Types of Biogas Systems Explained

Biogas is an abundant renewable resource with mostly untapped potential. It’s produced through anaerobic digestion (AD)—a process so widespread that it’s happening in your digestive tract, the local landfill, and tri-generation power plants at this very moment.

Scientists have studied biogas for decades and have gradually developed many technologies to harness and refine it for productive use. Some simply capture what’s naturally produced from existing waste infrastructure, while others take a more active approach to boosting production efficiency.

Such research and development efforts are warranted because biogas is produced with or without intervention. Left alone, biogas is pollution. Yet, if captured and refined, it can supplement or replace conventional natural gas using, boasting equal efficiency and cleaner emissions.

This article will highlight several types of biogas systems that demonstrate the versatility and power potential of renewable natural gas (RNG), including:

• Lagoon Systems
• Continuous Stirred Tank Reactors (CSTRs)
• Dry Digesters and Fermentors
• Upflow Anaerobic Sludge Blanket (UASB) Reactors

What Are Biogas Systems?

Biogas production comes in all shapes and sizes, lending to the renewable energy source’s versatility in meeting a range of needs.

While diverse, all biogas systems share two things in common:

1- They leverage anaerobic digestion, a natural process where microorganisms break down organic waste.

2- They collect the biogas and nutrient-rich digestate produced by anaerobic digestion.

Raw biogas is ready-to-use fuel but can also be refined into pipeline-grade gas or liquid.

Small biogas production systems can serve individual farms, turning manure and other organic waste into biogas fuel and fertilizer. Medium-sized models, meanwhile, can serve business districts, sourcing food waste from surrounding merchants as AD feed. Finally, larger units process wastewater from expansive industrial plants and entire cities—converting it into clean energy for use in any number of settings.

The wide range of biogas solutions creates near-limitless opportunities to cut greenhouse gas (GHG) emissions and support the transition to a net-zero future.

Types of Biogas Systems

Understanding the pros and cons of each system is essential to selecting the best model for your feedstock and energy needs.

In the broadest terms, biogas systems come in two varieties: wet and dry. These terms refer to the system’s process and not necessarily the type of waste it uses as feed. Wet and dry biogas systems both process a degree of wet and dry waste, but some models may be optimized for one or the other.

Another important term is retention time, or how long the feed materials are left to digest. Longer retention times generally require less labor and extract more energy and other valuable byproducts. Shorter periods allow for higher volume waste processing and faster biogas production at a lower energy conversion rate.

With that in mind, here are some of the most popular types of biogas systems in use today:

Lagoon Systems

These artificial pools are ideal for processing high-temperature or heavily polluted water. This process keeps pollutants out of the ecosystem while extracting biogas from the organic elements in wastewater.

There are many variations, but lagoons can be loosely categorized as covered or uncovered. Covered lagoons are enclosed by a secondary system that prevents fugitive gases from rising off the wastewater. Covered systems are a larger upfront investment but achieve higher biogas production volumes while capturing GHG emissions and odors.

Lagoon systems are most widely used in agricultural settings—especially those with high animal wastewater output—but are also found in some industrial sectors.

The main drawback of lagoons is that they are not suitable for processing large quantities of solid or dry waste. They also generally require a minimum retention time of 30 days or more, which is longer than most other systems.

Lagoon biogas systems are among the most cost-effective but require suitable space and land for a sizable trench.

Continuous Stirred Tank Reactors (CSTRs)

As the name suggests, these systems process waste materials in a tank with a mechanism that constantly stirs the contents. The stirring action processes feed materials into an even mixture, maximizing contact with the microorganisms responsible for AD and supercharging biogas production.

These systems are attractive options for their high efficiency in processing slurries of both solid and liquid wastes in a relatively short time. As enclosed systems, CSTRs also eliminate the risk of fugitive emissions when operated correctly.

CSTRs are uniquely adept at processing large amounts of waste, increasing in efficiency the more frequently they’re fed.

The main drawback of these types of biogas systems is that they’re more complex and expensive than other options. Operating and maintaining them requires more training than simpler production systems. The stirring mechanism also leads to higher energy consumption than more passive production methods.

However, CSTRs’ ability to process varied waste products in large batches makes them an ideal choice for many industries.

Dry Digestors and Fermentors

Dry biogas systems are sealed and heated chambers that extract byproducts from solid waste with minimal water use.

There are many varieties, with some providing columns of shelves to maximize airflow and capacity. Others more closely resemble a warehouse and are large enough for heavy-duty trucks to drive inside with waste deliveries. Once loaded, the feed materials are sprayed with warm water or other solutions that facilitate digestion.

These biogas systems are ideal for processing large volumes of solid waste—including plant clippings, food waste, and manure.

The ideal retention time for fermentation and dry digestion depends on the user’s goals. Some research suggests that a shorter retention time (two weeks) is optimal for biogas production, while a longer period is better for solid fertilizer production.

The main drawbacks to these types of biogas systems are their incompatibility with liquid waste and higher maintenance needs than other models.

Upflow Anaerobic Sludge Blanket (UASB) Reactors

These inventive biogas production systems force wastewater upward through a bed of granular sludge made of microorganisms. This so-called “sludge blanket” filters pollutants and thoroughly mixes waste products for maximum digestion.

UASB reactors achieve incredible efficiency with half the retention time of other systems. This method best suits high-strength wastewater with a diverse mix of organic compounds. The combination of sludge at the bottom, water in the middle, and air at the top creates a natural separation of solid, liquid, and gaseous byproducts.

These systems also show great potential for “co-digestion,” or processing complementary compounds together to achieve greater efficiency.

UASB reactors have some of the lowest energy demands and smallest environmental impacts of any biogas production system. They are also highly efficient, boasting one of the best energy conversion rates.

However, they aren’t without drawbacks. UASB reactors require a high initial investment, frequent monitoring, regular maintenance, and trained operators. They also typically function within multi-stage systems, offering less convenience than all-in-one digestion and storage units.

Though costly, few systems can process such vast quantities of wastewater within such a compact space. For these reasons, UASB systems are widely used for industrial and municipal wastewater solutions.

Benefits of Biogas Systems

Biogas is often discussed as a “fuel of the future,” and for good reason. Rerouting waste to systems that transform it into 100% renewable natural gas (RNG) is a scientific marvel.

Replacing or supplementing the consumption of natural gas (a fossil fuel) with biogas will play a leading role in the shift to carbon-neutral fuel sources. Biomethane (the refined form of biogas) can be used in all the same ways as conventional natural gas while burning significantly cleaner.

Harnessing biogas also provides the natural benefit of keeping pollutants out of the air, water, and environment. Microorganisms produce biogas with or without a system in place, making landfills and wastewater major contributors to GHG emissions. Diverting this harmful waste to state-of-the-art facilities protects the local community and ecosystem from exposure.

In short, biogas systems turn hazardous and polluting liabilities into energy assets.

Challenges in Biogas System Implementation

Though biogas touts many benefits, there are barriers to implementation worth noting.

As with most technological shifts, implementing a new system requires an initial investment of time, energy, labor, and money. Biogas systems also consume energy. While some have equipment to channel biogas energy back into production, others do not.

Eventually, the system will produce more energy than its implementation and operation consume, assuming it has a regular feed supply. Smaller farms and facilities may not produce enough waste for a biogas system to be worth the investment, but longer retention times may mitigate this shortcoming.

Despite these challenges, biogas infrastructure is gradually spreading and gaining wider global attention. Biogas systems’ dependence on biodegradable waste means it won’t replace entire power grids, but experts are optimistic that it can minimize our use of gases derived from fossil fuels.

Build a Sustainable Future with Biogas Systems

Now more than ever, leaders are making sustainability central to their growth and development goals. What better way to contribute to the push for renewable energy than transitioning to systems that transform trash into energy?

Partner with energy experts at FASTECH—the leader in renewable energy infrastructure EPC+M—to develop a custom plan for implementing biogas solutions.