The following sections detail the experimental procedures as well as the operational and analytical methods used to evaluate the performance of a small pilot-scale PFBR with respect to Mn removal.
6.2.1 Synthetic Groundwater (SGW) Preparation
All assays were conducted at pilot-scale (20 LPH) using a SGW. Before each assay, a volume of 200-400 L of SGW was prepared using deionized water. The required alkalinity and hardness were provided using an appropriate amount of 0.5 M NaHCO3 and 0.5 M CaCl2 solutions, respectively.
The concentration of Mn(II) in water was controlled by the addition of 0.1 M MnSO4.H2O stock solution. The pH of the SGW was adjusted by bubbling CO2 and/or N2 gas into the water prior to conduct the assay. SGW were not deoxygenated as preliminary assays indicated that performance was not significantly impacted by the presence of dissolved oxygen in the feed waters.
6.2.2 Lab-Pilot Experimental Set-up and Pyrolucite Characteristics
The FBR column had a cross-sectional area of 5.07 cm2 corresponding to an inner diameter of 2.54 cm, which was filled up with a 100 cm of a commercially available pyrolucite (LayneOx™ brand).
The physical properties of pyrolucite media are presented in Table 6.1. Short-term (four-hour) Mn(II) uptake capacity of media was determined at pH=7, 23oC, 1 mg/L Mn concentration and alkalinity and hardness of 200 mgCaCO3/L according to the procedure outlined by Tobiason et al.
(2008). Mn uptake capacity result of new media was 0.84 mg Mn/g media and increased to 0.93 ± 0.01 after regeneration (indicates regeneration condition).
Table 6.1. Summary of pyrolucite media characteristics.
Effective Size
6.2.3 Operation of the Experimental set-up
As assays were conducted both at 22oC and 9oC, the column was covered with insulation material for experiments conducted at 9oC in order to provide constant temperature throughout the PFBR.
Free chlorine concentration was controlled by the addition of dilute bleach (NaOCl) solution which was prepared daily using a 6% (W/V) NaOCl stock solution. The SGW and dilute NaOCl solution were mixed ahead of a screen located at the bottom of the reactor. For most experiments, an effluent free chlorine residual of 1 mgCl2/L was targeted. Unless mentioned otherwise, the PFBR was operated at a bed expansion of 10% corresponding to an hydraulic loading rate (HLR) of 40 m/h (16 gpm/ft2).
In order to determine the Mn removal profile within the PFBR, total and dissolved Mn concentrations were measured at different heights of the PFBR (0, 5, 17.5, 25, 50, 75, 100, 110, and 150 cm from the bottom of the column). Dissolved Mn was defined here as Mn filterable through 0.45 μm Millipore filter (CAT NO. HAWP02500). The PFBR was operated for periods of 4 hours (short term assays) or 12 days (long term assays). Samples were collected at increasing operation time. At each sampling time, turbidity and free chlorine concentration in the influent and effluent water were analyzed. Additionally, in each experiment, alkalinity, pH and temperature of influent and effluent waters were measured.
Each experimental trial first consisted in acclimatizing the PFBR for an hour with Mn-free SGW at the desired HLR. This step also allowed cooling down the PFBR to 9oC. At time zero, the valve was switched to the drum containing the desired level of Mn. Then, the first samples were collected at time zero (t = 0h) from each sampling port along the PFBR column. Additional water samples were collected at 1, 2 and 4 hours after initiation of the experiment (t = 0h). At the end of each experiment, the PFBR was expanded (up to around 50%) and the media was rinsed with tap water for around 10 min. In order to provide the same media condition for all tests, an overnight regeneration of the media with 8-liter of recirculating chlorine solution (100 mg Cl2/L) was conducted.
6.2.4 Experimental Matrix
The experimental plan was developed to test the water quality and operational variables expected to explain PFBR performance. Accordingly, four water quality characteristics and one operational parameters were measured and controlled in each experimental condition: the influent dissolved Mn concentration (0.5 – 3 mg Mn/L), Ca2+ hardness (0 – 200 mg CaCO3/L), pH (6.2 – 7.8), temperature (9 – 23oC), and HLR (24 – 63 m/h) (i.e., bed expansion (0 – 30%)). The reference groundwater characteristics was chosen as pH=7, 9oC, HLR of 40 m/h, alkalinity and hardness of 200 mgCaCO3/L and Mn concentration of 1 mg/L. The experimental matrix was realized by changing one test variable at a time.
6.2.5 PFBR Stability Experiments
In order to determine the long-term performance of the PFBR for Mn removal, assays were conducted for a period of up to 12 days. A schematic diagram of the experimental set-up used to
achieve this objective is shown in Figure 6.1. During these experiments, 400 L SGW was continuously pumped to the bottom of the PFBR. The SGW was renewed after 4-day of operation.
As shown in Figure 6.1, free chlorine and Mn were injected from the stock solutions into the water prior entering the column. Around 10% of the effluent water from the PFBR was discarded. The remainder water was pumped to a GAC filter cartridge to remove the residual free chlorine and then returned back to the drum. The pH, temperature, alkalinity, turbidity, and chlorine and Mn concentrations were monitored and controlled daily.
Figure 6.1. Schematic diagram of the long-term experimental plan.
6.2.6 Analytical Methods
Samples collected for total and dissolved Mn analysis were acidified to 0.5% HNO3 and stored at 4oC. Mn analysis was performed using an Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, model iCAP 6000) from Thermo Instruments Inc. with a 1 μg/L Mn detection limit. A calibration curve was run at the beginning of each set of analysis. A standard and a blank were measured for quality assurance after every 20 unknown samples.
The DPD (N, N-diethyl-p-phenylenediamine) colorimetric method, Standard Method 4500-Cl-G, (American Public Health Association (APHA), American Water Works Association (AWWA) 2005) was used to measure the free chlorine residual concentration. A spectrophotometer (Hach, model DR 5000) was used for the analysis. Alkalinity was measured by titration with 0.02 N sulfuric acid (Method 2320 (American Public Health Association (APHA), American Water Works
Association (AWWA) 2005). Solution pH and turbidity were measured with an UltraBasic pH meter, model UB-5 from Denver Instrument and a Hach 2100 N turbidimeter, respectively.