High Temperature Effects on Activated Sludge Treatment Performance and Sludge Quality in a Recycle Mill Pam Norris Richard Marshall, P.E. Michael Richard, Ph.D.*

McKinley Paper Co. Marshall Environmental The Sear-Brown GroupP.O. Box 100 Training and Consulting 201 South Meldrum StreetPrewitt, NM 87045 2870 Duchess Place Fort Collins, CO 80521USA Corvallis, OR 97330 USA

USA * author for correspondence

ABSTRACT Mill wastewater temperature has been increasing due to newer mill processes. This has resulted in biological upset andpoor performance of mill activated sludge systems at treatment temperatures above 35�C.

McKinley Paper Co., a 100% recycle mill, operates a sequencing batch reactor activated sludge system for treatment ofits wastewater. This mill has had frequent and severe biological upsets in wastewater treatment. One possible cause ofthese upsets is a hot mill wastewater above 35-40�C.

A bench scale study was conducted by the papermill personnel to evaluate the effect of high treatment temperature in therange 35-56�C on activated sludge performance and sludge quality.

The bench scale study findings duplicated and explained the treatment problems that frequently occur in the full scalesystem at higher temperature. As the treatment temperature was increased above 35�C in the bench scale study, sludgequality deteriorated with dispersed single bacteria and dispersed filament growth, poor floc formation, and filamentousbulking. COD removal also declined with increasing temperature above 45�C, paralleling the deteriorationin sludge quality. An important finding was the high temperature selection or elimination of specific filamentousbacteria causing sludge bulking, not previously reported.

The findings of this study have implications to all mill activated sludge systems operated at higher temperature.

INTRODUCTION

Mill wastewater temperature has been increasing due to newer mill processes. This has resulted in biological upset andpoor performance of mill activated sludge systems. A number of systems have experienced biological upset at treatmenttemperatures above 35�C.

The papermill, located near Prewitt, NM, is a 100% recycle linerboard mill producing 200,000 tons per day. The mill iscompletely closed with zero effluent discharge. Wastewater treatment involves three stages: primary, secondary andtertiary. A dissolved air flotation unit is used for primary treatment. The secondary or biological treatment step utilizesa two-basin sequencing batch reactor (SBR) system. The tertiary step includes microfiltration and reverse osmosis.

Historically, biological wastewater treatment has been difficult with poor sludge quality, extensive filamentous bulking,and frequent solids losses from the process. This has been due to high wastewater organic acid concentration, at timesover 2,000 mg/L, and high SBR temperatures in excess of 41�C. Poor biological treatment performance has frequentlymade it difficult to produce enough treated wastewater to meet mill production needs.

Studies were undertaken at the papermill to examine high temperature effects on sludge quality and treatmentperformance. A small scale SBR reactor was operated using mill wastewater as feed. Two studies were performed. Inthe first study, the reactor temperature was slowly raised over a 76-day period (approximately 8 sludge ages) from 25 to56�C and the effect on sludge quality and treatment performance monitored. In the second study, the reactortemperature was held relatively constant at 40 to 45�C for 37 days (4 sludge ages) and the effect on sludge quality andtreatment performance monitored to determine whether a stable treatment biomass could be obtained in this temperaturerange.

METHODS

A small-scale reactor was operated in the lab with mill wastewater supplemented with nutrients analogous to the fullscale system. The reactor consisted of a one cu.ft. plexiglass cube (28.3 L). The reactor temperature was controlled byplacing the reactor in a heated waterbath with a submergible heater in the reactor. Aeration was supplied using coarsebubble aerators. The reactor was operated as a SBR with a 12-hour treatment cycle (two cycles per day). Operationalphases were: preaeration 1 hour; feed plus aeration 9 hours; settle 1 hour; and decant 1 hour.

Daily measurements (each treatment cycle) included: influent wastewater chemical oxygen demand (COD), totalsuspended solids (TSS), pH and temperature; reactor mixed liquor suspended solids concentration (MLSS), pH,temperature, dissolved oxygen (DO) concentration and sludge settled sludge volume (SVI); and effluent COD, TSS,total inorganic nitrogen (ammonia plus nitrate) and ortho-phosphorus. Reactor samples were collected three times perweek (approximately every other day) and sent to the Sear-Brown laboratory in Fort Collins, CO for microbiologicaltests. Microbiological measures of sludge quality included: extent of floc formation and dispersed growth (singlebacteria or filaments); maximum floc diameter; filament abundance; identification of filaments present; presence andamount of zoogloea; and presence and type of higher life forms. Filament abundance and dispersed growth were scoredon a relative scale of 1 to 7 (1 = few; 2 = some; 3 = common; 4 = very common; 5 = abundant; 6 = excessive; and 7 =composed the entire culture).

All chemical and physical tests were done according to the procedures in Standard Methods (1). Microbiologicalmeasurements of sludge quality and filament identification were done using the methods of Jenkins et al. (2).

The influent wastewater COD ranged from about 1,500 to 5,500 mg/L, averaging 4,000 mg/L. The reactor DOconcentration was maintained in the range 0.5 - 5.0 mg/L, generally above 2.0 mg/L, and nutrients were in excess duringthe studies with effluent total inorganic nitrogen (ammonia plus nitrate) in the range 1.0 - 10.0 mg/L and ortho-phosphorus in the range 1.0 - 25.0 mg/L.

The target reactor MLSS concentration was approximately 6,000 mg/L and this was adjusted daily by manual sludgewasting to yield an approximate sludge age of 9 days. However, both the MLSS concentration and the sludge age variedsignificantly, due to frequent and extensive solids losses from the reactor.

RESULTS

Study 1

In study 1, the reactor temperature was gradually increased from 25�C to 56�C over a 76-day period (8 sludge ages). Reactor chemical and microbiological changes were quantitated as described in the methods. Following are thefindings.

The activated sludge SVI versus reactor temperature is shown in Figure 1. The type of filaments present and theirabundance in the activated sludge versus temperature are shown in Figure 2. The sludge SVI was relatively high at thestart of the study at 25-35�C, due to the occurrence of three filaments causing filamentous bulking: type 0914, ThiothrixII and Nostocoida limicola II. These filaments are caused by high organic acids, a characteristic of the wastewater at thepapermill. The sludge SVI then declined to the 100-200 ml/g range from 35 to 45�C. This coincided with a loss ofThiothrix II and N. limicola II from the sludge in this temperature range. The sludge SVI then increased to high valuesof 500 ml/g or greater above a temperature of 45�C, due to severe filamentous bulking. Filament growth at temperaturesabove 45�C was all due to type 0914, which grew well up to at least 56�C. These findings showed that filamentousbulking by type 0914 became a serious operational problem at temperatures above 45�C.

Effluent TSS values versus reactor temperature are shown in Figure 3 and the occurrence of dispersed single bacteriaand dispersed filaments is shown in Figures 4 and 5. Here it can be seen that effluent TSS values were frequently high,ranging from 500 to more than 5000 mg/L. This TSS was due to the high growth of both dispersed single bacteria anddispersed filaments (type 0914), as shown in Figures 4 and 5. There was a general trend of higher effluent TSS as thereactor temperature increased, with very high effluent TSS values in the range 1000 to 2000 mg/L above a temperatureof 50�C. It appeared that there were several temperature regions where dispersed growth and effluent TSS wererelatively low, e.g. at 35�C and 45�C. These may have been due to temperature regions where certain floc-formingspecies had a growth advantage.

The maximum floc size (diameter) in the activated sludge versus temperature is shown in Figure 6. Floc size remainedrelatively constant to about 40�C, then declined with increasing temperature. Floc formation was lost entirely above atemperature of 50�C, leaving only dispersed single bacteria and dispersed filaments as the reactor biomass. The increasein dispersed single bacteria and dispersed filament growth with increasing temperature coincided with reduced flocformation at higher temperature.

The occurrence of zoogloea and higher life forms (protozoa) in the sludge versus temperature is shown in Figure 7. Higher life forms were present in the activated sludge at temperatures to 35�C, then were lost at temperatures above this. Zoogloea occurred in the temperature range 30-40�C, then were lost above 40�C. These findings demonstrate themesophilic growth of both higher life forms and zoogloea, with their loss at temperatures above 35-40�C.

COD removal versus temperature is shown in Figure 8 and effluent COD values versus temperature are shown in Figure9. Although there was high variation in COD removal from day to day, COD removal showed a slight increase withincreasing temperature in the range 30 to 45�C. However, COD removal significantly declined above a temperature of45�C, to 60% or less removal. Effluent COD values varied significantly during the study, but were very high above atemperature of 45-50�C. These observations indicate adequate COD removal up to a temperature of 45�C, butdecreasing removal at temperatures above 45�C.

In summary, sludge quality started to deteriorate above a temperature of 35�C, with poor floc formation, high singlebacteria and filament dispersed growth, and severe filamentous bulking. Filament growth above 35-40�C was all due totype 0914, demonstrating its thermotolerant growth. COD removal was maintained up to a temperature of 45�C, butdeclined at higher temperature. These findings indicate sludge quality problems above a temperature of 35�C and lossof COD removal above a temperature of 45�C.

Study 2

In the first study, there was some indication of stable operation at a temperature of 35-45�C. A second study was doneto examine whether a stable biomass could be developed at 40-45�C over a longer time period. The bench scale reactorwas operated at 40-45�C for four sludge ages (approximately a 9-day sludge age) with measurement of sludge qualityand treatment performance as in the first study. Following are the findings.

The sludge SVI versus time at 40-45�C is shown in Figure 10. The type of filaments present and their abundance versestime are shown in Figure 11. The sludge SVI was relatively low during the first 2-3 sludge ages, but showed anincreasing trend with time thereafter, culminating in high SVI values to 500 ml/g during the fourth sludge age. Thetypes of filaments present in the sludge changed during this time. Initially, type 0914, Thiothrix II and N. limicola IIwere present in the sludge. Thiothrix II and N. limicola II declined after about two sludge ages, while type 0914increased to become the dominant filament present after three sludge ages. These findings indicate that at 40-45�C,filament growth increased to cause severe filamentous bulking, and that the filaments Thiothrix II and N. limicola IIwere lost at this temperature while type 0914 increased.

Effluent TSS values versus time at 40-45�C are shown in Figure 12. The occurrence of dispersed single bacteria anddispersed filaments versus time is shown in Figures 13 and 14. Effluent TSS values were relatively low through the first2-3 sludge ages, then significantly increased to high values after this. There was a large increase in dispersed singlebacteria and dispersed filaments that paralleled the effluent TSS increase. Sustained operation of the reactor at 40-45�Cresulted in high dispersed growth and high effluent TSS.

The maximum floc size in the activated sludge versus time at 40-45�C is shown in Figure 15. Maximum floc sizegradually decreased with time from 2000 to 400 um diameter at 40-45�C. This coincided with high dispersed growthand high effluent TSS.

The occurrence of zoogloea and higher life forms in the sludge versus time at 40-45�C is shown in Figure 16. Higher lifeforms only occurred briefly at the start of the study and were insignificant thereafter. This indicates their growth to belimited to <40�C. Zooglea were moderate in amount for the first 2-3 sludge ages, then declined to a low amount (andwere probably about to be lost entirely) after four sludges.

COD removal versus time at 40-45�C is shown in Figure 17 and effluent COD values versus time are shown in Figure18. COD removal was maintained through three sludge ages, but showed some decrease after this. Effluent CODvalues gradually increased over the four sludge age period from about 700 to 1600 mg/L. These findings indicate agradual loss of process efficiency at COD removal over time at 40-45�C. Stable COD removal was not obtained at thistemperature.

These findings indicate that thermal stress was starting to occur at 40-45�C, resulting in filamentous bulking by type0914; a smaller floc size, dispersed growth, and high effluent TSS; and reduced COD removal. A stable biomass withgood treatment was not achieved at 40-45�C.

DISCUSSION

COD removal efficiencies for pulp and papermill activated sludge systems have been reported to decrease at treatmenttemperatures above 35�C (3,4) and to remain stable at treatment temperatures above 35�C (5,6). In this study, CODremoval efficiency was maintained up to about 40�C, but declined above this temperature.

In the second study, a significant amount of time, 2-4 sludge ages, was needed for the effect of high temperature (40-45�C) to significantly impact sludge quality and COD removal. Sludge quality and COD removal were relatively goodfor the first 1-2 sludge ages following a temperature increase to 40-45�C, but these deteriorated after two sludge ages.This time lag has implication in practice for mills with wastewater temperature above 35�C. Changes in sludge qualityand treatment efficiency caused by high temperature may go unrecognized or may not be associated with treatmenttemperature, since these changes will occur relatively slowly over several sludge ages.

Little information exists on the temperature tolerance of filamentous bacteria in activated sludge. Most of these havebeen considered mesophilic, however, with little testing having been done due to a lack of pure cultures for testing.

Two filaments, Nostocoida limicola II and Thiothrix II, common filaments in activated sludge at lower temperature,were lost from the system at a temperature >35�C. Thus, these two filaments are mesophilic. Type 0914, in contrast,grew well and caused treatment problems to a temperature of 56�C. Type 0914 is thus thermotolerant. This is the firstreport of thermophilic growth for a filament in activated sludge.

In a survey of the filament types responsible for bulking in 29 pulp and papermill activated sludge systems for 1982-1990 (7), type 0914 was an uncommon filament, accounting for less than 3% of bulking episodes. In contrast, type 0914was ranked 4th and accounted for 24% of bulking episodes in a second survey of 80 pulp and papermill systemsconducted in 1996. The increase in the prevalence of type 0914 recently may in part be due to the trend in the industrytowards higher wastewater temperature due to mill process changes and the thermotolerant nature of type 0914.

A cooling tower was added to the treatment system in 1999 to maintain wastewater temperature below 32-33 �C in the

summer months. Sludge quality has improved dramatically since this time with less filament growth, better settlingsludge, and a stable process.

CONCLUSIONS

Bench scale studies duplicated full-scale system experience at high temperature operation >35�C. Operation of thebench scale sequencing batch reactor above 35�C lead to high filament growth and filamentous bulking, loss of flocformation, high dispersed single bacteria and dispersed filament growth, and high effluent TSS. Treatment temperatureabove 45�C resulted in a complete loss of floc formation, reduced COD removal, and high effluent COD. The benchscale study also showed the mesophilic nature of two common filaments (Thiothrix II and N. limicola II), zoogloea andhigher life forms. An unexpected finding was the thermotolerant growth of the filament type 0914 to at least 56�C. Astable activated sludge was not obtained in this study at a temperature above 35�C.

These findings have major significance to pulp and papermill activated sludge systems operated at high temperature>35�C. Sludge quality and treatment performance will probably be poor at high temperature. The only recourse for thisproblem is reduced wastewater temperature, using wastewater cooling.

References

1. Standard Methods for the Examination of Water and Wastewater, 20th Ed., American Public Health Association,Washington, D.C., 1998.

2. Jenkins, D., M.G. Richard and G.T. Daigger, Manual on the Causes and Control of Activated Sludge Bulking andFoaming, 2nd Ed., Lewis Publishers, Boca Raton, FL, 1993.

3. Flippen, T.H. and W.W. Eckenfelder, Jr., "Effect of Elevated Temperature on the Activated Sludge Process",proceedings of the 1994 TAPPI Environmental Conference, p. 947, Portland, OR.

4. Tripath, C.S. and G.D. Allen, "Feasibility Study of Thermophilic Aerobic Biological Treatment of Bleached KraftPulp Mill Effluent", preceedings of the 1998 TAPPI Environmental Conference, p. 1189.

5. Barr, T.A., J.M. Taylor and S.J.B. Duff, "Effect of HRT, SRT and Temperature on the Performance of ActivatedSludge Reactors Treating Kraft Mill Effluent", Water Res. 30(4):799, 1996.

6. Rintala, J. and R. Lepisto, "Thermophilic, Anaerobic-Aerobic and Aerobic Treatment of Kraft Bleaching Effluents",Water Sci. Technol. 28(11), 1993.

7. Richard, M.G., "Recent Changes in the Prevalence and Causes of Bulking Filamentous Bacteria in Pulp and PaperMill Activated Sludge Systems", proceedings of the 1997 TAPPI Environmental Conference, p. 553, Minneapolis-Saint Paul, MN.

Figure 1. Study 1:

Sludge SVI vs. Temperature

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Figure 4. Study 1:

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Figure 5. Study 1:

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Figure 6. Study 1:

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Figure 7. Study 1:

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Figure 8. Study 1:

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Figure 9. Study 1:

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Figure 10. Study 2:

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Figure 11. Study 2:

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Figure 12. Study 2:

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Figure 13. Study 2:

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Figure 14. Study 2:

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Figure 15. Study 2:

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Figure 16. Study 2:

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Figure 17. Study 2:

COD Removal % vs. Time at 40-45C

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Figure 18. Study 2:

Effluent COD vs. Time at 40-45C

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