Sulphate reduction in reactors

Jarno Mäkinen; VTT Technical Research Centre of Finland Ltd; P.O. Box 1000, FI-02044 VTT, Finland; jarno.makinen(at)vtt.fi

Introduction

Sulphate reduction in special bioreactors is relatively rapid, effective and well controllable method for reducing dissolved sulphate and heavy metals from waste waters. In contrast to other biologic sulphate reduction methods (e.g. permeable reactive barrier and mine shaft treatment) it can produce saleable products, like metal sulphides and elemental sulphur, but requires also more expensive apparatus, electricity and skilled personnel for monitoring.

Description of the technology

The method utilizes sulphate reducing bacteria (SRB), which require organic substrate and anoxic conditions to reduce sulphate to hydrogen sulphide, which precipitates, together with raised alkalinity and pH, various heavy metals from water as poorly soluble sulphides (Kaksonen et al. 2003, Papirio et al. 2013):

3SO₄²⁻ + 2CH₃CHOHCOOH → 3H₂S + 6HCO₃⁻

H₂S + M²⁺ → MS(s) + 2H⁺

Also hydrogen can be utilized by autotrofic SRBs (Papirio et al. 2013):

4H₂ + SO₄²⁻ + 2H⁺ → H₂S + 4H₂O

It is especially due to the nature of sulphide precipitation why the biologic method is seen as a better solution than e.g. lime treatment:

• the precipitation is efficient and produced precipitates are less soluble than oxides, reaching very low levels of SO₄, H₂S and metals in the effluent
• the precipitation is highly selective, capable to produce even “artificial metal concentrates”, e.g. ZnS, and
• produced precipitates have better thickening and dewatering characteristics (Janssen et al. 2001, Lewis 2010, Papirio et al. 2013).

On the other hand, produced metal sulphides cannot be stored or disposed of without caution, due to re-oxidation of sulphides, similarly as naturally occurring metal sulphides (Salomons 1995). Moreover, very low ambient temperature usually hampers greatly the SO₄ reduction efficiency (Tsukamoto et al. 2004, Vestola & Mroueh 2008).

Biologic sulphate reduction in the reactor is a commercial method, provided e.g. by Paques B.V. (Janssen et al. 2001).

Appropriate applications

Compared to passive SO₄ reduction methods (e.g. reactive barriers, mine pit treatment, wetlands) bioreactors need special equipment, maintenance, electricity, monitoring and skilled personnel, increasing the costs and need of infrastructure. For these reasons the technology is most suitable for sites that have very little land area to use, or very strict environmental permits for waste waters (MEND 1996, Vestola & Mroueh 2008).

It is stated that SRB require the temperature above ~8°C to reach high SO₄ reduction rates, being the main obstacle for the passive biologic technologies in cold climates. However, bioreactors can be heated to the temperature favoured by mesophilic bacteria (25-35°C), or even thermophilic area (e.g. 65°C) and therefore widen greatly the possibilities of biologic sulphate reduction (MEND 1996, Tsukamoto et al. 2004, Papirio et al. 2013). Heating costs can be reduced if some excess heat is produced in the vicinities of the site by industry. Another requirement for application is availability of suitable organic substrate for the SRBs. These can be agricultural biomasses, but if not available near enough, also ethanol, lactate and others can be transported to the site (Liameleam & Annachhatre 2007). Also H₂ can be used as a substrate (Janssen et al. 2010, Papirio et al. 2013).

The wastewater chemical characteristics play also an important role in biologic sulphate reduction. Main sulphate reducing bacteria require wastewater conditions of pH > 5.5 (García et al. 2001), but activity of some microbes is observed also in pH < 3.0 (Johnson 2003). Even though very acidic waters (pH 2.5-3) have been completely neutralized during biologic SO₄ reduction (Kaksonen et al. 2003, Kaksonen et al. 2004), proper laboratory testing of this kind of waters is essential before scale-up. For the removal of produced H₂S, the water should have equal amount or excess of heavy metals precipitating this highly toxic compound. If heavy metals are not present, sulphide can be oxidized to elemental sulphur (MEND 1996, Janssen et al. 2001, Liamleam and Annachhatre 2007).

The inhibition for SRB can be caused due to high concentration of dissolved sulphide, heavy metals, sulphate and organic acids (Vestola & Mroueh 2008, Papirio et al. 2013). The influent pH plays an important role as when moving to the acidic conditions, sulphides are present as H₂S_, which is more toxic than HS^– and S^2- (Figure 1). With pH > 7.5, sulphide tolerance can be over 1,000 mg/l, decreasing to less than 500 mg/l when moving to acidic conditions (Greben et al. 2004, Celis-García et al. 2007). According to the review of Papirio et al. (2013), SRB can tolerate heavy metal (Fe, Cu, Ni, Zn, Pb, Cd) concentrations from just few to hundreds of milligrams per litre. With active SO₄ reduction, metals are efficiently removed from the solution, increasing the possibilities of the technology for heavy metal laden waste waters – in addition, certain reactor configurations (e.g. fluidized-bed reactor, FBR) have dilution effect, protecting bacteria from high metal concentrations (Kaksonen et al. 2003, Vestola & Mroueh 2008, Papirio et al. 2013). Typical for all inhibition studies is the large variation in the results (Papirio et al. 2013), showing the importance of proper laboratory scale testing of the technology before scale-up.

Figure 1. Speciation of hydrogen sulphide as a function of pH at 35 C, 1 atm. H₂S(aq) (◊), HS^-1 (●) and S^2- (Δ) (Hol 2011).

Performance

The SO₄ removal rate of various bioreactors has been 1,690 – 6,860 g/m³d. The sulphate concentration of the influent in these experiments has varied from 1,200 – 8,150 g/l. More information is shown in Table 1. According to Janssen et al. (2001), many processes provided by Paques can reach < 300 ppm SO₄ levels for effluent.

Table 1. Biological sulphate reduction rates in various experiments (Kaksonen 2004, Liamleam & Annachhatre 2007).

FBR=Fluidized bed reactor, UASB=Upflow anaerobic sludge blanket reactor.

MEND (1996) presented costs of the bioreactor treatment, based on the 38 m³ fiberglass reactor, metering pump (75 – 100 litres/minute) and needed pipes. The equipment costs were 42,500 Cdn $; however, there is now mention about studies or demonstration of such a plant presented by MEND (1996). The operation costs consist of organic substrate, electricity, possible heating and agitation, monitoring and personnel costs.

Design requirements

Various reactor types have been introduced for biologic sulphate reduction, and the selection is usually related to organic substrate. For example, Paques provides GLR (gas-lifted bioreactors), where H₂ gas stream is simultaneous substrate for bacteria and agitator to the reactor. (Janssen et al. 2001) With organic substrates, traditional reactors are UASB (upflow granular sludge bed reactor) and FBR (fluidized-bed reactor), with the main difference whether to let microbes create free granules with slow wastewater flow through (UASB), or to utilize fluidized carrier materials for attachment of microbes (FBR). Also plenty of other variations, and possible carrier materials, are illustrated and reviewed by Papirio et al. (2013).

The selection and dosage of organic substrate (or H₂) is done firstly by stoichiometric evaluations and then proved/optimised in the laboratory-scale experiments. The easiest way is to utilise anaerobic sludge or animal manure to provide microbes, carbon, nitrogen and phosphorus all in the same feed. Theoretically, sulphate reducing bacteria require 0.67 mol of chemical oxygen demand (COD) or electron donors to remove 1 mol of sulphate, which serves as a basis for dosing calculations (Liamleam & Annachhatre 2007). However, in the nature utilisation ratio is never 100% and also rivalry is observed between sulphate reducers and other microbes. Therefore, the needed dosing is more or less higher than theoretical value. (Vestola & Mroueh 2008) Optimal performance of sulphate reducing bacteria is observed with C:N:P ratios of 100:7:1 (Cocos et al. 2002), while anaerobic sludges or manures contain clearly higher shares of nitrogen and/or phosphorus, compared to carbon. Therefore, these nutrients are not completely used by bacteria and they may be discharged into the surrounding environment. The C:N:P ratio of a feed can be improved by addition of e.g. ethanol into the bioreactor (Vestola & Mroueh 2008). The more detailed review of organic substrates is presented by Liamleam & Annachhatre (2007).

The hydraulic retention time (HRT) is an important parameter for SO₄ and heavy metals reduction with bioreactor treatment. With too high HRT treatment, capacity can exceed, resulting as poor reduction rates or even failure of the operation due to inhibition. With too low HRT, reactor sizes increase, which affects to CAPEX (equipment) and OPEX (agitation, pumping, heating). The HRT has been in various studies clearly less than 24 hours (Kaksonen 2004, Liamleam & Annachhatre 2007).

References

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