| Summary: | Pulp manufacture uses sulphur in a variety of forms and some form of these
sulphur compounds ultimately appears in the effluent. Under anaerobic conditions,
sulphate, sulphite and thiosuiphate are reduced to sulphide. This results in problems of
toxicity, odour, corrosion, and inhibition of the wastewater treatment microorganisms.
The fate of these inorganic sulphur compounds in a bleached chemithermomechanical
puip/thermomechanical pulp (BCTMPITMP) effluent mixture was examined in two phase
anaerobic reactors at 35 °C and 55 °C. The following sulphur management strategies were
investigated: 1) shifting the sulphide to the less toxic form by controlling the pH of the
acidogenic reactor, 2) inhibiting the sulphur reducing bacteria via molybdenum addition to
the feed tank, and 3) stripping the hydrogen sulphide dissolved in the methane phase
reactor liquor by recycling hydrogen sulphide-free scrubbed off gas. The laboratory scale
experimental apparatus consisted of upflow anaerobic sludge bed pre-treatment or
acidogenic reactors followed by hybrid upflow anaerobic sludge bed/fixed film
methanogenic reactors.
The sulphur management strategies which were investigated demonstrated
significantly improved treatment efficiencies.
At 35 °C, controlling the pH of the acidogenic reactors with sodium carbonate
from 5.5 (uncontrolled) to 8.0 in order to shift the formed sulphide species to the less
toxic ionic form appeared to be effective in promoting the wastewater treatment
efficiency. Sulphate reduction efficiencies were typically 75 to 90% irrespective of acid
phase reactor pH. Maximum total organic carbon (TOC) removals of 55 and 63% were
observed at an acid phase reactor pH of 7.5 for total hydraulic retention times (HRTs) of
1.2 and 1.8 days respectively.
Molybdate was added to the wastewater at levels from 0.1 to 1.0 mM. At 35 °C, it
was effective in two out of three effluent batches at the 1.0 mM level in decreasing sulphate reduction from 90% down to 23 to 40%. Maximum TOC removal efficiencies of
42% were observed at 0.5 mM molybdate. Molybdate additions greater than 0.5 mM
resulted in reduced TOC removals and gas production rates.
Hydrogen suiphide stripping, using ferric chloride scrubbed and recycled off gas,
resulted in lower dissolved suiphide levels and increased TOC removals. Sulphate
reduction was unaffected by the varying concentrations of dissolved suiphide. Stripping
resulted in TOC removal efficiencies of up to 57%, a significant improvement over the
unstripped control runs where the TOC removals were only approximately 24%. This
sulphur management strategy at 35 °C appeared to be the most effective means of sulphur
management for sulphur rich mechanical pulping effluents.
Thermophilic 55 °C anaerobic treatment was also studied using the same effluent,
inocula and sulphur management strategies. Overall, both the treatment efficiency and the
sulphate reduction were considerably lower for the thermophilic runs compared to the
mesophilic runs. Raising the acidogenic phase reactor pH from 7.0 to 7.5 to 8.0 appeared
to have no significant effect on the organic carbon removal efficiency (maximum 24%) or
on sulphate reduction efficiency where a maximum of only 51% was realized.
Thermophilic molybdate inhibition of sulphate reduction was not as marked as for the 1.0
mM level at 35 °C, perhaps due to the already low baseline sulphate reduction efficiency
(maximum of 50%) at 55 °C. Molybdate addition of up to 1.0 mM improved the TOC
removal efficiency, perhaps by decreasing the suiphide inhibition of the methanogenic
bacteria. Stripping hydrogen sulphide from the reactor liquor at 55 °C helped to promote
the treatment efficiency to a maximum of 39% and was effective in lowering the suiphide
levels. Similar to the 35 °C study, suiphide removal by gas stripping appeared to be the
most effective means of sulphur management for the thermophilic experiments.
Very high acetate concentrations and minute gas production rates were recorded
throughout the experimental program. Since the experiments resulted in the various
combinations of high and low levels of both sulphate and suiphide, this study demonstrated that compounds in addition to sulphate and suiphide inhibited the methane
producing bacteria from using acetate. Various wood extractives and chelating agents are
suspected since they are present in high concentrations in this effluent and they are known
to inhibit the anaerobic wastewater treatment microorganisms.
iv === Applied Science, Faculty of === Chemical and Biological Engineering, Department of === Graduate
|
|---|
