Mechanistic study of on-line bicarbonate (hydrogencarbonate) monitoring of anaerobic sewage sludge digesters

Philip J. C. Anstice, Heather Coleman, Donal J. Murphy, Paul A. Zuber, C. L. Paul Thomas, John F. Alder

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

A study is reported of a simple method for bicarbonate (hydrogencarbonate) alkalinity determination in an anaerobic sewage sludge digest based on the measurement of pressure increase in a constant-volume vessel on acidification of a sludge aliquot. The method provides the basis for a simpler and more robust instrument than the current constant-pressure designs, an important consideration for unattended operation in commerical installations. The factor that has led to the previous dismissal of constant-volume instruments has been the perceived problem of carbon dioxide solubility in water and the proportionality of the solubility with carbon dioxide partial pressure. Pre-saturation of the sample aliquot with carbon dioxide was thus inappropriate for such instruments. In fact the instability of aqueous sodium bicarbonate is shown to give rise to errors in bicarbonate alkalinity determination which are significantly larger than those resulting from failure to pre-saturate with carbon dioxide, provided that carbon dioxide partial pressures do not exceed typically 50 mbar over a 0.08 mol dm–3 aqueous sodium bicarbonate solution, typical of a digester sludge. Furthermore, carbon dioxide pre-saturation is shown to exacerbate significantly the effect of sludge sulfide content on the accuracy of bicarbonate alkalinity determinations. A theoretical basis is presented for this phenomenon and the conclusions are supported by the results of a factorial analysis experiment. A standard additions calibration of carbon dioxide pressure increase resulting from the neutralization of 10 cm–3 of a typical digested sludge, with additions of 6.756 g dm–3 aqueous sodium bicarbonate solution, was linear with a correlation coefficient of greater than 0.999 between sodium bicarbonate concentration and pressure increase. This calibration has been used in the monitoring of a digester by daily triplicate bicarbonate alkalinity analyses which have had an average relative standard deviation of approximately 2.7% over a 2 month period. Comment is made regarding a modification to this prototype instrument that will allow the simultaneous monitoring of sludge sulfide content by means of an electrochemical sensor, the sulfide content measurement to be applied as a correction to the bicarbonate alkalinity determination.
LanguageEnglish
Pages2873
JournalThe Analyst
Volume120
Issue number12
DOIs
Publication statusPublished - 29 Jun 1995

Fingerprint

Sewage sludge
Bicarbonates
Sewage
bicarbonate
Carbon Dioxide
Carbon dioxide
Sodium bicarbonate
Alkalinity
Sodium Bicarbonate
carbon dioxide
Monitoring
monitoring
alkalinity
Sulfides
Pressure
sludge
Partial Pressure
sodium
Partial pressure
Solubility

Keywords

  • hydrogen carbonate
  • sewage sludge
  • anaerobic digestion

Cite this

Anstice, Philip J. C. ; Coleman, Heather ; Murphy, Donal J. ; Zuber, Paul A. ; Thomas, C. L. Paul ; Alder, John F. / Mechanistic study of on-line bicarbonate (hydrogencarbonate) monitoring of anaerobic sewage sludge digesters. In: The Analyst. 1995 ; Vol. 120, No. 12. pp. 2873.
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abstract = "A study is reported of a simple method for bicarbonate (hydrogencarbonate) alkalinity determination in an anaerobic sewage sludge digest based on the measurement of pressure increase in a constant-volume vessel on acidification of a sludge aliquot. The method provides the basis for a simpler and more robust instrument than the current constant-pressure designs, an important consideration for unattended operation in commerical installations. The factor that has led to the previous dismissal of constant-volume instruments has been the perceived problem of carbon dioxide solubility in water and the proportionality of the solubility with carbon dioxide partial pressure. Pre-saturation of the sample aliquot with carbon dioxide was thus inappropriate for such instruments. In fact the instability of aqueous sodium bicarbonate is shown to give rise to errors in bicarbonate alkalinity determination which are significantly larger than those resulting from failure to pre-saturate with carbon dioxide, provided that carbon dioxide partial pressures do not exceed typically 50 mbar over a 0.08 mol dm–3 aqueous sodium bicarbonate solution, typical of a digester sludge. Furthermore, carbon dioxide pre-saturation is shown to exacerbate significantly the effect of sludge sulfide content on the accuracy of bicarbonate alkalinity determinations. A theoretical basis is presented for this phenomenon and the conclusions are supported by the results of a factorial analysis experiment. A standard additions calibration of carbon dioxide pressure increase resulting from the neutralization of 10 cm–3 of a typical digested sludge, with additions of 6.756 g dm–3 aqueous sodium bicarbonate solution, was linear with a correlation coefficient of greater than 0.999 between sodium bicarbonate concentration and pressure increase. This calibration has been used in the monitoring of a digester by daily triplicate bicarbonate alkalinity analyses which have had an average relative standard deviation of approximately 2.7{\%} over a 2 month period. Comment is made regarding a modification to this prototype instrument that will allow the simultaneous monitoring of sludge sulfide content by means of an electrochemical sensor, the sulfide content measurement to be applied as a correction to the bicarbonate alkalinity determination.",
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Mechanistic study of on-line bicarbonate (hydrogencarbonate) monitoring of anaerobic sewage sludge digesters. / Anstice, Philip J. C.; Coleman, Heather; Murphy, Donal J.; Zuber, Paul A.; Thomas, C. L. Paul; Alder, John F.

In: The Analyst, Vol. 120, No. 12, 29.06.1995, p. 2873.

Research output: Contribution to journalArticle

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T1 - Mechanistic study of on-line bicarbonate (hydrogencarbonate) monitoring of anaerobic sewage sludge digesters

AU - Anstice, Philip J. C.

AU - Coleman, Heather

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AB - A study is reported of a simple method for bicarbonate (hydrogencarbonate) alkalinity determination in an anaerobic sewage sludge digest based on the measurement of pressure increase in a constant-volume vessel on acidification of a sludge aliquot. The method provides the basis for a simpler and more robust instrument than the current constant-pressure designs, an important consideration for unattended operation in commerical installations. The factor that has led to the previous dismissal of constant-volume instruments has been the perceived problem of carbon dioxide solubility in water and the proportionality of the solubility with carbon dioxide partial pressure. Pre-saturation of the sample aliquot with carbon dioxide was thus inappropriate for such instruments. In fact the instability of aqueous sodium bicarbonate is shown to give rise to errors in bicarbonate alkalinity determination which are significantly larger than those resulting from failure to pre-saturate with carbon dioxide, provided that carbon dioxide partial pressures do not exceed typically 50 mbar over a 0.08 mol dm–3 aqueous sodium bicarbonate solution, typical of a digester sludge. Furthermore, carbon dioxide pre-saturation is shown to exacerbate significantly the effect of sludge sulfide content on the accuracy of bicarbonate alkalinity determinations. A theoretical basis is presented for this phenomenon and the conclusions are supported by the results of a factorial analysis experiment. A standard additions calibration of carbon dioxide pressure increase resulting from the neutralization of 10 cm–3 of a typical digested sludge, with additions of 6.756 g dm–3 aqueous sodium bicarbonate solution, was linear with a correlation coefficient of greater than 0.999 between sodium bicarbonate concentration and pressure increase. This calibration has been used in the monitoring of a digester by daily triplicate bicarbonate alkalinity analyses which have had an average relative standard deviation of approximately 2.7% over a 2 month period. Comment is made regarding a modification to this prototype instrument that will allow the simultaneous monitoring of sludge sulfide content by means of an electrochemical sensor, the sulfide content measurement to be applied as a correction to the bicarbonate alkalinity determination.

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