Simultaneous nitrate and perchlorate reduction from drinking water by elemental sulfur-based autotrophic and mixotrophic denitrification processes

Perchlorate (ClO4-) is the salt derived from perchloric acid. Perchlorate, can contaminate drinking water sources via natural and anthropogenic sources. Perchlorate is used in products such as airbags, fireworks, military weapons (eg. Missiles and rockets) and by the pharmaceutical industries (eg. For the treatment of hyperthyroidism). It naturally forms from the chemical reaction between chlorine gas and ozone and can be found in nitrate deposits in arid regions such as the Atacama Desert in Chile. With the utilization of nitrates as fertilizer, perchlorate might be more commonly found in the groundwater together with nitrate. Due to the high solubility and low adsorption properties of ammonium perchlorate, it can easily reach sources of drinking water. Perchlorate blocks the iodine uptake by the thyroid, decreasing the thyroid hormone (triiodothyronine-T3 and thyroxine-T4) concentrations in the body. These hormones regulate basal metabolisms in adults and normal development processes in children. Therefore the presence of perchlorate in drinking water might contribute to the onset of a variety of basal metabolic disorders. Current remediation methods are classified into two sub-groups: removal or destruction. Some removal methods include ion exchange, membrane filtration and adsorption. Destruction methods involve chemical, electrochemical or biological reduction. Besides the additional chemical requirements for these reduction processes, these removal methods are costly, and have low efficiencies when other contaminants are present in the solution. Most seriously, this removal method leads to the discharge of concentrated brine. Regardless, biological reduction is the most common method, due to its fast reaction rate, and the fact that it does not require expensive catalyst or chemicals. In many studies, ClO4- is sucessfully biologically reduced to CI- ion. However, these widely used conventional drinking water treatment processes are not effective in the treatment of perchlorate. In the last decade it has become possible to detect low concentrations of perchlorate, due to improvements in the analytical methods used. Since then, the presence of perchlorate has been detected in the drinking water of many countries, especially in the USA. However, perchlorate is not listed as hazardous by the standards of the World Health Organization (WHO), the European Union, TS266 (regulations on water intended on human consumption), or the US-EPA. While some US states have already established standards (Arizona, 14 µg/L; Massachusetts, 2 µg/L; Nevada, 18 µg/L), an Interim Health Advisory Level of 15 µg/L has been released by the EPA. Nitrate is one of the most commonly encountered pollutant in surface and groundwater. The most important sources of nitrate in groundwater are nitrogen containing fertilizers and the release of improperly treated wastewater from industrial and domestic sources. According to TS266, the maximum allowed concentrations of NO3--N and NO2--N are 11.3 and 0.15 mg/L, respectively. Additionally, many sources of groundwater may be contaminated with both nitrate and ClO4-, necessitating the development of processes able to simultaneously remove nitrate and perchlorate. Recently, many studies on autotrophic denitrification processes have been conducted. In sulfur-based autotrophic process (SLAD: sulfur-limestone autotrophic denitrification) sulfur and nitrate are used as electron donors and acceptors, respectively. As a result, sulfur is oxidized to sulfate, and nitrate is reduced to nitrogen gas. The reaction is given below (Equation 1), and CO2 is used as a carbon source. 55S0+50 NO3-+38H2O+20CO2+4NH4+ → 4C5H7O2N+55SO42-+25N2+64H+ (1) Although sulfur-based autotrophic denitrification has several advantages, its not widely used due to sulfate and acid formation. In heterotrophic denitrification, however, sulfate is not produced and 3.57 g CaCO3 is formed per gram of NO3--N reduced (Equation 2). NO3-+1.08CH3OH+0.24H2CO3 → 0.056C5H7NO2+0.47N2+1.68H2O+HCO3- (2) Even though there is no sulfate formed and the pH decreases in heterotrophic processes, these processes are highly sensitive to the quantity of (organic) electron donors. For example addition of more organic compounds may lead to effluent that still contains those compounds; whereas, nitrite formation may be observed in the case of lower amount of organic supplementation. Therefore, mixotorophic processes, that combine heterotrophic and autotrophic denitrification, can be used to control the amount of sulfate formation, the pH and the risk of residual organic compounds. Additionally, both nitrate and perchlorate reduction could be achieved at higher rates compared to autotrophic processes. Therefore the aims of this thesis are: (1) evaluation of sulfur-based autotrophic denitrification processes for the simultaneous reduction of nitrate and perchlorate from drinking water; (2) simultaneous reduction of both oxianions by methanol-based heterotrophic processes; (3) stimulation of the mixotrophic process by addition of organic matters to autotrophic process for simultaneous nitrate and perchlorate reduction; (4) evaluation of heterotrophic-autotrophic sequential process for nitrate and perchlorate reduction and (5) the determination of microbial community in the bioreactors under varying operational conditions. In addition to these aims, the effect of different alkalinity sources on autotrophic reactors was investigated. For this thesis project, two autotrophic, and one heterotrophic reactors were operated for more than 100 days. The mixotrophic reactor and the heterotrophic-autotrophic sequential system were operated for 174 and 100 days, respectively. Real groundwater polluted with nitrate was supplied to the sequential system for a 7 day period. To identify the nitrate and perchlorate removal mechanism, batch tests were conducted for 90 hours. The dominant microbial community was determined using molecular tools. The first autotrophic reactor was filled with elemental sulfur and limestone particles. The feed contained 25 mg NO3--N /L and various concentrations of perchlorate (50-1000 µg/L). Complete nitrate reduction was achieved in all tested conditions. Although high perchlorate removals were attained, perchlorate was always detected in the effluent; the concentrations varied from 21.88–85 µg/L depending on the perchlorate and nitrate loadings. Perchlorate was reduced from 1000 µg/L to 53±21.36 µg/L, corresponding to around 95% reduction with an HRT of 2 h. Simultaneous reduction of both anions produced sulfate in the effluent with 227±30 mg SO42-/L. The average influent alkalinity concentration was decreased from 155±23 to 96±30 mg CaCO3/L but not below this threshold because of the limestone. In the second autotrophic reactor, NaHCO3 was added to compare the effect of alkalinity sources. This reactor also highly reduced perchlorate and decreased perchlorate from 1000 µg/L to 33.23±30.4 µg/L. The influent nitrate concentration of 25 mg NO3--N/L was completely reduced. The effluent sulfate concentration of the autotrophic reactor averaged 259±87.70 mg/L throughout the study. For autotrophic reactors the maximum reduction rate of nitrate was 300 mg NO3--N/(L.d). The heterotrophic process reduced nitrate and perchlorate completely in all tested conditions (i.e., 25 mg NO3--N/L and 50-1000 µg ClO4-/L). However, 2 – 4 mg/L DOC was detected in the effluent. The complete reduction of 25 mg NO3--N/L and 1000 µg/L perchlorate was also accomplished in mixotrophic reactor and the effluent sulfate concentration was controlled by adjusting the C/N ratio in the influent. Mixotrophic denitrification was stimulated by the addition of 25 mg/L methanol. Fifty-three percent, of influent nitrate was reduced by heterotrophic process, which decreased the effluent sulfate concentration to half of the autotrophic counterpart. Effluent DOC concentrations were below 2 mg/L. Lastly a heterotrophic-autotrophic sequential system was investigated for the reduction of both anions. Heterotrophic effluent was pumped to autotrophic reactor. In this configuration, nitrate and perchlorate were reduced completely with maximum initial concentrations of 100 mg NO3--N/L and 1000 µg/L respectively. The C/N ratio varied between 1.24 and 2.77 throughout the study, effluent sulfate concentration was below the drinking water standard of 250 mg/L and the pH was neutral.