Algae-Based Wastewater Treatment: Critical Analysis and Review

Introduction

Wastewater comprises liquid or water-carried wastes from the sanitary facilities of residences, commercial buildings, and industrial facilities, as well as groundwater, surface water, and stormwater where present. Untreated water contains high levels of organic material, pathogenic organisms, nutrients, and many toxic elements. This type of wastewater poses both an environmental and public health danger, necessitating the removal of such waste from its sources and appropriate treatment before disposal [1]. The primary objective of wastewater management is to protect the environment in a manner consistent with public health and socio-economic concerns. For this reason, wastewater management is becoming increasingly vital because of diminishing water resources, rising wastewater disposal costs, and strict discharge regulations that have lowered permissible contaminant levels in water bodies.

The significance of water as a worldwide resource for human life is irrefutable. People across the globe recognize it as a requirement to protect this important resource, making water conservation a global priority. This awareness has driven advances in the efficiency, convenience, and sanitation of human society through the establishment and distribution of large-scale, reliable supplies of high-quality water. In doing so, however, those same systems also enable the convenient disposal of infectious and hazardous effluents from their sources into water bodies [2]. Wastewater—and the activities associated with addressing it—forms the basis of this paper.

Although many treatment practices exist, some generate large quantities of sludge requiring off-site disposal. In addition, many wastewater treatment processes cannot efficiently handle variations in the composition of wastewater. This means that a treatment process effective at one time of year may not be proficient at another. This paper analyzes research on the use of algae for wastewater treatment. Algae-based water treatment systems have demonstrated interesting advantages over conventional treatment methods, including cost-effectiveness, low energy requirements, reduced sludge formation, and the generation of useful algal biomass [3].

Algal-Bacterial Symbiosis in Wastewater Treatment

The concept of algae-bacteria culture as an engineered system for domestic and industrial wastewater treatment has attracted increasing attention over the past decades. This system performs well in regions that experience high solar radiation and temperature, because the removal process is essentially natural. When solar radiation falls on the algae, they respond by producing oxygen, which aerobic bacteria use to biodegrade pollutants, while the algae simultaneously absorb the carbon dioxide released through bacterial respiration [4]. In this way, the algae provide an affordable and safe alternative to mechanical aeration and contribute to carbon dioxide mitigation. This technology is efficient because nitrogen and phosphorus can accumulate in the algal-bacterial biomass during the removal process.

The primary disadvantage of this technology is the requirement for cost-effective biomass harvesting methods. A technical separation unit involving centrifugation may be needed, which raises costs. Using chemicals such as calcium or slaked lime results in secondary pollutants [5]. Incorporating immobilization techniques is one possible solution; however, all available media are expensive and tend to become ineffective over extended periods of operation. Therefore, an efficient biomass harvesting strategy—such as a settleable algae-bacteria system—is needed. Although previous studies addressed the identification and biometry of dominant algal species, they lacked sufficient information about the bacterial community associated with the process [4].

Algae play an important role in the natural self-purification of contaminated waters [6]. This natural algal treatment process cannot occur without biological intervention. Early studies described this kind of association as an interrelationship or mutual "symbiosis" between algae and bacteria. Research by Bartsch found that bacteria and algae are the most dominant organisms among the planktonic biota of oceans, and that their associated metabolism helps control pelagic energy flow and nutrient cycling in aquatic environments. The treatment of concentrated wastes depends on these microbiological processes to accomplish effective remediation.

A waste stabilization pond (WSP) serves as a reactor that intensifies waste concentrations, resulting in an accelerated rate of naturally occurring waste treatment and purification [6]. At the center of this natural biological process is the cyclic, synergistic relationship between algae and bacteria. Certain fungi also play a substantial role in the algal-microbial stabilization of organic effluents within WSPs, such that the metabolic elements of fungi, bacteria, and algae are interrelated.

In terms of the underlying biochemistry, heterotrophic microbial mineralization of incoming organic materials generates carbon dioxide (CO₂), ammonia-nitrogen (NH₃-N), phosphates (PO₄³⁻), and essential vitamins—all stable, inorganic, and oxidized by-products. Autotrophic algae then utilize these products of bacterial metabolism for their own development and growth through photosynthesis. The splitting of water molecules during algal photosynthesis provides oxygen for aerobic microbes, enabling the oxidative decomposition of wastewater organics, and the process continues in this positive feedback cycle. The aquatic chemistry involved accounts for the diurnal shifts in dissolved oxygen (arising from photosynthesis and respiration) and pH (via the carbonate-bicarbonate system) commonly observed in WSPs.

Using Aquatic Plants for Nutrient Removal

Several studies have documented the capacity of aquatic plants to extract nutrients from the water in which they grow. The rise in published research on the nutrient extraction potential of water plants over the past decade has been driven largely by growing awareness of the problems of water pollution—both freshwater and saltwater—resulting from population growth, industrial development, and the disposal of human, animal, and industrial wastes into water bodies [1]. Documented cases of devastating effects of wastewater on previously clean rivers and lakes have aroused both public and scientific concern, generating interest in reversing the process by actively extracting pollutants [1]. The exceptional ability of water plants to absorb compounds and elements from water is well established.

Boyd [7] proposed a method for reducing water pollution by harvesting water plants that have extracted nutrients from the water. All water plants can serve this purpose; however, small or submerged plants are more difficult and expensive to harvest than floating and emergent vascular plants. For example, the water hyacinth—when covering approximately 10% of a pond's surface—can effectively remove adequate nutrients to prevent excessive phytoplankton development. Boyd further demonstrated that the water hyacinth is capable of extracting nitrogen and phosphorus under favorable growing conditions [7].

Conclusion

The analysis presented in this paper demonstrates that algae-based treatment is effective in controlling water pollution. The evidence clearly shows that it is possible to treat domestic wastewater, industrial wastes, and agricultural wastes using algal systems. The literature review further confirms that algae can significantly improve water quality. Temperature, biological activity, and flow rate each exert considerable influence on nutrient removal. A settleable algal-bacterial culture enriched from municipal wastewater was shown to successfully treat wastewater in a stirred tank photobioreactor, reducing total suspended solids to 0.016 g/L within 20 minutes.

The average removal of COD, TKN, and phosphate were 1.3%, 1.6%, and 1.0%, respectively, over an eight-day batch period. Average biomass generation was 1.2 g/m²·day. Biomass uptake was identified as the primary mechanism involved in nutrient removal. The dominant algal species in the bioreactor was blue-green algae, and the dominant bacteria included Flavobacteria, Gammaproteobacteria, Bacteroidia, and Betaproteobacteria. This review provides new insight into settleable algal-bacterial culture enrichment strategies and contributes meaningfully to the understanding of microbial ecology and diversification in algal-bacterial culture systems.

References

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