Recipient Organization
COLORADO STATE UNIVERSITY
(N/A)
FORT COLLINS,CO 80523
Performing Department
Civil and Environmental Engineering
Non Technical Summary
Colorado is home to a multitude of animal feeding operations and food production facilities, including diaries, animal feed yards, as well as milk and meat processing facilities.Dairy and livestock production and processing companies are vital to Colorado's economy and food security, yet they are burdened with the challenge of handling waste that imposes limits on their operation and expansion. Solid and liquid animal waste products and byproducts from food processing facilities, pose water pollution risks. Farms and food processing plants can rely on anaerobic digestion (AD) to safely treat excess organic wastes while generating biogas (a methane rich fuel gas) and stabilized digestion effluent. Many agricultural AD systems utilize land application (on cropland) as a cost-effective disposal method for nutrient rich digestion effluent. Effluent, generated on a continual basis, must be stockpiled in large containment lagoons as cropland can only accept effluent irrigation during certain periods of the cropping cycle. Storage within such open lagoons leads to odorous, sulfur-derived, gaseous emissions. One of the largest agricultural digesters within the United States, the Heartland Biodigester (Greeley, Colorado) was closed due to a toxic level of gaseous hydrogen sulfide (H2S) emissions from its effluent lagoons.This event and the loss of $115M in investor capital has sent shockwaves through the agricultural AD community as the future of other planned projects are re-evaluated. Hydrogen sulfide emissions, once regarded simply as a nuisance, are now an undeniable financial risk factor for agricultural AD projects.On-farm and agricultural digesters often process agricultural byproducts with elevated levels of animal and plant proteins such as animal manure and dairy and meat processing wastes. These waste products are all rich in two sulfur containing amino acids: methionine, and cystein.The decomposition of methionine and cystine are responsible for much the pungent odor of rotten eggs or decaying animal tissue. During the AD process, as methionine and cysteine are converted into volatile fatty acids (a precursor to methane) reduced sulfur compounds (including hydrogen sulfide) are generated as a byproduct. The hydrogen sulfide gas exists in equilibrium with dissolved bisulfide (HS-) and sulfide (S-2). Hydrogen sulfide gas is highly toxic when inhaled, the US EPA lists levels for acute exposure at 70 parts per billion.In addition to odor and human health related issues H2S (and related sulfides) create noticeable issues for digester operations:Sulfides are inhibitory to anaerobic microbes limiting biogas yield and digester stability.Liquid handling equipment must tolerate dissolved sulfide ions as well as hydrogen sulfide gas which are corrosive to various metals, attacking carbon steel, brass, and even 304 stainless steel.Hydrogen sulfide contaminated biogas requires treatment before gas can be utilized for most purpose(combustion for energy generation, or pipeline injection).Current hydrogen sulfide removal processes focus on the biogas, but this approach does not directly impact sulfide concentration within the liquid digestate of the digester. This leads to continued emissions from the digestion system in the effluent itself. There is a need for a practical and cost-effective solution for sulfide recovery/treatment from the liquid digestate itself.There is promise for the development of a biologically mediated method to remove and recover sulfides from within digestion systems. The proposed approach utilizes anoxygenic purple sulfur bacteria (PSB) to metabolize sulfides within the digestion system. PSB belong to a diverse group of organisms known as chemolithoautotrophs that utilize carbon dioxide as their sole source of carbon. PSB utilize a unique metabolic pathway which derives energy from light and the oxidation of sulfides. PSB oxidize bi-sulfide and produce insoluble zerovalent sulfur crystals. This leads to a reduction of produced hydrogen sulfide, through the reduction of available sulfide, and the upward pH shift due to the uptake of bicarbonate by the autotrophic PSB. At CSU's demonstration scale anaerobic digestion facility operated by co-PI Loetscher, PSB have been observed forming an attached film within the leachate filled translucent reactor and filter bodies exposed to bright light.The objective of this work is to assess the potential for PSB to enable sulfide removal in the digestion of highly nitrogenous agricultural wastes within an engineered LED illuminated FPPBR. Funds received from AES will be applied to design and operate a working bench scale FPPBR for the removal of sulfide from process fluid and gas derived from agricultural anaerobic digestion systems.
Animal Health Component
100%
Research Effort Categories
Basic
(N/A)
Applied
100%
Developmental
(N/A)
Goals / Objectives
Assess the potential for purple sulfur bacteria (PSB) to enable sulfide removal in the digestion of protein rich agricultural wastes within an engineered light emitting diode (LED) illuminated flat plate photobioreactor (FPPBR). Funds received from AES will be applied to design and operate a workingbench-scaleFPPBR for the removal of sulfide from process fluid and gas derived from agricultural anaerobic digestion systems.Objectives are as follows:1)Determine the optimal illumination intensity for a substratum illuminated biofilm of PSB at targeted wavelengths delivered by LED (based on moles of oxidized sulfur per watt hour of energy) This will be separately evaluated for both synthetic and filtered anaerobic digestion effluent.2)Develop an LED illuminated FPPBR for the oxidation of sulfides within anaerobic digestion effluent.3)Validate FPPBR on digestion effluent from an AD system operating on agricultural substrates.
Project Methods
Objective 1: Determine the optimal illumination intensity for a substratum illuminated biofilm of PSB at targeted wavelengths delivered by LED (based on moles of oxidized sulfur per watt hour of energy) This will be separately evaluated for both synthetic and filtered anaerobic digestion effluent.Lab scale experiments will be conducted with synthetic growth media under controlled conditions:Determine optical absorbance of locally collected mixed PSB cultureDetermine the photonic efficiency (moles of photons per mole of sulfide recovered) of active biofilm at various illumination intensitiesDetermine photonic efficiency of biofilm as a function of time (as biofilm depth increases)Objective 2: Develop an LED illuminated FPPBR for the oxidation of sulfides within anaerobic digestion effluent.Design/process validation of bench scale PSB-FPPBR will include:Determine biomass growth rates within FPPBRMeasure reactor specific photonic efficiencyDetermine sulfur mass uptake per unit area ofilluminated surfaceareaObjective 3: Validate FPPBR on digestion effluent from an AD system operating on agricultural substrates.Process validation with AD process liquid derived from agricultural substratesAgricultural substrates:Dry-lot dairy and feedlot manure wastesDairy processing wastes (milk and cheese process wash water, solid wastes from cheese)Determine biomass growth rates within FPPBR while processing leachateMeasure photonic efficiency while processing leachateDetermine sulfur mass uptake per unit area ofilluminatedareaAgricultural wastes will be processed within CSUs demonstration scale agricultural digester. In this system, 200 L reaction vessels will be loaded with the above wastes, where they will be processed for 21 days. During this process, leachate will be circulated through the bench scale PSB-FFPBR before being returned to the process. Sulfide concentrations will be measured before and after the PSB-FFPBR and the sulfur contents of the harvested biomass will be measured. This configuration will enable the larger scale testing of the PSB-FFPBR system with realistic agricultural substrates.