Endotoxin release from biologically active dual-media (anthracite/sand) bench-scale drinking water filters was investigated. The biological filters were typically net producers of endotoxin during normal operation with mean concentrations increasing from 6 endotoxin units (EU)/mL to 16 EU/mL in filter influent and effluent, respectively. The filter operating condition resulting in the greatest release of endotoxin occurred upon shutdown and subsequent restart, with the highest observed filter effluent endotoxin concentration being 745 EU/mL in the first pore volume following the return of flow through the filter. Effluent samples collected following filter shutdowns were chlorinated to determine the effect of bacterial cell death on endotoxin release. Chlorination did not induce immediate ‘dumping’ of endotoxin, nor did holding the chlorinated samples for 5 d result in an increase in aqueous endotoxin concentration. In addition to endotoxin, measurable quantities of potassium were detected in the interstitial water in the biofilter during shutdown periods. In order to reduce potential risk to dialysis patients and humidifier users, it is recommended that, following even short biofilter shutdowns, filters should be immediately backwashed or, failing this, the first five pore volumes of effluent water be discharged to waste to allow endotoxin levels to return to pre-shutdown values.

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