Microalgal removal from aquaculture discharge using electro-flocculation

The Queensland prawn industry is valued at $167 million and is rapidly expanding with a 28% increase in production in the last 2 years. A primary factor limiting the expansion of land-based aquaculture is the need to dispose of vast quantities of wastewater with high levels of nitrogen. As most prawn farm sites in Queensland are located adjacent to the Great Barrier Reef, prawn farm discharges must meet stringent water quality limits, particularly for total nitrogen (TN) and dissolved inorganic nitrogen (DIN). Developing new technologies to improve the management of water discharge quality and ideally repurposing the waste material is a key objective for the Queensland prawn industry. Naturally occurring microalgae take up excess nitrogen but current practices to remove them through sedimentation are slow. However, microalgae can be efficiently captured using electro-flocculation, which will speed up the process and provide a flocculant that may have use as a commercial bio-product (e.g. in bio-fertiliser or as a supplement in cattle fodder). To test this, water was collected from Queensland prawn farm ponds containing three different microalgal assemblages: Pond 13 (P13) was dominated by cyanobacteria Gloeocapsa and Pseudanabaena, and two species of the diatom Bellerochea; Pond 25 (P25) was dominated by Gloeocapsa; and Pond 30 (P30) was dominated by both species of Bellerochea and Gloeocapsa. In trials, two carbon electrode configurations were used: 3 parallel spaced 100 mm apart vs. 5 parallel spaced at 50 mm apart with two water states: stationary in a single tank (60 L) vs. flowing in a dual tank system (240 L). The current was set at 19.6 amps and the voltage at 17.6 volts. Electro-flocculation success was greater with electrodes spaced closer together. The greatest reduction in total nitrogen (TN) was 83.19% (±1.65 stderr) in P30 from 10.93 to 1.83 mg.L-1 in 20 mins, using five electrodes in stationary water. The greatest reduction in dissolved inorganic nitrogen (DIN) was 90.64% (±3.06 stderr) from 3.19 to 0.30 mg.L-1 in 67 mins in P30 with five electrodes in flowing water. The greatest quantity of freeze-dried microalgal flocculant produced in 60 L stationary water was 7.83 gdwt in P13 and 15.17 gdwt from 120 L flowing water, also in P13. Flocculant consisted primarily of Bellerochea and Gloeocapsa spp. As the prawn farming industry intensifies production, nitrogen waste compliance is becoming increasingly challenging. This research has shown electro-flocculation can reduce TN and DIN to compliant levels with the potential to repurpose waste material as a bio-product. We have demonstrated this using nutrient rich pond water samples, prior to any treatment by farm settlement systems.