HEAVY METAL PRECIPITATION IN UASB

By: Wadodkar Ketan Kishor

Enrollment No. 10519013

Under Guidance of: Dr. A. A. Kazmi (Associate Professor)

UASB Technology: Worldwide 200 UASB are operational and around 23 in India, by Tare et al. (2003)

Widely used in India and around the world because of its following benefits;• Low energy requirement and Low cost technique.• less skilled labor required for operation and supervision.• Biogas generation, used for production of energy.• High treatment efficiency and a short HRT.• Heavy metals can be precipitated in UASB.• Heavy metals are not biodegradable and can cause treatment process

failure.• Sludge containing precipitated heavy metal can be disposed in Class

B landfill.

INTRODUCTION

NEED OF STUDY

Heavy metals are toxic also affects functioning of UASB

Sulphate reduction can facilitate heavy metal precipitation, so added advantage can be achieved

ORP, COD/SO42- and microbes (SRB) govern the reduction of sulphate.

So optimising and improving efficiency of metal retention is important

OBJECTIVE OF STUDY

Study of metal sulphide precipitation

Heavy metal concentration in digested sludgeDeveloping and understanding efficient technique for efficient

removal of heavy metals.To obtain mass balance of heavy metal precipitation

LITERATURE REVIEW

LITERATURE REVIEW

Metals are precipitated as sulphides, hydroxides and carbonates, of which sulphide precipitation is preferred because:

• Instantaneous metal sulphide complex are formed,• Low sludge production,• Metal sulphides are compact and have faster settling velocities.

Precipitation of metal is solubility product dependant in order of lower Ksp to higher.

SRB’s and MPB’s are important in decomposition of organic substrate.

Sulphates are reduced to sulphides and heavy metals are precipitated as metal sulphides.

Precipitated in dense black sludge form.

LOG CONCENTRATION DIAGRAM OF H2S, SAWYER ET AL. (2003)

SOLUBILITY OF METAL-SULPHIDE AS A FUNCTION OF PH (SOURCE: HTTP://CPE.NJIT.EDU)

FIGURE: PRECIPITATION OF HEAVY METAL w.r.t. pH (METCALF ET AL. 2003)

Competition between SRB’s and MPB’s investigated by Isa et al. (1986) and following reaction occurs:

COD/SO42- ratio = 0.67 (stochiometrically) for equal SRB and

MPB activity. At lower COD/SO4

2- ratio, SRB predominates over MPB. Hence ratio of 1.7-2.7 was suggested by Choi and Rim (1991) so that methane production and sulphate reduction are not affected.

Presence of other metal decreases both sorption and affinity of respective metal.

SRB’s acts as nucleation sites and by bio sorption through the cell wall causes removal of heavy metals.

Metals are precipitated in sludge.

The TS of sludge contains ASS and AIS.

The mobility, bio availability and toxicology effects of heavy metals are depends on its speciation.

• Metal:• Soluble metal in influent = residual metals in effluent + precipitated metal

sulphide in biomass (sludge)• Sulphide:• Soluble sulphide in influent = residual soluble sulphide in effluent +

precipitated metal sulphide in biomass + unaccountable sulphide oxidised by oxygen

Mass balance:

CASE STUDY

CASE STUDY 1: LEAD REMOVAL THROUGH BIOLOGICAL SULPHATE REDUCTION PROCESS, HOA ET AL. (2007)

The feasibility of lead removal through biological sulphate reduction process with ethanol as electron donor was investigated.

• Start-up and operation of sulphidogenic process• Lead sulphide precipitation

Experiment divided in 2 steps:

• 99%of available electron at COD/SO42- = 60

• 69% at COD/SO42- = 1.5

• 13% at COD/SO42- = 0.75

MPB consumes

Figure 3.2, 3.3, 3.4, 3.5, 3.6, (Hoa et al. 2007)

CASE STUDY 2: HEAVY METAL REMOVAL FROM SYNTHETIC WASTEWATERS IN AN ANAEROBIC BIOREACTOR USING STILLAGE FROM ETHANOL DISTILLERIES AS A CARBON SOURCE, GONCALVES ET AL. (2007)

Three main objective of this paper:

• to increase the number of SRB in sludge through the bioactivation procedure• to assess the removal efficiency of the main heavy metals• to look for additional mechanisms involved in the removal of metals not precipitated

as sulphide.

The experiment was carried out in 4 runs. Continuous bench scale reactor of 13l for 85 days was used.

Table 1 description of 4 runs, (Goncalves et al. 2007)

ORGANIC REMOVAL-SULPHATE REDUCTION, (GONCALVES ET AL. 2007)

Organic concentration in influent was 300 mg/L during bio activation, and 200 mg/L when metals were added to bioreactor.

Amount of sulphate reduced was lower in last run because of low organic matter. In this period 133±32 mg/L SO4

2- /L.d is reduced which <158mg/L SO42-

/L.d (stochiometric estimation).

Hence about 25 mg/L SO42- /L.d of sulphide was not reduced and H2S in reactor

was about 11±2 mg/L H2S /L.d. This shows total part of the sulphide necessary for removing the metallic load added is not being used.

Amount of sulphide in effluent= 30 mg/L SO42- /L.d, remaining is spent for

metal removal.

Hence, sulphide precipitation is not only form observed for removal of heavy metals.

The unreacted sulphide could be lost due to volatilisation during sampling for analyses, by (Nagpal et al.2000).

SRB reduces sulphate and produce sulphide along with H2S and bicarbonate alkalinity.

Alkalinity reduction was there during I to IV run, resulting in reduction in pH of the effluent.

These bicarbonates ions are used for metal removal.

SRB amount was reduced in sludge bed end of metal removal due to entrapment of cell in precipitates.

SRB play an additional role in metal precipitation, since they are able to act as nucleation sites, regarding the mechanism proposed by Beveridge et al. (1976), Jalali and Baldwin (2000).

FIGURE 3.8 METAL REMOVAL, (GONCALVES ET AL. 2007)

CASE STUDY 3: EFFECT OF SULPHUR SOURCE ON THE PERFORMANCE AND METAL RETENTION OF METHANOL-FED UASB REACTORS, ZANDVOORT ET AL. (2005)

The effect of sulphur source on the performance and metal retention of methanol-fed UASB was investigated.

Different sources of sulphur were supplied, eg. Sulphate or L-cysteine.

Methanosarcina barkeri perform assimilatory action. L-cysteine is a ligand that will complex with metals

present in the wastewater.

Sulphate can be reduced by

Dissimilatory by SRB

Assimilatory by methanogenic bacteria

2 UASB reactors were operated at HRT = 8h

Initially R1 was supplied with only substrate and R2 was supplied with substrate along with L-cysteine• During period I, in R1: no major reduction in COD was observed. Whereas in

R2 COD removal was about 80-90% reduction was possible.• Due to absence of sulphate source in R1 during period I.• When sulphate source was added to influent of R1, similar results were

observed in R1 and R2.• On day 70-78 in R2, due flocculent sludge the methanol conversion was

affected.• M. bakeri uses FeS and ZnS as sulphur source for growth on methanol. Sulphur

availability for M. bakeri depends upon Ksp of the metal sulphides.• Leaching of cobalt was not affected by sulphur source.• Molybdenum was added to influent as an oxyanion, which is 1st reduced by D.

desulphuricans and in presence of sulphide it is removed by extracellular precipitation.

FIGURE 3.9 (ZANDVOORT ET AL.2005)

Figure 3.9: Evolution of the reactor performance of R1 and R2 with time. (A) R1: influent methanol (♦), effluent methanol (■), and effluent VFA (▲) concentration. (B) R2: influent methanol (♦), effluent methanol (■), and effluent VFA (▲) concentration. (C) Methane production of R1 (―) and R2 (―) start of period II, L-cysteine addition to R1 (1st dotted line) and start of period III no sulphur source and trace element solution in both reactors (2nd dotted line). (D) Maximum specific methanogenic activity (pH 7; 30 °C) of UASB sludge with methanol as the substrate as a function of time. Activity of R1 (♦) and R2 (■) after 30 days and after 77 days of operation.

CONCLUSION AND GAPS IDENTIFIED:

It is an added feature of UASB along with COD removal.

80-90% removal of heavy metals can be possible.

SRB’s play vital role in sulphide reduction

Order of precipitation depends upon Ksp.

Minor amount of heavy metals are removed through carbonates.

Metals precipitated in sludge can be seperated by treatment with acids. As sludge containing heavy metals can not be used for soil conditioning.

No paper has suggested that sulphate addition should be preffered for improving performance of UASB.

Further studies:

• Metal mass balance is required.• Studies @ 111mld STP in Bhattian, Ludhiana has started and we have the heavy metals digested

sample, waiting for permission from IIC, IIT Roorkee, so we could do the analysis at various sampling point for heavy metals.

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Appl. Chem. 69(11), 2425-2429. Goncalves, M. M. M., da Costa, A. C. A., Leite, S.G.F., Sant’Anna Jr, G.L., (2007). "Heavy metal removal

from synthetic wastewaters in an anaerobic bioreactor using stillage from ethanol distilleries as a carbon source." Chemosphere 69, 1815-1820.

Hoa, T. T. H., Liamleam, W., Annachhatre, A. P., (2007). "Lead removal through biological sulfate reduction process." Bioresource Technology 98, 2538-2548.

Isa, Z., Grusenmeyer, S., Verstraete, W., (1986). "Sulfate Reduction Relative to Methane Production in High-Rate Anaerobic Digestion: Microbiological Aspects." Applied And Environmental Microbiology 51(3), 580-587.

Jong, T., Parry, D. L., (2003). "Removal of sulfate and heavy metals by sulfate reducing bacteria in short-term bench scale upflow anaerobic packed bed reactor runs." water research 37, 3379-3389.

Metcalf & Eddy inc., (2003). In: “ Wastewater engineering treatment and reuse”, Tata McGraw Hill Publishing Company Limited, New Delhi, p.515

Sawyer, C. N., McCarty, P. L., Parkin, G. F., (2003). chemistry for environmental engineering and science. New Delhi, Tata-Mcgraw Hill.

Velasco, A., Ramirez, M., Volke-Sepulveda, T., Gonzalez-Sanchez, A., Revah, S. (2008). "Evaluation of feed COD/sulfate ratio as a control criterion for the biological hydrogen sulfide production and lead precipitation." Journal of Hazardous Materials 151(2008), 407-413.

Yoshizaki, S., Tomida, T., (2000). "Principle and Process of Heavy Metal Removal from Sewage Sludge." Environ. Sci. Technol 34(8), 1572-1575.

Zandvoort, M.H., van Hullebusch, E. D., Gieteling, J., Lettinga, G., and Lens, P. N. L., (2005). "Effect of Sulfur Source on the Performance and Metal Retention of Methanol-Fed UASB Reactors." Biotechnol. Prog 21(3), 839-850.

THE END

Be clever with Heavy Metals

Thank you