Biotransformation of caffeine, gliclazide and prazosin in an up-flow anaerobic sludge blanket reactor

Abstract

Up-flow anaerobic sludge blanket (UASB) reactors are largely utilised as an anaerobic reactor configuration to efficiently treat various wastewater streams. However, previous UASB operations for the removal of pharmaceutical compounds at low hydraulic retention time (HRT) resulted in poor chemical oxygen demand (COD) and removal efficiencies for the investigated compounds. Anaerobic process performance has also been found to be vulnerable towards pharmaceutical compounds at high concentration levels but there is no evidence to indicate if the same effects will be induced when other compounds are present at trace concentration levels. This research aims to establish the relationship between pharmaceutical compound removal, influencing parameters, and anaerobic process performance. In this research, caffeine, gliclazide, and prazosin were selected based on their environmental occurrences, persistency, and toxicity. The compounds were investigated as a mixture to simulate their concurrent occurrences in the same actual wastewater stream. The first part of the research investigated the biodegradability of the three compounds in an anaerobic batch experiment. Synthetic wastewater and inoculum mixture were spiked with 1 mg/L of mixed pharmaceutical compounds and incubated at mesophilic condition (37°C) for 90 days. All compounds achieved removal through biotransformation between 44 - 99% by the end of the incubation period. The second part of the research was commenced by running five experimental phases in an acclimatised laboratory-scale UASB reactor. Phases I - V were carried out to assess the effect of pharmaceutical concentrations (0.1 – 1 mg/L), HRT (36 – 48 hours) and different reducing conditions (predominant methanogenic and simultaneously reducing conditions) to the removal of pharmaceutical compounds and UASB process performance. Overall, biotransformation remained the predominant removal pathway in the laboratory-scale experiments despite the changes in pharmaceutical concentrations and HRT. Only gliclazide recorded an improvement of biotransformation up to 99% under simultaneous reducing conditions compared to that in predominant methanogenic conditions. The UASB reactor was also operating in a stable condition (COD removal efficiency of 93 ± 2%) but fluctuations were recorded in the production of individual volatile fatty acids (VFAs) and biogas. Microbial assessment through 16s rRNA sequencing justified the changes in VFAs production based on the effect of the pharmaceutical compounds towards the growth of hydrolytic and fermentative bacteria. The compounds also shifted the composition of methanogenic archaea by favouring hydrogenotrophic methanogens (59 - 72%) over acetoclastic methanogens (15 - 31%) under predominant methanogenic conditions.