Manure Digesters Richard Ballenger

Overview Cattle Basics Cattle Basics Manure Management Manure Management Digester Overview Digester Overview Digester Chemistry Digester Chemistry Digester Flow Digester Flow Reactors Reactors Crave Farm Case Study Crave Farm Case Study Future Digesters Future Digesters

Cattle Basics Produce up to 100 lbs of milk/day ~10-12 gal Produce up to 100 lbs of milk/day ~10-12 gal Milked three times each day - 24 hr/day operation Milked three times each day - 24 hr/day operation Creatures of habit Creatures of habit Eat up to 90 lbs of food/day Eat up to 90 lbs of food/day Drink lbs of water/day ~ gal Drink lbs of water/day ~ gal Mass balance: lbs of waste/day Mass balance: lbs of waste/day These numbers are staggering considering there can be thousands of cattle per farm These numbers are staggering considering there can be thousands of cattle per farm

Manure Management How to handle the large amount? How to handle the large amount? Where does it go? Where does it go? How does it get there? How does it get there? What are its uses? What are its uses?

Typical Manure Management Some farms have slatted floors Some farms have slatted floors Manure is gravity fed to a holding lagoon Manure is gravity fed to a holding lagoon Can be used as fertilizer Can be used as fertilizer High organic N content – can’t be spread on live plants High organic N content – can’t be spread on live plants Can only be spread before planting Can only be spread before planting Odor Odor

Manure Composition Moisture content:88-91% Moisture content:88-91% Total solids (TS):9-12% Total solids (TS):9-12% Volatile solids (VS):7-9% Volatile solids (VS):7-9%

Digester Overview Converts methane from manure into power Converts methane from manure into power Completely removes odor Completely removes odor Separates solid from liquid waste Separates solid from liquid waste Solids can be sold as potting mix Solids can be sold as potting mix Solids can be used as cattle bedding Solids can be used as cattle bedding Liquid can be used as fertilizer on green plants Liquid can be used as fertilizer on green plants Reduces manure management problems Reduces manure management problems

Digester Myths Does not reduce manure volume Does not reduce manure volume Liquid does not have reduced nutrients Liquid does not have reduced nutrients ***Phosphorus exception ***Phosphorus exception

Digester Chemistry Anaerobic Process (No oxygen) Anaerobic Process (No oxygen) VS contain fasts, carbohydrates, proteins, and miscellaneous other nutrients VS contain fasts, carbohydrates, proteins, and miscellaneous other nutrients Two stage process Two stage process Acid-forming stage Acid-forming stage Methane forming stage Methane forming stage

Acid Forming Stage Performed by acid formers Performed by acid formers VS + bacteria = fatty acids VS + bacteria = fatty acids Not very sensitive to temperature and pH Not very sensitive to temperature and pH Excess acid formers lower the pH Excess acid formers lower the pH

Methane Forming Stage Performed by methane formers Performed by methane formers fatty acids + bacteria = methane and carbon dioxide fatty acids + bacteria = methane and carbon dioxide Optimal Temperature ~ °F Optimal Temperature ~ °F Optimal pH ~ Optimal pH ~ Lime sometimes added at startup to control pH from excess acid formers Lime sometimes added at startup to control pH from excess acid formers

General Digester Flow Manure enters floor slats and is gravity fed to underground tank Manure enters floor slats and is gravity fed to underground tank Manure fed into digester Manure fed into digester Methane and carbon dioxide collect under the digester cover Methane and carbon dioxide collect under the digester cover Fed to combustion engine for electricity generation Fed to combustion engine for electricity generation Solids and liquids are separated Solids and liquids are separated Solids stored in barn Solids stored in barn Liquids stored in a lagoon Liquids stored in a lagoon

Flow Diagram

Reactors Three types Three types Covered Lagoon Covered Lagoon Plug Flow Plug Flow Complete Mix Complete Mix

Covered Lagoon Digester Make by covering your holding lagoon Make by covering your holding lagoon Simplest type, but least effective Simplest type, but least effective In cold climates only used for odor control In cold climates only used for odor control

Plug Flow Middle of the road digester Middle of the road digester Can be used in warm or cold climates Can be used in warm or cold climates Has problems with plugging Has problems with plugging

Completely Mixed Best technology, but most expensive Best technology, but most expensive Warm and cold climates Warm and cold climates Usually cylindrical towers made of concrete or stainless steel Usually cylindrical towers made of concrete or stainless steel Up to about 90 ft in diameter Up to about 90 ft in diameter

Case Study Crave Brothers Farm – Waterloo, WI Crave Brothers Farm – Waterloo, WI Installation by Clear Horizons, a division of Peeper Electric Installation by Clear Horizons, a division of Peeper Electric Complete Mix Digester Complete Mix Digester 1,000 head of cattle 1,000 head of cattle Approximate energy production 200 kW Approximate energy production 200 kW

Structure 84 ft outside (82 ft inside) diameter 84 ft outside (82 ft inside) diameter Stainless steel construction Stainless steel construction 304 Stainless for solid contact 304 Stainless for solid contact 316 Stainless for Biogas contact 316 Stainless for Biogas contact Wrapped in insulation Wrapped in insulation

Roof Double membrane Double membrane Rigid outside Rigid outside Floating inside Floating inside Blower regulates pressure between the two membranes Blower regulates pressure between the two membranes Allows for differences in methane production Allows for differences in methane production

Heat Control Inside temperature °F Inside temperature °F Regulated by temperature coils Regulated by temperature coils Heat required comes from the combustion engine Heat required comes from the combustion engine

Agitation Agitated by one large and three small submersible motors

Pumping Manure pumped in just below solid/liquid level Manure pumped in just below solid/liquid level Methane pumped off the top Methane pumped off the top As bacteria break down manure, the density increases as sinks to bottom As bacteria break down manure, the density increases as sinks to bottom Solid/liquid pumped off bottom and separated Solid/liquid pumped off bottom and separated Maximum retention time ~ 3 days Maximum retention time ~ 3 days

Engine Internal combustion engine Internal combustion engine 200 kW 200 kW Effluent from digester is 60% methane 40% carbon dioxide Effluent from digester is 60% methane 40% carbon dioxide Electricity and Heat are utilized Electricity and Heat are utilized Electricity pumped back into power grid Electricity pumped back into power grid Heat used to heat the digester Heat used to heat the digester Excess heat can be used for hot water on the farm Excess heat can be used for hot water on the farm

DCS DCS (Distributed Control System) controls and trends all aspects DCS (Distributed Control System) controls and trends all aspects Completely automated Completely automated Only needs to be checked about once per week Only needs to be checked about once per week

Cost Analysis Barely economical in the United States Barely economical in the United States $0.06/kw-hr $0.06/kw-hr $1.5 million capital cost $1.5 million capital cost Pay back period of 7-8 years Pay back period of 7-8 years Very economical in Europe Very economical in Europe $ $0.40/kw-hr $ $0.40/kw-hr Carbon credits Carbon credits

Carbon Credits New concept – Linked to Kyoto New concept – Linked to Kyoto Destroy green house gas and receive credits Destroy green house gas and receive credits Less greenhouse gasses emitted with digester Less greenhouse gasses emitted with digester Companies that emit lots of greenhouse gasses can purchase carbon credits Companies that emit lots of greenhouse gasses can purchase carbon credits “Reduces” their output by only spending money “Reduces” their output by only spending money

Crave Farm Agreement Crave farm agrees to provide manure for ten years Crave farm agrees to provide manure for ten years Clear Horizons pays the bill for the digester Clear Horizons pays the bill for the digester They get all profits from sale of electricity and solid They get all profits from sale of electricity and solid Crave farm gets the liquid fertilizer and any excess heat from the combustion Crave farm gets the liquid fertilizer and any excess heat from the combustion

Clear Horizon - Financial Risk? Clear horizons investment – Not risk Clear horizons investment – Not risk Get tax cut for green energy Get tax cut for green energy With no change they will make their money back in 8 years With no change they will make their money back in 8 years If energy costs increase, pay back period a little shorter If energy costs increase, pay back period a little shorter If US needs carbon credits, pay back in about 1-2 years If US needs carbon credits, pay back in about 1-2 years

Why not own the digester? For a farm $1.5 million is a lot of money to finance for about 8 years For a farm $1.5 million is a lot of money to finance for about 8 years Farm has to do all of the work and upkeep instead of Clear Horizons Farm has to do all of the work and upkeep instead of Clear Horizons Already getting a tax break because they are a farm Already getting a tax break because they are a farm

Future of Digesters Everything about a digester helps a farm Everything about a digester helps a farm Only problem is cost Only problem is cost Clear horizons is virtually eliminating cost issue Clear horizons is virtually eliminating cost issue Not signing on because farmers like to be in total control Not signing on because farmers like to be in total control Some farmers use sand bedding which doesn’t work well with the digester Some farmers use sand bedding which doesn’t work well with the digester

Special Thanks To: Karl Crave Karl Crave Crave Brothers Farm Crave Brothers Farm Clear Horizons Clear Horizons

Sources ents/PowerPoints/SchmidtAndigest.pdf ents/PowerPoints/SchmidtAndigest.pdf od%20Waste%20fact%20sheet%202.pdf od%20Waste%20fact%20sheet%202.pdf engin/g01881.htm engin/g01881.htm West.pdf West.pdf