1. Introduction
Algae are plant-like, unicellular or multicellular aquatic organisms. Bioremediation performed by algae is termed phycoremediation. The use of algae to treat wastewater has been in vogue for more than 50 years with one of the first descriptions of this application being reported by Oswald in 1957. Phycoremediation is used to describe remediation of contaminants in a water body using algae (micro and macro algae). It is a branch of bioremediation that makes use of algae. It is a bio-restoration technology involving the use of algae and it is relatively new in Africa. Algae can fix carbon dioxide by photosynthesis and remove excess nutrients effectively at minimal cost. Phycoremediation is employed for improving water quality. In addition, photosynthetically produced oxygen can relieve biological oxygen demand (BOD) in the waste water. Microalgae are superior in remediation processes as a wide range of toxic, and other wastes can be treated with algae and they are non-pathogenic. The risk of accidental release of pollutants into the atmosphere can cause health, safety and environmental problems, but are avoided when algae are employed for remediation. Algae use the wastes as nutrient and enzymatically degrade the pollutants. The xenobiotics and heavy metals are known to be detoxified, transformed or volatilized by algal metabolism. They have the ability to take up various kinds of nutrients like nitrogen and phosphorus. They can utilize various organic compounds containing nitrogen and phosphorus from their carbon sources. Many researchers have studied microalgae as pos-Department of Botany, Centre for PG studies and Research, Sacred Heart College etc. Some other researchers such as, etc have also documented some studies on phycoremediation. The choice of microalgae to be used in wastewater treatment is determined by their robustness against wastewater and by their efficiency to grow in it and take up nutrients from wastewater. Some algae which are generally used for the waste water treatment are Chlorella, Scenedesmus, Synechoccystis, Gloeocapsa, Chroococcus, Anabaena, Lyngbya, Oscillatoria, Spirulina etc. Pollution has been a common feature in almost all rivers and lakes because of organic and industrial wastes. The use of microalgae to treat wastewater is an environmental friendly method with no secondary pollution as long as the biomass produced is reused and efficient nutrient recycling is allowed. The microalgae consume the minerals in the waste to optimize of their growth process. In addition to treating the water, the created biomass has a variety of applications including production of bio-diesel, animal feed, products for pharmaceutical and cosmetic purposes, or it can even be used as a source of heating or electricity. Algal biomass forms an important food source for shellfish or other aquatic species. This wide variety application of microalgae explains the interest in controlling their growth.
Microalgal biomass generated from remediation process offers more advantages compared to conventional biomass production because do not require arable land for cultivation. Innovations to microalgae production allow it to become more productive while consuming resources that would otherwise be considered as waste.
In this circumstance, wastewater can be considered as resources. Microalgae biomass can be produced at extremely high volumes and this biomass can yield a much higher percentage of oil than other sources. In recent years, many researchers have studied the potential of dual application of microalgae for wastewater treatment and biomass production. The high nitrogen level in wastewater had become a growing concern which has increased the necessity to develop simple, efficient, and cost effective nitrogen removal techniques. High nitrate wastes (>1000 ppm) are usually generated by fertilizer, metal finishing, cooking and organic chemical industry, nuclear industry and nitrified landfill leachate. Species of chlorophyta, Rhodophyta, Cyanophyta, Diatoms, Pheophyta, Charophyta e.t.c. can be utilized in this technology. Phycoremediation can be incorporated into secondary effluent treatment stage. Several industries in the world are utilizing this technology and examples of such companies include; Algae tech International (Malaysia,) Sunrise Ridge Algae, Inc. (USA), Snap Natural &Alginate Products LTD (India), Nutraville International (Chennai).
2. Algae-Based Waste Water Treatment Systems
To construct algae based wastewater treatment system, it is essential to consider both wastewater treatment as well as algal cultivation. Cell retention time, nutrient addition rate, water depth, and degree of mixing are parameters to be considered for growth of algae. In addition to these parameters, BOD reduction, TDS reduction, pH, nitrogen removal rate and phosphorus removal rate should be considered for wastewater treatment. Therefore, the system should be designed accordingly to allow both growth of algae and wastewater treatment. Main reasons for failure are;
1. Failure to consider all relevant local factors at the pre-design stage
2. A lack of technical knowledge
3. Inappropriate discharge standards.
Two types of wastewater treatment systems are available for algae based treatment.
1. Waste Stabilization Pond Systems
2. High Rate Algal Ponds
2.1. Waste Stabilization Pond Systems (WSPs)
They are large, shallow basins. Wastewater is treated entirely by natural processes involving both algae and bacteria. They are used in temperate and tropical climates and is one of the most cost-effective, reliable and easily-operated methods for treating wastewater. They are very effective in pathogen removal, e.g faecal coliform bacteria. Sunlight energy is solely required for its operation. Furthermore, it requires regular cleaning of the outlets and inlet. The temperature and sunlight in tropical countries offer a high efficiency and satisfactory performance for this water-cleaning system. The advantage of these systems in terms of removal of pathogens is one of the most important reasons for its use. Wastewater treatment in Waste Stabilization Ponds (WSPs) is "green treatment" which is achieved by mutualistic growth of microalgae and heterotrophic bacteria. The algae produce oxygen from water as a by-product of photosynthesis. This oxygen is used by the bacteria as they aerobically bio-oxidize the organic compounds in the wastewater. Carbon dioxide is the end-product of this bio-oxidation which is converted into cell carbon by the algae during photosynthesis.
3. Significance of Phycoremediation in Environmental Sustainability
Phycoremediaton is of great significance and offers several benefits in comparism with other bioremediation processes. These include; 1. No yielding of toxic products. 2. Pathogen removal 3. Reduction in carbon-effect and concentration 4. It is an eco-safe process 5. Detoxification and removal toxic wastes. 6. Green House Gas emission reduction
3.1. Advantages of Algae Wastewater Treatment
Using algae for wastewater treatment offers some interesting advantages over conventional wastewater treatment such as in;
1. Cost effectiveness and safety;
2. Green House Gas emission reduction;
3. Reductions in sludge formation and low energy requirement;
4. Production of algal biomass;
5. Oxygenation of the systems through photosynthesis thereby enabling effective decomposition;
6. Effective reduction of nutrient load and consequent total dissolved solids as these are used up as nutrient sources;
7. Production of high algal biomass which can be used as feed in aquaculture and as bio-fertilizer;
8. Simple operation and maintenance;
9. Potential for energy and nutrient recovery
3.2. Major Setbacks in Conventional Methods of Waste Water Treatment
1. Sludge formation is more often unavoidable which is difficult to dewater and dispose.
2. Physical methods such as reverse osmosis and other chemical methods are costly.
3. Addition of chemicals may increase the salinity and conductivity of water.
4. Ecological implications due to altered and increased water variables.
5. Problematic in the treatment of some effluents such as metal-bearing streams.
4. Conclusion
It is necessary to often invest in low cost and high effective phycoremediation method in treating wastewater from industries and agricultural lands before disposing. It offers eco-friendly method of waste water treatment before disposal or reuse. Government through her Environmental Protection Agency should ensure appropriate investment in this area of phyco-waste treatment. Research institutes, academic institutes, companies and industries should continue to conduct researches on the appropriateness of different algal species that can be used for efficiency in phyco-remediation.