Continuous co-treatment of mine drainage with municipal wastewater

Acid mine drainage (AMD) and municipal wastewater (MWW) are commonly co-occurring waste streams in mining regions. Co-treating AMD at existing wastewater facilities represents an innovative solution for simultaneous AMD reclamation and improved MWW treatment. However, unknowns related to biological processes and continuous treatment performance block full-scale use. The overarching goal of this work was to address questions related to efficacy and performance of continuous processing of AMD in a biological MWW treatment system. Synthetic AMD was co-treated with synthetic MWW in a continuously-operating bench-scale sequencing batch reactor (SBR). SBRs treated MWW with two strengths of AMD (91 and 720 mg/L as CaCO3 Acidity) to capture the variations of coal AMD chemistry and strength observed in the field. Each co-treatment phases lasted 40+ days, during which clarified effluent and settled sludge quality was routinely monitored to determine impacts of co-treatment relative to conventional MWW treatment performance. Co-treatment produced effluent that met key standards for secondary treatment including biochemical oxygen demand (BOD) < 5 mg/L, total suspended solids (TSS) < 20 mg/L, and pH ∼7.0. Addition of AMD also improved treatment performance, increasing Phosphate (PO4) removal by >60% and pathogen removal by an order of magnitude. Furthermore, AMD co-treatment did not exhibit any major impacts on the overall diversity of the wastewater microbial community. Co-treatment sludge had slightly higher settleability and a lower bound water content, but notable changes in sludge morphology was observed. This study demonstrates co-treatment allows for continuous mitigation of AMD without adversely impacting MWW treatment performance in conventional biological MWW processes.

Pilot-scale evaluation of sulfite-activated ferrate for water reuse applications

Ferrate is a promising, "green" (i.e., iron-based) pre-oxidation technology in water treatment, but there has been limited research on its potential benefits in a water reuse (wastewater recycling) paradigm. Recent studies have shown ferrate treatment processes can be improved by activation, the addition of reductants (i.e., sulfite) to the reaction. Prior bench scale experimentation suggests sulfite-activated ferrate may be a feasible option for water reuse applications; however, extent questions need to be addressed. This study evaluated the viability of sulfite-activated ferrate in water reuse treatment through continuous-flow experiments using synthetic and field-collected secondary wastewater effluents. The effluents were processed through the piloting system which included various physicochemical processes such as ferrate pre-oxidation, coagulation, clarification, and dual-media filtration. In each trial, the system was run continuously for eight hours with data collected via grab samples and online instrumentation with real-time resolution. Results demonstrate that reuse systems using activated ferrate pre-oxidation can produce effluents with water quality meeting most regulatory requirements without major impacts on downstream physicochemical processes. When compared to traditional ferrate pre-oxidation, activation showed several improvements such as lower byproduct yields. Operationally, activated ferrate does increase the development of headloss across the dual-media filter. In general, sulfite-activated ferrate is viable in a water reuse setting, resulting in several improved water quality outcomes. Results from this work create a pathway for adaptation at scale.

Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.

Sulfite-activated ferrate for water reuse applications

Ferrate is a promising, emerging water treatment technology. However, there has been limited research on the application of ferrate in a water reuse paradigm. Recent literature has shown that ferrate oxidation of target contaminants could be improved by "activation" with the addition of reductants or acid. This study examined the impact of sulfite-activated ferrate in laboratory water matrix and spiked municipal wastewater effluents with the goal of transforming organic contaminants of concern (e.g., 1,4-dioxane) and inactivating pathogenic organisms. Additionally, the formation of brominated disinfection byproducts by activated ferrate were examined and a proposed reaction pathway for byproduct formation is presented. In particular, the relative importance of reaction intermediates is discussed. This represents the first activated ferrate study to examine 1,4-dioxane transformation, disinfection, and brominated byproduct formation. Results presented show that the sub-stoichiometric ([Sulfite]:[Ferrate] = 0.5) activated ferrate treatment approach can oxidize recalcitrant contaminants by >50%, achieve >4-log inactivation of pathogens, and have relatively limited generation of brominated byproducts. However, stoichiometrically excessive ([Sulfite]:[Ferrate] = 4.0) activation showed decreased performance with decreased disinfection and increased risk of by-product formation. In general, our results indicate that sub-stoichiometric sulfite-activated ferrate seems a viable alternative technology for various modes of water reuse treatment.

Pollutant co-attenuation via in-stream interactions between mine drainage and municipal wastewater

Municipal wastewater (MWW) and mine drainage (MD) are common co-occurring sources of freshwater pollution in mining regions. The physicochemical interactions that occur after mixing MWW and MD in a waterway may improve downstream water quality of an impaired reach by reducing downstream concentrations of nutrients and metals (i.e., "co-attenuation"). A first-order stream (Bradley Run in central Pennsylvania), with coal MD and secondarily treated MWW entering the stream in the same location, was systematically monitored to determine in-stream water-quality dynamics. Monitored constituents included pH, nutrients (i.e., phosphorus and nitrogen), and metals (e.g., iron, aluminum, manganese). Mixing of the MWW, MD, and upstream water decreased concentrations of phosphate, aluminum, and iron by 94%, 91%, and 98%, respectively, relative to conservative mixtures at the 1400-m-downstream site. The pollutant co-attenuation resulted in water quality equivalent to that upstream of the pollutant sources and improved the phosphorus-based trophic status of the stream. Geochemical models indicate the primary mechanisms for P attenuation in the studied stream were precipitation as variscite (AlPO4:2H2O) or amorphous AlPO4 plus adsorption to hydrous ferric oxide, despite a much greater abundance of hydrous aluminum oxide. The results presented in this study suggest that in-stream mixing of MD with untreated or secondarily treated MWW may be an important, overlooked factor affecting downstream transport of common pollutants in mining regions. Decreased metals loading and increased pH resulting from natural attenuation and remediation of MD could affect the potential for retention of phosphate by stream sediment and could lead to the release of nutrients from legacy accumulations, highlighting the potential need to address high-nutrient discharges (e.g., improved MWW treatment) in concert with MD remediation.

Physicochemical implications of cyanobacteria oxidation with Fe(VI)

Increases in harmful algal blooms has negatively impacted many surface-sourced drinking water utilities. To control these blooms, many water utilities implement pre-oxidation with ozone, chlorine, or permanganate; however, pre-oxidation of algae has both positive and negative water quality outcomes. This study investigated ferrate (Fe(VI)) as an alternative oxidant by measuring its effect on cell lysing, surface characteristics, and coagulation in waters containing the cyanobacteria Microcystis aeruginosa. Bench scale studies were conducted to examine the complex combination of processes in a Fe(VI)-algae system. These processes were characterized by fluorescence index, surface charge, collision frequency modeling, particle counts and sphericity, total nitrogen, and ferrate decomposition measurements. Results showed that Fe(VI) lysed algal cells, but further oxidation of released organic matter is possible. The presence of algae did not significantly impact the rate of Fe(VI) decomposition. Fe(VI) pre-oxidation may also be capable of decreasing the formation of nitrogenated disinfection byproducts through subsequent oxidation of released nitrogen rich organic matter. Streaming current and zeta potential results indicate destabilization of the resulting algae and iron suspension was incomplete under most conditions. Particle collision frequency modeling indicates fluid shear to be an important aggregation mechanism of the resulting suspension. Overall, Fe(VI) is a viable alternative to other strong oxidants for water utilities struggling with harmful algal blooms, but the final fate of the resulting organic matter must be further studied.

Abatement of circumneutral mine drainage by Co-treatment with secondary municipal wastewater

Acid mine drainage is a persistent and problematic source of water pollution. Co-treatment with municipal wastewater at existing wastewater treatment plants has several advantages; however, potential impacts on plant physicochemical and biological processes have not been well explored. The primary purpose of this bench-scale study was to examine the impact of co-treatment by combining a mild acid mine drainage at various ratios with municipal wastewater, followed by sludge settling and supernatant comparative analysis using a variety of effluent water quality parameters. These measurements were combined with carbonate system and adsorption isotherm modeling to elucidate the mechanisms underlying the experimental results. Acid mine drainage addition decreased municipal wastewater effluent PO₄^3- concentrations below 0.2 mg/L with greater than 97% removal, demonstrating co-treatment as an alternative solution for municipal wastewater nutrient removal. Biochemical oxygen demand remained similar to controls with <10% variation after co-treatment. Coagulation from metals in acid mine drainage was incomplete due to PO₄^3- adsorption, confirmed by comparing experimental results with Langmuir isotherm behavior. Sweep flocculation was the dominating particle aggregation mechanism, and co-treatment led to improved particle clarification outcomes. Improved clarification led to up to 50% Fe removal. Final pH had little variation with all conditions having pH > 6.0. Carbonate system modeling adequately explains pH effects, and can also be applied to varying acid mine drainage matrices. The impact of acid mine drainage addition on the municipal wastewater microbial community was also investigated which provided evidence of microbial adaptation. This study demonstrates post-aeration co-treatment enables mitigation of mild acid mine drainage without adversely affecting wastewater treatment plant processes. Reported results also frame required future studies to address extant questions prior to full-scale adaptation.