Influence of solids residence time and carbon storage on nitrogen and phosphorus recovery by microalgae across diel cycles.

Microalgal treatment systems could advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology, but their successful implementation requires an understanding of how design decisions influence nutrient uptake over daily (i.e., diel) cycles. This work demonstrates the ability to influence microalgal N:P recovery ratio via solids residence time (SRT) while maintaining complete nutrient removal across day/night cycles through carbon storage and mobilization. Experiments were conducted with two microalgal species, Scenedesmus obliquus and Chlamydomonas reinhardtii, in photobioreactors (PBRs) operated as cyclostats (chemostats subjected to simulated natural light cycles) with retention times of 6-22 days (S. obliquus) and 7-13 days (C. reinhardtii). Nutrient loading and all other factors were fixed across all experiments. Elevated SRTs (>8 days) achieved limiting nutrient concentrations (either N or P) below the detection limit throughout the diel cycle. N:P mass ratio in algal biomass was linearly correlated with SRT, varying from 9.9:1 to 5.0:1 (S. obliquus) and 4.7:1 to 4.3:1 (C. reinhardtii). Carbohydrate content of biomass increased in high irradiance and decreased in low irradiance and darkness across all experiments, whereas lipid dynamics were minimal over 24-h cycles. Across all nutrient-limited cultures, specific (i.e., protein-normalized) dynamic carbohydrate generally decreased with increasing SRT. Nighttime consumption of stored carbohydrate fueled uptake of nutrients, enabling complete nutrient limitation throughout the night. Dynamic carbohydrate consumption for nutrient assimilation was consistent with dark protein synthesis but less than that of heterotrophic growth, underscoring the need for algal process models to decouple growth from nutrient uptake in periods of low/no light. The ability to tailor microalgal N:P uptake ratio and target an optimal energy storage metabolism with traditional engineering process controls (such as SRT) may enable advanced nutrient recovery facilities to target continuous and reliable dual limitation of nitrogen and phosphorus.

Navigating environmental, economic, and technological trade-offs in the design and operation of submerged anaerobic membrane bioreactors (AnMBRs).

Anaerobic membrane bioreactors (AnMBRs) enable energy recovery from wastewater while simultaneously achieving high levels of treatment. The objective of this study was to elucidate how detailed design and operational decisions of submerged AnMBRs influence the technological, environmental, and economic sustainability of the system across its life cycle. Specific design and operational decisions evaluated included: solids retention time (SRT), mixed liquor suspended solids (MLSS) concentration, sludge recycling ratio (r), flux (J), and specific gas demand per membrane area (SGD). The possibility of methane recovery (both as biogas and as soluble methane in reactor effluent) and bioenergy production, nutrient recovery, and final destination of the sludge (land application, landfill, or incineration) were also evaluated. The implications of these design and operational decisions were characterized by leveraging a quantitative sustainable design (QSD) framework which integrated steady-state performance modeling across seasonal temperatures (using pilot-scale experimental data and the simulating software DESASS), life cycle cost (LCC) analysis, and life cycle assessment (LCA). Sensitivity and uncertainty analyses were used to characterize the relative importance of individual design decisions, and to navigate trade-offs across environmental, economic, and technological criteria. Based on this analysis, there are design and operational conditions under which submerged AnMBRs could be net energy positive and contribute to the pursuit of carbon negative wastewater treatment.

Energy positive domestic wastewater treatment: the roles of anaerobic and phototrophic technologies.

The negative energy balance of wastewater treatment could be reversed if anaerobic technologies were implemented for organic carbon oxidation and phototrophic technologies were utilized for nutrient recovery. To characterize the potential for energy positive wastewater treatment by anaerobic and phototrophic biotechnologies we performed a comprehensive literature review and analysis, focusing on energy production (as kJ per capita per day and as kJ m(-3) of wastewater treated), energy consumption, and treatment efficacy. Anaerobic technologies included in this review were the anaerobic baffled reactor (ABR), anaerobic membrane bioreactor (AnMBR), anaerobic fluidized bed reactor (AFB), upflow anaerobic sludge blanket (UASB), anaerobic sequencing batch reactor (ASBR), microbial electrolysis cell (MEC), and microbial fuel cell (MFC). Phototrophic technologies included were the high rate algal pond (HRAP), photobioreactor (PBR), stirred tank reactor, waste stabilization pond (WSP), and algal turf scrubber (ATS). Average energy recovery efficiencies for anaerobic technologies ranged from 1.6% (MFC) to 47.5% (ABR). When including typical percent chemical oxygen demand (COD) removals by each technology, this range would equate to roughly 40-1200 kJ per capita per day or 110-3300 kJ m(-3) of treated wastewater. The average bioenergy feedstock production by phototrophic technologies ranged from 1200-4700 kJ per capita per day or 3400-13 000 kJ m(-3) (exceeding anaerobic technologies and, at times, the energetic content of the influent organic carbon), with usable energy production dependent upon downstream conversion to fuels. Energy consumption analysis showed that energy positive anaerobic wastewater treatment by emerging technologies would require significant reductions of parasitic losses from mechanical mixing and gas sparging. Technology targets and critical barriers for energy-producing technologies are identified, and the role of integrated anaerobic and phototrophic bioprocesses in energy positive wastewater management is discussed.

Life cycle assessment applied to wastewater treatment: state of the art.

Life cycle assessment (LCA) is a technique to quantify the impacts associated with a product, service or process from cradle-to-grave perspective. Within the field of wastewater treatment (WWT) LCA was first applied in the 1990s. In the pursuit of more environmentally sustainable WWT, it is clear that LCA is a valuable tool to elucidate the broader environmental impacts of design and operation decisions. With growing interest from utilities, practitioners, and researchers in the use of LCA in WWT systems, it is important to make a review of what has been achieved and describe the challenges for the forthcoming years. This work presents a comprehensive review of 45 papers dealing with WWT and LCA. The analysis of the papers showed that within the constraints of the ISO standards, there is variability in the definition of the functional unit and the system boundaries, the selection of the impact assessment methodology and the procedure followed for interpreting the results. The need for stricter adherence to ISO methodological standards to ensure quality and transparency is made clear and emerging challenges for LCA applications in WWT are discussed, including: a paradigm shift from pollutant removal to resource recovery, the adaptation of LCA methodologies to new target compounds, the development of regional factors, the improvement of the data quality and the reduction of uncertainty. Finally, the need for better integration and communication with decision-makers is highlighted.

The use of qualitative system dynamics to identify sustainability characteristics of decentralized wastewater management alternatives.

In order to pursue more sustainable alternatives in wastewater management, it is vital that we understand how a given infrastructure alternative will impact the various aspects of sustainability. A set of qualitative tools (force field diagrams and causal loop diagrams (CLDs)) for the assessment of wastewater management alternatives is proposed and demonstrated in the context of a decentralized wastewater infrastructure upgrade. The objective for the application of these tools is to improve decision makers' understanding of how a given alternative will impact the economic, environmental/ecological, social, and functional aspects of sustainability. In the proposed method, each aspect of sustainability is treated as a stock, and its movement (up or down) can be inferred using both qualitative and quantitative data. By incorporating these tools into a participatory planning process, project-specific CLDs can be developed and loops of interest can be identified to help elucidate stakeholder values. The ultimate goal of this methodology is to facilitate the pursuit of sustainability in wastewater management by allowing decision makers to address specific sustainability challenges without creating new ones.