The algae-bacterial systems work according to the principle of the symbiotic relationship between algae and bacteria. The algae-bacterial systems have the potential to remove micropollutants due to the diverse possible interactions between algae-bacteria. The schematic illustration of the quorum-sensing and algal-bacterial interactions is shown in Figure 2. The complex interactions of these signaling molecules are expected to take place in the phycosphere, a diffuse boundary layer region around the algal cell into which the algae secrete. influence co-occurring organisms [78]. for bacteria and stimulate their proliferation by secreting substances that can also promote the formation of biofilms [79].
Schematic representation of algal-bacterial systems (a) high-speed algal pond (b) flat plate PBR (c) tubular PBR (d) bubble column PBR (e) inner ring column PBR (f) membrane PBR.
Physio-chemical factors affecting algal-bacterial systems
Cyclotella sp.-24--40 (TN)50 (TP)[121] DWW-Domestic sewage; SWW-Synthetic wastewater; UWW-Urban wastewater; COD-Chemical Oxygen Demand; TOC-Total Organic Carbon; TN-Total Nitrogen; YES. Several studies have shown that light intensity and the period of the light-dark cycle are the main factors affecting the productivity of microalgae [35,142]. It is well documented that changes in light intensity cause an immediate effect on the rate of organic removal, algal photosynthesis and the nitrification process by bacterial cells [143].
The result shows that carbon removal was positively related to the length of the dark period and inversely related to nitrogen and phosphorus removal. An increase in blue light intensity was reported to cause photo-. Therefore, it can be understood that the light intensity and the light/dark cycle should be optimized for the actual use of algae and bacteria systems for wastewater treatment.
Several studies have shown that the photooxygen generated by algal biomass is sufficient enough for heterotrophic bacterial oxidation of organic matter under no aeration conditions and satisfactory carbon removal has been achieved [154,155]. However, contrary to this observation, other researchers have shown that the addition of external aeration improves the removal efficiency [54]. Usually, domestic wastewater contains a low C/N ratio compared to algal biomass; therefore, nutrient removal can be improved by adding external aeration, which supplements the required carbon in the form of carbon dioxide [156].
Micropollutant or ‘emerging contaminant’
The schematic illustration of different mechanisms involved in EC removal by algal-bacterial systems is shown in Figure 4. Therefore, photodegradation may also play an important role in EC removal in algal-bacterial systems. 185] also reported that more than 99% caffeine removal was observed in combined algal-bacterial systems as only 17% removal in sole microalgae incubation.
164] studied the removal of 28 ECs (pharmaceuticals, bisphenols, neonicotinoids and selected transformation products) in Chlorella Vulgaris monoculture and algal-bacterial mixed culture. They reported that the algal-bacterial mixed culture performed better in removing bisphenols than the algal monoculture. In algal-bacterial systems, interaction between different species and co-metabolism can increase the rate of pollutant transformation.
In a recent study, it was shown that the removal of LAS and caffeine was more algal bacterial trickling photobioreactor compared to bacterial trickling filter treating the same wastewater [126]. Various studies on the removal of micropollutants in algal-bacterial systems are summarized in Table 6. Enhanced removal of micropollutants can be achieved in algal-bacterial systems due to photodegradation and symbiotic interaction between algae and bacteria.
Algal-bacterial systems modeling
Different algal-bacterial models available, their properties, processes, components and their limitations are summarized in Table 7. The model was executed in MATLAB by considering 6 processes and 11 components as shown in Table 7. The sensitivity analysis of the model shows that light is the most sensitive parameter for algal growth.
The results show that the model was able to accurately predict nutrient levels, pH change, dissolved oxygen and biomass concentrations. 211] developed a model of the algal-bacterial system by extending the ASM3 model with modified algal biokinetics. 212] have developed a comprehensive ALBA model for describing the long-term dynamics of the algal-bacterial ecosystem in a 56 m2 wastewater treatment pond.
In comparison with other models, we evaluated algal-bacterial systems for a longer period with the ALBA model, taking into account the data of 443 days of operation. The model was able to accurately predict dissolved oxygen and pollutant removal profiles in a photo-sequencing batch reactor. Most models of algae and bacteria assumed ideal mixing conditions in reactors, which is not true in real-scale systems.
Biomass harvesting
213] developed an algal bacterial model based on the ASM3 model by including processes associated with heterotrophic bacteria and algae. 214] have analyzed the carbon fluxes in algal-bacterial flocs under different growth conditions (photoautotrophic, mixotrophic and heterotrophic) using a model developed from respirometric-titrimetric data. The model was able to take into account EPS production and consumption and accurately predict heterotrophic bacterial and algal growth under photoautotrophic, mixotrophic and heterotrophic conditions.
Computational Fluid Dynamics (CFD) simulation will provide detailed insight into the distinct hydrodynamic pattern of the system. The integration of the developed models with the CFD platform can help predict the accurate biochemical process parameters such as pH, dissolved oxygen and other components [192]. In complete systems, the components are not uniformly distributed, leading to optical absorption and light shading of the cells.
In their study, it was stated that the axenic culture of Chlorella vulgaris has only 2% flocculation activity compared to algal culture associated with bacteria, which has a flocculation efficiency of about 94%. 224] showed that Chlorella Vulgaris cultured with sewage-associated bacteria achieved a flocculation activity of > 92% in bioflocculation. The high settling rate of algal-bacterial aggregates makes biomass harvesting simple in algal-bacterial systems.
Resource recovery
After the lipid extraction from algal bacterial biomass, the remains of the biomass are rich in carbohydrates and proteins, making anaerobic digestion a possible way of resource recovery. Biohydrogen production during fermentation depends on the available fermentative organisms and biomass pretreatment. 236] have recently investigated the effect of gamma radiation pretreatment on biohydrogen production from microalgae Laminaria Japonica biomass using dark fermentation.
237] have shown the feasibility of combining urban wastewater treatment with biohydrogen production in an integrated approach. They reported a biohydrogen production yield of 56.8 mL/g volatile solids from the dark fermentation of Scenedesmus obliquus grown in wastewater. Co-fermentation of algal biomass supplemented with co-substrate in the presence of a catalyst can further increase biohydrogen production.
238] have reported a 37.14% increase in cumulative biohydrogen production during dark fermentation of Lyngbya limnetica biomass supplemented with glucose as co-substrate in the presence of Fe3O4 nanoparticles as catalyst. Although biohydrogen production from microalgal biomass is environmentally friendly, the amount of biohydrogen produced is low for commercialization. HTL has been performed on algal-bacterial biomass (harvested from wastewater bioreactors) at 300°C temperature, (10–12) MPa pressure and 30 min reaction time [239].
![Figure 5. Conversion technologies for biofuel production from algal-bacterial biomass [225].](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10483125.0/29.890.149.737.82.331/figure-conversion-technologies-biofuel-production-algal-bacterial-biomass.webp)
Challenges and future prospective of the algal-bacterial system for wastewater
The major challenge in using algal bacterial systems for real-scale systems is the stability of the consortia. The stability of the algal-bacterial consortia depends on the communication pattern between the individual species (exchange of metabolites and molecular signals) and the division of labor. The dynamics of algal-bacterial systems are largely unexplored, which makes them difficult to handle.
So there is a need for the development of robust models that can accurately simulate the bioprocesses in algal-bacterial systems. Coupling the computational fluid dynamics and biokinetic parameters of the algal bacteria systems could be used to model the system and implement it in real-scale systems. Most of the investigated algal-bacterial systems are on a laboratory scale and were operated for a relatively very short period of time.
The main drivers for treating wastewater using algal bacterial systems are reducing greenhouse gas emissions, improving energy and recovering nutrients. However, a detailed techno-economic feasibility analysis of these systems is needed to understand the economic feasibility, energy balance and productivity of the algal bacterial biomass as feedstock for biofuel synthesis, thus convincing end users to adopt these systems. technologies [21]. The algal bacterial biomass grown on domestic wastewater can be used for resource recovery through biodiesel synthesis, methane production or conversion into biohydrogen to reduce the cost of treatment and ultimately lead to a circular bio- economy.
Conclusion
Ability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study. Minimizing greenhouse gas emissions from wastewater treatment process by integrating activated sludge and microalgae processes. Integration of microalgal systems at municipal wastewater treatment plants: implications for energy and emission balances.
Evaluation of the dynamics of microalgal population structure and process performance during swine wastewater treatment. Increased pond depth improves algal productivity and nutrient removal in wastewater treatment with high algal ponds. Influence of light intensity on bacterial nitrifying activity in algal bacterial photobioreactors and its implications for microalgae-based wastewater treatment.
Effect of solids retention time on the performance of algae-activated sludge association in municipal wastewater treatment and biofuel production. Wastewater treatment and algae production in high-rate algae ponds with carbon dioxide addition. Performance of a membrane-coupled high-rate algal pond for demonstration-scale urban wastewater treatment.
Influence of pH and CO2 source on the performance of microalgae-based secondary treatment of domestic wastewater in outdoor pilot circuits. Influence of solar radiation levels on the formation of microalgae-bacteria aggregates for municipal wastewater treatment.
