Abstract
Improving the ecological status of water sources is a growing focus for many developed and developing nations, in particular with reducing nitrogen and phosphorus in wastewater effluent. In recent years, mixotrophic micro-algae have received increased interest in implementing them as part of wastewater treatment. This is based on their ability to utilise organic and inorganic carbon, as well as inorganic nitrogen (N) and phosphorous (P) in wastewater for their growth, with the desired results of a reduction in the concentration of these substances in the water. The aim of this review is to provide a critical account of micro-algae as an important step in wastewater treatment for enhancing the reduction of N, P and the chemical oxygen demand (COD) in wastewater, whilst utilising a fraction of the energy demand of conventional biological treatment systems. Here, we begin with an overview of the various steps in the treatment process, followed by a review of the cellular and metabolic mechanisms that micro-algae use to reduce N, P and COD of wastewater with identification of when the process may potentially be most effective. We also describe the various abiotic and biotic factors influencing micro-algae wastewater treatment, together with a review of bioreactor configuration and design. Furthermore, a detailed overview is provided of the current state-of-the-art in the use of micro-algae in wastewater treatment.
Highlights
The aim of this review is to provide a critical account of microalgae as an important step in wastewater treatment for enhancing the reduction of N, P and the chemical oxygen demand (COD) in wastewater, whilst utilising a fraction of the energy demand of conventional biological treatment systems
The current evidence is that integrating micro-algae as an alternative biological wastewater treatment option is technologically and environmentally feasible. It is economically competitive if it is borne in mind that the huge cost associated with the cultivation of a micro-algae plant discussed here can be regarded as “installation” cost, but conventional systems have installation costs
The use of micro-algae to reduce nitrogenous, phosphorous and carbonaceous material does have the potential to operate at a lower footprint in terms of energy consumption and greenhouse gas generation compared to conventional biological wastewater treatment processes
Summary
Micro-algae, including eukaryotic algae and cyanobacteria, have demonstrated to be an environmentally friendly and sustainable alternative to energy-intensive and conventional biological treatment processes that are widely used today (Singh et al, 2015; Oswald, 2003). Wastewater treated by an algal-bacterial co-culture does not need to transition between different operating environments to facilitate inorganic N and P removal, requiring only a single-step treatment stage and thereby reducing the complexity and energy of the treatment process (Sturm and Lamer, 2011; Gouveia et al, 2016). This is because micro-algae assimilate ammonia (NH3) and phosphate (PO4) directly and in concert for cell growth and metabolic function (Falkowski and Raven, 2007; Borowitzka et al, 2016). Overall, furnishing wastewater with dissolved O2 through micro-algae photosynthesis is a sure bet for significant savings in energy demand and reductions in associated greenhouse gas emissions
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