Effect of substrate characteristics on microbial community structure, function, resistance, and resilience; application to coupled photocatalytic-biological treatment

Advanced oxidation (AO) coupled with biodegradation is an emerging treatment technology for wastewaters containing biologically recalcitrant and inhibitory organics, including those containing chlorinated aromatic compounds. The composition of the AO effluent organics can vary significantly with reaction conditions, and this composition can affect the performance of subsequent biodegradation. Three synthetic effluents were used to mimic varying degrees of AO of 2,4,5-trichlorophenol: 4-chlorocatechol to mimic light transformation, 2-chloromuconic acid to mimic moderate transformation, and acetate to mimic extensive transformation. The substrates were fed to identical chemostats and analyzed at steady state for removal of chemical oxygen demand (COD) and dissolved organic carbon (DOC), biomass concentration, and bacterial diversity. The chemostat fed acetate performed best at steady state. The 2-chloromuconic acid chemostat was next in terms of steady-state performance, and the 4-chlorocatechol reactor performed worst, correlating with degree of AO transformation. A spike of 100 μM 2,4,5-trichlorophenol was then applied to each chemostat. The chemostat fed 4-chlorocatechol exhibited the best resistance to the perturbation in terms of maintaining consistent community structure and biomass concentration, whereas the performance of the acetate-fed chemostat was severely impaired in these categories, although it quickly regained capacity to remove organics near pre-perturbation levels demonstrating good resilience. The opposing trends for steady-state versus perturbed performance highlight tradeoffs inherent in coupled chemical-biological systems.

An ultra-low energy method for rapidly pre-concentrating microalgae

This study demonstrates that Nannochloropsis sp. can be effectively separated from its growth medium (0.2-0.3g/L) using electro-coagulation-flocculation in a 100mL batch reactor with nickel electrodes and a treatment time of only 4s. Minimum energy density input for effective separation is 0.03 kWh/m(3). Both energy input and treatment time are much smaller than reported elsewhere. The process results in rapid separation of microalgae (over 90% in 120 min) with minimal damage to algal cells (>90% still alive after processing). At around 4V input, algae can be effectively separated even in very low concentrations. Pulsing is equally effective in separating microalgae as continuous direct current of same magnitude and total exposure time. Algae can separate from their growth medium even if the suspension itself is not treated, but is mixed with treated saltwater with same conductivity. The described method has significant advantages including applicability to continuous processing and water reuse.

Coupled photocatalytic-biodegradation of 2,4,5-trichlorophenol: effects of photolytic and photocatalytic effluent composition on bioreactor process performance, community diversity, and resistance and resilience to perturbation

Sequentially coupled advanced oxidation-biodegradation systems have proven effective for treating a variety of wastewaters, but in several cases the pretreatment did not improve, or even hindered, subsequent biodegradation. Therefore, investigating the relationship between advanced oxidation pretreated effluent and subsequent bioreactor performance can help to optimize these systems. Here, a photocatalytic reactor was used to produce four unique effluents from 2,4,5-trichlorophenol (TCP) by varying light wavelength, catalyst presence, and reaction time, demonstrating that the conditions of photocatalytic pretreatment can be tuned to achieve a variety of treatment objectives. The photocatalytic effluents were characterized for chemical oxygen demand (COD), chloride release, aromaticity, and residual TCP concentration. The four effluents were normalized to 40 mg COD/L, combined with biological medium components, and fed to continuous bioreactors. Bioreactors were assayed for COD removal, TCP removal, optical density (OD), and microbial diversity via denaturing gradient gel electrophoresis. In general COD removal in the bioreactors increased as aromatic character of the photoeffluent decreased, but the least aromatic effluent performed poorly indicating the nuanced relationship between photoreactor effluent composition and bioreactor performance. While neither indicator of community diversity, richness nor evenness, correlated with COD removal or biomass accumulation, each effluent produced a unique community as indicated through similarity indices. All conditions demonstrated strong overall TCP removal. After two weeks at steady state, the reactors were perturbed with a 120-μM spike of TCP. Overall the most aromatic photoeffluent produced the most resistant community to the perturbation, while the optimum effluents at steady state produced communities with poor resistance in terms of biomass accumulation and COD removal. These results highlight the tradeoffs between steady state performance and resistance to perturbation that are necessary to optimize a combined advanced oxidation-biodegradation treatment strategy.

Thermal pretreatment of algae for anaerobic digestion

The objective of this work was to determine the benefit of thermal pretreatment on biogas yield from microalgae-fed anaerobic digester mesocosms. Replicate Nanochloropsis oculata cultures were heated for 4h at 30, 60, and 90°C, as well as at a constant temperature of 90°C for 1, 3.5, and 12h. Net biogas production increased from 0.28L biogas/g volatile solids added (VSa) for the control to 0.39 L biogas/g VSa (p<0.01) when heated at 90°C, but there was no improvement at 30 or 60°C. Increased biogas production correlated with increased soluble chemical oxygen demand (COD). Net biogas production increased as a function of heating time, from 0.32 L biogas/g VSa for the control, to 0.41, 0.43, and 0.44 L biogas/g VSa (p<0.05 for all combinations vs. control) when preheated at 90°C for 1, 3.5, and 12h, respectively. However, despite enhanced biogas production the energy balance is negative for thermal pretreatment.

Biodegradation of 2,4,5-trichlorophenol by aerobic microbial communities: biorecalcitrance, inhibition, and adaptation

Chlorinated aromatic compounds challenge our environment and wastewater treatment processes due to their biorecalcitrance and inhibition. In particular, 2,4,5-trichlorophenol (TCP) seems to demonstrate greater resistance to biodegradation than other trichlorophenols and is a known uncoupler of the electron transport chain, although little work addresses this compound specifically. Here, we investigate the biorecalcitrance, inhibition, and adaptation to 2,4,5-trichlorophenol by aerobic mixed microbial communities. We show that 2,4,5-trichlorophenol is strongly resistant to biodegradation at concentrations greater than 40 microM, demonstrates inhibition to respiration in direct proportion to 2,4,5-trichlorophenol concentration (with 50% inhibition projected near 85 microM 2,4,5-trichlorophenol), and does not sustain biomass in continuous reactors, even when all input 2,4,5-trichlorophenol is degraded. Communities showed consistent adaptation patterns to 2,4,5-trichlorophenol at concentrations of 10 microM and 20 microM, but these patterns diverged at concentrations greater than 40 microM. Finally, thermodynamic approximations were used to estimate the yield of 2,4,5-trichlorophenol as 0.165 gVSS/gCOD, a low value that partially explains why biodegradation of 2,4,5-trichlorophenol did not sustain the biomass. In particular, we estimated that the minimum concentration to support steady-state biomass (S (min)) is approximately 180 microM, a value much larger than the 40-microM concentration that is strongly resistant to biodegradation. Thus, readily biodegradable concentrations of 2,4,5-trichlorophenol are too low to sustain the biomass that biodegrades it.