Journal of Environmental Chemical Engineering 1 (2013) 73–78

Pilot-scale operation of enhanced anaerobic digestion of nutrient-deficientmunicipal sludge by ultrasonic pretreatment and co-digestion of kitchen garbage

Zai-li Zhang a,b, Ling Zhang a,b, Ya-liang Zhou a,b, Jin-can Chen a,b, Yong-mei Liang a,b, Liu Wei a,b,*a School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, Chinab Guangdong Province Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China

A R T I C L E I N F O

Article history:

Received 14 February 2013

Received in revised form 22 March 2013

Accepted 27 March 2013

Keywords:

Nutrient-deficient activated sludge

Anaerobic digestion

Ultrasonication

Co-digestion

Kitchen garbage

A B S T R A C T

A pilot-scale study was carried out to evaluate the in situ anaerobic digestion of nutrient-deficient

activated sludge (AS) from southern China. A single and two-stage system were compared and the

performance of ultrasound pretreatment and co-digestion with kitchen garbage was evaluated by

comparing volatile solids removal and biogas production. The results showed that two-stage systems

performed better than single-stage systems at high feeding concentration and short retention time.

Ultrasonic pretreatment significantly enhanced sludge solubilization though cell lysis and subsequently

increased biogas yields. Mixture with kitchen garbage resulted in substantial volatile solids reductions in

both single and two-stage systems of up to 47% as well as significant increases in biogas yields. Processes

including digestion, dewatering, drying and incineration of this type of sludge were proposed, and mass

balance calculation showed that these processes were feasible and had the potential to provide

environmental and economic benefits for wastewater treatment plants in southern China.

� 2013 Elsevier Ltd All rights reserved.

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Journal of Environmental Chemical Engineering

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / jec e

Introduction

In the last few years, the number of wastewater treatmentplants (WWTP) in China has increased significantly, giving a hugerise to the production of activated sludge (AS). At the end of 2010,the national urban sewage treatment capacity reached 45.6 bil-lion m3/year [1]. In prosperous southern China, combined sewersystems have been adopted, resulting in dilution of wastewater byrainfall; moreover, industrial wastewater is also accepted bydomestic sewage treatment plants, resulting in lower COD (130–185 mg/L) and ammonia nitrogen (13–22 mg/L) levels in waste-water [2]. Due to above reasons, AS produced through biologicaltreatment processes in southern China are nutrient deficient (VS/TS < 58%). Considering urban sustainable development andenvironmental protection, to find an effective and economicsolution to nutrient deficient sludge is imminent and of greatsignificance.

Anaerobic digestion is theoretically not available for treat-ment of low organic content sludge. However, among biologicaltreatments, anaerobic digestion is frequently the most cost-effective because of the high energy recovery linked to theprocess and its limited environmental impact [3]. In fact, more

* Corresponding author at: School of Environmental Science and Engineering, Sun

Yat-sen University, Guangzhou 510275, China. Tel.: +86 20 39332690;

fax: +86 20 39332690.

E-mail address: esslw@mail.sysu.edu.cn (L. Wei).

2213-3437/$ – see front matter � 2013 Elsevier Ltd All rights reserved.

http://dx.doi.org/10.1016/j.jece.2013.03.008

than 36,000 anaerobic digesters are currently in operation inEurope, treating around 40–50% of the sludge generated [3].However, the organic content (VS/TS) of sludge is high in Europe,reaching 70–80% [4]. There has been a lack of studies focused onanaerobic digestion of excess sludge with low nutrient levels.Therefore, it is important to enhance anaerobic digestion ofnutrient-deficient municipal sludge to promote sludge treat-ment in China.

Generally, four major steps can be distinguished in anaerobicdigestion of particulate material and macromolecules: hydrolysis,acidogenesis, acetogenesis and methanogenesis [5]. In the case ofsewage sludge digestion, biological hydrolysis has been identifiedas the rate-limiting step [6,7]. Various chemical, mechanical andbiological sludge disintegration methods have been investigated toimprove rate limiting hydrolysis and digestion performance [1].Among these methods, sonication is the most widely appliedmethod to lyse microbial cells for extraction of the intracellularmaterial [8]. Tiehm et al. [9] conducted an experiment toinvestigate the effect of ultrasound pretreatment on sludgedegradability using ultrasound at a frequency of 31 kHz andshowed that VS removal increased from 45.8% to 50.3%. Enhance-ment of biogas production ranges from 24% to 140% in batchsystems and from 10% to 45% in continuous or semi-continuoussystems [1,10–12]. Thus, ultrasonic treatments were applied as apretreatment of anaerobic digestion to investigate its performancein low content sludge.

In addition to its resistance to biodegradation, the low C/N ratioof AS of 6/1-16/1 is also regarded as a problem in anaerobic

Nomenclature

AS activated sludge

COD chemical oxygen demand (mg O2/L)

VS volatile solids (g/L)

TS total solids (g/L)

VS/TS degree of organic content of sludge (%)

C/N carbon to nitrogen ratio (%)

WWTP wastewater treatment plant

LCS low concentration sludge

CSTR continuous stirred tank reactors

OLR organic loading rate (kg VS/(m3 d))

SRT sludge retention time (d)

VFA volatile fatty acids (mg CH3COOH/L)

MLVSS mixed liquor volatile suspended solids

VSR volatile solids removal (%)

SBP specific biogas production (L/(g VS fed))

CODS soluble COD (mg O2/L)

SCOD soluble COD (mg O2/L)

CODP COD in particles (mg O2/L)

SCOD COD solubilisation (%)

DDCOD degree of disintegration (%)

Fig. 1. Diagram of pilot-plant digester.

Z.-l. Zhang et al. / Journal of Environmental Chemical Engineering 1 (2013) 73–7874

digestion [13]. The co-digestion of AS with a substrate containing ahigh level of C/N, such as kitchen garbage, to overcome difficultiesassociated with treating nutrient-deficient AS and to adjust itsunbalanced C/N ratio constitutes an interesting solution. The largequantities of kitchen garbage generated by the catering industryare characterized by easily biodegradable organic matter with ahigh C/N ratio and water content [14].

This study was conducted to provide references for the designand operation of an AS anaerobic digestion system for a uniqueWWTP. To accomplish this, three types of pilot experiments wereconducted: (a) comparison of the activity of a conventionalmesophilic two-stage and single-stage digester, (b) examination ofthe effects of ultrasonic pretreatment on the anaerobic process and(c) evaluation of the performance of co-digestion of kitchengarbage and AS.

Material and methods

Waste activated sludge characteristics

Activated sludge used in this study was obtained from themunicipal treatment plant of Foshan, Southern China. The maincharacteristics of the feed used are summarized in Table 1. Theworking ranges for TS and VS content in low concentration sludge(LCS) were 17–28 g/L and 13–21 g/L [15], respectively. Therefore,sludge obtained from the Foshan WWTP is LCS.

Pilot-scale digestion system

Experiments were carried out in two perspex continuouslystirred tank reactors (CSTR) that each had a working capacity of

Table 1Main characteristics of raw sludge.a

Parameter pH Moisture content (100%) TS (g/L

Mean value 7.04 97.25 27.5

a The samples were tested three times and the error range was from �3% to 3%.

20 tons (Fig. 1). The two-stage anaerobic system contained anacidogenic reactor and methane reactor with working volumes of4.5 and 15.5 tons, respectively. The reactor was operated with a gascollection unit and an influent and effluent line over a period ofapproximately 350 days. The reactor medium was mechanicallystirred with stainless steel paddles on a central shaft operated atconstant speed by an electric motor with a speed controller.Temperature was maintained within the mesophilic range(35 � 1 8C) using a temperature digital controller. The reactor wasoperated in an intermittent fashion, with a mixture of primary andsecondary activated sludge being fed to the reactor once a day.

The ultrasonic apparatus used was an ultrasonic homogenizerwith an operating frequency of 40 kHz and a supplied power ofabout 500 W. Treated samples had a volume of 250 L. Theoperating frequency (40 kHz) was selected based on a researchof Tiehm [10], which indicated that the disintegration of AS wasmost effective at the lower end at frequencies between 41 and3127 kHz.

Experimental procedures

The organic loading rate (OLR) was 0.85 kg VS/(m3 d) with anSRT of 20 days. Subsequently, the SRT was decreased to 15, 10 and8 days as the OLR was increased from 1.35 to 2.145 kg VS/(m3 d).For both the single-stage and two-stage systems, the reactors wereoperated for 8 days when the OLR was 0.85 and 1.35 kg VS/(m3 d),respectively, while they were operated for 7 days in the followingtwo states.

The specific energy was applied at 0–15,000 kJ/kg TS toinvestigate the degree of disintegration obtained with differentultrasonic treatments. Specific energy (Es) [16] is calculated usingultrasonic power (P), ultrasonic time (t), sample volume (v) andinitial total solid concentration (TS0) as follows:

Es ¼Pt

vTS0(1)

The kitchen garbage obtained from school canteens wascomposed of rice, meat and a small amount of vegetables. The

) VS (g/L) VFA (mg CH3COOH/L) SCOD (mg/L)

16.9 47 130

∞

a

Z.-l. Zhang et al. / Journal of Environmental Chemical Engineering 1 (2013) 73–78 75

garbage was diluted to a VS of 82.75 g/L with tap water, while theexcess sludge was obtained from a municipal WWTP in Foshan. Asubstrate blend of waste activated sludge and kitchen garbage witha ratio of 5:1 by volume based on volatile solids loading percentagewas applied and treated using a reactor operated at an SRT of 20days.

Analytical methods

The progress of the digestion was determined by monitoringVS and COD reduction, gas production, pH, alkalinity and volatilefatty acids (VFAs) levels. VS and COD reduction are commonlyadopted to measure the performance of anaerobic digestion. Thevolume of biogas production in the reactor was determineddirectly using a mass flow-sensor. VFA was measured by gaschromatography (Shimadzu GC-17A) using a previously de-scribed method [17]. Ammonium nitrogen (N-NH4

+) was mea-sured in the soluble fraction by colorimetric dosage [18]. Analysesof total solids (TS), VS, COD and alkalinity were performed daily bystandard methods [18].

Results and discussion

WWTP and sludge

The in situ pilot-scale process for municipal sludge treatmentwas conducted in the Foshan wastewater treatment plant (size390,000 P.E.), which employs three process trains constructed inthree stage projects. The A/O (Anoxic/Oxic) process was selected inthe first stage project, with the primary focus being COD reduction.The second and third stages were designed to remove nitrogen andphosphorous; therefore, the A/A/O (Anaerobic/Anoxic/Oxic) pro-cess was employed. All three process trains had been operated andmaintained in very healthy condition. Activated sludge wasdelivered after a centrifugal thickening step.

The main figures and performance of the wastewater treatmentprocess trains during the first half of 2012 are reported in Table 2.According to the data, the first and second stages of the projecthave been operating at the designed loads of pollutants, while thethird stage process train has been running at under half of the loadit was designed for. Conversely, the biomass concentrationsapplied in all project stages were quite similar. The food to

Table 2Profile and performance of the wastewater treatment lines.

Process train

1 2 3

Applied process A/O A/A/O A/A/O

Loads

Design flow rate (m3/d) 100,000 100,000 50,000

Actual flow rate (m3/d) 95,600 106,200 22,100

Organic loading (kg COD/d) 12,444 10,437 2607

Nitrogen loading (kg N/d) 2279 2032 511

Phosphorous loading (kg P/d) 305 201 55

Solids loading (kg SS/d) 6027 5416 1501

Removal efficiencies

COD (%) 87.70 86.90 89.90

Nitrogen (%) 57.20 53.20 64.50

Phosphorous (%) 84.60 83.20 81.60

Suspended solids (%) 93.70 92.20 94.10

Operational parameters

Biomass (g/L) 2.526 2.256 2.536

F/M (kg COD/kg MLVSS d) 0.28 – 0.11

HRT (h) 7 9 9.5

A/O: anoxic/oxic process; A/A/O: anaerobic/anoxic/oxic process.

microorganisms ratios, F/M, in first and third stage were 0.28 and0.111 kg COD/kg MLVSS d, respectively.

The removal efficiencies were good for both the COD and thesuspended solids, and generally >86%. Phosphorous removal wasequal to 82%, while the nitrogen removal efficiency was low(average 58.3%) in the WWTP.

Single and two-stage mesophilic anaerobic digestion

In this study, the sludge strain of the two-stage system wascultivated first; however, the single-stage system was started byinoculation with a strain from the methane reactor after the two-stage system was operated under stable conditions. After 50 daysof sludge domestication, both systems were operated at differentorganic loading rates to compare their treatment performances.

The organic removal efficiency (as a percentage of initial VS)and biogas production (L/(g VS fed)) are illustrated in Fig. 2. Thevolatile solid reduction (VSR) in both systems at different SRT werelower than 40% because of the low organic content (VS/TS = 59.73,VS = 16.87 g/L). When comparing Fig. 2a and b, there were noobvious differences between systems during the 20-day and 15-day SRT (OLR = 0.84 and 1.10 kg VS/(m3 d), respectively). However,when the SRT decreased to 10 days or 8 days and the OLR increasedto 1.85 or 2.145, respectively, the VSR of the two-stage system washigher than that of the single system.

∞

b

Fig. 2. VS reduction and specific biogas production of (a) two-stage and (b) single-

stage system.

∞

a

b

Fig. 4. Performance of anaerobic digestion after sonication (ES = 10,000 kJ/kg TS) as

demonstrated by (a) volatile solids removal and (b) biogas production.

Z.-l. Zhang et al. / Journal of Environmental Chemical Engineering 1 (2013) 73–7876

Conversely, the biogas production per gram VS reduction of thetwo-stage systems was higher than that of the single-stagesystems at each SRT. Therefore, in terms of biogas resources, two-stage systems have an incomparable advantage over single-stagesystems.

Anaerobic sludge digestion following ultrasonic disintegration

COD solubilization

COD solubilization [16] was calculated based on the differencebetween soluble COD (CODs) and initial soluble COD (CODS0),which is compared with the initial particulate COD (CODP0):

SCOD ¼ðCODs � CODS0Þ

CODP0� 100% (2)

The degree of disintegration (DDCOD) was defined by Muller andPelletier [19] as the comparison between the ultrasonic processand a maximum soluble chemical demand CODNaOH obtained byalkaline hydrolysis:

DDCOD ¼ðCODS � CODS0ÞðCODNaOH � CODS0Þ

� 100% (3)

The solubilization and degree of disintegration are reported inFig. 3. CODs increased strongly at specific energies between 0 and9000 kJ/kg. For specific energy below 1000 kJ/kg, the solubilizationand degree of disintegration were low (SCOD = 3%, DDCOD = 8%). Forenergy above 1000 kJ/kg, the solubilization and degree ofdisintegration increased strongly, with values of ES = 15,000 kJ/kg TS, SCOD = 28% and DDCOD = 51% being observed.

Sludge digestion following ultrasonic treatment

Ultrasonic pretreatment which was conducted withES = 10,000 kJ/kg TS in this study resulted in better anaerobicdegradation of the WAS. In Fig. 4, this is demonstrated bycomparison of the VS reduction (Fig. 4a) to the biogas production(Fig. 4b) of disintegrated and untreated samples. The VS reductionwas slightly lower for the disintegrated WAS than the untreatedcontrol. This effect might have been due to changes in the sludgecomposition. Owing to the higher soluble COD (SCOD) content ofthe disintegrated sludge and the preference of microorganisms forSCOD, the degradation of VS remained inactive.

However, the total biogas production increased significantly by172.56% relative to the control, indicating that the break-up ofmicrobial cells occurred during sonication. The specific biogas

0 2000 4000 6000 8000 10000 12000 14000 16000

0

10

20

30

40

50

60

solubilisation

degree of disintegration

SC

OD

or

DD

CO

D(%

)

specific energy(kJ/kg)

Fig. 3. Solubilization and degree of sludge disintegration (DDCOD) vs. specific energy.

production, i.e. the biogas production related to the mass of VSdegraded, also increased to 677, 758 and 714 L/kg VS degraded.

Co-digestion of waste activated sludge and kitchen garbage

pH is an important parameter affecting the performance ofanaerobic digestion. pH in the single-system reactor and methanereactor was kept at a constant level of 6.8–7.2 throughout theprocesses by adding hydrochloric acid at a concentration of 10%.However, during the process of co-digestion, the pH in theacidogenic reactor was maintained at about 5.0, while that of thesingle-stage system and methane reactor remained stable ataround 7.0 without the addition of acid (Fig. 5). The self-regulationof the systems indicates that they tended to be more steady afterblending of the WAS and kitchen garbage, which indicates greatpotential for application of this type of waste treatment.

Fig. 6 shows the enhanced anaerobic digestion after mixingkitchen garbage with waste activated sludge as demonstrated by abetter VS reduction and increased production of biogas. Theaverage VS reduction of both systems was increased to around 47%,and reached as high as 54.32%, which surpassed that of aconventional mesophilic anaerobic system with the same organicloading by 30%. Moreover, production of biogas increasedsignificantly during the co-digestion process when compared withthe normal process. Taken together, these improvements indicatedthat optimization of the sludge composition by mixing it withkitchen garbage has the potential for use in anaerobic digestion ofnutrient-deficient sludge. Currently, almost all kitchen garbage is

0 1 2 3 4 5 6 7 84

5

6

7

8

pH

t/d

single -ph ased system

ac idogenic reac tor

metha ne reac tor

Fig. 5. Variation of pH in both single-stage and two-stage reactors during co-digestion.

0.0

0.4

0.8

1.2

1.6

two-stage

co-di gestion system

single -stage

co-di gestion syste

con vent ion al

two-stage system

con ventional

single -sta ge syste m

48.04

0.44

46.38

0.96

26.56

0.92

25.20

SBP(sp ecific biogas production)

VSR(volatile so lid s re moval)

0.16

SB

P(L

/(g

VS

fed

))

0

10

20

30

40

50

60

VS

R(%

)

Fig. 6. Enhanced anaerobic digestion after mixing kitchen garbage with waste

activated sludge.

Z.-l. Zhang et al. / Journal of Environmental Chemical Engineering 1 (2013) 73–78 77

recycled to produce fertilizer and feedstuff; therefore, co-digestionof sewage sludge with kitchen garbage will only be practical if thegovernment changes its policies regarding the disposal of kitchenwaste. Despite the difficulties of applying this process widely, co-digestion with kitchen garbage should be considered as aregulation measure when digestion systems are operatingunsteadily because of a low sludge organic content.

Design and operation

Based on our previous experiments, a mesophilic anaerobicdigestion and incineration and material use scheme was proposedto solve the problem of nutrient-deficient sewage sludge (Fig. 7).There were no obvious differences between the single-stagesystem and two-stage system when sludge from the Foshan WWTPwas used; therefore, the single-stage system was employedbecause of its efficiency and reduced operation cost. In addition,ultrasonication was applied as a pretreatment for anaerobic

Fig. 7. Process flow diagram of sludge treatm

digestion. The consideration of thermal application as a sludgewaste management solution is of increasing importance today, andincineration of sewage sludge is perhaps the most establishedthermal method for the disposal of sludge [20].

After 3 months’ debugging and operation of the designedscheme, a steady condition has been achieved. A mass balance ofthe entire process was studied (Fig. 7) and then used for thefollowing economic estimation:

(1) Profits from methane generation: as every cubic meterproduces 1.6–2.0 kW h, the system generated 5954.4 � 1.8 =10,717.92 kW h, resulting in a profit of $1049.35 (US dollars) atan electricity price of $0.10 kW�1 h.

(2) Fuel cost: there are 119,088 MJ/d and 79,732.8 MJ/d heatproduced in the anaerobic digestion unit and incineration unit,but 234,130.96 MJ/d heat is needed by the drying unit, so35,310 MJ/d heat must be added to the process. This requires1.686 t raw coal, which will cost $162.36/d.

ent and utilization with mass balance.

Z.-l. Zhang et al. / Journal of Environmental Chemical Engineering 1 (2013) 73–7878

(3) Savings during sludge treatment: the sludge volume reductionreached 30% after anaerobic digestion, resulting in a decreasefrom 206 m3/d to 144 m3/d and a savings of $39 �(206 � 144) = $2418.

In summary, this process could result in a profit of $886.92 eachday. Furthermore, the profit would be higher when benefits fromproducing building materials using ash are considered. It isanticipated that the environmental and economical benefits of thissystem will result in shifts in policy to support sludge anaerobicdigestion.

Conclusion

Enhanced digestion for municipal sludge with low organiccontent from southern China was evaluated in a pilot-scale studythrough comparison of a single and two-stage digestion systemthat employed ultrasonic sludge treatment and co-digestion ofsludge with kitchen garbage. The results revealed that aconventional two-stage anaerobic digestion system performedbetter than a single-stage system when the concentration of feedsludge was increased and the SRT was decreased. However, interms of the specific conditions (SRT as long as 15 days and ORL aslow as 1.37 kg VS/(m3 d)), the single-stage system would be abetter choice. Subsequent anaerobic digestion of the ultrasonicallydisrupted sludge can improve biogas production significantly withreduced sludge quantity, which is vital to the economical operationof wastewater treatment plants. The system stability wasimproved after the blending of WAS and kitchen garbage, andtreatment performance was enhanced as demonstrated by a betterVS reduction and increased production of biogas. Finally, a processincluding digestion, dewatering, drying and incineration of thiskind of sludge was proposed, and mass balance calculation showedgreat application potential in southern China.

Conflicts of interest

The authors declared that they have no conflicts of interest tothis work.

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