• Energy Research

Hydrogen sulfide (H2S) is a corrosive trace gas present in biogas produced from anaerobic digestion systems that should be removed to reduce engine-generator set maintenance costs. This study was conducted to provide a more complete understanding of two H2S scrubbers in terms of efficiency, operational and maintenance parameters, capital and operational costs, and the effect of scrubber management on sustained H2S reduction potential. For this work, biogas H2S, CO2, O2, and CH4 concentrations were quantified for two existing H2S scrubbing systems (iron-oxide scrubber, and biological oxidation using air injection) located on two rural dairy farms. In the micro-aerated digester, the variability in biogas H2S concentration (average: 1938 ± 65 ppm) correlated with the O2 concentration (average: 0.030 ± 0.004%). For the iron-oxide scrubber, there was no significant difference in the H2S concentrations in the pre-scrubbed (450 ± 42 ppm) and post-scrubbed (430 ± 41 ppm) biogas due to the use of scrap iron and steel wool instead of proprietary iron oxide-based adsorbents often used for biogas desulfurization. Even though the capital and operating costs for the two scrubbing systems were low (<$1500/year), the lack of dedicated operators led to inefficient performance for the two scrubbing systems.

Anaerobic digestion (AD) is an economically viable manure treatment option for large dairies (>500 cows) in the U.S. However, roughly 90% of U.S. dairies have less than 200 cows, making this technology economically inaccessible to the vast majority of U.S. dairies. While there have been case studies of individual small dairies with anaerobic digesters, there are no comparative studies using cost data from these systems. The objectives of this study were to (1) determine the economic viability of small-scale U.S. digesters using cost data from nine existing 100 to 250-cow dairies and seven theoretical systems and (2) reevaluate the minimum size dairy farm needed for economically feasible AD in the U.S. Cash flow analysis results showed that total capital costs, capital costs per cow, and net costs per cow generally decreased with increasing herd size in existing systems. Among existing revenue streams, use of digested solids for bedding generated the highest revenue

Abstract A life cycle assessment (LCA) approach was used to identify the processes and stages in organic waste composting that have the largest environmental impacts. The LCA included impacts associated with the collection of feedstock, production and distribution of compost, and its use as a replacement for peat for soil conditioning. The use phase of the compost product has not been included in previous LCA studies in the United States. Nine LCA impact categories were analyzed (global warming potential, ozone depletion, smog, acidification, eutrophication, carcinogens, non-carcinogens, respiratory effects, and ecotoxicity) using TRACI 2 methodology. The functional unit was defined as the collection, processing, transportation, and application of one tonne of compost that meets USEPA composting standards. The compost was produced at the Organics Material Processing and Education Center (OMPEC) at The Pennsylvania State University. The data used in the assessment was collected from seven composting windrows over thirteen consecutive months (December 2010–January 2012). Given the wide range of decomposition emission factors reported in the literature for methane (CH 4 ), nitrous oxide (N 2 O) and ammonia (NH 3 ), three emission scenarios were calculated: average, minimum, and maximum emission scenarios. Carbon dioxide (CO 2 ) emissions from the compost were considered biogenic and not included in the assessment. For all scenarios, compost processing was the stage with the largest environmental impact, with decomposition emissions contributing the most to global warming potential, acidification and eutrophication impact categories under the average and maximum emissions scenarios. To account for the avoided environmental impacts of peat mining and transport, these values were subtracted from the composting life cycle. The avoided impacts from peat replacement were higher than the impacts from composting for all categories, illustrating that using compost instead of peat results in net environmental gains. This study highlights the importance of minimizing life cycle impacts associated with CH 4 , N 2 O and NH 3 emissions during the decomposition process and the need for more consensus in the literature on emission values from composting processing.

Anaerobic digestion (AD) is a biological-based technology that generates methane-enriched biogas. A microbial electrolysis cell (MEC) uses electricity to initiate bacterial oxidization of organic matter to produce hydrogen. This study determined the effect of energy production and waste treatment when using dairy manure in a combined AD and MEC (AD-MEC) system compared to AD without MEC (AD-only). In the AD-MEC system, a single chamber MEC (150 mL) was placed inside a 10 L digester on day 20 of the digestion process and run for 272 h (11 days) to determine residual treatment and energy capacity with an MEC included. Cumulative H2 and CH4 production in the AD-MEC (2.43 L H2 and 23.6 L CH4) was higher than AD-only (0.00 L H2 and 10.9 L CH4). Hydrogen concentration during the first 24 h of MEC introduction constituted 20% of the produced biogas, after which time the H2 decreased as the CH4 concentration increased from 50% to 63%. The efficiency of electrical energy recovery (ηE) in the MEC was 73% (ηE min.) to 324% (ηE max.), with an average increase of 170% in total energy compared to AD-only. Chemical oxygen demand (COD) removal was higher in the AD-MEC (7.09 kJ/g COD removed) system compared to AD-only (6.19 kJ/g COD removed). This study showed that adding an MEC during the digestion process could increase overall energy production and organic removal from dairy manure.

The effects of metal nanoparticle (NP) addition during anaerobic digestion (AD) of poultry litter was tested using two sequential experiments: Exp. A) four NPs (Fe, Ni, Co, and Fe3O4) at three concentrations; and Exp. B) NP combinations (Fe, Ni, and Co) at four concentrations. Scanning electronic microscopy (SEM) and elemental analysis were used to confirm NP inclusion after dispersion (before AD) and track nanoparticles post-AD, and new technique for NP extraction post-AD was developed. Before AD, NPs ranged from 30.0 to 80.9 nm for Fe, Ni, and Co, and 94.3 to 400 nm for Fe3O4. Methane production increased with NPs addition compared to poultry litter-only, with the highest increases observed with NPs concentrations (in mg/L) of 12 Ni (38.4% increase), 5.4 Co (29.7% increase), 100 Fe (29.1% increase), and 15 Fe3O4 (27.5% increase). Nanoparticle mixtures greatly decreased H2S production. The SEM post-AD detected Fe, Ni, and Fe3O4 at concentrations ≥100 mg/L.

Bioplastics have emerged as a viable alternative to traditional petroleum-based plastic (PET). Three of the most common bioplastic polymers are polyhydroxybutyrate-valerate (PHBV), polylactide (PLA), and cellulose-based bioplastic (CBB). This study assessed biodegradation through anaerobic digestion (AD) of these three bioplastics and PET digested with food waste (FW) at mesophilic (35 °C) and thermophilic (55 °C) temperatures. The four plastic types were digested with FW in triplicate batch reactors. Additionally, two blank treatments (inoculum-only) and two PHBV treatments (with FW + inoculum and inoculum-only) were digested at 35 and 55 °C. The PHBV treatment without FW at 35 °C (PHBV-35) produced the most methane (CH4) normalized by the volatile solids (VS) of the bioplastics over the 104-day experimental period (271 mL CH4/g VS). Most bioplastics had more CH4 production than PET when normalized by digester volume or gram substrate added, with the PLA-FW-55 (5.80 m3 CH4/m3), PHBV-FW-55 (2.29 m3 CH4/m3), and PHBV-55 (4.05 m3 CH4/m3) having 848,275 and 561%, respectively, more CH4 production than the PET treatment. The scanning electron microscopy (SEM) showed full degradation of PHBV pellets after AD. The results show that when PHBV is used as bioplastic, it can be degraded with energy production through AD.

Abstract Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm 3 ) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH 4 and H 2 S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH 4 potential than dairy manure on a volatile solids basis. While no synergistic effect on CH 4 production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH 4 production. Initial H 2 S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H 2 S concentrations (0.10–0.14%) and higher CH 4 content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH 4 yield (372 ± 12 L kg −1 VS). The co-digestion mixture containing 40% radish had a lower specific CH 4 yield (345 ± 2 L kg −1 VS) but also showed significantly less H 2 S production at start-up and high quality biogas (58% CH 4 ). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH 4 production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH 4 production of manure digesters in the fall/winter.

Biological desulfurization of biogas from a field-scale anaerobic digester in Peru was tested using air injection (microaeration) in separate duplicate vessels and chemical desulfurization using duplicate iron filters to compare hydrogen sulfide (H2S) reduction, feasibility, and cost. Microaeration was tested after biogas retention times of 2 and 4 h after a single injection of ambient air at 2 L/min. The microaeration vessels contained digester sludge to seed sulfur-oxidizing bacteria and facilitate H2S removal. The average H2S removal efficiency using iron filters was 32.91%, with a maximum of 70.21%. The average H2S removal efficiency by iron filters was significantly lower than microaeration after 2 and 4 h retention times (91.5% and 99.8%, respectively). The longer retention time (4 h) resulted in a higher average removal efficiency (99.8%) compared to 2 h (91.5%). The sulfur concentration in the microaeration treatment vessel was 493% higher after 50 days of treatments, indicating that the bacterial community present in the liquid phase of the vessels effectively sequestered the sulfur compounds from the biogas. The H2S removal cost for microaeration (2 h: $29/m3 H2S removed; and 4 h: $27/m3 H2S removed) was an order of magnitude lower than for the iron filter ($382/m3 H2S removed). In the small-scale anaerobic digestion system in Peru, microaeration was more efficient and cost effective for desulfurizing the biogas than the use of iron filters.