Liquid Digestate Research Articles

Natural zeolite pretreatment of anaerobic digestate as efficient and promising detoxification strategy to improve microalgae biomass production

This work describes a new detoxification approach for the valorization of anaerobic digestate through microalgal cultivation. In this context, adsorption columns packed with different doses of natural zeolite (clinoptilolite) were used to pretreat liquid digestate and remove recalcitrant compounds such as ammonium nitrogen. After 24 h of treatment with 100 g/L of zeolite, the initial ammonium concentration was successfully reduced from 163.2 to 37.5 mg NH4-N/L, achieving 77.0 % of NH4-N removal efficiency and 1.3 mg NH4-N/g of adsorption capacity. As opposed to the untreated digestate, the zeolite pretreatment showed a significant positive effect on Tetradesmus obliquus microalgae consortium growth, with an average specific growth rate of 0.16 d−1 combined with the removal of 99.1 and 73.3 % of NH4-N and PO4-P, respectively. The experimental findings of this study have the potential to unlock the development of microalgae-based processes for the bioremediation of liquid digestate with the concomitant production of valuable algal biomass.

The liquid by-product of biogas production: characterisation and impact on soil fungi

ABSTRACT A liquid digestate (LD) obtained from the anaerobic digestion of mixed organic waste was characterised and tested on the fungi Pleurotus eryngii, Irpex lacteus and Trametes versicolor. Aqueous mixtures of LD at doses of 0.5, 1 and 2% (v/v) were tested directly or after interaction with 5% (w/v) biochar (BC-LD) and/or 100 mg L−1 soil humic acid (HA-BC-LD and HA-LD). Total luminescence (TL) analysis of LD showed the presence of fluorophores typical of scarcely aromatic matter, while the Fourier transform infrared (FTIR) spectrum evidenced absorption bands typical of labile and non-condensed material. Some spectroscopic variations of the LD sample were observed after its interaction with the other materials. All LD treatments markedly promoted hyphal extension of P. eryngii whose growth rate increased up to 38% at the highest LD dose. The LD alone had no influence on the other fungi, whose growth was stimulated by some combinations of LD with the other materials. In facts, the growth rate of I. lacteus increased in BC-LD 1 and BC-LD 2 (P ≤ 0.05) and in all treatments with HA (up to 6% in HA-LD 2, P ≤ 0.01), while the growth of T. versicolor was promoted by HA-BC-LD 2 treatment (P ≤ 0.05) and slightly inhibited by all BC-LD combinations (up to 7% by BC-LD 1, P ≤ 0.05). The overall results obtained encourage the addition of LD to the soil and suggest a possible use of this material as substrate ingredient for the cultivation of edible mushrooms such as P. eryngii.

Towards efficient N cycling in intensive maize: role of cover crops and application methods of digestate liquid fraction

AbstractDigestate, a by‐product of biogas production, is widely recognized as a promising renewable nitrogen (N) source with high potential to replace synthetic fertilizers. Yet, inefficient digestate use can lead to pollutant N losses as ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and nitrate () leaching. Cover crops (CCs) may reduce some of these losses and recycle the N back into the soil after incorporation, but the effect on the N balance depends on the CC species. In a one‐year field study, we tested two application methods (i.e., surface broadcasting, BDC; and shallow injection, INJ) of the liquid fraction of separated co‐digested cattle slurry (digestate liquid fraction [DLF]), combined with different winter cover crop (CC) options (i.e., rye, white mustard or bare fallow), as starter fertilizer for maize. Later, side‐dressing with urea was required to fulfil maize N‐requirements. We tested treatment effects on yield, N‐uptake, N‐use efficiency parameters, and N‐losses in the form of N2O emissions and leaching. CC development and biomass production were strongly affected by their contrasting frost tolerance, with spring‐regrowth for rye, while mustard was winter killed. After the CCs, injection of DLF increased N2O emissions significantly compared with BDC (emission factor of 2.69% vs. 1.66%). Nitrous oxide emissions accounted for a small part (11%–13%) of the overall yield‐scaled N losses (0.46–0.97 kg N Mg grain−1). The adoption of CCs reduced fall leaching, being 51% and 64% lower for mustard and rye than under bare soil. In addition, rye reduced leaching during spring and summer after termination by promoting N immobilization, thus leading to −57% lower annual leaching losses compared with mustard. DLF application method modified N‐loss pathways, but not the cumulative yield‐scaled N losses. Overall, these insights contribute to inform an evidence‐based design of cropping systems in which nutrients are recycled more efficiently.

Anaerobic Fluidized-Bed Membrane Bioreactor for Treatment of Liquid Fraction of Sludge Digestate: Performance and Agricultural Reuse Analysis

The treatment of sludge digestion liquid is a big challenge in wastewater treatment. If treated as normal wastewater, large amounts of nitrogen and phosphorus present in the sludge digestion liquid might be wasted when they could be reused in agricultural irrigation and reduce the consumption of artificial fertilizers. Thus, it is of utmost importance to deliver a simple and feasible strategy to treat sludge digestion liquid for agricultural reuse. In this study, a novel type of anaerobic fluidized bed membrane bioreactor system (US-AnFMBR) was developed by combining an ultrasonic processing unit and biochar in AnFMBR. The improvement of sludge properties, removal of pollutants performance and membrane fouling mitigation were achieved in this novel system. The optimal dose of BC (biochar) was 2.5 g·L−1, and the optimal ultrasonic treatment conditions were 30 min at 26 W. The main contribution of ultrasound was to improve the activity of sludge microorganisms to adsorb and degrade more organic matter present in sewage. The system achieved the removal efficiencies of COD, NH4+-N and PO43−-P up to 89.41%, 49.29% and 54.83%, respectively, and had a better mitigation effect in terms of membrane fouling. On the one hand, the biochar addition for COD removal performance was mainly manifested in membrane rejection performance. On the other hand, the combination of low-cost biochar and AnFMBR can also provide new ideas for the recycling of agricultural waste for biochar production. However, regarding the removal efficiency of NH4+-N and PO43−-P, the US-AnFMBR system promoted the activity of starved sludge to preferentially absorb NH4+-N compared with PO43−-P by statistical analysis. The US-AnFMBR can reduce the viscosity of sludge and release more small molecular substances, thus better mitigating membrane fouling. Long-term operation performance also revealed the excellent stability of the sludge digestion liquid treatment. The US-AnFMBR system achieves the recovery of nitrogen and phosphorus resources for subsequent agricultural recycling, and avoids the eutrophication of water ecosystems. Reclaimed water meets the nutrient requirements of typical crops during the growing season. To a certain extent, carbon emission reductions in agriculture can be achieved.

Recovering, Stabilizing, and Reusing Nitrogen and Carbon from Nutrient-Containing Liquid Waste as Ammonium Carbonate Fertilizer

Ammonium carbonates are a group of fertilizer materials that include ammonium bicarbonate, ammonium carbonate hydrate, and ammonium carbamate. They can be synthesized from diverse nutrient-bearing liquid waste streams but are unstable in a moist environment. While extensively utilized several decades ago, their use gradually decreased in favor of large-scale, facility-synthesized urea fertilizers. The emergence of sustainable agriculture, however, necessitates the recovery and reuse of nutrients using conventional feedstocks, such as natural gas and air-derived nitrogen, and nutrient-containing biogenic waste streams. To this extent, anaerobic digestion liquid presents a convenient source of solid nitrogen and carbon to produce solid fertilizers, since no significant chemical transformations are needed as nitrogen is already present as an ammonium ion. This review describes detailed examples of such feedstocks and the methods required to concentrate and crystallize solid ammonium carbonates. The technologies currently proposed or utilized to stabilize ammonium carbonate materials in the environment are described in detail. Finally, the agricultural efficiency of these materials as nitrogen and carbon source is also described.

Changes in the proteomics of exhaled breath condensate under the influence of inhaled hydrogen in patients with post-COVID syndrome

Aim. To study the effect of inhalation therapy with an active hydrogen (AH) on the protein composition of exhaled breath condensate (EBC) in patients with post-COVID syndrome (PCS).Material and methods. This randomized controlled parallel prospective study included 60 patients after coronavirus disease 2019 (COVID-19) with PCS during the recovery period and clinical manifestations of chronic fatigue syndrome who received standard therapy according to the protocol for managing patients with chronic fatigue syndrome (CFS). The patients were divided into 2 groups: group 1 (main) — 30 people who received standard therapy and AH inhalations (SUISONIA, Japan) for 10 days, and group 2 (control) — 30 medical workers who received only standard therapy. Patients in both groups were comparable in sex and mean age. All participants in the study were sampled with EBC on days 1 and 10. Samples were subjected to tryptic digestion and high-performance liquid chromatography combined with tandem mass spectrometry analysis using a nanoflow chromatograph (Dionex 3000) in tandem with a high-resolution time-of-flight mass spectrometer (timsTOF Pro).Results. A total of 478 proteins and 1350 peptides were identified using high resolution mass spectrometry. The number of proteins in samples after AH therapy, on average, is 12% more than before treatment. An analysis of the distribution of proteins in different groups of patients showed that only half of these proteins (112) are common for all groups of samples and are detected in EBC before, after, and regardless of hydrogen therapy. In addition to the qualitative difference in the EBC protein compositions in different groups, quantitative changes in the concentration of 36 proteins (mainly structural and protective) were also revealed, which together made it possible to reliably distinguish between subgroups before and after treatment. It is worth noting that among these proteins there are participants of blood coagulation (а-1-antitrypsin), chemokine- and cytokine-mediated inflammation, and a number of signaling pathways (cytoplasmic actin 2), response to oxidative stress (thioredoxin), glycolysis (glyceraldehyde-3- phosphate dehydrogenase), etc.Conclusion. The use of hydrogen therapy can contribute to the switching of a number of physiological processes, which may affect the success of recovery in PCS patients. In particular, the obtained results indicate the activation of aerobic synthesis of adenosine triphosphate in mitochondria by hydrogen therapy, which correlates well with the decrease in the blood lactate level detected by laboratory studies. At the same time, this therapy can inhibit pro-inflammatory activity, negatively affecting the coagulation and signaling pathways of integrins and apoptosis, and, in addition, activate protective pathways, tricarboxylic acid cycle, FAS signaling, and purine metabolism, which may be essential for effective recovery after COVID-19.

Insights into the microbial community structure in the biodegradation process of high-strength ammonia digestate liquid fraction in conventional activated sludge system

Biodegradation of digestate liquid fraction was performed in the activated sludge system with acetic acid, flume water, and molasses as external carbon sources. High-throughput sequencing was used to gain in-depth insight into the activated sludge microbial community. The type and amount of carbon source in influent (COD/TN ratio) significantly influenced microbial community structure, especially at the genus level, and thus the biodegradation performance of digestate liquid fraction. The highest total nitrogen and chemical oxygen demand removal efficiencies averaging 85.3% and 88.3%, respectively, were achieved in series with acetic acid and flume water and COD/TN ratio of 10.7 and 11.2, respectively. The microbial diversity in these series averaged at 3.08 and 3.65. The dominant bacteria at the phylum level in series with acetic acid were Proteobacteria and Bacteroidota, and at the genus level Azospira, while in series with flume water they were Bacteroidota and Firmicutes, and Macellibacteroides, respectively.

Optimization of Liquid Digestate Fertigation and Light Intensity for Lettuce Cultivation in Closed Plant Production System

Optimization of Liquid Digestate Fertigation and Light Intensity for Lettuce Cultivation in Closed Plant Production System

Membrane distillation for separation and recovery of valuable compounds from anaerobic digestates

Digestate - the wet by-product of the anaerobic digestion (AD) of organic waste - is composed of a mixture of residual organic matter, microbial biomass and inorganic compounds. Although digestate is rich in nutrients, the occurrence of high levels of salinity and soluble ammoniacal nitrogen (N-NH4+) limits its direct exploitation as organic fertilizer and imposes a complex disposal.This study assesses at laboratory-scale the potential of Direct Contact Membrane Distillation (DCMD) for the separation of nutrients and the recovery of water from the liquid phase of the digestate produced by a digester that treats cattle manure and agricultural residues. Experimental tests, carried out at different feed temperature (50–70 °C) and initial pH (6–8.2) of the liquid digestate, aimed at investigating the effect of the operating conditions on the transmembrane flux of water vapor, the concentration of N-NH4+, P-PO3-, and Chemical Oxygen Demand (COD) in both permeate (also referred to as distillate) and retentate streams.For raw digestate, the initial transmembrane flux enhanced from 1.83 L·m−2·h−1 to 5.55 L·m−2·h−1, in response to increasing temperature gradient. Overall, the highest achieved water recovery factor was 40% at a feed temperature of 70 °C. The pH had a limited impact on the transmembrane flux, whereas notably affected the DCMD performance in terms of separation efficiency. In particular, a pH reduction from 8.2 (raw digestate) to 6 was able to completely avoid the transfer of N-NH4+ into the distillate. Compared to the concentration in the raw digestate, the reduction of COD and P-PO3- in the distillate was 99.0% and 99.9%, respectively. Regardless feed temperature, the performance of the process was subjected to a significant decline of evaporation rate due to occurrence of membrane fouling; however, cycles of cleaning with citric acid allowed to partially regenerate the membrane by removing the reversible cake of foulants accumulated on the surface. The results demonstrate an adequate performance of MD process for recovering water and nutrients, leading to a dehydrated fraction of the digestate phase that can be easily stored and used in agronomic practices.

Selection of photosynthetic microorganisms grown in artificial saline industrial effluents with liquid digestate: From screening to consortium cultures

The objective of this study was to determine the feasibility of using saline industrial streams as a culture medium to grow microalgae and cyanobacteria. Experiments were performed to determine the extent of the growth in artificial saline produced water and aquifer water supplemented with liquid digestate. Tests were performed in 96-wells microplates. Media were composed with different proportion of saline artificial produced water or aquifer water supplemented with 5% v/v liquid digestate (final concentrations: 149–195 mgN·L−1, 1.5–2.7 mgP·L−1). Media were completed to 100 % with artificial seawater, corresponding to final salinities of 40, 70 and 100 g·L−1. D. salina, N. oceanica and T. suecica showed the best growth rates. They were selected to perform mixed cultures in 80 mL tubes in the same culture media. Population evolutions were followed for 19 days. Depending on salinity and industrial effluent used, different species became predominant over the two others (N. oceanica, T. suecica and D. salina. at 40, 70 and 100 g·L−1, respectively). It appears that mixed culture is a good solution to have a biomass production during a culture process where the culture media will evolve in terms of salinity and composition.

Ammonia recovery from anaerobic-fermentation liquid digestate with vacuum membrane distillation

Vacuum Membrane Distillation (VMD) is systematically investigated herein, for recovering ammonia after phosphates precipitation, towards development of a sustainable process to valorize anaerobic-fermentation liquid-digestates (separated from the sludge). Initially, batch tests were performed, with representative synthetic solutions, to identify a narrow range of conditions favoring optimal ammonia (i.e. total ammonia nitrogen-TAN) separation, mainly regarding selectivity and recovery in H2SO4 scrubbing-solution. Next, guided by the preceding test-results, targeted batch experiments with digestate samples from an industrial plant, led to determination of near-optimum conditions for ammonia recovery. Specifically, for the small TAN concentration (<200 mg/L) encountered in the digestate feeds, VMD at modest temperature (T< ∼45 °C) and driving force Δp (<20 mbar) allowed high ammonia removal fraction r (greater than 0.85) with fair selectivity. Under such conditions, the (ammonia-water) distillate flux J exhibited linear dependence on Δp; moreover, J and pH of feed solution remained practically constant for a significant distillate removal-fraction R (under constant Δp and T). Based on theoretically supported correlation of ammonia removal-fraction r with distillate removal-fraction R, and other results, an approach is proposed for realistic preliminary design of a continuous VMD-based process, thus facilitating further steps towards large-scale process development.

A farm-scale sustainability assessment of the anaerobic digestate application methods

Digestate is the anaerobic digestion by-product that can be used as an organic fertilizer, but some agronomic and environmental concerns still hinder its application on land. This study aims to evaluate the agronomic performances (i.e., silage maize dry yield, protein content, and N uptake) and environmental sustainability of solid and liquid digestate fraction application in a field experiment involving two farms with different textures covering about 47 ha in North Italy. The best available distribution methods included mineral fertilizer (MF), mineral fertilizer in variable rate application (VRA) (VRA-MF), liquid digestate with a nitrification inhibitor (LD+), liquid digestate in VRA (VRA-LD), liquid digestate with a nitrification inhibitor in VRA (VRA-LD+), and solid digestate (SD) and were applied to silage maize (Zea mays L.) in 2019 and 2020 cropping seasons. Results showed that both digestate fractions gave satisfying agronomic performances (i.e., dry biomass > 13 t ha-1 and protein content > 6.8%), comparable to those of mineral fertilizers, irrespective of soil type and application techniques. On the contrary, system sustainability investigated with a spatial evaluation of nitrogen use efficiency (NUE) revealed a strong interaction between NUE and soil texture. Indeed, in fine-textured soil only the adoption of both VRA and the nitrification inhibitor allowed the liquid digestate to reach a NUE between 50% and 90% while SD exhibited poor NUE (e.g., < 50%). In conclusion, liquid digestate fraction might be an effective substitute for mineral fertilizers in silty soils meeting also environmental criteria when VRA or nitrification inhibitors are applied. Contrarily, longer-term experiments are requested to evaluate SD fraction sustainability.

Phosphorus recovery as struvite and hydroxyapatite from the liquid fraction of municipal sewage sludge with limited magnesium addition.

Phosphorus (P) is an essential element to produce feed and fertilizers but also a nonrenewable resource. Both the predicted exhaustion of phosphatic rocks and the risk of eutrophication lead to an increasing necessity for P recovery methodologies to be applied in municipal wastewater treatment plants (WWTPs). One of the most promising solutions involves the precipitation of P-based minerals reusable as slow-release fertilizers. In this study, P recovery as struvite and hydroxyapatite from a municipal WWTP digestate liquid fraction (centrate) was investigated at varying pH (8-10), reagent typologies (MgCl2 , NaOH, Ca(OH)2 , and CaCl2 ), and concentrations under limiting magnesium doses through liquid- and solid-phase analyses and thermodynamical modeling. A maximum P recovery of 87.3% was achieved at pH 9 by adding NaOH and MgCl2 at a dose of 656mg/L (the higher tested). According to these data, it was estimated that 92.0 tons/year of struvite and 33.2 tons/year of hydroxyapatite could be recovered from the WWTP centrate with a cost for reagent consumption being almost 50% of the mean P market value. An increase in P precipitation was observed while comparing experiments with the same pH values but with a higher Mg2+ dose. Ca2+ addition led to extensive P precipitation but mainly as amorphous phases that interfere with struvite formation.

Comparison of the Effect of Solid and Liquid Digestate on the Growth of Lettuce (Lactuca sativa L.) Plants

As a consequence of the Russian invasion of Ukraine, Europe is facing a shortage of chemical fertilizers for agriculture. Therefore, the use of byproducts of biomass anaerobic digestion, e.g., solid (SD) and liquid (LD) digestate, could be a key solution to cope with this problem. In this framework, the present study aimed to evaluate the effect of both SD and LD, derived from the same feedstock, on the biometric, physiological, and biochemical parameters of lettuce (Lactuca sativa L.) plants. Compared to the controls, the plants treated with 3% (w/w) SD showed a reduction in leaf fresh biomass, chlorophyll content, performance index, fractal dimension, and antiradical activity, while vitamin C increased by 18.8%. An opposite response was observed for the lettuce plants treated with 3% (v/w) LD, which showed an increase in all the above-mentioned parameters, except for vitamin C, which decreased by 39.8%. Interestingly, the content of malondialdehyde, which is correlated with cell membrane lipid peroxidation, increased in the SD-treated plants (+39.7%) and decreased (−42.1%) in the LD-treated plants. These results strongly support the use of LD in agriculture as a valuable product to improve the productivity and nutritional quality of crop plants.

The effect of fertilization with microfiltered liquid digestate on the quality parameters of Citrus fruits

Nowadays, the adoption of sustainable agricultural practices, including the reduction of synthetic fertilizers, has become a challenge for the agriculture sector. In this experimental work, the effect of the liquid fraction of digestate (by-product of the anaerobic digestion process) as a fertilizer was evaluated. The aim of the research was to verify to which extent digestate can affect growth and quality parameters of orange fruits, comparing the results to those obtained for fruits grown on soil treated with conventional mineral fertilizers. To assess the effectiveness of the treatments, different qualitative and quantitative parameters of Citrus fruits were measured. In particular, the results showed slight differences between the two treatments, suggesting that digestate may be used for the production of high-quality fruits. Moreover, in some orchards, the Citrus fruits of the plants treated with digestate showed a higher concentration of health-promoting compounds, such as vitamin C, flavonoids, phenolic content, when compared to the control group. Thus, digestate can be considered an optimal source of plant nutrients and can be used as a crop growth promoter, since it represents an effective strategy for reducing the mineral fertilizers input.

Quality of fish sludge as fertiliser to spring cereals: Nitrogen effects and environmental pollutants

The aim of this study was to contribute to development of organic fertiliser products based on fish sludge (i.e. feed residues and faeces) from farmed smolt. Four dried fish sludge products, one liquid digestate after anaerobic digestion and one dried digestate were collected at Norwegian smolt hatcheries in 2019 and 2020. Their quality as fertilisers was studied by chemical analyses, two 2-year field experiments with spring cereals and soil incubation combined with a first-order kinetics N release model. Cadmium (Cd) and zinc (Zn) concentrations were below European Union maximum limits for organic fertilisers in all products except one (liquid digestate). Relevant organic pollutants (PCB7, PBDE7, PCDD/F + DL-PCB) were analysed for the first time and detected in all fish sludge products. Nutrient composition was unbalanced, with low nitrogen/phosphorus (N/P) ratio and low potassium (K) content relative to crop requirements. Nitrogen concentration in the dried fish sludge products varied (27–70 g N kg-1 dry matter), even when treated by the same technology but sampled at different locations and/or times. In the dried fish sludge products, N was mainly present as recalcitrant organic N, resulting in lower grain yield than with mineral N fertiliser. Digestate showed equally good N fertilisation effect as mineral N fertiliser, but drying reduced N quality. Soil incubation in combination with modelling is a relatively cheap tool that can give a good indication of N quality in fish sludge products with unknown fertilisation effects. Carbon/N ratio in dried fish sludge can also be used as an indicator of N quality.

Treatment and Valorization of Agro-Industrial Anaerobic Digestate Using Activated Carbon Followed by Spirulina platensis Cultivation

The increased production of biogas through the anaerobic digestion (AD) process has raised several concerns regarding the management of liquid digestate, which can present some environmental risks if not properly handled. Among the different techniques to treat AD digestate, microalgae and cyanobacteria cultivation has emerged as a sustainable approach to valorizing digestate while producing valuable biomass for production of biofuels and high value bioproducts. However, the intrinsic parameters of the liquid digestate can strongly limit the microalgae or cyanobacteria growth as well as limit the uptake of residual nutrients. In this study, the detoxification potential of activated carbon (AC) was evaluated on agro-industrial liquid digestate prior to Spirulina platensis cultivation. Different doses of AC, ranging from 5 to 100 g/L, were tested during adsorption experiments in order to determine the adsorption capacity as well as the removal efficiency of several compounds. Experimental results showed the high reactivity of AC, especially towards phosphate (PO4-P), total phenol (TP) and chemical oxygen demand (COD). At a dosage of 50 g/L, the AC pretreatment successfully achieved 54.7%, 84.7% and 50.0% COD, TP and PO4-P removal, corresponding to adsorption capacity of 94.7 mgDCO/g, 17.9 mgTP/g and 8.7 mgPO4-P/g, respectively. Even if the AC pretreatment did not show significant effects on Spirulina platensis growth during toxicity assays, the AC adsorption step strongly participated in the digestate detoxification by removing hardly biodegradable molecules such as phenolic compounds.

Valorization of Anaerobic-Fermentation Liquid Digestates—Membrane-Based Process Development

Complete valorization of various wastes and effluents, with significant organic content, remains a great challenge in the pursuit of a circular economy. The approach based on anaerobic fermentation, leading to valuable biogas production, has been broadly accepted and employed as an attractive processing scheme. However, despite notable research efforts, complete valorization of the digestates (involving recovery of nutrients/by-products and full recycling/reuse of treated water) requires additional work for sustainable process development. This study aims to make a contribution in this direction by demonstrating a systematic methodology for valorizing the liquid digestate. The proposed membrane-based processing scheme involves UF-membrane pretreatment of the liquid digestate (for sludge separation) and subsequent NF/RO membrane treatment for reuse/recycling of the permeate; the concentrate, enriched in "nutrients" (phosphate and ammonium compounds), can be utilized for soil fertilization, with further conditioning/processing. By performing targeted laboratory experiments and advanced simulations, the membrane-based process was developed to a relatively high technology-readiness level, including a pilot unit design/construction and preliminary testing with satisfactory results. Through pilot testing in industrial environment, further process development and optimization will be pursued, towards practical applications. The demonstrated methodology is also considered appropriate for systematic development of membrane-based processes to valorize/treat a variety of similar effluents.

Cultivation of the PHB-producing cyanobacterium Synechococcus leopoliensis in a pilot-scale open system using nitrogen from waste streams

PHB-producing cyanobacteria may provide the raw material for bio-based and biodegradable plastics. To commercialize photoautotrophic PHB production, their cultivation needs to be scaled up in open systems and to reduce costs and increase sustainability, nutrients must be obtained from waste streams. Here, the feasibility of these steps was verified. Different PHB-producing cyanobacteria were compared in laboratory-scale cultivations using either water from recirculating aquaculture systems or pre-processed liquid digestate as nutrient sources. Then, Synechococcus leopoliensis was cultivated in an open thin-layer photobioreactor (18 m2, 200 L), were growth in mineral Z-medium was again compared to said waste streams. All cultivations were successful. Cultivation in mineral medium resulted in both the highest final biomass yield (6 g L−1) and productivity (0.7 g L−1 d−1). Both waste stream-based media showed lower biomass yields and productivities (2 g L−1 and 0.25–0.3 g L−1 d−1). However, due to differences in the cultivation conditions (e.g., temperature, nutrient supply), final biomass yield and productivity do not represent the performance of the cultivations adequately. Relative parameters such as nitrogen and energy conversion ratios indicate that cultivation with aquaculture water suffered from insufficient nitrogen supply to the culture, whereas the use pre-processed liquid digestate resulted in a substantial loss of nitrogen due to volatilization. All cultivations in the open system were continued in the laboratory, where cultures were starved for ten days under nutrient-depleted conditions (without nitrogen, phosphorus, or both). While PHB accumulation occurred, concentrations were comparatively low (< 1%dw). The comparison of the results suggests that PHB yields were influenced more by the initial cultivation condition than by the specific type of nutrient depletion. Thus, while the cultivation with waste streams in an open system is feasible, environmental parameters seem to influence PHB yields considerably and must be considered for the optimization of the complete process.

Effect of photocatalysis on the physicochemical properties of liquid digestate

Antibiotic residues pose a risk to the agricultural application of liquid digestate. In our previous study, photocatalysis was employed to degrade the antibiotics in liquid digestate and observed that the removal efficiency of TC, OTC, and CTC was up to 94.99%, 88.92%, and 95.52%, respectively, at the optimal experimental level, demonstrating the feasibility of this technology. In this study, the liquid digestate after photocatalysis was analyzed to evaluate the effect of photocatalysis on the nutrients, phytotoxicity, and bacterial community of liquid digestate. The results showed that photocatalysis had little effect on the major nutrients TN, TP, and TK in liquid digestate. However, photocatalysis could cause an increase in tryptophan substances as well as soluble microbial by-products and a decrease in humic acid substances in the liquid digestate. The toxicity of liquid digestate after photocatalysis exhibited an increasing trend followed by a decreasing trend, and the liquid digestate after photocatalysis for 2 h had a promoting effect on seed germination and root growth. The richness, diversity, and evenness of bacterial communities in liquid digestate were decreased as a result of photocatalysis. The dominant species in the liquid digestate was dramatically changed by photocatalysis, and the antibiotic concentration also had a major effect on the dominant species in the liquid digestate after photocatalysis. After photocatalysis for 2 h, the dominant species in the liquid digestate changed from Firmicutes to Proteobacteria.