two-stage thermophilic anaerobic–aerobic digestion of waste-activated sludge

ENVIRONMENTAL ENGINEERING SCIENCEVolume 21, Number 5, 2004© Mary Ann Liebert, Inc.

Two-Stage Thermophilic Anaerobic–Aerobic Digestion ofWaste-Activated Sludge

M. Ros* and G.D. Zupancic

National Institute of ChemistryDepartment of Chemistry, Biology and Technology of Water

Ljublijana, Slovenia

ABSTRACT

High-rate two-stage thermophilic anaerobic–aerobic sludge digestion was studied and compared to the re-cent published research. The studied process is, in fact, a combination of two single-stage anaerobic andaerobic processes in the most favorable way possible. The goal was to combine the benefit of anaerobicdigestion—biogas production, with the benefits of aerobic digestion—better COD and VSS removal. Sev-eral combinations were made, from 313 days (anaerobic 1 aerobic), 316 days, 515 days, 3112 days to10110 days HRT. The best process was 3112 days, which showed a VSS removal of 61.8% and CODremoval of 57.4% in just 15 days HRT. Comparison of the processes with recently published researchshowed that the 3112 process is better than most published two-stage processes. All of the two-stage pro-cesses in this paper were performed completely in the thermophilic temperature range in contrast to pub-lished two-stage processes where the second stage was always in the mesophilic temperature range. Thereason for using a completely thermophilic process is in the discovery made in this study, that any fur-ther stage after a first thermophilic stage requires only 2–3% more heat for operation than if the next stagewould be mesophilic. For this little additional input, the processes are worth operating in the thermophilictemperature range completely.

Key words: aerobic digestion; anaerobic digestion; biogas production; heating energy; thermophilic sludgedigestion; two-stage sludge digestion; waste activated sludge

617

*Corresponding author: National Institute of Chemistry, Department of Chemistry, Biology and Technology of Water, Haj-drihova 19, P.O. Box 660, SI-1001 Ljubljana, Slovenia. Phone: 386 1 4760 237; Fax: 386 1 4760 300; E-mail: [email protected]

INTRODUCTION

THE SLUDGE GENERATED in biological treatment pro-cesses of municipal and industrial wastewaters must

be properly treated and disposed of in a manner accept-able to the community and the environment. The two topoptions of the Urban Wastewater Treatment Directive(91/271/EEC, 1991) are avoidance and minimization.

Minimization is especially important in urban areas dueto large quantities of sludge and their transport to the fi-nal disposal site. Accepting the standards in the directive,also a certain level of solids degradation will have to beachieved and sludge pasteurization will be required in thefuture. Today, the most common process for treatingwaste sludge from biological wastewater treatment plantsis still the mesophilic anaerobic digestion process at

35°C. Also used, but less common, is the aerobic diges-tion process. Generally anaerobic processes are currentlythe subject of research, since biogas is a byproduct ofsuch process, and therefore considered advantageous.

Reported degradation of volatile suspended solids(VSS) in the conventional mesophilic anaerobic processis about 40% at retention times (HRT) between 30 and40 days (Borchardt, 1981; Cook, 1986; Parkin and Owen,1986). Reported VSS degradation in the Domzale-Kam-nik WWTP (the largest operating WWTP in Slovenia,200,000 PE) is 44% at a retention time of 50 days(Domzale-Kamnik WWTP, 2001, 2002). Biogas produc-tion averages around 400 L/kg volatile suspended solidsinserted. Some wastewater companies tend to work to 20days retention time, but such processes may bring diffi-culties when temporarily higher loads occur. At the mu-nicipality of Velenje, Slovenia, the anaerobic digesterworks at 20 days retention time with substantial troublesduring the season of high load wastewater, and the di-gester has problems with acidification. Also, the digesterdegradation of solids is less efficient, about 35% in re-tention time of 19 days (Velenje Municipality report,2002).

Anaerobic digestion has two ranges (Cook, 1986). Thefirst range at 35°C is the mesophilic range, and is todaythe most common for sludge digestion. The second rangeat 55°C is the thermophilic range. In this range digestionis much faster and thorough, although specific biogas pro-duction is about the same. Considering much shorter re-tention times the biogas yield is therefore larger.

The aerobic process is today still used at smallerWWTPs and mostly at ambient temperatures; its degra-dation rate is even smaller, about 30% at 50 days reten-tion time (Borchardt, 1981; Aasheim, 1985; EPA, 1990),although there is substantial knowledge about autother-mal thermophilic processes (Messenger et al., 1993; Ri-ley and Forster, 2002). The temperature dependence of

aerobic digestion is shown in Fig. 1. Figure 1 shows thatthe reaction rate is exponentially increasing with tem-perature. However, at some point in the thermophilicrange the process must be inhibited. Research in aerobicdigestion conducted by Ros and Zupancic (2002a) in Fig.2 shows that the exponential behaviour is only valid to45°C. Above this temperature a different behaviour wasrecorded with a maximum degradation at 50°C.

Research in recent years has introduced new advancedmethods of high-rate digestion. Because sludge is ahighly voluminous waste, the goal of such high ratesludge digestion is to remove as much VSS as possiblein the shortest time and also meet the pasteurization re-quirements, which are defined as 70°C/30 min or 55°C/4h of retention time (Roberts et al., 1999). Recent researchand the high-rate processes described in it can be dividedinto two groups. The first, more common one, representsmainly phase-separated anaerobic digestion processes(separated to the acidogenic and methanogenic phase)and in the second group the first phase of the process isaerobic and the second is mesophilic anaerobic. Thesetwo phases then represent the two stages of the advancedsludge digestion processes. All these processes have acommon property. The first stage is mainly thermophilic,either anaerobic or autothermal aerobic, and the secondone is always mesophilic anaerobic. The retention timein the acidogenic phase is generally much shorter (1–3days) than in the methanogenic phase (10 to 20 days).

With such two-phase processes better VSS removal isachieved, and also the requirements for pasteurization aremet. The described two-stage processes demonstrated asignificant reduction in the time required to achieve aspecified removal of VSS compared with a single-stageanaerobic mesophilic process. The results of various au-thors are collected in Table 1.

At the National Institute of Chemistry, Ljubljana,Slovenia, in the Laboratory for Chemistry Biology and

618 ROS AND ZUPANCIC

Figure 1. Aerobic digestion reaction rate as a function of temperature.

Technology of Water thermophilic sludge digestion wasstudied in recent years. The goal of the research was todevelop a high-rate sludge digestion process that wouldcompletely meet the requirements of the future trends in-dicated in the Urban Wastewater Treatment Directive.Combining research results from VSS removal studies inanaerobic and aerobic sludge digestion processes with theidea of a two-stage digestion process, a new two-stageprocess was developed and patented (Ros and Zupancic,2002b). Anaerobic and aerobic process studies led us tothe combined two-stage process, where we tried to unitethe benefits of both types of process. The two-stage pro-cess is actually a combination of two single-stage pro-cesses, both performed in the thermophilic temperaturerange. The objective of this paper is to present the ben-efits and disadvantages of the developed two-stage pro-cess and to compare it to recent research in this field.

MATERIALS AND METHODS

In the experiments an anaerobic and an aerobic reac-tor were used. Figure 2 shows a schematic diagram ofthe anaerobic and aerobic reactors.

The anaerobic reactor was a cylindrically shaped re-actor, with a mixing device, a gas outlet, and a sludgeoutlet all mounted on the top. A dosage flask was con-nected to the sludge inlet, which was on the bottom ofthe reactor, which had an operating volume of 20 L. Thegas outlet was equipped with a water trap and gas wasmeasured with an OPTIFLOW 420 bubble meter device.

Gas composition (vol. % of CO2) was determined witha specially designed burette, where biogas was separatedin NaOH solution. The reactor was also equipped with atemperature regulation device and heated by an electricheater.

The aerobic reactor was a cube-shaped Plexiglas reac-tor with a pyramidal bottom having a mixing device onthe top of the reactor, a temperature regulation device anda dosage flask. The total volume of the reactor was 22.0L, although only 18.2 L were used in practice because ofextensive foaming that appeared while operating underhigh loads. The reactor inlet was on the top, and the out-let at the side. Aeration was introduced in the pyramidalbottom. The maximum flow rate of air into the reactorwas 550 L/h. The reactor was heated by hot water.

The main parameters considered in the experimentswere VSS, chemical oxygen demand (COD), biogas vol-ume, and composition and oxygen concentration. Tem-perature was also monitored. VSS were analyzed by Stan-dard Methods (APHA, 1998), COD was analyzed bySIST ISO 6060 (1996) standard method. For evaluationof the degree of degradation (digestion) of suspendedsolids in the system, the following expression was used:

TWO-STAGE DIGESTION OF WASTE-ACTIVATED SLUDGE 619

ENVIRON ENG SCI, VOL. 21, NO. 5, 2004

Figure 2. Anaerobic and aerobic reactors used in the experiments.

removal rateof volatile 5suspended solids

volatile suspended volatile suspendedsolids supplied 2 solids leaving the system

[volatile suspended solids supplied]

��

��

��

��

620R

OS

AN

DZ

UPA

NC

IC

Table 1. Results of high-rate two-stage sludge digestion processes published in recent years.

Maximum VSS Retention timeAuthors Type of digestion Temperature removal (%) (1st 1 2nd stage) Sludge type

Roberts et al. (1999) Thermophilic—mesophilic anaerobic (55–35°C) 45% 4 h 1 12 days Primary 1 WAS (1;1)Han et al. (1997) Thermophilic—mesophilic anaerobic (55–35°C) 50% 8 1 20 days Primary 1 WAS (1;1)

54.3% 2 1 10 days Primary 1 WAS (1;1)Bhattacharya et al. (1996) Mesophilic—mesophilic anaerobic (35–35°C) 42.7% 2 1 10 days WAS onlyCheunbarn and Pagilla (2000b) Thermophilic—mesophilic anaerobic (62–37°C) 61% 1 1 14 days Primary 1 WAS (1;1)Cheunbarn and Pagilla (2000a) Thermophilic aerobic—mesophilic (62–37°C) 63% 1 1 14 days Primary 1 WAS (1;1)

anaerobicMessenger et al. (1993) Thermophilic aerobic (autothermal)— (above 60– 50% Total 20 days Primary and human

mesophilic anaerobic 37–44°C) First stage 1–3 days sludge (biofiltration)(ratio not specified)

The sludge used for experiments was waste activatedsludge only, collected from Domzale-Kamnik WWTP of200,000 PE. Sludge for each study was from one singlecollection and was stored in a cool room at 4–5°C. Sludgewas fed to the reactors once a day at a concentration of10 g/L. Using such a low concentration of sludge avoidedthe clogging of tubes. As already mentioned, all of theexperiments were performed in the thermophilic temper-ature range. The anaerobic process was carried out at55°C and the aerobic one at 50–55°C.

To perform the two-stage sludge digestion successfullyas described in this paper, preliminary studies had to becarried out. In these preliminary studies VSS degrad-ability was determined in anaerobic and aerobic batch ex-periments. In further preliminary experiments the anaer-obic stage was studied in semicontinuous experiments todetermine the biogas kinetics. On the basis of the resultsthe two-stage process was then constructed and tested.The order of the stages was always anaerobic–aerobic.The basic reason for such an order was the biogas pro-duction, the anaerobic stage has to come first to maxi-mally utilize the biogas potential of the sludge.

RESULTS AND DISCUSSION

Preliminary batch studies showed that aerobic degra-dation is more complete. The study was performed withthe same sludge for both experiments. Observing thetrend lines of degradation in the batch studies (Fig. 3), inthe beginning the anaerobic degradation is slightly faster,but later it is apparent that anaerobic degradation is notcapable of such a high level of degradation as the aero-bic process. This fact also strongly contributed to the de-cision to put the two stages together in the order an-aerobic–aerobic. A further study was dedicated to theanaerobic process in the first stage of the two-stage di-

gestion process. The goal of this study was to establishthe maximum biogas yield for given conditions as a func-tion of HRT. The experiments conducted were semicon-tinuous at HRT of 1, 2, 3, 4, 6, 8, and 10 days. Sludgewas added once daily (except for the 1-day retention timewhen it was added twice daily). The results are presentedin Fig. 4. Specific biogas production was naturally higherat longer retention times. At 10 days HRT the value ofspecific biogas production was 565 L/kg VSS inserted,which well exceeds the average of 400 L/kg VSS in-serted. This average was already reached at a HRT of 3days. The maximum biogas production, expressed inL/day per reactor volume, also occurs at 3 days HRT.This value in a way expresses the fastest biogas produc-tion, considering specific biogas production and the vol-ume of the reactor. It is also evident that at this point themicrobiological conditions are the optimum for biogasproduction. Process stability was observed by monitor-ing the pH of the sludge. The pH was the quickest indi-cator of process quality and stability. If the value wasabove 7.2, the process was good and stable. Between 7.0and 7.2 the process was relatively good but unstable. Be-low 7.0 the process shifted completely into the acido-genic phase and biogas production literally stopped. Thegeneral stability of the process was good. Precautions instarting the procedure had to be considered. Startup re-quired a relatively large amount of sodium bicarbonate.Usually after a period of 1.5 retention times the processwas in equilibrium and the conditions were constant.

Based on the results of preliminary research, the HRTof the two-stage processes were chosen. The first stageof the process was anaerobic and the second one was aer-obic. More thorough digestion under aerobic conditionsand biogas production dictated the anaerobic–aerobic or-der of the stages. The basic idea was to win as much bio-gas as reasonably possible from anaerobic digestion, andthen to achieve the best degradation level with aerobic



Figure 3. Preliminary batch studies for designing the two-stage process.

digestion. The two-stage process was performed withHRT of 313 days (3 days anaerobic 1 3 days aerobic),316 days, 515 days, 10110 days, and 3112 days. Themost often used HRT of the anaerobic stage was 3 days,because at this point the fastest biogas production oc-curred. The aerobic stage was then adjusted to see howdegradation occurs. The two-stage process of 313 and316 were also compared to the 6 and 10 days anaerobicprocess only, to see the beneficial effect of the combi-nation. The 515 days experiment was performed to com-pare it to the 316 experiment and observe the impact ofdifferent HRT of the aerobic stage. The 10110 days pro-cess was performed to see the maximum level of degra-dation: 3112 was performed to see the effect of a longerHRT of the aerobic stage on the two-stage process. Theresults of the two-stage digestion experiments are shownin Table 2.

Comparing these values to single-stage experiments,the improvement is substantial. In the 313 days experi-ment VSS removal (38%; Table 2) is only slightly smallerthan 6 days anaerobic digestion only (39.7%; Fig. 6).

However, COD removal at 6 days HRT in anaerobic di-gestion (29.0%) is much less than in the two-stage pro-cess (41.8%; Table 2). In the 316 days experiment VSSremoval is slightly smaller compared to the 10-day sin-gle-stage experiment, but COD removal is much better.This shows the benefit of the two-stage process. In the515 experiment it is evident that the longer aerobic stagehas a beneficial impact on overall digestion. The 316 ex-periment gives equal results with 1-day shorter retentiontime (Table 2). The only disadvantage is that the specificbiogas production is much lower, as is obvious becauseof the shorter anaerobic stage HRT. The favorable im-pact of a longer aerobic stage is even more obvious if the3112 and 10110 days experiments are compared. Theresults are equal at a difference of 5 days in HRT.

Comparing two-stage thermophilic anaerobic-aerobicsludge digestion to conventional mesophilic sludge di-gestion, the improvement is enormous. Conventionalmesophilic sludge digestion achieves 40% VSS degra-dation in 30- to 40-days HRT. Two-stage sludge diges-tion achieves 61.8% VSS degradation in just 15 days.


Figure 4. Biogas production in the first anaerobic stage of the two-stage process.

Table 2. Results of two-stage anaerobic–aerobic digestion experiments.

Two stage process (anaerobic 1 aerobic HRT; in days)

3 1 3 3 1 6 5 1 5 3 1 12 10 1 10

Total HRT (days) 6 9 10 15 20

VSS removal (%) 38.0 49.0 49.0 61.8 62.0VSS removal (%)—anaerobic stage 33.4 37.4 42.9 38.1 49.2VSS removal (%)—aerobic stage 7.30 17.5 9.80 38.4 25.1Specific biogas (L/kg VSS inserted) 395 373 422 432 565Specific biogas production 0.52 0.35 0.36 0.23 0.22

(L/day per liter reactor volume)COD removal (%) 41.8 51.0 53.9 57.4 57.5

COD removal of anaerobic digestion only (%) 29.0 — 43.8 — —

Forty percent VSS degradation can be achieved with a6- to 7-day retention time. This is four to five times fasterthan conventional mesophilic digestion. This means thatreactor volumes can be reduced by 75 to 80%.

Comparison of the two-stage thermophilic anae-robic–aerobic sludge digestion to high-rate two-stagesludge digestion processes recently published (Table 1)shows that the results of the two-stage digestion describedin this paper are in the same range. The two-stage di-gestion described in this paper reached 61.8% of VSS re-moval in 15 days, which is better than most of the pro-cesses in Table 1. Only Cheunbarn et al. (2000a) reportedbetter results. However, to clearly specify which processwould be best, many more parameters would have to becompared. VSS removal, for instance, under the sameconditions can vary very much depending on the sludgetype and its consistency. Doubtless, all of the reportedprocesses are very good; however, differences can alsooriginate in the type of sludge (its consistency andbiodegradability). This means that even if the two-stageprocess described in this paper shows, for example, 62%VSS removal; in some other case when applied to an-other sludge this VSS removal could be very different.The improvement that may indicate that the two-stageprocess described in this paper has potential for betterVSS removal is in the fact, that all of the two-stage pro-cess is operating in the thermophilic zone. In the ther-mophilic zone all digestion processes generally operatefaster.

The only real problem in thrmophilic digestion is thehigh energy and heat requirements. Compared to con-ventional mesophilic sludge digestion these requirementsare much greater. This is also one of the reasons whyonly the first stage of the recently published two-stageprocesses (Table 1) is thermophilic.

The sludge digestion energy requirements usually con-sist of mixing, pumping, and aeration (Metcalf and Eddy,

1991) if the digestion is aerobic. These are generally thesame as in conventional mesophilic digestion. The heat-ing requirements of sludge digestion usually consist ofcovering the heat losses of the digestion process and heat-ing the sludge to the operating temperature. The heatingrequirements in thermophilic sludge digestion are muchhigher than in conventional mesophilic digestion (usuallyby a factor of 2). The solution to the high heating re-quirements is heat regeneration between the warm out-flow sludge and the cold inflow sludge. With this proce-dure the heating requirements of thermophilic digestioncan be brought to the same level as in mesophilic diges-tion (see Fig. 5).

To illustrate the size of the heating requirements andthe ratio between the heat losses of digestion and sludgeheating the case of a WWTP of 500,000 PE was studied(a similar detailed study is described in Zupancic and Ros, 2003). A two-stage sludge digestion as described inthe present paper was applied to the case of a WWTP of500,000 PE. The first and main conclusion is that the heatlosses of digestion are a very small portion of the sludgeheating requirements (Fig. 7), being only 2–3%. The heatlosses are mostly dependent on the structure of the di-gester. In our calculations the same structure of the di-gesters was applied, as it is present in the WWTP ofDomzale-Kamnik (200,000 PE). Once a structure type ischosen, then the heat losses depend only on the surfaceof the transformation, and the size of the surface dependson the digester size and the digester size depends on theHRT. Therefore, the heat losses are dependent on theHRT, when the structure is chosen.

The finding that the heat losses of digestion are onlya very small portion of the sludge heating requirementsleads to the following conclusions.

No matter how short the first stage of the two-stagedigestion is, the heat requirements are practically thesame, because the main part of the heat requirement is



Figure 5. Comparison of heat requirements of various types of digestion for a WWTP of 500,000 PE.

sludge heating, and the sludge has to be heated to the op-erating temperature. The duration of first stage affects theheating requirements very little.

The heat losses of the digestion process are very small;therefore, the duration of the overall process affects theheat requirements very little. This means that any fol-

lowing stages after the first stage are very easily kept atthe thermophilic temperature. Even more, when after thefirst stage the sludge is led directly to the second stage,and this stage has no heating, sludge cools down by only3% of the heat that was needed for sludge heating (thisamount represents the heat losses of the digestion). This


Figure 6. Scheme of the two-stage sludge digestion system.

Figure 7. Comparison of heat losses of digestion and sludge heating at thermophilic conditions.

means that the sludge cools by only 1–5°C. In our case,where the second aerobic stage was operating at50–55°C, the aerobic digester has no heat requirements.Considering the last conclusion, it is sensible to keep thetwo-stage process in the thermophilic temperature rangeat all times. The additional heat required is only 2–3%,but the benefits of the thermophilic process are substan-tial.

As mentioned above, thermophilic sludge digestionhas much higher heating requirements than mesophilicsludge digestion. This was considered the main problemin thermophilic sludge digestion. However, a solution canbe provided by the installation of a plain heat exchanger.The task of this regenerative heat exchanger is to drawheat from the warm outflow sludge and release it to thecold inflow sludge (details are shown in Fig. 6). In sucha case more than 50% of the heat requirements can beregenerated. An illustration of heat requirements formesophilic and thermophilic digestion is shown in Fig.5. When 50% of the heat requirements are recovered,thermophilic digestion is brought to the same level as themesophilic one as concerns the heat requirements.

CONCLUSIONS

The subject of this paper is two-stage thermophilicanaerobic-aerobic sludge digestion. The goal of the re-search was to combine anaerobic and aerobic treatment,to achieve better sludge degradation than in a single-stagetreatment. For successful performance of two-stage di-gestion preliminary studies were carried out. The resultsshowed that aerobic digestion is better than anaerobic.This fact, and the fact that anaerobic digestion producesbiogas, which is essential for sustaining the heat and en-ergy requirements of the digestion, dictated the anaero-bic–aerobic order of sludge digestion. The two-stageanaerobic–aerobic sludge digestion was performed at313, 316, 515, 10110, and 3112 days (anaerobic1aerobic) HRT. The most often chosen anaerobic reten-tion time was 3 days, because at that retention time bio-gas production was the highest. The 515 days experi-ment was carried out for comparison with 316 days, todetermine the impact of different HRT of the aerobicstage on the overall process. The 10110 process was car-ried out to achieve maximum sludge degradation. The3112 process was again performed for establishing theimpact of longer HRT of the aerobic stage to the two-stage process. The process studied in this paper is all op-erated in the thermophilic temperature range. This pro-cess showed a VSS optimal removal efficiency of 61.8%at 15 days retention time. Comparison of VSS removalof the 31 6 to 515 process and 10110 and 3112 pro-

cesses showed that a longer aerobic digestion time actu-ally improves the overall efficiency. The results of thetwo-stage digestion described in this paper are all in thesame range as the ones cited in the literature (Table 1).The real gain of this research is that all of the digestionoccurs in the thermophilic range and therefore has thepotential for better digestion. A comparison of heatingrequirements for thermophilic and mesophilic digestionconfirmed that thermophilic digestion requires abouttwice the heat for sustaining the operation. This is thereason why thermophilic digestion never really evolvedand even the two-stage processes developed so far usethermophilic digestion only in the first, shorter stage.However, analysis of the heating requirements revealedthat the major portion of the heating requirements is re-quired for sludge heating (97–98%). That means that ifthe sludge is already heated to thermophilic temperature,it takes only 2% additional heat to maintain the next stagein the thermophilic zone. If this additional heat is notadded to the system the next stage is cooled by only1–5°C. Therefore, it is sensible to have all of the diges-tion in the thermophilic zone, not only the first stage.Even more, with introduction of heat regeneration, whichcan be performed with a plain heat exchangers the heat-ing requirements are brought down to the same level asin mesophilic digestion.

REFERENCES

AASHEIM, S.E. (1985). Sludge Stabilization: Manual of Prac-tice No. FD-9. Washington, D.C.: Water Pollution ControlFederation: Task Force on Sludge Stabilization.

APHA, AWWA, WEF. (1998). Standard Methods for the Ex-amination of Water and Wastewater. Washington, D.C.: Au-thor.

BHATTACHARYA, K.S., RICHARD M.L., VALLING, D.A.,FARREL, J.B. (1996). Volatile solids reduction in two phaseand conventional anaerobic sludge digestion. Water Res.30(5), 1041–1048.

BORCHARDT, J.A. (1981). Sludge and Its Ultimate Disposal.Collingwood, MI: Ann Arbour Science.

CHEUNBARN, T., and PAGILLA, R.K. (2000a). Aerobic ther-mophilic and anaerobic mesophilic treatment of sludge. J.Environ. Eng. 126(9), 790–795.

CHEUNBARN, T., and PAGILLA, R.K. (2000b). Anaerobicthermophilic/mesophilic dual-stage sludge treatment. J. En-viron. Eng. 126(9), 796–801.

COOK, E.J. (1986). Anaerobic Sludge Digestion: Manual ofPractice No. 16. Alexandria VA: Water Pollution ControlFederation: Task Force on Sludge Stabilization.

DOMZALE-KAMNIK WWTP. (2001). Annual WWTP Work-



ing Report for 2000 (in Slovene: Porocilo za delo v CCN zaleto 2000. Domzale: Wastewater Treatment Plant Domzale-Kamnik.

DOMZALE-KAMNIK WWTP. (2002). Annual WWTP Work-ing Report for 2001 (in Slovene: Porocilo za delo v CCN vletu 2001. Domzale: Wastewater Treatment Plant Domzale-Kamnik.

EPA. (1990). Autothermal Thermophilic Aerobic Digestion ofMunicipal Wastewater Sludge. Washington, D.C.: Risk Re-duction Engineering Laboratory.

HAN, Y., SUNG, S., DAGUE, R.R. (1997). Temperature-phased anaerobic digestion of wastewater sludges. Water Sci.Technol. 36(6–7), 367–374.

MESSENGER, J.R., DE VILLIERS, H.A., and EKAMA, G.A.(1993). Evaluation of the dual digestion system: Part 1:Overview of the Milnerton experience. Water SA 19(3),185–192.

METCALF AND EDDY. (1991). Wastewater Engineering:Treatment, Disposal, and Reuse. Boston, MA: Irwin/Mc-Graw-Hill.

PARKIN, F.G., and OWEN, F.W. (1986). Fundamentals ofanaerobic digestion of wastewater sludges. J. Environ. Eng.112, 867–920.

RILEY, D.W., and FORSTER, C.F. (2002). An evaluation ofan autothermal aerobic digestion system, Trans. I. Chem. E.80(Part B) 100–104.

ROBERTS, R., DAVIES, W.J., and FORSTER, C.F. (1999).Two-stage thermophilic-mesophilic anaerobic digestion ofsewage sludge. Trans I. Chem E. 77(Part B), 93–97.

ROS, M., and ZUPANCIC, D.G. (2002a). Thermophilic aero-bic sludge digestion of waste activated sludge. Acta Chem.Slovenica 49, 931–943.

ROS, M., and ZUPANCIC, D.G. (2002b). Procedure and de-vice for stabilization and mineralization of sludges fromwastewater treatment plants in thermophilic range. Sloven-ian Intellectual Property Office, Patent Declaration No.200200254, Ljubljana 2002.

URBAN WASTE WATER TREATMENT DIRECTIVE.(1991). 91/271/EEC. Official Journal of the European Com-munities, L135/40-52, Brussels, 30 May 1991.

VELENJE MUNICIPALITY. (2002). Annual WWTP WorkingReport for 2002 (in Slovene: Porocilo za delo v cistilninapravi Velenje v letu 2002). Slovenia Wastewater TreatmentPlant Velenje (Electronic version).

ZUPANCIC, D.G., and ROS, M. (2003). Heat and energy re-quirements in thermophilic anaerobic sludge digestion. Re-newable Energy 28, 2255–2267.


two-stage thermophilic anaerobic–aerobic digestion of waste-activated sludge

Documents