combined mesophilic anaerobic and thermophilic aerobic digestion process: effect on sludge...

Combined Mesophilic Anaerobic and ThermophilicAerobic Digestion Process: Effect on Sludge Degradationand Variation of Sludge Property

Jiehong Cheng & Yuehong Ji & Feng Kong & Xian Chen

Received: 18 June 2013 /Accepted: 20 August 2013 /Published online: 31 August 2013# Springer Science+Business Media New York 2013

Abstract One-stage autothermal thermophilic aerobic digestion (ATAD) is effective for thereduction of volatile solids (VSs) and pathogen in sewage sludges. A novel process ofcombining mesophilic (<35 °C) anaerobic digestion with a thermophilic (55 °C) aerobicdigestion process (AN/TAD) occurred in a one-stage digester, which was designed for aerationenergy savings. The efficiency of sludge degradation and variation of sludge properties by batchexperiments were evaluated for the AN/TAD digester with an effective volume of 23 L for30 days compared with conventional thermophilic aerobic digestion (TAD). The AN/TADsystem can efficiently achieve sludge stabilization on the 16th day with a VS removal rate of38.1 %. The AN/TAD system was operated at lower ORP values in a digestion period withhigher contents of total organic compounds, volatile fatty acids, protein, and polysaccharide inthe soluble phase than those of the TAD system, which can rapidly decreased and had lowvalues in the late period of digestion for the AN/TAD system. In the AN/TAD system,intracellular substances had lysis because of initial hydrolytic acidification.

Keywords Sewage sludge . Anaerobic digestion . Thermophilic aerobic digestion .

Biodegradation . Cell lysis

Introduction

Thermophilic aerobic digestion (TAD) processes, particularly autothermal thermophilicaerobic digestion (ATAD), perform better in terms of stabilization and pasteurization thanother sludge treatment processes [1–5]. The use of thermophilic digestion (45 to 65 °C) toproduce class A biosolids in a municipal sludge has attracted growing interest after theimplementation of 40 CFR 503 Part (b) sludge reuse and disposal regulations (USEPA 1992)in the USA. The increased rate of degradation, pathogen inactivation, low hydraulic

Appl Biochem Biotechnol (2013) 171:1701–1714DOI 10.1007/s12010-013-0453-2

J. Cheng (*) : F. Kong :X. ChenSchool of Chemical and Environmental Engineering, Jiangsu University of Technology,Changzhou, Chinae-mail: [email protected]

Y. JiSchool of Environmental Science and Engineering, Suzhou University of Science and Technology,Suzhou, China

retention time, and less demand of energy increase the feasibility of ATAD to be used innumerous wastewater treatment plants (WWTPs) in Europe and North America in a large-scale application [6].

As a new method in China, pilot-scale operation studies focusing on the ATAD of sewagesludge have been achieved [7–9]. Aside from one-stage ATAD systems with volumes less than2.3 m3 in 2005 [7, 10], a one-stage ATAD process with a large-scale digester and a volume of12.3 m3 designed in 2009 has been developed since [8, 9]. The previous engineering applicationresults showed that this one-stage ATAD system can achieve the same effect for pathogenreduction and volatile solid (VS) reduction as two-stage or multistage ATAD systems [8, 9].However, during TAD, the high cost of providing a large aeration quantity makes the ATADprocess less popular than other digestion processes. In previous studies, the ATAD digestionsystem showed a low oxidation-reduction potential (ORP; below 0 mV) throughout the entiredigestion process, which implies that the digestion system is not entirely aerobic [1, 9, 11]. Duringinitial thermophilic digestion, the ATAD system combines anaerobic, fermentative, and aerobicconditions with an ORP value below 0 mV in the digester [1, 9, 11]. Researchers have studiedtwo-stage or multistage unit systems containing an aerobic thermophilic stage [3, 12–14].However, two-stage or multistage systems occupy a large area and are more complicated interms of operation than single-stage systems. Hence, strategies based on selected combinations ofunit processes that can be applied to different required treatments are preferred. Thus, a practicalalternative to simplify operation and save aeration energy is to apply a combination of mesophilicanaerobic digestion followed by a thermophilic aerobic digestion (AN/TAD) in a one-stagedigester. Previous research results of one-stage ATAD system at optimal operation conditionswere selected to attain the system.

The main goal of this work is to evaluate the effectiveness of the AN/TAD and to providerecommendations for its practical operation. Based on previous optimum operation param-eters [7, 9, 11], a batch-mode experiment was carried out. The effectiveness of the AN/TADsystem was investigated to elucidate the stabilization process in terms of the removal rate ofVS and oxidation-reduction potential (ORP), which were utilized as surrogate indicators inmonitoring oxygen quantity. Other performance measures, such as variation of nitrogen,phosphorus, carbon, and so on in the supernatant, were also analyzed.

Materials and Methods

Experimental Facilities

Two similar digesters exist: one of them is operated as an anaerobic-TAD (AN-TAD) reactorand another as a TAD reactor. The main body of the digestion reactor was a cylindrical stainlesssteel digester submerged in a water bath to maintain a specific digestion temperature. Thediameter and height of the digester were 300 and 500 mm, respectively, and the effectivevolume was 23 L. On top of the reactor, an odor controller was used to collect the effluent gas.Other equipment include temperature probes in the middle of the reactor and level monitors.

Start Up of Digestion Process and Operation

The sludge was first pre-dewatered to the required total solid (TS) level at a solid content ofapproximately 6 % [15] and was then fed in batches. Based on the optimal temperature of thepilot-scale ATAD [8, 9], the water bath temperature was increased from 20 to 55 °C at a rate of2.5 °C every 12 h [8, 9] and kept at 55 °C afterward during the whole digestion process. For the

1702 Appl Biochem Biotechnol (2013) 171:1701–1714

AN-TAD digester, the mesophilic anaerobic process (≤35 °C) was operated for 3 days [11] inbatch mode by the startup of the stirring sludge to promote mixing and to achieve completesuspension of the sludge. After the anaerobic process, the thermophilic aerobic process wasinitiated when air was provided to the digester with a flow rate of 2.5 L L−1 h−1 by means of amicroporous diffuser. A constant stirring speed of 35 rpm was used during the entire digestionprocess [8, 9]. For the TAD digester, air was supplied from the beginning up to the final phase inbatch mode. The AN-TAD and TAD digesters were all operated in batch mode by stirring thesludge to promote mixing and to achieve complete suspension of the sludge for 30 days.Evaporation losses were recovered by adding distilled water before each sampling.

Sludge Sample

The sludge used was from a municipal WWTP in Changzhou, China. The plant performedan anaerobic-anoxic-oxic process to treat domestic wastewater. The raw sludge employedbefore digestion was from a thickening tank and a mixture of secondary and primary sludge.The characteristics of the feed activated sludge that was fed to the AN-TAD and TADdigesters were the same and are shown in Table 1.

Analysis Method

The pH was not regulated during the entire digestion process. The TAD system used an ORPas an indicator of aerobic character because the monitoring of dissolved oxygen in the TADsystem has proven to be quite difficult. Therefore, the pH and ORP, as well as total organiccompound (TOC), were all monitored every 24 h. Sludge samples were collected every 48 hto determine TSs and VSs, as well as other indicators such as protein (PN), polysaccharide(PS), volatile fatty acid (VFA), total nitrogen (TN), NH4

+–N, total phosphorus (TP),orthophosphate (ortho-P), and SO4

2− in the supernatant of the sludge sample.All the above mentioned indicators, except pH and ORP, were determined in triplicate,

and the standard deviations of all analyses were less than 5 %. The pH of the sludge wasmeasured using a pH meter (pHs-3C; Leici Co. Ltd., Shanghai). Analyses of TSs and VSswere performed based on the standard methods [16]. Destruction of VSs was calculated asthe fraction of VSs destroyed via the constant ash method. The samples were previouslycentrifuged at 12,000×g for 15 min, and the supernatant was then filtered through a 0.45-μmprewashed membrane filter (Advantec GC50 Glass Fiber) to determine the soluble analysesas described below. The filtrate was collected to measure the concentrations of PN, PS, TOC,VFA, TN, NH4

+–N, NO3−–N, NO2

−–N, TP, ortho-P, and SO42−. The PN in the sludge

solution filtrate was determined using the Coomassie Brilliant Blue G-250 method [17], andcasein (Shanghai Sangon Biotechnology Co., Ltd., China) was used as the standard. The PS inthe filtrate was measured using the anthrone–sulfuric acid method [18], and glucose was used as

Table 1 Physicochemical indexes of the feed sludge

Parameter pH TS(%)

VS(g L−1)

VS/TS(%)

STN(mg L−1)

NH4+–N

(mg L−1)Ortho-P(mg L−1)

STP(mg L−1)

Value 7.43 6.01 39.2 65.2 78.7 40.3 40.9 47.1

TN concentration of total nitrogen in the supernatant of the sludge sample, NH4+ –N concentration of

supernatant ammonia nitrogen in the supernatant of the sludge sample, Ortho-P concentration of orthophos-phate in the supernatant of the sludge sample, TP concentration of total phosphorus in the supernatant of thesludge sample

Appl Biochem Biotechnol (2013) 171:1701–1714 1703

the standard. TOC and TN were determined using a TOC/TN analyzer (Analytik Jena MultiN/C 3000, Germany). NOx

−–N, NH4+–N, TP, ortho-P, and VFA were measured using the

distillation-boric acid absorption–titration method [19]. The concentration of SO42− was deter-

mined using ion chromatography (MIC Dionex DX Series, Metrohm Corp., Switzerland).

Results and Discussion

Variation of VS with Digestion Time

The TAD process can stabilize the sludge by destroying VS to produce a class A finalproduct that is suitable for safe disposal and reuse. Therefore, the removal efficiency of VS isa significant indicator for sludge digestion systems [5]. Figure 1 shows the VS removal attwo different processes of simulated AN-TAD and TAD systems.

Although anaerobic digestion was observed on the third day in the primary period andtemperature of the digested sludge were lower than TAD system, the achieved VS removalof the digested sludge was better than the expected results in auto-heated TAD. As shown inFig. 1, the VS removal rate continuously increased up to 50.4 % at 30 days with a VSremoval of 38.1 % at 16 h, which met the class A biosolid definitions in Part 503 of the USEPA 40 CFR (i.e., removal efficiency of VS must exceed 38 % when aerobic digestion isapplied to stabilize the sewage sludge from municipal WWTP) [20]. Compared with theresults of the TAD system with a VS removal rate of 39.7 % at 20 days, the VS removal rateof the AN-TAD process exceeded that of the TAD system from a retention time of 2 daysand the sludge was stabilized after 16 days.

In the one-stage ATAD system, an aeration flow rate of 12.0 to 16.0 m3 h−1 is necessary todigest the sludge by a batch-mode operation [8]. In this study, an initial mesophilic anaerobicdigestion of 3 days saved aeration energy. Anaerobic digestion and auto-heated TAD can operatein the same digester. Thus, a simple structure can be achieved, and only a small area is occupied.

Variation of Oxidation-Reduction Potential (ORP) and pH with Digestion Time

The changes in oxygen quantity in the ATAD digester cannot be monitored via the specificoxygen uptake rate. Thus, ORP was monitored instead to investigate the performance inaerobic, anoxic, and anaerobic biological systems [21]. Studies have indicated that ORP

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VS

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(%)

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Fig. 1 Variation in concentrations of VS in AN-ATAD and TAD digesters


often vary below 0 mV during the stable operation period in a one- or two-stage ATADsystem [4, 9, 11]. In this study, the values of ORP in the AN-TAD digester were observed,and the results are shown in Fig. 2.

In the AN-TAD digester, the ORP values continuously decreased following the initialdecline up to a minimum value of −291 mVat 5 days, which resulted in anaerobic digestionat an initial digestion time of 4 days. The value then fluctuated and increased to −152 mVat30 days. On the whole, the ORP ranged between −291 and −106 mV during the entire AN-TAD process. Although the AN-TAD system maintained the continuous aeration after aninitial anaerobic digestion treatment for 3 days, the ORP had low values below 0 mV fromthe beginning to the end of the digestion process. Correspondingly, the ORP values of theTAD systems were low between −112 and −283 mV. Such results implied that the digestionsystem operated at oxygen-limiting (or anoxic) conditions [9, 22] while providing air to theAN-TAD and TAD digesters. Kelly et al. [1] reported that the ORP often varied between−250 and −50 mV in a two-stage ATAD system. Furthermore, the ORP in the one-stageATAD system was still low (i.e., −180 mV after 240 h) [7, 9, 11]. Such phenomenonsuggests that aeration factor in an AN-TAD system has no significant influence on theORP and resulted in small differences between AN-TAD and TAD systems.

The ORP values were low because the oxygen demand of the biomass is greater than theoxygen supply of the aeration equipment with a high solid content of feed sludge. However,within the thermophilic range from 45 to 65 °C, the maximal solubility of oxygen drops toapproximately 4 mg L−1 [15]. The drop in the maximal oxygen solubility at thermophilictemperatures (55 °C) and a high concentration of TS at 6 % also work against O2 transfer. Inan AN-TAD or TAD system, microaerobic or anaerobic conditions occur because of a lackof oxygen [23, 24]. Hence, a combined process of initial mesophilic anaerobic digestionfollowed by TAD is feasible.

Figure 2 shows that the pH varied and changed from 6.20 to 7.40 for the AN-TAD systemand from 6.86 to 7.63 for the TAD system. The pH finally reached 7.10 and 7.47 at the endof the digestion for the AN-TAD and TAD systems, respectively. Chang et al. [21] found thatthe profiles of the ORP value reveal an opposite trend to the pH value. In this paper,however, no statistically significant correlation (R2<0.31) was observed between pH andORP [21]. From the other research, pH is related to hydrolytic release [15] or tended to be

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pH

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OR

P(m

v)

AN/TAD-pH TAD-pHAN/TAD-ORP TAD-ORP

Fig. 2 Variation in ORP and pH with retention time in the AN-TAD and TAD systems


alkaline in the ATAD system [25]. Accordingly, at anaerobic or microaerobic condition, thechange in pH in the TAD system is complicated and is usually affected by a combination ofvarious factors relevant to acidity and alkalinity, such as the production of VFA and therelease of ammonia in a batch-mode operation system.

Variation of TOC and VFA in the Supernatant

The results of TOC and VFA in the supernatant for the AN-TAD and TAD systems areshown in Figs. 3 and 4, respectively. In the AN-TAD digester, Fig. 3 shows that the TOCconcentration in the supernatant of the sludge rapidly increased up to 8 days with amaximum value of 8,265 mg L−1 and decreased to 2,218 mg L−1 at 30 days. However,the TOC content of the TAD digester was lower than that of the AN-TAD system from 0 to20 days, and the highest value of 5,233 mg L−1 was observed at 8 days, which was 37 %lower than the highest concentration of AN-TAD. Compared with a conventional TADsystem, an increase in the sludge intrinsic biodegradability is also observed (37 %) in theAN-TAD process.

The concentrations of VFAs in the supernatant were also investigated, and the results areshown in Fig. 4. The VFA concentration in the liquid phase of the sludge varied in a similarmanner. The VFA concentration increased to a maximum value of 272 mg L−1 for the AN-TAD digester at 11 days and 240 mg L−1 for the TAD digester at 15 days and then declinedto 58 and 66 mg L−1 at 30 days, respectively. In the AN-TAD system, the initial anaerobicdigestion process resulted in a rapid accumulation of VFA similar to oxidation and fermen-tation at microaerobic or anaerobic conditions in the initial phase of the ATAD system [15,25, 26]. Moreover, the initial anaerobic digestion process caused either rapid disintegrationor solubilization of abundant organic matter. The accumulation of VFA because of hydro-lytic acidification resulted in a decrease in pH, as shown in Fig. 2. The pH constantlydecreased from the beginning of the digestion up to the minimum value of 6.2 at 6 days forthe AN-TAD system. The value then gradually increased afterward. Higher concentrationsof SCOD and VFA in AN-TAD system were presented before 19 and 13 days of digestiontime. These processes contributed to the high concentration of SCOD and VFA duringdigestion time [27, 28]. Moreover, oxygen concentration is often limiting in full-scale TAD

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AN/TAD-TOC TAD-TOC

Fig. 3 Variation in TOC with retention time in the AN-TAD and TAD systems


processes, which contributed to a high demand for oxygen by the rapidly growing thermo-philic populations [29]. However, the dissolved oxygen content in sludges is due to thelimited solubility of oxygen at high temperatures and solid contents of the sludge, limitationsin equipment capability, and high energy demand for aeration [23, 30]. Oxygen limitation isbelieved to cause the accumulation of products of fermentative metabolism (VFAs) in amanner akin to the pockets of facultative and anaerobic metabolism that occur duringcomposting [31]. Therefore, the system cannot possibly retain the aerobic condition thoughcontinuous aeration from the beginning to the end of the digestion treatment because of thehigh solid concentrations of the feed sludge. As a result, the TAD process can be modified toa combination process of initial anaerobic digestion and TAD. Based on the removal rate ofVS (Fig. 1), the AN-TAD digester can efficiently stabilize the sludge compared with theresults of the TAD digester. Accordingly, the use of the AN-TAD system is feasible.

As digestion continued, thermophilic aerobic microbes (bacteria) make full use of thereleased organic compounds to achieve rapid growth and develop predominant populationsin the digestion system [9, 28], whereas less temperature-tolerant microorganisms die. Thus,with decreasing content of organic compounds in the solution, the oxygen demand ofthermophiles decreased and the demand was even lower than the aeration capacity [15],which resulted in an increase in ORP (as shown in Fig. 2). Meanwhile, the TOC valuedecreased (Fig. 3). Similarly, VFA production rapidly decreased and had low values in thelate period of digestion for the AN-TAD system. As a result, TOC and VFA in thesupernatant of digestion sludge effluent were not high at the end of the digestion, whichrelieve negative effect in dewaterability.

Variation of Protein and Polysaccharide with Digestion Time

The results of PN and PS contents in the supernatant for the AN-TAD and TAD systems arepresented in Figs. 5 and 6. The contents of PN for the AN-TAD system gradually increasedup to the highest value of 163 mg L−1 at 18 days and then fluctuated up to a final content of140 mg L−1 at 30 days. On the other hand, PN concentrations in the TAD system were stilllower than in the AN-TAD system after a retention time of 8 days. Similarly, the results ofPS content had similar profiles to that of PN, and the highest value was 2,208 and

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VF

A(m

g/L

)

AN/TAD-VFA TAD-VFA

Fig. 4 Variation in VFA with retention time in the AN-TAD and TAD systems


1,565 mg L−1 at 16 days for the AN-TAD and TAD systems, respectively. Thus, theapplication of thermophilic temperature causes partial or total destruction of cell walls[32], which releases intracellular PS and PN into the liquid phase of the sludge, therebyleading to an increase in PN and PS contents in solution. As a whole, the PN and PS contentsof the AN-TAD-digested sludge were higher than that of the TAD sludge, which indicatethat the initial anaerobic hydrolytic acidification promoted cell lysis of less temperature-tolerant microorganisms. Thus, some nonbroken cell substances had cell lysis and werebioavailable.

Moreover, the concentrations of PS and PN in the liquid at the end of digestion were stillhigher than that of the initial phase, which indicate that some organic matters did not entirelydegrade at the end of digestion. Hence, researchers have suggested that thermophilicallydigested sludge should be given mesophilic aerobic treatment afterward to reduce the

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AN/TAD-PN TAD-PN

Fig. 5 Variation in PN with retention time in the AN-TAD and TAD systems

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Fig. 6 Variation in PS with retention time in the AN-TAD and TAD systems


contents of PN and PS in the liquid phase because mesophilic temperatures (25 and 35 °C)had a significant influence on the hydrolysis of PN and carbohydrates, and the contents ofPN and PS in the sludge supernatant decreased [18, 33–35].

Change of Total Nitrogen, Ammonia, and Nitrate Nitrogen in the Supernatant with DigestionTime

The concentrations of TN, NH4+–N, NO3

−, and NO2− in the supernatant were monitored to

investigate the variation in nitrogen in the AN-TAD and TAD processes. The results areshown in Figs. 7 and 8. As seen in Fig. 7, the TN concentrations for the AN-TAD and TADsystems varied and increased during an initial retention time of 14 and 10 days and reached amaximum of 2,399 and 2,583 mg L−1, respectively, whereas the NH4

+–N concentrationcontinually increased up to the highest value for the AN-TAD- and TAD-digested sludge at10 days. TN fluctuated and rapidly decreased after 10 and 14 days for the AN-TAD and TADsystems, respectively, whereas the contents of NH4

+–N continuously declined after 10 days.These results indicate that cell lysis at thermophilic conditions contributed to a large amountof released intercellular nitrogen to the liquid phase, which resulted in a sharp increase in TNconcentration.

For the AN-TAD and TAD systems, the concentrations of NO3−–N were always less than

5.5 mg L−1 (Fig. 8) and significantly higher than NO2−–N, which implied that thermophilic

digestion inhibited nitrification and denitrification. This result has also been supported byLiu et al. [28], who reported that no nitrification and denitrification were observed in theATAD system. The transformation of nitrogen in the digestion process is characterized bythe degradation of organic nitrogen compounds in the extracellular polymeric substance,which is mainly released in the form of ammonia [36]. Based on the dynamic equilibrium inthe reaction NH3+H2O ⇔ NH4

+ + OH− [7], the concentration of NH4+–N increased because

of the conversion of soluble organic nitrogen in the supernatant to ammonia. As a result, theNH4

+–N concentration increased during an initial digestion time of 10 days. However,constant aeration will promote the release of ammonia as gas from the digester, which willdecrease the NH4

+–N concentration after 10 days. The reason that the contents of NH4+–N in

the AN-TAD system are higher than in the TAD system during the first 10 days is that theAN-TAD system lacks air supply because of an initial anaerobic phase for 3 days.

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cent

rati

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AN/TAD-TN AN/TAD-

TAD-TN TAD- NH4+-N

NH4+-N

Fig. 7 Variation in TN and NH4+–N with retention time in the AN-TAD and TAD systems


Change of Total Phosphorus and Orthophosphate in the Supernatant with Retention Time

The variation in TP and ortho-P with digestion time for the AN-TAD and TAD systems areshown in Figs. 9 and 10, respectively. In the AN-TAD system, the TP in the supernatant ofsludge increased to 878 mg L−1 at 8 days and declined to 95.8 mg L−1 at 11 days (Fig.9). Thevalue slightly increased to 368 mg L−1 at 20 days and then fluctuated at about 300 mg L−1 atthe end of the digestion. The change in ortho-P at different digestion temperatures varied in asimilar manner. Similarly, the TP in the sludge supernatant in the TAD system had amaximum value of 581 mg L−1 at 10 days, which is lower than the maximum of878 mg L−1 in the AN-TAD system. The TP in the sludge supernatant then decreased tothe lowest value of 61 mg L−1 at 14 days (Fig. 10).

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NO3--N

NO3--N

NO2--N

NO2--N

Fig. 8 Variation in the ratio of NO3−–N and NO2

−–N in the supernatant with retention time in the AN-TADand TAD systems

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Fig. 9 Variation in TP and ortho-P with retention time in the AN-TAD system


In this paper, feed sludge is a mixture of secondary and primary sludge from a municipalWWTP, in which an anaerobic-anoxic-oxic process was applied to remove nitrogen andphosphorus in domestic wastewater containing large amounts of phosphorus compounds.Abundant intercellular substrates in the sludge can be rapidly released into the supernatant ata thermophilic temperature because less temperature-tolerant microorganisms undergo celllysis, thereby causing large quantities of phosphorus to be released into the liquid phase,which caused the concentration of ortho-P to increase. The AN-TAD digester had a highconcentration of TP in the early digestion process because of initial anaerobic digestionfollowed by thermophilic temperature and oxygen deprivation. Afterward, TP andortho-P sharply decreased after 8 and 10 days for the AN-TAD and TAD systems,respectively, because the released phosphorus began to precipitate. The levels of Ca2+

and Mg2+ in the liquid and solid phases of the sewage sludge were very high [37, 38].Figure 7 shows the high concentrations of NH4

+–N. The high copresence of Ca2+ and Mg2+

together with high concentrations of ammonia nitrogen and phosphates can lead to theformation of particular salts [39, 40], hydroxyapatite (Ca5(OH)(PO4)3, HAP), and struvite(MgNH4PO4·6H2O, MAP) [38]. Hence, TP and ortho-P in the soluble phase of the sludgedecreased at 8 and 10 days for the AN-TAD and TAD systems, respectively. When thedigestion time was continued after 12 days, phosphorus was continuously releasedbecause of cell lysis at a thermophilic temperature condition, thereby causing the concen-tration to increase.

For these two kinds of system, the ratios of ortho-P to TP had high values from 62 to98 % during the entire digestion time, which implies that other forms of phosphorus(including polyphosphate and organic phosphorus) were present in the digested sludge inaddition to ortho-P because no gaseous phosphorous substances were observed in thesupernatant. However, further investigation is required.

Variation of Content of SO42− in the Supernatant with Digestion Time

As seen in Fig. 11, sulfate content slightly increased within the first 4 days, then rapidlyincreased from 74 mg L−1 at 4 days to a maximum value of 860 mg L−1 at 14 days. Amoderate fluctuation occurred between 680 and 860 mg L−1 followed by a slow decline at15 days. Accordingly, the sulfate production rate increased with increasing digestion time.

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Fig. 10 Variation in TP and ortho-P with retention time in the TAD system


Within an anaerobic digestion period of 3 days, the temperature in the digester wasbelow 35 °C, and the contents of most of the released organic macromolecularcompound (including PN) remained low. As a result, sulfate content showed nodistinct increase, which resulted in the degradation of few sulfur-containing proteins(the existing form of organic sulfur). When the digestion time was changed from 3 to15 days, aeration began to be supplied in the digester. However, sulfate was producedin the liquid, and the content gradually increased up to a maximum value of 860 mg L−1 at14 days. At the same time, ORP showed values between −345 and −185 mV whichresulted in oxygen-deprived conditions and suggest that oxygen deprivation in thedigester had less inhibition on sulfate formation. However, sulfur-oxidizing microor-ganisms survived in the thermophilic oxygen-limiting condition, which is consistentwith the report of Jain et al. [41]. Although the air was sparged at a flow rate of2.5 L L−1 h−1 to ensure good aeration and constant mixing of the sludge, ORP stillhad values below −100 mV (Fig.2). This result showed significant difference withother results, which found that ORP increased from −250 to −180 to 220 to 370 mV after28 days of digestion [42] because of different solid contents of the feed sludge and aerationquantities. A solid content of 6 % in this research is significantly higher than 0.5 to 4 % in thereport of Chen et al. [42], which indicates that a high solid content is a main reason for relativelylow ORP values.

Conclusion

In TAD, the ATAD system exhibited anaerobic or microaerobic conditions because of a highsolid content of the feed sludge. However, excessive aeration was applied to the digester.Considering that an initial anaerobic condition existed in the ATAD system, a combinationprocess of anaerobic and TAD (AN-TAD) can be utilized, which can achieve the same VSremoval efficiency as that of the ATAD system. Compared with the TAD system, the AN-TAD system had lower ORP values as well as higher contents of TOC, VFA, nitrogen,phosphorus, protein, and polysaccharide in the soluble phase during middle phase of theentire digestion, which they can easily decreased and had low values in the late period ofdigestion at the end of the digestion. This result indicates that the initial anaerobic processdestroyed more organic matters and caused faster biodegradation because of hydrolyticacidification.

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Fig. 11 Variation in SO42− with digestion time in the AN-TAD system


Acknowledgments This study was supported by Jiangsu Overseas Research and Training Program forUniversity Prominent Young and Middle-aged Teachers and Presidents, the Item of Jiangsu Province “333Hi-class personnel training project” (no. BRA2011185), and the Environmental Protection Agency of JiangsuProvince (no. 2008020).

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