Techno-economic assessment of a novel algal-membrane system versus conventional wastewater treatment and advanced potable reuse processes: Part II.

This study developed a comprehensive techno-economic assessment (TEA) framework to evaluate an innovative algae resource recovery and near zero-liquid discharge potable reuse system (i.e., the main system) in comparison with a conventional potable water reuse system (i.e., the benchmark system). The TEA study aims to estimate the levelized costs of water of individual units and integrated processes including secondary wastewater treatment, advanced water purification for potable reuse, and sludge treatment. This would provide decision-makers valuable information regarding the capital and operational costs of the innovative main system versus a typical potable water reuse treatment train, along with possible routes of cost optimization and improvements for the design of full-scale facilities. The main system consists of (i) a novel algal-based wastewater treatment coupled with a dual forward osmosis and seawater reverse osmosis (Algal FO-SWRO) membranes system for potable water reuse and hydrothermal liquefaction (HTL) to produce bioenergy and subsequent nutrients extraction from the harvested algal biomass. The benchmark system includes (ii) an advanced water purification facility (AWPF) that consists of a conventional activated sludge biological treatment (CAS), microfiltration (MF), brackish water reverse osmosis (BWRO), ultraviolet/advanced oxidation process (UV-AOP), and granular activated carbon (GAC), with anaerobic digestion for sludge treatment. Capital expenditures (CAPEX) and operational expenditures (OPEX) were calculated for each unit of both systems (i.e., sub-systems). Based on a 76% overall water recovery designed for the benchmark system, the water cost was estimated at $2.03/m3. The highest costs in the benchmark system were found on the CAS and the anaerobic digester, with the UV-AOP combined with GAC for hydrogen peroxide (H2O2) quenching as the driving factor in the increased costs of the system. The cost of the main system, based on an overall 88% water recovery, was estimated to be $1.97/m3, with costs mostly driven by the FO and SWRO membranes. With further cost reduction and optimization for FO membranes such as membrane cost, water recovery, and flux, the main system can provide a much more economically viable alternative in its application than a typical benchmark system.

Life cycle energy use and greenhouse gas emissions for a novel algal-osmosis membrane system versus conventional advanced potable water reuse processes: Part I.

This study applied a life cycle assessment (LCA) methodology for a comparative environmental analysis between an innovative algae resource recovery and near zero-liquid discharge potable reuse system (i.e., the main system) versus a conventional potable reuse system (i.e., the benchmark system) through energy use and greenhouse gas (GHG) emissions. The objective of this study is to demonstrate that pilot-scale data coupled with LCA would provide valuable information for system optimization, integration, and improvements for the design of environmentally sustainable full-scale systems. This study also provides decision-makers valuable information regarding the energy demand and environmental impact of this innovative main system compared to a typical tried-and-true system for potable water reuse. The main system consists of a novel algal-based wastewater treatment coupled with a dual forward osmosis and seawater reverse osmosis (Algal FO-SWRO) membranes system for potable water recovery and hydrothermal liquefaction (HTL) to recover biofuels and valuable nutrients from the harvested algal biomass. The benchmark system refers to the current industry standard technologies for potable water reuse and waste management including a secondary biological treatment, microfiltration (MF), brackish water reverse osmosis (BWRO), ultraviolet/advanced oxidation process (UV-AOP), and granular activated carbon (GAC), as well as anaerobic digestion for sludge treatment. Respective energy and GHG emissions of both systems were normalized and compared considering 1 m3 of water recovered. Based on an overall water recovery of 76% designed for the benchmark system, the energy consumption totaled 4.83 kWh/m3, and the system was estimated to generate 2.42 kg of CO2 equivalent/m3 with most of the emissions coming from the biological treatment. The main system, based on an overall water recovery of 88%, was estimated to consume 4.76 kWh/m3 and emit 1.49 kg of CO2 eq/m3. The main system has high environmental resilience and can recover bioenergy and nutrients from wastewater with zero waste disposal. With the application of energy recovery devices for the HTL and the SWRO, increase in water recovery of the FO membrane, and replacement of the SWRO membrane with BWRO, the main system provides an energy-competitive and environmentally positive alternative with an energy demand of 2.57 kWh/m3 and low GHG emissions of 0.94 kg CO2 eq/m3.

Impacts of seasonality and operating conditions on algal-dual osmosis membrane system for potable water reuse: Part 2.

This study investigated the impact of seasonal variation and operating conditions on recovery of potable quality water from municipal wastewater effluent using an integrated algal treatment process with a dual forward osmosis (FO)-reverse osmosis (RO) membrane system. Pilot study of the algal process treating primary effluent validated the technical viability and seasonal performance during warm weather (May to October, 25-55 °C) using an extremophilic algal strain Galdieria sulphuraria, and during cold weather (November to April, 4-17 °C) using polyculture strains of algae and bacteria. Algal effluents from both seasons were used as the feed solution for the laboratory FO-RO study. In addition, pilot-scale FO-RO experiments were conducted to compare the system performance during treatment of algal effluent and secondary effluent from the conventional treatment facility. At 90% water recovery, the FO-RO achieved over 90% overall rejection of major ions and organic matter using the bench-scale system and over 99% rejection of all contaminants in pilot-scale studies. Detailed water quality analysis indicated that the product water from the integrated system met both the primary and secondary drinking water standards. This study demonstrated that the FO-RO system can be engineered as a viable alternative to treat algal effluent and secondary effluent for potable water reuse independent of seasonal variations and operating conditions.

Impacts of seasonality and operating conditions on water quality of algal versus conventional wastewater treatment: Part 1.

Municipal wastewater is a reliable source from which water, renewable energy, and nutrients could be recovered for beneficial use. Our previous efforts have documented that an innovative algal-based wastewater treatment (WWT) system could recover energy and nutrients from wastewater while having a lower energy footprint than conventional WWT processes. As a biological treatment process, the algal WWT can be affected by algal species, operating conditions, and meteorological factors. This study aimed to identify suitable algal cultures to treat municipal wastewater during warm and cold weather. The algal system achieved the secondary effluent discharge standards for biochemical oxygen demand and nutrients within 2-3 days during warm weather (May to October, 25-55 °C) using an extremophilic algal strain Galdieria sulphuraria; and within 1-2 days in winter (November to April, 4-17 °C) using polyculture strains of algae with bacteria. The impact of seasonal variation and operating conditions on the water quality of pilot-scale algal bioreactors was compared with a full-scale conventional WWT system. The treatment performance of the algal system (NH4-N: 1.3 ± 1.25 mg/L in winter and not detected in summer and conventional system; PO4-P: 0.89 ± 0.6 mg/L in winter, 0.02 ± 0.03 mg/L in summer and, 5.93 ± 1.32 mg/L in conventional system) was comparable or better than that of the conventional WWT in nutrients removal and other contaminants were below the discharge standards. This study indicates that the algal system can be engineered for reliable wastewater treatment independent of seasonal variations.

Metagenomic profiling of antibiotic resistance and virulence removal: Activated sludge vs. algal wastewater treatment system.

Conventional activated sludge-based (CAS) wastewater treatment plants are known to be a source of antibiotic resistance genes (ARGs) and virulence genes (VGs). As an alternative, a single-step mixotrophic algal wastewater treatment (A-WWT) system is proposed here to effectively reduce ARGs and VGs in the final effluent while meeting all the discharge standards. In this study, we applied the metagenomic profiling approach to compare the A-WWT system against the CAS system in terms of removal efficacy of ARG and VGs. A total of 111 ARG and 93 VG subtypes belonging to 10 antibiotic resistant classes and 19 virulence classes were detected in this study. Although the CAS system reduced the relative abundance of most classes of ARGs (7 of 10) and VGs (11 of 19), 3 ARG classes and 7 VG classes had increased abundances. On the other hand, the A-WWT system reduced the relative abundance of all classes of ARGs and VGs, and effectively eliminated most subtypes of ARGs and VGs. In the CAS system, the bacterial genera carrying ARGs and VGs was expanded, and the diversity index was increased greatly, suggesting the occurrence of horizontal gene transfer (HGT). In contrast, the A-WWT system narrowed down the potential host range and decreased their diversity substantially. Results of this study highlight the potential risk of ARGs and VGs in CAS system and demonstrate the feasibility of the algal-based system in removing ARGs and VGs.

Pilot Demonstration of Reclaiming Municipal Wastewater for Irrigation Using Electrodialysis Reversal: Effect of Operational Parameters on Water Quality.

The modification of ion composition is important to meet product water quality requirements, such as adjusting the sodium adsorption ratio of reclaimed water for irrigation. Bench- and pilot-scale experiments were conducted using an electrodialysis reversal (EDR) system with Ionics normal grade ion-exchange membranes (CR67 and AR204) to treat the reclaimed water in the Scottsdale Water Campus, Arizona. The goal is to investigate the impact of operating conditions on improving reclaimed water quality for irrigation and stream flow augmentation. The desalting efficiency, expressed as electrical conductivity (EC) reduction, was highly comparable at the same current density between the bench- and pilot-scale EDR systems, proportional to the ratio of residence time in the electrodialysis stack. The salt flux was primarily affected by the current density independent of flow rate, which is associated with linear velocity, boundary layer condition, and residence time. Monovalent-selectivity in terms of equivalent removal of divalent ions (Ca2+, Mg2+, and SO42-) over monovalent ions (Na+, Cl-) was dominantly affected by both current density and water recovery. The techno-economic modeling indicated that EDR treatment of reclaimed water is more cost-effective than the existing ultrafiltration/reverse osmosis (UF/RO) process in terms of unit operation and maintenance cost and total life cycle cost. The EDR system could achieve 92-93% overall water recovery compared to 88% water recovery of the UF/RO system. In summary, electrodialysis is demonstrated as a technically feasible and cost viable alternative to treat reclaimed water for irrigation and streamflow augmentation.

Bacteria and virus reduction in secondary treatment: Potential for minimizing post disinfectant demand.

Today's wastewater utilities are facing the dilemma of balancing pathological threats of bacteria and viruses in their effluent against health threats associated with the byproducts of disinfection. A possible solution to this dilemma is to adopt secondary treatment technologies capable of concurrent pathogen reduction, minimizing the demand for external disinfectants. Towards this end, bacterial and viral reductions possible in algal wastewater treatment (WWT) systems are highlighted here and compared with those in conventional activated sludge (AS) systems and membrane bioreactor (MBR) systems. High log reduction values (LRV) of E. coli [>5] and fecal coliform [>7] have been achieved without any external disinfectants in the classical photoautotrophic algal WWT systems and in an emerging mixotrophic algal WWT system. LRVs of E. coli, fecal coliform, and somatic coliphages in the mixotrophic system are higher than those in AS systems and, comparable to those in MBRs. But, LRVs of F-specific coliphages, Enterovirus and Norovirus GI are greater in MBRs than in the mixotrophic and AS systems. The low-energy algal WWT systems providing high inherent reductions of bacteria and viruses can serve as affordable alternatives to the capital- and energy-intensive AS and MBR systems for greener WWT, meeting several of the United Nation's Sustainable Development Goals.

Removal of antibiotic resistance genes in an algal-based wastewater treatment system employing Galdieria sulphuraria: A comparative study.

In this study, we compared removal of antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs) in two wastewater treatment systems fed with the same primary effluent: a conventional wastewater treatment system (consisting of a trickling filter followed by an activated sludge process) versus an algal-based system, employing an extremophilic alga, Galdieria sulphuraria. Our results demonstrated that the algal system can reduce concentrations of erythromycin- and sulfamethoxazole-resistant bacteria in the effluent more effectively than the conventional treatment system. A decreasing trend of total bacteria and ARGs was observed in both the treatment systems. However, the relative ratio of most ARGs (qnrA, qnrB, qnrS, sul1) and intI1 in the surviving bacteria increased in the conventional system; whereas, the algal system reduced more of the relative abundance of qnrA, qnrS, tetW and intⅠ1 in the surviving bacteria. The role of bacteriophages in horizontal gene transfer (HGT) of ARGs in the two systems was indicated by a positive correlation between ARG absolute abundance in bacteriophage and ARG relative abundance in the bacteria. Four of the five detectable genes (qnrS, tetW, sul1 and intI1) were significantly reduced in the algal system in bacteriophage phase which signified a decrease in phage-mediated ARG transfer in the algal system. Results of this study demonstrate the feasibility of the algal-based wastewater treatment system in decreasing ARGs and ARB and in minimizing the spread of antibiotic resistance to the environment.

Potable-quality water recovery from primary effluent through a coupled algal-osmosis membrane system.

A coupled algal-osmosis membrane treatment system was studied for recovering potable-quality water from municipal primary effluent. The core components of the system included a mixotrophic algal process for removal of biochemical oxygen demand (BOD) and nutrients, followed by a hybrid forward osmosis (FO)-reverse osmosis (RO) system for separation of biomass from the algal effluent and production of potable-quality water. Field experiments demonstrated consistent performance of the algal system to meet surface discharge standards for BOD and nutrients within a fed-batch processing time of 2-3 days. The hybrid FO-RO system reached water productivity of 1.57 L/m2-h in FO using seawater as draw solution; and permeate flux of 3.50 L/m2-h in brackish water RO (BWRO) and 2.07 L/m2-h in seawater RO (SWRO) at 2068 KPa. The coupled algal-membrane system achieved complete removal of ammonia, fluoride, and phosphate; over 90% removal of calcium, sulfate, and organic carbon; and 86-89% removal of potassium and magnesium. Broadband characterization using high resolution mass spectrometry revealed extensive removal of organic compounds, particularly wastewater surfactants upon algal treatment. This study demonstrated long-term performance of the FO system at water recovery of 90% and with membrane cleaning by NaOH solution.

Bio-crude oil from hydrothermal liquefaction of wastewater microalgae in a pilot-scale continuous flow reactor.

To explore the feasibility of scaling up hydrothermal liquefaction (HTL) of algal biomass, a pilot-scale continuous flow reactor (CFR) was operated to produce bio-crude oil from algal biomass cultivated in urban wastewater. The CFR system ran algal slurry (5 wt.% solids loading) at 350 °C and 17 MPa for 4 h without any clogging issues. Bio-crude oil chemistry was characterized by high-resolution Fourier transform mass spectroscopy (FT-MS), proton nuclear magnetic resonance spectroscopy (1H NMR), bomb calorimetry, and elemental analysis. Bio-crude oil yield of 28.1 wt% was obtained with higher heating values of 38-39 MJ/kg. The quality of light bio-crude oil produced from the CFR system was comparable in terms of molecular structures to bio-crude oil produced in a batch reactor.

Hydrothermal liquefaction of Galdieria sulphuraria grown on municipal wastewater.

Two strains of Galdieria sulphuraria algae, 5587.1 and SOOS, were grown on municipal wastewater to develop energy-positive treatment systems. Hydrothermal liquefaction (HTL) of 5-10 wt% algal biomass solids was conducted at 310-350 °C for 5-60 min to produce bio-crude oil. HTL product yields and energy recovery were compared to those from previous studies using G. sulphuraria grown on a modified Cyanidium medium. Total bio-crude oil yields were lower (11.2-23.0 wt%) and char yields were higher (22.6-36.4 wt%) for HTL of algae grown on actual wastewater compared with that grown on media (31.4 wt% and 4.8 wt%, respectively), indicating a potential limitation for using yields from media-based studies. High-resolution mass spectroscopy of bio-crude oil provides new insights into differences in composition based on growth media. Energy recovery in total bio-crude oil and char at 350 °C was 17-28% and 14-19%, respectively, for the 5587.1 strain, and 23-27% and 14-25%, respectively, for the SOOS strain.

Hydrothermal liquefaction of high- and low-lipid algae: Mass and energy balances.

Hydrothermal liquefaction (HTL) of high-lipid microalgae Nannochloropsis salina (N. salina) and low-lipid microalgae Galdieria sulphuraria (G. sulphuraria) were run under subcritical conditions (310-350 °C and 10-17 MPa) in a 1.8 L batch autoclave system. HTL mass and energy balances for both species were compared under different operating conditions to predict the optimum reaction conditions for new algae strains based on their feedstock composition. Bio-crude oils and chars were characterized by bomb calorimetry, elemental analysis, inductively coupled plasma optical emission spectrometry (ICP-OES), and thermogravimetric analysis (TGA). Under the optimized conditions, 59 wt% and 31 wt% bio-crude oil yields were obtained from HTL of N. salina and G. sulphuraria, while 85% and 59% of the feedstock energy were partitioned into N. salina-derived and G. sulphuraria-derived bio-crude oils, respectively. More favorable energy balances were related to shorter reaction times and higher algal solid contents.

Hydrothermal liquefaction of Cyanidioschyzon merolae and the influence of catalysts on products.

This work investigates the hydrothermal liquefaction (HTL) of Cyanidioschyzon merolae algal species under various reaction temperatures and catalysts. Liquefaction of microalgae was performed with 10% solid loading for 30min at temperatures of 180-300°C to study the influences of two base and two acid catalysts on HTL product fractions. Maximum biocrude oil yield of 16.98% was obtained at 300°C with no catalyst. The biocrude oil yield increased to 22.67% when KOH was introduced into the reaction mixture as a catalyst. The algal biocrude and biochar has a higher heating values (HHV) of 32.22MJkg-1 and 20.78MJkg-1 respectively when no catalyst was used. Gas chromatography time of flight mass spectrometry (GC/TOFMS) was employed to analyze the biocrude oil composition, and elemental analysis was performed on the algae, biocrude and biochar samples. Analysis of the HTL aqueous phase revealed the presence of valuable products.

Evaluation of internally illuminated photobioreactor for improving energy ratio.

The internally illuminated photobioreactor (IIPBR) design has been shown to be more efficient in utilizing the incident light energy than the externally illuminated designs. This study evaluated (i) optimal sparging of the IIPBR with CO2-enriched air (CEA) to enhance biomass productivity; and, (ii) single-stage and two-stage operation of the IIPBR to enhance lipid productivity. Growth data from two algal cultures-Scenedesmus sp. and Nannochloropsis salina, cultivated in an 18-L prototype version of the IIPBR were used to establish the optimal conditions for the two goals in terms of the energy ratio. Based on the optimized results under sparging with CEA, the energy ratio in the IIPBR in the first stage with Nannochloropsis salina was at least 6 times higher due to optimal performance of the IIPBR at lower energy input than typical literature results for other PBR designs, whereas the energy ratios in the second stage were comparable to literature results.

In situ ethyl ester production from wet algal biomass under microwave-mediated supercritical ethanol conditions.

An in situ transesterification approach was demonstrated for converting lipid-rich wet algae (Nannochloropsis salina) into fatty acid ethyl esters (FAEE) under microwave-mediated supercritical ethanol conditions, while preserving the nutrients and other valuable components in the algae. This single-step process can simultaneously and effectively extract the lipids from wet algae and transesterify them into crude biodiesel. Experimental runs were designed to optimize the process parameters and to evaluate their effects on algal biodiesel yield. The algal biomass characterization and algal biodiesel analysis were carried out by using various analytical instruments such as FTIR, SEM-EDS, TLC, GC-MS and transmission electron microscopy (TEM). The thermogravimetric analysis (TGA) under nitrogen and oxygen environments was also performed to examine the thermal and oxidative stability of ethyl esters produced from wet algae. This simple in situ transesterification process using a green solvent and catalyst-free approach can be a potentially efficient route for algal biodiesel production.

Modeling microalgal growth in an airlift-driven raceway reactor.

In previous proof-of-concept studies, feasibility of a new airlift-raceway configuration and its energetic advantage and improved CO2 utilization efficiency over the traditional raceways and photobioreactors have been documented. In the current study, a mathematical model for predicting biomass growth in the airlift-raceway reactor is presented, which includes supply and transfer of CO2 and the synergetic effects of light, CO2, nitrogen, and temperature. The model was calibrated and validated with data from prototype scale versions of the reactor on two test species: Nannochloropsis salina and Scenedesmus sp., cultivated under indoor and outdoor conditions. Predictions of biomass concentrations by the proposed model agreed well with the temporal trend of the experimental data, with r(2) ranging from 0.96 to 0.98, p<0.001. A sensitivity analysis of the 10 model parameters used in this study revealed that only three of them were significant, with sensitivity coefficients ranging from 0.08 to 0.13.

Energy-efficient photobioreactor configuration for algal biomass production.

An internally illuminated photobioreactor (IIPBR) design is proposed for energy-efficient biomass production. Theoretical rationale of the IIPBR design and its advantages over the traditional bubble column photobioreactors (PBRs) are presented, followed by experimental results from prototype scale cultivation of freshwater and marine algal strains in an 18L IIPBR. Based on theoretical considerations, the proposed IIPBR design has the potential to support 160% higher biomass density and higher biomass productivity per unit energy input, B/E, than a bubble column PBR of equal incident area per unit culture volume. Experimental B/E values recorded in this study with fresh water algae and marine algae (1.42 and 0.37 gW(-1)d(-1), respectively) are at least twice as those reported in the literature for comparable species cultivated in bubble column and airlift PBRs.

Modeling algal growth in bubble columns under sparging with CO2-enriched air.

A theoretical model for predicting biomass growth in semi-continuous mode under sparging with CO(2)-enriched air was developed. The model includes gas-to-liquid mass transfer, algal uptake of carbon dioxide, algal growth kinetics, and light and temperature effects. The model was validated using experimental data on growth of two microalgal species in an internally illuminated photobioreactor: Nannochloropsis salina under gas flow rates of 800 and 1200 mL min(-1) and CO(2) enrichments of 0.5, 1, and 2%; and Scenedesmus sp. at a gas flow rate of 800 mL min(-1) and CO(2) enrichments of 3 and 4%. Temporal algal concentration profiles predicted by the model under semi-continuous mode with harvesting under the different test conditions agreed well with the measured data, with r(2) values ranging from 0.817 to 0.944, p<0.001. As demonstrated, this model can be beneficial in predicting temporal variations in algal concentration and in scheduling harvesting operations under semi-continuous cultivation mode.

Optimizing net energy gain in algal cultivation for biodiesel production.

An approach based on energy gain was utilized to optimize algal cultivation in bubble columns. Net energy gain was estimated considering the energy input for mixing and providing carbon dioxide, and the energy that can be generated from the lipids extracted from the algal biomass. Energy input for sparging was minimized based on the gas-to-culture volume ratio and energy output from lipid production was maximized based on nitrate and CO(2) levels. Sparging at a gas-to-culture volume ratio of 0.18 min(-1) with CO(2)-enrichment of 0.5% and initial nitrate concentration of 1mM was optimal for improving net energy gain with Nannochloropsis salina. Sparging with CO(2)-enriched air of 0.5% along with nitrogen starvation resulted in 50% more lipid productivity than sparging with ambient air.

Feasibility of microalgal cultivation in a pilot-scale airlift-driven raceway reactor.

A Scenedesmus sp. was cultivated in a 23-L airlift-driven raceway reactor under artificial lighting and laboratory conditions, in batch and continuous modes. In batch mode, a maximum volumetric biomass productivity of 0.085 dry g L(-1) day(-1) was achieved under sparging at a CO(2)-to-air ratio of 1%, and a maximum CO(2) utilization efficiency of 33% was achieved at a CO(2)-to-air ratio of 0.25%. In continuous mode, the maximum volumetric biomass productivity was 0.19 dry g L(-1) day(-1). Biomass productivities per unit power input achieved in this reactor configuration (0.60-0.69 dry g W(-1) day(-1)) were comparable to or better than those reported in the literature for different photobioreactor designs (0.10-0.51 dry g W(-1) day(-1)). Based on the energy-efficient productivity and the high CO(2) utilization efficiency demonstrated in this study, the proposed airlift-driven raceway design holds promise for cost-effective algal cultivation.