Assessment of the performance of an anoxic-aerobic microalgal-bacterial system treating digestate

Microalgae-bacteria consortia dynamics in a long term operated membrane-coupled high-rate algal pond (MHRAP)

Traditional activated sludge-based technologies have significant drawbacks, including high energy requirements and greenhouse gas emissions. Microalgae-based processes offer a promising, low-cost, and environmentally friendly alternative. However, the knowledge of treatment systems based on microalgae-bacteria consortia is limited, and even more so is their microbial composition and its relationship with operational parameters. Thus, this study explores the dynamics of microalgae-bacteria consortia in a long-term operated membrane-coupled high-rate algal pond (MHRAP) for wastewater treatment. For this, a pilot-scale MHRAP plant, located in a wastewater treatment plant in Valencia (Spain), was monitored under various hydraulic retention times (HRT) and wastewater influents: i) effluent from a primary settler and ii) effluent form pre-treatment. The biomass retention time was kept constant at 6 days. The composition of the bacterial community was studied through 16S rDNA sequencing, while 18S rDNA sequencing was used to study the microalgae. The results indicate that shorter HRTs significantly increased bacterial diversity, but not eukarya. Principal Co-ordinates Analysis (PCoA) revealed that the HRT and the incoming wastewater quality control the type of the bacterial populations. However, this effect was not observed in eukaryotic organisms. The dominant microalgae genera identified were Desmodesmus and Coelastrella, with Coelastrella becoming more prevalent at shorter HRTs. For bacteria, Verrumicrobiota dominated (18-56%) at high HRT while Proteobacteria was dominant (28-44%) at HRTs below 6 days. The changes observed in the bacterial composition, including the ammonia oxidizing bacteria (AOB) community (mainly Nitrosomonas), suggest that photo-inhibition could be taking place. The nitrite oxidizing bacteria (NOB) community was dominated by Nitrospira and Candidatus Nitrotoga. Operational parameters such as light intensity, pH, and nitrite concentration were found to significantly influence the microbial community structure. Higher light intensity and alkaline pH favored the growth of Desmodesmus, while Coelastrella thrived under lower HRTs. Bacterial diversity plays a crucial role in the treatment process, while microalgae primarily support aerobic bacterial processes by providing oxygen. These findings contribute to a deeper understanding of the complex biological processes in microalgae-bacteria consortia and offer insights into improving wastewater treatment technologies.

Assessing the potential of Chlorella sp. phycoremediation liquid digestates from brewery wastes mixture integrated with bioproduct production

Digestates from different anaerobic digesters are promising substrates for microalgal culture, leading to effective wastewater treatment and the production of microalgal biomass. However, further detailed research is needed before they can be used on a large scale. The aims of this study were to investigate the culture of Chlorella sp. in Digestate_M from anaerobic fermentation of brewer's grains and brewery wastewater (BWW) and to explore the potential use of the biomass produced under different experimental conditions, including diverse cultivation modes and dilution ratios. Cultivation in Digestate_M initiated from 10% (v/v) loading, with 20% BWW, obtained maximum biomass production, reaching 1.36 g L^-1 that was 0.27g L^-1 higher than 1.09 g L^-1 of BG11. In terms of Digestate_M remediation, the maximum removal of ammonia nitrogen (NH₄ ⁺-N), chemical oxygen demand, total nitrogen, and total phosphorus reached 98.20%, 89.98%, 86.98%, and 71.86%, respectively. The maximum lipid, carbohydrate, and protein contents were 41.60%, 32.44%, and 27.72%, respectively. The growth of Chlorella sp. may be inhibited when the Y(II)-F_v/F_m ratio is less than 0.4.

Long term operation of a phototrophic biological nutrient removal system: Impact of CO2 concentration and light exposure on process performance

The utilization of non-aerated microalgae-bacterial consortia for phototrophic biological nutrient removal (photo-BNR) has emerged as an alternative to conventional wastewater treatment. Photo-BNR systems are operated under transient illumination, with alternating dark-anaerobic, light-aerobic and dark-anoxic conditions. A deep understanding of the impact of operational parameters on the microbial consortium and respective nutrient removal efficiency in photo-BNR systems is required. The present study evaluates, for the first time, the long-term operation (260 days) of a photo-BNR system, fed with a COD:N:P mass ratio of 7.5:1:1, to understand its operational limitations. In particular, different CO₂ concentrations in the feed (between 22 and 60 mg C/L of Na₂CO₃) and variations of light exposure (from 2.75 h to 5.25 h per 8 h cycle) were studied to determine their impact on key parameters, like oxygen production and availability of polyhydroxyalkanoates (PHA), on the performance of anoxic denitrification by polyphosphate accumulating organisms. Results indicate that oxygen production was more dependent on the light availability than on the CO₂ concentration. Also, under operational conditions with a COD:Na₂CO₃ ratio of 8.3 mg COD/mg C and an average light availability of 5.4 ± 1.3 W h/g TSS, no internal PHA limitation was observed, and 95 ± 7%, 92 ± 5% and 86 ± 5% of removal efficiency could be achieved for phosphorus, ammonia and total nitrogen, respectively. 81 ± 1.7% of the ammonia was assimilated into the microbial biomass and 19 ± 1.7% was nitrified, showing that biomass assimilation was the main N removal mechanism taking place in the bioreactor. Overall, the photo-BNR system presented a good settling capacity (SVI ∼60 mL/g TSS) and was able to remove 38 ± 3.3 mg P/L and 33 ± 1.7 mg N/L, highlighting its potential for achieving wastewater treatment without the need of aeration.

A Systematic Study of Ammonia Recovery from Anaerobic Digestate Using Membrane-Based Separation

Ammonia recovery from synthetic and real anaerobic digestates was accomplished using hydrophobic flat sheet membranes operated with H₂SO₄ solutions to convert ammonia into ammonium sulphate. The influence of the membrane material, flow rate (0.007, 0.015, 0.030 and 0.045 m³ h⁻¹) and pH (7.6, 8.9, 10 and 11) of the digestate on ammonia recovery was investigated. The process was carried out with a flat sheet configuration at a temperature of 35 °C and with a 1 M, or 0.005 M, H₂SO₄ solution on the other side of the membrane. Polytetrafluoroethylene membranes with a nominal pore radius of 0.22 µm provided ammonia recoveries from synthetic and real digestates of 84.6% ± 1.0% and 71.6% ± 0.3%, respectively, for a membrane area of 8.6 × 10⁻⁴ m² and a reservoir volume of 0.5 L, in 3.5 h with a 1 M H₂SO₄ solution and a recirculation flow on the feed side of the membrane of 0.030 m³ h⁻¹. NH₃ recovery followed first order kinetics and was faster at higher pHs of the H₂SO₄ solution and recirculation flow rate on the membrane feed side. Fouling resulted in changes in membrane surface morphology and pore size, which were confirmed by Atomic Force Microscopy and Air Displacement Porometry.

C. vulgaris growth batch tests using winery waste digestate as promising raw material for biodiesel and stearin production

The recovery of high added value compound from waste stream is fundamental to keep biotechnological processes sustainable. In this study, anaerobic digestion of two highly produced organic waste was integrated with microalgae-based processes both to treat liquid digestate and recover high value compounds. Chlorella vulgaris growth was assessed for lipids accumulation and subsequent recovery, using two types of digestate: organic waste and sewage sludge digestate (DIG-OFMSW) and wine lees digestate (DIG-WL). Growth tests were carried out in batch mode and results showed a slightly higher final biomass concentration from DIG-WL (1.36 ± 0.09 g l^-1) compared to DIG-OFMSW (1.05 ± 0.13 g l^-1) and a clearly different lipids accumulation yield (28.86 ± 0.05% in DIG-WL compared to 6.1 ± 0.2% of DIG-OFMSW, on total solids). Lipid characterization showed a high oleic acid accumulation (69.52 ± 0.50%w/w in DIG-WL) that positively influence biodiesel properties and a low linolenic acids content (below 0.30%w/w) that comply with European law EN14214 for biodiesel (linolenic acid content lower than 12%w/w). In addition, due to the high concentration of palmitic and stearic acids detected at the end of test, this oil can be used as new substrate to produce stearin, normally produced from palm oil.

Converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia: A critical review

Conventional wastewater treatment using activated sludge cannot efficiently eliminate nitrogen and phosphorus, thus engendering the risk of water eutrophication and ecosystem disruption. Fortunately, a new wastewater treatment process applying microalgae-bacteria consortia has attracted considerable interests due to its excellent performance of nutrients removal. Moreover, some bacteria facilitate the harvest of microalgal biomass through bio-flocculation. Additionally, while stimulating the functional bacteria, the improved biomass and enriched components also brighten bioenergy production from the perspective of practical applications. Thus, this review first summarizes the current development of nutrients removal and mutualistic interaction using microalgae-bacteria consortia. Then, advancements in bio-flocculation are completely described and the corresponding mechanisms are thoroughly revealed. Eventually, the recent advances of bioenergy production (i.e., biodiesel, biohydrogen, bioethanol, and bioelectricity) using microalgae-bacteria consortia are comprehensively discussed. Together, this review will provide the ongoing challenges and future developmental directions for better converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia.