Browsing by Author "Duncan, Carla S."
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- Carbon and nitrogen dynamics on a forest site receiving continual papermill sludge applications: a soil column studyDuncan, Carla S. (Virginia Tech, 1992-12-15)Successful renovation of wastewater and sludge via land application depends upon sludge-induced soil changes associated with carbon (C) and nitrogen (N) cycles within the soil/plant system. The C, N, and hydrologic cycles within a soil/sludge system receiving a year-round, daily application of paper mill sludge were studied. Soil samples were collected from three locations on a land application site in the Piedmont of Virginia that had received papermill sludge applications for six, two, and no prior years. The average application rate was 4.4 cm/wk, each week of the year, with a N loading of 700 kg N ha⁻¹yr-⁻¹. The column study showed that C and N were still accumulating on the land application site after 6 years, but at a decreasing rate. Based on this study, C accumulation will level out after 13 years of application, but N will continue to accumulate for almost 30 years. As application period increased, soil bulk density increased in the O, A, and B horizons, the percentage of non-capillary porosity fell below 10% in the A horizon and approached zero in the B horizon, and there was a dramatic decrease in the soil's hydraulic conductivity in both the A and B horizons. Nitrogen leaching is expected to increase with time due to high amounts of N in the papermill sludge, a continued narrowing of the C:N ratio, a high percentage of nitrification, and low denitrification rates. Experimental timing and rates of sludge additions were imposed to alter the aerobic/anaerobic properties of the soil system to determine the conditions under which optimum C and N mineralization, nitrification, and denitrification would occur. Application rates were factorially arranged for single or multiple doses on a daily or alternating schedule. The C decomposition and N mineralization processes were both optimized with an increase in the length of cycle; they were maximized with an alternating 9 days on/9 off application schedule. The nitrification potential also increased with the length of cycling, with an average nitrification rate of 96%. Denitrification was minimal in all treatments, with an average denitrification rate of 16%. This was primarily attributed to movement of nitrate-N below the most biologically active zone in the soil column. Sludge renovation will ultimately depend upon the excess N being sequestered in plant biomass or denitrified. Proper management of these processes will ensure that wastes decompose, and that N is stored or evolved as a benign gas rather than leached at unacceptable levels.
- Wastewater renovation with soil depth as influenced by additional treatment of septic tank effluentDuncan, Carla S. (Virginia Tech, 1994-07-15)Many soils are marginally suited for installation of on-site wastewater disposal systems. With soil limitations, additional wastewater treatment prior to soil application may allow for a reduction in soil depth. Undisturbed 20-cm-diameter soil columns (fine loamy, mixed, mesic Typic Hapludult), in a factorial arrangement between depth of soil (15, 30, and 45 cm) and type of effluent (septic tank, constructed wetlands, and recirculating sand filter), were used in this study. Effluent (670 cm³/d) was applied 6 times daily. Additional treatment of septic tank effluent by a constructed wetland and a recirculating sand filter resulted in 30 and 70% higher average soil infiltration rates, 92 and 96% reduction in fecal coliforms, 34 and 44% reduction in total nitrogen, and a 60 and 94% reduction in BOD₅, respectively. Fecal coliforms were present only in soil leachate from the 15 and 30 cm soil depths receiving septic tank effluent and the 15 cm depth that received constructed wetland effluent. Average soil leachate NO₃⁻-N concentrations were 19, 10 and 14 mg/L from soil columns receiving septic tank, constructed wetland, and recirculating sand filter effluents, respectively. Soil leachate contained <5 mg/L TKN and 1.8 mg/L NH4⁺-N. Total nitrogen losses were 55, 73, and 66 for the septic tank, constructed wetland, and recirculating sand filter treatments, respectively. BOD₅ averaged less than 4 mg/L in the soil column leachate, despite a 10 fold difference among influent types. In comparing the 1993 and 1994 growing seasons, average plant tissue dry weight, percent nitrogen, and percent phosphorus were greater during the 1994 growing season. The results from this study indicate that additional treatment of septic tank effluent can be substituted for soil depth.