A novel option for reducing the optical density of liquid digestate to achieve a more productive microalgal culturing

Engineered algal systems for the treatment of anaerobic digestate: A meta-analysis

The objective of this review was to provide quantitative insights into algal growth and nutrient removal in anaerobic digestate. To synthesize the relevant literature, a meta-analysis was conducted using data from 58 articles to elucidate key factors that impact algal biomass productivity and nutrient removal from anaerobic digestate. On average, algal biomass productivity in anaerobic digestate was significantly lower than that in synthetic control media (p < 0.05) but large variation in productivity was observed. A mixed-effects multiple regression model across study revealed that biological or chemical pretreatment of digestate significantly increase productivity (p < 0.001). In contrast, the commonly used practice of digestate dilution was not a significant factor in the model. High initial total ammonia nitrogen suppressed algal growth (p = 0.036) whereas initial total phosphorus concentration, digestate sterilization, CO2 supplementation, and temperature were not statistically significant factors. Higher growth corresponded with significantly higher NH4-N and phosphorus removal with a linear relationship of 6.4 mg NH4-N and 0.73 mg P removed per 100 mg of algal biomass growth (p < 0.001). The literature suggests that suboptimal algal growth in anaerobic digestate could be due to factors such as turbidity, high free ammonia, and residual organic compounds. This analysis shows that non-dilution approaches, such as biological or chemical pretreatment, for alleviating algal inhibition are recommended for algal digestate treatment systems.

Innovative and sustainable cultivation strategy for the production of Spirulina platensis using anaerobic digestates diluted with residual geothermal water

The following study investigates the possibility of growing the Spirulina platensis (S. platensis) cyanobacteria on two agro-industrial anaerobic digestion (AD) digestates diluted with geothermal water. The two digestates (FAWD: Food and Agricultural Wastes Digestate and CDD: Cheese Diary Digestate) were selected based on their different chemical characteristics, attributed to the type of feedstock and the operating conditions used during the AD process. In the first part of the study, a screening experiment was performed in 200 mL glass tubes to evaluate the appropriate dilution factor to generate the maximum S. platensis growth using both AD digestates individually and geothermal water as sustainable alternative dilution agent. Based on the different growth parameters measured, dilution rates of 5x and 40x were chosen for CDD and FAWD respectively, as a trade-off between growth performances and quantity of water to use. Volumetric productivities of 33 ± 1 mg/L/d and 56 ± 8 mg/L/d combined with maximal concentrations of 0.52 ± 0.02 g/L and 0.69 ± 0.02 g/L were achieved when cultivating S. platensis on CDD and FAWD, respectively. In the second part, the selected experimental results were scaled-up to 6 L flat panels bioreactors and S. platensis biomass productivities of 71 and 101 mg/L/d were obtained for CDD and FAWD, respectively using sodium bicarbonate as inorganic carbon source. When regulating the pH to 8.5 with carbon dioxide (CO2) injection, cultures were able to produce up to 1.13 g/L and 0.79 g/L of S. platensis corresponding to biomass productivities of 81 and 136 mg/L/d for CDD and FAWD, respectively. In addition, S. platensis properly assimilated the ammonium present in the digestate-based culture media, with removal efficiency up to 98% in the case of the CDD substrate. The characterization of the final S. platensis biomass revealed the presence of high concentration of carbohydrates (48.6-70.3 % of dry weight) in the culture supplemented with both AD digestates. The experimental findings show the potential of reusing liquid digestate, CO2 as well as geothermal water for the sustainable production of carbohydrate-rich S. platensis biomass.

Algal Biomass Accumulation in Waste Digestate after Anaerobic Digestion of Wheat Straw

Cultivation of microalgae in waste digestate is a promising cost-effective and environmentally friendly strategy for algal biomass accumulation and valuable product production. Two different digestates obtained as by-products of the anaerobic fermentation at 35 °C and 55 °C of wheat straw as a renewable source for biogas production in laboratory-scale bioreactors were tested as cultivation media for microalgae after pretreatment with active carbon for clarification. The strains of microalgae involved were the red marine microalga Porphyridium cruentum, which reached 4.7 mg/mL dry matter when grown in thermophilic digestate and green freshwater microalga-Scenedesmus acutus, whose growth was the highest—7.3 mg/mL in the mesophilic digestate. During cultivation, algae reduced the available nutrient components in the liquid digestate at the expense of increasing their biomass. This biomass can find further applications in cosmetics, pharmacy, and feed. The nitrogen and phosphorus uptake from both digestates during algae cultivation was monitored and modeled. The results led to the idea of nonlinear dynamic approximations with an exponential character. The purpose was to develop relatively simple nonlinear dynamic models based on available experimental data, as knowing the mechanisms of the considered processes can permit creating protocols for industrial-scale algal production toward obtaining economically valuable products from microalgae grown in organic waste digestate.

Microalgae production for nitrogen recovery of high-strength dry anaerobic digestion effluent

Dry anaerobic digestion (D-AD) generates nitrogen-rich effluents that are normally neglected in the circular bioeconomy. The high turbidity and ammonium content hamper nitrogen recovery from these effluents via biological processes, such as microalgae culture. The goal of this study was to demonstrate microalgae growth viability in high-strength D-AD effluents in order to recover nitrogen (N) as microalgae biomass. According to the experimental factorial design conducted in batch reactors, ammonium was identified as the critical inhibitory compound for microalgae growth while turbidity did not exhibit a significantly negative effect. Instead, turbidity resulted advantageous since it promoted high nitrogen uptake rates and biomass production. The presence of organic turbidity resulted in a positive effect that boosted Chlorella growth in a stream with higher ammonium (350 mg NH₄⁺-N L^-1) and turbidity (175 NTU) than the inhibition thresholds reported in the literature, reaching 98.7% of N recovery as microalgae biomass. When microalgae culture was scaled up in a photobioreactor operated in continuous mode, microalgae biomass was effectively produced while recovering 100% of N at a hydraulic retention time of 10 days. By imposing long exposure times and high turbidity, Chlorella adaptation to high-strength D-AD effluent resulted in high N uptake and biomass production. This study demonstrated not only the most influencing factor and the optimal NH₄⁺-N and turbidity combination, but also the viability of using D-AD effluents as culture media for microalgae biomass production.

Attempts to alleviate inhibitory factors of anaerobic digestate for enhanced microalgae cultivation and nutrients removal: A review

Anaerobic digestion is a well-established process that is applied to treat organic wastes and convert the carbon to valuable methane gas as a source of energy. The digestate that comes out as a by-product is of a great challenge due to its high nutrient content that can be toxic in case of improper disposal to the environment. Several attempts have been done to valorize this digestate. Digestate has been considered as an interesting medium to cultivate microalgae. The nutrients available in the digestate, mainly nitrogen and phosphorus, can be an interesting supplement for microalgae growth requirement. The main obstacles of using digestate as a medium to cultivate microalgae are the dark color and the high ammonium-nitrogen concentration. The focus of this review is to discuss in detail the major attempts in research to overcome inhibition and enhance microalgae cultivation in digestate. This review initially discussed the obstacles of digestate as a medium for microalgae cultivation. Different processes to overcome inhibition were discussed including dilution, supplying additional carbon source, favoring mixotrophic cultivation and pretreatment. More emphasis in this review was given to digestate pretreatment. Among the pretreatment methods, filtration, and centrifugation were of the most applied ones. These strategies were found to be effective for turbidity and chromaticity reduction. For ammonium nitrogen removal, ammonia stripping and biological pretreatment methods were found to play a vital role. Adsorption could work both ways depending on the material used. Combining different pretreatment methods as well as including selected microalgae stains were found interesting strategies to facilitate microalgae cultivation with no dilution. This study recommend that more study should investigate the optimization of microalgae cultivation in anaerobic digestate without the need for dilution.

Valorising nutrient-rich digestate: Dilution, settlement and membrane filtration processing for optimisation as a waste-based media for microalgal cultivation

Digestate produced from the anaerobic digestion of food and farm waste is primarily returned to land as a biofertiliser for crops, with its potential to generate value through alternative processing methods at present under explored. In this work, valorisation of a digestate resulting from the treatment of kitchen and food waste was investigated, using dilution, settlement and membrane processing technology. Processed digestate was subsequently tested as a nutrient source for the cultivation of Chlorella vulgaris, up to pilot-scale (800L). Dilution of digestate down to 2.5% increased settlement rate and induced release of valuable compounds for fertiliser usage such as nitrogen and phosphorus. Settlement, as a partial processing of digestate offered a physical separation of liquid and solid fractions at a low cost. Membrane filtration demonstrated efficient segregation of nutrients, with micro-filtration recovering 92.38% of phosphorus and the combination of micro-filtration, ultra-filtration, and nano-filtration recovering a total of 94.35% of nitrogen from digestate. Nano-filtered and micro-filtered digestates at low concentrations were suitable substrates to support growth of Chlorella vulgaris. At pilot-scale, the microalgae grew successfully for 28 days with a maximum growth rate of 0.62 day^-1 and dry weight of 0.86  g⋅L^-1. Decline in culture growth beyond 28 days was presumably linked to ammonium and heavy metal accumulation in the cultivation medium. Processed digestate provided a suitable nutrient source for successful microalgal cultivation at pilot-scale, evidencing potential to convert excess nutrients into biomass, generating value from excess digestate and providing additional markets to the anaerobic digestion sector.

Production of Microalgal Slow-Release Fertilizer by Valorizing Liquid Agricultural Digestate: Growth Experiments with Tomatoes

Anaerobic Digestion (AD) is a process that is well-known and fast-developing in Europe. AD generates large amounts of digestate, especially in livestock-intensive areas. Digestate has potential environmental issues due to nutrients (such as nitrogen) lixiviation or volatilization. Using liquid digestate as a nutrient source for microalgae growth is considered beneficial because digestate could be valorized and upgraded by the production of an added value product. In this work, microalgal biomass produced using liquid digestate from an agricultural biogas plant was investigated as a slow-release fertilizer in tomatoes. Monoraphidium sp. was first cultivated at different dilutions (1:20, 1:30, 1:50), in indoor laboratory-scale trials. The optimum dilution factor was determined to be 1:50, with a specific growth rate of 0.13 d−1 and a complete nitrogen removal capacity in 25 days of culture. Then, outdoor experiments were conducted in a 110 dm3 vertical, closed photobioreactors (PBRs) in batch and semi-continuous mode with 1:50 diluted liquid digestate. During the batch mode, the microalgae were able to remove almost all NH4+ and 65 (±13) % of PO43−, while the microalgal growth rate reached 0.25 d−1. After the batch mode, the cultures were switched to operate under semi-continuously conditions. The cell densities were maintained at 1.3 × 107 cells mL−1 and a biomass productivity around 38.3 mg TSS L−1 d−1 during three weeks was achieved, where after that it started to decline due to unfavorable weather conditions. Microalgae biomass was further tested as a fertilizer for tomatoes growth, enhancing by 32% plant growth in terms of dry biomass compared with the control trials (without fertilization). Similar performances were achieved in tomato growth using synthetic fertilizer or digestate. Finally, the leaching effect in soils columns without plant was tested and after 25 days, only 7% of N was leached when microalgae were used, against 50% in the case of synthetic fertilizer.

Effect of iron and magnesium addition on population dynamics and high value product of microalgae grown in anaerobic liquid digestate

In this study, FeSO4 supplementation ranging from 0 to 4.5 mM, and MgSO4 supplementation ranging from 0 to 5.1 mM were investigated to observe the effect on the population dynamics, biochemical composition and fatty acid content of mixed microalgae grown in Anaerobic Liquid Digestate (ALD). Overall, 3.1 mM FeSO4 addition into ALD increased the total protein content 60% and led to highest biomass (1.56 g L-1) and chlorophyll-a amount (18.7 mg L-1) produced. Meanwhile, 0.4 mM MgSO4 addition increased the total carotenoid amount 2.2 folds and slightly increased the biomass amount. According to the microbial community analysis, Diphylleia rotans, Synechocystis PCC-6803 and Chlorella sorokiniana were identified as mostly detected species after confirmation with 4 different markers. The abundance of Chlorella sorokiniana and Synechocystis PCC-6803 increased almost 2 folds both in iron and magnesium addition. On the other hand, the dominancy of Diphylleia rotans was not affected by iron addition while drastically decreased (95%) with magnesium addition. This study helps to understand how the dynamics of symbiotic life changes if macro elements are added to the ALD and reveal that microalgae can adapt to adverse environmental conditions by fostering the diversity with a positive effect on high value product.

Effect of organic carbon and nutrient supplementation on the digestate-grown microalga, Parachlorella kessleri

Digested effluents are usually deprived of the appropriate levels of organic carbon or macro- and micro-nutrients to effectively sustain microalgal growth. In this regard, Parachlorella kessleri was cultivated in an agro-waste digestate supplemented with different glucose concentrations, magnesium and trace metals and alternatively with cheese whey (CW), with view to enriching digestate with organic and inorganic constituents and decreasing freshwater demand. Between the conditions tested, CW addition resulted in the highest biomass concentration, 2.68 g L^-1 within 18 days of cultivation. Chlorophyll content significantly decreased under 5 g L^-1 glucose addition, in contrast to MgSO₄ co-addition and CW supplementation. The latter also induced high photosynthetic activity and better-preserved vitality of the photosynthetic apparatus, compared to sole glucose addition. Concerning lipid accumulation, in the presence of high glucose concentration, % of total fatty acids decreased, and the saturated fraction increased over polyunsaturated fatty acids (PUFAs).

Microalgae and Phototrophic Purple Bacteria for Nutrient Recovery From Agri-Industrial Effluents: Influences on Plant Growth, Rhizosphere Bacteria, and Putative Carbon- and Nitrogen-Cycling Genes

Microalgae (MA) and purple phototrophic bacteria (PPB) have the ability to remove and recover nutrients from digestate (anaerobic digestion effluent) and pre-settled pig manure that can be Utilized as bio-fertilizer and organic fertilizer. The objective of this study was to compare the effectiveness of MA and PPB as organic fertilizers and soil conditioners in relation to plant growth and the soil biological processes involved in nitrogen (N) and carbon (C) cycling. To this end, a glasshouse experiment was conducted using MA and PPB as bio-fertilizers to grow a common pasture ryegrass (Lolium rigidum Gaudin) with two destructive harvests (45 and 60 days after emergence). To evaluate the rhizosphere bacterial community, we used barcoded PCR-amplified bacterial 16S rRNA genes for paired-end sequencing on the Illumina Mi-Seq. Additionally, we used phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) analysis for the detection of putative functional genes associated with N and soil-C cycling. There was a significant increase in plant growth when the soil was amended with PPB, which almost performed as well as the chemical fertilizers. Analysis of the rhizosphere bacteria after the second harvest revealed a greater abundance of Firmicutes than in the first harvest. Members of this phylum have been identified as a biostimulant for plant growth. In contrast, the MA released nutrients more slowly and had a profound effect on N cycling by modulating N mineralization and N retention pathways. Thus, MA could be developed as a slow-release fertilizer with better N retention, which could improve crop performance and soil function, despite nutrient losses from leaching, runoff, and atmospheric emissions. These data indicate that biologically recovered nutrients from waste resources can be effective as a fertilizer, resulting in enhanced C- and N-cycling capacities in the rhizosphere.

Cultivation of microalgal biomass using swine manure for biohydrogen production: Impact of dilution ratio and pretreatment

This study assessed the impact of swine manure (SM) dilution ratio on the microalgal biomass cultivation and further tested for biohydrogen production efficiency from the mixed microalgal biomass. At first, various solid/liquid (S/L) ratio of the SM ranged from 2.5 to 10 g/L was prepared as a nutrient medium for the algal biomass cultivation without addition of the external nutrient sources over a period of 18 d. The peak biomass concentration of 2.57 ± 0.03 g/L was obtained under the initial S/L loading rates of 5 g/L. Further, the cultivated biomass was subjected to two-step (ultrasonication + enzymatic) pretreatment and evaluated for biohydrogen production potential. Results showed that the variable amount of hydrogen production was observed with different S/L ratio of the SM. The peak hydrogen yield of 116 ± 6 mL/g TS_added was observed at the 5 g/L grown SM mixed algal biomass.

Biocapture of CO₂ by Different Microalgal-Based Technologies for Biogas Upgrading and Simultaneous Biogas Slurry Purification under Various Light Intensities and Photoperiods

Co-cultivation of microalgae and microbes for pollutant removal from sewage is considered as an effective wastewater treatment method. The aim of this study is to screen the optimal photoperiod, light intensity and microalgae co-cultivation method for simultaneously removing nutrients in biogas slurry and capturing CO₂ in biogas. The microalgae–fungi pellets are deemed to be a viable option because of their high specific growth rate and nutrient and CO₂ removal efficiency under the photoperiod of 14 h light:10 h dark. The order of both the biogas slurry purification and biogas upgrading is ranked the same, that is Chlorella vulgaris–Ganoderma lucidum > Chlorella vulgaris–activated sludge > Chlorella vulgaris under different light intensities. For all cultivation methods, the moderate light intensity of 450 μmol m⁻² s⁻¹ is regarded as the best choice. This research revealed that the control of photoperiod and light intensity can promote the biological treatment process of biogas slurry purification and biogas upgrading using microalgal-based technology.

Photoautotrophic cultivation of mixed microalgae consortia using various organic waste streams towards remediation and resource recovery

In this study, mixed microalgae consortia was cultivated using digestate (D), animal manure (AM) and textile wastewater (TW) as growth medium providing mainly N (nitrogen) and P (phosphorous) sources without any extra nutrient addition. The corresponding total nitrogen (TN) and total phosphorous (TP, PO3-P) concentrations were noted as 323 and 21 for AM, 481 and 31 for TW and 747 and 55mg/L for D, respectively. After a cultivation period of 13days, P were completely removed (100%), however, N was still remain and attained the removal rate of 70.1, 72.3 and 16.7% for TW, AM and D, respectively. The peak growth rate and biomass production of 0.419d^-1 and 0.4g/L (in terms of volatile solids, VS) was achieved using TW as growth medium.

Cascading biomethane energy systems for sustainable green gas production in a circular economy

Biomethane is a flexible energy vector that can be used as a renewable fuel for both the heat and transport sectors. Recent EU legislation encourages the production and use of advanced, third generation biofuels with improved sustainability for future energy systems. The integration of technologies such as anaerobic digestion, gasification, and power to gas, along with advanced feedstocks such as algae will be at the forefront in meeting future sustainability criteria and achieving a green gas supply for the gas grid. This paper explores the relevant pathways in which an integrated biomethane industry could potentially materialise and identifies and discusses the latest biotechnological advances in the production of renewable gas. Three scenarios of cascading biomethane systems are developed.