ENVIRONMENTAL ENGINEERING SCIENCEVolume 24, Number 3, 2007© Mary Ann Liebert, Inc.DOI: 10.1089/ees.2005.0046

Aerobic vs. Anaerobic-Aerobic Biotreatment:Paper Mill Wastewater

M. Lerner,1 N. Stahl,2 and N. Galil1

1Department of Civil and Environmental EngineeringTechnion—Israel Institute of Technology

Haifa 32000, Israel2American-Israeli Paper Mills (AIPM)

Hedera 38101, Israel

ABSTRACT

The operation of existing activated sludge treatment plants at paper mills in the world is often characterizedby disturbances, caused by bad biosolids settling, sludge bulking, and reduction of biomass activity. This pa-per presents a comparison between the results obtained in a case study based on a full-scale activated sludgetreatment (AST) system working as the only biotreatment and the AST working with anaerobic pretreatmentby upflow anaerobic sludge blanket (UASB). The study was performed at paper mills in Hedera, Israel. Thedata was collected during 8 years. The anaerobic/aerobic system (2002–2004) provided steady operation per-formance, while the AST system without anaerobic pretreatment (1997–2001) produced effluent character-ized by oscillatory values. The results indicate much lower levels of total suspended solids in the anaero-bic/aerobic treatment system effluent, 5–10 mg/L, compared to 50–85 mg/L in the AST system only. Similarimprovement was observed in terms of the organic matter removal: 220–250 mg/L vs. 80–120 mg/L as CODt,and 20–40 mg/L vs. 4–7 mg/L as BODt with low corresponding fluctuations. After the operation of the anaer-obic pretreatment, the sludge age increased from 8–10 days to 30–40 days and the activated sludge perfor-mance substantially improved: the sludge volume index decreased from 200 to 80 mL/g with significant re-duction of the excess sludge amount. The data collected during the operation monitoring clearly indicates thatthe anaerobic process followed by AST considerably reduced electrical and chemicals consumption in the bi-ological treatment plant and provided low operational cost.

Key words: activated sludge; anaerobic pretreatment; UASB; paper mill

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*Corresponding author: Department of Civil and Environmental Engineering, Technion—Israel Institute of Technology, Haifa32000, Israel. Phone: �972-4-8292645; Fax: �972-4-8293629; E-mail: galilno@tx.technion.ac.il

INTRODUCTION

THE PAPERMAKING INDUSTRY is one of the biggest usersof water and generates large quantities of highly pol-

luted wastewater. Great pressure from environmental au-

thorities, local regulations, consumers, and economic is-sues has forced the papermaking industry to apply tech-nologies for wastewater minimization and proper waste-water treatment. Water recycle/reuse (closing the loops)is the most common technique used for fresh water and

wastewater reduction in paper industry. However, watersystem closure without appropriate treatment is associ-ated with an increase in organic and inorganic contami-nant concentration in mill process water streams. It mayhave a negative impact on product quality, and could re-sult in accelerated scaling and corrosion problems of pa-permaking equipment, odors in water and paper, depositsof sticky material, increase in biological activity in themill process water (slime growth), and influence on ef-ficiency of paper making chemicals (Malmqvist et al.,1999; Berard, 2000). On the other hand, the increasingreuse of water and the reduction in fresh water con-sumption by paper mills are causing extensive problemsin the biological wastewater treatment, especially in theactivated sludge process. Therefore, an important part ofthe paper mill water closure strategy is the upgrading ofexisting biological wastewater treatment units and/or ap-plying additional technologies.

Biological wastewater treatment has been traditionallywidely utilized in paper industry in order to purify theirwastewater containing lost cellulose fibers, resin acids,plant sterols, lignin, starch, chlorinated organic com-pounds, fatty acids, sulphur compounds, dyes, and oth-ers. This wastewater is characterized by relatively highchemical and biochemical oxygen demand (COD, BOD)and suspended solids (mainly fibers). During the past 2decades, activated sludge treatment (AST) has becomemore popular for pulp and paper mill wastewater purifi-cation showing high organic matter removal efficiency.However, AST is highly sensitive to external distur-bances (both physical and chemical nature) created dur-ing the purification (Hynninen, 1998). In case of paper

industry wastewaters, these disturbances may vary fromsudden changes in pH or organic loading rate (becauseof chemical spills, for example) to toxicity caused byresins acids or chlorinated organic compounds. Sarlin etal. (1999) reported that unusual changes in the paper millwastewater characteristics (because of spillage of bio-cides, oils, dyes, acids, and others) can reduce biomassactivity and have a negative effect on biosolids set-tleability. The surface active material accumulationand/or certain species of bacteria could encourage foam-ing in the aerated basin of the biological plant and affectboth AST performances and effluent quality (Richard,2003). Another typical problem of the AST plant that de-teriorates effluent quality is sludge bulking caused by ex-cessive growth of filamentous bacteria, especially Mi-crothrix parvicella (Richard, 2003). Webb (1994)reported that bulking in AST plant of paper mills is of-ten connected with wastewater composition containingeasily biodegradable materials such as sugars andstarches. In addition to the low organic load (i.e., lowF/M ratio), a limited supply of nutrients (phosphorousand/or nitrogen) and some essential metals is typical forpaper wastewater. These limitations could also be the rea-son for sludge bulking and loss of “light” biosolids withthe effluent (Hynninen and Ingman, 1998).

In the 1990s, the anaerobic biological treatment pro-cesses became very popular for purification of paper millswastewater. Webb (2002) and Stahl et al. (2004) reportedthat combination of anaerobic pretreatment with aerobicbiotreatment provided excellent results in the removal oforganic matter and other problematic substances presentin paper mill effluent. Kortikaas et al. (1994) and Eroglu

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Table 1. AIPM wastewater composition and UASB effluent quality.

Parameter Wastewater UASB effluent

1 pH 6.73 � 0.58 6.59 � 0.172 Total COD, mg/L 2300 � 590 760 � 3603 Soluble COD, mg/L 1950 � 530 575 � 2304 Total BOD, mg/L 1180 � 270 410 � 2805 Soluble BOD, mg/L 990 � 260 300 � 2406 TSS, mg/L 160 � 110 170 � 1407 Ca Hardness, mg/l as CaCO3 540 � 140 490 � 808 Alkalinity, mg/l as CaCO3 170 � 110 940 � 1509 TKN, mg/L 22.9 � 6.9 22.4 � 6.810 NH4-N, mg/L 5.7 � 2.5 9.1 � 4.311 Total P, mg/L 6.5 � 5.1 13.7 � 10.912 PO4-P, mg/L 1.3 � 1.3 6.2 � 3.713 SO4, mg/L 140 � 52 65 � 3014 VFA, mg/L 410 � 207 6 � 515 Temperature, °C 34.8 � 3.1 35.5 � 2.9

Abbreviations: BOD, biochemical oxygen demand; COD, chemical oxygen demand; TKN, total Kjeldohl nitrogen; TSS, totalsuspended solids.

et al. (1994) have found that anaerobic-aerobic treatmentsystems produced effluent of much better final quality atlower capital and operational cost than anaerobic or aer-obic processes separately.

It should be noted that most of the anaerobic-aerobictreatment studies and reports based on full-scale instal-lations have been carried out for wastewater purificationoriginating from one specific (single) paper mill produc-ing one type of paper. These wastewaters might be char-acterized by a relatively uniform composition, regardingorganic and hydraulic loads, and this could enable goodbiomass acclimation. However, there are some paper-making mills operating several lines for producing dif-ferent kinds of paper on the same site. The general waste-water stream of such mills is characterized by unstable

complicated chemical composition and by fluctuating hy-draulic load during short time periods. The aim of thework was to study technological, environmental, and eco-nomic issues connected with the operation of an anaero-bic system as pretreatment for an aerobic process bothinvolved in the treatment of wastewater from a paper millcomplex producing three different types of paper.

EXPERIMENTAL PROTOCOLS

All of the tests and measurements were carried out atthe American Israel Paper Mills (AIPM group) full-scalewastewater treatment plant (WWTP) in Hedera, Israel.The mills produce approximately 300,000-ton tissue, fine

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Figure 1. Schematic description of the full-scale wastewater treatment plant, before and after 2002.

Table 2. Average activated sludge treatment (AST) performances.

AST before anaerobic AST after anaerobicParameter system installation system installation

1 MLSS, mg/L 5800 � 1300 6200 � 14002 MLVSS, mg/L 4500 � 1000 4200 � 10003 DO in aeration basin, mg O2/L 1.8 � 0.8 2.5 � 1.54 F/M, kg BOD5/kgMLVSS* day 0.14 � 0.06 0.06 � 0.035 OUR, mg O2/L*h 23 � 11 16 � 66 SOUR, mgO2/gr MLVSS*h 5.2 � 2.4 3.9 � 1.47 SRT, days 8 � 2 31 � 58 HRT, h 30 � 4.1 30 � 1.99 SVI, mL/g 196 � 122 80 � 19

Abbreviations: DO, dissolved oxygen; MLSS, mixed liquor suspended solids; HRT, hydraulic retention times; SRT, sludge retention time; SVI, sludge volume index.

and packaging paper per year and require about 2,600,000m3 of fresh water per year. The wastewater average flowis 250 m3/h, and its composition is shown in Table 1.

WWTP consists of an anaerobic pretreatment stage ac-complished and operated since April 2002, and an acti-vated sludge system, operated since 1980 (Fig. 1).

The aerobic plant includes a rectangular completelymixed aeration basin (8,500 m3) equipped with surfaceaerators and followed by four circular clarifiers forbiosolids separation. The anaerobic system is an upflowanaerobic sludge blanket [UASB, Paques’ internal circu-lation (IC) reactor] with a 24-meter cylindrical, slim de-sign reactor (1,200 m3). The average hydraulic retentiontime in the UASB system is 4.2 h. The overall amountof anaerobic bacteria in the IC reactor is about 32 ton,expressed as volitile suspended solids (VSS). In the

anaerobic reactor the organic matter is converted into bio-gas (methane and carbon dioxide) producing a “gas lift.”Through gas-lift riser pipes the sludge/gas mixture up-ward to a sludge/gas separator on the reactor top. Thebiogas leaves the reactor through this separator and theanaerobic granules return to the reactor bottom via adowner pipe. The biogas production varies from 3,300 to6,200 m3/day depending on the influent organic matterconcentration. Nitrogen and phosphorous deficiency inraw mill wastewater created the need for controlled ad-dition of H3PO4 and CO(NH2)2 to the inlet of the UASBsystem in order to keep the ratio of COD:N:P in level of350:5:1, required for proper anaerobic treatment of thepaper mill wastewater according to the Paques experi-ence. It should be noted that the operation of the UASBallowed decreasing the organic load on the AST by about

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Figure 2. COD (a) and BOD (b) in effluent and removal efficiency (until June 2005).

(a)

(b)

65% as COD (see Table 1). The average performancesof the AST plant before and after the operation of theanaerobic system are given in Table 2.

All the analyses were performed in accordance withStandard Methods (1997). The AST performance wasmonitored during 8 years (from 1997 to 2005). Duringthis period approximately 1,100 and 2,700 samples werecollected for COD and BOD analysis accordingly. Solu-ble concentrations of COD and BOD have been per-formed by filtration through the standard glass microfiberGF/C filter with a 1.2-micron pore size.

RESULTS AND DISCUSSIONS

Organic matter removal

The results of organic matter removal expressed asCOD and BOD (both total and soluble) are summarized

in Fig. 2. These parameters were measured two to threetimes per week and from 2001, the COD concentrationswere daily determined.

Total COD concentrations in the AST effluent during1997 to 2001 (before UASB operation) were significantlyhigher (220 to 250 ppm) in comparison with 2002–2004,after UASB operation (80 to 120 ppm). Similar resultswere obtained also in terms of BOD removal: effluent to-tal BOD concentrations before and after UASB operationwere 20 to 40 and 4 to 7 ppm, accordingly.

Additionally, the period from 1997 to 2001 was char-acterized by high effluent COD and BOD fluctuationswhich were caused by high sensitivity of the AST tochanges in hydraulic and organic loading connected withthe wastewater composition. A significant part of the timethe AST was operated under strong bulking sludge con-ditions, low dissolved oxygen (DO) concentration, lowF/M ratio and nutrient (P and N) deficiency. Moreover,

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Figure 3. CODt in effluent vs. cumulative probability.

occasionally spillage of oils and some other chemicals(especially starches in the year 2000) resulted in tempo-rary biomass activity inhibition, foaming over the aera-tion basin and development of filamentous bacteria. Allthe above-mentioned abnormal conditions lead to insta-bility of the AST performances and relative low averageorganic matter removal (Fig. 2).

The situation drastically changed in 2002 with the in-stallation and operation of the UASB. Both total and sol-uble average COD and BOD levels in the effluent con-siderably dropped off (90 and 7 ppm for total COD andBOD, respectively) in comparison with previous years.Effluent organic matter concentrations became more uni-form (see Fig. 3) and the overall organic compounds re-moval efficiency increased (95.8% for total COD and99.5% for total BOD). The organic removal improvementwas associated with the reduction of the total organicloading on AST after UASB operating at about 65% asCOD.

It should be noted that after the operation of the anaer-obic pretreatment, events of sludge bulking, and foam-ing have never been observed.

Suspended solids removal

During 1997–2001 the average effluent total sus-pended solids (TSS) concentration varied from 50 to 85ppm (Fig. 4) corresponding to average TSS removal ef-ficiency of 85%.

It should be noted that during this period the daily ef-fluent TSS concentration fluctuated within 80–90%.

However, with the operation of UASB, since 2002, theTSS removal improved (95%) with effluent average of 8ppm and relatively low variation (Fig. 5). The resultsdemonstrate that the application of the anaerobic pre-treatment for paper mill wastewater purification im-proved the activated sludge settling characteristics andeliminated sludge bulking problems.

Activated sludge properties

Since the separation between final effluent and bio-mass is performed in the secondary clarifier of the ASTplant, the settling characteristics of the mixed liquor sus-pended solids (MLSS) is one of the most important pa-rameters. The average activated sludge performances(Fig. 6) indicate that after the operation of the UASB pro-cess, the settling characteristics of activated sludge im-proved from 200 to 80 mL/g, based on average values ofsludge volume index (SVI). The improved MLSS settlingability not only improved the AST effluent quality, butalso influenced the sludge age (sludge retention temper-ature: SRT). SRT increased from 8–10 days (beforeUASB running) to 30–40 days. It is well known that thelonger the sludge age, the amount of biosolids will belower and more stable.

During 1997–2001 the average quantity of excesssludge was 4–7 ton/day with ash content of 20%. Af-ter the operation of the anaerobic system (2002–2004),the average daily excess sludge amount decreased to1–2 ton and contained about 35% ash, indicating sta-ble sludge.

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Figure 4. TSS in effluent and removal efficiency (until June 2005).

It should be noted that after UASB operation the bio-mass in the aeration basin changed color from a yellowto dark brown and odor problems disappeared.

Chemical consumption and cost comparison

Table 3 presents average daily electricity and chemi-cals consumption as well as annual costs for the biolog-ical treatment plant during the period 1999–2004.

The data in Table 3 demonstrate that the anaerobic pre-treatment improved AST performances, and final efflu-ent quality lowered electricity requirements by 30% andsubstantially reduced the chemicals consumption.

Polymer consumption was reduced by 50% due tolower production of waste biosolids. The nutrient (N andP) demand in the biological systems is correlated with

the organic load. The nutrient demand of the anaerobictreatment was about 60% of the AST plant, as shown inTable 3.

Caustic soda is essential for controlling the pH levelof the anaerobic process, and therefore, the UASB oper-ation is connected with relatively large amounts of it (3.6ton/day). When the pH in the reactor drops below 6, themethanogenic bacteria stop their activity (methane pro-duction) resulting in VFA accumulation and acidificationof the reactor. Therefore, it is very important to keep thepH in the reactor always above 6 (in fact, set point of pHis 6.7) and for this purpose the caustic soda has to beadded. Lately the facilities to strip the CO2 were addedto the anaerobic reactor, and since then the caustic sodaconsumption has been quite lowered (1 ton/day).

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Figure 5. Cumulative probability vs. TSS in effluent.

The total reduction of electricity and chemical con-sumption resulted in a reduction of the operating ex-penses by about 50%, despite of the addition of causticsoda.

SUMMARY

The obtained results have shown that anaerobic pre-treatment, performed by UASB high-rate internal circu-lation reactor (Paques), allowed successful purificationof complicated wastewater generated by mixing ofstreams from producing different kinds of paper. The op-

eration of the anaerobic process significantly improvedthe performance of the aerobic biomass. The paper millanaerobic/aerobic effluent quality, in terms of COD,BOD, and TSS, as well as their fluctuation, dramaticallyimproved compared to the single aerobic treatment. Thiswas achieved due to improved settleability of thebiosolids and higher SRT in the activated sludge process.

The data collected on the full-scale system during amonitoring period of over 2 years demonstrated thatanaerobic/aerobic wastewater treatment was more cost-effective compared to AST only. Electrical and chemi-cals consumption as well as waste sludge production con-siderably reduced wastewater treatment costs.

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Table 3. The wastewater treatment plant (WWTP) main expenses.

Anaerobic � aerobicAerobic treatment only treatment

Parameter Unit 1999 2000 2001 2002 2003 2004

Electricity kWh/day 10,800 10,100 10,200 9,800 7,100 7,000Polymers kg/day 74 64 67 34 19 31Nutrients (PN) kg/day 783 1,464 1,186 679 747 708Caustic soda kg/day — — — 3,600 1,900.a 1,000.a

Annual cost k$/year 752 743 624 607 308 421Cost per m3 $/m3 0.284 0.320 0.291 0.289 0.143 0.177

aAfter addition of CO2 stripping system to the anaerobic tank.

Figure 6. Activated sludge properties (until June 2005).

Anaerobic/aerobic treatment can be used as an impor-tant component of wastewater preparation for further in-ternal paper mill water reuse/recycle (“closing waterloops”).

ACKNOWLEDGMENTS

The project was carried out at the American Israeli Pa-per Mill (AIPM), Hedera, Israel. Mark Lerner is a Ph.D.student, Nathan Stahl is project engineer at AIPM, NoahI. Galil is Professor of civil and environmental engineer-ing at the Technion.

REFERENCES

BERARD, P. (2000). Filling in the holes after closing the loop.PPI 4, 44.

EROGLU, V., OZTURK, I., UBAY, G., DEMIR, I., and KORKURT, E.N. (1994). Feasibility of anaerobic pre-treat-ment for the effluents from haedboard and laminated boardindustry. Water Sci. Technol. 29(5–6), 391.

HYNNINEN, P. (1998). Environmental Control, book 19. Fin-land: Fapet Oy.

HYNNINEN, P., and INGMAN, L.C. (1998). Improved con-trol makes activated sludge treatment more viable. Availablefrom http://www.paperloop.com/db_area/archive/p_p_mag/1998/9811/focus2.htm

KORTIKAAS, S., DOMA, H.S., POTAPENKO, S.A., FIELD,J.A., and LETTINGA, G. (1994). Sequenced anaerobic-aer-obic treatment of hemp black liquors. Water Sci. Technol.29(5–6), 409.

MALMQVIST, Å., TERNSTROM, A., and WELANDER, T.(1999). In-mill biological treatment for paper mill closure.Water Sci. Technol. 40(11–12), 43.

RICHARD, M. (2003). Activated sludge microbiology problems and their control. Available from www.dec.state.ny.us/website/dow/bwcp/DrRichard.pdf

SARLIN, T., HALTTUNEN, S., VUORIRANTA, P., andPUHAKKA, J. (1999). Effects of chemical spills on activatedsludge treatment performance in pulp and paper mills. Wa-ter Sci. Technol. 40(11–12), 319.

STAHL, N., TENENBAUM, A., and HABETS, L. (2004).Finding a better way. PPI 11, 29.

WEBB, L. (1994). Legislative pressure means more changes inthe pipeline. PPI 2, 31.

WEBB, L. (2002). Kidney technology brings success. PPI 4,28.

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