AMMONIA REMOVAL FROM HIGH-STRENGTH WASTEWATERS BY STEAM STRIPPING

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1 AMMONIA REMOVAL FROM HIGH-STRENGTH WASTEWATERS BY STEAM STRIPPING lw 'p PJf by G. B. Wickramanayake, Ph.D., P.E. Senior Engineer ENVIRON Corporation Princeton, New Jersey INTRODUCTION, OBJECTIVES, AND SCOPE Extracting metal values from some ores requires the use of hydrometallurgical techniques that employ ammoniacal lixiviants. The metal values are recovered from the pregnant liquors, leaving an aqueous wastewater which is high in dissolved solids and ammonia. This wastewater requires treatment for removal of ammonia as well as dissolved solids to meet the discharge standards. The purpose of this study was to evaluate the effectiveness of steam stripping on removal and recovery of ammonia from metal smelting and refining industry wastewaters. (r The overall objective was pursued as two discrete tasks. Task 1 involved performing theoretical and laboratory studies to determine the effects of wastewater composition on the equilibrium of ammonia (gas) and water and gas-liquid mass transfer rates, especially within the temperature range of interest in actual plant operation. In Task 2, laboratory tests were performed using a small pilot-scale steam stripping apparatus. Experiments were conducted using two representative synthetic waste streams; ph adjustment was accomplished using lime or caustic. METHODS One of the objectives of task 1 was to investigate how the distribution of inorganic species and changes in temperature affect the equilibrium of ammonia (gas) and the wastewater. Effects of different electrolytes such as Nat, Mg", Ca", SO=,, and C1- on 703

2 the Henry s Law constant (He) were evaluated. This theoretical study was based on information given in the published literature and results obtained by data analyses using the geochemical computer code EQ3/6. The second part of Task 1 involved experiments to study the effect of varying SO, concentration, molal strength, and the ph adjustment method [NaOH or Ca(OH),J on the gas transfer rates for ammonia. Composition of the synthetic wastewaters evaluated in this study is given in Table 1. Wastewater Stream A contained a high SO=, level than B, whereas both A and C had the same SO=, level. The molal strength of Waste Streams B and C were the same and were brought to such conditions by adjusting C1- concentrations. The experiments were conducted in identical completely mixed batch reactors at temperatures near 90 C. The change in ammonia concentration with time was studied in the solutions where ph was adjusted to 11.5 using NaOH or Ca(OH),. During Task 2, a small pilot-scale steam stripping unit was designed and constructed (Figure 1). Wastewater was released at the top of the packing materials in the column. Steam was released from the bottom of the column to establish the counter-current stripping effect. Experiments were conducted using two different synthetic waste streams (Solutions B and C of Table 1). The ph of each waste stream was adjusted to 11.5 or above using caustic or lime. Ammonia removal was studied under different ph adjustment methods and different ratios of steam-to-wastewater flow. Steam and wastewater flow rates were varied to determine the differences in ammonia removal efficiency. RESULTS AND DISCUSSION TASK 1: Theoretical and Laboratom Studies on the Equilibrium and Mass Transfer of Ammonia in Wastewater The analyses of estimated Henry s Law constant and changes in solubilities of ammonia indicated that adding caustic, compared with slaked lime, could result in higher Henry s Law constants and lower solubilities for the three waste streams considered. These effects may be attributed to the relatively high ionic strength found in wastewaters when ph was 704

3 adjusted with NaOH. Between Solutions B and C, where the sulfate concentration was varied without changing the solution strength (total number of moles), no significant difference in Henry's Law constant or ammonia solubility could be seen for either of the two ph adjustment methods. Highest Henry's Law constant and corresponding lowest ammonia solubility were observed in Solution A which has the highest molal strength. Experimental studies to determine the effects of dissolved species and ph adjustment methods on the mass transfer rate coefficient (K) indicated that those effects were relatively low and the maximum changes in mass transfer rate coefficient did not exceed 25 percent. In two of the three different solutions tested, however, mass transfer rate coefficient was higher for the waste stream where ph was adjusted using caustics. For solutions with approximately the same SO', concentration but different molal strength, the mass transfer rate coefficients were comparable when NaOH was used a s the ph adjustment method. For Ca(OH), however, the K values were comparable for those where both the molal strength and SO=, level were different. The overall analysis of data indicate that, when compared with lime, addition of NaOH can promote ammonia removal for solutions with relatively low SO', levels (5,OOOmg/L). TASK 2 Ammonia Removal Studies Using Steam StriDpine Unit The results of the steam stripping study are summarized in Table 2. Data show that when the steam-to-wastewater flowrates are low (1.3 lb/gal), the ammonia removal efficiency was as low as 93 percent. By increasing steam-to-wastewater flow rates to 3.8 Ih/gal, removals of more than 99.9 percent were observed. the removal efficiency in one study was improved by 2 percentile points when the temperature of influent waste stream was raised by ahout 20 C. The addition of lime resulted in more than a 10 C increase in temperature because the hydration of lime is an exothermic process. For wastewater with low initial SO', level (5,000 mg/l), ammonia removal was 3 percent higher when ph was adjusted with caustic rather than lime. These observations agree with the trends predicted from the estimated Henry's Law constant and mass transfer rate coefficient. In the experiments conducted with wastewaters using higher initial SO', 705

4 levels (20,000 mg/l), ammonia removal was slightly higher when ph was adjusted with lime instead of caustic. These observations agree with the conclusions reached from the corresponding mass transfer rate studies, which did not agree with the theoretical estimates of solubilities based on Henry s Law constant. In summary, more than 99.9 percent removal of ammonia can be achieved by introducing high steam-to-wastewater flowrates such 3.8 Ib/gallon. Varying chemical constituents such as SO=, and the molal strength has only a little effect on net NH, removal. Higher removal efficiencies can be achieved by preheating wastewaters and operating the stripping tower at high temperatures by increasing steam-to-wastewater flowrates. ACKNOWLEDGEMENT This study was performed by Battelle Memorial Institute, Columbus, Ohio under Contract No for US. Environmental Protection Agency, Cincinnati, Ohio. This abstract has not been subjected to the review of the US EPA, and therefore does not necessarily reflect the view of the Agency. 706

5 TABLE 1. WASTEWATER CHARACTERISTICS FOR GAS TRANSFER RATE EXPERIMENTS 707

6 I I TABLE 2. SUMMARY RESULTS OF LABORATORY STUDY ON STFAM SIRWF"NG Wastewater Temp, 'C NHrN eone Wastewater Adjustment Method I I,' Cao C (high SO=,) NaOH Column Intlueut Colunn Effluent Steam to Wastewater Influent 5,200 5,200 5,100 5,m 4,750 4,700 3,950 3,825 Average NH,

7 I I Synthetic 100 gal. wastevrater Tank Pump z T r Thermocouple P = Pressure Gauge V = Valve Figure 1. Experimental setup for amnia removal by ste?m stripping

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