Chemical treatment of high pressure membrane concentrate for improved residuals management
Removal of specific contaminants from high ionic strength process residuals, such as concentrate from desalting membranes, may increase the number of alternatives for residual disposal, reuse, and/or resource recovery. In the case of disposal, treatment of these waste streams could expand the number of feasible options while potentially lessening the environmental impacts of the residuals. With regards to increasing utilization of resources, treatment of membrane concentrate prior to the use of crystallization processes such as those used for zero-liquid discharge might increase the overall efficiency of the recovery system as well as improve the quality of the recovered salts by removing seed crystal poisons and potentially toxic constituents. Alternatively, reduction of key constituents to an acceptable level may allow for the treated residuals to be used as irrigation water for salt tolerant plants, saline aquaculture (farming of brine shrimp or other salt water species), or for options not yet identified because of the developing nature of this problem. Finally, treatment of the concentrate may allow for some portion of the residuals stream to be incorporated back into the overall treatment process leading to higher treated water recoveries.
This research explored removal of specific pollutants from simulated and real high-pressure membrane concentrates by treatment with ferric chloride or calcium oxide (lime). Ionic strength effects were important in terms of particle aggregation for ferric iron treatments; this phenomenon was reflected in better solid liquid separation at higher ionic strengths (> 0.2 M) than at 0.008 M. Decreasing pH by acid addition to near 6.5 prior to iron treatments generally improved arsenic uptake compared to uptake where pH was not adjusted (pH ∼ 8).
Arsenic uptake by solids formed during lime softening increased linearly with increasing lime dose is matrices consisting of calcium only or calcium and magnesium. Based on the results presented in this study, it would seem that precipitation with calcium can be an important mechanism of arsenic removal. Ionic strength effects were not significant during lime softening tests because, unlike the case of in-situ iron precipitation, solids were already present due to the addition of lime as a Ca(OH)2 slurry.
Removal of arsenic from full-scale concentrates spiked with arsenic was somewhat similar for lime softening and in-situ iron precipitation. Arsenic was detected by EDX analysis of the solids formed during iron precipitation in the presence of arsenic, however, background noise during analysis of lime softening solids did not allow for arsenic detection. Organic carbon removal from full-scale concentrates was much less than the removals observed for similar doses during treatment of the simulated residuals.
Predictions of arsenic removal from the full-scale concentrates (with and without arsenic spike) was relatively good, although more extensive modeling is required and better model parameters should be determined. Based on the economic evaluation of the treatment options for arsenic removal from full-scale residuals, iron coagulation with pH adjustment was cheaper than iron coagulation without pH adjustment option or lime softening for all three full-scale membrane concentrates (with and without arsenic spike).
0775: Environmental engineering