The purpose of this study was to investigate the role of low-temperature conditions in potentially impacting soluble Mn2+ removal in MnOx(s)-coated media systems. The study involved an extensive examination of water quality and process data for a 10-year period (2005–2014) from the Harwood's Mill Water Treatment Plant in Newport News, VA. Additionally, laboratory-scale shallow depth column studies were performed employing MnOx(s)-coated media for soluble Mn2+ removal. Data were evaluated for the impacts of low temperature conditions, water pH, soluble Mn2+ concentration, and free chlorine concentration on the ability of MnOx(s)-coated media to promote effective soluble Mn2+ removal. The results show that the uptake of soluble Mn2+ onto MnOx(s)-coated filter media is significantly impacted by water temperatures below 15 °C, with impacts being very substantial under lower temperature (T < 10 °C) conditions. Also, the adverse effects of low temperature on MnOx(s)-coated treatment processes can be at least partially mitigated by an increase in solution pH.
INTRODUCTION AND RESEARCH OBJECTIVES
The removal of soluble manganese (Mn) in water treatment facilities is often necessary to minimize the potential for water discoloration and complaints by consumers. While aesthetic (discoloration) issues have been the primary motivation historically for practising Mn removal research, studies have documented potential health effects due to elevated Mn in drinking water (Liu 2005; Menezes-Filho et al. 2011). Such concerns have prompted Health Canada (2016) to propose a legally enforceable, health-based drinking water standard of 0.10 mg/L. The US Environmental Protection Agency likewise added Mn to its most recent promulgation of the EPA Critical Contaminants List (CCL4).
A common treatment process for soluble Mn2+ removal is the natural greensand effect (NGE) during granular media filtration, which has proven to be very effective over a wide range of raw water conditions (Griffin 1960; Knocke et al. 1988, 1991; Coffey et al. 1993; Hu et al. 2004; Brandhuber et al. 2013). During the NGE process, soluble Mn2+ ions are adsorbed to manganese oxide-coated (MnOx(s)) media. The adsorbed Mn2+ is then oxidized by chlorine, forming more MnOx(s) to become part of the coating structure. The MnOx(s) coating level is increased over time through a continuous process of Mn2+ adsorption and oxidation, and the resulting media is available for the adsorption of additional soluble Mn2+ ions (Merkle et al. 1997a, 1997b). Studies have shown that the adsorptive uptake process for soluble Mn2+ on MnOx(s)-coated media depends on: (1) the presence of a free chlorine residual in the filter-applied water, (2) a filter-applied pH of greater than 6.0, and (3) the presence of soluble Mn2+ for adsorptive uptake (Brandhuber et al. 2013). While such coatings typically develop naturally over time, researchers have demonstrated means by which MnOx(s) coatings can be deposited on anthracite coal and sand media at significantly faster rates than occur naturally in water treatment situations (Merkle et al. 1997b; Bazilio et al. 2016).
The primary purpose of this study was to further investigate the potential of low-temperature conditions to impact the removal of soluble Mn2+ in MnOx(s)-coated media systems. The specific objectives of the study were as follows:
Evaluate the ability of MnOx(s)-coated media to promote effective soluble Mn2+ removal as a function of water temperature, and to determine how observed Mn removal varies with pH conditions and free chlorine concentration.
Identify a potential way to mitigate any potential negative temperature effects through process modifications. The primary process modification considered was pH adjustment of the water being applied to the MnOx(s)-coated filter media.
To accomplish the stated objectives, water quality and process data for a 10-year period (2005 to 2014) were obtained from the Harwood's Mill Water Treatment Plant in Newport News, VA. Additionally, data were obtained from laboratory-scale column studies that employed shallow depths (up to 25 cm (10 inches)) of MnOx(s)-coated filter media for soluble Mn2+ removal. Data were evaluated for the impacts of low temperature conditions, water pH, and free chlorine concentration on the ability of MnOx(s)-coated media to promote effective soluble Mn2+ removal.
EXPERIMENTAL METHODS AND MATERIALS
This research effort was closely coordinated with the Newport News Water Authority in Newport News, Virginia. Harwood's Mill Water Treatment Plant is one of two major water treatment facilities operated by the Authority. The unit operations utilized at the Harwood's Mill plant include pre-oxidation/reaction (seasonal use), flash mixing/coagulation, ultra-high rate clarification, high rate filtration, pH adjustment and corrosion inhibitor chemical addition, and finished water storage. The filters consist of approximately 1 m of media, with approximately two thirds of the media depth being anthracite coal and the remaining depth in silica sand. The hydraulic loading rate (HLR) under normal operations is approximately 5 m/hr (2 gal/min-ft2). Treatment at the Harwood's Mill plant includes the addition of free chlorine to the filter-applied water, resulting in the uptake of soluble Mn2+ across the filters and a substantial accumulation of MnOx(s) coating on the filter media. This coating helps promote very effective soluble Mn2+ removal (Brandhuber et al. 2013). While the US Environmental Protection Agency has promulgated a secondary (suggested; non-enforceable) maximum contaminant level for Mn in drinking water of 0.05 mg/L, the Authority has set its own operational goal for finished water Mn concentration of 0.015 mg/L. This Mn goal was established to lower the total Mn loading released annually to the distribution system, with hopes to reduce the potential for chronic buildup of Mn in the system and minimize the potential for consumer complaints related to water discoloration. Such an operational goal is based upon the published work of Sly et al. (1990) and Kohl & Medlar (2006) who recommended water utilities keep finished water Mn concentrations at or below 0.02 mg/L to address water discoloration complaint potential.
Water quality and process data for a 10-year period (2005–2014) were obtained from plant personnel at the Harwood's Mill Water Treatment Plant. Data were evaluated for the impacts of low temperature conditions, water pH, soluble Mn2+ concentration, and free chlorine concentration on the ability of MnOx(s)-coated media to promote effective soluble Mn2+ removal. The data collected were stored in Microsoft Excel, which was used for data organization and comparison. JMP 12.1.0 statistical discovery software was used for statistical analysis (SAS Institute).
A statistical analysis (one-way analysis of variance) determined that differences in free chlorine residual (after water filtration) did not significantly impact the filter effluent soluble Mn2+ concentration as long as the free chlorine concentration was at least 0.5 mg/L. As such, the analysis for the Harwood's Mill treatment facility utilized only data wherein the free chlorine concentration was above 0.5 mg/L (which represented 98% of the overall values in the 10-year data set, since the Authority seeks to maintain a filtered water free chlorine concentration above 0.5 mg/L). Values for finished water soluble Mn2+ concentrations under conditions with a filtered water free chlorine residual less than 0.5 mg/L were not included in the data evaluation.
Laboratory-scale filter column experiments were also conducted as part of this research effort. Small-diameter (1.1 cm (7/16 inch) ID) glass columns were filled to a depth of 25 cm (10 in) with fully regenerated (using contact with free chlorine), MnOx(s)-coated anthracite coal media from the Harwood's Mill Water Treatment Plant. The media had approximately 40 mg of extractable Mn present in the MnOx(s) coating per g of media (Mn extraction was performed using sample processing and analysis methods described by Tobiason et al. (2008)).
Two separate feed solutions were prepared and stored for feeding to the test columns. One contained the soluble Mn2+ feed and associated background ion concentrations. The second solution contained the same background ion concentrations and free chlorine at a concentration to produce the desire free chlorine dosage once the solutions were mixed just prior to addition to the column of media. The combined flowrate resulted in a HLR to the columns of approximately 10 m/hr (4 gpm/ft2). This HLR yielded an empty bed contact time (EBCT) of 0.6 minutes in the 10-cm media depth column, and 1.6 minutes in the 25-cm media depth column. The columns were set up initially under room temperature (20–22 °C) for the first part of the experiment. An incubator and temperature control room were utilized to evaluate lower temperature conditions.
Column influent water quality characteristics were controlled to ensure that water applied to the MnOx(s)-coated media was similar to filter-applied water for the Harwood's Mill facility. Influent soluble Mn2+ concentrations varied between 0.15–0.20 mg/L and involved addition of Mn from a stock solution prepared by dissolving manganese chloride tetrahydrate (MnCl2·4H2O) in high-purity distilled/deionized water. The alkalinity (from sodium bicarbonate addition) was targeted at 30–40 mg/L as CaCO3. The desired range of free chlorine concentrations was 2.0–2.5 mg/L in order to achieve an effluent free chlorine concentration of approximately 1.0 mg/L. The pH values ranged from 5.7 to 7.9 and involved addition of dilute hydrochloric acid as needed to achieve the desired pH value in a given column experiment.
A HACH DR/2700 benchtop spectrophotometer was used to measure free chlorine concentration using the N,N-diethyl-p-phenylene-diamine method (HACH Method 8021) and reagents with a maximum detection limit of 2.0 mg/L as Cl2. A Thermo Electron Corporation X-Series inductively coupled plasma mass spectrometer (ICP-MS) was used for soluble Mn2+ analysis. Ten mL of sample were collected in capped plastic ICP sample tubes and acidified to 2% nitric acid. Samples were analyzed using the laboratory method, which uses standard SM-1517-014. Limited testing of samples collected following passage through the column of media showed that it was not necessary to further process the samples by filtration prior to Mn analysis; instead, these tests demonstrated that it was reasonable to assume that any Mn present in the column effluent was soluble Mn2+.
The solution pH and temperature were measured with a portable HACH multi-parameter Model HQ40d meter and probe. The meter was calibrated before each use with buffers of pH 4, 7, and 10. For the low-temperature experiments, buffer solutions were stored at the same temperature as the corresponding column experiment, and the pH meter was calibrated at that temperature. Data from ICP-MS output as well as data collected in the laboratory were stored in a carbon copy notebook as well as in Microsoft Excel for organization and analysis.
Evaluation of Mn data from the full-scale Harwood's Mill water treatment facility
Laboratory-scale column results
A series of experiments was conducted in small-scale filter columns that contained samples of MnOx(s)-coated anthracite coal that was obtained from a full-scale filter at the Harwood's Mill Water Treatment Plant. The media was initially exposed to a high-strength (at least 10 mg/L as Cl2) free chlorine solution to regenerate the MnOx(s) coating present; likewise, the media was maintained with approximately 1 mg/L free chlorine residual in the filtrate water during all experiments as this free chlorine concentration helps to ensure effective MnOx(s) surface regeneration (Brandhuber et al. 2013). Samples were collected for analysis at both 10-cm and 25-cm media depths during these column experiments. These media depths were well below the standard media depths used in full-scale, dual-media water filters (typically from 0.7 to 1 m (24 to 36 inches) of media); however, they were selected to help ensure the presence of significant residual Mn concentrations after exposure to the MnOx(s)-coated media in the columns so that the effect of various parameters on soluble Mn2+ removal could be better evaluated.
|.||Soluble Mn concentration (μg/L)|
|pH range .||10-cm media depth .||25-cm media depth .|
|.||Soluble Mn concentration (μg/L)|
|pH range .||10-cm media depth .||25-cm media depth .|
ENGINEERING SIGNIFICANCE OF RESEARCH FINDINGS
A primary goal of this study was to provide useful guidance to water process engineers and water treatment plant personnel on the conditions that will allow MnOx(s)-coated treatment systems to operate effectively for soluble Mn2+ control even under low temperature conditions. For example, one process control parameter in the NGE is the concentration of free chlorine present in the water applied to MnOx(s)-coated filter media. Conceivably a response to colder water temperature conditions could be to increase the free chlorine dosage prior to the coated media. However, analysis of the 10 years of Harwood's Mill Water Treatment Plant operational data showed that for the same pH condition the filter effluent Mn concentrations for residual free chlorine concentrations between 0.5 and 1.0 mg/L were not statistically different to those with a free chlorine residual greater than 1.0 mg/L. Hence, as long as the filter free chlorine residual was maintained at or above 0.5 mg/L there was no discernible impact on Mn removal within the filter media.
In comparison, experimental results showed that both solution pH and temperature were significant for defining soluble Mn2+ removal using the MnOx(s)-coated media process; further, the impacts of pH on soluble Mn2+ removal were magnified under low temperature (<10 °C) conditions. Information presented in Table 2 shows the percentage of finished water soluble Mn2+ concentration values that exceeded the recommended Newport News Water Authority goal of 0.015 mg/L when sorted by pH and temperature for the full-scale Harwood's Mill plant data. As shown in Table 2, the percentage of finished water soluble Mn2+ concentration values exceeding the goal generally decreased as the solution temperature increased. Likewise, the percentage of finished water soluble Mn2+ concentration values exceeding the goal of 0.015 mg/L decreased as the pH increased within each temperature range. Finally, results presented in Table 2 also demonstrate that an increase in solution pH may at least partially mitigate the negative effects of low water temperature on Mn removal using MnOx(s)-coated filter media treatment systems.
|.||Solution pH value|
|Temperature .||5.7–6.0 .||6.0–6.3 .||6.3–6.6 .||> 6.6 .|
|.||Solution pH value|
|Temperature .||5.7–6.0 .||6.0–6.3 .||6.3–6.6 .||> 6.6 .|
The water treatment significance of these findings is of specific importance to a limited number of treatment situations. The reality is that elevated soluble Mn concentrations in surface water treatment situations are typically seen during the warmer months of the year (e.g., summer and early to mid-fall, prior to overturn and subsequent mixing in a stratified lake or reservoir). At these times of the year, water temperatures are typically at or above 15 to 20 °C, a temperature condition that would predict highly effective soluble Mn2+ removal by the MnOx(s)-coated media. For example, at the Harwood's Mill treatment facility the filtered water Mn concentration in the non-winter months is typically below 0.010 mg/L, and sometimes below the analytical detection limit for Mn. However, on occasion during the winter months (when water temperatures are typically well below 12–15 °C) there will be upsets in the Harwood's Mill Reservoir (e.g., storm events) that result in a limited-duration episode of elevated soluble Mn2+ entering the treatment plant. The full-scale data as well as the laboratory-scale column studies demonstrate that, under low water temperature conditions, this Mn removal process is less effective and can yield finished water Mn concentrations that significantly exceed the Authority's 0.015 mg/L target concentration. The data also indicate that, under these treatment conditions, in the future the Authority could consider pH adjustment in the filter-applied water to improve soluble Mn2+ removal across their filters.
This research effort was not designed to evaluate the specific mechanisms by which low temperature conditions negatively impact soluble Mn2+ removal on MnOx(s)-coated filter media; instead, the primary focus was on documenting that such an impact does exist. However, it is possible to at least speculate on how temperature could affect the process. Soluble Mn2+ removal on MnOx(s)-coated media is known to be a two-step process involving initial soluble Mn adsorption onto the MnOx(s) surface followed by subsequent surface-catalyzed oxidation of the Mn by free chlorine (Knocke et al. 1991; Coffey et al. 1993; Merkle et al. 1997b). Prior research (Tobiason et al. 2008) showed that the Mn adsorptive capacity of MnOx(s)-coated media from the Harwood's Mill Water Treatment Plant was quite substantial, owing to the large amount of MnOx(s) surface coating present on the media at the facility. This large capacity typically resulted in almost complete soluble Mn uptake within the first 6–12 inches of media depth in the filter. Thus, even if lower temperature conditions reduced the Mn adsorptive capacity of the media to some degree, the overall depth of coated media (typically 30 to 36 inches) should have been sufficient to still achieve the 0.015 mg/L Mn level in the treated water. Instead, it may be more likely that lower temperature conditions negatively impact the surface-catalyzed Mn oxidation step. If this regeneration step is slowed, then the adsorptive capacity of the MnOx(s)-coated media would in fact decline over time and could lead to increased Mn levels in the filtered water. Again, these thoughts are purely speculative as an evaluation of the direct effect of temperatures on the mechanisms of the removal process was beyond the scope of this research.
The following conclusions were formulated based upon the results obtained in this research effort:
The uptake of soluble Mn2+ onto MnOx(s)-coated filter media is negatively impacted by water temperature conditions below 12–15 °C, with the most extreme impacts observed under very low water temperature conditions (4–7 °C).
The adverse effects of low water temperature on soluble Mn2+ removal within MnOx(s)-coated media systems may be at least partially mitigated by increasing the pH of the water applied to the MnOx(s)-coated media.
Finished water soluble Mn concentration is not significantly affected by free chlorine residual concentration as long as it is greater than 0.5 mg/L.
The authors wish to acknowledge the important help from W. Scott Dewhirst and Randy Hawkins from the Newport News Waterworks Authority, especially in reference to providing both the 10 years of full-scale operating data from the Harwood's Mill Water Treatment Plant but also providing media samples for use in the laboratory-scale Mn uptake studies. Ms Cooley's financial support during her time as a graduate student was provided through the Via Department of Civil and Environmental Engineering at Virginia Tech.