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Effect of sulphate concentration and sulphide desorption on the combined removal of organic matter and sulphate from wastewaters using expanded granular sludge bed (EGSB) reactors Freddy
Valdés Edmundo
Muñoz Rolando
Chamy Gonzalo
Ruiz Christian
Vergara David
Jeison* *Corresponding author Financial support: Research Project FONDECYT 1020201. Keywords: anaerobic digestion, EGSB, sulphate reduction, sulphide.
During the last 30 years, the anaerobic process has been successfully used at big scale for the treatment of wastewaters. Nowadays, it can be considered as an established technology and it is successfully used for the treatment of sewage and many kinds of industrial wastewaters. It offers the possibility of an efficient treatment with low operational costs. However, during anaerobic treatment of high sulphate concentration wastewaters, operational problems may arise as a result of the formation of hydrogen sulphide, produced due to sulphate reduction. Sulphate reduction is the result of the activity of sulphate-reducing bacteria, which form a group of generally anaerobic microorganisms that are able to use sulphate as final electron acceptor, for the oxidation of H2 and a wide variety of organic compounds. Several industrial processes that use sulphuric acid in high amounts, or sulphate rich substrates, generate wastewaters with high sulphate and organic matter content. This is the case of fermentation and marine food processing industries. Both pollutants can also be found in productive activities that use reduced sulphur compounds like tanneries and Kraft pulp bleaching. The impact of sulphate over anaerobic digestion of organic matter depends on the sulphate concentration, and specifically on the ratio between chemical oxygen demand (COD) and sulphate. Values of COD/SO4-2 over 10 should not represent a threat to process stability. Below this value, sulphate reduction becomes important and a large fraction of the organic matter present in the wastewater begins to be consumed through sulphate reduction. Sulphide generation in anaerobic wastewater treatment produces several difficulties: reduction in the methane production, odour problems, corrosion, increase of effluent chemical oxygen demand, among others. However, despite these difficulties, sulphate reduction can be used as a tool for sulphate removal. If adequate measures are undertaken to control sulphide concentration, anaerobic technology may be used as a tool to remove both sulphate and organic matter, becoming a biotechnological alternative for the combined removal of both pollutants. The present research is focused on the study of Expanded Granular Sludge Bed (EGSB) reactors for the combined removal of sulphate and organic matter from wastewaters. Digester high mixing level may be useful to enhance sulphide transfer to the gaseous phase, reducing inhibition risks, and improving treated water quality. Hydrogen sulphide can then be removed from the gas phase by a chemical process. A High sulphate removal levels were attained during all the experimental periods. Outlet sulphate concentrations were in the range 10-20 mg/L for most of the tested conditions. COD removal reduced as the inlet sulphate concentration increased. This was produced at a high extent by the increasing sulphide concentration on reactor effluent: sulphide increases the oxygen demand of the treated water, notoriously reducing its quality. Reduction of COD removal is not therefore generated by a decrease in organic matter removal. Under all studied conditions most of the generated sulphide remained in the liquid phase, reducing effluent quality. Changes in pH had a small effect in increasing sulphide desorption displacing acid-base equilibrium towards non-dissociated sulphide. Anyhow this effect was small, due to the narrow window of pH values that can be safely applied to the biological reactor. Therefore, under the conditions of this study (inlet sulphate concentration up to 0.9 g/L), the limitation of anaerobic EGSB reactors to treat high sulphate content wastewater may not related with inhibition considerations, but with low treated water quality due to sulphide content. An alternative to improve effluent quality by promoting sulphide desorption, is through a mass transfer equipment to enhance sulphur desorption. During the second phase of the reactor operation, a desorption column was placed in the recirculation line. Nitrogen was used to simulate the biogas stream that has passed through a physical-chemical step to remove H2S. Such a configuration would enhance H2S desorption and promote lower total sulphide concentration in the liquid phase. 3 levels of sulphate concentration were tested during this period (900, 1600 and 2100 mg/L). Sulphate removal was again very high, close to 98%, except for 2100 mgSO4-2/L. The reduction in removal for the last sulfate concentration is likely to be due to the high applied sulfate load. Sulfide effluent concentrations were notoriously lower than those observed in the absence of the desorption column. Anyhow they were far from the minimum theoretical values, that can be evaluated by mass balances, aassuming equilibrium conditions. This is due to a low performance of the desorption column which is probably related with the use of coarse gas diffuser and its small size, which means a low contact time between the phases.
Results showed that high sulphate and organic matter removals can be attained, with no signal of inhibition at high concentrations of sulphate (2100 and 900 mgSO4-2/L, with and without the desorption unit, respectively). The pH showed an effect over sulphide distribution in gaseous and liquid phases, but due to the narrow range of pH operation that biological wastewater treatment imposes, most of the sulphide is anyway present in liquid effluent reducing treated water quality. The use of an adsorption column to reduce concentration of sulphide improves effluent quality, by reducing notoriously TS concentration in the liquid phase. Optimum design of the column is necessary to increase mass transfer coefficient, in order to achieve an efficient treatment process. If this condition is fulfilled, the proposed system, in conjunction with a sulphide removal step from the biogas, can be successfully used to combine high rate removal of organic matter and sulphate from wastewaters. |
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