Evaluation of Dissolved Chitosan for Suspended Solids Removal

In a preliminary study conducted at The Conservation Fund Freshwater Institute (Shepherdstown, WV, USA), dissolved chitosan was added to a recirculating system to determine if the chitosan would coagulate particulate matter and consequently increase solids removal. The recirculating water became visibly clearer and the culture tank total suspended solids (TSS) concentration dropped from 10.7 to 2.9 mg/L within 2 hours after dosing had been initiated. However, fish showed symptoms of distress and the chitosan treatment was discontinued. In subsequent studies conducted to determine the particle capture mechanism associated with chitosan addition, effluent treated with dissolved chitosan was not returned to the system. The results of two jar test studies indicated that dissolved chitosan did not enhance particle capture by settling or by microscreen filtration when mixed with a fish culture system effluent containing * 10 mg/L of TSS. However, these jar tests indicated that an additional 44% of TSS could be removed from the water that had already passed through a microscreen filter if this water was treated by a mixing and settling step, even without addition of dissolved chitosan. Additional studies using small-scale fluidized-sand biofilters indicated that the reduction in TSS observed in our initial experiment was due to TSS capture in the fluidized sand biofilter. TSS concentrations were reduced from 5.1-7.4 mg/L at the biofilter inlet to 1.7-2.2 Ihg/L at the biofilter outlet. Thus, adding dissolved chitosan to water flowing into a fluidized-sand biofilter turned the biofilter into a novel type of upflow 'sludge blanket clarifier,' which appears to be both non-plugging and relatively simple to operate. In addition, dissolved chitosan did not change nitrification occurring within the fluidized-sand biofilter. Therefore, adding a coagulant (such as dissolved chitosan or a non-toxic polymer) to the flow entering a fluidized sand biofilter has the potential to create a unit process that reduces TSS while simultaneously treating dissolved wastes.