Performance and application of a fluidized bed limestone reactor designed for control of alkalinity, hardness and pH at the Warm Springs Regional Fisheries Center

Abstract

Springs serving the Warm Springs Regional Fisheries Center, Warm Springs, Georgia, have pH, alkalinity, and hardness levels that lie under the range required for successful fish propagation while free CO2 is well above allowable targets. We evaluate a pretreatment process that exploits limestone’s (CaCO3) ability to react away hydrogen ions (H+) and carbon dioxide (CO2) while increasing alkalinity (HCO3−) and calcium (Ca2+) concentrations, i.e.CaCO3+H+↔HCO3−+Ca2+CaCO3+CO2+H2O↔Ca2++2HCO3−Limestone sand was tested in both pilot and full scale fluidized bed reactors (CycloBio®).11Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. We first established the bed expansion characteristics of three commercial limestone products then evaluated the effect of hydraulic flux and bed height on dissolution rate of a single selected product (Type A16×120). Pilot scale testing at 18C showed limestone dissolution rates were relatively insensitive to flux over the range 1.51–3.03m3/min/m2 but were sensitive (P<0.001; R2=0.881) to changes in bed height (BH, cm) over the range 83–165cm following the relation: (Alkalinity, mg/L)=123.51−(3788.76 (BH)). Differences between filtered and non-filtered alkalinity were small (P>0.05) demonstrating that limestone was present in the reactor effluent primarily in the form of dissolved Ca(HCO3)2. Effluent alkalinity exceeded our target level of 50mg/L under most operating conditions evaluated with typical pilot scale values falling within the range of 90–100mg/L despite influent concentrations of about 4mg/L. Concurrently, CO2 fell from an average of 50.6mg/L to 8.3mg/L (90%), providing for an increase in pH from 5.27 to a mean of 7.71. The ability of the test reactor to provide changes in water chemistry variables that exceeded required changes allowed for a dilution ratio of 0.6. Here, alkalinity still exceeded 50mg/L, the CO2 concentration remained well below our limit of 20mg/L (15.4mg/L) and the pH was near neutral (7.17). Applying the dilution ratio of 0.6 in a full scale treatment plant at the site reduced by 40% the volume of spring water that is directed through each of three parallel reactors that combined react away 49,000kg of limestone/yr.