Bioresource Technology 143 (2013) 96–103

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Bioresource Technology

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Reduced temperature hydrolysis at 134 �C before thermophilicanaerobic digestion of waste activated sludge at increasing organic load

0960-8524/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.biortech.2013.05.069

⇑ Corresponding author. Tel.: +39 0690672799; fax: +39 0690672787.E-mail address: gianico@irsa.cnr.it (A. Gianico).

A. Gianico ⇑, C.M. Braguglia, R. Cesarini, G. MininniWater Research Institute, CNR, Area di Ricerca RM1-Montelibretti, Via Salaria km 29.300, 00015 Monterotondo, Roma, Italy

h i g h l i g h t s

� Low temperature hydrolysis as pretreatment of WAS anaerobic thermophilic digestion.� Long semi-continuous tests were carried out at increasing organic load.� Solubilization due to pretreatment increased degradability and CH4 production rate.� Thermophilic digestion worsened dewaterability, especially feeding untreated sludge.� Heat balance of the enhanced digestion process was strictly related to organic load.

a r t i c l e i n f o

Article history:Received 19 March 2013Received in revised form 17 May 2013Accepted 19 May 2013Available online 4 June 2013

Keywords:Thermal hydrolysisOrganic loading rateMethaneDewaterabilitySupernatants

a b s t r a c t

The performance of thermophilic digestion of waste activated sludge, either untreated or thermal pre-treated, was evaluated through semi-continuous tests carried out at organic loading rates in the rangeof 1–3.7 kg VS/m3 d.

Although the thermal pretreatment at T = 134 �C proved to be effective in solubilizing organic matter,no significant gain in organics degradation was observed. However, the digestion of pretreated sludgeshowed significant soluble COD removal (more than 55%) whereas no removal occurred in control reac-tors. The lower the initial sludge biodegradability, the higher the efficiency of thermal pretreated diges-tion was observed, in particular as regards higher biogas and methane production rates with respect tothe parallel untreated sludge digestion. Heat balance of the combined thermal hydrolysis/thermophilicdigestion process, applied on full-scale scenarios, showed positive values for direct combustion of meth-ane. In case of combined heat and power generation, attractive electric energy recoveries were obtained,with a positive heat balance at high load.

� 2013 Published by Elsevier Ltd.

1. Introduction

Since the development of the biogas sector, waste activatedsludge (WAS) is considered as renewable energy source becomingan interesting substrate for anaerobic digestion, despite its lowkinetics of degradation. Besides energy recovery, digested WAS isone of the best sources of agricultural soil conditioner and a sourceof slow-release nutrients and microelements. The only potentialconcern is that sludge originating from wastewater treatmentplants contains a wide diversity of pathogens, therefore its useon land could be restricted or banned (Riau et al., 2010). Conven-tional mesophilic digestion systems are not able to produce sludgethat is consistently free of pathogens while under thermophilicconditions the inactivation of pathogens can be easily achieved.

With respect to mesophilic, thermophilic anaerobic digestionincreases methane production, reduces the retention time neededfor sludge treatment and provides sludge hygienization. However,thermophilic digestion has some disadvantages, including ammo-nia, COD and propionate accumulation in supernatants (De la Ru-bia et al., 2006; Speece et al., 2006) and worse dewaterability.The main problem in thermophilic sludge digestion compared tomesophilic one is the higher heating requirement for sustainingthe process (Zupancic and Roš, 2003).

It is well known that enzymatic hydrolysis is rate-limiting inthe anaerobic digestion of substrates as sewage sludge. Duringthe hydrolysis step, both solubilization of particulate matter andbiological decomposition of organic polymers take place. Accord-ing to the recent WAS-floc agglomeration concept, extracellularpolymeric substances (EPS) and divalent cations may be the mostimportant parameters governing WAS hydrolysis and determiningfloc structure, integrity and strength. Moreover Novak et al. (2003)have shown that EPS produce short chain organic compounds

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during fermentation which are the substrate for methanogenicpopulation during the following methanation step.

Thermal, chemical, biological and mechanical processes havebeen studied as possible pretreatments to disintegrate the EPS net-work, enhance the rate of sludge biodegradability (Braguglia et al.,2011; Carrère et al., 2010; Eskicioglu et al., 2006) and potentiallyincrease dewaterability (Ormeci and Vesilind, 2000) afterdigestion.

Among these pretreatments, ultrasonic cavitation, ozone, andhigh-pressure homogenization are largely targeted at the floc dis-integration, while thermal hydrolysis process (THP) has the poten-tial to produce Class A pathogen-free biosolids as defined byUnited States CFR 40 Part 503.32 US EPA.

Because the thermal hydrolysis process holds the combinedbenefits of decreasing biosolids quantity and increasing biosolidsacceptability, it is of great interest to the wastewater industry(Wilson and Novak, 2009).

Nevertheless, few authors have investigated the effect of ther-mal hydrolysis on thermophilic digestion of waste activated sludgealone, and these studies are mostly carried out in batch systems(Bougrier et al., 2008; Liu et al., 2012; Mottet et al., 2009).

There is however an upper limit of temperature (around 180–200 �C) beyond which the effectiveness of THP not only starts todrop but rather results in the production of hardly degradablecompounds. Pilot scale studies reported the range of 165–180� asthe optimal one to enhance biodegradability and biogas productionin mesophilic conditions (Bougrier et al., 2006; Neyens and Baey-ens, 2003), despite these higher temperatures may negatively af-fect heat balances. The increase of methane production dependson the initial biodegradability of the sludge, with higher impactson hardly biodegradable sludge and better results on waste acti-vated sludge than on primary sludge.

As regards WAS dewaterability after THP, several studies areavailable in literature reporting a deterioration of sludge filterabil-ity applying temperatures lower than 150 �C caused by the in-crease of colloidal and soluble particles. On the contrary, fortemperatures higher than 150 �C, filterability resulted improved.This could be explained by the modification of sludge structureand the released of linked water (Neyens et al., 2004).

Because organic loading rate (OLR) of a digester, related to thehydraulic retention time (HRT) and the volatile solids content, isa key factor in controlling anaerobic digestion systems (Apul andSanin, 2010; de la Rubia et al., 2006), the objective of this workwas to investigate the effect of the thermal pretreatment onsemi-continuous thermophilic WAS digestion at different OLRs.

The digestion performances were therefore evaluated with re-gard to the following: process stability and inhibition, organic mat-ter reduction and methane production, quality of effluentsupernatant, and dewaterability efficiency of thermophilic di-gested sludge.

Moreover, this paper deals with an evaluation of overall perfor-mances and heat requirements of thermophilic digestion, appliedon untreated or thermally pretreated WAS, at different OLRs.

Table 1Characteristics of raw activated sludge.

Parameter Average value

TS (mg/L) 20,800VS (mg/L) 14,400VS/TS (%) 69%pH 6.5Soluble COD (mg/L) 45Total COD (mg/L) 20,100Soluble Ntot (mg/L) 22Soluble NH4-N (mg/L) 14

2. Methods

2.1. Sludge

Sludge was sampled from the municipal ‘‘Roma-Nord’’ waste-water treatment plant, serving about 780,000 P.E. with an averageflow rate of 4.1 m3/s. Average influent water quality to the WWTPwas 250 mg COD/L, 20 mg N-NHþ4 /L and 4 mg Ptot/L.

The plant includes primary clarification and secondary treat-ment of activated sludge without nutrients removal. Table 1 re-ports the average characteristics of the waste activated sludge

(WAS), collected directly from the oxidation tank operating at anaverage sludge age of 20 d. The anaerobic inoculum was collectedfrom the full-scale digester of the plant, fed with primary and sec-ondary sludge.

2.2. Matter composition

Total and volatile solids (TS and VS) were determined accordingto standard. VS were measured in triplicate. A preliminary sample,used to determine dry solid concentrations, was dried at 105 �C for24 h. In the next step, the dry solids were brought to 550 �C for 2 h.The resulting residue represents the inorganic part of the dry sol-ids. The difference between the total dry solid concentrationsand the inorganic concentrations gives the volatile solids. To ana-lyze the soluble phase, the particulate sludge matter was removedby centrifugation (10 min at 5000 rpm), and the resulting centratewas filtrated through 0.45 lm pore size membrane filters.

Soluble COD (sCOD), measured in duplicate, was determined byphotometric determination of the chromate consumption by or-ganic compounds (COD Cell Test by Spectroquant Merck) subse-quent to digestion in a concentrated sulfuric acid solution for 2 hat 148 �C (EPA method 410.4). Pre-thickening of sludge in test #3was performed by centrifugation for 2 min at 1100 rpm.

Total nitrogen was determined in triplicates by photometricdetermination using the nitrogen (EN ISO 25663) cell test by Spec-troquant (Merck); ammonia nitrogen was determined according tomethod 4500-NH3 C of APHA Standard Methods, 18th edition(1992).

The pH was detected by a portable pH-meter (WTW, pH 330/SET-1) with a single probe consisting of two measuring electrodesand a temperature sensor.

Sludge filterability was estimated using a capillary suctionapparatus supplied by Triton Electronics Ltd., England. A stain-less-steel tube with an inner radius of 0.535 cm and filter paper(Whatman No. 17) was used. Each sludge was analyzed 5 timesand the results were averaged before the results were standardizedto the TS concentration.

The charge density determinations were performed by a Parti-cle Charge Detector PCD02 (Mütek GmbH, Herrsching). The PCDoperates on the principle of the ‘‘streaming current detector’’. Sincethe streaming current is proportional to the electric charge of thecolloids, it may provide an indication of charge-related particledestabilization in a manner similar to zeta potential (Abu-Orfand Dentel, 1997). The method adopted to measure the surfacesludge density on the liquid fraction of sludge samples is fully de-scribed in Braguglia et al. (2009).

2.3. Thermal pretreatment

Thermal pretreatment was carried out by a bench scale auto-clave Laboklav 25b, with total capacity of 25 L, allowing a temper-ature increase by steam mode up to T = 134 �C and p = 3.12 bar.

Table 2Operating conditions of anaerobic tests.

Test #

1 2 3

Test duration (d) 103 102 51OLR (kg VS m�3 d�1) 1.0 1.8 3.7HRT (d�1) 15 8 8

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Once temperature was reached, the pretreatment time lasted20 min, on 400 mL of sample. The degree of sludge disintegration(DDCOD) was calculated as the ratio of the soluble COD increaseafter pretreatment to the maximum possible soluble COD increase(Braguglia et al., 2006).

The effect of thermal pretreatment on sludge has been investi-gated in terms of COD and ammonia release, specific surface chargeand filterability alteration.

2.4. Digester system

Each digestion test was performed using two anaerobic digest-ers operated in semi-continuous mode under the same conditions.One reactor, as control unit, was fed with untreated WAS, and thesecond one was fed with the same sludge after thermal pretreat-ment. Untreated or pretreated WAS influents were fed manuallyto the digesters once a day after withdrawing the same volumeof digested sludge. In the case of test #1 carried out at HRT 15 dthe volume of sludge fed daily was 0.467 L, while in the case of test#2 and test #3 at HRT 8 d the volume was 0.875 L.

Both jacketed reactors (7 L) were completely mixed and main-tained at the constant temperature of 55 �C. Table 2 lists the oper-ating conditions of the anaerobic digestion tests.

All tests were carried out using the same WAS; the first twotests were carried out with gravity thickened WAS (TS = 20.8 g/L)while test #3 was carried out by increasing the solids concentra-tion with a dynamic pre-thickening step (TS = 41 g/L).

Data were collected during steady state conditions, i.e., afterreaching constant specific biogas production. The produced biogaswas collected by water displacement in a biogas collection unit.The gas meter consisted of a volumetric cell for gas–liquid dis-placement, a sensor device for liquid level detection, and an elec-tronic control circuit for data processing and display. Themethane content in the biogas was measured using a PerkinElmerAuto System Gas Chromatographer equipped with a thermal con-ductivity detector (TCD). The GC analyses were performed underthe following conditions: stainless steel column (Supelco),150 � 1/800 (2.1 mm ID) packed with 60/80 mesh Carboxen 1000.The temperatures for injector, column and detector were set at200, 150 and 200 �C, respectively. Nitrogen was used as a carriergas at a flow rate of 17 mL/min.

Table 3Effects of thermal pre-treatment.

Gravity thickened WAS

Untreated P

TS (g/L) 20.8 2VS/TS (%) 69% 6Soluble COD (mg/L) 45 ± 6 2DDCOD (%) – 1Soluble Ntot (mg/L) 22 4Soluble NH4

+-N (mg/L) 14 1Charge density (mC/gTS)a 54 1CST (s L g TS�1)a 0.6 2

a In order to compensate for the different amounts of dry solids contained in sludg(Vesilind, 1988).

3. Results and discussion

3.1. Thermal hydrolysis

The effects of thermal pre-treatment on WAS samples at differ-ent solids concentration have been evaluated.

As shown in Table 3 total and volatile solids concentration didnot change during pretreatment probably because of the low tem-perature, suggesting that, in these conditions, thermal hydrolysisdid not lead any mineralization effect. As such, the hydrolysis pro-cess in no way diminished the available resource for methane pro-duction (Morgan-Sagastume et al., 2011).

COD solubilization resulted consistent with other authors find-ings, in fact Wilson and Novak (2009) reported a disintegration de-gree around 10% after 2 h (including initial heating time) ofhydrolysis carried out on pre-thickened WAS (TS = 45 g/L) at130 �C with a muffle furnace.

Moreover a huge solubilization of ammonia from proteins, rele-vant issue in terms of risks of methanogenic inhibition, was observedafter thermal hydrolysis of both gravity and dynamic thickenedsludges. Ammonification of WAS proteins by thermal hydrolysiswas also observed by Wilson and Novak (2009), a NHþ4 -N concentra-tion of 148 mg/L was measured after THP at 130 �C.

Untreated WAS samples carried a net negative surface charge(Table 3), mainly due to ionization of functional groups of the poly-meric substances. An increase of the specific charge occurred afterpre-treatment, due to the increased amount of colloidal and poly-meric material released in solution (Fig. S1).

Untreated sludge presented in fact the typical morphology withfilamentous bacteria surrounding the matrix of compact flocs(Fig. S1a). Thermal treatment did not significantly modify thisstructure probably because of the low energy applied, not suffi-cient to disintegrate the flocs (Fig. S1b). Nevertheless, solubiliza-tion phenomena and changes on surface properties occurredduring thermal pretreatment, although the floc structure remainedunchanged.

This floc deterioration and solubilization resulted in a worsen-ing of the sludge filterability, as shown by the increase of the CSTvalues reported in Table 3. A strict linear correlation between par-ticle charge density and CST was observed, indicating that the in-crease of colloidal particles in solution due to pre-treatmentaffected negatively the filterability of sludge, according with previ-ous studies (Braguglia et al., 2009; Neyens et al., 2004).

3.2. Digestion performances

Untreated and pretreated WAS anaerobic digestion perfor-mances, in terms of organic matter removal, specific biogas pro-duction, methane production rate and biodegradability arereported in Table 4. As regards the untreated WAS, the VS removals

Dynamic thickened WAS

retreated Untreated Pretreated

0.5 41.4 40.29% 71% 71%834 ± 320 206 ± 15 5908 ± 5654% – 13%82 37 146692 32 279527 24 16831.4 1.0 50.5

e, CST and surface charge were ‘‘normalized’’ with respect to total solids content

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observed during steady state conditions were in the range of38–43%, lower than the typical values observed in thermophilicdigesters treating mixed sludges (Bolzonella et al., 2005). Theselow removals are principally due to the low VS/TS ratio (lower than0.7) of the feed that clearly indicated a significant stabilization de-gree attributable to the high SRT of the activated sludge process.

3.3. Effect of OLR

The specific biogas productions with respect to fed or removedVS did not vary significantly by increasing OLR in the range of 1–3.7 kg VS/m3 d indicating that the conversion rate was not affectedby the changing load in the digester, assuring the stability of theanaerobic tests in these conditions. Methane content in the biogas,measured during steady state periods, ranged between 60% and70%, as typically reported in the literature for thermophilic sludgedigestion (de la Rubia et al., 2006; Ferrer et al., 2008, 2010).

Methane production rate varied between 0.15 and 0.80 Nm3/m3

digester d and increased logarithmically by increasing the OLR,for both untreated and pretreated WAS digestion. This result sug-gested that higher OLR (and lower HRT) might have resulted in fur-ther methane production improvement.

Moreover, the methane production was used to calculate theinitial sludge biodegradability (Table 4), determined by the ratioof the volume of methane produced per gCOD introduced dividedby the theoretical methane production of 350 mL/gCOD at normaltemperature and pressure conditions (Carrère et al., 2008). Thesludge biodegradability was significantly low (�30%) in the caseof untreated digestion of test #1, whereas was around 45% in thecase of test #2 and test #3, carried out at higher OLRs. Taking intoaccount that the sludge sampling point was always the same, andthat the organics content of the sludge during the entire experi-mentation did not change significantly, the cause of the low biode-gradability of digestion test #1 may be likely attributed to the loworganic load with respect to the anaerobic biomass in the digester.Organics degradation in terms of volatile solids reduction did notchange by halving the HRT from test #1 to test#2, but decreased

Table 4Main performances of anaerobic digestion tests at different OLRs.

Test #1 OLR = 1.0 kg VS/m3 d

Untreated Pretreated

VSdestruction (%) 43 ± 1.5 46 ± 1.8Specific biogas production (Nm3/kgVSfed) 0.26 ± 0.02 0.36 ± 0.04Specific biogas production (Nm3/kg VSremoved) 0.61 ± 0.05 0.77 ± 0.06Methane production rate (Nm3/(m3

digester d) 0.15 ± 0.01 0.22 ± 0.02Specific methane production (Nm3/kgCODfed) 0.11 ± 0.01 0.16 ± 0.01Initial sludge biodegradability (%) 32 47

Fig. 1. Soluble COD of feed and digested

significantly in test#3, carried out at the same short HRT, feedinga more concentrated substrate.

De la Rubia et al. (2006) found that in the case of thermophilicdigestion of mixed sludge, the percentage of VS removed decreasedfrom 73% to 49% as the OLR increased from 0.4 to 2.2 kg VS/m3 d,whereas Toreci et al. (2009) in the case of mesophilic waste acti-vated sludge digestion found that the VS removal decreased from49.9% at OLR = 1 kg VS/m3 d to 39.5% at OLR = 2 kg VS/m3 d.

Other studies reported that the thermophilic anaerobic diges-tion of pre-thickened secondary sludge carried out on pilot scalewith HRT 20 d and OLR of 2.2 kg VS/m3 d permitted to reach amethane production rate of 0.64 Nm3/m3 d and a specific biogasproduction of 0.9 Nm3/kg VSremoved (Bolzonella et al., 2012), consis-tent with the values reported in Table 4.

At the same time Ferrer et al. (2010) reported that the specificbiogas production with respect to VSfed remained constant in theOLR range 0.5–5.2 kg VS/m3 d for the thermophilic digestion ofmixed sludge, confirming the same finding of this study.

3.4. Effect of pre-treatment

The solubilization effect of thermal pretreatment on waste acti-vated sludge (summarized in Table 3) was assessed in terms ofdigestion performances, referring to a parallel control reactor, fedwith the same raw sludge, without pretreatment. It is importantto note that the disintegration degree was maintained the same(around 14%) during the whole duration of the digestion tests. Inthe case of the pretreated digestion tests the soluble organic loadranged from 0.196 g sCOD L�1 d�1 up to 0.724 g sCOD L�1 d�1,whereas in the case of the untreated digestion the load was negli-gible (0.004–0.013 g sCOD L�1 d�1).

The pretreatment did not improve significantly the volatile sol-ids removal in the OLRs range investigated (Table 4). These resultsare borne out in other author’s findings: Ferrer et al. (2008) re-ported a VS removal of 37% after thermophilic digestion of a ther-mal pretreated (70 �C, 9 h) mixed sludge compared to 33% ofuntreated sludge. Bougrier et al. (2007) showed a VS removal of

Test #2 OLR = 1.8 kg VS/m3 d Test #3 OLR = 3.7 kg VS/m3 d

Untreated Pretreated Untreated Pretreated

42 ± 1.4 45 ± 1.6 38 ± 1.1 37 ± 1.30.31 ± 0.03 0.32 ± 0.03 0.31 ± 0.02 0.32 ± 0.030.73 ± 0.06 0.70 ± 0.07 0.81 ± 0.07 0.87 ± 0.080.38 ± 0.01 0.41 ± 0.01 0.74 ± 0.02 0.80 ± 0.020.15 ± 0.01 0.15 ± 0.02 0.15 ± 0.01 0.15 ± 0.0142 44 42 44

sludge at steady state conditions.

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41% after mesophilic digestion of a 30 min thermal pretreatedWAS, with respect to 39% of untreated WAS. However, all thedigesters fed with pretreated sludge had more than 55% solubleCOD removal whereas no removal occurred in the control reactors(Fig. 1).

Because of this high removal of soluble organic material, biogasproduction and in particular methane production rates always in-creased in the reactors fed with pretreated sludge in comparison tothe control reactors (+47%; +8% and +8% in tests #1; #2 and #3,respectively). The gain was very high in the case of test #1, con-firmed by the high specific biogas production of 0.36 Nm3/kg VSfed

obtained during this test, too. This result evidenced the need towork at high HRT when digesting pretreated sludge with high sol-uble COD load. It is important to note that a gain in biogas produc-tion of pretreated sludge was not linearly related to a gain in solidsdegradation evidencing the changes in the biochemical fermenta-tion process of pretreated sludge with respect to the untreatedone (Braguglia et al., 2011).

Biodegradability enhancement was evaluated by the ratio ofbiodegradability of treated samples divided by the one of un-treated samples. Only in the case of test #1 the enhancementdue to the pretreatment was relevant, around 1.5, whereas byincreasing the load, the biodegradability enhancement was around1.1, evidencing that the lower the initial sludge biodegradability,the higher the efficiency of thermal pretreatment. The increasedsolubilization via thermal pretreatment has probably converted afraction of the more recalcitrant degradable material and increasedthe amount of more readily biodegradable material, by increasingthe rate of digestion rather than the extent of VS destruction.

3.5. Digestate quality

Clearly, enhancing sludge stabilization processes in the sludgeline, some problems should be taken into account: the increaseddegree of floc- and cell-lysis caused an increase in nutrients con-tent and COD in supernatants from the anaerobic digesters to berecirculated and treated in the wastewater line. Table 5 showsthe organic and nitrogen content of the supernatants obtainedafter digestion of untreated or pretreated sludge during the threetests. Dewaterability parameters as specific surface charge andCST values of the digested sludge are reported, too. In order tocompensate for the different amounts of dry solids contained ofthe sludge, CST and surface charge were ‘‘normalized’’ with respectto total solids content (Vesilind, 1988).

By comparing the soluble COD trends during digestion tests(Fig. 1), a marked solubilization of organic matter was observedin the reactors fed with untreated WAS highlighting a predomi-nance of the biological hydrolysis phase before methanogenesis(Braguglia et al., 2011).

On the contrary, the parallel reactors fed with pretreated sludge,rich in soluble organics, showed an inverse trend characterized by asignificant removal of sCOD during digestion. The soluble COD inthe reactors was not affected by varying the OLR from 1 to1.8 kg VS/m3 d, and was almost constant around 650 and

Table 5Characterization of anaerobic supernatants and sludge dewatering properties, at steady st

Test #

1 (OLR = 1.0 kg VS/m3 d)

Untreated Pretreated

CODsol (mg/L) 621 ± 44 995 ± 122Soluble Ntot (mg/L) 890 900Soluble NH4-N (mg/L) 767 757Charge density (mC g�1TS) 1428 1890CST (s L g�1TS) 30.4 35.5

1100 mg/L, for untreated and pretreated sludge, respectively.Increasing OLR up to 3.7 the soluble COD in the reactors was around1600 mg/L, independently on the pretreatment.

As well as COD, also ammonia showed similar trend duringdigestion depending on the OLR. In fact, working at the highestload a significant release of ammonia was observed, derivingmostly from intense protein degradation typical of thermophilicdegradation. However, no inhibitory effects were observed becausethe ammonia level in all the digesters resulted always lower withrespect to the methane inhibition level of 2000 mg/L (Han et al.,2007) and, at the same time, the pH measured during the digestionprocesses was not extremely alkaline (average pH value of8.1 ± 0.2).

Although ammonia concentration in the pretreated feed in-creased also due to thermal hydrolysis, organic loading rate appearedto be the most significant parameter affecting ammonia levels inanaerobic supernatants, according with Wilson and Novak (2009).

Because of the bacterial hydrolysis releasing soluble and colloi-dal fine particles, anaerobic digestion of untreated sludge led to aworsening of sludge filterability, independently from the appliedorganic load. In fact, the CST value of the sludge increased around30 times after digestion (Houghton et al., 2000), combined with asteep increase of the specific surface charge. Thermophilic diges-tion resulted therefore in rapid deterioration in the dewaterabilityof secondary sludge, which has been attributed to the increasedEPS in the sludge (Murthy et al., 2000). In fact proteins and poly-saccharides have negatively charged functional groups which af-fect sludge filterability through sequence specific interactions,hydrophobic interactions and hydrogen bonding (Novak et al.,2003). Increasing these soluble biopolymers due to hydrolysismight contribute to deteriorate the dewaterability parameters ofthe digested sludge, as indicated by the increase of the CST.

Regarding the digestate deriving from the pretreated feed, aslight deterioration of sludge filterability was observed after diges-tion in tests #1 and #2, while in the case of the digestion carriedout at high load (as test #3) an amelioration was even observed,with the CST of digested sludge around 36 against the 50 s L g�1TSof the pretreated feed. The pretreated sludge, due to thermal pre-treatment, was rich in soluble and colloidal fines when fed to thedigesters, so the abatement of these compounds was strictly re-lated to the efficiency of the digestion process. These results con-firm what reported by Lawler (1986) highlighting the importanceof the digestion process to reduce the amount of fine particlesand the specific surface of the digested sludge.

Nevertheless, thermal pretreatment did not lead any improve-ment in filterability of the final digested sludge with respect tothe untreated one, and this result may be attributed to the highamount of water attached to the large surfaces provided by the re-duced particle size after thermal pre-treatment.

3.6. Heat balance based on full scale scenario

The experimental lab-scale data deriving from the integratedprocess thermal hydrolysis/thermophilic digestion at different

ate conditions.

2 (OLR = 1.8 kg VS/m3 d) 3 (OLR = 3.7 kg VS/m3 d)

Untreated Pretreated Untreated Pretreated

697 ± 82 1132 ± 40 1580 ± 165 1570 ± 151974 1068 1362 1303939 1031 1344 12251157 1818 1713 170224.7 39.1 33.5 36.2

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operating conditions were used to assess the heat balance in a full-scale scenario for a large WWTP serving a population equivalent(P.E.) of 500,000. The heat balance of this advanced process wasthen compared to the one calculated for the conventional thermo-philic digestion of waste activated sludge (WAS).

Assuming a WAS pro-capita production (Pwas) of 38 gTS/P.E.⁄d, the‘‘volumetric sludge flow (Qw)’’ was obtained by the following Eq. (1):

Q w ¼ ðPwas � P:E:Þ=TS ½m3=d� ð1Þ

The digester volumes (Vd) were designed according to the dif-ferent HRT’s used in lab-scale tests, therefore (Eq. (2)):

Vd ¼ Q w �HRT ½m3� ð2Þ

The thermal hydrolysis reactor volumes (Vth) were designedaccording to the pretreatment time used during lab-scale tests(Tth = 20 min); the security factor used in the design was 1.5 (Eq. (3)).

V th ¼ Q w � Tth � 1:5 ½m3� ð3Þ

The results are shown in Table S1.Heat is necessary to (i) compensate for heat loss through tank

walls and (ii) raise the incoming sludge to the temperature of theanaerobic digestion (55 �C) or thermal hydrolysis (134 �C) tank.

Heat losses from anaerobic digester and thermal hydrolysisreactor, were estimated assuming a 0.56 �C (1�F) decrease in tem-perature each 24 h (Heat losses = V� � �0.56).

Feed sludge heating requirement (Qw� � �DT) was calculatedassuming an average external temperature of 15 �C.

Therefore, the DT amounted to 40 �C for the sludge fed to anaer-obic digester and 120 �C for the sludge fed to thermal hydrolysistank. By inserting a heat exchanger between the inlet and outletsections of digester, a reduced DT of 20 �C can be achieved; by

Fig. 2. Thermophilic untreated WAS digestion (Scheme 1) a

inserting a heat exchanger between the inlet and outlet sectionsof thermal hydrolysis tank, it’s moreover possible to reduce theDT from 120 to 40 �C (Fig. 2).

Typical thermal hydrolysis full-scale applications (Dwyer et al.,2008) are equipped with a flash tank with the aim to partially re-cover the hot steam. When hot condensate, under pressure, is re-leased to a lower pressure, part of it is re-evaporated, becomingwhat is known as flash steam. The heat content of flash is identicalto that of live steam at the same pressure; with proper sizing andinstallation of a flash recovery system, the latent heat content offlash steam may be used for pre-heating feed sludge. In this studya heat content of flash steam equal to 60% of the total heatingrequirement was assumed, according with Pickworth et al. (2005).

The heat consumptions of the different units are reported inTable S2.

Heat production was evaluated considering the total amount ofsludge fed to the digesters in the full-scale scenarios and the SGP(specific gas production, Nm3/kg VSfed) obtained in the lab-scaletests. The amount of volatile solids fed to the treatment processeswas calculated with the following Eq. (4), on the basis of the exper-imental VS/TS ratio:

VSfed ¼ Pwas � ðVS=TSÞ � P:E:½kgVSfed=d� ð4Þ

Total biogas production (Pbiogas) is therefore defined in Eq. (5):

Pbiogas ¼ VSfed � SGP ½Nm3=d� ð5Þ

The theoretically amount of heat that can be produced frommethane utilization (Hmethane) was obtained using the experimen-tal data on methane content (%CH4) measured during the tests andconsidering the inferior calorific power (ICPCH4) of methane equalto 9.89 kWh/Nm3 (Eq. (6)).

nd thermophilic pretreated WAS digestion (Scheme 2).

Fig. 3. Net thermal energy productions from untreated and pretreated sludge digestion, direct biogas combustion scenario (a) and CHP scenario (b).

102 A. Gianico et al. / Bioresource Technology 143 (2013) 96–103

Hmethane ¼ Pbiogas �%CH4 � ICPCH4 ½kWh=d� ð6Þ

In order to evaluate the theoretically achievable heat, two dif-ferent way of utilizing biogas were considered: direct combustionin boilers or burners, and combined heat and power (CHP) genera-tion. In this model, a 90% thermal efficiency was assumed for directcombustion, while a 45% thermal efficiency coupled with an elec-trical efficiency of 35% was estimated for the CHP system. On thebasis of these assumptions, the different energy productions re-lated to different scenarios are reported in Table S3.

The heat gained from the biogas produced by the pretreatedsludge was higher for the test #1 carried out at high HRT, andamounted to 0.71 kWh/kg VSfed against 0.06 and 0.10 for test #2and #3, respectively (Table S3).

A balance between heat production and heat loss was carriedout on the basis of the results obtained for each scenario. In caseof direct combustion of methane (Fig. 3a), the heat produced bypretreated WAS digestion resulted always sufficient to supply thewhole heat requirements, while the untreated WAS digestionshowed negative balances, except for test #3 at high OLR.

The net thermal energy increased by increasing the OLR; re-duced digester volumes and reduced inlet sludge flow rates causedin fact a decrease of heating requirements.

As regards the hypothesis of methane utilization in a CHP sys-tem (Fig. 3b), besides attractive electric energy productions(Table S3), the heat balance resulted positive only in the case ofpretreated WAS digestion at high OLR. Sludge pre-thickening istherefore confirmed to be a pre-requisite to attain a positive heatand energy balance allowing to perform a self-sufficient anaerobicprocess, with the added benefit of a surplus of green electricenergy.

4. Conclusion

Because of high soluble COD removal, up to 55%, evidenced forall the digesters fed with thermal pretreated sludge (no removaloccurred in the control reactors), methane production rates alwaysincreased (up to +47%). The highest methane gain obtained in test#1 evidenced the need to work at low OLR when digesting disinte-grated sludge. Nevertheless, thermophilic conditions resulted indeterioration of dewaterability, especially for untreated sludge.

The assessment of this combined process on full scale scenario,using methane in a CHP system, highlighted attractive electric en-ergy gains, but heat balance resulted positive only in the case ofpretreated thickened sludge.

Acknowledgements

This work was supported by the EU ROUTES project (ContractNo 265156, FP7 2007–2013, THEME [ENV.2010.3.1.1-2] Innovativesystem solutions for municipal sludge treatment and manage-ment). The authors thank Dr. Maria Cristina Gagliano for the sludgemicrographs and the GC analysis, performed at IRSA-CNR of Rome(Italy).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2013.05.069.

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