Combined alkaline and ultrasound pre-treatment of thickened pulp mill waste activated sludge for improved anaerobic digestion
Highlights
► Pre-treatment increased VS removal in both the thickened (by 4%) and non-thickened sludge (by 9%). ► Pre-treatment did not increase the overall methane yield, but did increase the initial rate of methane production. ► 80% of total methane/biogas production was reached 5.5–6.5 days faster when pre-treated; therefore reducing the hydraulic retention time. ► Pre-treatment decreased the dewaterability of the sludge.
Introduction
The production of wood pulp for paper manufacturing uses large amounts of water. Pulp and paper effluents are mostly treated at the primary and secondary treatment levels producing large amounts of secondary sludge. Disposal of secondary sludge is costly due to its poor dewaterability [1]. The production of secondary sludge will continue to increase as wood pulp production increases and as environmental regulations pertaining to effluent quality become more stringent [1]. Sludge disposal and management can amount to 60% of the total cost of pulp mill wastewater treatment; for this reason the minimization of sludge production is important [2].
One method that is often used to reduce sludge volume is anaerobic digestion (AD). Anaerobic digestion is the biological breakdown of organic waste in the absence of oxygen. Some of the benefits of anaerobic digestion include; reduced sludge volume, sludge stabilization, sludge disinfection, and energy recovery in the form of methane. There have only been four anaerobic treatment system utilized in Canada for the treatment of pulp and paper sludge [3]. Of these four instillations only two are currently operational [3]. Sludge produced from the pulp and paper industry is mainly composed of microbial cell mass that forms during secondary treatment and lignin and chemical residuals from the pulping process [4]. These substrates are difficult to degrade by AD. Consequently, AD of pulp sludge requires long retention times, has a low degradation efficiency, and poor economics. For these reasons, AD is not widely used in the pulp and paper industry [3], [5].
Increasing sludge digestion through sludge disintegration can increase methane production, decrease sludge volume, reduce sludge retention time, and thereby improve the overall economics of the process. Disintegration helps to increase the sludge hydrolysis rate by mechanically, chemically, thermally, or biologically aiding in the solubilization of the sludge by reducing floc particle size and rupturing cell walls/membranes.
Alkaline addition has been shown to increase solubilization of pulp and paper sludge and improve biogas yield [5], [6]. Sodium hydroxide is most often used over other chemicals such as KOH and Ca(OH)2 due to its high rate of COD solubilization [7]. Hydroxy anions weaken cell walls and break a part floc structure rendering the sludge amenable to further treatment. Alkaline pre-treatment has been combined with ultrasound, microwave, thermal, high pressure gradients, to further sludge breakdown [8], [9], [10], [11].
Ultrasound treatment of wastewater sludge has been studied at length due to its ease of operation and effectiveness [12]. Ultrasound treatment involves using high frequency sound waves generated by a vibrating probe. As the sound waves travel through the liquid, gas bubbles are formed and collapse violently producing localized high temperatures and pressures. Cell walls and floc structures are destroyed making their contents' susceptible to anaerobic digestion. Combinations of both ultrasound and alkali pre-treatment have been shown to improve sludge digestion over either treatment alone [13], [8]. Improvement of sludge sonication efficiency has been improved by increasing the solids content of the sludge [14], [15].
Much of the research conducted on the pre-treatment of secondary sludges has been utilizing municipal wastewater sludge. Much less has been reported on the pre-treatment of sludges produced from pulp mill wastewater treatment [3]. Some pre-treatment of pulp sludge have been reported in the literature, however; few have reported combined treatment methods, or have pre-treated thickened pulp sludge [5], [16], [17].
The objective of this experiment was to determine the methane potential of secondary sludge produced by the Quesnel River Pulp (QRP) mill. Thickened (65 g kg−1 TS) and non-thickened sludges (25 g kg−1 TS) were subject to alkaline pre-treatment followed by ultrasonication in order to improve sludge AD efficiency and compared to non pre-treated sludge. Pre-treatment effectiveness was analysed by comparing the soluble COD, soluble TS and soluble VS fraction in treated and non-treated sludges. AD efficiency was based on methane production and VS reduction. Hydrogen sulphide production was also calculated by comparing sludge sulphur content before and after digestion. QRP sludge is known to be high in S due to sulphite addition during the pulping process.
Section snippets
Pulp sludge sample
Secondary sludge was collected from the Quesnel River Pulp mill, in Quesnel, B.C. Canada. The QRP mill produces bleached chemi-thermo-mechanical pulp (BCTMP) and thermo mechanical pulp (TMP). Wastewater is composed of approximately 2:1 BCTMP: TMP effluent. The plant typically uses 60% white spruce and 40% lodgepole pine. Wastewater is treated using a moving bed biological reactor followed by activated sludge treatment. Pulp mill sludge (PMS) samples were stored at 4 °C and used within 24 h of
Effect of sludge pre-treatment
As seen in Table 1, the VS TS−1 content of the pulp sludge's is high, even larger than the municipal primary sludge used as inoculum, suggesting that it is ideal for anaerobic digestion [24]. FS content of the pre-treated sludge increased due to the added NaOH during the pre-treatment process as well as a small amount of possible sludge mineralization caused by pre-treatment. Pre-treatment reduced the COD value of both sludges. This may have been due to loss of volatile components in the sludge
Conclusions
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Methane production (average of pre-treated and non-treated) from QRP sludge produced 340 ± 150 mL g−1 VSconsumed.
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Pre-treatment increased volatile solids removal in both the thickened (by 4%) and non-thickened sludge (by 9%).
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Pre-treatment did not increase the overall methane yield, but did increase the initial rate of methane production.
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80% of total methane/biogas production was reached 5.5–6.5 d faster when pre-treated.
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Pre-treatment decreased the dewaterability of the sludge.
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Thickening of
Acknowledgements
The authors of this paper would like to thank West Fraser, the Quesnel River Pulp Mill, and Nick Finch for supplying sludge samples. The City of Prince George helped to supply sludge inoculum samples. Also, we would like to thank Clive Dawson at the British Columbia Ministry of Forests laboratory in Victoria, B.C. for completing some of the analysis. This research was funded by the National Sciences and Engineering Research Council of Canada.
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