EFFECTIVE MANAGEMENT OF SEWAGE SLUDGE

Sequential anaerobic/anaerobic digestion for enhanced sludgestabilization: comparison of the process performance for mixedand waste sludge

M. Concetta Tomei & Nicola Antonello Carozza

Received: 19 March 2014 /Accepted: 29 May 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Sequential anaerobic-aerobic digestion has beendemonstrated as a promising alternative for enhanced sludgestabilization. In this paper, a feasibility study of the sequentialdigestion applied to real waste activated sludge (WAS) andmixed sludge is presented. Process performance is evaluatedin terms of total solid (TS) and volatile solid (VS) removal,biogas production, and dewaterability trend in the anaerobicand double-stage digested sludge. In the proposed digestionlay out, the aerobic stage was operated with intermittentaeration to reduce the nitrogen load recycled to the wastewatertreatment plant (WWTP). Experimental results showed a verygood performance of the sequential digestion process for bothwaste and mixed sludge, even if, given its better digestibility,higher efficiencies are observed for mixed sludge. VS removalefficiencies in the anaerobic stage were 48 and 50 % for wasteand mixed sludge, respectively, while a significant additionalimprovement of the VS removal of 25 % for WAS and 45 %for mixed sludge has been obtained in the aerobic stage. Thepost-aerobic stage, operated with intermittent aeration, wasalso efficient in nitrogen removal, providing a significantdecrease of the nitrogen content in the supernatant: nitrifica-tion efficiencies of 90 and 97% and denitrification efficienciesof 62 and 70 % have been obtained for secondary and mixedsludges, respectively. A positive effect due to the aerobic stagewas also observed on the sludge dewaterability in both cases.Biogas production, expressed as Nm3/(kgVSdestroyed), was0.54 for waste and 0.82 for mixed sludge and is in the rangeof values reported in the literature in spite of the low anaerobicsludge retention time of 15 days.

Keywords Sequential digestion . Anaerobic-aerobicdigestion . VS removal . Nitrogen removal . Sludgedewaterability . Intermittent aeration . Nitrogen fate

Introduction

Sludge stabilization is one of the critical steps in sludgemanagement because its performance affects both thesludge amount and its quality: requirements for efficientstabilization are a significant reduction of the solid frac-tion and, at the same time, the improvement of sludgequality both in terms of toxic pollutants, pathogens, andviruses. Optimization of the energy balance in the diges-tion process has led to prefer anaerobic digestion to theaerobic one given the possibility of energy recovery withthe produced biogas. However, anaerobic process is morecomplex and difficult to manage and control, so cost-effective only for medium-high size plants (an indicativepotentiality limit value is 30,000 PE). On the other hand,aerobic digestion requires additional energy for aerationbut is more effective in removal of residual toxic pollut-ants in the sludge and in sludge sanitation. So both pro-cesses have positive and weak features to deal with and toconsider for performance optimization. In light of thesecharacteristics, the combination of two stages in sequence,anaerobic-aerobic has been proposed and investigated as apromising strategy to improve the sewage sludgedigestibility. Kumar et al. (2006a, b) pointed out that thepotential advantages of this approach arises from thedifferent reaction environments (anaerobic and aerobic)provided to attain optimal biodegradability conditionsfor the different volatile solid (VS) sludge fractions. Inthe last decades, dual-step digestion has received in-creased interest and previous studies have demonstratedthe validity of this approach to improve the performance

Responsible editor: Philippe Garrigues

M. C. Tomei (*) :N. A. CarozzaWater Research Institute, C.N.R., Via Salaria km 29.300,C.P. 10-00015 Monterotondo Stazione Rome, Italye-mail: tomei@irsa.cnr.it

Environ Sci Pollut ResDOI 10.1007/s11356-014-3130-2

of conventional digestion processing (Kumar et al. 2006a;Parravicini et al. 2008; Zupancic and Ros 2008) due toincreased VS removal eff ic iency and improveddewatering properties of the biosolids.

An additional positive effect of the post-aerobic digestionstep is the ammonia nitrogen removal in the supernatantstream that can significantly reduce the nitrogen load recycledto the wastewater treatment plant (WWTP) as observed formixed sludge digestion by Zupancic and Ros (2008) withnitrification efficiency up to 85 % and by Parravicini et al.(2008) and Kim and Novak (2011) reporting ammonia remov-al efficiency higher than 90 %.

In the previous scientific literature, reported investiga-tions on anaerobic-aerobic digestion are mainly related tomixed sludge (Novak et al. 2011; Parravicini et al. 2008;Tomei et al. 2011a) but the worse digestibility of second-ary sludge suggests good potentialities of this technolog-ical solution also for waste activated sludge (WAS). Thisapplication is of interest for high-size wastewatertreatment plants where the separation of primary andwaste sludge could be feasible and advantageous.According to Mininni et al. (2004), the rationale forsludge separation comes from the different characteristicsof primary and secondary sludges (in terms of content ofpollutants and nutrients), digestibility and dewateringcharacteristics. WAS is less polluted than primary sludge,so its separate treatment can be advisable for safety inagricultural reuse, while primary sludge can be muchbetter thickened, digested, and mechanically dewatered(Koop and Dichtl 2001) with a consequent positive ener-gy balance in terms of energy demand and recovery.Sludge separation can be more easily implemented inhigh-size WWTPs where it is more likely the availabilityof two or more digestion units to be converted forseparate digestion of primary and secondary sludgesand/or for the implementation of the aerobic post-treat-ment unit. Preliminary promising results on the feasibilityof the sequential digestion applied to secondary sludge arereported in Tomei et al. (2011b).

Objective of this paper is to present a complete feasi-bility study of the sequential digestion applied to realwaste and mixed sludge by testing the process perfor-mance in terms of total solids (TS), VS, and chemicaloxygen demand (COD) removal, biogas production, anddewaterability trend in the double-stage digested sludge.Nitrogen removal through simultaneous denitrification,achieved by operating the aerobic stage with intermittentaeration, is also investigated. Although sequential diges-tion has been investigated in previous studies on mixedsludge, the application to secondary sludge was not ex-tensively investigated, and the comparison of the sequen-tial digestion performance, including the nitrogen removalin the aerobic phase for WAS and mixed sludge, produced

in the same full scale WWTP, is presented for the firsttime in this paper.

Materials and methods

Sludge

Primary and secondary sludges utilized in this study wereprovided by the Rome North Wastewater Treatment Plant.The plant is a conventional activated sludge system includingscreening, primary clarification, and secondary treatment andserves about 700,000 P.E. The influent COD average value is200 mg/L that is quite low in comparison to the range of CODvalues of 210–740 mg/L reported in Henze et al. (1987) forsettled wastewater in EU countries and to the COD values of500 and 650mg/L reported in Gray (1992) for raw wastewaterin USA and UK, respectively.

Secondary sludge was obtained for each feed step from theaeration basin, then thickened for 18–24 h, while primarysludge, due to practical reasons of accessibility, was obtainedonce a week and was stored at 4 °C until used. Sludgethickening allowed to reach the high VS loads utilized in theexperiments. The mixed sludge sample utilized in the exper-iments was prepared by mixing primary and secondarysludges in the ratio 1:1 on VS basis. This ratio has beenchosen according to the data reported in Vesilind andSpinosa (2001) and can be considered as representative ofthe primary and secondary sludge production.

Reactors

Lab scale reactors utilized in this study were 7.4-l cylindricalglass vessels. Both of the reactors were equipped with me-chanical stirrers. The reactors were operated in series; theanaerobic digester was fed daily with real WAS or mixedsludge (provided by the full scale plant). Depending on thesludge concentration and on the established organic loads, thesludge samples were thickened for different times (in therange of 18–24 h) prior feeding. The aerobic reactor was feddaily with the anaerobic digested sludge withdrawn from thefirst reactor. In both cases, the feed phase is very fast (it can beconsidered practically instantaneous) and the reactors aremixed during the feed phase.

The lab scale plant lay out is shown in Fig. 1.The first reactor, operated under anaerobic conditions, was

equipped with a thermostatic jacket and a control devicekeeping the temperature at 37±0.5 °C. The working volumewas 7 L and the sludge retention time (SRT) was controlled at15 days. The biomass for the inoculum was taken from a fullscale anaerobic sludge digester. At the beginning of the start-up phase, it was filled with the inoculum and sewage sludge(ratio inoculum/sludge 1:1).

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The second reactor, which was operated under aerobicconditions, had a working volume of 4.5 L, and air wassupplied by a compressor connected to a medium (4–9 mm)bubble glass diffuser able to maintain the concentration ofdissolved oxygen (DO) in the range of 3–4mg/L. This intervalof values was established according to literature data(Zupancic and Ros 2008), suggesting 3 mg/L as the minimumvalue required to have nitrification in a concentrated sludgesuspension where mass transfer resistances are significantlyhigher than in the mixed liquor of the WWTP aeration thankwhere 2 mg/L concentration of dissolved oxygen is generallysuggested. The air flow rate, in order to reduce additionalenergy consumption, was kept at the minimum value neces-sary to maintain the dissolved oxygen in the reactor at theprefixed interval of values. Such an operating strategyprevented ammonia stripping. The reactor was operated atroom temperature at SRTof 12 days. During the winter period,it was activated as a control system to maintain the tempera-ture at values ≥20 °C. Intermittent aeration (40 min ON and20 min OFF) was applied in order to achieve simultaneousdenitrification.

The distribution of the aerated and not aerated times hasbeen defined to ensure high VS removal and nitrification/denitrification efficiencies. The higher aerobic time (respectto the anoxic one) is justified because the primary objective ofthe second stage in the sequential digestion process is toensure the completion of the aerobic biodegradation for theVS fractions not biodegradable under anaerobic conditionsand efficient nitrification. According to this criterium, the timedistribution 2/3 aerated and 1/3 anoxic has been chosen andvalidated with preliminary tests (data not shown).

As aerobic inoculum, a mixed liquor sample from thebiological reactor of the Rome North Plant was utilized.

pH was regularly monitored to verify that its valuesremained around neutrality (range of values for both reactors,7.2–7.8) to avoid negative effects on the most sensitive bac-terial species, i.e., methanogens in the anaerobic stage andnitrifiers in the aerobic one. When necessary, pH has beencorrected by addition of alkaline solution.

Bioreactors were operated in series in two subsequentexperimental periods of 6 and 2.5 months with secondary(i.e., WAS) and mixed sludges, respectively. Applied organicload rates (OLRs) were in the range of 1.4–2.5 kgVS/(m3 days).

Analysis

Regular sample collection and analysis were initiated afterstart up once stable operating conditions were reached.

Stable operating conditions were assumed when the diges-tion process exhibited and maintained stable performance(compatibly with the unavoidable variation of the fed sludge)in terms of daily recorded performance parameters, i.e., VSand TS removal efficiencies for both anaerobic and aerobicphases and biogas production for the anaerobic stage.

Sampling from both reactors was performed under well-mixed conditions. Feed, anaerobic digested, and aerobicdigested sludges were analyzed for TS, VS, COD, capillarysuction time (CST), ammonia, nitrite, and nitrate nitrogen.Biogas production and methane fraction were continuouslymonitored. Analytical methods and related devices are report-ed in the following. Sampling and analysis of VS and TS wereperformed daily.

Volatile solidsVolatile solid concentration was measured according

to standard methods (APHA 1998).Nitrogen

Ammonia, nitrites, and nitrates were determined ac-cording to the standard methods (APHA 1998).Capillary suction time

Sludge for CST measurement was prepared by accu-rately mixing the sample (~3 min in a flask equipped witha magnetic stirrer) in order to achieve proper homogeni-zation. The sample was then poured in the test cell and theparameter determined according to the Method 2710G(APHA 1998). No polymers was added.

Air ON/OFF

Anaerobic digested sludge

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V= 4.5 L SRT=12 d T ≥ 20 °C

V= 7 L SRT=15 d T = 37+0.5°C

Biogas

Fig. 1 Lab scale plant lay out andoperating conditions of thebioreactors

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MethaneMethane in the biogas was determined by a gas chro-

matograph Perkin Elmer AutoSystem equipped with aCarboxen 1000 (Supelco) column and a TCD detector.Biogas detection device

The flow rate of biogas produced by the anaerobicreactor was measured by a volumetric counter using aclosed water displacement system with electrical contactsand with an electromagnetic valve to discharge the pro-duced biogas to the atmosphere (Mata-Alvarez et al.1986). The measurement device was controlled by aProgrammable Logic Controller that also provides therecording of signals.

Results and discussion

Performance of the proposed sequential digestion processhas been assessed with the classical reference parametersnamely VS removal efficiency in the two process steps,biogas production, and dewaterability of the digestedsludge. In addition, being the aerobic stage operated withintermittent aeration, the nitrification and denitrificationefficiencies were also evaluated. Additional data of TSremoval and soluble COD fate are reported to give a morecomplete characterization of the process.

A first preliminary consideration for data analysis is relatedto the operating conditions for the two reactors: the anaerobicphase was operated at 15 days SRT while the subsequentaerobic one at 12 days. The first value is derived from theneed to have a sufficiently good anaerobic performance toensure appreciable biogas production and energy recovery, soit was defined in the low range of values reported in thespecialized literature for mesophilic anaerobic digestion(Vesilind and Spinosa 2001). The aerobic SRT has beenchosen taking into account the objective of achieving simul-taneous nitrification denitrification, so it is higher than theprevious values proposed for the aerobic post-treatment(Kumar et al. 2006a; Subramanian et al. 2007) which wereessentially finalized to improve the solid removal and thedewaterability of the digested sludge.

VS and TS removal

Figure 2a–d shows the experimental results for the firstseries of test performed with WAS. Figure 2a, b shows theVS and TS concentration profiles in the feed, anaerobicdigested, and aerobic digested sludges, while in Fig. 2c, d,the removal efficiencies, determined on weekly base andthe related standard deviations, are reported. The samedata for mixed sludge are shown in Fig. 3a–d, while

Fig. 2 VS (a) and TS (b) concentration profiles in the feed, anaerobic, and aerobic digested sludges. VS (c) and TS (d) removal efficiencies and relatedstandard deviations calculated on weekly base. Feed: WAS

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average removal efficiencies for the anaerobic, aerobic,and two-stage process, referring to the entire experimentalperiods, are shown in Table 1.

Figure 2a–d highlights stable performance of the anaerobicreactor fed with WAS with an average VS removal efficiencyof 48±7 %. The observed variation can be attributed to thevariability of the feed VS concentration, which was 2.60±0.4(expressed as % w/w) and to the intrinsic variability of thebiological digestion processes. In the subsequent aerobicstage, an additional VS removal of 25±5 % was achieved,which can be considered a very good result given the lowdigestibility of the waste sludge. Good removal efficiencieswere also observed for TS in the entire experimental period asreported in Fig. 2b. The notable stabilization performancerelated to WAS in both the anaerobic and aerobic phasesmay be attributable to the applied VS load significantly higherthan the average value of ~1 kgVS/(m3 d) generally employed

in anaerobic mesophilic digestion of sewage sludge(Bolzonella et al. 2005).

Improved performance was observed for mixed sludge, asshown in Fig. 3a–d, with VS removal efficiencies of 50±8 and45±5 % for the anaerobic and aerobic phase, respectively.Variability in this case also is mainly due to the variation in theinfluent VS concentration equal to 3.62±0.8 %. This is con-sistent with the better digestibility of mixed sludge due to thepresence of a primary sludge fraction. It is worth noting that inthis case, the removal efficiency in the aerobic stage is com-parable to the anaerobic one. This may be explained with thehigher OLRs employed in this second series of tests (seeTable 1): due to the high VS load, in the anaerobic digestedsludge, there is still a residual VS fraction available for bio-degradation in the subsequent aerobic stage (as it is alsoproved by the higher aerobic OLR) which, consequently,operated at higher efficiency. This finding suggests the

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Fig. 3 VS (a) and TS (b) concentration profiles in the feed, anaerobic, and aerobic digested sludge. VS (c) and TS (d) removal efficiencies and relatedstandard deviations calculated on weekly base. Feed: mixed sludge

Table 1 OLR and removal effi-ciencies of VS and TS detected inthe two digestion phases and inthe sequential process

Feed Parameter Anaerobic reactor Aerobic reactor Sequential process

WAS OLR (kgVS/m3 days) 1.42±0.5 0.83±0.3 –

VS removal efficiency (%) 48±7 25±5 61±7

TS removal efficiency (%) 36±5 19±6 48±9

Mixed sludge OLR (kgVS/m3 d) 2.54±0.6 1.49±0.2 –

VS removal efficiency (%) 50±8 45±5 72±8

TS removal efficiency (%) 41±8 40±7 64±10

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suitability of this technological solution for highly loadeddigesters or in the upgrading of WWTPs when an increaseof the served population occurs.

A comparison with literature data showed that for bothWAS and mixed sludge, the anaerobic VS removal efficiencywas in the intermediate range of values (27–71 %) reported ina survey of 30 anaerobic sludge digesters of municipalWWTPs (Speece 1988) and in the high range of valuesreferred by Bhattacharya et al. (1996) who found percent VSreductions in the range of 26–50 % in conventional anaerobicdigestion of mixed sludge of three WWTPs. Additional VSremoval in the aerobic stage for WAS is comparable to valuesreported in Parravicini et al. (2008) (16 %) and Kumar et al.(2006a) (20 %) for mixed sludge, while in our case, presum-ably because of the higher SRT, higher aerobic VS removalefficiency has been observed for mixed sludge in comparisonto the mentioned literature data.

Furthermore, it is worth noting that given the anaerobicperformance comparable to the maximum efficiency datareported for conventional mesophilic anaerobic digestion,the additional VS removal achieved in the aerobic stageconstitutes a net gain in the reduction of the solids content ofthe stabilized sludge.

Soluble COD

The evolution of soluble COD (CODsol) for the two experi-mental periods is shown in Fig. 4. It is observed a markeddifference in the feed concentration: the mixed sludge is richerthan WAS of soluble COD due to the contribution of theprimary sludge fraction. Hydrolysis of particulate matter inthe anaerobic stage gives rise to a COD increase in theanaerobic digested sludge (from 110±47 to 724±160 mg/Lfor WAS and from 673±148 to 734±120 mg/L for mixedsludge). In the subsequent aerobic phase, a slight decrease isobserved, the still high residual soluble COD in the aerobicdigested sludge (548±180 and 587±151 mg/L for WAS and

mixed sludge, respectively) may be attributable to the hydro-lysis of the particulate COD associated to VS: the high VSremoval causes a marked production of soluble COD which isnot completely biodegraded in the aerobic bioreactor. SolubleCOD pattern is determined by biopolymers (proteins andpolysaccharides) release in the hydrolysis process and, conse-quently, it is related to the sludge dewaterability properties(see dewaterability trend paragraph).

Nitrogen removal

Simultaneous nitrification-denitrification achieved in the aer-obic stage due to intermittent aeration can be of relevance ifwe consider that the supernatant from anaerobic sludge diges-tion is characterized by a high nitrogen content and canrepresent a significant fraction (up to 50 %) of the nitrogenload of a wastewater treatment plant (Zupancic and Ros2008). In Fig. 5a, b, the nitrogen speciation in the supernatant(interesting for the recycle to the plant) from the aerobicdigester detected in subsequent periods of experimentation isreported for secondary and mixed sludges, respectively. Theevolution of the nitrification-denitrification performance ishighlighted in Fig. 5a: there is a progressive improvement ofnitrification after the switching from continuous to intermit-tent aeration mode.

The nitrogen species pattern in Fig. 5a is representative ofthe progressive modification of the culture following theswitching to intermittent aeration. The first days are represen-tative of the first experimental period just after the switchingand, in the subsequent period, we observe a progressiveevolution of the culture and of the consequent nitrogen speciesdistribution.

On the contrary, in Fig. 5b for mixed sludge, we observe astable performance because this experimental run follows thefirst with secondary sludge, so the biomass was grown inalternate aeration conditions and the nitrifying anddenitrifying bacteria are already present in the biocenosis.

Fig. 4 Soluble COD trend in thetwo series of tests

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Ammonia nitrogen in the feed was 852±52 and 896±118 mg N-NH4/L for WAS and mixed sludge, respectively.

Table 2 shows a summary of nitrification-denitrificationefficiencies evaluated from the mass balance of nitrogen inthe supernatant and associated to the VS. In particular, nitrifi-cation efficiency has been evaluated as the percent ratio be-tween the nitrified nitrogen and the ammonia nitrogen avail-able for nitrification, i.e., present in the influent to the aerobicdigester and produced in the aerobic hydrolysis of removedVS. The nitrogen released in the aerobic hydrolysis of VS wasevaluated by assuming an unitary stoichiometric coefficientaccording to the approach proposed in the ASM1 model(Henze et al. 1987) (and considered in all the followingASM series models) for hydrolysis of particulate COD. Itfollows that the released nitrogen is equal to the nitrogencontent of the hydrolyzed VS. Reported values for nitrogencontent in anaerobic digested sludge with reference toEuropean Union (EU 2001) are in the range of 2.5–14 % of

VS depending on the characteristics of the fed sludge and onthe operating conditions of the digestion. In our case, anintermediate value of 10 %, confirmed by experimental mea-surement (data not shown), has been assumed to evaluate thereleased nitrogen to be included as input value for thenitrification/denitrification mass balances.

Denitrification efficiency was evaluated as the ratio be-tween the denitrified nitrogen and the N-NOx available fordenitrification that is the nitrogen present in the influent addedof the amount produced in nitrification.

The aerobic SRT of 12 days was demonstrated to be suit-able for achieving efficient nitrification (up to 97 % ammonianitrogen removal for mixed sludge), while the applied inter-mittent aeration was able to achieve remarkable denitrificationefficiencies (62–70 % for mixed and secondary sludges,respectively).

For practical application of sequential digestion, this pointhas to be taken into account for the design criteria of theaerobic stage. In the literature (Kumar et al. 2006a), lowSRTs are suggested for the aerobic phase that can be consid-ered to be a finishing step to complete the removal processinitiated in the anaerobic one. Instead, if we include theadditional objective of reducing the nitrogen load recycled to

Table 2 Nitrogen removal efficiencies detected in the aerobic reactoroperated at intermittent aeration

Feed Process Efficiency (%)

WAS Nitrification 90±6

Denitrification 62±11

Nitrogen removal 35±7

Mixed sludge Nitrification 97±1.3

Denitrification 70±7

Nitrogen removal 49±5 Fig. 6 Specific biogas production (SGP; Nm3/(kgVSdestroyed) detectedfor WAS (a) and mixed sludge (b)

Fig. 5 Nitrogen speciation in the feed and supernatant from the aerobicdigester detected in subsequent characteristic periods for WAS (a) andmixed sludge (b)

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theWWTPwith the supernatant from the digester, the optimalSRT should be high enough to ensure efficient nitrificationand denitrification.

Biogas production

Specific biogas production detected in the two series ofexperiments is reported in Fig. 6 (panel a for WAS andpanel b for mixed sludge). As above specified, the twoseries of tests have been performed in subsequent periods:WAS was utilized in the first period, then the feed wasswitched to mixed sludge. In Fig. 6a (for WAS digestion),we observe an average SGP value of 0.54±0.15 Nm3/(kgVSdestroyed), while biogas profile in Fig. 6b (reportingdata after the change of the fed sludge from WAS tomixed) shows a progressive SGP increase in the first10 days up to 0.82±0.15 Nm3/(kgVSdestroyed) value whichcan be considered representative of the biogas productionfor mixed sludge. The difference in the specific biogasproduction for WAS and mixed sludge can be explainedwith the better digestibility of the mixed sludge given bythe particulate COD fraction present in primary sludgeand derived from influent wastewater. Methane fraction(representative of potential energy recovery from the bio-gas) is comparable in the two cases (0.66 and 0.67 forWAS and mixed sludge, respectively) and consistent withliterature data (Bousková et al. 2005). The average spe-cific biogas production is in both cases within the rangeof 0.19–1.6 Nm3/(kgVSdestroyed) reported in the literature(Speece 1988; Bolzonella et al. 2005) for mesophilicdigestion of sewage sludge so, even for secondary sludgeat a low anaerobic SRT of 15 days, the experiencedoperating conditions for the sequential digestion ensure agood energy recovery in the anaerobic stage.

Dewaterability trend

A first assessment of the effects of the sequential diges-tion approach on sludge dewaterability was performedthrough CST measurement: data reported in Fig. 7 showfor both examined sludges an increase of CST after theanaerobic stage (i.e., negative effect on dewaterability)from 30±10 to 470±45 s for WAS and from 154±53 to538±78 s for mixed sludge, followed by a decrease afterthe aerobic phase with values of 350±60 and 253±72 sfor WAS and mixed sludge, respectively. CST pattern inthe sequential process can be explained by the evolutionof biopolymers (mainly polysaccharides and proteins) inthe sequential process: anaerobic hydrolysis increases thebiopolymer content which are released in soluble andcolloidal form in the bulk phase. This causes the observedincrease of soluble COD in the anaerobic effluent (seeFig. 4). Biopolymers have a significant affinity for water,exerting a negative effect on dewaterability, and this canjustify the CST increase observed after the anaerobicstage. Conversely, degradation of biopolymers, associatedto the soluble COD decrease and consequent release ofthe bound water, can improve sludge dewaterability(Wang et al. 2006): we observe this in the aerobicdigested sludge characterized by lower soluble COD andreduced CST.

The positive effect is more evident for mixed sludge andconfirms the data reported by of Kumar et al. (2006a) of asubstantial reduction in the CST after sequential mesophilic-aerobic digestion even at low aerobic SRTs (3 days).

This is an important feature positively affecting the ener-getic performance of the sequential process: a betterdewaterability implies a reduced energy demand fordewatering which can partially recover the additional energyconsumption required for aeration.

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Conclusions

Sequential anaerobic-aerobic digestion has been demon-strated effective for enhanced stabilization of both sec-ondary and mixed sludges. Results showed that the com-bination of the two digestion phases can be advantageousin that can mitigate some of the drawbacks characterizingthe two individual digestion methods and offer moredegree of freedom in terms of parameters to be modifiedin process optimization. This peculiarity is important be-cause the required characteristics of the digested sludgeare different, depending on its final destination (i.e., land-scape, agricultural reuse), so the possibility of modifyingthe operating variables to meet the different sludge qualitytargets is certainly an added value. Results confirmed thebetter digestibility of mixed sludge derived by the pres-ence of particulate influent wastewater COD in the pri-mary sludge fraction which is more easily degradable thanthe WAS mainly constituted by bacterial cells. This pos-itively affects the main performance evaluation parame-ters as VS removal, biogas production, dewaterability ofthe digested sludge, and also nitrification and denitrifica-tion efficiency in the aerobic stage. Stated the excellentperformance observed for mixed sludge, it is also worthnoting that good performance is also detected for secondarysludge with significant gain in VS removal (+25 %), CSTdecrease (−30 %), and nitrogen removal (90 and 62 % nitri-fication and denitrification efficiency, respectively).

These findings make this alternative digestion process alsosuitable for high potentiality WWTPs where the separation ofthe sludge treatment lines of primary and secondary sludges isproposable and potentially advantageous. It is worth notingthat the feasibility of the sludge separation strategy should beevaluated for the specific case, depending on the plant char-acteristics, namely potentiality, units of the sludge treatmentline, and final destination of the sludge.

When the sludge separation is not cost-effective (i.e., formedium and small size plants), the sequential digestion hasalso been demonstrated to be a valid alternative in enhancingmixed sludge digestion.

Acknowledgments This work was supported by the EU ROUTESproject funded from the European Union’s Seventh Programme forresearch, technological development, and demonstration under grantagreement No. 265156.

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