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

Authors: WATTEN, BARNABY - Us Geological Survey (USGS); MUDRAK, VINCENT - Us Fish And Wildlife Service; ECHEVARRIA, CARLOS - Us Fish And Wildlife Service; SIBRELL, PHILIP - Us Geological Survey (USGS); SUMMERFELT, STEVEN - Freshwater Institute; BOYD, CLAUDE - Auburn University

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2017
Publication Date: 3/21/2017
Citation: Watten, B.J., Mudrak, V.A., Echevarria, C., Sibrell, P.L., Summerfelt, S.T., Boyd, C.E. 2017. Performance and application of a fluidized bed limestone reactor designed for control of alkalinity, hardness and pH at the Warm Springs Regional Fisheries Center. Aquacultural Engineering. 77:97-106.

Interpretive Summary: A relatively acidic and extremely soft water supply at the Warm Springs Regional Fisheries Center, Warm Springs, Georgia, has required relatively expensive and challenging chemical treatments to adjust the water to a pH and hardness suitable for fish propagation. A less expensive and simpler water pretreatment process was developed to exploit limestone's ability to destroy acidity while simultaneously increasing water hardness. This innovative process used reactor vessels to fluidize crushed limestone sand, allowing the limestone to slowly dissolve and create alkaline water with healthy levels of calcium-magnesium hardness. After proving the concept with a pilot-scale setup, three full-scale reactors were installed to treat 60% of the total water required, while bypassing 40% of the flow, yet still maintaining suitable alkalinity and hardness levels. This limestone-based treatment process has provided effective water quality control and reduced operating costs; it has great potential as a pretreatment for other sites with broadly similar water quality.

Technical 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+ reacts in a reversible acid-base chemical equilibrium to form HCO3-+ Ca2+ CaCO3 + CO2+ H2O reacts in a reversible acid-base chemical equilibrium to form Ca2+ + 2HCO3- Limestone sand was tested in both pilot and full scale fluidized bed reactors (CycloBio®). 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 18°C showed limestone dissolution rates were relatively insensitive to flux over the range 1.51 -3.03 m3/min/m2 but were sensitive (P less than 0.001; R2= 0.881) to changes in bed height (BH, cm) over the range 83 -165 cm following the relation: (Alkalinity, mg/L) = 123.51 - (3788.76 (BH)). Differences between filtered and non-filtered alkalinity were small (P greater than 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 50 mg/L under most operating conditions evaluated with typical pilot scale values falling within the range of 90 -100 mg/L despite influent concentrations of about 4 mg/L. Concurrently, CO2 fell from an average of 50.6 mg/L to 8.3 mg/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 50 mg/L, the CO2 concentration remained well below our limit of 20 mg/L (15.4 mg/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 49,000 kg of limestone/yr.