How Does an Anaerobic Digester Work?

A digester is a tank, whose size and appearance vary considerably depending on what it is used for.

A household digester for a single family can have a volume of less than one cubic meter, whereas an industrial-scale unit can easily surpass 5,000 cubic meters. A digester on a farm or a ranch typically has a capacity of a few dozen cubic meters.

Plastic Fusion explores the inner workings of such technologies, providing detailed insights into how anaerobic digesters function and their role in sustainable waste management. So, how does it work, you ask? Well, you’re in the right place to find out.

What is an Anaerobic Digestion?

Anaerobic digestion is a process in which bacteria break down organic matter such as food waste, without oxygen. As the bacteria consume the food waste, they give off biogas which rises to the top of the digester. This consists mostly of methane, the primary component of natural gas.

The solid content of the food waste falls to the bottom of the digestion chamber and this is extracted to make a nutrient-rich organic fertiliser.

In simple terms, the system is much like a huge stomach and, like all stomachs, it requires regular feeding and a healthy balanced diet. To provide just the right recipe for the bacteria, the food must be prepared and have any packaging removed.

Digesters come in a wide range of shapes and sizes and each one is slightly different. Getting the precise mix of nutrients to keep the microbacteria well-nourished and efficient is quite an art which can involve adding ingredients such as manure or other specific trace elements to the feedstock.

Getting this right ensures that the plant maximises the energy recovered from the food waste.

How Does an Anaerobic Digester Work?

All Anaerobic digestion systems adhere to the same working principle.

• Loading. The particles of solid or liquid matter are crushed to obtain a homogenized substrate that is fed by a hopper into an anaerobic, or oxygen-free vessel of cylindrical form. Feedstocks include grains (the most efficient), molasses, animal fat, cattle and chicken manure, household waste and sewage sludge.
• Fermentation. The biomass is heated to approximately 37°C to 38°C (sometimes more than 50°C) and stirred continuously. After at least 20 days and a series of bacteria-induced chemical transformations, the fermented biomass produces biogas. The biogas contains 40% to 70% methane(CH₄), the same as natural gas from a hydrocarbon deposit. The remaining gas is carbon dioxide (CO₂), plus small amounts of sulfur.
• Biogas uses. The biogas may be used on-site in a cogeneration engine for producing heat and power or it can be purified with membranes to extract the methane for injection into the public natural gas network or for use as transportation fuel.
• Removal. The Digestate, or material left over following the conversion of the substrate, is typically used as fertilizer for crops.

Step-by-Step Process of Anaerobic Digestion

It’s a Biological Process, and there are 4 stages: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis.

• Hydrolysis – the first step in the conversion of organic material to biogas. In this stage, certain bacteria break down organic polymers like carbohydrates into simple sugars so that the next group of bacteria can further process the material.
• Acidogenesis – the second step in the conversion of organic material to biogas. In this stage, certain bacteria called acidogenic bacteria convert the simple sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids.
• Acetogenesis – the third step in the conversion of organic materials to biogas. In this stage, certain bacteria called acetogenic bacteria convert the organic acids into acetic acid, carbon dioxide, and hydrogen.
• Methanogenesis – the final step in the conversion of organic materials to biogas. In this stage, certain single-celled organisms called methanogens convert the intermediate products produced in the preceding stages into biogas (primarily methane and carbon dioxide). The solid and liquid leftover from this process, digestate, consists of material that the microbes cannot use as well as dead bacteria.

Temperature and pH Effect on the Digestion Process

Maintaining optimal conditions in the digester is crucial for the successful completion of each step. Temperature and pH levels are among the most significant parameters.

An optimal pH range between 6.8 and 7.4 usually works best. A balanced pH maintains the health and activity of the microbial communities inside the digester.

Temperature also impacts the rate and efficiency of the digestion process. Most processes prefer the mesophilic range (15°C-45°C) or thermophilic conditions (45°C-80°C). The choice between these two depends on the specific goals and resources of the waste management project.

Adjusting these parameters allows us to facilitate the different steps of anaerobic digestion efficiently. It ensures a steady production of biogas and supports sustainable waste management and renewable energy efforts.

Types of Anaerobic Digesters

Here, we identify and discuss the key distinctions between wet and dry digesters and high-solids and low-solids digesters.

Wet and Dry Digesters

Within the realm of anaerobic digesters, two principal types exist: wet and dry digesters. Recognized for their moisture content variation, these digesters hold different characteristics and applications.

Wet digesters, containing 90-95% moisture content, suit liquid waste processing, commonly used in sewage treatment plants. This variant stimulates bacteria at optimized temperatures, sparking the breakdown of waste into soluble, biogas-generating compounds.

Conversely, Dry digesters, containing 65-70% moisture content, cater to the processing of solid waste like food scraps and yard waste.

Emphasizing a mechanically mixed environment, they secure an equitable distribution of bacteria and heat, assisting in efficient waste processing into energy-producing biogas.

High Solids and Low Solids Digesters

Pivoting to the intensity of the process, anaerobic digesters are also into high solids and low solids digesters. Reinforcing operational flexibility, these variants target the substance’s total solid (TS) value, influencing the system’s operational dynamics.

High solids digesters, operating at 15-40% TS, specialize in managing solid waste with low water content, such as food waste, garden waste, and manure. They deliver excellent yield per volume and demand lower water usage and energy input, fostering a lower environmental impact.

Low solids digesters, conversely, work efficiently with 0.5-10% TS, making them appropriate for digesting liquid waste like sewage sludge and effluent. They exhibit shorter retention times, resulting in quicker turnover rates and operational efficiency.

However, they may require more energy input and careful monitoring, given the potential for inhibitory substances present in liquid waste.

Benefits of Using an Anaerobic Digester

The utilization of anaerobic digesters results in multiple environmental and economic benefits.

These include:

• production of renewable energy;
• reductions in odor;
• reductions in greenhouse gas emissions;
• improved water quality; and
• reductions in viable weed seeds.

Energy

Anaerobic digestion produces energy that can be used for heating or generating electricity, both of which change a waste management problem into an income-generating enterprise.

Odor

The effluent and gas (when combusted and utilized) is almost odor-free15,16. In these times of encroaching urbanization, that is a major advantage of farm-based digesters and is the sole purpose for some installations.

Greenhouse Gas

Because the methane is consumed to produce energy, the high greenhouse gas (GHG) impact from methane being released into the atmosphere from traditional manure management (i.e. lagoons) is greatly diminished (methane has 21 times the global warming potential of carbon dioxide).

In a life-cycle assessment (LCA), a conventional manure handling system had almost five times the greenhouse gas impact as an anaerobic digestion system.

Because the methane is combusted to produce heat, the greenhouse gas (GHG) impact from methane is greatly diminished when the methane is utilized (combusted in a generator or directly for heat/cooking).

Anaerobic digestion provides an energy source with no net increase in atmospheric carbon. Additionally, the need for fossil fuel is displaced and the economic burden from purchasing fossil fuel is decreased.

Water Quality

Solids are reduced by 50-90%. Reduced solids translate into reduced biochemical oxygen demand (the amount of dissolved oxygen needed by aerobic biological organisms in the water to break down organic material), which improves the downstream water quality.

Practical Applications of Anaerobic Digesters

Anaerobic digesters on livestock farms have many benefits compared to traditional manure management systems, including:

Diversified Farm Revenue

Looking for ways to diversify revenue is critical to the long term financial viability of many farms. While manure has always traditionally been a resource on the farm as a fertilizer, alternative products can be produced creating new income generating opportunities.

Maximizing the value of manure increases farmers’ resiliency to the uncertainty of commodity product markets.

Rural Economic Growth

As farmers look to innovative ways to economically and environmentally manage their manure, there are opportunities for rural economic growth. Constructing and operating digesters offers new local job opportunities and increases local tax revenue.

Conservation of Agricultural Land

Healthy soil and water is important to farmers, their families and the community. The vast majority of farms in America are family-owned and active conservation practices can continue to pass this legacy from one generation to the next.

Improve soil health by converting the nutrients in manure to a more accessible form for plants to use.

Help protect the local water resources by reducing nutrient run-off and destroying pathogens.

Energy Independence

Many early farms and other agricultural operations, such as mills, were often energy independent, getting their power from wind or water sources. Manure provides an opportunity for farm energy independence.

Biogas from manure is composed of more than 50% methane, the primary component of renewable natural gas.

Considerations When Installing an Anaerobic Digester

Building upon the valuable benefits of anaerobic digesters in waste transformation and energy production, we delve into the vital considerations to make when installing these mechanisms.

We focus on the technical requirements and the aspect of cost and return on investment.

Technical Requirements

In installing an anaerobic digester, understanding the technical needs proves indispensable. You’d primarily need sufficient space, as the size of the digester depends upon the volume of waste you aim to process.

Onsite waste generation capability, like livestock manure or crop residue, is a significant determinant of the intended scale of the digester. Second, consider the environmental conditions as temperature and pH influence the bacterial activity inside the digester and thus its operation.

Additionally, anaerobic digesters demand robust systems for waste loading and unloading, gas collection, and storage. For instance, efficient mixing systems ensure uniform heat distribution and promote bacteria’s effective digestive performance.

Attention to specific equipment, such as gas cleaning units, heat exchangers, and gas storage units, plays a crucial role, too.

Ancillary infrastructure, such as a power generator to convert biogas into electricity, may also be necessary if you intend to utilize the biogas for power generation.

Cost and Return on Investment

Mapping the path of total costs versus the projected return on investment encompasses another critical aspect of installing an anaerobic digester.

These costs include initial expenses like procurement and setup of the digester and ancillary equipping, operating costs like replenishment and labor, and potential disposal costs of remaining solids after digestion.

To calculate the potential return on investment, consider the value of methane-rich biogas produced: it could serve as a part of the facility’s energy requirement, substituting for expensive traditional energy sources.

Also, digestate, a byproduct of anaerobic digestion, acts as a reliable source of biofertilizer. Its application helps reduce the farm’s dependence on externally supplied fertilizers, resulting in significant cost savings.

Return on investment is also enhanced by the reduction in waste management costs and potential regulatory compliance costs, as anaerobic digestion helps in reducing waste volume and greenhouse gas emissions.

As a result, the cumulative return could, over time, surpass the initial and ongoing investment.

However, bear in mind that return on investment may vary significantly based on factors such as local energy prices, waste disposal costs, regulatory conditions, and incentives for renewable energy practices.

It’s important to conduct a thorough financial study factoring in these variables to assess feasibility.

Conclusion

Anaerobic digestion of recycling biosolids and sewage sludges has been highly successful in the past and presently the growing interest in this field is primarily to ensure that the biodegradability of materials going to the landfill is significantly reduced.

One must remember that AD is a multi-option sophisticated process that provides options for improved recycling of nutrients and organics and also has the potential for the reclamation of a considerable proportion of the inherent energy of organic waste.

FAQs.