Bioreactor Sulphate

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Picavet et al. EJEAFChe, 2 (2), 2003. [297-302]

Electron. J. Environ. Agric. Food Chem.ISSN 1579-4377

298

The sulphide produced can be readily used to precipitate heavy metals as their respective

sulphides.

22 2 2HS Me MeS H + ++ +

In this way metals such as zinc and cadmium can precipitated down to very low concentrations.

This principle was successfully put into practice in 1992 at zinc refinery Budelco BV, The Netherlands,

for the treatment of polluted groundwater (Scheeren et al. (1993)). Here sulphate is reduced using Upflow

Anaerobic Sludge Blanket (UASB) reactors. Simultaneously metals are precipitated as metal sulphides

and separated using tilted plate settlers (TPS). Figure 1 shows a picture of this water treatment plant.

Figure 1 - Biological groundwater treatment plant at Budelco BV, The Netherlands.

This plant has run without any difficulties at a mean flow of 350 m3/h. Typical analyses of in andeffluent are shown in Table 1.

Table 1 - Typical analyses of in and effluent

Compound Unit Influent Effluent

SO42-

mg/l 1000


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DESIGN CONSIDERATIONS

Mixing in the UASB reactors is achieved by applying a liquid upflow velocity. This way, mixing

is obtained under low shear conditions preserving the granular sludge and thus retaining active biomass in

the reactor. Mixing can be improved by increasing the upflow velocity. The most straightforward way is

decreasing the reactor surface area at a same volumetric flow rate leading to higher reactors. However,

this leads to an increased chance of biomass washout and activity loss. Therefore, improved biomassretention is needed. This can be achieved by equipping the reactor with a tilted plate settler (TPS).

The taller reactors are not new. These are the Expanded Granular Sludge Blanket (EGSB)

reactors. These are designed for methane production. Adding a TPS to have better biomass retention is

new, however, and distinguishes the BEST from the rest. This reactor concept has been developed by

Paques and recently put into practice in South Africa for sulphate removal from mine water. The first

pilot scale experience, however, stems from 1998 and was carried out at ViaSystems Mommers BV, the

Netherlands.

PILOT PLANT

From March until September 1998 the BEST was tested on a pilot scale level for the treatment of

wastewater containing sulphate and metals from ViaSystems Mommers, a factory producing integratedcircuits (Picavet (1998)). They treat their water using a physical/chemical treatment plant. Because of

more stringent demands by the government and their striving to reuse industrial water, they were looking

for new means of treating their water. Biological sulphate reduction and simultaneous metals precipitation

as metal sulphides was considered a likely option.

At this site 7 different wastewaters were available with a total flow of 80 m3 /hr. The meancomposition of all the wastewaters combined is shown in Table 2.

Table 2 - Typical analysis of the wastewater

Compound Unit Influent

SO42- mg/l 840

Cu2+ mg/l 90

Sn2+ mg/l 11Pb2+ mg/l 15

Ni2+ mg/l 2

Zn2+ mg/l 1.5

Figure 2 shows a schematical representation of the pilot plant. The influent flow was set at 1

m3 /hr. The influent is fed to a buffer tank. Here, the pH is controlled at 8 and the water is brought to

temperature. From the buffer tank water is pumped to the BEST where sulphate is reduced to sulphide

using ethanol as an electron donor. Metals present precipitate as their respective metal sulphides. Part of

the effluent of the BEST is recycled to the buffer tank to be able to reach a wide range of upflow

velocities without overloading the BEST. The rest of the effluent goes to an aerobic reactor where excess

sulphide is oxidised to elemental sulphur. The elemental sulphur produced is separated using a tilted plate

settler. Part of the sulphur slurry is recycled to the aerobic reactor to preserve biological activity; the rest

is discharged. The TPS overflow is polished using a dynamic sand filter.

The main advantages envisaged were:Lower residual metal concentrations since they precipitate as their respective sulphides.

Very low sulphate concentrations are possible (


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Buffer Tank

TPSAerobic

Reactor

BEST

Sand Filter

Influent

Air

Ethanol

Sulphur Sludge

Effluent

Nutrients

Buffer Tank

TPSAerobic

Reactor

BEST

Sand Filter

Influent

Air

Ethanol

Sulphur Sludge

Effluent

Nutrients

Figure 2 - Schematical block diagram of the Mommers pilot plant.

RESULTS AND DISCUSSION

Sulphate was removed down to concentrations of 400 mg/l, which was well below the effluent

demand. Metals removal was satisfactory as well. Typical effluent concentrations of less than 1 mg/l were

reached. Mean effluent concentrations are shown in Table 3.

Table 3 - Typical analysis of the process effluent

Compound Unit Influent

SO42- mg/l 400

Cu2+ mg/l < 0.8

Sn2+ mg/l < 1.0

Pb2+ mg/l < 0.8

Ni2+ mg/l < 0.2

Zn2+ mg/l < 0.6

Figure 3 shows a photograph of the BEST. The BEST performed according to the expectations.

Stable operation was no problem. Upflow velocities as high as 6 m/h could be applied without significant

washout of sludge. Thus, the biomass retention was more than satisfactory. Furthermore, this implies that

sludge was accumulated inside the BEST.

The sludge over the BEST reactor was characterised using four sample points at different heights

over the reactor. Table 4 shows the composition at each sample point for samples taken 1.5 months apart.

It shows solids accumulated in the BEST over time and mainly resided in the bottom part of the reactor.

Both the TSS and VSS concentration increased over time. However, the organic fraction of the sludge

decreased showing that most metals sulphides remained in the BEST and contributed to pellet formation.

This is confirmed by the fact that practically no suspended solids were washed out. The organic fraction

was the lowest at the bottom. Since no significant washout was noticed, eventual sludge discharge is best

done from the lower part of the reactor.


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Figure 3 - Pilot plant at Mommers with the BEST at the back.

Table 4 - Sludge characterisation of BEST sludge at different heights.

Sample at t = 0 Sample at t = 1.5 months

TSS (kg/m3)2)VSS

(kg/m3)3)TSS

(kg/m3)

VSS

(kg/m3)

Sample point 11) 67 46 106 47

Sample point 2 19 14 57 35

Sample point 3 0 0 0 0

Sample point 4 0.6 0.1 0 0

Mean over BEST 21 14 42 22

1) Lowest sample point

2) Total Suspended Solids

3) Volatile Suspended Solids

One of the essential features of the BEST is increased biological activity compared to UASB

reactors leading to smaller reactors. The pilot reactor showed that this is possible using the BEST. The

increased upflow velocity led to at least a doubling in the sulphate reducing activity compared to a

standard UASB. Next to the upflow, important parameters influencing the activity appeared to be the pH

and a high enough sulphide concentration. The preferred sulphide concentration lay between 200 and 500

mg/l. In this range, methanogenic activity is inhibited and sulphidogenic activity induced leading to

efficient electron donor usage.

Metal sulphides precipitation might interfere with biological activity due to encapsulation of the

biomass. Such problems were not encountered during this pilot experiment.

FULL SCALE PLANT

Since the first BEST pilot experiment more pilot projects were run on a smaller scale. One of

these projects has led to the actual implementation on a full-scale level. At the moment, the first full scale

BEST is started up in South Africa.

CONCLUSIONS

Paques has developed a novel reactor, the BEST, specific for biological sulphate reduction using

organic electron donors. Pilot experiments have shown that higher sulphidogenic activities are achievable

than with UASB reactors due to better mixing. Furthermore, methanogenic activity is limited due to


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operation at relatively high sulphide concentration. Thus a more efficient process is achieved, since the

electron donor is mainly used for sulphate reduction.

Metals precipitation in the BEST showed to be no problem.

REFERENCES

Boonstra, J., Dijkman, H. and Buisman, C.J.N., 2001. Novel Technology for the Selective

Recovery of Base Metals, Waste Processing and Recycling in Mineral and Metallurgical Industries IV,

Eds. S.R. Rao et al., MetSoc, pp. 317-323.

Copini, C.F.M. et al., 2000. Recovery of sulfides from sulfate containing bleed streams using

a biological process,Lead-Zinc 2000, Eds. J.E. Dutrizac et al, TMS, pp. 891-901.

Dijkman, H. et al., 2002. Optimization of metallurgical processes using high ratebiotechnology, Sulfide Smelting 2002, Eds. R.L. Stephens and H.Y. Sohn, TMS, pp. 113-123.

Peters, A.S., 1999. The Selective Removal of Copper and Arsenic from Electrolyte Bleed

Development and Design of a Sulfide Precipitation Process, (confidential), nr. PM88.02.005.

Picavet, M.A., 1998. Biologische zuivering van afvalwater afkomstig van de printplaten-

industrie, confidential internal report about pilot project (confidential).

Ruitenberg, R. et al.,2001. Copper electrolyte purification with biogenic sulfide,

Electrometallurgy 2001, Eds. J.A. Gonzales et al., MetSoc, 2001, 33-43.Scheeren, P.J.H., Koch, R.O. and Buisman, C.J.N., 1993. Geohydrological Containment

System and Microbial Water Treatment Plant for Metal-Contaminated Groundwater at Budelco, World

Zinc 93, Hobart, Tasmania, October 10-13, 373-384.

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