Bioresource Technology 140 (2013) 187–191

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Bioresource Technology

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Alkaline post-treatment for improved sludge anaerobic digestion

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.04.093

⇑ Corresponding author. Tel.: +86 755 26036105; fax: +86 755 26036709.E-mail address: lihuansz@qq.com (H. Li).

Huan Li ⇑, Shuxin Zou, Chenchen Li, Yiying JinShenzhen Environmental Microbial Application and Risk Control Key Laboratory, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

h i g h l i g h t s

� Alkaline post-treatment was applied to sludge anaerobic digestion.� Optimized post-treatment conditions were concluded.� The biogas production increased by 33%.

a r t i c l e i n f o

Article history:Received 6 March 2013Received in revised form 23 April 2013Accepted 25 April 2013Available online 3 May 2013

Keywords:SludgeAlkaline treatmentAnaerobic digestion

a b s t r a c t

Alkaline post-treatment was tested in order to improve sludge anaerobic digestion. Between the 8th andthe 12th hour of a 24-h digestion cycle, 5% of sludge was extracted from a semi-continuous digester witha sludge retention time of 20 days. The sludge was then disintegrated with 0.1 mol/L NaOH and returnedto the digester after neutralization. The results showed that alkaline post-treatment increased the level ofsoluble organic substances in the extracted sludge, particularly of volatile fatty acids and polysaccharides.This process resulted in a 33% enhancement of biogas production in comparison with the control. Whenthe ratio of the recycled sludge was further increased to 10% or 15%, the increment of biogas yield wasreduced, due to excessive inactivation of anaerobic bacteria in the digester. Alkaline post-treatmenthad a minimal impact on the dewaterability of digested sludge.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

A continuous increase in the levels of waste biomass (includingprimary sludge and excess sludge) discharged from wastewatertreatment processes has been noted worldwide. Since thesesludges typically contain heavy metals, pathogens and persistentorganic pollutants, the need for a safe and cost-effective meansof disposal is becoming increasingly important. Various techniquesfor sludge decrement have been developed, including anaerobicdigestion (AD), a traditional technology for sludge stabilizationand reduction. Such digesters are characterized by their large vol-umes and long sludge retention times. The hydrolysis of sludgeparticles is recognized as the rate-limiting step (Bougrier et al.,2006) and many disintegration technologies have been applied inorder to accelerate hydrolysis or increase the degree of degradationwith a fixed duration time. Mechanical treatment (Hwang et al.,1997; Nah et al., 2000), thermal treatment (Bougrier et al., 2008;Appels et al., 2010; Nielsen et al., 2011), chemical treatment (Linet al., 1997; Navia et al., 2002; Devlin et al., 2011; Li et al., 2012)and ultrasonic treatment (Neis et al., 2000; Hogan et al., 2004;Kim and Lee, 2012) can work alone or combined together (Vlys-sides and Karlis, 2004; Takashima and Tanaka, 2010; Vigueras-Car-

mona et al., 2011; Saha et al., 2011) prior to the anaerobic digestionof sludge. These pretreatments can disrupt sludge flocs and cells,release inner organic matter, accelerate sludge hydrolysis and, con-sequently, improve the performance of subsequent anaerobicdigestion (Weemaes and Verstraete, 1998; Kim et al., 2003).

The above-mentioned retreatment methods are effective for theanaerobic digestion of excess sludge or its mixture with primarysludge, because such methods can disrupt cell walls and zoogleain excess sludge. Nevertheless, when the influent sludge of anaer-obic digesters is mainly composed of primary sludge, the effect ofpretreatments may be limited because many organic substances inprimary sludge are easily dissolved and bio-utilized during anaer-obic digestion. Under these conditions, disintegration methodsmay be applied to the partly-digested sludge extracted fromdigesters, rather than to the influent sludge. During semi-continu-ous anaerobic digestion, the degradation of organic matter slowsdown gradually after the influent sludge is fed into digesters, i.e.,most organic substances are degraded during the first half of adigestion cycle, and the production of biogas is very limited duringthe second half of the cycle – with the result that, at the end of theprocess, there are still some biodegradable organic substancesremaining inside microbial cells and anaerobic sludge particles.These organic substances can be released by disintegration meth-ods, thus enhancing the subsequent anaerobic digestion process.This means that some of the partly-digested sludge can be ex-

188 H. Li et al. / Bioresource Technology 140 (2013) 187–191

tracted from digesters, disintegrated and then returned back to thedigesters. Thus, instead of pretreatment of influent sludge, thetreatment of partly-digested sludge can be regarded as a usefulpost-treatment method for improved anaerobic digestion.

Among the above-mentioned disintegration methods, alkalinetreatment has several advantages: it is simple to manufacturethe device; it is easy to operate and is highly efficient. Most inves-tigations have indicated an increase of biogas production and a de-crease in volatile solids, especially during low-dose alkalinetreatment (Lin et al., 1997; Lin et al., 2009; Navia et al., 2002; LópezTorres and Espinosa Lloréns, 2008). In most cases, the preferred al-kali was sodium hydroxide (NaOH), which yields greater solubili-zation efficiency than calcium hydroxide (Ca(OH)2) (López Torresand Espinosa Lloréns, 2008). In most cases, the optimum dose ofNaOH was at a level of 0.08–0.16 g/g total sludge solids (TS) (Linet al., 1997; Lin et al., 2009) or 0.25 g/g TS (Navia et al., 2002).Higher doses led to a limited increase in solubilised sludge organicmatter, but the residual NaOH would require more HCl for neutral-ization and higher concentration of Na+ may also inhabit the activ-ity of microorganisms (Li et al., 2012). Although much attentionwas paid to alkaline sludge treatment, most of the related researchhas only focused on alkaline pretreatment or its combination withother methods prior to anaerobic digestion of sludges (Lin et al.,1997; Lin et al., 2009; Navia et al., 2002; López Torres and EspinosaLloréns, 2008). The information on alkaline posttreatment duringsludge anaerobic digestion was not found.

To the best of the knowledge, the work focused on the feasibil-ity of improving sludge anaerobic digestion by alkaline posttreat-ment. The variation of biogas generation was analyzed duringcommon semi-continuous anaerobic digestion in order to selectoptimal times for extraction, disintegration and reflux of a certainamount of partly-digested sludge. The effects of alkaline disinte-gration with NaOH on the extracted sludge were examined. Opti-mal conditions for NaOH post-treatment were also investigated,so as to enhance the disintegration of extracted sludge duringthe second half of a digestion cycle. Research has also focused onthe biogas production during the subsequent anaerobic digestionand the dewaterability of digested sludge (following the additionof the disintegrated sludge into the digesters).

2. Methods

2.1. Sludge samples

Sewage sludge used in this study was collected from a local full-scale municipal wastewater treatment plant (WWTP), to which abiological aerated filter process was applied. The excess biofilmsludge and the primary sludge were mixed, thickened, conditionedwith polyacrylamide (PAM) and dewatered by centrifugation. Thesludge was composed of about 80% primary sludge and 20% biofilmsludge. The dewatered sludge was collected and stored at 4 �C in arefrigerator. The dewatered sludge was diluted and used as theinfluent sludge for anaerobic digestion. The pH of the influentsludge was 7.0, and the concentrations of total suspended solid(TS) and volatile solids (VS) were maintained at 20 g/L and 11.3–12.9 g/L, respectively.

2.2. Anaerobic digestion

Two laboratory semi-continuous anaerobic digesters were usedfor experiments. One was operated as an alkaline post-treatment-assisted anaerobic digestion (APAD) system; the other as a conven-tional anaerobic digestion (CAD), i.e., without alkaline post-treat-ment. The two digesters both had effective volumes of 6 L. In

both systems the digested sludge was discharged and then theinfluent sludge was added once a day. Sludge retention time(SRT) was 20 days. A set of electric heating devices and tempera-ture sensors were used to control the temperature inside thedigesters at 35 ± 2 �C. The biogas yields of the two digesters wererecorded by means of wet air flowmeters.

During the APAD process, a certain volume of sludge was drawnout from the digester, disintegrated with NaOH, and then returnedto the digester after neutralization with HCl. This treatment pro-cess was carried out once at the beginning of the second half of adigestion cycle. The recycling ratio (RR) was defined as the ratioof the volume of the recycled sludge against the effective volumeof the digester.

2.3. Alkaline sludge treatment

The partly-digested sludge extracted from the APAD digesterwas disintegrated with NaOH in a 2.0 L beaker while stirring withan automatic device. After the addition of NaOH, the sludge wasstirred at 240 rpm for 30 min, 60 min, 90 min and 120 min. Anoptimum NaOH dose of 0.l mol/L was used during the alkalinesludge treatment (Navia et al., 2002; Li et al., 2008). Followingthe alkaline treatment, the sludge was neutralized to pH 7–8 with6 mol/L HCl. The solubilization of sludge organic matter was as-sessed in terms of the soluble chemical oxygen demand (SCOD).

2.4. Analytical procedures

Water content, TS, VS, and chemical oxygen demand (COD)were measured according to standard methods (MEP, 2002).Sludge pH was measured with a pH meter (EUTECH Cyberscan510,Singapore). To measure SCOD, sludge samples were centrifuged at5000g for 10 min, after which the supernatant was filtered througha membrane of 0.45 lm mesh. The filtrate was used to determinesoluble COD (SCOD).

Protein content was measured using the coomassie brilliantblue method, which is not subject to interference by K+, Na+,Mg2+, small peptides or free amino acids (Wang and Qin, 2003).Polysaccharides were measured using 3,5-dinitrosalicylic acid col-orimetry (Wang and Qin, 2003). Ammonia nitrogen was deter-mined by the method of Nessler’s reagent photometry (MEP,2002). Volatile fatty acids (VFA) were measured with the titrationmethod and the results were expressed as acetic acids (Wang et al.,2008).

The dewaterability of the digested sludge, discharged from theAPAD and CAD digesters, was measured by means of dewateringtests, carried out in a set of vaccum filtration devices. The digestedsludge was first conditioned with 10 mg/L cationic polyacrylamide(PAM). A 100 ml of the digested sludge was placed into a 250 mlconical flask, and PAM was gradually added whilst gently swirlingthe flask until coagulation occurred. Once coagulated, the samplewas placed into a vacuum filter complete with a medium-velocityquantitative filter paper. Cake solids were determined after thenegative pressure remained at 0.045 MPa for 10 min, and the tur-bidity of the filtrate was detected with a turbidimeter (WGZ-B,Cany, China).

To determine the total nitrogen (TN) and total phosphorus (TP)in the supernatant of the digested sludge, the digested sludge wasfirst centrifuged, then the supernatant was filtered through amembrane with a mesh size of 0.45 lm, and finally the filtratewas used for TN and TP determination. TN was measured with aTOC/TN analyzer (TOC-Vcph, Shimadzu), and TP was measuredwith Ammonium molybdate spectrophotometric method (MEP,2002).

Fig. 1. Biogas production during two 24-h cycles from a semi-continuous sludgeanaerobic digester.

H. Li et al. / Bioresource Technology 140 (2013) 187–191 189

3. Results and discussion

3.1. Alkaline disintegration of recycled sludge

The optimum time for extracting sludge from the APAD digestershould be determined prior to the application of alkaline post-treat-ment to anaerobic digestion. An illustration of the biogas produc-tion of semi-continuous anaerobic digestion in a 24-h cycle isgiven in Fig. 1. After the addition of influent sludge, biogas produc-tion increases rapidly during the initial stage, and then slows downafter 8–12 h. The two experiments indicated that half of the diges-tion time was spent on a low-efficiency operation, although thisoperation may be necessary to meet the final requirements forsludge decrement and stabilization. Biogas production was foundto be dependent on the distribution of biodegradable organic sub-stances in the influent sludge. Soluble organic substances, or eas-ily-dissolved organic substances in the outer layer of sludgeparticles, were initially degraded by anaerobic microorganisms.Thus, the rapid production of biogas occurred during the initialstage of a digestion cycle but residual organic substances withinsludge particles were not easily utilized, due to the protection of cellwalls or zooglea. For this reason, biogas production was severelylimited during the second half of a digestion cycle. Similar observa-tions on biogas production during semi-continuous anaerobicdigestion have been noted by other researchers (Devlin et al., 2011).

Borja et al. (2005) proposed that the hydrolysis of sludge parti-cles during anaerobic digestion approximates a first-order reactionmodel, which fits with the observation that biogas productionslows down gradually with decreasing levels of sludge organicmatter. Consequently, alkaline post-treatment can be introducedto anaerobic digestion so as to increase the amount of substratefor anaerobic microorganisms by disintegrating a certain amountof partly-digested sludge.

Fig. 2. SCOD of the extracted sludge after alkaline treatment with 0.1 mol/L NaOH.

By the end of the tenth hour, the biogas yield had reached about80% of the total yield in a 24-h cycle. Alkaline post-treatment wastherefore tested at the end of the tenth hour. A certain quantity ofpartly-digested sludge was extracted from the APAD digester, andtreated with 0.1 mol/L NaOH. The treated sludge was neutralizedand finally added back into the APAD digester. The TS and VS ofthe extracted sludge were determined to be 17.1 g/L and 7.4 g/L,respectively, at a stage when the degradation rate of sludge organicmatter was calculated as 34.5%. The disintegration effect of the ex-tracted sludge can be demonstrated by the changes in the SCOD(Fig. 2). Increased levels of SCOD indicated the release and dissolu-tion of organic substances from sludge particles. After 30 minNaOH treatment, SCOD of the extracted sludge increased by threetimes. When the alkaline treatment duration was extended to 60–120 min, the further increase of SCOD was very limited. Since thetreated sludge should reflow into the APAD digester timely, thetreatment duration was controlled at 30 min. Compared with theeffects of alkaline treatment on other kinds of sludge (Li et al.,2012), it was found that the SCOD increase in the extracted sludgewas lower than that of the raw sludge and higher than that of thedigested sludge. The comparison showed that the effect of NaOHtreatment was also influenced by the content of dissolvable organ-ic matter.

The obvious increase of sludge SCOD verified the effect of alka-line treatment on the dissolution of sludge organic matter. Furtheranalyses on the components of the alkaline lysate were carried out(Table 1). Alkaline treatment was reported to result in a high levelof protein and polysaccharide dissolution from excess sludge (Caiet al., 2004; Li et al., 2009; Peng et al., 2012). Nevertheless, onlya small amount of protein was found in the supernatant of the ex-tracted sludge, and the increase of protein concentration was alsovery limited after alkaline treatment. Considering that the coomas-sie brilliant blue method only reflected the concentration of pro-tein instead of small peptides or free amino acids (Wang andQin, 2003), the results indicated that most of the protein was de-graded at different degrees during the first half of a 24-h digestioncycle. Moreover, alkaline treatment alone can also enhance thedecomposition of protein into small peptides or amino acids. Con-sequently, the concentration of soluble protein in the extractedsludge was not increased obviously after alkaline treatment. Incontrast, the concentration of polysaccharides in the supernatantof the extracted sludge was relatively high, and alkaline treatmentresulted in a higher solubilisation of polysaccharides. Althoughalkaline treatment can not result in the decomposition of polysac-charides into oligosaccharides or monosaccharides, the disruptionof sludge particles and microbial cells led to the release of polysac-charides from cells. The increase of soluble polysaccharides wascertainly much lower than that when alkaline treatment was ap-plied to excess sludge (Wei and Liu, 2010).

The concentration of volatile fatty acids (VFA) in a well-run di-gester is normally between 200 mg/L and 400 mg/L (Grady andLim, 1980). The concentration of VFA in the supernatant of the ex-tracted sludge was relatively low, which indicated that the anaer-obic digestion was almost completely after a 10-h reaction period.This situation was consistent with the biogas production, andhence the time point for extracting partly-digested sludge wasproved to be correct. VFA was generated from micromolecular or-ganic substances inside acidogenic bacteria, and then excreted byacidogenic bacteria. Alkaline sludge treatment can destroy someacidogenic bacteria and release the VFA contained in these acido-genic bacteria. Moreover, alkaline treatment resulted in the releaseof fats from sludge particles and subsequent hydrolysis into glycer-ine and fatty acids, with a consequent increase in the concentrationof VFA. This increase was beneficial to biogas production duringthe subsequent anaerobic digestion stage.

Table 1Effects of 30 min alkaline treatment with 0.1 mol/L NaOH on the sludge extractedfrom the digester.

Beforetreatment

Aftertreatment

d

SCOD (mg/L) 490.6 ± 69.5 1630.2 ± 215.8 1139.6a

Protein (mg/L) 2.2 ± 0.1 4.4 ± 0.4 2.2a

Polysaccharide (mg/L) 71.9 ± 7.8 136.0 ± 14.1 64.1a

VFA (mg/L, based on aceticacid)

84.0 ± 4.0 132.0 ± 6.7 48.0a

NHþ4 -N (mg/L) 280.0 ± 6.5 279.5 ± 14.0 0.5b

a P 6 0.05.b P > 0.05.

190 H. Li et al. / Bioresource Technology 140 (2013) 187–191

The concentration of ammonia nitrogen in the supernatant ofthe extracted sludge was close to that of some types of com-pletely-digested sludge (Mei et al., 2012) and this was also in ac-cord with the trend of biogas production. After alkalinetreatment the concentration of ammonia nitrogen indicated verylittle change. The variation in the above parameters showed that,after most of soluble and easily-biodegradable organic substanceswere consumed during the initial stage of anaerobic digestion,alkaline treatment and acid neutralization enhanced the solubilisa-tion of inner organic substances and did not significantly increasethe overall level of inhibitory factors.

3.2. Effects of alkaline post-treatment on biogas production

Besides VFA and NHþ4 -N, the concentration of Na+ may also havean impact on anaerobic digestion. In general, 3.5–5 g/L Na+ canmoderately inhibit the activity of mesophilic methanogens, and8 g/L Na+ can lead to strong inhibition (McCarty, 1964). Alkalinetreatment of the recycled sludge would increase the concentrationof Na+ in the APAD digester. Since the extracted sludge was treatedwith 0.1 mol/L NaOH, the concentration of Na+ in the APAD diges-ter would finally reach 2.3 g/L when the concentration achievedstability in the digester. This means that the inhabitation of Na+

was negligible. If the dose of NaOH were to be increased to0.3 mol/L or 0.5 mol/L, the alkaline sludge treatment would dis-solve more organic substances and the concentration of Na+ wouldincrease to levels of 6.9 g/L and 11.5 g/L, respectively. A high-salin-ity environment would thus lead to a strong inhabitation of anaer-obic bacteria. It can thus be shown that alkaline post-treatment,with 0.1 mol/L NaOH and HCl conditioning, is suitable for APADdigesters.

On the other hand, alkaline treatment can also inactivate anaer-obic microorganisms directly. Hence, the APAD digester coulddeteriorate, or even fail, if the recycling ratio (RR) is too high. Theeffects of RR on the biogas production of the APAD digester in a24-h cycle are shown in Fig. 3. In the contrast digester (CAD), the

Fig. 3. Biogas production in a 24-h cycle from the CAD and APAD digesters withdifferent recycling ratios (RR).

biogas production slowed down gradually, from the tenth hour,due to a lack of soluble organic substrate. When alkaline post-treatment was applied to the APAD digester, the biogas productionwas enhanced after the treated sludge was added back into the di-gester. When the RR was 5%, the accumulative biogas yield in-creased by 26.5% in a 24-h cycle. When the RR was 10% or 15%,the biogas yield also increased, but the increment was lower thanthat when the RR was 5%. The decrease in biogas yield may beattributed to the excessive inactivation of anaerobic bacteria.

The comprehensive effect of NaOH post-treatment on sludgeanaerobic digestion depended on the increase of releasable organicsubstrate and the inhibition of increased Na+. Considering the per-formance of the APAD digester, the RR of 5% was sufficient for alka-line post-treatment with 0.1 mol/L NaOH. Under these conditions,the degradation rate of sludge organic matter increased from 39.8%of the CAD digester to 44.5% of the APAD digester, the biogas yieldincreased from 364 ml/g VS to 381 ml/g VS, and the methane pro-portion in biogas remained constant at a level of 54.1–57.1%. Withthe help of NaOH post-treatment, the specific biogas yield in-creased slightly, due to a higher level of dissolution of polysaccha-rides and fats, which can produce more biogas than is the case forprotein (Wang, 1997). The increase of biogas yield and theenhancement of sludge decrement demonstrated that sludgeanaerobic digestion had improved.

The continuous performances of the APAD digester and the CADdigester are compared in Fig. 4. During the initial stage, the twodigesters were run without alkaline post-treatment, until the24th day. The biogas yields of the two digesters were very similarand both decreased slowly day-by-day, possibly because a slowfermentation had occurred when the influent sludge samples werestored in a refrigerator. From the twenty-fifth day, alkaline post-treatment was applied to the APAD digester. During this period,alkaline post-treatment increased the biogas yield of the APAD di-gester compared with that of the CAD digester. Similarly, the bio-gas yields of the two digesters continued to decrease graduallydue to the slow fermentation of the influent sludge samples duringits storage. From the forty-fourth day, new sludge was collectedfrom the WWTP as the influent sludge, and the sludge had an or-ganic content of 64.5%. The higher organic content of the influentsludge led to higher biogas yields. Certainly, the APAD digester stillproduced more biogas. It seems that the APAD digester showed agreater advantage, in terms of biogas production, when the organiccontent of the influent sludge increased. Results, however, indi-cated that alkaline post-treatment increased the total biogas yieldby 33.6% during the period from the twenty-fifth day to the forty-third day, and by 32.0% during the period from the forty-fourth dayto the sixtieth day. This indicates that the organic content of theinfluent sludge had low impact on the effects of alkaline post-treatment.

Fig. 4. Biogas production from the CAD and APAD digesters with continuousoperation.

Table 2Characteristics of the digested sludges derived from the CAD and APAD digesters.

CAD APAD d

Water content of sludge cake (%) 81.6 ± 3.2 82.7 ± 2.0 1.1b

Turbidity of the dewatering effluent(NTU)

17.4 ± 5.6 17.0 ± 5.0 0.4b

SCOD (mg/L) 342.9 ± 27.1 348.6 ± 4.3 5.7b

TN (mg/L) 346.1 ± 26.0 298.9 ± 3.7 47.2a

TP (mg/L) 1.6 ± 0.3 1.2 ± 0.1 0.4a

a P 6 0.05.b P > 0.05.

H. Li et al. / Bioresource Technology 140 (2013) 187–191 191

3.3. Effects of alkaline post-treatment on the characteristics of digestedsludge

The characteristics of the digested sludge from the APAD diges-ter and the CAD digester were tested during the last 4 days of theexperiment (Table 2). For the two kinds of digested sludge, thewater contents of sludge cake and the turbidity values of dewater-ing effluent were similar. The results indicated that the effect ofalkaline post-treatment on the dewaterability of digested sludgewas almost negligible. The alkali-treated sludge was only 5% ofthe total sludge in the digester. After the alkali-treated sludgewas added back into the digester, it was mixed completely withthe residual sludge. Hence the alkaline post-treatment had very lit-tle influence on the dewatering performance of digested sludge.

After alkaline post-treatment, the increase of SCOD in digestedsludge was almost negligible. Compared to the CAD-derived di-gested sludge, the supernatant of the APAD-derived digestedsludge had lower levels of TN and TP, although the decrement ofTN and TP in the supernatant of digested sludge was very limited.It was deduced that some ammonia, phosphate and Mg2+ formedsmall amounts of struvite precipitation under alkaline conditions(Stratful et al., 2001).

4. Conclusions

Alkaline post-treatment, with optimal doses of NaOH and aproper recycling ratio of sludge, can improve sludge anaerobicdigestion. During the eighth to the twelfth hour in a 24-h digestioncycle, 5% of sludge can be extracted from a digester, disintegratedwith 0.1 mol/L NaOH, and finally returned to the digester afterneutralization. Under these conditions, biogas production in-creased by 33%, and the degradation rate of sludge organic matterincreased from 39.8% to 44.5%, compared to the values of theseparameters in the control. Alkaline post-treatment was shown tohave very little influence on the dewatering performance ofdigested sludge.

Acknowledgements

Financial support for this project was obtained from the ChinaMajor Science and Technology Program for Water Pollution Controland Treatment (No. 2011ZX07317), the Natural Science Foundationof China (51008174), and the Shenzhen Science and TechnologyResearch and the Development Fund.

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