Joana Fradinho

Mercury removal from water streams through the ion exchange membrane bioreactor concept

Mercury is a highly toxic heavy metal that causes human health problems and environmental contamination. In this study, an ion exchange membrane bioreactor (IEMB) process was developed to achieve Hg(II) removal from drinking water and industrial effluents. Hg(II) transport through a cation exchange membrane was coupled with its bioreduction to Hg(0) in order to achieve Hg removal from concentrated streams, with minimal production of contaminated by-products observed. This study involves (1) membrane selection, (2) demonstration of process effectiveness for removing Hg from drinking water to below the 1ppb recommended limit, and (3) process application for treatment of concentrated water streams, where >98% of the Hg was removed, and the throughput of contaminated water was optimised through membrane pre-treatment. The IEMB process represents a novel mercury treatment technology with minimal generation of contaminated waste, thereby reducing the overall environmental impact of the process.

Optimisation of glycogen quantification in mixed microbial cultures

This study addressed the key factors affecting the extraction and quantification of glycogen from floccular and granular mixed microbial cultures collected from activated sludge, nutrient removal systems and photosynthetic consortiums: acid concentration, hydrolysis time and concentration of biomass in the hydrolysis. Response surface modelling indicated that 0.9 M HCl and a biomass concentration of 1 mg mL(-1) were optimal conditions for performing acid hydrolysis. Floccular samples only needed a 2-h hydrolysis time whereas granular samples required as much as 5 h. An intermediate 3 h yielded an error of 10% compared to the results obtained with the hydrolysis times specifically tailored to the type of biomass and can thus be recommended as a practical compromise.

Photosynthetic mixed culture polyhydroxyalkanoate (PHA) production from individual and mixed volatile fatty acids (VFAs): Substrate preferences and co-substrate uptake

This work studied the effect of the substrate feeding composition on the polyhydroxyalkanoate (PHA) accumulation capacity of an acetate enriched photosynthetic mixed culture (PMC). From the six tested organic acids - malate, citrate, lactate, acetate, propionate and butyrate - only the three volatile fatty acids (VFAs) enabled PHA production, with acetate and butyrate leading to polyhydroxybutyrate (PHB) formation and propionate leading to a HB:HV copolymer with a 51% fraction of hydroxyvalerate (HV). Also, results showed an acceleration of butyrate and propionate consumption when fed in the presence of acetate, suggesting that the latter can act as a co-substrate for butyrate and propionate uptake. Furthermore, results suggest that some PMC bacterial groups present a substrate preference for butyrate in relation to acetate and propionate. These findings indicate the possibility of feeding the PMC with cheap VFA rich fermented wastes, leading to a more cost-effective and environmentally sustainable PHA production system.

The effect of carbon source on the biological reduction of ionic mercury

Mercury (Hg) is the most highly toxic heavy metal, and must be removed from waterways to very low levels. Biologically mediated mercury removal is an emerging technology that has the potential to be robust, efficient and cost-effective. In this study, the impact of carbon source on the behaviour and microbial community composition of mixed microbial cultures was evaluated, and their performance was compared with a pure culture of Pseudomonas putida spi3. Glucose and acetate, two carbon sources that are commonly present in wastewaters, were chosen for this study. Distinct microbial populations were enriched with each carbon source. Glucose led to a more suitable microbial culture for Hg(2+) bioreduction that was able to reduce Hg(2+) at faster rates when compared to acetate. Furthermore, acetate consistently led to poorer process performance, irrespective of the microbial culture, possibly due to the formation of mercuric acetate complexes. It is proposed that glucose can be a more beneficial carbon source than acetate for the successful operation of Hg bioremediation systems.

Polyhydroxyalkanoates production by a mixed photosynthetic consortium of bacteria and algae

For the first time, a mixed photosynthetic culture (MPC) consisting of a consortium of bacteria and algae was investigated for its capacity to accumulate polyhydroxyalkanoates (PHA). The culture was subjected to a feast and famine regime in an illuminated environment without supplying oxygen or any other electron acceptor. The MPC accumulated PHA during the feast phase and consumed it in the famine phase, where the PHA consumption was made possible due to oxygen production by algae. The internal cycling of carbohydrates was also observed, which was likely linked to bacterial glycogen being used as an additional source of energy for acetate uptake during the feast phase, and restored in the famine phase via PHA degradation. The MPC reached a PHA content of 20%, with a PHA storage yield per acetate similar to aerobic systems, opening up the possibility of a new sunlight-driven PHA production process without the need for aeration.

Effect of dark/light periods on the polyhydroxyalkanoate production of a photosynthetic mixed culture

This work studied the possibility of operating a viable polyhydroxyalkanoate (PHA) producing photosynthetic mixed culture (PMC) under dark/light periods without aeration. The culture was subjected to a feast and famine regime, being fed in the dark phase and entering into famine during the light phase. Throughout consecutive feast and famine dark/light periods, the PMC became enriched in PHA accumulating organisms, where non-PHA producing algae that can grow under continuous illumination were out-competed. The very low algae levels enabled greater light and carbon source availability for PHA accumulating bacteria, leading to higher metabolic rates and PHA levels. The PMC reached a PHA content of 30% PHA/VSS, and doubled its specific PHA production rate in relation to PMCs operated previously under continuous illumination. This new process takes a further step towards operating a more cost effective PMC system for PHA production, opening up the possibility for direct sunlight utilization in the future.