Loading...
Removal of nitrate in raw water using a vertical roughing filter with an external carbon source
Author(s)
Mohobane, Lekhooa Ford
Date Issued
2022
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
High accumulation of nitrate above the recommended maximum guideline value has become a
common problem in most water supply sources. According to the World Health Organization
(WHO), about 30% of water supply sources in the world exceed the maximum nitrate
contamination level of 11 mg/L-N / 50 mg/L-NO3. Consumption of water with high nitrate
concentration poses health hazards to both humans and livestock. Several technologies such as
reverse osmosis and electrodialysis, have been adopted in removing nitrate from raw water.
However, they have drawbacks that include the production of high strength residual brine and low
efficiency. Nonetheless, biological denitrification has proved to be an effective technology for
nitrate removal and the process can be enhanced by adding an external carbon source.
Denitrification in roughing filters has not been widely studied, except in bio-filters and slow sand
filters. This research aimed to investigate the efficacy of roughing filters enhanced by an external
carbon source in removing nitrate in raw water. Two upward vertical roughing filters in series
(UVRFs) were used, one was a vertical roughing filter with ethanol as a carbon source (VRFwt)
and the other was a vertical roughing filter without a carbon source (VRFwo). The inflow and
outflow of nitrate and other physicochemical parameters were monitored to evaluate their
influence on a roughing filter’s performance in removing nitrate in raw water. The carbon: nitrogen
ratios (C/N ratios) of 1.05, 1.08 and 1.1, were investigated, coupled with a nitrate removal kinetic
model. Furthermore, filter design parameters and the effect of biomass on flow rate were also
studied.
The average nitrate removal efficiency in a vertical roughing filter with a carbon source was 88%,
70%, and 83%, for carbon: nitrogen ratios (C/N ratios) of 1.05, 1.08, and 1.1, respectively. The
drop-in flow rate was 27% for a vertical roughing filter with a carbon source (VRFwt) and was
attributed to the biological layer growth, whereas a 15% decline was observed in the vertical
roughing filter without a carbon source (VRFwo). The decrease in flow rate was evident at 30-35
days from the start of the filter operation. The removal efficiency was 75%, 43%, and 46% at C/N
ratios of 1.05,1.08 and 1.1, respectively. The residual ethanol measured as chemical oxygen
demand (COD) in the filter with an external carbon source (VRFwt) ranged between 85 mg/L to
632 mg/L during the filter run. The average residual ethanol measured as COD during the filter
rest period ranged between 41 mg/L and 561 mg/L with a removal efficiency of 88%, 49% and
53% at C/N ratios of 1.05,1.08 and 1.1, respectively. The overall average reduction of dissolved
oxygen (DO) in the VRFwt at C/N ratios of 1.05, 1.08 and 1.1 was 42%, 54%, and 51%
respectively, while DO reduction in the VRFwo was 17% 18% and 17%, respectively. A decline in
DO was profound in the VRFwt compared to the VRFwo.
The VRFwt showed a high potential for removing nitrate in raw water for potable use. Therefore,
when the VRFwt is applied at large scale, it will increase access to water sources that were initially
rendered unsuitable to many water utilities due to high nitrate concentrations; thereby increasing
their water supply. Importantly, the lack of nitrate in potable water would minimize water-related
diseases induced by the use of high nitrate-rich water. Again, the reaction rate order (n) and
reaction rate constant (k) determined from the nitrate removal kinetic model can help in assessing
the total nitrate removal rate and efficiency in a vertical roughing filter, without the need to operate
the filter, thus saving time and money.
common problem in most water supply sources. According to the World Health Organization
(WHO), about 30% of water supply sources in the world exceed the maximum nitrate
contamination level of 11 mg/L-N / 50 mg/L-NO3. Consumption of water with high nitrate
concentration poses health hazards to both humans and livestock. Several technologies such as
reverse osmosis and electrodialysis, have been adopted in removing nitrate from raw water.
However, they have drawbacks that include the production of high strength residual brine and low
efficiency. Nonetheless, biological denitrification has proved to be an effective technology for
nitrate removal and the process can be enhanced by adding an external carbon source.
Denitrification in roughing filters has not been widely studied, except in bio-filters and slow sand
filters. This research aimed to investigate the efficacy of roughing filters enhanced by an external
carbon source in removing nitrate in raw water. Two upward vertical roughing filters in series
(UVRFs) were used, one was a vertical roughing filter with ethanol as a carbon source (VRFwt)
and the other was a vertical roughing filter without a carbon source (VRFwo). The inflow and
outflow of nitrate and other physicochemical parameters were monitored to evaluate their
influence on a roughing filter’s performance in removing nitrate in raw water. The carbon: nitrogen
ratios (C/N ratios) of 1.05, 1.08 and 1.1, were investigated, coupled with a nitrate removal kinetic
model. Furthermore, filter design parameters and the effect of biomass on flow rate were also
studied.
The average nitrate removal efficiency in a vertical roughing filter with a carbon source was 88%,
70%, and 83%, for carbon: nitrogen ratios (C/N ratios) of 1.05, 1.08, and 1.1, respectively. The
drop-in flow rate was 27% for a vertical roughing filter with a carbon source (VRFwt) and was
attributed to the biological layer growth, whereas a 15% decline was observed in the vertical
roughing filter without a carbon source (VRFwo). The decrease in flow rate was evident at 30-35
days from the start of the filter operation. The removal efficiency was 75%, 43%, and 46% at C/N
ratios of 1.05,1.08 and 1.1, respectively. The residual ethanol measured as chemical oxygen
demand (COD) in the filter with an external carbon source (VRFwt) ranged between 85 mg/L to
632 mg/L during the filter run. The average residual ethanol measured as COD during the filter
rest period ranged between 41 mg/L and 561 mg/L with a removal efficiency of 88%, 49% and
53% at C/N ratios of 1.05,1.08 and 1.1, respectively. The overall average reduction of dissolved
oxygen (DO) in the VRFwt at C/N ratios of 1.05, 1.08 and 1.1 was 42%, 54%, and 51%
respectively, while DO reduction in the VRFwo was 17% 18% and 17%, respectively. A decline in
DO was profound in the VRFwt compared to the VRFwo.
The VRFwt showed a high potential for removing nitrate in raw water for potable use. Therefore,
when the VRFwt is applied at large scale, it will increase access to water sources that were initially
rendered unsuitable to many water utilities due to high nitrate concentrations; thereby increasing
their water supply. Importantly, the lack of nitrate in potable water would minimize water-related
diseases induced by the use of high nitrate-rich water. Again, the reaction rate order (n) and
reaction rate constant (k) determined from the nitrate removal kinetic model can help in assessing
the total nitrate removal rate and efficiency in a vertical roughing filter, without the need to operate
the filter, thus saving time and money.
Additional information
Thesis (MEng (Civil Engineering))--Cape Peninsula University of Technology, 2022
File(s)![Thumbnail Image]()
Loading...
Name
Mohobane_Lekhooa_216011280.pdf
Size
12.11 MB
Format
Adobe PDF
Checksum
(MD5):4022ffb0b0de9a707df0bb9237a4b3d5