Urban stormwater treatment using chitosan

Abstract

[Abstract]: The need to study the treatment of urban stormwater was identified as Toowoomba Regional Council in partnership with the University of Southern Queensland investigates the feasibility of a stormwater harvesting scheme. Urban runoff would be captured, treated to a non-potable standard and reused for local irrigation and industrial applications. The study tests chitosan as a coagulant to remove suspended solids from the captured stormwater. Reduced inflows to Toowoomba’s three major reservoirs and the depletion of basalt bores have emphasized the need to utilise urban stormwater. The presence of contaminants including suspended solids, heavy metals, hydrocarbons and nutrients combined with a history of proven alternatives has seen stormwater harvesting projects generally overlooked. The study tested the performance of chitosan against more traditional coagulants. Chitosan is a biodegradable chemical compound produced from the pulverisation and dissolution of crustacean shells. FlocClear BioPolymer™ has been sourced from Los Angeles, USA for the project. FlocClear is a solution containing 2% chitosan acetate by weight. Chitosan will be compared to Magnasol 589, the chemical of choice at Toowoomba’s Mt Kynoch water treatment plant. Samples were taken from a variety of urban catchments to ensure a representative range of stormwater turbidity, pH and particle size characteristics were tested. Jar testing was employed using a rapid mixing speed of 100 rpm for 2 minutes. Flocculation followed at 30 rpm for 20 minutes. Settling time was 30 minutes. The initial and final turbidity were measured. Total suspended solids (TSS) tests were also conducted. A relationship between turbidity and TSS was plotted to validate the results. The results were analysed for compliance with the Queensland Water Recycling guidelines for non-potable class-A water. Chitosan proved effective in the removal of suspended solids from urban stormwater particularly from high turbidity stormwater. The maximum efficiency was achieved using a 5.0mg/L chitosan acetate dose. The turbidity of the stormwater was reduced from 260.0 NTU to 8.9 NTU. Chitosan is also capable of treating less turbid water which is seen by reducing a 19.5 NTU influent to 2.5 NTU using a 3.5 mg/L dose. A form of sand filtration is required to further reduce turbidity below 2 NTU, to comply with the Queensland Water Recycling Guidelines.