Christopher Y. Choi

Application of microbial risk assessment to the development of standards for enteric pathogens in water used to irrigate fresh produce.

Microbial contamination of the surfaces of cantaloupe, iceberg lettuce, and bell peppers via contact with irrigation water was investigated to aid in the development of irrigation water quality standards for enteric bacteria and viruses. Furrow and subsurface drip irrigation methods were evaluated with the use of nonpathogenic surrogates, coliphage PRD1, and Escherichia coli ATCC 25922. The concentrations of hepatitis A virus (HAV) and Salmonella in irrigation water necessary to achieve a 1:10,000 annual risk of infection, the acceptable level of risk used for drinking water by the U.S. Environmental Protection Agency, were calculated with a quantitative microbial risk assessment approach. These calculations were based on the transfer of the selected nonpathogenic surrogates to fresh produce via irrigation water, as well as previously determined preharvest inactivation rates of pathogenic microorganisms on the surfaces of fresh produce. The risk of infection was found to be variable depending on type of crop, irrigation method, and days between last irrigation event and harvest. The worst-case scenario, in which produce is harvested and consumed the day after the last irrigation event and maximum exposure is assumed, indicated that concentrations of 2.5 CFU/100 ml of Salmonella and 2.5 x 10(-5) most probable number per 100 ml of HAV in irrigation water would result in an annual risk of 1:10,000 when the crop was consumed. If 14 days elapsed before harvest, allowing for die-off of the pathogens, the concentrations were increased to 5.7 x 10(3) Salmonella per 100 ml and 9.9 x 10(-3) HAV per 100 ml.

Effect of relative humidity on preharvest survival of bacterial and viral pathogens on the surface of cantaloupe, lettuce, and bell peppers.

The purpose of this study was to compare the effects of humidity on the preharvest survival of microbial pathogens on cantaloupe, lettuce, and bell peppers. An additional goal was to evaluate Clostridium perfringens as an indicator of fecal contamination on produce. The microorganisms used in this study included Escherichia coli, E. coli O157:H7, Shigella sonnei, Salmonella enterica subsp. enterica, Clostridium perfringens, hepatitis A virus (HAV), feline calicivirus (FCV), and coliphage PRD1. The study took place in a controlled environment chamber that allowed for the control of temperature (18 to 26 degrees C) and relative humidity. Survival rates under high (mean, 85.7 to 90.3%) and low (mean, 45.1 to 48.4%) relative humidity were compared. The surfaces of the edible portion of each plant were inoculated with the study microorganisms. Samples were collected throughout 2 weeks. More microorganisms survived significantly longer (P < 0.05) on cantaloupe than on lettuce and bell peppers. The type of produce on which each organism experienced the highest inactivation rate tended to change with relative humidity. The survival of microorganisms on produce surfaces was not uniformly affected by relative humidity. Of the studied microorganisms, HAV, PRD1, and C. perfringens were found to have the lowest inactivation rates, whereas FCV and E. coli ATCC 25922 tended to become inactivated most rapidly. C. perfringens generally survived longer than all other bacteria and FCV in all experiments. This trend suggests that C. perfringens may be an acceptable indicator of bacterial contamination and survival in various environments and on different types of crops.

Mixed convection in an inclined channel with a discrete heat source

Abstract The effects of laminar forced flow on buoyancy-induced natural convection cells throughout the regions of natural, mixed, and forced convection have been numerically investigated for a parallel planes channel with a discrete heat source. Emphasis is placed on the influence of the inlet flow velocity and the inclination angle of the channel, and the local buoyancy induced by the discrete source. The results indicate that the overall Nusselt number of the source strongly depends on the inclination angle (γ) in the natural and mixed convection regimes when γ > 45° . On the other hand, the changes in Nu and θ s.max are negligible when the channel is from 0 to 45°, i.e. there is no significant penalty in heat transfer due to the inclination of the channel up to γ = 45° . As Gr increases at a fixed Re, the entrainment of the air from the downstream exit is observed for the case of aiding flow.

Convective heat transfer in porous and overlying fluid layers heated from below

Abstract The onset of convection when a porous layer underlying a fluid layer is heated from below has been numerically investigated. In order to validate the interface boundary conditions along with the numerical scheme, the present study has focused on the critical Rayleigh number and the corresponding number of cells. In addition, the effects of the Rayleigh number, aspect ratio and thickness ratio on supercritical convection in the composite layer have been investigated. The results show that the number of cells at the critical Rayleigh number ( Ra pc ) is in good agreement with the previous report based on the linear stability theory. The abrupt change in convective flow patterns accurately verifies the precipitous drop of Ra pc with the increasing the depth ratio ( d ) and the rapid change of the wave number ( a p ) near d = 0.12. As Ra p increases, the cell size changes for all depth ratios ( d = 1.0, 0.5, 0.2, 0.1 and 0). In particular, heat transfer rate changes dramatically due to the effects of the aspect ratio and the corresponding number of cells when d = 0.1.

GROUND-BASED REMOTE SENSING OF WATER AND NITROGEN STRESS

A ground–based remote sensing system (Agricultural Irrigation Imaging System, or AgIIS) was attached to a linear–move irrigation system. The system was used to develop images of a 1–ha field at 1 U 1 m resolution to address issues of spatial scale and to test the ability of a ground–based remote sensing system to separate water and nitrogen stress using the coefficient of variation (CV) for water and nitrogen stress indices. A 2 U 2 Latin square water and nitrogen experiment with four replicates was conducted on cotton for this purpose. Treatments included optimal and low nitrogen with optimal and low water. ANOVA was not an adequate method to assess the statistical variation between treatments due to the large number of data points. In general, the coefficient of variation of water and nitrogen stress indices increased with water and nitrogen stress. In fact, the coefficient of variation of stress indices was a more reliable measurement of water and nitrogen status than the mean value of the indices. Differences in coefficient of variation of stress indices between treatments were detectable at 3 m grid resolution and finer for water stress and at 7 m grid resolution and finer for nitrogen stress.

Role of Irrigation Water in Crop Contamination by Viruses

Foods traditionally eaten raw or receiving minimal processing provide an ideal route for the transmission of viruses. Fruits and vegetables can potentially become contaminated before harvesting by irrigation water, water used for spray application of pesticides, or water used in processing (e.g., washing, hydrocooling with ice, etc.). An increase in the number of produce-associated outbreaks corresponds with the increased consumption of fresh fruits and vegetables and with the expanded global sources of these products over the past two decades (Sivapalasingam et al., 2004). Produceassociated outbreaks have increased from 0.7% of all outbreaks in the 1970s to more than 6% in the 1990s in the United States. In 2002, the number of cases of produce-associated illnesses was almost equal to all of those reported for beef, poultry, and seafood combined (Center for Science in the Public Interest, 2002). Several known and suspected food-borne outbreaks have been ascribed to crops contaminated in the field, suggesting contamination by irrigation or during harvesting (Dentinger et al., 2001; CDC, 2003). Perhaps more significant is the low-level transmission of viruses by food contaminated with irrigation water. Quantitative microbial risk analysis has suggested that low levels of virus in irrigation water can result in a significant level of risk of infection to consumers (Petterson et al., 2001). Stine et al. (2005c) estimated that less than one hepatitis A virus per 10L of irrigation water could result in a risk exceeding 1 :10,000 per year considering the efficiency of transfer of the virus to crop and its survival till harvest time. The 1 :10,000 risk of infection per year is currently the acceptable level used by the United States Environmental Protection Agency for Drinking Water (Regli et al., 1991). The largest use of freshwater in the world is in agriculture with more than 70% being used for irrigation.About 240 million ha, 17% of the world’s cropland, are irrigated, producing one third of the world’s food supply (Shanan, 1998). Nearly 70% of this area is in developing countries. In the United States, California and Arizona are the major producers of lettuce, carrots, broccoli, and cantaloupe (Arizona Farm Bureau, 2003).All of these crops are grown almost entirely by irrigated agriculture. It is thus surprising that we know little about the microbial quality of irrigation water. Most studies have dealt with the occurrence and fate of enteric pathogens in reclaimed water used for irrigation and not the quality of surface waters currently in use.

A numerical investigation of conjugate heat transfer from a flush heat source on a conductive board in laminar channel flow

Abstract A numerical investigation was conducted on the heat transfer from a uniformly powered strip source of heat located on the surface of a two-dimensional conducting substrate. The upper and lower surfaces of the substrate are cooled by forced laminar flow that is two-dimensional, steady and with constant properties. The problem is a paradigm for the investigation of the competing effects of substrate conduction and fluid convection in the cooling of electronic components, i.e. chips or chip carriers, on boards or substrates that are cooled by air flowing parallel to the surface. The objectives of the study were to investigate the conjugate heat transfer mechanisms in great detail and in a methodical way, such as to use the results as a baseline for successively more complex situations of air-cooling of on-board components. Results are: presented for the substrate conductivity to fluid conductivity ratio, ks/kf from 0.1 to 100, channel Reynolds number from order 100 to order 1000, corresponding to air velocities of order 1 m s−1, and for both developing and fully developed laminar, parallel-plane channel flow.

SUBSURFACE DRIP IRRIGATION FOR BERMUDAGRASS WITH RECLAIMED WATER

Subsurface drip irrigation was compared to sprinkler irrigation of bermudagrass turf in a multi-year experiment using reclaimed water. The experimental plots were established with sprinkler irrigation. During the following three irrigation seasons, plots were irrigated when the average soil water content in the upper 30 cm of soil decreased below 20% by volume, which corresponded to a management-allowed depletion value of 50%. The amount of irrigation water, the visual appearance of the turf, shoot biomass production, soil salinity, and health risks due to reclaimed water reuse were examined. Management problems such as emitter clogging were identified. No emitters were completely clogged, and emitter clogging was not serious enough to impact visual quality. Statistical uniformity of emitters was reduced from 91.8% (for new emitters) to 85.3% after the first year and 86.2% after the third year, while flow rates remained at 3.75, 3.78, and 3.89 L/h, respectively. The amount of water applied in the subsurface drip irrigated plots to maintain acceptable visual quality was similar to that applied in the sprinkler irrigated plots. Increases in the electrical conductivity of the soil surface of the subsurface drip irrigated plots were noted after the first and second seasons. However, these increases were not high enough to negatively affect turf appearance. Inspection of the emitters at the end of the irrigation seasons found signs of root intrusion into the emitters, which may pose a threat to the long-term use of subsurface drip irrigation.

COMPARISON OF SUBSURFACE DRIP IRRIGATION AND SPRINKLER IRRIGATION FOR BERMUDA GRASS TURF IN ARIZONA

A subsurface drip irrigation (SDI) system is potentially efficient because it provides water directly to the root zone, minimizing evaporative loss, especially in arid lands. In this study, subsurface drip irrigation was compared to standard overhead sprinkler irrigation of Bermuda grass turf using reclaimed water. Research focused on the response of Bermuda grass to two irrigation treatments, subsurface drip irrigation and overhead sprinkler irrigation. Soil moisture content was calculated via time domain reflectometry (TDR) and neutron probe data. The remotely sensed crop water stress index (CWSI) could not be used to schedule irrigation. When the average soil moisture of all eight plots was depleted to 50% of readily available water, they were irrigated until the soil moisture content reached field capacity. No significant differences were observed between the two irrigation systems in total irrigation depth, relative root weight, dry clipping mass per unit area, or visual quality. The electrical conductivity (EC) of a soil water extract measured at the beginning and end of the season indicated salt accumulation near the ground surface in the subsurface plots, but there was not sufficient accumulation to affect the appearance of turf. Visual inspection of emitters after one irrigation season showed signs of root intrusion because of water stress in certain plots with high surface sand content.

TRANSPORT PHENOMENA AT INTERSECTIONS OF PRESSURIZED PIPE SYSTEMS

The movement of chemicals or biological agents in a water distribution system is examined via computational fluid dynamics simulations. A series of computational simulations using selected intersecting geometries are carried out at various Reynolds numbers. Boundary conditions, turbulence intensities, convergence criteria, and mesh sizes are thoroughly evaluated. The present parametric study focuses particularly on pipe intersections to characterize complex mixing phenomena in pressurized water distribution pipe networks. Selected computational results are compared with experimental results. The water quality model integrated with an existing computer program (EPANET) was re-evaluated based on the computational and experimental data. Initial computational fluid dynamics simulations consistently underestimated mixing, and experimental data are utilized to reexamine turbulent mixing by adjusting the turbulent Schmidt number. Corrections based on computational results are incorporated into the existing code as an example case study. The improvement of the existing code may be important not only to predict concentrations of chemical species such as chlorine in water distribution systems, but also to prepare for potential intentional and accidental contamination events. Computational results must be further calibrated and verified through lab- and field-scale experiments.