Sequential anaerobic/aerobic digestion of waste activated sludge: analysis of the process performance and kinetic study

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  • New Biotechnology Volume 29, Number 1 December 2011 RESEARCH PAPER





    erM. Concetta Tomei, Sara Rita and Giuseppe Mininni

    Water Research Institute, C.N.R., Via Salaria km 29,300, C.P. 10 - 00015 Monterotondo Stazione (RM), Italy

    Sequential anaerobicaerobic digestion was applied to waste activated sludge (WAS) of a full scale

    wastewater treatment plant. The study was performed with the objective of testing the sequential

    digestion process on WAS, which is characterized by worse digestibility in comparison with the mixed

    sludge. Process performance was evaluated in terms of biogas production, volatile solids (VS) and COD

    reduction, and patterns of biopolymers (proteins and polysaccharides) in the subsequent digestion

    stages. VS removal efficiency of 40%, in the anaerobic phase, and an additional removal of 26%, in the

    aerobic one, were observed. For total COD removal efficiencies of 35% and 25% for anaerobic and

    aerobic stage respectively, were obtained. Kinetics of VS degradation process was analyzed by assuming

    a first order equation with respect to VS concentration. Evaluated kinetic parameters were

    0.44 0.20 d1 and 0.25 0.15 d1 for the anaerobic stage and aerobic stage, respectively. With regardto biopolymers, in the anaerobic phase the content of proteins and polysaccharides increased to 50%

    and 69%, respectively, whereas in the subsequent aerobic phase, a decrease of 71% for proteins and 67%

    for polysaccharides was observed. The average specific biogas production 0.74 m3/(kgVS destroyed),

    was in the range of values reported in the specialized literature for conventional anaerobic mesophilic

    WAS digestion.

    IntroductionAlthough biological processes are an effective way of treating

    wastewater and ensuring minimum residual impact on the aquatic

    environment, they have the serious drawback of producing high

    amounts of excess sludge. Historically, it was common to see plant

    layouts that showed the water treatment scheme in detail with all

    of the process units and an arrow at the end that simply said

    sludge to disposal. This approach does not represent anymore the

    reality and today it is recognized that without a reliable disposal

    method for the produced sludge, the actual concept of water

    protection would fail. Currently, production of excess sludge is

    one of the most serious challenges in biological wastewater treat-

    ment. Treatment and disposal of sewage sludge from wastewater

    treatment plants (WWTPs) account for about half, even up to 60%,

    of the total cost of wastewater treatment [1]. In addition to the

    high costs, the current legal constraints and public sensitivity for

    sewage sludge disposal have provided considerable interest to

    explore and develop strategies and technologies for minimization

    of sludge production. To this aim one of the considered strategies

    is the optimization of the digestion stage. Anaerobic mesophilic

    digestion is extensively applied for sludge stabilization and was

    demonstrated to be effective in reducing pathogens and destroy-

    ing organic matter, but solid reduction above 50% is often difficult

    to achieve [2]. A promising solution to improve the sludge diges-

    tion performance, in terms of solid abatement, is the combined

    anaerobicaerobic process that has been investigated in the recent

    years by several researchers in different configurations [24]. The

    principle of operation, in the combined digestion, is the potential

    improvement of biosolid degradation, due to the specificity

    of metabolic pathways, aerobic and anaerobic, required by the

    different sludge fractions. Moreover, combining the two digestion

    methods can also be advantageous to reduce the drawbacksCorresponding author: Tomei, M.C. (

    1871-6784/$ - see front matter 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nbt.2011.03.006 17Sequential anaerobicwaste activated sludgprocess performanceerobic digestion ofe: analysis of thend kinetic study

  • RESEARCH PAPER New Biotechnology Volume 29, Number 1 December 2011



    erattributed to the two single processes. In fact, aerobic digestion is

    characterized by high energy demand but is simple to manage.

    Instead anaerobic digestion is more complex to manage and less

    stable, but allows the energy recovery with the produced methane.

    Concerning the sequence of the process phases, both anaerobic

    aerobic and aerobicanaerobic schemes have been proposed.

    Pagilla et al. [5] observed a positive effect in terms of solid reduc-

    tion and coliform abatement by adding a thermophilic aerobic

    pre-treatment stage before the anaerobic digestion. Subramanian

    et al. [6] focused their investigations on the dewatering properties

    of the digested sludge and found a significant improvement when

    the anaerobic digested sludge was post-treated in a subsequent

    aerobic phase.

    Advantages of the anaerobicaerobic sequential digestion were

    also highlighted in previous studies [4,7]. In comparison to the

    conventional anaerobic digestion, the combined process con-

    firmed the better solid removal efficiency, the improvement in

    dewatering characteristics and a marked removal of the ammonia

    nitrogen in the aerobic phase. Nitrogen removal in combined

    digestion was also investigated by Zupancic and Ros [8] at different

    intervals of temperatures and utilizing pure oxygen and air. They

    found that the addition of an aerobic stage with pure oxygen

    aeration to the conventional anaerobic digestion enhances ammo-

    nium nitrogen removal that reached 85% efficiency at 8 days of

    hydraulic retention time for the aerobic stage.

    The objective of this study was to verify the performance of the

    sequential anaerobicaerobic digestion on real waste activated

    sludge (WAS) by evaluating: the removal efficiencies of volatile solids (VS) and COD in the

    anaerobic and aerobic stages, the removal efficiencies of colloidal proteins and polysacchar-

    ides as specific sludge components determining the demand for

    polymer conditioning agents, the biogas production, the VS degradation kinetics in both anaerobic and aerobic


    Elements of novelty in the present paper are the testing of the

    sequential digestion process on WAS, which is characterized by

    worse digestibility in comparison with the mixed (primary and

    WAS) sludge [9], and the evaluation of the kinetic parameters for

    VS degradation in the two digestion phases.

    Materials and methodsSludgeThe WAS utilized in this work was provided by the Rome North

    wastewater treatment plant. The plant is a conventional activated

    sludge system including screening, primary clarification and sec-

    ondary treatment, and serves about 700,000 P.E. It is operated with

    a relatively high sludge age (20 d) and it is characterized by a

    diluted influent sewage. The influent COD average value is

    200 mg/L that is in the very low end of the range of values,

    210740 mg/L COD, reported in Henze et al. [10] for settled waste-

    water in EU countries.

    Secondary sludge was obtained for each feed step from

    the recycle stream, and then thickened for 18 h before utiliza-

    tion, whereas the anaerobic inoculum was taken from the fullscale digester of the plant fed with primary and secondary


    18 reactors utilized in this study are cylindrical glass vessels of

    7.4 L volume.

    The reactors were operated in series, the first reactor was oper-

    ated under anaerobic conditions and was maintained at

    37 0.58C by a thermostated jacket. The work volume was 7 Land the sludge retention time (SRT) was controlled at 15 days. The

    second reactor was operated under aerobic conditions with a work

    volume of 4.5 L. Air was supplied by a compressor able to maintain

    the concentration of dissolved oxygen at levels 3 mg/L. Theaerobic reactor worked at room temperature and the SRT was

    controlled at 12 days.

    In both reactors mixing was ensured by mechanical stirrers

    equipped with helicoidal blades.

    WAS was fed to the anaerobic reactor once per day, whereas an

    equivalent volume of digested sludge was extracted from the

    reactor and fed to the following aerobic reactor.

    AnalysisRegular sample collection and analysis were started after one week

    from the start up. Feed, anaerobic digested sludge and aerobic

    digested sludge were analyzed for total solids (TS), VS, COD,

    proteins and polysaccharides. Analytical methods and devices

    are reported in the following.

    Total and volatile solids TS and VS were measured according to

    Standard Methods APHA 2540B and APHA 2540E, respectively


    Chemical oxygen demand (COD) COD Cell Tests (MERCK-refer-

    ring to EPA 410.4 method), based on potassium dichromate oxida-

    tion and spectrophotometric determination (Spectroquant

    Nova30), have been employed.

    Proteins and polysaccharides Samples were centrifuged for

    10 min at 4000 rpm then the supernatant was filtered at 0.2 mm

    and analyzed for the biopolymer concentration. Protein concen-

    tration was determined according to the spectroscopic Bradford

    method [12], based on the addition and reaction with the Comas-

    sie Brilliant Blue and absorbance detection at 595 nm wavelength.

    Polysaccharide concentration was evaluated by the Dubois

    method [13] based on the reaction of the sample with phenol

    and sulphuric acid. Absorbance of the treated sample was mea-

    sured at 490 nm wavelength.

    Biogas detection Flow rate of biogas produced by the anaerobic

    reactor was measured by a volumetric counter using a closed water

    displacement system with electrical contacts and with an electro-

    magnetic valve to discharge the produced biogas to the atmo-

    sphere [14]. The measurement device is controlled by a

    Programmable Logic Controller that also provides the recording

    of signals.

    Methane Methane in the biogas was determined by a gas-

    chromatograph PerkinElmer AutoSystem equipped with a Car-

    boxen 1000 (Supelco) column and a TCD detector. Sample volume

    was 50 mL, transport gas was helium (3 bar) and operating tem-

    peratures were 1808C for the oven, 1508C for the injector and2508C for the detector.

    Results and discussion

    The two reactors were operated in series in semi-continuous mode

    for 2.5 months to have a work period long enough to be repre-

  • sentative of the achievable performance in a real system. The start

    up phase, that is the time required to have stable performance in

    the two reactors, was quite short (15 d), and this could bereasonably explained with the use in both anaerobic and aerobic

    reactors of biomass inocula already acclimatized. After the start-

    up, even if the removal efficiencies are affected by the variability of

    the fed sludge, the performance of the system confirmed the

    beneficial effect of the additional aerobic stage on the VS and

    COD removal for the entire operation period.

    One of the key parameters affecting the performance of the

    anaerobic digestion is the SRT, and different criteria can be con-

    sidered in its choice, depending on the goals to be achieved. In

    previous investigations on sequential digestion [2,4] the SRT of the

    anaerobic phase was in the range of 1015 d for mesophilic and 15

    iumhigh size plants, there is a lack of kinetic parameter data for

    this substrate and this makes difficult the setting up of reliable

    process models. To give a contribution to this aspect, the kinetic

    analysis of VS removal in the anaerobic phase was performed by

    correlating the experimental data with the following equation:


    k Xs

    that is a first order kinetics with respect to the VS concentration,

    where XS is the particulate substrate concentration (VS) and k the

    kinetic constant. In spite of its simplicity, the first order equation is

    the most utilized for modeling anaerobic sewage sludge degrada-

    tion. According to [17] this empirical equation, if adequately

    supported by experimental data, is potentially able to consider

    the cumulative effects of many processes playing a role in a so

    complex system where is really difficult to distinguish the active

    biomass from the sludge volatile solids representing the substrate.

    The kinetic coefficient k was evaluated by integrating the equation

    New Biotechnology Volume 29, Number 1 December 2011



    er20 d for thermophilic digestion. Optimal SRT values reported in

    [15] for conventional mesophilic anaerobic digestion are in the

    range of 1518 days. The 15 d SRT value utilized in this study was

    chosen taking into account these data with the objectives of

    ensuring a good performance of the anaerobic digestion and, at

    the same time, having a reduced reactor volume.

    Concerning the aerobic phase, Kumar et al. [4], in their experi-

    ments on secondary aerobic digestion of anaerobically digested

    mixed sludge, worked with aerobic SRT values in the range of 39

    d, so in this study, considering the poorer digestibility of WAS, a

    more cautionary SRT value of 12 d was utilized.

    Moreover, being the anaerobic SRT utilized in this study in the

    range of optimal values for the conventional anaerobic digestion,

    the characteristics of the anaerobically digested sludge in our

    experiments are comparable to the ones obtainable with the single

    anaerobic digestion. As a consequence, all the performance

    improvements achieved with the additional aerobic phase can

    be considered as advantages of the sequential digestion process.

    VS and TS removalTS and VS concentration profiles, detected in the fed and digested

    sludge, from anaerobic and aerobic reactors, are shown in Figs 1

    and 2. Reported data are referring to one-month period (from the

    20th to 50th day) whereas the removal efficiencies, as discussed

    below, refer to the entire operation period.

    The results must be read considering the low VS/TS ratio (0.4

    0.5) in the influent WAS sludge. Quite stable performance was

    achieved in the anaerobic phase and the observed variation of VS








    50454035302520time (d)


    l Sol



    feed anaerobic effluent aerobic effluent

    FIGURE 1Total solids concentration profiles in the fed WAS and in effluent from theanaerobic and aerobic digesters.removal efficiency can be reasonably attributed to the marked

    variability of the VS concentration in the feed that, in the period of

    reference, was 1.54 0.17 (expressed as %). Average anaerobic VSremoval efficiency, evaluated after the first start up period, was

    40 10% within the range of values, 3045%, reported for thedigestion of waste activated sludge [16]. In the subsequent aerobic

    stage an additional VS removal of 26 9% was achieved, so theglobal VS removal efficiency was significantly improved reaching

    levels (56% average value) comparable with the values (6065%),

    reported in [2,4], for mixed sludge. The slightly lower efficiency, in

    our case, is justified by the poorer WAS digestibility with respect to

    the mixed sludge and by the operating conditions of the WWTP

    that, because of the high SRT, produces a partially digested sec-

    ondary sludge.

    A similar trend in terms of removal efficiency was observed for

    TS with a percentage of 31 8% for the anaerobic phase and21 9% for the aerobic one.

    Kinetic analysisIt...


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