PI: Timothy Townsend, University of Florida 

Drinking water treatment residues (WTR) offer potential advantages when beneficially recycled through land application. The current guidance in Florida allows for unrestricted land application of lime softening WTR; alum and ferric WTR require additional evaluation of total and leachable concentrations of select trace metals prior to land application. In some cases a mixed WTR is produced when lime softening is accompanied by the addition of a coagulant or other treatment chemicals. Limited data on the characterization of these WTRs are available. The objective of this research was to characterize the total and leachable chemical content of WTR from facilities that utilize multiple treatment chemicals; WTR samples were collected from 18 water treatment facilities in Florida, including facilities which only utilized lime softening and those employing lime and other treatment additives. Management implications were assessed by comparing the results to risk-based regulatory thresholds and the results from the previous Florida study. Additional leaching tests were conducted to evaluate leaching of trace metals from WTR when placed below the water table as a clean fill under low dissolved oxygen (DO) conditions.

 

Total metal concentrations of WTR were found to be below Florida Soil Cleanup Target Levels, with Fe found in the highest abundance at a concentration of 3,650 mg/kg-dry. When assessing leaching risk through the use of the synthetic precipitation leaching procedure (SPLP), several Al concentrations (SPLP 95% Upper Confidence Limit (UCL) =0.24 mg/L) did exceed the Florida Ground Water Cleanup Target Level (GWCTL=0.2 mg/L). To further evaluate leaching, a subset set of samples from five facilities was subjected to a suite of additional leaching tests. Leachate concentrations from pH static leaching tests (EPA method 1313) demonstrated that Al, As, Ba, Cr, Fe, Mn, Ni, and Zn exceeded the GWCTLs in certain samples, the majority of which were at low pH values. During up-flow percolation column testing (EPA method 1314), Al was found to exceed its GWCTL at liquid to solid ratios (L/S) above 2.0. Tank leaching tests of compacted WTR (EPA method 1315) found that As was the only element that exceeded its respective GWCTL; this was seen in samples from 4 out of the 5 facilities tested at leaching times of 2 hours and of one day. Characterization of mixed WTR demonstrated slight chemical differences between lime, lime-ferric, and lime-alum WTR. Comparison of mixed lime WTR to ferric and alum WTR from the previous study demonstrated significant differences in total and leachable concentrations of trace metals. Overall, it can be established that mixed WTR are most adequately represented as lime WTR.

 

Further testing was conducted under reducing conditions to evaluate the impact of low DO environments on trace metal release from WTR. In column tests under anaerobic conditions, Fe was detected in 2 out of 5 facilities, while it was found to be below instrument detection limits (0.045 mg/L) under aerobic conditions for all 5 facilities. In the anaerobic column tests, concentrations of Al were found to be elevated above GWCTLs at high liquid to solid ratios in samples from 2 of the 5 facilities studied. SPLP tests under reducing conditions showed elevated concentrations of Fe and Mn in comparison to standard SPLP leachates. In the natural water leaching test, Al concentrations and pH of leachates were found to be lower in comparison with SPLP results. The Fe concentrations in the natural water leachates were higher than the SPLP results; this was possibly caused by the background Fe concentration in the natural water samples. Testing of lime and mixed lime WTR under reducing conditions demonstrated the potential for release of certain trace metals (Fe, Mn) above applicable regulatory thresholds. Batch leaching tests using natural water samples demonstrated little difference compared with SPLP leaching results overall.

 

University of FloridaFlorida international universityUSFMiami UniversityFlorida A&MUCFFlorida StateFAUUniversity of West FloridaFlorida Institute of Technology