Algae can play a positive role in wastewater treatment by recycling of nutrients, by taking up carbon, nitrogen and phosphorus, by reducing the nutrient load through stripping and precipitation, by producing oxygen for bacterial decomposition of organic matter, by eliminating pathogenic bacteria through bactericide action, and last but not least by solving odor problems. If sufficient insolation, and space are available, intensive algal wastewater treatment-systems proved to be valuable and promising alternatives to conventional treatment systems based on activated sludge or trickling filters. The various technologies developed for algal wastewater treatment are commented in this review. They include simple stabilization-oxidation ponds, as well as more advanced high rate oxidation ponds (= HIROP), high rate algal ponds (HIRAP), and activated algae ponds. Depending on the objective of the system, algae are a part of the secondary or the tertiary treatment or of both. Treatment efficiency and nutrient removal are described and commented on in function of algal growth, wastewater characteristics (nutrient imbalance, chemical and biological toxicity), and operational parameters such as detention time, pH, temperature, light, depth, mixing, and system configuration. Some design criteria and conceptual considerations are given with respect to microalgal tertiary treatment systems. The differences in design parameters are illustrated, depending whether the objective is biomass production or nutrient removal. Several mechanisms for N- and P-removal are summarized. Complete design models and methods for mixed secondary-tertiary treatment are also discussed and illustrated. Distinction is made between conventional lagooning or ponding and HIROP systems. Process stability, optimization and control strategies are emphasized for each system. Microalgal biomass may be valorized either directly in aquaculture and mariculture as live food for growing fish, shellfish and crustaceans, or indirectly (after concentrating, harvesting, and processing) , as animal feedstuff for broilers, pigs, and fish, or as agricultural fertilizers, fuels (methane, ethanol, hydrogen) , chemical feedstuff and raw materials for chemical processes. The potentials of these various ways of resource recovery are analyzed. Physical, chemical, and biological methods for harvesting and processing microalgae are also reviewed. These methods consist in centrifugation, microstraining, flocculation, flotation, sedimentation, filtration, drying, sterilization, and preservation. Finally, technical, economical, and political problems encountered in the mass culturing of algae are discussed. Major areas of concern for the future development of large-scale algal wastewater treatment-systems are: production costs, economic harvesting of microalgae, species control as related to harvestability and nutritional value for consumers, predator infestation, intoxication by diseases and pollutants of different nature. |
