Evaluation of arsenic and iron transport from sediments of a potable water treatment plant wastewater stabilization pond system
Date
2021-05-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
The creation of clean drinking water within Drinking Water Treatment Plants (DWTP) leads to residual wastewaters which need to be properly treated before being released into the environment. Treated wastewater effluents must meet effluent guidelines which can be difficult for DWTPs that use groundwater as their sole source of raw water, since their wastewaters typically contain elevated concentrations of metal(loid)s, such as arsenic (As) and iron (Fe). A treatment strategy for DWTP wastewaters is the use of Wastewater Stabilization Ponds (WSP), in which most of the metal(loid)s are either settled in the sedimentation process by adsorption to the suspended particles or precipitated by producing insoluble compounds. However, these metal(loid)s are not perpetually fixed in the sediments and can become resuspended and released back into the residual wastewater over time. This potential for release shows the importance of evaluation of metal(loid)s in sediments.
The studied pond system has been in use for more than 50 years and consists of five sequential ponds currently designated as Ponds 1 through 5. The pond system receives wastewaters from a DWTP that has been recently upgraded from treating a mixture of surface and groundwater with electro-dialysis reversal (EDR) technology to treating only groundwater with reverse osmosis (RO) technology with an increased production capacity to meet the growing needs of the municipality. The pond system effluent was found to have frequently elevated concentrations of As and Fe and exceeded guideline values for As prior to the start of this study. This study monitored both the overlying water and sediment As and Fe concentrations, among other parameters, in each of the individual ponds over the spring and summer of 2019 (before the new DWTP commissioning) and spring and summer of 2020 (after the January 2020 commissioning of the new DWTP). The collected pond sediments on one sampling occasion in 2020 were also used in laboratory experiments used to better understand the fate and transport potential of As and Fe between sediments and water. These laboratory experiments included determination of solid-water partitioning coefficients (Kd) and leaching rates for both As and Fe using a 16 day duration to mimic the overall WSP hydraulic retention time.
The results from this study indicate that the individual ponds and the pond system effluents had elevated As and Fe concentrations. These concentrations did not meet the guidelines for As concentrations, while were close to exceeding new Fe guidelines currently under consideration. It is interesting to note that the historic Fe guidelines were often exceeded but were more stringent than the new guidelines which vary based on organic carbon concentrations and pH of the water. Release of polluted effluent has resulted in As contamination in sediments downstream of the pond system outlet causing the sediments to exceed As guidelines. The pond system sediments were found to contain elevated As and Fe contents in the ranges of about 25 to 400 and 10,000 to 45,000 mg/kg, respectively, and with variations more significant between the ponds than in each individual pond over time. Thus far, the ponds sediment As and Fe concentrations in 2019 and 2020 suggest that the change in the DWTP treatment process did not affect the ponds’ sediments significantly.
Results from the sediment leaching experiments indicated that each of the ponds can potentially release leachable As and Fe contents over their individual retention times, as well as during the overall WSP retention times. The As concentrations resulted in sediment log Kd values ranging widely from 2.21 to 4.31 L/kg among the ponds with Kd values of Pond 1 ≫ Pond 3 > Pond 4 > Pond 5 ≫ Pond 2. The Fe concentrations resulted in sediment log Kd values having a range of 3.32 to 5.53 L/kg with the values being in the following order: Pond 5 ≫ Pond 1 > Pond 4 > Pond 3 ≫ Pond 2. For the leaching rate experiments, As concentrations increased from Day 0 to Day 9 for all ponds with a range of concentrations from ~0.005 to 0.015 mg/L. From Day 9 to Day 16 the water concentrations stabilized resulting in the highest concentration for Pond 2 at ~0.02 mg/L. For Fe, the initial leaching rate was high reaching the highest concentrations in the Day 0 sampling with Pond 2 being the highest at ~0.18 mg/L followed by Pond 1 at ~0.15 mg/L and Pond 4 at ~0.14 mg/L. Unlike As, the Fe concentrations reduced after initial peaks until approximately Day 6 or Day 9 when the concentrations reached a steady state of under 0.05 mg/L for all ponds.
The conclusion of this research is that the DWTP WSP system has elevated concentrations of As and Fe in pond sediments that can potentially impact the treatment process via leaching of these pollutants into the overlying waters. Clearly the sediments are already shown to be releasing into the overlying waters given the increasing concentrations found for both As and Fe through the WSP system on numerous sampling occasions. These results may be useful to inform potential updates to the WSP system in the future.
Description
Keywords
Sediments, Arsenic, Iron, Water Treatment Plants, Wastewater stabilization pond, redox potential, microbial activity
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering