Microalgae allow reducing aeration requirements and improve the efficiency of granular sludge treating coastal aquaculture streams aimed at recirculation

Water pollution has become a global issue of increasing environmental concern. In the journey towards the energy and carbon neutrality of wastewater treatment processes, an increasing interest has been placed in microalgal-bacterial granular sludge (MBGS) systems. The symbiotic relationship between microalgae and bacteria in granules in terms of gas exchange could lead to energy savings and greenhouse gases emission reduction. This study aimed to ascertain on the minimum dissolved oxygen content needed to efficiently remove pollutants and allow for water recycling in coastal aquaculture facilities. For that, a photo-sequencing batch reactor was inoculated with microalgae-bacterial granular sludge and operated under a stepwise decrease of the airflow rate from 3.0 to 1.5 L min-1 for 134 days. The removal efficiency of chemical oxygen demand (COD) was kept at high level (47 to 77%). Complete ammonium removal was achieved throughout operation, independently of the applied airflow rate whilst nitrite and nitrate removal improved at lower airflow rates (< 2.0 L min⁻¹). In fact, the mass balance on nitrogen species further revealed that the overall nitrogen removal improved indicating that water of quality for recirculation can be obtained at lower airflow rates (<2.0 L min⁻¹). Although the reduction of the airflow rate till ca. 1.5 L min⁻¹ benefited the reactor performance, outgrowth of filamentous microorganisms started to occur, compromising the quick and efficient separation of the biomass from treated water. The MBGS system operated under extremely low airflow rates represents a promising solution in recirculation aquaculture systems, rendering water with chemical quality that complied with marine fish toxicity limits, whilst potentially reducing the aquaculture farm operational costs. Nevertheless, an airflow rate threshold should not be surpassed to prevent the outgrowth of filamentous microbes.