Anaerobic treatment of kraft pulp-mill waste activated-sludge: Gas production and solids reduction

Bioresource Technology 39 (1992) 61-68

Anaerobic Treatment of Kraft Pulp-Mill Waste Activated-Sludge: Gas Production and Solids Reduction Jaakko A. Puhakka* Water and Environmental Engineering, Department of Civil Engineering, Tampere University of Technology, PO Box 527, SF-33101 Tampere, Finland

Matti Alavakeri Plancenter Ltd, SF-00520 Helsinki, Finland

& Wen K. Shieh Department of Systems, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6315, USA (Received 23 December 1990; accepted 7 January 1991) Abstract

Anaerobic digestion of kraft pulp-mill waste activated-sludge was studied in a pilot-scale unit. The anaerobic digester was operated for 21 months over a wide range of volatile solids (VS) loading rates (1"5-5.2 kg VS/m 3 day; Hydraulic Retention Time, HRT, 24-8 days) to establish the optimum operational conditions for solids reduction and biogas production. The process started up in 6 weeks using a mixture of municipal-digester sludge and sediment slurry, which was acclimated to kraft pulpmill effluent, as seed material. Digestion of sludge containing about 38% lignin showed the median VS removal of 40% with the median biogas production of 220 litres/kg VS added or 570 litres/kg VS removed. The respective 90 percentiles were 60% for VS reduction and 350 litres/kg VS added or 780 litres/kg VS removed for biogas production. Optimal process performance was obtained at the VS loading of 2.2 kg VS/m 3 day. Alkali addition (13 g NaOH/kg VS) to feed sludge, and sludge recycle (with a ratio of 0"25) were required for stable operation at VS loadings exceeding 1.5 kg VS/m 3 day. *Present address: Department of Civil Engineering, Universityof Washington,Seattle, WA 98195, USA.

The anaerobic process remained stable even at the VS loading of 5"2 kg/m 3 day. Key words: Activated-sludge, anaerobic digestion, biogas production, kraft pulp-mill, solids reduction. INTRODUCTION The annual production of c. 9 million tons of pulp and 8.7 million tons of paper and board products in Finland involves the production of 1 x 109 m 3 of wastewater with an organic load of 140 x 103 tons of BOD 7 (Leppfinen, 1989). This wastewater is treated by activated-sludge plants producing 30 000 tons (dry weight) of waste activated-sludge per year (Saunamfiki, 1989). The first activatedsludge plant for the pulp and paper industry was started in 1984. Since then, 20 activated-sludge plants have been constructed and the remaining mills will have their plants in operation before 1995. Many of these plants have been designed to operate under high organic-loading conditions to generate waste activated-sludge at rates ranging from 0"8 to 1"2 kg solids/kg BOD 7 removed (Saunam/iki, 1988). In addition to the large sludge volume, high-loading operations produce sludges

61 Bioresource Technology 0960-8524/92/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

62

J. A. Puhakka, M. Alavakeri, W. K. Shieh

difficult to settle and dewater. The elimination of waste activated-sludge is difficult with the method available. Today these sludges are generally combined with primary sludges, mechanically dewatered, and disposed of in landfills. Anaerobic digestion has been widely employed for stabilizing municipal wastewater sludges and reducing the volume of solids for disposal. Primary sedimentation sludges, excess sludges produced in secondary treatment, and mixtures of both are amenable to anaerobic digestion. Significant biogas production has been detected at paper-mill sludge landfill areas (Wardwell, 1982) and in anaerobic digestion experiments with pulpand paper-mill sludges (Boman & Bergstr6m, 1985; Puhakka et al., 1988). Anaerobic digestion would reduce sludge volumes and improve sludge dewaterability (Lawler et al., 1986; Parkin & Owen, 1986). In combined anaerobic-aerobic treatment for pulp- and paper-industry wastewaters, a portion of the excess sludges produced from aeration has been recycled to the anaerobic reactor as nutrient supplements (Huss et al., 1986). To the best of our knowledge, however, anaerobic digestion has not yet been applied for full-scale treatment of pulp- and paper-mill sludges. The purpose of this study was to evaluate the performance of anaerobic digestion for treatment of the kraft pulp-mill waste activated-sludge, with special reference to solids reduction and biogas production. The paper presents and discusses the results of the pilot-scale experimentation.

METHODS Digester design and operation The experimental set-up was placed in a temporary shelter to maintain the ambient temperature at 10-15°C during winter time. In the summer the temperature inside the shelter varied from 20 to 30°C. Sludge digestion experiments were performed using a pilot-scale unit consisting of the following components (Fig. 1 ): (1) Two stainless steel, rectangular, sludgestorage tanks (800 mm × 800 mm × 900 mm), each with a bottom hopper, giving a working volume of 0.5 m 3. These tanks were employed for feed-sludge storage and pretreatment. Each was equipped with a mechanical mixer. (2) A stainless steel cylindrical (diameter 1410 mm) anaerobic digester with a mixer

(3)

(4)

and bottom collector (diameter 510 mm; height 200 mm) for sludge recycling. The active digester volume was 2-5 m 3. Internal sludge recycling at a recycle ratio of 1.0 was practised for all experimental runs. A stainless steel, rectangular, storage tank (1000 m m x 1000 m m x 1100 ram) with a bottom hopper for digested sludge. A portion of the digested sludge was recycled to the anaerobic digester during pseudo steady-state experiments (at VS loadings exceeding 1.5 kg VS/m 3 day) to improve digester performance. A gas metering system consisting of a water trap, a gas vent, and a Kimmon Aluminum Case PK-II domestic gas meter (Kimmon Manufacturing Co. Ltd, Tokyo). (PI in Fig.

1). Allweiler (ANP 6.2) pumps were used for feeding the digester, recycling the sludge, and disposing of the digested sludge. The feed pump was equipped with a regulator giving pumping rates ranging from 0.06 to 0.3 m3/h. Sludge recycle and disposal pumps had a pumping rate of 0.25 m3/h at 290 rpm. The waste activated-sludge was fed semicontinuously into the digester using a feed timer equipped with a relay interrupter. During the startup period the feed system operated 8 h per day. During the pseudo steady-state operation sludge was fed 24 h per day. The feed sludge storage tanks were refilled each 1-3 days. The sludge pH was adjusted using a 10% NaOH solution. The digester was wrapped with electrical heating cable, insulated with a mineral-wool layer (100 mm, heat transfer coefficient: 0-045 W/K m2), and covered with aluminium foil. The overall heat transfer coefficient was 0-40 W / K m 2. Two thermostats with temperature sensors were used to maintain the digester temperature at 36°C. The digester was mixed mechanically by two submerged impellers at 1 rpm and internal sludge recycling. The sludge was collected in the bottom collector using two scrapers, connected to the mixer. Digester sludge was allowed to overflow to the digested-sludge tank. During the recirculation period digested sludge was recycled to the digester at a recycle ratio of 0"25. At the beginning 2 m 3 water was pumped into the digester, which was then heated up to 32°C and the seed material was added. The digester temperature was allowed to rise to 36°C prior to startup. A mixture of municipal digester sludge (2

Anaerobic digestion of pulp-mill activated-sludge FEED-SLUDGE STORAGE

63 DIGESTED SLUDGE STORAGE

DIGESTER

WASTE ACTIVAT] SLUDG:

Fig. 1. Pilot-plant scheme: T1, point of temperature measurement; PI, point of gas measurement; TS, temperature control; FIQ, point of samples for gas quality.

m 3) and sediment slurry (0.8 m 3) collected from the recipient-water tank of the pulp-mill effluent was used as the seed material. The characteristics of the seed material are given in Table 1. Before continuous feeding, small amounts of feed sludge were pumped into the digester to calibrate and test the pumps, during a two-week period. During the continuous experiments the pumps were calibrated at least once a week.

Sludge characteristics The waste activated-sludge originated from an activated-sludge plant treating a bleached kraft pulp-mill composite wastewater. The pulping process shifted periodically between hardwood and softwood causing fluctuation in the wastewater characteristics as well as the sludge quality. A special feature of this plant was the prolonged thickening of the secondary sludge (hydraulic retention time at about 24 h) to release sludge phosphorus which was then recycled to the aeration basins. The waste activated-sludge in the thickener was in the acidogenic phase. The characteristics of the feed sludges are shown in Table 2. The mean VS/TS-ratio of the sludge was 0-6,

Table I. Seed-sludge characteristics

Parameter (mg/litre)

SS VS ALK VA COD total COD soluble

Recipient sediment slurry Sample 1 (0"6 m 3)

Sample 2 (0.2 rrfl)

65 000 40 000 1 500 1 000 ND 3 300

50 000 32 000 1 700 1 350 39 000 3 700

Municipal-digester sludge (2 m "~)

8 300 4 200 1 500 260 7 300 300

ND = not determined. Demand (CODs) and volatile acids (VA). The samples for COD s and VA were filtered using 10 /~m Whatman GF/A glass-fibre filters. The analyses were performed according to the following Finnish Standard Procedures (SFS, 1976-1981): pH (SFS 3021, 1979), ALK (SFS 3005, 1981), COD (SFS 3020, 1979), SS (SFS 3008, 1976), TS and VS (SFS 3008, 1981). Volatile acids were measured by direct titration (DiLallo & Albertson, 1961 ).

RESULTS

Analytical procedures The anaerobic digestion performance was monitored by the quantity and quality of biogas (APHA, 1985) and by the analysis of sludge samples for pH, alkalinity (ALK), Suspended Solids (SS), Total Solids (TS), total Chemical Oxygen Demand (CODT), soluble Chemical Oxygen

Startup After seeding the digester and calibrating the mechanical components of the pilot-unit, semicontinuous (8 h/day) feeding of waste activatedsludge was started to achieve the 1.5 kg VS/m 3 day loading commonly used for design of munici-

64

J. A. Puhakka, M. Alavakeri, W. K. Shieh

Table 2. Characteristicsof kraft-millwaste activated-sludgefrom thickener Parameter

N°

Range of values b

Mean

Standard deviation

TS VS ALK VA C O D total C O D soluble pH

375 372 101 375 375 375 102

16 5 0 0 - 1 0 6 500 8 9 0 0 - 63 000 1101 800 3 0 - 2 000 13 4 0 0 - 1 0 0 000 6 1 0 - 6 400 5.8-7.0

53 100 32 600 1 070 590 53 300 2 500

15 600 9 000 320 360 15 000 1 100

aNumber of samples. bmg/litre except pH.

pal digesters. The VS loading was increased by decreasing the hydraulic retention time as illustrated in Fig. 2. The digester performed well during the startup period, as indicated by the gas production rates (Fig. 2) and the daily VA/ALK ratios of 0-05-0.12. The digester pH remained at 6.9-7.4 and the average VA concentration was 90 mg/litre, whereas these values for feed sludge varied from 5.8 to 6.8 and from 200 to 2000 mg/ litre, respectively. At the end of the startup period, the gas production rate was approximately 250 litres/kg VS added. These results indicate that the chosen seed successfully started up the kraft pulpmill sludge-digestion process. While solids accumulation in the digester took place during the startup period, the VS removal could not be accurately determined. However, the removal of soluble COD indicated active substrate utilization during this period (Fig. 2). After 66 days of semicontinuous operation the solids in the digester were more uniformly distributed than on the first day of operation (Fig. 3). Overall process performance After startup the digester was operated at a constant hydraulic retention time of 25 days for two months. However, due to large variations in feedsludge characteristics (Table 2), steady digester operation was not possible using a constant feed volume. Therefore, the VS loadings were maintained constant by daily adjustment of the feedsludge volume according to its VS content (i.e.. pseudo steady-state operation). The overall results of digester performance from the 21 months of operation are presented in Fig. 4. These probability plots showed that the digester had little sensitivity towards the wide variations in feed COD s and VA. The median and 90 percentile effluent COD s were 750 and 1250

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mg/litre, respectively. The respective values of the feed sludge were 2100 mg/litre and 3800 mg/ litre. The data indicate good COD s removal over the wide range of VS loadings (0.5-5.2 kg VS/m 3 day). Since the feed sludge was already in the acidogenic phase, it contained a high concentration of VA (i.e. the median value at 500 mg/litre and the

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where P-- percent reduction of volatile solids VS1 = VS/TS ratio of the feed sludge VS2 = VS/TS ratio of the digested sludge The probability plots of overall gas production rates are presented in Fig. 5. The median gas production rate was 220 litres/kg VS added or 570 litres/kg VS removed. The respective 90 percentiles for gas production were 350 litres/kg VS added and 780 litres/kg VS removed, with an average methane content of 56% in the biogas. The median VS reduction was 40% and the 90 percentile reduction was 60% (Fig. 5). As has been described elsewhere (Kyll6nen et al., 1988) the lignin content of this sludge was about 38%. Since lignin is known to be anaerobically nonbiodegradable, the results show that the potentially biodegradable 62% portion of volatile solids was mainly anaerobically degraded.

Pseudo steady-state performance Pseudo steady-state operation was studied using VS loadings of 1"5, 2.2, 3.0, 4.0, and 5.2 kg WS/m 3 day. After 1-3 months of pseudo steady-state operation at a constant VS loading the loading

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was gradually increased to a higher level over a period of 1-1.5 months. The first pseudo steady-state experiment with the VS loading of 1.5 kg VS/m 3 day was conducted without sludge recycling. During the subsequent periods a recycle ratio of 0.25 was used. An improved process performance was observed with sludge recycling as shown in Table 3: the median and 90 percentile VS reduction and gas production results were lower at 1.5 kg WS/m3 day than the results at 2.2 kg VS/m 3 day. No thickening of recycled sludge was observed in the digested-sludge storage tank. Hence, the recycle

J. A. Puhakka, M. Alavakeri, W. K. Shieh

66

did not increase the active biomass retention in the digester but possibly enhanced digester performance by improving the mixing conditions. The process operation was disturbed twice during the study (see process upsets). The VS loading was increased to 3 kg VS/m 3 day after the second

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process upset. The pseudo steady-state period at this VS loading was started less than 10 days after the upset (Fig. 6). This may explain why the VS reductions were smaller at 3 kg VS/m 3 day than at the other pseudo steady-state conditions with sludge recycle. The gas contained 56 to 57% methane at all VS loadings. Process upsets During this investigation two major process upsets occurred as indicated by the abnormally high VA/ALK ratios illustrated in Fig. 6. The first process upset (day 150-180) took place during a pseudo steady-state period at the VS loading rate of 1.5 kg VS/m 3 day. The upset was indicated by an increase in VA/ALK ratios up to 0"55 (Fig. 6). The VA concentration of the digester sludge increased from 100 to 600 mg/litre and the pH varied between 6.4 and 6.7 during this period. The average gas production dropped from the normal 260 to 130 litres/kg VS added during this period. To bring the process back to normal, caustic extract was added to the feed sludge once a day at an average dosage of 13 g of NaOH/kg VS added. This practice increased the alkalinity of the digester sludge from 1000 to about 2000 mg CaCO3/litre within two weeks, at which time the process recovered. Afterwards, the daily NaOH addition was continued to maintain proper alkalinity and pH in the digester. This indicated that the waste activated-sludge tested did not have enough alkali to maintain the pH at 6.5-7.5. The second upset (day 350-360) in the digester performance was due to a dramatic change in the feed-sludge quality during a period where the VS loading rate was increased from 2.2 to 3"0 kg VS/m 3 day. The activated-sludge process failed and started to produce foaming sludge. The

Table 3. Pseudo steady-state results of anaerobic digestion system for kraft-mill waste

activated-sludge

Organic loading rate (kg VS/m ~ day)

VS reduction (%)

(Litresflitre day) Median

1'5 b 2'2 3"0 4"0 5"2

Gas Production

37 55 35 42 49

Litres/kg VS,dded

901~ 52 67 44 53 61

Median

90p

Median

90p

0"36 0"78 0"68 0"77 0"92

0"45 1"0 0"9 1"0 1"1

230 360 230 200 180

320 435 295 250 225

~90% of observations were equal to or less than stated value. bNo sludge recycle.

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050O.t,Ov 030-

--3 .,<

,~ o2o>

OlOoooo

16o

zoo 36o Time (clays)

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digester pH dropped from 6"8 to 6"5 within one day with simultaneous decrease in gas production. The VS loading rate was lowered from 2.8 to 0-6 kg VS/m 3 day immediately after the pH drop. After 8 days the VA/ALK ratio of the digester sludge started to return to normal as shown in Fig. 6, and the VS loading rate was reset back to 2"4 kg VS/m 3 day within two weeks.

DISCUSSION The results of this study show that kraft pulp-mill waste activated-sludge was amenable to anaerobic digestion although the quality of wastewater and sludge varied widely. VS loadings over 5 kg VS/ m 3 were achieved with steady process performance. Rapid startup of the digester was achieved by using a mixture of municipal digester sludge and sediment slurry from the kraft mill as seed material. Inhibition of methane production from kraftmill waste activated-sludge by low pH caused the accumulation of volatile acids and increased the VA/ALK ratio above the critical value of 0-35 (MERL, 1979). Sodium hydroxide is used in kraft pulping and it is inexpensive for sludge pH adjustment in dosages used in this study (about 2500 US Dollars/m 3 of 50% NaOH) (Alavakeri & Puhakka, 1990). Waste activated-sludge consists of microbial biomass, cell-decay products and non-biodegradable lignin precipitates (38%). In addition to maintaining a proper VA/ALK ratio, sodium hydroxide may have hydrolyzed some sludge VS constituents and thereby improved digester performance. Overall process performance of the pilot digester showed an efficient volatile solids removal (median VS reduction 40%) which is important in mills landfilling the final sludge. The

67

biogas (median gas production 220 litres/kg VS added) produced can be used as an energy substitute in wastewater treatment operations. These results compared favourably with those observed in municipal applications (Parkin & Owen, 1986). The pilot results were better than those of our earlier results obtained with laboratory-scale digester with the same sludge (median VS removal 28% at 2.2 kg SS/m 3 day) (Puhakka et aL, 1988). The improved results were possible due to the 'once a day' feed pattern in the laboratory digesters while the pilot digester was fed semicontinuously. One short-term pilot experiment (overall test period 50 days with 29 days of constant organic loading) by Boman and Bergstrrm (1985) indicated 45% VS removal and 0"56 m 3 gas/kg VS removed at 1"0 kg VS/m 3 day for paper-mill waste activated-sludge. Thickening of feed sludge, efficient digester mixing and an input of excess alkali were necessary to obtain stable digestion with higher VS loadings than reported earlier for waste activated-sludge from pulp- or paper-mill origin. Digestion affected the sludge dewaterability and the filtrate quality as described in detail elsewhere (Puhakka et al., 1991). The full-scale design and cost analysis of the process is presented by Alavakeri and Puhakka (1990).

ACKNOWLEDGEMENTS This work was financially supported by the Ministry of Trade and Industry, Finland, the Academy of Finland and the Heikki and Hilma Honkanen Foundation. We thank Prof. Mirja SalkinojaSalonen for critically reading the manuscript.

REFERENCES Alavakeri, M. & Puhakka, J. A. (1991). Anaerobic treatment of kraft pulp mill waste activated sludge:full-scaledesign and cost analysis.(Submittedfor publicationin TAPPIJ.). APHA (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. APHA-AWWA-WPCF, Washington,DC. Boman, B. & Bergstr6m,R. (1985). Anaerobic treatment of fibre sediment and forest industry wastewater sludge.1VL Rapport B792, Swedish Environmental Research Institute. DiLallo, R. & Albertson, O. E. (1961). Volatile acids by direct titration. J. Water Poll. Control Fed., 33,978-95. Huss, L., Sievert, P. & S~irner,E. (1986). Anamet full scale anaerobic treatment experiences at three pulp and papermills.In Proc. PIRA Key Event, Cost Effective Treatment of Papermill Effluents Using Anaerobic Technologies, PIRA, Leatherhead, UK.

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Kyll6nen, H. L., Lappi, M. K., Thun, R. T. & Mustranta, A. H. (1988). Treatment and characterization of biological sludge from the pulp and paper industry. Water Sci. Tech., 20, 183-92. Lawler, D. F., Hung, Y. J., Hwang, S. J. & Hull, B. A. (1986). Anaerobic digestion: effects on particle size and dewaterability. J. WaterPoll. ControlFed., 58, 1107-17. Lepp~non, T. (1989). Preliminary yearly statistics. National Board of Waters and Environment, Helsinki, Finland. MERL (1979). Process Design Manual: Sludge Treatment and Disposal. Municipal Environmental Research Labo rato ry, Cincinnati, Ohio, 625 / 1- 79-011. Parkin, G. E & Owen, W. E (1986). Fundamentals of anaerobic digestion of wastewater sludges. J. Env. Eng. Div., ASCE, 112, 867-920. Puhakka, J. A., Viitasaari, M. A., Latola, P. K. & M~i/itt/i, R. K. ( 1988). Effect of temperature on anaerobic digestion of

pulp and paper industry wastewater sludge. Wat. Sci. Tech., 20,193-201. Puhakka, J. A., Alavakeri, M. & Shieh, W. K. (1991). Anaerobic treatment of kraft pulp-mill waste activated-sludge: sludge dewaterability and filtrate quality. Bior. Tech., 39, 69-75. Saunam/iki, R. (1988). Sludge handling and disposal at Finnish activated sludge plants. Wat. Sci. Tech., 20, 171-82. Saunam/iki, R. (1989). Biological wastewater treatment in the Finnish pulp and paper industry. Paperi ja Puu - Paper and Timber, 71,158-64. SFS (1976-1981 ). SFS Standards: 3005, 3008, 3020, 3021, and 3037. Finnish Standardization Society, National Board of Waters and Environment, Helsinki, Finland. Wardwell, R. E. (1982). Bioconversion of papermill sludge. In Proc. Symp. Long Range Disposal Alternatives for Pulp and Paper Industry Sludges, Bangor, Maine, pp. 130-52.