Desionización capacitiva

7.1 Overview

The primary objective of this dissertation was to expand and advance available tools to recover resources from wastewater. The dissertation specifically focused on using anaerobic biotechnology for direct energy recovery from domestic wastewater, a waste stream that has long been considered incompatible with the goal of direct energy recovery. This work began with a preliminary investigation of anaerobic membrane bioreactor (AnMBR) treatment of simulated domestic wastewater and actual domestic wastewater at 15°C (Chapter 3; (Smith et al. 2013)). This work suggested that the membrane foulant layer or biofilm may play a role in treatment and motivated an in-depth investigation of the biofilm’s ability to improve effluent quality (Chapter 4; (Smith et al. 2014b)). We then applied this approach to gauge the lower temperature limits of AnMBR domestic wastewater treatment (Chapter 5; (Smith et al. 2014a)). Finally, an assessment framework was developed to assess the life cycle environmental and economic impacts of AnMBR systems compared to conventional wastewater treatment systems with a focus on highlighting operational and design targets AnMBR needs to achieve for the technology to move to full-scale (Chapter 6; (Smith et al. 2014c)).

7.2 The membrane biofilm can significantly improve AnMBR treatment

The primary objective of AnMBR is to treat wastewater to protect the aquatic environment while recovering energy. This requires that AnMBR consistently produces a high quality effluent under stresses such as variable wastewater strength and composition, and seasonal and daily temperature

an additional barrier of treatment such as a membrane biofilm. We originally observed an indication that fouled membranes improve effluent quality (Chapter 3; (Smith et al. 2013)). However, improvements in effluent quality were minor, likely due to the limited biodegradable substrates (e.g., acetate and propionate) available during this operational period. We hypothesized that the biofilm could significantly improve treatment under conditions in which biodegradable substrates are more available such as when suspended biomass treatment is inadequate due to low temperature, a sharp increase in organic loading rate (OLR), or other factors.

Improving AnMBR treatment through biofilm development was explored by operating a bench- scale AnMBR with independent membrane housings under different levels of fouling as determined by transmembrane pressure (TMP) (Chapter 4; (Smith et al. 2014b)). During this operational period, the AnMBR was running under conditions that led to relatively high availability of acetate and propionate to the biofilm due to insufficient treatment by suspended biomass. Membranes with the highest level of fouling almost completely removed acetate and propionate and significantly decreased permeate chemical oxygen demand (COD) relative to membranes operated under low fouling conditions. However, COD removal in the biofilm corresponded to substantial dissolved methane oversaturation in the permeate, suggesting a downside to this approach to improve effluent quality.

16S rRNA sequencing indicated that controlled membrane fouling led to development of a biologically active membrane biofilm enriched in highly active aceticlastic and hydrogenotrophic methanogens and syntrophic bacteria. The increase in methanogenic activity was confirmed using reverse transcription quantitative PCR (RT-qPCR) targeting the methyl coenzyme-M reductase (mcrA) gene. DNA-based molecular analyses (16S rDNA sequencing) were insufficient to describe microbial community activity and functional significance in this study. This work enabled

us to recommend a combinations of RNA and DNA-based molecular analyses to study AnMBR and other systems in which microbial growth is limited due to low temperatures, low OLR, or other factors.

Membranes operated under the highest level of membrane fouling were returned to a near zero TMP to evaluate if biological activity in the biofilm could be maintained in the absence of TMP. Low effluent COD was maintained in further operation indicating a negligible impact on biofilm treatment. This suggests that the active biofilm is tightly adhered to the membrane surface and potentially distinct from the layer of foulants contributing to high TMP. Dissolved methane oversaturation persisted suggesting that oversaturation is primarily driven by methanogenesis in the biofilm and not by high TMP.

7.3 AnMBR can produce a high quality effluent at temperatures as low

as 6°C

Increasing the potential adoption of AnMBR technology requires demonstration of treatment at low temperatures, which occur during winter in most temperate climates. We explored the lower temperature limits of AnMBR treatment of domestic wastewater by operating a bench-scale AnMBR at temperatures of 12, 9, 6, and 3°C (Chapter 5; (Smith et al. 2014a)). Membranes were operated under conditions that supported biofilm development based on previous observations (Chapter 4; (Smith et al. 2014b)) to maximize overall treatment performance.

COD removal > 95% was maintained at temperatures as low as 6°C. COD removal was not affected until temperature was reduced to 3°C, after which it fell to 86 ± 4.0%. An increase in the biofilm’s role in treatment was observed as temperature decreased suggesting that suspended biomass was more sensitive than the biofilm to temperature decreases. This greater reliance on the

methanogenesis in the biofilm. Membrane fouling became more severe as temperature decreased indicating a potential concern for AnMBR implementation at such low temperatures.

High-throughput sequencing of 16S rRNA indicated a diversification of metabolisms as temperature decreased (i.e., reduced relative activity of methanogens and syntrophic bacteria and increased relative activity of fermenters). A concurrent increase in permeate dissolved methane oversaturation as temperature decreased suggests that methanogenesis in the biofilm increased, despite lower relative activity of methanogens, and therefore, that the overall biological activity in the biofilm also increased. Hydrogenotrophic methanogenesis as opposed to aceticlastic methanogenesis was the preferred pathway in the biofilm but not in suspended biomass, possibly due to better spatial microbial organization in the biofilm supporting syntrophy.

7.4 Full-scale implementation requires dissolved methane recovery

and reduction in membrane fouling control energy demands

Dissolved methane in AnMBR permeate represents a significant fraction of the energy produced during treatment and would result in greenhouse gas emissions if released to the atmosphere. Failing to recover dissolved methane from AnMBR permeate thus decreases the favorability of the energy balance while also increasing concerns regarding the environmental impacts of treatment. During a preliminary investigation of AnMBR domestic wastewater treatment at 15°C, dissolved methane oversaturation of approximately 1.5 times that predicted by Henry’s Law was observed (Chapter 3; (Smith et al. 2013)). Due to this level of oversaturation, dissolved methane represented 40-50% of methane produced during treatment. We further linked dissolved methane oversaturation directly to methanogenesis in the biofilm by operating a bench-scale AnMBR under different levels of fouling (i.e., biofilm treatment). Dissolved methane oversaturation as high as 3 times that predicted by Henry’s Law was observed under the highest level of biofilm treatment

(Chapter 4; (Smith et al. 2014b)). We further observed a strong dependence of dissolved methane oversaturation on operational temperature when relying on biofilm treatment (Chapter 5; (Smith et al. 2014a)). Dissolved methane concentration in the effluent increased both because of the decrease in temperature, which increased methane solubility, and the increase in oversaturation. Dissolved methane oversaturation approached 7 times that predicted by Henry’s Law during operation at 3°C. Essentially all of the methane produced at this temperature remained in the dissolved form. Therefore, both biofilm treatment and low operational temperature are detrimental to energy recovery and global warming potential of AnMBR if adequate dissolved methane recovery technologies are not in place.

An environmental and economic evaluation framework was established to compare AnMBR with conventional aerobic wastewater treatment systems considering the impacts related to dissolved methane release to the atmosphere (Chapter 6; (Smith et al. 2014c)). AnMBR had significantly greater global warming impact than aerobic treatment systems with 75% of this impact from effluent dissolved methane. This analysis considered a dissolved methane oversaturation of 1.5 times. Therefore, AnMBR could potentially have even greater global warming impacts based on the work described above.

The environmental and economic evaluation framework also highlighted the significance of fouling control energy demands in AnMBR. During treatment of medium strength domestic wastewater, AnMBR was unable to recover net energy because energy recovery was far outweighed by energy demands associated with biogas sparging. AnMBR is currently better suited to higher strength domestic wastewater treatment. For medium strength domestic wastewater, biogas sparging flow rates need to be reduced to those currently used in full-scale aerobic

membrane bioreactors to achieve net energy recovery. Alternatively, employing intermittent biogas sparging or increasing membrane flux could achieve net energy recovery.

7.5 Future research directions

This dissertation research demonstrated that AnMBR can produce a high quality effluent at temperatures as low as 6°C through membrane biofilm development. The practicality of doing so, considering the majority of methane remains dissolved in the effluent, is questionable if efficient dissolved methane recovery is not in place. One option to reduce dissolved methane concentration is to limit the reliance on biofilm treatment. To do so, novel approaches to improve suspended biomass activity need to be developed such that a high quality effluent can be produced at low temperatures without biofilm treatment.

One approach could include supplying biofilm carriers within the reactor (e.g., granular activated carbon (GAC) as previously demonstrated (Yoo et al. 2012) or a plastic media such as those used in moving bed biofilm bioreactors). However, the underlying mechanisms behind the high biological activity observed in the biofilm, particularly at such low temperatures, is poorly understood. We hypothesize that spatial organization of microbes within the biofilm enhances syntrophic interactions by reducing intercellular distances between syntrophic bacteria and hydrogenotrophic methanogens. High shear within suspended biomass due to biogas sparging may concurrently disrupt these syntrophic relationships in suspension. This hypothesis should be explored using fluorescence in situ hybridization (FISH) targeting these specific populations and comparing their spatial juxtapositioning in suspended and biofilm biomass. However, mass transfer limitations and substrate availability may also play a role or may even be the primary driver in the high biofilm activity observed. If so, adding biofilm carriers to an AnMBR may not appreciably improve suspended biomass activity. Alternative approaches to improving activity by