involves the solids being settled. Finally, effluent is drawn off during the decant stage. The ASBR digester may repeat the cycle up to four times per day, resulting in almost constant gas production.

The liquid retention time for an ASBR digester may be as short as five days. This quick turnaround calls for frequent liquid tank cleaning to remove sludge and enable the ASBR digester to operate at its full capacity. The zero-human entry Submersive Robotic Cleaning System from Bristola cleans the liquid storage facility without interfering with the digester's everyday production.

How Different Types of Anaerobic Digesters Are Used

The following sections discuss anaerobic digesters commonly used for farms, water resource recovery facilities and manufacturing plants.

Anaerobic Digesters on Farms

An on-farm digester can add considerable value to farms and their surrounding communities. These digesters can aid farmers in managing nutrients, reducing odors and generating extra revenue. Most often, dairy, poultry and swine farms include an anaerobic digester of some sort. In addition to animal waste, on-farm digesters can accept outside food waste as feedstock.

The most common on-farm digesters include:

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• Covered lagoons

• Plug flow

• Complete mix

The type of anaerobic digester found on a farm is usually determined by the farm's manure management practices and the kind of animal waste available to feed into the digester. Typically, the biogas produced from the anaerobic digester systems is used for generating electricity, providing combined heat and power or fueling furnaces or boilers.

Anaerobic Digesters at Water Resource Recovery Facilities

A water resource recovery facility serves to produce clean water, recover nutrients and reduce the community's dependence on fossil fuel by generating renewable energy. These facilities rely on anaerobic digesters to assist in their operations. Primarily, digesters treat wastewater solids, making them subject to EPA biosolids regulations.

To comply with regulations and fit with the facility, digesters at water resource recovery facilities can vary in the following ways:

• Size

• Shape

• Processing rate

• Number of stages to the process

• Operating temperature

• Extent of pre-digestion processing

• Types of mixing strategies

Depending on a specific water resource recovery facility's needs, it could require any type of anaerobic digester. Currently, more than 1,200 water resource recovery facilities in the country use anaerobic digesters to produce biogas by treating wastewater solids. Many of them use the biogas as an energy resource for generating electricity or heat, which can be used for operations at the facility, sold to the grid or injected into natural gas pipelines.

Anaerobic Digesters at Manufacturing Plants

Known as stand-alone digesters, anaerobic digesters made for manufacturing plants or processing plants accept feedstocks from one or more sources for a tipping fee. These types of digesters can process industry-specific wastes, manage residential food waste or operate as community-based operations or organics recycling businesses.

Frequently, stand-alone digesters process food waste as their feedstock. However, some stand-alone digesters are designed to co-digest other organic materials like manure, yard waste and wastewater solids. Many anaerobic stand-alone anaerobic digesters are high rate systems to assist with co-digestion and increase methane production from difficult-to-digest or low-yielding materials.

Keep Anaerobic Digesters Clean with Bristola

Any type of operation with an anaerobic digester needs to have its digester cleaned routinely. Regardless of what type of anaerobic digester is on the grounds, regular anaerobic digester cleaning is imperative. Bristola offers professional digester cleaning services to ensure digesters are clean, well-maintained and able to perform at their full capacity.

Bristola services anaerobic digesters with a cutting-edge, zero-human entry Submersive Robotic Cleaning System that uses sonar robotic vacuums to clean tanks without removing their contents, halting production or risking human life. Bristola's unique manhole cover technique replaces the tank's existing cover to allow the robotic vacuums to enter without removing the cover.

Choosing the Bristola Submersive Robotic Cleaning System enables you to clean your anaerobic digester without shutting down your facility. In this way, our services are hassle-free and cost-effective. Additionally, our full-service system will collect data about your tank to evaluate its condition and performance.

We also offer a unique entry portal that is placed on the manhole of tanks. This valve will allow us to easily re-enter and service your tank in the future. 

Contact Bristola today to learn more about our Submersive Robotic Cleaning System or schedule a demo.

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All anaerobic digestion systems adhere to the same basic principles whether the feedstock is food waste, animal manure or wastewater sludge. Although the process is the same, designs will vary from digester to digester. Design variations are described below followed by descriptions of digester categories. The main categories of digesters are stand-alone digesters, on-farm digesters, and digesters at wastewater treatment plants. 

Common Ways to Describe Digesters

Digester systems are built for many different reasons and there are many ways to describe them. The most common distinctions include:

Operating Temperature

Digesters are designed to run at different target temperature ranges. The temperature ranges are typically 86 - 100&#; F for mesophillic and 122 - 140&#; F for thermophilic. There are different populations of anaerobic microbes that thrive in these temperature zones.

Generally, thermophilic anaerobic digestion would be used when greater pathogen kill is necessary. This temperature range can produce "Class A Biosolids". Class A Biosolids is a designation for dewatered and heated sewage sludge that meets U.S. EPA guidelines for land application with no restrictions. Thus, Class A Biosolids can be legally used as fertilizer on farms, vegetable gardens, and can be sold to home gardeners as compost or fertilizer. Thermophilic digesters require less time to process feedstocks but may have higher costs and be more difficult to operate.

In general, mesophillic digesters are easier to operate and maintain, but will not result in sufficient pathogen kill to produce Class A Biosolids.

Learn more about biosolids.

Feedstock Variation

Some digesters are designed to process one type of feedstock and other digesters are designed to process multiple feedstocks. Co-digestion is often a driver for the development of anaerobic digesters. Many feedstocks require or can benefit from pre-processing before digestion (e.g., blending, screening, thermal conditioning, etc.).

Wet (low-solids) and Dry (high-solids)

Wet and dry classifications of digesters refer to the moisture content of the feedstocks. Wet digesters are more common that dry. A wet digester or low solids AD system generally processes feedstock with less than 15 percent solids content. The feedstocks for a wet digester are typically in slurry form and can be pumped.

A dry digester or a high solids AD system generally processes feedstock with greater than 15 percent solids content. The feedstocks for a dry digester are often described as stackable. 

Batch versus Continuous Flow

In a batch digester, feedstocks are loaded into the digester all at once. Following loading there is a set period of time for digestion to occur. Following this time period, the digester is manually emptied and reloaded.

In a continuous flow digester, feedstocks are constantly fed into the digester and digested material is continuously removed.

Stand-Alone Digesters

Most stand-alone digesters accept and process feedstocks from one or more sources for a tipping fee. Stand-alone digesters can be operated as organics recycling businesses, community-based operations or built by a municipality to manage residential food waste.

Other stand-alone digesters are built to process industry specific wastes. This is common in the food and beverage industry. These digesters are co-located at processing plants and are designed to process a certain kind of material. These units typically do not accept other feedstocks from offsite sources. 

The primary feedstock processed in stand-alone digesters is food waste. However, digesters built to process food waste can also co-digest other organic materials, such as yard waste, manures and wastewater solids.

Demand for stand-alone digesters is increasing to address the increase of diversion of food waste from landfills. 

On-Farm Digesters

On-farm digesters can bring great value to farmers

Photo of a dairy farm with anaerobic digestion system tanks in the background and cows in the foreground. 

and the surrounding community. Digesters can help farmers manage nutrients, reduce odors, and generate additional farm revenue. Dairy, swine and poultry are the primary animal types for farms with digesters. On-farm digesters can also accept outside food waste as a feedstock. 

The most common digester system technologies on farms in the United States are: plug flow, complete mix, and covered lagoons.

Learn more about on-farm digesters and how value can be recovered from waste (manure).

The type of digester used on farms is typically determined by the manure management practices in place and type of animal manure that is fed into the digester. Biogas collected from the anaerobic digester systems is often used to generate electricity, to fuel boilers or furnaces, or to provide combined heat and power.

As of January there were 248 operating digesters on livestock farms in the United States. This data is tracked by EPA&#;s AgSTAR program. Learn more about AgSTAR Data and Trends.

The profiles listed below represent real-world examples of projects that demonstrate the benefits of using anaerobic digestion systems for manure management. In addition, accepting organic feedstocks from offsite sources can generate revenue from tipping fees and boost biogas production which can be used as energy for the farm or sold. EPA's AgSTAR program promotes the use of anaerobic digestion on farms in the United States. According to AgSTAR, there are 242 farms with active digesters.

Digesters at Water Resource Recovery Facilities

Plant operators, industry associations, research institutions,

Aerial photo of the two anaerobic digesters at the Central Marin Sanitation Agency treatment plant in San Rafael, California.

and state and federal agencies are now using the term &#;Water Resource Recovery Facility (WRRF)&#; to refer to Wastewater Treatment Facilities (also known as Publicly Owned Treatment Works or POTWs). The term &#;Water Resources Recovery Facility&#; acknowledges their ability to produce clean water, recover nutrients (such as phosphorus and nitrogen), and reduce our dependence on fossil fuel through the production and use of renewable energy.

The use of anaerobic digestion at WRRFs in the United States dates back to the early &#;s. Over the last century, there have been both advancements and setbacks in the development of the technology. Anaerobic digestion is both a biological process and an engineered system that requires expertise in both disciplines for success.

The primary purpose of anaerobic digesters at WRRFs is to treat wastewater solids. As a result, these digesters are subject to EPA biosolids regulations (Title 40 of the Code of Federal Regulations, Part 503). WRRF digesters vary in a number of ways including:

• Size and shape.
• Processing rate.
• Number of stages to the process.
• Operating temperature.
• Extent of pre-digestion processing.
• Types of mixing strategies. 

In the United States, over 1,200 WRRFs have anaerobic digesters that treat wastewater solids and produce biogas. While a number of these WRRFs flare-off the biogas produced in this process, more than half use the biogas they produce as an energy resource for producing electricity or usable heat. Of the facilities using their biogas for energy, about one third are generating electricity that is used for operations at the facility. Of the WRRFs generating electricity from biogas, almost 10 percent sell this electricity to the grid. About two percent of the WRRFs with digesters process the biogas into a form that is pure enough to inject into natural gas pipelines.  

These uses of biogas generated at WRRFs are documented in a database maintained by the Water Environment Federation (WEF). Access to WEF Database.