Simultaneous nitrate and perchlorate reduction in an elemental sulfur-based denitrifying membrane bioreactor

https://doi.org/10.1016/j.ibiod.2019.104741Get rights and content

Highlights

  • Simultaneous nitrate and perchlorate reduction was achieved in 4 L membrane bioreactor.

  • Maximum denitrification rate was 400 mg NO3-N/(L.d) at HRT 6 h.

  • Complete reduction of perchlorate was achieved when the influent concentration was 3000 μg/L at HRT 12 h.

  • Membrane fouling was not significant at 12.5 L/(m2.h) membrane flux.

Abstract

Perchlorate is a man-made and naturally occurring anion that causes hypothyroidism and is often present in groundwater together with nitrate. In this study, a novel elemental sulfur-based denitrifying membrane bioreactor was tested for simultaneous nitrate and perchlorate reduction. A bench scale 4 L bioreactor equipped with polyethersulfone (PES) ultrafiltration membranes (0.04 μm of pore size) was used. Influent nitrate was varied between 25 and 100 mg NO3-N/L. Complete denitrification of 100 mg NO3-N/L was achieved at HRT as low as 6 h corresponding to 400 mg NO3-N/(L.d) denitrification rate. Complete reduction of perchlorate was also achieved when the influent concentration was 3000 μg/L and HRT of 12 h. Further increase in perchlorate load resulted in elevated effluent perchlorate concentration and maximum perchlorate reduction rate was 15.1 mg/(L.d). Sulfate was produced as a result of sulfur based denitrification. Influent was supplemented with NaHCO3 (1000 mg CaCO3/L) to meet the alkalinity requirement of the process. Membrane fouling was not significant at 12.5 L/(m2.h) membrane flux.

Introduction

Nitrate and perchlorate are the anions present in water, groundwater and several other matrices all over the world (Srinivasan and Sorial, 2009). Discharge of inappropriately treated domestic wastewaters and excess fertilizer utilization can be the main source of nitrate in the groundwater. Perchlorate, on the other hand, is used in rocket and missile propellants, highway safety flares, air-bag inflators, fireworks and matches. It can also be formed naturally by the reactions between chlorine gas and ozone in the atmosphere (Ucar et al., 2017).

Both contaminants can be found in water sources with varying concentrations. A recent study illustrated 1480 μg/L perchlorate in surface water at Chile (Cao et al., 2019). Although perchlorate does not exist in any regulations belonging to World Health Organization or European Union regulations, US-EPA declared a temporary reference dose as 24.5 μg/L. However, some states in the USA have already determined their own reference perchlorate dose which varies between 1 and 18 μg/L depending on state and regulation type (Srinivasan and Sorial, 2009).

Nitrate, on the other hand, is the most encountered anion in groundwater. In a study conducted in Harran Plain/Turkey, up to 180 mg NO3–N/L was measured and average concentration in all plain was 35 mg NO3–N/L (Yesilnacar et al., 2008). According to World Health Organizations guidelines, nitrate limit is 50 mg/L. For other authorities and countries, nitrate limit in the drinking water varies between 10 and 50 mg/L (Della Rocca et al., 2007).

The presence of nitrate may cause blue baby syndrome and perchlorate causes a competitive inhibition in the thyroid gland. This inhibition results in decreased levels of T3 (Thyroxine) – T4 (Triiodothyronine) hormones causing decreased metabolism rate (Ucar et al., 2017).

Biological reduction of these anions is one of the most efficient and cost effective methods due to several reasons. For example, there is no need for expensive catalysts (in electrochemical reduction) and there is no secondary pollution (membrane filtration or ion exchange) to be dealt with (Srinivasan and Sorial, 2009). In the biological reduction, microorganisms utilize nitrate and perchlorate as electron acceptors in the presence of an electron donor (Liebensteiner et al., 2014). Organic electron donors such as methanol, ethanol, acetic acid, sucrose, acetate, as well as some inorganic electron donors such as hydrogen, reduced sulfur compounds, and iron are reported electron donors (Matějů et al., 1992; Kapoor and Viraraghavan, 1997; Bardiya and Bae, 2011).

Nitrate and perchlorate removals were investigated extensively in anaerobic column reactors filled with granular sulfur (Ucar et al., 2015, 2016b). Elemental sulfur, due to its low solubility, provides necessary electrons in situ on demand, thus eliminating continuous addition and issues regarding dose adjustment. Approximate stoichiometries of elemental sulfur based nitrate and perchlorate reduction is presented in (1), (2) below.1.1S0 + NO3 + 0.76H2O + 0.4CO2 + 0.08NH4+ → 0.08C5H7O2N + 1.1SO42− + 0.5N2 + 1.28H+0.1ClO4 + 0.036CO2 + 0.07NO3 + 0.167S0 + 0.188H2O → 0.1Cl- + 0.07C5H7O2N + 0.167SO42− + 0.326H+

For nitrate and perchlorate, up to 300 mg NO3-N/(L.d) and 11.4 mg ClO4/(L.d) reduction rates were reported in granular sulfur based up-flow autotrophic reactor, respectively (Ucar et al., 2016b). Elemental sulfur can also be used together with some organic electron donors such as methanol. In such applications higher nitrate and perchlorate reduction rates can be obtained. In a study, methanol and sulfur based mixotrophic reactor up to 400 mg NO3-N/(L.d) reduction corresponding to 15.5 mg/(L.d) (Ucar et al., 2015) was reported. High nitrate and perchlorate removals are possible in bioreactors connected in series, i.e. a heterotrophic process followed with an autotrophic process as polishing step (Ucar et al., 2016a, 2017).

Membrane bioreactors (MBRs), on the other hand, have gained popularity in drinking water treatment (Maeng et al., 2015). High quality effluent and less area requirement are among the advantages of membrane bioprocesses. It is well known that the denitrification rate is a function of elemental sulfur particle size. However, the use of powder material in column reactors is problematic due to clogging problem. MBR allows the use of powdered elemental sulfur using micro- or ultra-filtration membranes to keep all the particles, including biomass, in the bioreactor. Sahinkaya et al. (2015) studied a MBR equipped with hydrophilic flat sheet polyethersulfone (PES) membranes (0.45 μm). They used powdered sulfur and reported almost complete denitrification at 0.24 g NO3-N/(L.d) loading rate (Sahinkaya et al., 2015). The same reactor was also operated under mixotrophic conditions in which methanol and sulfur were simultaneously used as electron sources for simultaneous nitrate and chromium reduction (Sahinkaya et al., 2016).

The use of powder sulfur in MBRs can provide more efficient denitrification performance due to higher surface area. To the best of author's knowledge, simultaneous nitrate and perchlorate reduction has never been reported in a powdered elemental sulfur based MBR. Therefore, the aim of this study is to reduce nitrate and perchlorate simultaneously in a completely stirred MBR. An ultrafiltration membrane (0.04 μm flat sheet polyethersulfone membranes) was used and denitrification performance was evaluated together with perchlorate reduction, sulfate generation, and alkalinity changes according to their mass balances based on (1), (2).

Section snippets

Membrane bioreactor

Continuously fed anoxic MBR made of plexiglas with the dimensions of 15 cm × 10 cm x 30 cm was used in the study. The total and active volumes were 4.5 and 3 L, respectively. To ensure complete mixing and perform cross flow velocity, membrane was placed on a magnetic stirrer and continuously stirred at 150 rpm. A double sided membrane module (10 cm × 10 cm and total area is 0.02 m2) was placed in the reactor and 0.04 μm pore sized flat sheet polyethersulfone (PES) ultrafiltration membrane was

Nitrate reduction

Nitrate was added to the feed at a concentration of 25 mg NO3-N/L and its concentration was gradually increased to 100 mg NO3-N/L. High nitrate removal performance was observed throughout the study (Fig. 1). Effluent NO3-N/L concentration was generally below 1 mg NO3-N/L up to HRT of 3 h. There was a temporary decrease in performance due to operational problems occurring in days 15–37. Similarly, a significant decrease in performance was also observed in days 232–252. These performance

Discussion

In the autotrophic process, there is less risk of organic contamination of effluent. Inorganic electron sources are usually inexpensive and may even present as waste products from some industries (Hanein et al., 2018). However, sulfur-based denitrification requires alkalinity. Alkalinity can be supplied to the reactor by using limestone as filling material or as dissolved form in the influent. In an autotrophic reactor filled with limestone and elemental sulfur nitrate and perchlorate removal

Conclusions

Elemental sulfur based nitrate and perchlorate reduction of drinking water was investigated in a bench scale membrane bioreactor. Up to 400 mg NO3-N/(L.d) and 15.1 mg ClO4/(L.d) perchlorate reductions were achieved. Sulfate was generated as a result of elemental sulfur based denitrification and average effluent sulfate was 187 ± 68 mg/L and 911 ± 225 mg/L when influent nitrate was 25 and 100 mg NO3-N/L. Alkalinity decreased from approximately 1,120 mg CaCO3 to 660 ± 77 mg CaCO3/L as a result

Acknowledgment

This research was supported by The Scientific and Technological Research Council of Turkey (TUBITAK/Project no: 117Y014).

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