Abstract
Anaerobic digestion (AD) is an important component in the drive towards delivering renewable energy for all; however as with all biological processes there are a range of compounds that can interfere with the process. Pharmaceuticals, an emerging group of pollutants, in different environmental compartments such as water, waste, sludge,sediments etc., may have a harmful effect on the organisms in these environs. Pharmaceuticals are rarely found individually in wastewater, thus the interaction between categories and types need to be evaluated. The overall aim of this project was to both investigate the performance of the anaerobic processes in the presence of common pharmaceuticals found in sewage sludge and as a secondary aim to evaluate the potential for their removal by AD treatment.The effect of pharmaceuticals on AD was studied using syringe batch tests using either the individual compounds or mixtures added as a single dose, as well as in bioreactors operated in a semi-continuous manner utilising a mixture of nine selected pharmaceuticals (ciprofloxacin, clarithromycin, clotrimazole, erythromycin, fluoxetine, ibuprofen, sertraline, simvastatin and tamoxifen). These nine pharmaceuticals represent five therapeutic classes.Conventional parameters such as biogas production, accumulation of volatile fatty acids,and volatile solids removal were used as endpoints for evaluating any effect.
The selected pharmaceuticals at the three different concentrations tested (normal predicted environmental level, lOx this level and lOOx) were significantly reduced in syringe batch tests and none of the pharmaceuticals inhibited biogas production. However, in the semicontinuous AD, with the same complex synthetic sludge as the batch tests, the combined mixture of the selected pharmaceuticals added daily at normal environmental predicted concentrations had no effect initially but began to inhibit biogas production around 80 days during the second HRT cycle. This was associated with an accumulation of volatile fatty acids(VFA) and a drop in pH. Thus clearly after prolonged exposure to pharmaceuticals the AD reactor became unstable indicating that the bacteria were adversely affected.
In combination, the compounds had an effect on the AD process - biogas dropped from an average of 560 mL.gCODadded-1d-1 to 94 mL.gCODadded-1d-1 over the duration of the pharmaceutical closing experiment. This was accompanied by a drop in pH (from >7.4 to5.62); a build-up of VFA demonstrated by an increase in the Intermediate Alkalinity/Total Alkalinity ratio from 0.2 during the pre-trial period to >0.9 and reduction in COD removal,dropping from over 85% to only 6.02%.
Exposure to pharmaceuticals affected the structure of the bacterial population within the dosed reactor, with Clostridiaceae increasing and Bacteroidetes decreasing in the reactor with pharmaceuticals added. Additionally, the compositions of Archaea were affected by exposure to pharmaceuticals, in the control reactor acetoclastic methanogens accounted for> 98% of the Archaea but in the dosed reactor the ratio of acetoclastic to hydrogenotrophic methanogens was basically 50:50 at the end of the trial.
Phytotoxicity testing was used to evaluate the indirect impact of pharmaceuticals on the natural ecosystems. Digestate diluted to an equivalent application rate of 170 kg nitrogen ha-1 did not seriously affect the germination of Lactuca sativa seeds but root elongation in the lettuce seedlings was significantly retarded with time. This was associated with the collapse of the reactor and the build-up of VFA, drop in pH, etc., thus this effect is probably due to low pH, presence of organic acids rather than the result of toxicity by pharmaceuticals compounds.
This study has demonstrated that when combined with an artificial substrate, a mixture of the selected nine pharmaceuticals have a serious impact on the AD performance, seemingly affecting all the stages. These data highlight the complex interactions that can occur between common pharmaceuticals, even at environmentally expected concentrations, and the intricate bacterial interrelationships that occur within the AD treatment.
| Date of Award | 2019 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Colin Hunter (Supervisor) & Ole Pahl (Supervisor) |