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This paper presents the results of DNA-based molecular analyses of the microbial community responsible for biological iron (Fe) and manganese (Mn) removal in slow sand filters (SSF). A lab-scale SSF was operated in 55-day sets under different operating conditions in order to evaluate long-term performance of the filter. The concentrations of Fe and Mn in synthetic feed water were increased from 1 mg/L to 2 mg/L at two different filtration rates (0.1 and 0.3 m/h). Daily samples were taken from influent and effluent for turbidity and Fe-Mn concentration measurements. 90-95% removal efficiencies were achieved with very low effluent concentrations. PCR-DGGE analyses were performed on samples, and Gallionella, Leptothrix, Crenothrix, and Hyphomicrobium were identified as the main microbial strains responsible for iron and manganese oxidation in SSF. Results also revealed that microbial activity was the main mechanism for Fe and Mn removal in the early stages of operation.
This paper presents results from a research study in which the effects of steepness coefficient (S) for the activation function of a back propagation neural network (BPNN) were investigated, and optimum values of S for each activation function were suggested for environmental modeling purposes. A BPNN algorithm was implemented in Excel Visual Basic for Applications with built-in activation functions of sigmoid, hyperbolic tangent, and sinc. Various steepness coefficients were employed for modeling cyclone Euler numbers for pressure drop estimation with three different activation functions. Best results for sigmoid function were obtained for S = 1.00 with a median value of mean square errors (MSEs) of 4.33*10-4. For hyperbolic tangent function, the optimum value of S was found as 0.2 with a median MSE value of 2.02*10-4. The median value of MSEs obtained with BPNN sinc function was 1.20*10-3 for S = 0.50. Results showed, for environmental modeling problems, that any activation function can be used with satisfactory results provided that an optimized value of the steepness coefficient is used, which is considered problem specific.
A total of 162 cyclones with distinct geometries were used to obtain experimental pressure drops at six different inlet velocities between 10 and 24 m/s. Pressure drops were measured between 84 and 2,045 Pa. Pressure drop coefficients were calculated by the well-known formulation of a cyclone pressure drop. The values ranged between 1.09 and 9.07, with an average of value of 3.76. A backpropagation neural network algorithm was implemented in Visual Basic for Applications with nine built-in activation of linear, rectified linear, sigmoid, hyperbolic tangent, arctangent, Gaussian, Elliot, sinusoid, and sinc functions to test their ability to satisfactorily explain the complex relationship between cyclone geometry and the pressure drop coefficient. The neural network was run 25 times for each activation function with randomly selected 70% of data set as the ratios of inlet height, cylinder height, cone height, vortex finder diameter, and vortex finder length-to-body diameter being the independent variables, and the pressure drop coefficient being the dependent variable. Neural network results showed that sigmoid was the one activation function that explains the complex relationship between cyclone geometry and pressure drop coefficient with an average mean square error (MSE) of 0.00085. The coefficients of determination between measured and predicted values of pressure drop coefficient were over 0.99. Also, the percent residuals from sigmoid activation function concentrated around the mean value of zero, with very small standard deviation.
A laboratory-scale slow sand filtration (SSF) system was used to investigate biomass formation in different depths of SSF depending on various operating conditions in regard to filtration rate and influent iron-manganese concentrations. Results suggest that biomass formation occurs mainly in the uppermost 1.5 cm of the filter bed with slight contributions from layers between 1.5 cm and 14.5 cm. The highest volatile solids (VS) accumulation was observed in the uppermost layer as 16.93±0.07 mgVS/g dry sand, and the accumulation was found to be a function of both filtration rate and influent iron-manganese concentrations. Hydraulic conductivities were tested as a measure of biomass formation. The highest initial value of hydraulic conductivity was measured as 13.7 μm/s, while the lowest values ranged from 3.28 to 6.62 μm/s at the end of 55 days of operation. Hydraulic conductivities of the upper layers decreased quickly with time, while slight reductions were observed in hydraulic conductivities of the lower layers.
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