Manure is an excellent feedstock for producing biogas through anaerobic digestion. Biogas generation from manure has many benefits including renewable energy production, reduced greenhouse gas emissions, and improved waste management. This comprehensive guide will explain everything you need to know about utilizing manure for biogas production.
What is Biogas?
Biogas is a clean burning, renewable fuel that is produced through the anaerobic digestion or fermentation of organic matter including manure, food scraps, agricultural waste, and sewage sludge. The main components of biogas are:
- Methane (CH4) – combustible gas that is used for energy production. Methane makes up 50-75% of biogas.
- Carbon dioxide (CO2) – noncombustible gas. CO2 comprises 25-50% of biogas.
During anaerobic digestion, bacteria break down organic material in an oxygen-free environment. This produces biogas along with a nutrient-rich digestate that can be used as fertilizer.
The benefits of biogas include:
- Renewable energy source – can be used for electricity, heat, and fuel.
- Reduced waste – provides productive use for manure and other organic waste.
- Lower greenhouse gas emissions – prevents methane release from manure to the atmosphere. Captures methane as clean energy source.
- Digestate fertilizer – leftover material is excellent agricultural fertilizer.
Manure as a Biogas Feedstock
Manure is one of the best substrates for biogas production because:
- High organic matter and nutrient content – provides excellent food for the anaerobic bacteria.
- Readily available on farms and livestock operations.
- Regular waste product that requires better management.
- Has high methane production potential.
The types of manure that can be used for biogas generation include:
- Cattle manure
- Pig manure
- Poultry litter (chicken/turkey manure mixed with bedding material)
- Horse manure
- Sheep and goat manure
Cattle and swine manures generally produce the highest biogas yields.
Biogas Digester Systems
There are several types of anaerobic digester systems that can process manure for biogas production:
Covered Lagoon Digester
- Outdoor ponds or tanks covered with a flexible cover.
- The floating cover traps biogas.
- Low cost and simple to operate.
- Lower gas yields than other systems.
- Requires large land area.
Complete Mix Digester
- Steel or concrete tank with heating system.
- Continuously mixed to evenly distribute bacterial cultures.
- More controlled digestion provides maximum gas production.
- Higher capital investment but faster payback.
Plug Flow Digester
- Long narrow tank that manure flows through.
- Minimal mixing required.
- Produces medium to high gas yields.
- Moderate building cost.
Attached Growth Digester
- Bacteria grow on fixed surfaces inside the tank.
- Allows for very high-rate digestion.
- Operates in sequential batches.
- High capital cost but very high gas yields.
Sizing the Biogas Digester
Properly sizing the digester is crucial for optimum biogas generation. The main factors determining digester size are:
- Manure volume – Amount of daily manure input influences tank size.
- Solids content – Manure with higher solids requires longer retention time and larger volume.
- Temperature – Heated systems have faster digestion so smaller volumes.
- Gas production goals – High energy needs require maximizing digester capacity.
As a general guideline, the digester size should be based on the volume of manure produced in a 1 to 3 day period. Optimal sizing ensures adequate retention time for biogas production. Oversizing the digester leads to unused capacity.
Digester Feedstock Preparation
Proper feedstock preparation is key to maximizing biogas yields:
- Remove large solids – Screen out bedding, rocks, course fibers which can clog piping.
- Mix with water – Dilute thick or dry manure to optimal solids content.
- Maintain ideal C:N ratio – Balance carbon and nitrogen by co-digesting with high-N organic waste.
- Sample and analyze – Evaluate manure composition to identify optimization strategies.
Adjusting the manure characteristics through blending, dilution, or additives can substantially boost biogas generation.
Maintaining Proper Digester Conditions
To achieve high biogas productivity, the digester environment must be continuously monitored and controlled:
Temperature – Mesophilic bacteria operate best from 30°C to 38°C. Thermophilic bacteria prefer 49°C to 57°C. Heating or cooling may be required to maintain optimal temperature range.
pH – The ideal pH level is between 6.8 and 7.2. pH can be adjusted by adding lime or acid solutions.
Alkalinity – Sufficient alkalinity (over 1000 mg/L) buffers pH changes. Supplementing with sodium bicarbonate prevents volatile fatty acid accumulation.
Nutrients – Key nutrients like nitrogen, phosphorus, and trace elements must be adequate to support high microbial growth and activity.
Loading rate – Hydraulic retention time of 15 to 30 days is typical. Organic loading rate depends on digester type and temperature.
Mixing – Mixing prevents stratification and evenly distributes bacterial populations. Excessive mixing can inhibit methanogen activity.
Routine sampling and analysis ensures any needed adjustments are made quickly to maintain optimal digester function.
The methane-rich biogas generated from manure digestion can be beneficially used in several ways:
- Biogas can fuel engine-generator sets to produce electricity.
- The heat from biogas combustion can provide process heating or space heating.
- Waste engine heat can be recovered for heating greenhouses, barns, and digesters.
- After moisture removal and moderate compression, biogas can fuel natural gas vehicles.
- Biogas can be upgraded to pure biomethane for use as vehicle fuel or gas grid injection.
- The remaining digestate is an excellent organic fertilizer that can be applied on crops and pastures or sold.
Careful assessment of available biogas utilization options and costs is recommended to select the best energy recovery strategy. On-farm electricity generation typically offers the fastest payback on investment.
Case Study: Dairy Farm Anaerobic Digester
Maple Dell Farm is a 800 cow dairy operation. They installed a complete mix digester to process the manure from their herd and maximize biogas production.
Feedstock – The digester processes 18,000 gallons of raw dairy manure daily. Additional food waste from the farm is co-digested to improve C:N ratio.
Digester – A 625,000 gallon concrete complete mix tank with flexible cover was constructed onsite. It is heated to 36°C and continuously mixed.
Biogas Use – The raw biogas fuels a generator producing 125 kW of electricity. This powers the farm’s facilities and excess power is sold to the local utility.
Impact – The digester generates over 50,000 kWh of green electricity monthly while preventing methane emissions. Annual revenue from electricity sales reached $200,000 providing a 2 year payback.
- Manure provides an excellent high-strength feedstock for biogas generation through anaerobic digestion.
- Multiple digester designs can be implemented at commercial scales providing renewable energy and sustainable waste management.
- Critical factors for success include appropriate digester sizing, optimizing manure characteristics, and maintaining proper digester conditions.
- As demonstrated in the case study, manure digestion can provide significant environmental and economic benefits for livestock farms.
Implementing a biogas plant provides a “win-win” solution enabling sustainable energy production from an abundant on-farm waste stream. With proper planning and management, manure biogas systems can provide attractive returns on investment while advancing renewable energy goals.