Department of Water Resources Engineering

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Browsing Department of Water Resources Engineering by Author "Bigambo, T."

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    Nitrogen Transformation in Horizontal Subsurface Flow Constructed Wetlands I: Model Development
    (Elsevier, 2005) Mayo, Aloyce W.; Bigambo, T.
    In this paper a mathematical model for prediction of nitrogen transformation in horizontal subsurface flow constructed wetlands was developed. Two horizontal subsurface flow constructed wetlands were designed to receive organic loading rate below 50 kg/ha/d and hydraulic loading rate of 480 m3 /ha/d from a primary facultative pond. Two rectangular shaped units each 11.0 m long,3.7 m wide and 1.0 m deep and bottom slope of 1% were constructed and filled with 6–25 mm diameter gravel pack to a depth of 0.75 m. Each unit was planted with Phragmites mauritianus with an initial plant density of 29,000 plants/ha. The plants were allowed to grow for about four months before sampling for water quality parameters commenced. Samples were collected daily for about three months. Dissolved oxygen,pH and temperature were measured in situ and ammonia,total Kjeldahl nitrogen,nitrates,nitrite and Chemical Oxygen Demand were measured in the laboratory in accordance with Standard Methods. The mathematical model took into account activities of biomass suspended in the water body and biofilm on aggregates and plant roots. The state variables modelled include organic,ammonia,and nitrate–nitrogen,which were sectored in water,plant and aggregates. The major nitrogen transformation processes considered in this study were mineralization,nitrification,denitrification,plant uptake, plant decaying,and sedimentation. The forcing functions,which were considered in the model,are temperature,pH and dissolved oxygen. Stella II software was used to simulate the nitrogen processes influencing the removal of nitrogen in the constructed wetland. One of the two-wetland units was used for model calibration and the second unit for model validation. The model results indicated that 0.872 gN/m2 d was settled at the bottom of the wetland and on gravel bed and roots of the plants. However,0.752 gN/m2 d (86.2%) of the settled nitrogen was regenerated back to the water body,which means that only 13.8% of the settled nitrogen was permanently removed. Denitrification and nitrification were responsible for transformation of 0.436 gN/m2 d and 0.425 gN/m2 d,respectively. Uptake of nitrogen by plants was 0.297 gN/m2 d out of which 0.140 gN/m2 d was returned to the water body as plants decay. It was found that the major pathways leading to permanent removal of nitrogen in a horizontal subsurface flow constructed wetland system in descending order are denitrification (29.9%),plant uptake (10.2%) and net sedimentation (8.2%). A total nitrogen removal of 48.9% was achieved in this study.
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    Nitrogen Transformation in Horizontal Subsurface Flow Constructed Wetlands II: Effect of Biofilm
    (2005) Bigambo, T.; Mayo, Aloyce W.
    In this paper the significance of the biofilm biomass present in horizontal subsurface flow constructed wetland in removal of nitrogen was demonstrated. The model was developed and optimised using data obtained in a horizontal subsurface flow constructed wetland planted with Phragmites mauritianus and filled with 6–25 mm diameter gravel pack. The effects of biofilm biomass activities were studied by removing the effects of plant and gravel bed biofilm in an already calibrated model and re-run the same. Research results indicate that total nitrogen removal was largely influenced by growth of biofilm on plants than on aggregates. When plant biofilm and suspended biomasses were considered total nitrogen removal of 38.1% was observed compared with 25.1% when aggregate-biofilm and suspended biomasses were considered because plants have more surface areas, which are active sites for the effective biofilm activities than aggregates. However, in a natural wetland where the soil grain size is smaller, the effect of biofilm on plants may be smaller than biofilm on soil particles. There was no significant difference in organic-nitrogen effluent concentrations when biofilm biomass was considered or rejected. The averages in organic-nitrogen effluent concentrations were 0.39, 0.41 and 0.53 gN/m2 for suspended alone, aggregate-biofilm and suspended; and suspended and plant-biofilm, respectively. This indicates that the removal of organic-nitrogen in wastewater is not significantly influenced by biofilm activities. Sedimentation and mineralization processes are the major factors influencing the concentration of organic-nitrogen in the effluent. On the other hand, biofilm activities had significant influence on ammonia–nitrogen and nitrate–nitrogen transformation. The developed model output indicates that the effluent ammonia concentration was 2 gN/m2, but in absence of biofilm the effluent ammonia concentration increases to 3.5 gN/m2. Statistical analysis indicates that the mean average nitrogen–nitrogen in the effluent was 0.71 gN/m2 when aggregate-biofilm was considered, but increased to 0.83 gN/m2 when it was not considered.