Process design and economic analysis for the production of microalgae from anaerobic digestates in open raceway ponds.

A model based framework was established for large scale assessment of microalgae production using anaerobically digested effluent considering varied climatic parameters such as solar irradiance and air temperature. The aim of this research was to identify the optimum monthly average culture depth operation to minimize the cost of producing microalgae grown on anaerobic digestion effluents rich in ammoniacal nitrogen with concentration of 248 mg L-1. First, a productivity model combined with a thermal model was developed to simulate microalgae productivity in open raceway ponds as a function of climatic variables. Second, by combining the comprehensive open pond model with other harvesting equipment, the final techno economic model was developed to produce a microalgae product with 20 wt% biomass content and treated water with <1 mg L-1 ammoniacal nitrogen. The optimization approach on culture depth for outdoor open raceway ponds managed to reduce the cost of microalgae production grown in anaerobic digested wastewater up to 16 %, being a suitable solution for the production of low cost microalgae (1.7 AUD kg-1 dry weight) at possible scale of 1300 t dry weight microalgae yr-1.

Microalgae cultivation for treating agricultural effluent and producing value-added products.

Wastewater generated within agricultural sectors such as dairies, piggeries, poultry farms, and cattle meat processing plants is expected to reach 600 million m3 yr-1 globally. Currently, the wastewater produced by these industries are primarily treated by aerobic and anaerobic methods. However, the treated effluent maintains a significant concentration of nutrients, particularly nitrogen and phosphorus. On the other hand, the valorisation of conventional microalgae biomass into bioproducts with high market value still requires expensive processing pathways such as dewatering and extraction. Consequently, cultivating microalgae using agricultural effluents shows the potential as a future technology for producing value-added products and treated water with low nutrient content. This review explores the feasibility of growing microalgae on agricultural effluents and their ability to remove nutrients, specifically nitrogen and phosphorus. In addition to evaluating the market size and value of products from wastewater-grown microalgae, we also analysed their biochemical characteristics including protein, carbohydrate, lipid, and pigment content. Furthermore, we assessed the costs of both upstream and downstream processing of biomass to gain a comprehensive understanding of the economic potential of the process. The findings from this study are expected to facilitate further techno-economic and feasibility assessments by providing insights into optimized processing pathways and ultimately leading to the reduction of costs.

Nitrogen dynamics and biological processes in soil amended with microalgae grown in abattoir digestate to recover nutrients.

The use of microalgae for nutrient recovery from wastewater and subsequent conversion of the harvested biomass into fertilizers offers a sustainable approach towards creating a circular economy. Nonetheless, the process of drying the harvested microalgae represents an additional cost, and its impact on soil nutrient cycling compared to wet algal biomass is not thoroughly understood. To investigate this, a 56-day soil incubation experiment was conducted to compare the effects of wet and dried Scenedesmus sp. microalgae on soil chemistry, microbial biomass, CO2 respiration, and bacterial community diversity. The experiment also included control treatments with glucose, glucose + ammonium nitrate, and no fertilizer addition. The Illumina Mi-Seq platform was used to profile the bacterial community and in-silico analysis was performed to assess the functional genes involved in N and C cycling processes. The maximum CO2 respiration and microbial biomass carbon (MBC) concentration of dried microalgae treatment were 17% and 38% higher than those of paste microalgae treatment, respectively. NH4+ and NO3- released slowly and through decomposition of microalgae by soil microorganisms as compared to synthetic fertilizer control. The results indicate that heterotrophic nitrification may contribute to nitrate production for both microalgae amendments, as evidenced by low amoA gene abundance and a decrease in ammonium with an increase in nitrate concentration. Additionally, dissimilatory nitrate reduction to ammonium (DNRA) may be contributing to ammonium production in the wet microalgae amendment, as indicated by an increase in nrfA gene and ammonium concentration. This is a significant finding because DNRA leads to N retention in agricultural soils instead of N loss via nitrification and denitrification. Thus, further processing the microalgae through drying or dewetting may not be favorable for fertilizer production as the wet microalgae appeared to promote DNRA and N retention.

Lactic acid production from food waste at an anaerobic digestion biorefinery: effect of digestate recirculation and sucrose supplementation.

Low lactic acid (LA) yields from direct food waste (FW) fermentation restrict this production pathway. However, nitrogen and other nutrients within FW digestate, in combination with sucrose supplementation, may enhance LA production and improve feasibility of fermentation. Therefore, this work aimed to improve LA fermentation from FWs by supplementing nitrogen (0-400 mgN·L-1) as NH4Cl or digestate and dosing sucrose (0-150 g·L-1) as a low-cost carbohydrate. Overall, NH4Cl and digestate led to similar improvements in the rate of LA formation (0.03 ± 0.02 and 0.04 ± 0.02 h-1 for NH4Cl and digestate, respectively), but NH4Cl also improved the final concentration, though effects varied between treatments (5.2 ± 4.6 g·L-1). While digestate altered the community composition and increased diversity, sucrose minimised community diversion from LA, promoted Lactobacillus growth at all dosages, and enhanced the final LA concentration from 25 to 30 g·L-1 to 59-68 g·L-1, depending on nitrogen dosage and source. Overall, the results highlighted the value of digestate as a nutrient source and sucrose as both community controller and means to enhance the LA concentration in future LA biorefinery concepts.

Long term outdoor microalgal phycoremediation of anaerobically digested abattoir effluent.

Sufficient and reliable long-term field data on the growth, productivity and nutrient removal rates of microalgal based wastewater treatment system is essential to validate its overall techno-economic feasibility. Here, we investigated the semi-continuous microalgal cultivation of Scenedesmus sp. in anaerobically digested abattoir effluent (ADAE) for 13 months in outdoor raceway ponds operated at 20 cm depth. This study was initiated with three different cultures consisting of 1) monocultures of Chlorella sp., 2) Scenedesmus sp., and 3) an equal mixed concentration of both microalgae species. However, after 15 weeks, Scenedesmus sp. was found to be the most dominant microalgae species in all the different cultures, even completely taking over the Chlorella sp. monoculture. Over the course of summer and early autumn, the average weekly biomass productivity of Scenedesmus sp. cultures was 12.5 ± 0.6 g m-2 d-1 which was 16% and 30% higher than productivities recorded in spring and winter, respectively. All available ammoniacal nitrogen (NH3-N) was found to be exhausted during each growth period with an average 33.6% nitrogen assimilation rate. The average rate of phosphate and COD (chemical oxygen demand) removals were 85.2% and 37.5% throughout the cultivation period. No significant differences were found in carbohydrate, lipid and protein content of Scenedesmus sp. during different seasons of the year. Over 53% increase in biomass productivity can be achieved if CO2 is added to control culture pH at pH 6.5. Here, we successfully demonstrated reliability of continuous long-term cultivation of microalgae in ADAE for simultaneous wastewater treatment and algal biomass production.

Monochromatic light filters to enhance biomass and carotenoid productivities of Dunaliella salina in raceway ponds.

Monochromatic blue and red wavelengths are more efficient for light to algal biomass conversion than full-spectrum sunlight. In this study, monochromatic light filters were used to down-regulate natural sunlight to blue (400-520 nm) and red (600-700 nm) wavelengths to enhance biomass productivity of Dunaliella salina in outdoor raceway ponds. Growth indices such as cell size, pigment concentrations, biomass yield, photosynthetic efficiency, and major nutritional compositions were determined and compared against a control receiving unfiltered sunlight. Results showed that red light increased biomass productivity, lipid, and carotenoid contents but decreased cell volume, chlorophyll production, and cell weight. Conversely, blue light increased cell volume by 200%, cell weight by 68%, and enhanced chlorophyll a and protein contents by 35% and 51%, respectively, over red light. Compared to the control treatment, photoinhibition of D. salina cells at noon sunshine was decreased 60% by utilizing optical filters on the pond's surface.

Microalgae cultivation for the treatment of anaerobically digested municipal centrate (ADMC) and anaerobically digested abattoir effluent (ADAE).

The successful cultivation of microalgae in wastewater establishes a waste to profit scenario as it combines treatment of a waste stream with production of valuable end-products. Here, growth and nutrient removal efficiency of three different locally isolated microalgal cultures (Chlorella sp., Scenedesmus sp., and a mixed consortium) cultivated in anaerobically digested municipal centrate (ADMC) and anaerobically digested abattoir effluent (ADAE) was evaluated. No significant differences (P > 0.05) in specific growth rate and biomass productivity were recorded between Chlorella monocultures and the mixed culture grown in both effluents. Scenedesmus sp. monocultures was found incapable of growth in both ADMC and ADAE throughout the cultivation period resulting in the collapse of cultures and no further measurements on the growth, biomass production and nutrient removal efficiency of this alga in both effluent. Fq´/Fm´ values which represent the immediate photo-physiological status of microalgae found to be negatively inhibited when Scenedesmus sp. was grown in both effluents throughout the cultivation period. Fq´/Fm´ values of Chlorella sp. monocultures and the mixed cultures recovered back to normal (≈0.6) after an initial drop. Ammonium removal rates was found to be significantly higher (≈2 folds) for Chlorella sp. monocultures grown in both ADMC and ADAE when compared to the mixed cultures. Nonetheless, no significant differences were observed in the removal of phosphate for both cultures in the different effluents. The total protein and carbohydrate content of the biomass produced was similar for both microalgae cultures grown using ADAE and ADMC. However, chlorophyll a and total carotenoids content were found to be higher (P < 0.05) for the cultures grown in ADAE than ADMC. Overall, Chlorella sp. monoculture was the most efficient option for treating both ADMC and ADAE while simultaneously generating protein rich biomass (up to 49%) that can be potentially exploited as aquaculture feedstock.

Ammonia stress on a resilient mesophilic anaerobic inoculum: Methane production, microbial community, and putative metabolic pathways.

Short term inhibition tests, 16S rRNA tag sequencing and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt), were employed to visualise the effects of increasing total ammoniacal nitrogen (TAN) concentration (3400-10166 ppm TAN) on microbial community structure and metabolic pathways for acetate degradation. The rate of methane production on acetate was significantly reduced by TAN concentrations above 6133 ppm; however, methane continued to be produced, even at 10166 ppm TAN (0.026 ±â€¯0.0003 gCOD.gVS-1inoculum.day-1). Hydrogenotrophic methanogenesis with syntrophic acetate oxidation (SAO) was identified as the dominant pathway for methane production. A shift towards SAO pathways at higher TAN concentrations and a decrease in the number of 'gene hits' for key genes in specific methanogenesis pathways was observed. Overall, the results highlighted potential for inhibition activity testing to be used together with PICRUSt, to estimate changes in microbial metabolism and to better understand microbial resilience in industrial AD facilities.