Anion control through biological sulfate reduction

A unique method for controlling sulfate-based scalants is through the use of microorganisms to reduce sulfate (SO4") to sulfide (S2"). This reduction process is known as BSR. The resulting S2" can then be reoxidized to elemental sulfur (Ss) via another unit process, resulting in a solid end-product. The reduction of SO;j_ via BSR would effectively lower the potential for the precipitation of BaSO4, CaSO4, and SrSO4, thereby reducing the scaling potential of the primary RO concentrate for further treatment by another unit process. BSR has been used in the industrial setting for a number of years [38-43]. The general reactions for sulfate reduction are as follows [44]:

SO4" + Organic carbon ) S2" + CO2 + H2O (4)

SO2" + 6H2 + CO2 ) S2" + Biomass + 6H2O (5)

From Eqs. (4) and (5), the basic process variables of the BSR process are (1) finding a suitable electron donor and carbon source, whether it be organic carbon or hydrogen (H2) and carbon dioxide (CO2); (2) selecting the appropriate biological community to reduce SO4"; and (3) devising a means by which S2" is rendered harmless, both in terms of inhibiting the biological reduction of sulfate, as well as to the environment in general.

4.2.1 Electron donor and carbon source

Low-molecular-weight organic compounds, such as acetate, propionate, ethanol, glucose, glycerol, malate, lactate, and sucrose, as well as H2 and CO2 are known electron donors and carbon sources for SO24" reduction [37,41]. Molasses has also been shown to be effective carbon source in SO4" reduction processes [45]. The use of appropriate carbon source is an important consideration owing to several reasons: cost, speed of assimilation, and by-product formation. For instance, lactate is assimilated more rapidly by SO4" -reducing bacteria than acetate or ethanol, and might therefore be used initially as a carbon source to reduce the induction times for reactor operation [37,41]. However, lactate could be more expensive than ethanol or acetate, and SO4" -reducing bacteria utilizing lactate would yield acetate as major by-product and propionate as a minor by-product in the treated effluent [46,47]. Ethanol would also yield acetate as a reaction by-product. These organic products would be poorly rejected to the RO membranes and may potentially cause organic and biological fouling. Hence, the use of lactate or ethanol may entail additional costs for acetate removal in the finished water. However, the use of H2 and CO2 for supporting the SO4— -reducing bacteria might be initially expensive, but could prove advantageous in view of the fact that no organic by-products or residual will be produced requiring additional aerobic treatment [37].

4.2.2 Sulfate-reducing bacteria

The SO;j—-reducing bacteria represent a specialized group of microorganisms that use SO4— as terminal electron acceptor for their respiration, although many species of microorganisms are capable of generating hydrogen sulfide (H2S) metabolically, SO;j— is the primary source. In a review on SO4—-reducing bacteria, Madigan [48] has listed 10 genera of dissimilatory organisms, divided into two broad physiological subgroups. The genera in the first group include Desulfovibrio, Desulfomonas, Desulfotomaculum, and Desulfobulbus; these bacteria utilize lactate, pyruvate, ethanol, and certain fatty acids as carbon sources, and in turn reduce SO24— to S2—. The genera in the second group include Desulfobacter, Desulfococcus, Desulfosarcina, and Desulfonema, and these bacteria specialize in the biochemical oxidation of fatty acids, especially acetate, during the concomitant reduction of SO;j— to S2—. All the 10 genera are obligate anerobes, which strictly require an anerobic environment for their growth and cultivation.

4.2.3 Hydrogen sulfide control

Several researchers reported that the effects of toxicity diminished process performance due to increased levels of S2— and H2S [49-51]. It must be noted that H2S acts as an inhibitor for the SO42— reduction process at elevated concentrations of 16 mM or 544 ppm H2S [50], but fluidization of the reactor bed may help in preventing the concentration from reaching inhibitory limits. These high free-H2S concentrations caused reversible inhibition rather than acute toxicity [37]. The elimination of H2S from BSR processes is an important consideration.

Several control methods for reducing effluent H2S concentrations include: off-gas scrubbing, precipitation of sulfide by magnesium addition, two-phase biological processes, and enhanced partitioning of H2S into the gas phase at thermophilic temperatures [52]. In addition to reducing aqueous phase H2S concentrations by gas stripping, low oxygen concentrations can create selective conditions for SO24— reduction to Ss, and less frequently to thiosulfate. Another control method would be the careful regulation of pH within the BSR process [53]. Because total sulfide exists as H2S, HS—, and S2—, the dissolution of H2S in water forms the following equilibrium system [45]:

The chemical equilibrium of these species is pH-dependent. At pH 8, most of the total sulfides are in HS— form, whereas at pH 6, most are in H2S form. Low pH (< 5-6) often inhibits SO;j— reduction activity and increases the solubility of metal sulfides already formed.

Example

BSR was evaluated at the bench-scale in terms of reducing SO2— concentrations of synthetic RO concentrate when using acetate, ethanol, and H2 and CO2 as the electron donor and carbon sources [10], Fig. 6 shows the influent and effluent SO4— concentrations and percentage of SO2— removal from fluidized bed reactors used during this study. Each of the BSR fluidized bed reactors underwent a conditioning perioc whereby the sulfate-reducing bacteria were exposed to ever increasing concentrations of sulfate (data not shown for acetate- and ethanol-fed reactors). In general terms, the SO4—-reducing bacteria acclimatized faster to the high SO4— feed when using H2 and CO2, followed by acetate and then ethanol. Maximum observed SO4— removal rates were 93%, 90%, and 96% for acetate, ethanol, and H2 and CO2, respectively. For the acetate- and ethanol-fed reactors, periods of suboptimal performance resulted from either the operational pH falling below pH 7.5 or maintaining less than 1.0 C/S ratios. Problems associated with insufficient H2 gas transfer and biological clogging of the media lead to an extended period of poor performance (60% SO42— removal) for the remainder of the H2/CO2 test. It is important to note that when using H2 and CO2, mass transfer and solubility limitations of H2 are important operational considerations. However, reactor performance was recoverable upon reverting to their respective operational set points. Lastly, acetate and ethanol consumption for each column ranged from 82% to 94% and 90% to 100%, respectively. These high levels of organic residuals would be poorly rejected by downstream RO processes and may lead to premature organic and biological fouling. However, the operational results using H2 and CO2 were encouraging from two standpoints: namely, achieving high process efficiency for SO4— reduction and maintaining low organic residuals in the treated effluent.

Table 1 shows saturation indices of BSR effluent for BaSO4 and CaSO4 calculated using a thermodynamic solubility model (LabAnalyzer 2.0 software, OLI Systems, Morris Plains, NJ) and water quality data from the previous section's CRW primary RO concentrate (Section 3.2). The data show that given the levels of SO42— removal achieved (greater than 90%) for any of the electron donor sources, the saturation ratios for BaSO4 and CaSO4 are either well within the ability of an antiscalant (less than 100) to retard BaSO4 precipitation or less than unity for CaSO4 scaling. Therefore, while only considering BaSO4 and CaSO4 scaling, BSR may be a viable technology not only to achieve high RO recoveries, but also to achieve a solid Ss end-product. -•

1200

20 30

Time (Days)

a

o

Oo<

a

a

ad □ ^

a a

a

a

a

□ -

1

i

P

nnn i

Figure 6 Influent and effluent sulfate concentration profiles and removal efficiencies for acetate (top), ethanol (middle), and H2 and CO2 (bottom). Periods for SO2"-reducing bacteria to acclimatize for acetate and ethanol reactors not shown.

Table 1 Theoretical reduction in BaSO4 and CaSO4 saturation ratios after biological sulfate reduction

Saturation indices

BaSO4 CaSO4

Primary RO concentrate

101

0.9

% Sulfate reduction

Secondary RO concentrate at 70% water recovery

70

157

1.4

80

36

0.31

90

3.7

0.03

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