Abstract
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.
Highlights
Organic suspended solids encountered in aquaculture systems will contain phosphorus, can contain undesirable organisms, and may cause gill irritation in salmonids (Noble and Summerfelt 1996)
Chitosan dosed into a coldwater recirculating system In the preliminary study, where dissolved chitosan was added to a recirculating system (Figure 1), the recirculating water had become visibly clearer within 2 hrs of initiation of chitosan addition, and the culture tank TSS levels had dropped from 10.7 to 2.9 mg/L
Dissolved chitosan should not be added to aquaculture systems containing rainbow trout
Summary
Organic suspended solids encountered in aquaculture systems will contain phosphorus, can contain undesirable organisms, and may cause gill irritation in salmonids (Noble and Summerfelt 1996). Suspended solids must be removed from recirculating aquaculture systems to improve water quality. Suspended solids must be removed from their effluents in order to meet state and federal effluent discharge limits. Sedimentation and microscreen filtration are the primary mechanisms used to remove particulate matter from coldwater recirculating systems and their effluents. Sedimentation and microscreen filtration units typically do not remove particles much smaller than about 75 mm (Timmons et al 2002), which might not be adequate because particles that can contribute to gill irritation and mortality may be in the 5-10 mm range (Chapman et al 1987). Other options that can be used to increase the removal of fine particles include foam fractionation (Weeks et al 1992), ozonation (Summerfelt et al 1997), and possibly the addition of flocculation aids such as ferric chloride, alum, and/or polymers (Ebeling et al In Review)
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