Heekwon Ahn

Determination of thermal properties of composting bulking materials

Thermal properties of compost bulking materials affect temperature and biodegradation during the composting process. Well determined thermal properties of compost feedstocks will therefore contribute to practical thermodynamic approaches. Thermal conductivity, thermal diffusivity, and volumetric heat capacity of 12 compost bulking materials were determined in this study. Thermal properties were determined at varying bulk densities (1, 1.3, 1.7, 2.5, and 5 times uncompacted bulk density), particle sizes (ground and bulk), and water contents (0, 20, 50, 80% of water holding capacity and saturated condition). For the water content at 80% of water holding capacity, saw dust, soil compost blend, beef manure, and turkey litter showed the highest thermal conductivity (K) and volumetric heat capacity (C) (K: 0.12-0.81 W/m degrees C and C: 1.36-4.08 MJ/m(3) degrees C). Silage showed medium values at the same water content (K: 0.09-0.47 W/m degrees C and C: 0.93-3.09 MJ/m(3) degrees C). Wheat straw, oat straw, soybean straw, cornstalks, alfalfa hay, and wood shavings produced the lowest K and C values (K: 0.03-0.30 W/m degrees C and C: 0.26-3.45 MJ/m(3) degrees C). Thermal conductivity and volumetric heat capacity showed a linear relationship with moisture content and bulk density, while thermal diffusivity showed a nonlinear relationship. Since the water, air, and solid materials have their own specific thermal property values, thermal properties of compost bulking materials vary with the rate of those three components by changing water content, bulk density, and particle size. The degree of saturation was used to represent the interaction between volumes of water, air, and solids under the various combinations of moisture content, bulk density, and particle size. The first order regression models developed in this paper represent the relationship between degree of saturation and volumetric heat capacity (r=0.95-0.99) and thermal conductivity (r=0.84-0.99) well. Improved knowledge of the thermal properties of compost bulking materials can contribute to improved thermodynamic modeling and heat management of composting processes.

Pile mixing increases greenhouse gas emissions during composting of dairy manure

The effect of pile mixing on greenhouse gas (GHG) emissions during dairy manure composting was determined using large flux chambers designed to completely cover replicate pilot-scale compost piles. GHG emissions from compost piles that were mixed four times during the 80 day trial were approximately 20% higher than emissions from unmixed (static) piles. For both treatments, carbon dioxide (CO(2)), methane (CH(4)), and nitrous oxide (N(2)O) accounted for 75-80%, 18-21%, and 2-4% of GHG emissions, respectively. Seventy percent of CO(2) emissions and 95% of CH(4) emissions from all piles occurred within first 23 days. By contrast, 80-95% of N(2)O emissions occurred after this period. Mixed and static piles released 2 and 1.6 kg GHG (CO(2)-Eq.) for each kg of degraded volatile solids (VS), respectively. Our results suggest that to minimize GHG emissions, farmers should store manure in undisturbed piles or delay the first mixing of compost piles for approximately 4 weeks.

Effects of a livestock manure windrow composting site with a fly ash pad surface and vegetative filter strip buffers on sediment, nitrate, and phosphorus losses with runoff

This study quantified the effects of a livestock manure-based windrow composting practice with a fly ash composting pad surface and vegetative filter strip (VFS) buffers on losses of runoff, runoff percent of rainfall, total solids, nitrate-nitrogen, ortho-phosphorus (PO4-P), and total-phosphorus during natural rainfall events. Runoff data from six events were collected during June and July (early season) and August and September (late season) 60-day duration composting periods from 2002 through 2004 at an Iowa State University research farm near Ames, central Iowa, USA. The research site was selected on uneven terrain with average slopes of 5% and 2% on the VFS buffer and composting pad plot areas, respectively. Runoff treatments were comprised of three compost windrows:VFS buffer area ratios that included 1:1, 1:0.5, and 1:0 (no buffer) control. The 1:1 and 1:0.5 area ratios represented a 6.0-m (20-ft) wide × 23-m (75-ft) long fly ash composting pad area compared to VFS buffer areas of equal and one-half size, respectively. All treatments had three replications for a total of nine runoff plots in a randomized complete block design. Results from the study indicate significantly higher levels (p < 0.05) of runoff, runoff percent of rainfall, total solids, nitrate-nitrogen, PO4-P, and total-phosphorus from the 1:0 control plots compared to the 1:1 and 1:0.5 VFS buffer plots. Results also show the 1:1 and 1:0.5 VFS buffer treatments were not significantly different (p < 0.05) and that average runoff loss reductions from the 1:1 and 1:0.5 VFS buffer plots were 98% and 93%, respectively, compared to the 1:0 control plots. These results reflect the effectiveness of VFS buffers for reducing runoff and contaminant losses from a windrow composting site. Compost nutrient mass balance analysis results indicate 41% and 26% of PO4-P were lost from the compost windrows during the 2004 early season and late season composting periods, respectively. However, only 0.1% to 0.4% of PO4-P was lost to runoff from the 1:0 control plots during the respective 2004 early season and late season composting periods. We hypothesize the relatively lower PO4-P losses in runoff may be attributed to potential chemical and physical effects of the fly ash composting pad material.

Evaluation of the biodegradability of animal carcasses in passively aerated bio-secure composting system

Composting livestock carcasses is a viable method for on-site treatment and disposal. Properly estimated carcass biodegradability is valuable for designing and controlling animal mortality composting systems. However, it is still difficult to assess the biodegradability inside composts. In this study, approximately 250kg of swine carcasses were composted in each of nine 2m X 2m weighable composting test units using three different envelope materials: corn silage, ground cornstalks, and ground oat straw. Total weight of compost material was measured monthly to observe the carcass decomposition trend with composting time. The most significant weight loss occurred during the first 6 weeks of composting. Biodegradability of the swine carcasses was estimated by comparing the mass of carcass remains after 16 weeks composting with the total carcass weight placed in the pile during the time of construction. Based on these results the influence of envelope material type on the biodegradability of swine carcasses was evaluated. The carcass decomposition within silage test units was only 66% of the initial carcass mass, while carcasses in cornstalk and oat straw test units decomposed 86% and 79% respectively.

Performance Evaluation of a Passively-Aerated Plastic-Wrapped Composting System Designed for Emergency Disposal of Swine Mortalities

Monitoring of a passively-aerated plastic-wrapped mortality composting system designed for emergency disposal of diseased swine highlighted the importance of the physical characteristics of materials used to envelop the carcasses. Inadequate moisture was a problem when using envelope materials such as ground cornstalks or straw having low density and high air-filled porosity. High O2 concentrations throughout these materials, and significantly higher moisture levels in the top layers than in the materials surrounding the carcasses, suggested significant air movement and transport of carcass moisture away from the carcasses, resulting in carcass desiccation and incomplete decay. Although internal temperatures and moisture levels in test units constructed with corn silage were much more favorable than in those constructed with cornstalks or straw, less carcass decomposition occurred. Settling and compaction, resulting in high bulk density and low air-filled porosity, caused low O2 concentrations that appeared to impair carcass decay in the silage test units.

Environmental Impacts of Emergency Livestock Mortality Composting—Leachate Release and Soil Contamination

A 3-year study was conducted in Iowa to evaluate the feasibility of using composting for emergency disposal of cattle mortalities. During the study, 49 metric tons of 450 kg cattle carcasses were composted in 27 replicated unturned windrow test units constructed during three different seasons of the year. Each test unit contained 1.8 metric tons of carcasses enveloped in one of 5 different materials: corn silage, ground cornstalks, straw/manure, leaves, or a soil/compost blend. Due to their water absorbing capacity and ability to evaporate absorbed water, the volume of leachate released into the soil was generally less than 5% of the 500-600 mm of precipitation that fell on the test units. Chemical analysis of 1.2 m deep soil cores collected from beneath the composting test units prior to and following composting showed statistically significant increases in chloride concentrations at all depths beneath composting test units constructed from silage, cornstalks, straw, and the soil/compost blend. Statistically significant increases in % total carbon (silage test units only) and % total nitrogen (silage, cornstalk, straw/manure test units) were limited to the top 15 cm of soil. Increases in these pollutants were moderate, amounting to less than 5X, 0.2X and 0.4 X respectively of chloride, % total carbon, % total N concentrations prior to composting. Statistically significant increases in total ammonia-nitrogen were noted at depths of up to 90 cm beneath test units constructed with silage or leaves, and at 30 cm and 15 cm depths respectively beneath test units constructed with straw/manure and cornstalks. The ammonia-nitrogen increases were large, ranging from 40-160 X of pre-composting levels of ammonia in the topsoil. When compared with the groundwater pollution potential of carcass burial, however, the estimated total mass of N contained in the composted cattle carcasses was 4-10 X the increases in total N measured in the soil beneath the composting test units.

Effects of Thermal Pretreatment Temperature on the Solubilization Characteristics of Dairy Manure for Dry Anaerobic Digestion

The effect of thermal pretreatment conditions on hydrolysis characteristics of dairy manure and sawdust mixtures has been evaluated. Thermal pretreatment temperature varied between 35 and 120°Cand the period of the treatment changed between 30 and 1440min (24h). As thermal pretreatment temperature and duration increased, organic material solublization rates were improved. Maximum solubilizations of chemical oxygen demand (SCOD), carbohydrates, and volatile fatty acids(VFAs) were observed when dairy manure treated for one day at 120°C. Although one day treatment duration at 120°Cshowed the highest SCOD, soluble carbohydrates, and VFAs concentration, its hydrolysis rate was only about 12%. The results reveal that the thermal pretreatment conditions tried in this study are not enough to solubilize the organic matter contained in dairy manure and sawdust mixtures. In order to maximize hydrolysis performance, the further research needs to determine the factors influences on organic material solubilization in addition to thermal pretreatment temperature and duration.

Effect of envelope material on biosecurity during emergency bovine mortality composting.

The biosecurity of composting as an emergency disposal method for cattle mortalities caused by disease was evaluated by conducting full-scale field trials begun during three different seasons and using three different envelope materials. Process biosecurity was significantly affected by the envelope material used to construct the composting matrix. Internal temperatures met USEPA Class A time/temperature criteria for pathogen reduction in 89%, 67%, and 22%, respectively of seasonal test units constructed with corn silage, straw/manure, or ground cornstalks. In trials begun in the winter, survival times of vaccine strains of avian encephalomyelitis and Newcastle disease virus were noticeably shorter in silage test units than in the other two materials, but during summer/spring trials survival times in ground cornstalk and straw/manure test units were similar to those in test units constructed with silage.

Livestock Manure Windrow Composting Runoff And Infiltration Characteristics from Laboratory Rainfall Simulations

Windrow-composted livestock manure has been shown to be less hazardous to the environment compared to manure directly applied to cropland and other agricultural areas. Although offsite contaminant losses through runoff and leaching can occur during the composting process, these losses are suspected to increase under different compost moisture conditions and as composted materials mature. This research quantified the effects of windrow-composted livestock manure and straw bedding components on runoff and infiltration characteristics from laboratory rainfall simulations. Compost samples collected on three dates at approximately the beginning (day 0), middle (day 30), and end (day 60) of a June-July 2004 field research windrow composting period were used for this rainfall simulation study. Replicated compost windrow-shaped cross-section samples were constructed in a specially-designed Plexiglas container apparatus for viewing and recording infiltrated leachate wetting front position boundary movement from simulated rainfall events. Runoff and leachate samples were collected and analyzed for drainage volumes and concentrations and total mass losses of sediment, nitrate-nitrogen (NO3-N), and ortho-phosphorus (PO4-P) during and following rainfall simulation trials. Leachate wetting front position boundary movement was significantly lower for day 60 compost samples compared among day 0 and day 30 compost sample material. Drainage volume analysis results indicated significantly higher average runoff versus leachate volumes within all compost sampling dates, and runoff volumes were significantly higher among day 30 and day 60 compost samples compared to runoff volumes from day 0 compost samples. Average sediment, NO3-N, and PO4-P concentrations were significantly higher in leachate versus runoff within all compost sampling dates. Conversely, the total mass losses of these contaminants were significantly higher in runoff compared to leachate within all compost sampling dates. Results of this study suggest that biological and mechanical functions of the composting process reduced compost sample aggregates and increased compost bulk density. We hypothesize that these changes in compost material structure and porosity volume decreased infiltration and increased runoff sediment, NO3-N, and PO4-P losses during the second and final compost sampling stages of a field windrow composting period.

Evaluation of Optimum Moisture Content for Composting of Beef Manure and Bedding Material Mixtures Using Oxygen Uptake Measurement

Moisture content influences physiological characteristics of microbes and physical structure of solid matrices during composting of animal manure. If moisture content is maintained at a proper level, aerobic microorganisms show more active oxygen consumption during composting due to increased microbial activity. In this study, optimum moisture levels for composting of two bedding materials (sawdust, rice hull) and two different mixtures of bedding and beef manure (BS, Beef cattle manure+sawdust; BR, Beef cattle manure+rice hull) were determined based on oxygen uptake rate measured by a pressure sensor method. A broad range of oxygen uptake rates (0.3 to 33.3 mg O2/g VS d) were monitored as a function of moisture level and composting feedstock type. The maximum oxygen consumption of each material was observed near the saturated condition, which ranged from 75% to 98% of water holding capacity. The optimum moisture content of BS and BR were 70% and 57% on a wet basis, respectively. Although BS’s optimum moisture content was near saturated state, its free air space kept a favorable level (above 30%) for aerobic composting due to the sawdust’s coarse particle size and bulking effect.