Co-cultivation of microalgae and macroalgae for the efficient treatment of anaerobic digestion piggery effluent (ADPE)

Abstract

Microalgal and macroalgal phytoremediation has been proposed as a practical green solution for the treatment of anaerobically digested piggery effluent (ADPE). This is mainly due to the algae’s inherent ability to strip away and convert inorganic nutrients, especially nitrogen and phosphorous efficiently from various effluents. Our previous Pork-CRC (4A-106 and 4A-108) studies showed the potential of a microalgae consortium that could grow efficiently on undiluted ADPE (up to 1600 mg L-1 of ammonium) and that of a macroalgae consortium (4A-107) which could treat diluted ADPE (below 250 mg L-1 of ammonium). The main advantage of macroalgae over microalgae is their ease of harvest, especially if the aim is to use the generated biomass as a source of animal feed. There is a potential in co-culturing cultures of microalgae and macroalgae to increase the overall efficiency of ADPE treatment and improve the economics related to algal biomass production. In accordance, we evaluated the co-cultivation of both microalgae and macroalgae together in two distinctive studies. For both studies, previously isolated consortium of microalgae consisting of Chlorella and Scendesmus sp. was initially grown on undiluted ADPE until the concentration of ammonium was reduced to desired levels. In order to identify the most suitable and efficient macroalgal species for co-cultivation with microalgae, a preliminary study was conducted to evaluate the growth and nutrient removal of four locally isolated macroalgae on ADPE. In the first co-cultivation study, the ADPE grown microalgae was directly utilized as a cultivation media for the propagation of macroalgae (Cladophora sp.) which was found capable of growing in ADPE up to 150 mg L-1 NH4+. However, despite the different conditions evaluated, the growth and photo-physiology of Cladophora sp. was found to decline and eventually led to its death due to the dominancy of microalgal culture during the co-cultivation period. Subsequently, based on this outcome, an outdoor inclined reactor was customized to evaluate the potential use of attached macroalgal culture as a way of scrubbing available nutrients and microalgae biomass from ADPE post microalgal treatment. Although, the inclined system was very efficient in scrubbing and harvesting microalgae biomass, nevertheless, nutrient removal rates (i.e. ammonium and nitrate) of the co-cultivated system was much lower than the control which was operated using macroalgae only. In this work, despite multiple different approaches and cultivation systems, both algal groups were unable to co-exist for efficient growth in ADPE due to direct competition for available resources and the negative interaction of both algal groups. Nevertheless, through this study, it has been demonstrated that macroalgae could be potentially used for harvesting microalgae grown in ADPE