Preventing biofilm development on DGT devices using metals and antibiotics

The DGT technique has potential as a tool for monitoring reactive phosphorus in freshwater aquaculture effluents. Because those waters have high concentrations of suspended matter and nutrients, biofilms may form on the surface of the DGT devices. Those biofilms may hinder the movement of reactive phosphorus and hence interfere with the DGT measurements. We tested two antibiotics, glutaraldehyde and chloramphenicol, two metal-iodides, copper and silver and also two alternative filter types, nucleopore membrane and silver-based filters, to evaluate their respective potential to prevent the formation of algae. The treatment with silver iodide seems to affect the properties of the diffusive gel and changes the flux measurements of the DGT device. The DGT response observed using the copper iodide and chloramphenicol treatments was not significantly different from the control. Glutaraldehyde changed the macroproperties of the diffusive gel and interfered with the phosphorus detection using spectrophotometric determinations. The effect of the anti-biofilm treatments on the DGT measurements was independent of pH and ionic strength of the water. For the field deployment in fish farms, copper and silver were the best anti-biofilm agents. Copper prevented algal colonisation for 14-days post-deployment and the response was unaffected by the anti-biofilm agent throughout this period. Silver was even better and prevented biofilm formation up to a 21-days post-deployment. Conversely, chloramphenicol did not prevent algal colonisation for the 14- and 21-days deployments. However, for deployments longer than 14 days, it was difficult to obtain consistently good results for all of anti-biofilm agents tested, due to the high concentration of suspended matter in the freshwater effluents of the fish farms tested. This approach suggests a metal pre-treatment of the membrane filters is useful to prevent biofilm formation for DGT deployments aimed at P measurements. DGT deployments for metal measurements would likely require a different approach.