This study used pilot-scale high-rate algae ponds to assess algal–bacteria biomass productivity and wastewater nutrient removal as well as the impact of mechanical and hydrothermal pretreatments on biomass disintegration, methane production kinetics, and anaerobic digestion (AD) energy balance. Mechanical pretreatment had a minor effect on biomass disintegration and methane production. By contrast, hydrothermal pretreatment significantly reduced particle size and increased the solubilized organic matter content by 3.5 times. The methane yield and production rate increased by 20–55% and 20–85%, respectively, with the highest values achieved after pretreatment at 121 °C for 60 min. While the 1st-order and pseudo-1st-order reaction equation models fitted methane production from untreated biomass best (R2 > 0.993), the modified Gompertz sigmoidal-type model provided a superior fit for hydrothermally pretreated algae (R2 ≥ 0.99). The AD energy balance revealed that hydrothermal pretreatment improved the total energy output by 25–40%, with the highest values for volume-specific and mass-specific total energy outputs reaching 0.23 kW per digester m3 and 2.3 MW per ton of biomass volatile solids. Additionally, net energy recovery (energy output per biomass HHV) increased from 20% for untreated algae to 32–34% for hydrothermally pretreated algae, resulting in net energy ratio and net energy efficiency of 2.14 and 68%, respectively.

Olive cake, the solid byproduct of three-phase centrifugation olive oil production, has a high organic and polyphenol content, rendering it an environmental threat when landfilled as well as limiting its animal feed potential. This residue can be a good candidate for biomethane production due to its rich polysaccharide content (pectin, hemicellulose, and cellulose). Two strategies were compared to maximize biomethane production: destoning (i.e., removal of the seed fragments via mechanical means) and enzymatic pretreatment of the pulp. After 30 days of batch anaerobic digestion at 35 °C, both enzymatically pretreated and destoned olive cakes produced similar amounts of methane (~295 mL CH4/g volatile solids (VS)), 42% more than the control. A comparison of olive cake’s biomethane yields with a broad range of agricultural residues in the literature demonstrated its suitability for biomethane production. Additionally, the digestate recovered from the anaerobic digestion of olive cake had high Kjeldahl nitrogen contents (3.6%, db) and low polyphenol concentrations (0.02 mg gallic acid equivalent (GAE)/g), qualifying it as an ingredient for soil amendment. This study demonstrated olive cake can be diverted from landfills for second-generation biofuel production, and that the resulting digestate may have value for soil amendment.

Photosynthetic algae represent a large, diverse bioresource potential. Yellow-green algae of the genus Tribonema are candidates for production of biofuels and other bioproducts. We report on a filamentous isolate from an outdoor raceway polyculture growing on municipal reclaimed wastewater which we classified as T. minus. Over one year of cultivation in 3.5 m2 raceway ponds fed by reclaimed municipal wastewater, T. minus cultures were more productive than the native algal polycultures, with annual average productivities of 15.9 ? 0.3 and 13.4 ? 0.4 g/m2/day, respectively. The biochemical composition of T. minus biomass grown outdoors was constant yearround, with 28.3 ? 0.4% carbohydrates, 37.6 ? 0.7% proteins, and 6.1 ? 0.3% fatty acids (measured as methyl esters), with up to 4.0% of the valuable omega-3 eicosapentaenoic acid, on an ash-free dry-weight basis. In summary, T. minus was more productive, easier to harvest and produced higher quality biomass than the native polycultures.

The feasibility of generating a lipid-containing algal-bacterial polyculture biomass in municipal primary wastewater and enhancing biomethanation of lipid-extracted algal residues (LEA) through hydrothermal pretreatment and co-digestion with sewage sludge (SS) was investigated. In high-rate algal ponds, the polyculture of native algal and bacteria species demonstrated a monthly average net and gross biomass productivity of 30 +/- 3 and 36 +/- 3 gAFDW m(-2) day(-1) (summer season). The algal community was dominated by Micractinium sp. followed by Scenedesmus sp., Chlorella sp., pennate diatoms and Chlamydomonas sp. The polyculture metabolic activities resulted in average reductions of wastewater volatile suspended solids (VSS), carbonaceous soluble biochemical oxygen demand (csBOD(5)) and total nitrogen (N-total) of 63 +/- 18%, 98 +/- 1% and 76 +/- 21%, respectively. Harvested biomass contained nearly 23% lipid content and an extracted blend of fatty acidmethyl esters satisfied the ASTM D6751 standard for biodiesel. Anaerobic digestion of lipid extracted algal residues (LEA) demonstrated long lagphase inmethane production of 17 days and ultimatemethane yield of 296 +/- 2 mL/gVS (or similar to 50% of theoretical), likely because to its limited biodegradability and toxicity due to presence of the residual solvent (hexane). Hydrothermal pretreatment increased the ultimatemethane yield and production rate by 15-30% but did not mitigate solvent toxicity effects completely leading to less substantial improvement in energy output of 5-20% and diminished Net Energy Ratio (NER < 1). In contrast, co-digestion of LEA with sewage sludge (10% to 90% ratio) was found to minimize solvent toxicity and improve methane yield enhancing the energy output similar to 4-fold, compared to using LEA as a single substrate, and advancing NER to 4.2. (c) 2018 Elsevier B. V. All rights reserved.

The objective of this project was to develop and demonstrate methods of recycling of water and nutrients for algal biofuels production. Recycling was accomplished both internal to the system and, in a broader sense, through import and reuse of municipal wastewater. Such an integrated system with wastewater input had not been demonstrated previously, and the performance was unknown, particularly in terms of influence of recycling on algal productivity and the practical extent of nutrient recovery from biomass residuals. Through long-term laboratory and pilot research, the project resulted in the following: 1. Bench-scale pretreatment of algal biomass did not sufficiently increase methane yield of nutrient solubilization during anaerobic digestion to warrant incorporation of pre-treatment into the pilot plant. The trial pretreatments were high-pressure orifice homogenization, sonication, and two types of heat treatment. 2. Solubilization of biomass particulate nutrients by lab anaerobic digesters ranged from 20% to nearly 60% for N and 40-65% for P. Subsequent aerobic degradation of the anaerobically digested biomass simulated raceways receiving whole digestate and resulted in an additional 20-55% N solubilization and additional 20% P solubilization. 3. Comparisons of laboratory and pilot digesters showed that laboratory units were reasonable proxies for pilot-scale. 4. Pilot-scale anaerobic digesters were designed, installed, and operated to digest algal biomass. Nutrient re-solubilization by the digesters was monitored and whole digestate was successfully used as a fertilizer in pilot algae raceways. 5. Unheated, unmixed digesters achieved greater methane yield and nutrient solubilization than heated, mixed digesters, presumably due to longer the solids residence times in unmixed digesters. The unmixed, unheated pilot digesters yielded 0.16 LCH4/g volatile solids (VS) introduced with 0.15 g VS/L-d organic loading and 16oC average temperature. A conventional heated mixed lab digester yielded 0.22 LCH4/g VS with 0.25 g VS/L-d and 30oC. The highest yield (0.30 LCH4/g VS) was achieved by the unmixed lab digesters operated at a constant 20oC. All digesters were operated with a 40-d hydraulic residence time. 6. In general, 50-75% of initial particulate N and P could be solubilized during anaerobic digestion and available for subsequent rounds of algae cultivation. 7. Bench-scale experiments showed the recovery from hydrothermal liquefaction (HTL) wastewater of carbon via anaerobic digestion and of nutrients to grow algae. To satisfy the nitrogen demand of algae cultivation, HTL wastewater would be diluted 400-fold, which was found to eliminate inhibition of algae growth by HTL wastewater. 8. Anaerobic digestion methane yield was lower for algal biomass containing coagulants such as would be used to aid harvesting or dewatering. Depending on doses, starch-based coagulant decreased yield by 10-14% and aluminum chlorohydrate decreased it by 14-26%. The lowest yield was 0.28 L CH4/g volatile solids introduced to the digesters. 9. Algae harvested from raceways operated on recycled water had methane yields 13% higher than algae from raceways operated on both recycled water and nutrients provided by algae digestate. The slightly lower yield was expected due to the presence of previously digested biomass from the digestate fertilizer. 10. Defined media was replenished with nutrients and recycled repeatedly in sequential batch growth of Chlorella sorokiniana (DOE 1412). This laboratory study tested for inhibition and accumulation of inhibiting compounds (allelopathic or auto-inhibitory substances), information that would help estimate the blowdown ratio needed for an integrated system. In laboratory experiments in which water was recycled a total of five times, each successive round of reuse resulted in an average 4±3% reduction in log-phase specific growth rates. However, linear-phase growth inhibition was only observed in the final fifth round of reuse. 11. No decline in productivity was detected after 15 rounds of water recycling with nutrients provided by whole digestate in lab cultivation. Lab tests allowed for steady light and temperature, increasing the ability to detect inhibition. 12. In initial pilot inhibition studies, wastewater growth media was reused once while productivity was monitored. Media reuse was accomplished with triplicate sets of 33-m2 raceways operated in series. First-round gross productivity (based on effluent biomass flow) averaged 23 g/m2-day annually while second-round gross productivity averaged 19 g/m2-day annually. In terms of net productivity (based on raceway effluent biomass minus influent biomass), the first-round productivity averaged 15 g/m2-d and second round averaged 13 g/m2-d during June-September operation. The higher productivity in the first-round ponds was likely due to heterotrophic/mixotrophic growth on the wastewater organic matter. 13. In a culminating pilot experiment, coagulant was used to decrease the carry-over of unsettled algae into subsequent rounds of growth. Over nearly 8 months, 93% of the media (the equivalent of 14 rounds of water reuse) was recycled without significant productivity loss compared to controls. Ponds receiving both recycled water and nutrients had net productivities of 14-24 g/m2-d during fall and mid-summer, respectively. 14. Techno-economic analysis of the proposed facility found minimum fuel selling price to range from $7.01/gallon gasoline equivalent without revenue other than fuel to $3.85/GGE with revenue from wastewater treatment fees and LCFS and RIN (Low Carbon Fuel Standard and Renewable Identification Numbers) credits. 15. Life cycle assessment indicated GHG emissions of 40.7 g CO2/MJ fuel and a net energy ratio (energy required/energy produced) of 0.37.

Algal-bacteria high-rate ponds represent an energy-efficient wastewater treatment approach and a source for affordable and sustainable biomass feedstock for production of renewable energy through anaerobic digestion (AD). However, there is still a need for more data on wastewater treatment efficiency, biomass productivity and settleability from outdoor treatment facilities, as well as on impact of variability in biomass composition and digestibility on methane yield and energy output. Hydraulic retention time (HRT) and wastewater quality fed into 30 m(2) raceway ponds had a major effect on algal-bacteria polyculture productivity, settleability, phylogenetic and biochemical compositions, digestibility and methane yield. While Micractinium, Scenedesmus, Chlorella, and pennate diatoms were always among the key species observed, the gross productivity and 2-hour settle-ability during summer cultivation varied in the first-stage ponds treating primary wastewater/ from 29 +/- 5 to 54 +/- 12 g(AFDW)/m(2)/d and from 88 +/- 8 to 94 +/- 4% for HRT of 3 and 2 days, respectively. For these conditions, the effluent had csBOD of 3.4 or 3.3 mg/L and N-total (mostly NO3- -N) of 9.2 or 8.2 mg/L, respectively. The second-stage algal ponds (HRT 3 days) showed lower productivity of 16 +/- 6 g(AFDW)/m(2)/d, settleability of 84 +/- 11%, and effluent csBOD 3.7 mg/L and N-total 0.8 mg/L. Biomass composition from different ponds was 34-38% protein, 18-28% total lipids and 6-14% FAME. The methane yield varied about 30% with largest value of 0.34 +/- 0.01 L/gVS and showed a positive correlation with biomass lipid content (R-2 = 0.93). First-order and pseudo-parallel first-order rate kinetic models exhibited a better fit for methane production (most R-2 > 0.993) than the modified Gompertz model. The variation in biomass composition led to significant differences in energy output (varied by about 60%), Net Energy Ratios (ranged from 1.6 to 2.2) and Net Energy Efficiency (from 60% to 70%) when projecting the energy balance for a large-scale continuous AD process with an optimal HRT of 20-30 days.

Renewable fuels can be more environmentally friendly than conventional fuels and can be produced locally, supporting energy independence. Liquid renewable fuels are of special interest due to their high energy density. Algae are a promising prospective feedstock for low carbon intensity liquid biofuels, due to their high productivity and potential for high lipid or carbohydrate content. Their ability to grow on wastewater and waste nutrients and on non-arable land are cost and sustainability advantages. The organic matter content of wastewater promotes mixotrophic and/or heterotrophic growth of both algae and bacteria, boosting biomass productivity further. However, economic and technologic challenges persist that prevent the scale-up of algal biofuels to commercial relevance. These challenges include algal harvesting, drying, dewatering, and the conversion of algal biomass to usable fuels (Hannon et al 2010, Dunlap and Shaw 2009, Pienkos et al. 2009). The US Department of Energy (DOE) Bioenergy Technologies Office (BETO) supports research to overcome these challenges and outlines the research and development goals for algal biofuels in their Multi-Year Program Plan (MYPP). The long-term goals of the MYPP for algal biofuels research are briefly as follows: develop the domestic ability to produce algal biofuels at a scale of 5 billion gallons per year (BGY) by 2030 and demonstrate technologies that produce biofuel intermediates from algae at a cost of $3/gallon of gasoline equivalent (GGE) by 2022 (BETO MYPP 2016). This research reported herein was funded to investigate methods of meeting an intermediate milestone of 2,500 gallons of biofuel intermediate per acre per year for a non-integrated process by improving algal productivity, harvesting efficiency, and conversion to biofuel intermediates. Use of wastewater and wastewater nutrients was a key distinguishing characteristic of this project. In pursuit of 2,500 gallons of biofuel intermediate (BFI) per acre per year, California Polytechnic State University, San Luis Obispo (Cal Poly) sought to develop the capability to produce biofuel intermediates from microalgae grown on municipal wastewater at a 20-acre algae-based wastewater treatment facility in Delhi, California (37.43° N), which includes 7 acres of raceway ponds. Coupling algal biofuel production with wastewater treatment capitalizes on the abundant waste nutrients and carbon present in wastewater and reduces use of clean water in algal biorefinery systems. In this research, the performance of the full-scale wastewater treatment plant was characterized by measuring productivity, wastewater treatment performance, hydraulic characteristics, and energy consumption. Nine 1,000-L pilot raceways with CO2 addition were used in experiments to maximize productivity. Low-cost, energy-efficient algae bioflocculation and settling were monitored. Population genetics were monitored in an attempt to correlate taxa with superior productivity and/or settling. Algae-bacterial biomass was converted to biofuel intermediates via bench-scale hydrothermal liquefaction (HTL), and the resulting fuel quality was characterized. To develop rapid strain screening capabilities, a climate simulating photobioreactor ("LEAPS") was developed and validated against outdoor raceways. A techno-economic analysis (TEA) and a lifecycle assessment (LCA) were performed to model process economics and sustainability. The major outcomes of this research included demonstration of 33 g/m2-day annual average algal-bacterial productivity in pilot raceways, and a biofuel intermediate yield of 0.35 g intermediate/g algae biomass via HTL, for a yield of 4,100 gallons BFI per acre per year, assuming a 90% harvest efficiency and ignoring other minor losses. This exceeded the project goal of 2,500 gallons of BFI per acre per year and nearly met the 2030 program goal, highlighting the promise of integrating wastewater treatment and biofuels production. The biocrude BFI produced by the process is refined to diesel and naphtha fuels. These fuels are normalized to the energy content of gasoline (gallons gasoline equivalent, GGE, for comparison to other fuels). Results from the TEA of the this complete "well-to-wheel" process demonstrated that, at a 400-ha scale, a minimum fuel selling price (MFSP) of $12.55/GGE, which could be decreased 57% to $7.14/GGE when including revenue from coupling the biofuel production with the co-products of wastewater treatment services and low carbon fuel credits from California and Federal programs. Due mainly to the high capital costs of thickener centrifuges and hydrothermal processing equipment, the fuel cost is sensitive to scale, with significantly lower costs expected for larger farms. In this research project, significant advances were made in improving raceway algal productivity and fuel production from algae biomass, while also improving process economics. These outcomes brought the algal biofuel technology closer to being a sustainable and marketable process. The remainder of this executive summary is organized into the following six primary project tasks: (1) to optimize biomass productivity for a selected strain at Delhi, (2) to maximize algal productivity and harvesting efficiency in Delhi pilot ponds, (3) a full-scale raceway hydraulic characterization, (4) biomass processing to biofuel intermediates, and (5) scale-up engineering analysis, modeling, and planning.

Options for Energy from Algae at Wastewater Treatment FacilitiesThe development of algae biofuels is a priority for the U.S. Department of Energy, Department of Defense, and many other organizations. Large-scale algae production facilities are already in the planning stage. Algal biomass harvested from wastewater treatment ponds can also potentially be used as a feedstock for renewable fuels such as biogas, biodiesel, bio-oil or synthesis gas. Despite the...Author(s)Matt HuttonRuth SpierlingDan HeimelIan WoertzTryg LundquistSourceProceedings of the Water Environment FederationSubjectSession 113: Algae for Treatment and BiofuelsDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2011ISSN1938-6478SICI1938-6478(20110101)2011:7L.7283;1-DOI10.2175/193864711802793669Volume / Issue2011 / 7Content sourceWEFTECFirst / last page(s)7283 - 7293Copyright2011Word count259Subject keywordsAlgaebiofuelanaerobic digestionbiodieselwastewater treatment