Optimizing the dissolved oxygen requirements for effective pollutant removal from coastal aquaculture wastewater aiming at water recycling

Wastewater treatment presents a pressing global challenge, emphasizing the urgent need for more sustainable solutions. In the pursuit of energy and carbon neutrality, microalgal-bacterial granular sludge (MBGS) systems have emerged as a promising alternative, leveraging the symbiotic relationship between microalgae and bacteria within granules in terms of gas exchange. MBGS systems offer efficient treatment but also hold the promise for substantial energy savings and greenhouse gas emissions reduction.
The present study aimed to ascertain on the dissolved oxygen threshold required for efficient pollutants removal from coastal aquaculture aiming at water recycling in industrial settings. To accomplish this, a MBGS system was applied for the treatment of aquaculture wastewater and underwent a gradual reduction in the airflow rate from 3.0 to 1.5 L min⁻¹ over 134 days. Regardless of the airflow rate, complete ammonium removal was consistently achieved, while lower airflow rates appeared to enhance nitrite and nitrate removal. The composition of treated effluents met toxicity limits for fish, enabling water reuse in aquaculture facilities. However, if the airflow rate was reduced to about 1.5 L min⁻¹, outgrowth of filamentous microorganisms started to occur on granules surface, compromising their efficient separation from the treated water.
Aeration typically contributes significantly to energy consumption in wastewater treatment processes. Utilizing MBGS systems can effectively reduce aeration needs, up to a certain level, without compromising the treatment performance, thus improving the ecological footprint of the treatment process.