Separated absorption and bacterial degradation of microcystins in GAC filtration

Abstract

For removal of microcystins, live tumor promoting cyanotoxins, from the final drinking water product granular activated carbon (GAC) filtration has shown to be a very promising option. It is not only an efficient adsorbent for microcystins it can also be operated as a biological reactor in which bacterial degradation of the toxins could be expected and consequently extends the life-time of this application. However, the competitive adsorption of the coexistent natural organic matters (NOM) in all natural water sources would cause early breakthrough of microcystins. And before initiation of the biodegradation, a lag-phase with uncertain length usually appears for acclimatisation of the microorganisms. Therefore, this study aimed to investigate the GAC adsorption of microcystins in water treatment plant. Three columns, including a sterile and a non-sterile GAC column and a sand column were studied for separation of the adsorption and biodegradation mechanisms in GAC filtration. The results from the study indicate that the effective lifetime of a virgin GAC bed for microcystin adsorption was considerably short under non-sterile conditions, only 10-15 days (900 – 1400 bed volumes) based on the prescribed m-LR concentration of <1 μg/L recommended (WHO, 1998). However, the same toxin breakthrough occurred after more than one month in the sterile GAC filter. Enhanced or complete removal reappeared when the biodegradation of microcystins commenced in the filter, which became the primary function for toxin removal rather than adsorption. Also, GAC appeared to be a better substrate for biofilm attachment and development when compared with the non-porous clean sand of the same particle size in this study.