**4. Potential of the ABGS systems in the remediation of textile wastewater treatment**

Algal-bacterial granular sludge (ABGS) process is considered a cost-effective and environmental friendly alternative to conventional technologies for the treatment of wastewater. In addition, this technology is viewed as an attractive alternative for resource recovery due to the presence of the algae consortium [4, 46]. In the last decade, ABGS processes have been intensively studied due to the inherent operational advantages, such as lower energy demands, higher nutrient removal ratio, and potential for resource recovery potential. These operational advantages have positioned the ABGS-based process as a promising technology to improve textile wastewater treatment.

### **4.1 Environmental advantages**

The ABGS processes are considered as a highly efficient technology for nutrient removal and degradation of both inorganic and organic pollutants in wastewaters, compared to classical biological treatments. The use of microalgal-bacteria consortia for wastewater treatment processes provides advantages at several levels. For instance, through oxygenic photosynthesis, microalgae generate the O2 required for the aerobic degradation of organic molecules by heterotrophic bacteria, while uptake the CO2 released during the bacterial aerobic mineralization of the organic substrates. This symbiotic interaction contributes to the prevention of greenhouse gas emissions during the operation of wastewater treatment plants [45].

Moreover, the developed granules are composed of different layers-phases (aerobic, anoxic, and anaerobic) that combined with the microalgae are able to remove toxic pollutants which conventional biological processes usually cannot remove. Other advantages offered by microalgae-bacteria granulation include—(1) the capacity of degrading priority pollutants, (2) enhanced settlement rate, (3) improvement of microalgae separation, and (4) lower operation and maintenance cost than conventional processes [47, 48].

On the other hand, as microalgae are photosynthetic in nature, pollutants are used for producing food and release oxygen into the system, facilitating aerobic pollutant degradation. In toxic environments, microalgae are able to acclimatize themselves to these extreme conditions, with an increasing tolerance to the pollutant toxicity, which allows its removal from water bodies [49]. On the other hand, microalgae-bacteria symbiotic associations contribute toward species microbiological growth by playing an integral part in environmental ecosystems [50].

Several studies have shown the potential of symbiotic algal-bacterial consortia to decolorize dyes and metabolize the aromatic amines typically released during the physicochemical oxidation of dyes. These compounds are considered even more hazardous than their predecessors' dyes [4, 51]. For instance, it was shown that ABGS systems used for the treatment of synthetic textile wastewater may attain efficiencies in dyes decolorization of 99 ± 1% and 96 ± 3% for dispersing orange-3 and dispersing blue-1, respectively [51].

These efficiencies could be explained by the fact that microalgae present three different mechanisms for decolorization or assimilation of the colored compounds. The chromophores are utilized—(1) for the production of algal biomass, carbon dioxide, and water; (2) for the transformation of the colored compounds to uncolored ones; and (3) for the adsorption of the dye on the algal biomass. It has been reported that *Chlorella* and *Oscillatoria* are able to degrade azo dyes to aromatic amines to simple compounds and subsequently to CO2 [1]. Besides, bacteria also have contributed to reaching the highest decolorization and mineralization rates of dyes present in the textile wastewaters. Species belonging to the genera *Pseudomonas*, *Bacillus*, *Aeromonas*, and *Proteus* are some of the most studied bacteria for the degradation of dyes and other toxic effluents [22, 52]. However, despite the high potential of algal-bacterial processes for wastewater treatment, only a few studies have dealt with the bioremediation of textile wastewaters [4].

### **4.2 Economic benefits**

The microalgal-bacterial symbiotic interactions based on the mutualistic exchange of O2 and CO2 between microalgae and bacteria also allow the system to operate without an additional oxygen source, representing an economic advantage during the treatment of xenobiotic pollutants. Aeration from photosynthetic metabolism is, therefore, especially interesting to reduce operation costs. In addition, many recalcitrant and toxic compounds are much easier to degrade under aerobic conditions than anaerobically [47, 53].

Several economic advantages linked to the resource recovery from microalgal can be highlighted. For instance, since microalgae are naturally found in the environment, at the proper temperature and light intensity microalgae development is possible with minimal costs, allowing the recovery of high-value products with minimum costs [49].

Consequently, ABGS-based processes arise as an alternative technology to AOPs for the treatment of textile wastewater. Tolerances to high pollutant loads, large removal efficiencies, as well as effective operating costs have been highlighted as the main advantages of ABGS technology in the treating of textile wastewater. Pollutant adsorption in the wall of microalgae cells facilitates the mineralization of dyes present in textile wastewater. Moreover, bacterial communities associated with microalgal cultures also can simulate microalgae growth by releasing growthpromoting factors. For instance, bacterial consortia can provide vitamins for improving microalgal growth which may result in lower cost for microalgal biomass production and, therefore in greater production efficiency [54].

### *A Critical Review on Algal-Bacterial Granular Sludge Process as Potential Economical… DOI: http://dx.doi.org/10.5772/intechopen.99973*

An essential factor that must be considered for the development of the stable microbial consortia is related to the compatibility between the selected species/ strains. In the particular case of microalgae-bacteria consortia, a balanced exchange of CO2 and O2 is essential for attaining optimal performance. Under high CO2 concentrations, a decreased pH is occurred, causing inhibitory episodes in some microalgal strains. The amount of CO2 required for microalgae growth varies according to the selected species, and it also depends on the specific configuration of the cultivation system.

On the other hand, the O2 accumulation produced by photosynthesis must be avoided, since high levels of dissolved oxygen may induce photooxidative damage in microalgae. Therefore, maintaining both CO2 and O2 under optimal concentration ranges is essential to guarantee stable removal efficiencies and low-operating costs throughout the process [45].

### **4.3 Resource recovery potential**

ABGS-based processes can be considered as potential technology where microalgae can generate valuable resources that can be recovered during the textile wastewater treatment. In the last decade, many efforts have been put to obtain axenic algal monocultures aimed at developing biomass production processes. However, the interactions between microalgae and microorganisms are currently recognized for the potential to improve algal biomass production and to enrich this biomass with valuable chemical and energy compounds with industrial interest such as lipids and carbohydrates [55]. In this respect, the general attributes of bacterial consortia are viewed with a higher interest due to the interactions with microalgae, which may affect algae growth, including motility, chemotaxis, type IV secretion systems, quorum sensing systems, and synthesis of growth promoters [54].

Microalgae are capable of synthesizing several biofuels as lipids and carbohydrates which represent the major energy storage molecules in the microalgae. In contrast, proteins in microalgae are generally not considered as substrates for biofuel production but rather for both food and feed use in human and animal nutrition [55]. In particular, the use of microalgae for the production of biodiesel has focused considerable attention in the past decades, since some species are able to accumulate hydrocarbons up to 30–70% of their dry weight [45].

**Table 3** presents several studies on microalgae and microalgae-bacteria cultures used for the treatment of different wastewater. As it can be noted, the obtained resources depend on the culture systems and type of treated wastewater. These studies showed that microalgae tend to produce a large number of proteins independently from the cultivation method used, as well as other high-value resources with large-energy content such as lipids compounds.

Algal lipids are used to generate biodiesel, a sustainable alternative to fossil fuels. Through photosynthesis, algae can convert CO2 and water into the organic matter like carbohydrates and lipids. Under ideal conditions, algae can produce carbohydrates, while external stress by limitation of nutrients arises, the algae tend to accumulate lipids. These lipids can be converted into fatty acyl methyl esters through transesterification reactions and can be used as fuel because of their excellent energy density [70].

On the other hand, recent studies have shown that under adequate operating conditions, microalgae and bacteria can form aggregates showing good settleability [71, 72]. The latter facilitates biomass harvesting for its use as feedstock for other energy-producing bioprocesses. Therefore, ABGS systems show a promising future as zero or even negative-energy systems [73]. In addition, the suitable control of the biological interactions between microalgae and bacteria could help to improve


*PBR, photobioreactor; PMFC, photosynthetic microbial fuel cell; SAR'CENA, synergistic algal refinery for circular economy using nutrient analogs.*

### **Table 3.**

*Microalgae growth and resource recovery potential according to the cultivation method.*

microalgae-based biomass and biofuel production in the future. Finally, all these economic, environmental, and resource recovery advantages allow considering the ABGS-based process as a sustainable technology.

## **5. Conclusions**

Although many studies have proposed different treatment strategies using chemical, and biological methods for treating textile effluents, no technique is suitable or all-around appropriate for treating the wide range of pollutants present in the textile and clothing industry effluents, causing severe impacts on the environment and human health.

In order to reduce the impact caused on the environment, these substances need to be oxidized by AOPs to become them in biodegradable components, and finally

### *A Critical Review on Algal-Bacterial Granular Sludge Process as Potential Economical… DOI: http://dx.doi.org/10.5772/intechopen.99973*

can be degraded through biological treatments. Although the combination of typical biological techniques with AOPs can provide good efficiencies for the removal of complex high-strength textile wastewaters, the overall process is characterized by high-operating costs. ABGS-based processes have is able to effectively remove nutrients and refractory organic pollutants, with several advantages over any typical individual treatment process. ABGS advantages are attributed to microalgaebacteria interactions taking place during the process which result in high removal efficiencies under relatively low-operating costs. The treatment efficiencies reached by these processes are also achieved with lower footprint requirements concerning to AOPs. These operating and efficiency advantages allow considering the ABGS technology as an alternative to AOPs concerning the economic and environmental aspects.

ABGS-based processes also promote algal biomass production, and thus, the generation of high-value products. Although ABGS processes have shown a great potential for resource recovery, the number of works evaluating the algal-bacterial symbiosis potential for textile wastewater treatment is still limited. Therefore, more studies on the performance mechanisms, removal efficiency, and cost-effectiveness as an alternative for textile wastewaters treatment are required.
