Cold water effects on enhanced coagulation of high DOC, low turbidity water

Prairie farm reservoirs are usually low in turbidity and contain high conce ntrations of dissolved organic carbon (DOC). Some are treated with aluminum sulfate in late fall when the water drops below 3 degreesC, often with poo r turbidity reduction results. jar tests using aluminum sulfate were conduc ted to study the effect of water temperature on enhanced coagulation of a t ypical high DOC prairie water. far tests showed that temperature affects tu rbidity particle counts and total residual aluminum but does not affect DOC , UV254 absorbance and colour reduction. Turbidity of the treated water inc reased consistently as the temperature decreased. Particle counts in the 1 to 10 mum range were reduced by two logs at 20 degreesC but only by one log at 1.5 degreesC indicating inferior oocyst removal at low temperatures. Be ntonite addition at 1.5 degreesC did not affect the treated water particle counts but because the bentonite increased the raw water particle count, th e reduction of the particles could be increased by one log. Coagulant deman d for the water tested without pH adjustment is one to two mg Al per mg of DOC at all temperatures tested. The optimum pH for all temperatures was abo ut 6.0 for DOC removal and about 6.5 for turbidity removal based on a dosag e increment of 0.5 pH units. UV254 absorbance is well suited to predict opt imum aluminum sulfate dosage for DOC removal.