Anaerobic Digestionof Dairy Manure:Implications for NutrientManagement PlanningNEBASA/SSSA Annual MeetingJuly 13, 2005Stoors, CTBrian AldrichManure Management ProgramDept. of Biological & Environmental Engineering
Anaerobic digestion is notnew. What is new is it’sapplication to animalwaste management on alarge scale.
“We are just at the beginningof the methanogenic era.”Prof. William Jewell,Dept. of Biological & Environmental EngineeringCornell U. - Sept. ’04
Definition of AnaerobicDigestion• Anaerobic digestion is a microbialoxidation process which occurs in theabsence of oxygen, and convertsorganic carbon to methane and carbondioxide (a mixture called “biogas”).
Benefits of Anaerobic Digestion• Odor control• Pathogen kill• Weed seed kill• Biogas heat digester + other uses• Improved consistency for liquid-solidseparation• Electricity production (optional)
Advantages of Treated Manurewith Less Odor for CNMP Planning• Can be spread in summer on hay with fewercomplaints from neighbors.• “Summer’ means drier soil, less runoff.• Near immediate crop uptake.• Summer application may mean access tosteeper or wetter fields.• May be able to spread on fields previouslyunavailable for spreading, due to proximityto neighbors.
Spring Application of Digested Manureon Corn Ground with Spray Gun
Composting of Separated Solids
Diesel EngineRetrofitted to run on biogas
Combined Heat & PowerCombined heat and power (“CHP”)systems both:1. Combust biogas to produce electricity2. Capture heat from the engine block
Heat ExchangerHeat is captured from the engine block using a heat exchanger,and used to heat the manure in the digester
Definition of Volatile Solids“Those solids in water or otherliquids that are lost on ignition ofthe dry solids at 550° centigrade.”In anaerobic digestion, “volatilesolids” are the fraction of themanure solids that are easilyconverted into biogas. (Theorganic fraction.)
Definition of HydraulicRetention Time“Hydraulic Retention Time” (HRT) isthe time it takes manure to passthrough the digester.
Three Microbial Populations1. Hydrolysis– Extracellular enzymatic breakdown ofcomplex organic molecules (producessugars, fatty acids, amino acids)2. Acid fermentation– Intracellular conversion of above productsinto organic acids3. Methane fermentaion– Methanogens that are obligate anaerobes useorganic acids to produce CH 4 , CO 2 , H 2 S, NH 4
What’s in Biogas?• ~ 40% carbon dioxide• ~ 60% methane• + trace elements, includinghydrogen sulfide (highlycaustic!)
Biogas Produced from One Cow per DayRaw Manure100 lbs.87 lbs. Water13 lbs.TotalSolidsPotential forproducingbiogas2lbsAsh11 lbs.VolatileSolids(Availablefor biogasproduction)Efficiencyof mostdigestersforproducingbiogas7 lbs. notconvertedto biogas4 lbs convertedto biogas
Take-Home Message #1:• Anaerobic digestion alone does notresult in a large decrease in mass!– Aside from the effluent, the only way formass to leave the digester is in the form ofbiogas.– Typically ~ 5% reduction in mass left toreturn to cropland
Implication for CNMP Planners:• Even with anaerobic digestion, you stillhave a lot of “stuff” (mass, nutrients) tofind a home for!!!• The remaining solids are usuallyseparated from the liquids postdigestion.Planners need to find outwhat is done with those solids:– Spread on cropland?– Composted and exported off the farm?– Recycled as bedding?
Take-Home Message #2:• If a farm is considering a digester, it isextremely important that theappropriate phosphorus index becalculated for all fields on the farm.• Whole-farm P balance• Need to make sure that the farm has anadequate land base for spreadingmanure (or a plan to export nutrients ifit does not).
What happens to N-P-Kin the digester?• Influent and effluent samples weretaken from three digesters over aperiod of two years• ~ monthly intervals• Analyzed for Total N, Total P,Ammonia-N and Ortho-P
Sampling raw manure from transfer pit prior to digestion(“influent”)
Sampling digested manure from the exit end of thedigester (“effluent”)
Take-Home Message #3:• The total amounts of N and P remainabout the same!• But there is a shift from organic toinorganic forms (comparing digesterinfluent to effluent)– 37% more N in the ammonia form– 26% more P in the Ortho-P form
Percent Increase in Ammonia andOrtho-P in Digested Dairy Manure706050403020100NH3Ortho-PFarm AAFarm DDIFarm NH
Can digester effluentnutrient values be applieddirectly to crop fertilityrecommendations?The answer is no because ……
Take-Home Message #4:Manure goes from the digester tostorage !
Things that can happen todigested manure in storage:• Untreated manure may bypass thedigester, going directly to the storage.• Washwater may be pumped directly to thestorage.• Gaseous loss processes may continue …• Does treated manure activated withmethanogens stimulate methaneproduction in the storage? (We don’tknow.)
Take-Home Message #5:• To make crop fertilizer recommendations,manure samples are still neededfrom the storage and/or from themanure application equipment as closeas possible to the time of spreading.
Implications of the ShiftTowards Ammonia• If manure tests from the storage or themanure application equipment bear outan increase in the ammonia fraction,then the need for rapid incorporation tominimize volatile losses of N will beeven more significant.• Presidedress Soil Nitrate Test
Food Waste Delivery to DigesterNutrient management plans for farms with digesters that accept food waste need toinclude food waste P imports in the whole-farm P balance.
Cornell Manure Management ProgramBrian AldrichDavid BelcherJean BonhotalCurt GoochScott InglisNorman Scottwww.manuremanagement.cornell.eduThanks to the New York State Energy Research & Development Authority for partialfunding in support of this work.