Augmenting the Applicability and Efficiency of Ecological Engineering Solutions for Water Quality Improvement

Abstract

The design options available to the ecological engineer were expanded by exploring new sustainable water quality improvement techniques, specifically the applicability of passive treatment of acid mine drainage (AMD) in high-altitude, arid environments, passive co-treatment of AMD and municipal wastewater (MWW), and ecologically engineered floating vegetation mats (EFVM).

Field studies at Cerro Rico de Potosí, Bolivia demonstrated that AMD must be addressed to render local waters safe for agricultural use. AMD discharges from both operating and abandoned portals as well as tailings-related deposits displayed a high degree of heterogeneity with total metal concentrations ranging from 0.11-7,48l, <0.022-889, <0.0006-65.3, <0.001-310, 0.12-72,100, 0.3-402, <0.012-34.8, and 0.24-19,600 mg/L of Al, As, Cd, Cu, Fe, Mn, Pb and Zn, respectively. Net acidity and pH ranged from -10 to 246,000 mg/L as CaCO3 equivalent and 0.90-6.94 standard units, respectively. In-stream waters contained total metals concentrations of up to 16 mg/L As, 4.9 mg/L Cd, 0.97 mg/L Co, 1100 mg/L Fe, 110 mg/L Mn, 4.1 mg/L Pb, and 1500 mg/L Zn with pH ranging from 2.8-9.5. AMD-impacted streams contained elevated concentrations of the same major ecotoxic constituents present in AMD discharges at concentrations statistically greater than in those stream unimpacted by AMD. The data indicate that historic and current mining activities have transformed these key natural resources into potential human and environmental health hazards.

To assess the viability of passive water quality improvement approaches for treating AMD from Cerro Rico, alkalinity production, acidity neutralization and metals removal were tracked for incubations of AMD in the presence of limestone (LS), a 1:1 mix of AMD and raw MWW, and a 1:1 mix of AMD and WW in the presence of LS. Three AMD sources from abandoned adits on Cerro Rico, raw WW from the city of Potosí and locally available LS were incubated in-situ for 72 hr in 1-L cubitainers. Although locally sourced LS can increase final alkalinity up to 397 mg/L as CaCO3, it is a prospective source for Mn and a few other potentially undesirable elements. Relevant to the prospects of AMD and WW passive co-treatment, mixing AMD with WW had relatively little effect on the final alkalinity achieved by LS dissolution. Accounting for dilution, dissolved concentrations of Ag, Al, As, Cd, Cr, Fe, Pb, Sb, Se, Sn, V and Zn decreased with AMD and WW incubation.

In laboratory studies, passive co-treatment of AMD and MWW was further explored, resulting in a system that efficiently removed key constituents of both effluents. A laboratory-scale, four-stage continuous-flow reactor system was constructed to test the viability of simulated Cerro Rico high-strength AMD and MWW passive co-treatment. The synthetic AMD had pH 2.6 and 1860 mg/L acidity as CaCO3 equivalent and with 46, 0.25, 2, 290, 55, 1.2 and 390 mg/L of Al, As, Cd, Fe, Mn, Pb and Zn, respectively. The AMD was mixed at a 1:2 ratio with raw MWW from the City of Norman, Oklahoma containing 265 ± 94 mg/L BOD5, 11.5 ± 5.3 mg/L PO4-3, and 20.8 ± 1.8 mg/L NH4+-N and introduced to the system which had a total residence time of 6.6 d. During the 135 d experiment, dissolved Al, As, Cd, Fe, Mn, Pb and Zn concentrations were consistently decreased by 99.8, 87.8, 97.7, 99.8, 13.9, 87.9 and 73.4%, respectively, pH increased to 6.8 ± 0.1, and net acidic influent was converted to net alkaline effluent. PO4-3 and NH4+-N were decreased to <0.75 and 7.4 ± 1.8 mg/L, respectively. BOD5 was generally decreased to below detection limits. Nitrification increased NO3- to 4.9 ± 3.5 mg/L NO3--N, however relatively little denitrification occurred. Sulfate reducing bacteria were able to maintain a relatively high level of sulfate reduction (0.56 mol/m3-d). A 100% reduction of all fecal indicator bacteria was observed. Results indicated that passive AMD and MWW co-treatment is a viable ecological engineering approach for the developed and developing world that can be optimized and applied to improve water quality with minimal use of fossil fuels and refined materials.

Field studies of EFVM illustrated that these systems could encourage water quality and temperature changes conducive to the passive treatment of various constituents. Four EFVM designs were constructed of drainpipe, burlap, mulch, utility netting, and reused polyethylene bottles then planted with Typha spp. and Juncus effusus. The water column beneath the EFVM in two test ponds was compared to that in an open water control pond. Dissolved oxygen concentrations and pH were lower, diurnal temperature range was dampened, and sulfate/nitrate reduction was greater under the EFVM with respect to the control. Alkalinity was also greater under EFVM. Although plant propagation was limited, results suggest that EFVM may be applied to encourage reducing, thermally insulated conditions for passive treatment of AMD and a wide range of other pollutants. Specifically, they may be employed to improve immediate and long-term performance of vertical flow bioreactors for AMD treatment by lowering dissolved oxygen concentrations in the water column and providing a continual source of organic carbon to the underlying substrate.