Anaerobic digestion of polyelectrolyte flocculated waste activated sludge

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Anaerobic digestion of polyelectrolyte ﬂocculated

waste activated sludge

C.P. Chu

a

, D.J. Lee

a,c,*

, Bea-Ven Chang

b

, C.H. You

b

, C.S. Liao

b

, J.H. Tay

c a

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b

Department of Microbiology, Soochow University, Taipei 111, Taiwan c

Division of Environmental and Water Resources Engineering, School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798

Received 5 August 2002; received in revised form 21 May 2003; accepted 21 May 2003

Abstract

This work examined how adding one of three polyelectrolyte flocculants (T3052: cationic, T2000: non-ionic, and T1052: anionic) affected the anaerobic digestion of wastewater sludge. Methane production, floc characteristics (morphology and f-potential) and process parameters (soluble chemical oxygen demands (SCODs) and reductive potentials) were monitored along the digestion tests. The digestion rates of T2000- and T1052-conditioned sludge re-sembled that for original sludge. The T3052-flocculated sludge generated methane at a higher rate during the first 6 days of digestion than did the original one. In the following stage, the digestion rate of sludge flocculated with T3052 at dosage exceeding 15 g/kg dried solids declined. For example, at 40 days of digestion the methane production amounts for original, 15 g/kg DS flocculated, and 40 g/kg flocculated sludge were of 136, 105, and 85 g/kg DS, respectively. The role of flocculants could change in different stages of digestion.

The dosed polymers had no apparent toxicity to the inoculum used. The changes in SCOD, adenosintriphosphate concentrations, oxidative and reductive potential, and f-potentials did not correlate with the noted hindered digestion for T3052-conditioned sludge. Microphotographic observation revealed that the ﬂocs of T3052-conditioned sludge were not only of a large size, but also were resistant to structural deterioration during digestion. Therefore, mass transfer resistance was proposed to account for the hindered digestion eﬃciency observed for T3052-conditioned sludge. 2003 Elsevier Ltd. All rights reserved.

Keywords: Digestion; Flocculation; Methane; Activated sludge; Size

1. Introduction

Anaerobic digestion is a widely used stabilization process for reducing sludge volume and producing bio-gas (Parkin and Owen, 1986). During the digestion process, eﬀective inactivation of the pathogenic

micro-organisms can be achieved (Carrington et al., 1991; Gavaghan et al., 1993; Stadterman et al., 1995; Jepsen et al., 1997).

Chemical ﬂocculants are added to sludge to enhance its sedimentation rate before digestion. The ﬂocculant dose leading to the fastest settling is referred to as the ‘‘optimal dose’’. Though the weight fraction of poly-electrolyte in the conditioned slurry is generally small, Dentel et al. (2000) demonstrated that the applied polyelectrolyte may be attached onto and carried out with the conditioned sludge at an amount up to 10 kg/ ton dry solids (DS). Although the fraction of polyelec-trolyte in the sludge is small, its role on the performance www.elsevier.com/locate/chemosphere

*

Corresponding author. Address: Department of Chemical Engineering, National Taiwan University, Taipei 10617, Tai-wan. Tel.: +886-2-2362-5632/2363-5230; fax: +886-2-2302-3040/ 2362-3040.

E-mail address:[email protected](D.J. Lee).

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of subsequent treatment/disposal processes has not been satisfactorily examined. Chu et al. (2000, 2001) reviewed that ﬂocculating sludge to charge neutralization point could markedly enhance the rate of subsequent thermal pyrolysis process.

Controversial results were reported with respect to the role of chemical conditioners on the anaerobic di-gestion of sludge. Gossett et al. (1978) noted that me-thane production and volatile solids destruction ratio were reduced in an anaerobic digested wastewater sludge in the presence of organic coagulant. El-Mamouni et al. (1998) found that the synthetic polyelectrolyte not only has no inhibition eﬀects to biomass activity, but also could enhance the metabolite transfer rate and reduce the inhibition of acetogenic and methanogenic bacteria in an upﬂow anaerobic sludge blanket reactor. Dis-crepancy clearly exists in the literature works on the role of organic conditioner on methane production.

This work investigates the role of polyelectrolyte flocculant on the anaerobic digestion efficiency of waste-water waste activated sludge. We monitored the meth-ane production together with the changes in floc characteristics and process parameters during digestion. Some observations on the influence of floc size on di-gestion efficiency were made.

2. Experimental

2.1. Sample and ﬂocculation conditioning

Waste activated sludge was taken from a wastewater treatment plant in the Presidential Enterprise Co., Taoyuan, Taiwan. Sludge sediment after 1-h settling was the test material. The dried solids of this material was measured as 9390 mg/l. Accupyc Pycnometer 1330 (Micromeritics, USA) was used to measure the density

of dried solid in sludge, giving a result of 1470 kg/m3

with a relative deviation of less than 0.5%. The pH value of original sludge was approximately 6.7.

Cationic polyelectrolyte T3052, non-ionic flocculant T2000, and anionic polyelectrolyte T1052 were provided by Kai–Guan Inc., Taiwan. The physical properties of these polymers were listed in Table 1. These polymers exhibited a similar molecular weight but a very different surface charge. Sludge sample was first placed in a mixing vessel equipped with a paddle mixer. Flocculant

solution (0.2% w/w) was then gradually poured into the mixing vessel with 200 rpm of stirring for 5 min, fol-lowed by 50 rpm for another 20 min. The ﬂocculation stage did not notably change the pH value of the sus-pension.

2.2. Anaerobic digestion and related tests

Anaerobic digestion of the sludge was conducted following the procedure proposed by Chang et al. (1996, 1998). The compositions of the gas in the glove box were

maintained at 99% N2, 0.05% O2, 0.75% N2, 0.01% CH4

and 0.2% CO2, to simulate an anaerobic environment in

nature. Eight millilitres sludge (original or ﬂocculated) mixed with 1 ml anaerobes and 1 ml medium (compo-sitions as listed in Table 2) were added to the serum bottles. Chang et al. (1998) isolated strains from the sediment at 12 sites of Tam–Shui River in Taipei County degrading organics to produce products including methane. The one collected at site eight, named strain K8, had the highest methane productivity among the other strains. Since this paper aimed at exploring the implications of polyelectrolyte ﬂocculation of sludge to its methane productivity during anaerobic digestion, the K8 strain was used as the seed microorganisms in sub-sequent digestion tests.

Serum bottles were then capped with butyl rubber stoppers and wrapped in aluminum foil to prevent

possible photolysis. The bottles were incubated at 35C

without shaking. Chang et al. (1996, 1998)

recom-mended 35C as the optimal temperature at which

an-aerobes grew, which was used in digestion tests. At each sample time (day 2, 4, 6, 8, etc.) three serum bottles were removed from the batch of 400 bottles total, analyzed,

Table 1

The properties of polyelectrolyte ﬂocculants used in this study

Components Ionic property Molecular weight Charge density

T3052 Polyacrylamide Cationic 107 _20%

T2000 Polyacrylamide Non-ionic 5· 106 _–

T1300 Polyacrylamide Anionic 107 _40%

Table 2

The components of medium used during incubation

Components Concentration (g/l) K2HPO4 0.35 KH2PO4 0.27 NH4Cl 2.7 CaCl2 2H2O 0.1 MgCl2 2H2O 0.1 FeCl2 6H2O 0.02 Yeast extract 12 Reszurin (reductants) 0.001

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and discarded. The mean results from these analyses were used to represent data with standard deviation calculated as less than 1%. For comparison, anaerobic digestion tests on pure ﬂocculant solution (0.2%) were also undertaken to demonstrate the biodigestability of the ﬂocculant.

2.3. Characterization of the sludge

A particle sizer (LS230, Coulter) measured the vol-ume average floc size using light scattering technique. The volume average floc size of original sludge was ap-proximately of 58 lm. The f-potentials of sludge flocs were measured by a zetameter (Zeta-Meter System 3.0, Zeta-Meter Inc., USA). The result for original sludge

was)13.6 mV. The f-potential of sludge did not change

much in the presence of anaerobe K8. The surface charges of sludge flocculated with T3052 would be neutralized at a dose of approximately 10–15 g/kg DS. For the T2000- or T1052-flocculated sludge, the surface charge remained unchanged or became more negative after flocculation. The microscopic photographs were obtained with the assistance of a phase contrast micro-scope (ECLIPSE E800, Nikon) at a magnification of 400·.

Methane production was quantiﬁed with a gas chromatography (Sigma 3B Perkin–Elmer) equipped

with a packed column (200· 0.5 cm, 80/100 porapal Q;

Supecol, Bellefonte, PA) and ﬂame-ionization detector.

Column temperature was kept at 90C while those for

injector and the detector were set at 55C. Nitrogen at a

ﬂow rate of 20 ml/min was the carrying gas. Integrator (HP3396 Series II) was used to integrate the peak area, hence quantifying the methane concentration.

The oxidative and reductive potential (ORP) of the sludge was measured with an ORP meter (HI 9017 Hanna). Total chemical oxygen demand (TCOD) of the sludge was measured with direct reading spectropho-tometer (DR/2000, HACH, USA). The TCOD of origi-nal sludge (with K8 and the medium) was measured as 7990 mg/l. The chemical oxygen demand of the ﬁltrate was measured and is referred from here to as soluble chemical oxygen demand (SCOD), which was read as 479 mg/l for the original sample.

2.4. ATP measurement

The Luceferin–Luciferase method was utilized to quantitatively measure the adenosintriphosphate (ATP) concentrations in the sludge. The ATP concentration was an index for the microbial density level in the sus-pension. An aliquot of activated sludge sample (pre-mixed with K8 and medium) of 0.5 ml was automatically pipetted along with the necessary reagents into a cuvette. The reagents used here include NRB (extracting ATP

from the microbial cell) and LUMIT PM (luciferase dissolved in LUMIT buﬀer solution). The enzyme– substrate complex, luciferase–luciferin, converted the chemical energy associated with ATP into light, and a luminometer (BIOCOUNTER M 2500, LUMAC, The Netherlands) measured the emitted bioluminescence light in the relative luminescence units (RLUs).

Using the tests with standard ATP solutions, 1 lg/l of ATP yielded a light intensity of 1036 RLU. The ATP concentration in the suspension containing only the anaerobe K8 (without sludge) was 0.42 lg/l. All the other testing samples reported herein were the sludge (original or ﬂocculated) mixed with K8 and the medium.

3. Results

3.1. Methane production

Figs. 1 and 2 depict the methane production amount as function of digestion time.

Fig. 1 illustrates the methane productions of the original sludge (without ﬂocculants) and the three ﬂoc-culants solutions (without sludge) under investigation (0.2% w/w). The methane gas production was expressed

in unit of g CH4/kg DS. As Fig. 1 reveals, the methane

production increased with time, indicating the progress of the digestion process. For the original sludge, the methane production increased to nearly 75 g/kg DS in 10 days. Chang et al. (2000) demonstrated that the poly-electrolyte ﬂocculant could be partially digested under aerobic and anaerobic conditions. However, the meth-ane generated from digestion of sample containing only

Day 2 4 6 8 10 Methane production (g/kg DS) 0.1 1.0 10.0 100.0 Original sludge T1052 solution (2000 mg/l) T2000 solution (2000 mg/l) T3052 solution (2000 mg/l)

Fig. 1. Methane production amount for anaerobic digestion of original sludge and three ﬂocculants.

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ﬂocculant polymer was only 5% in amount of that from digestion of unconditioned (original) sludge. The con-tribution of anaerobic digestion of ﬂocculants to the methane production could hence be ignored in the sub-sequent tests.

Fig. 2(a) illustrates the methane production amounts for the original and the T3052-flocculated sludge. As Fig. 2(a) illustrates, during the first 6 days of digestion, the T3052-flocculated sludge generated methane at a higher rate than did the unconditioned sludge. As time elapsed, the digestion rate of sludge flocculated at dos-age exceeding 15 g/kg DS declined. After 10 days of digestion, the methane production for the original and the flocculated sludge at a dosage less than 5 g/kg DS

converged to a single curve. For example, at 40 days of digestion the methane production amounts for original, 1 g/kg DS flocculated, and 5 g/kg flocculated sludge were of 136, 133, and 132 g/kg DS, respectively, which could be regarded identical considering the involved statistical errors. Meanwhile, the corresponding production for the 15 g/kg DS flocculated and 40 g/kg DS flocculated sludge generated methane only of 105 and 85 g/kg DS, respectively, corresponding to a 23% and 38% reduction. Fig. 2(b) demonstrates the methane production amounts from sludge flocculated with T1052 or with T2000. Resembling Fig. 2(b), digestion was enhanced compared to unconditioned sludge during the first 10 days. In the subsequent stages, however, all methane production curves converged to a common one, as dem-onstrated in the cases with T3052 flocculated sludge at a low polymer dose.

3.2. Other tests

During the digestion the methanogenic bacteria continuously transformed the soluble organic com-pounds into bio-gas that reduced the SCOD value, while the hydrolysis reaction by acetogenic bacteria kept re-plenishing it. In all tests the soluble COD in medium increased to a plateau value approximately 200–250% of its original value after 6 days of digestion (data not shown). The hydrolysis reaction dominated during the initial stage of digestion (up to 6 days). Afterward the rate of hydrolysis and the rate of methane produc-tion reached at a comparable level, hence with a nearly constant SCOD over time. The more ﬂocculant that was added into the sludge, the higher level of SCOD could appear in the medium. After 10 days of digestion all SCOD/TCOD ratios approached a constant around 0.11–0.14 (data not shown).

During anaerobic digestion the microorganisms re-duced the ORP of the medium to a level suitable for volatile fatty acids and methane to form. Fig. 3 dem-onstrates the changes in ORP values during digestion. At the start of digestion the ORP values for the sludge

samples ranged from)250 to )200 mV. Meanwhile, the

ﬂocculation with T3052 yielded a less reductive envi-ronment (less negative initial ORP value for ﬂocculated sludge).

As the digestion proceeded, the ORP soon became more negative in the ﬁrst 4–6 days of digestion. For instance, the ORP for the original sludge and those ﬂocculated with T1052 or T2000 dropped to a level of )320 to )330 mV in the 4th to 6th day, and then the trend reversed and the potential increased back to

ap-proximately)260 to )290 mV. The high-dosed T3052

flocculated sludge experienced a much less drop in ORP value during the first 6 days. The ORP values for the sludge flocculated with T3052 at a dose of 15 or 40 g/kg

M eth ane pr oduc ti on (g/ k g DS) 0 50 100 150 Original sludge T3052, 1 g/kg DS T3052, 5 g/kg DS T3052, 15 g/kg DS T3052, 40 g/kg DS Day 0 10 20 30 40 M eth ane pr oduc ti on (g/ k g DS) 0 50 100 150 Original sludge T1052, 1 g/kg DS T1052, 15 g/kg DS T2000, 1 g/kg DS T2000, 15 g/kg DS (a) (b)

Fig. 2. Methane production amount for anaerobic digestion of sludge: (a) the original and T3052-ﬂocculated sludge; (b) T1052-, and T2000-ﬂocculated sludge.

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DS merely dropped to a value around)280 to )300 mV, and then reversed back.

A close correlation between the initial ORP drop and the methane production rate was noticeable. Restated, in the cases where the ORP in the 4th to 6th days

dropped to lower than)320 mV, the methane

produc-tion would not be affected in the presence of the poly-electrolyte flocculant. After that, the polypoly-electrolyte appeared to hinder the digestion efficiency, as noted in the cases of high T3052 dosage (15 and 40 g/kg DS).

Chang et al. (1996) proposed that the change in ORP might be attributed to the growth of anaerobic consortium in sludge. Fig. 4 demonstrates the mea-sured ATP concentration, which represented the an-aerobic consortium density level. In all tests the ATP

increased in concentration during the first 10 days and then reached a plateau value ranging 3–4 lg/l. No correlation with the presence of polymer flocculants to the microbial activity could hence be concluded. The present three flocculants had no apparent toxicity to the applied anaerobes K8. The distinct trends noted for ORP changes in Fig. 3 thereby could not be in-terpreted with the changes in growth phase of anaer-obes.

Fig. 5 illustrates the time evolutions of the f-poten-tials for sludge ﬂocculated with the three polymers. The f-potential of sludge was neutralized while conditioned with cationic ﬂocculant T3052 at a dose among 5–15 g/kg

ORP (mV) -350 -300 -250 -200 Original sludge T3052, 1 g/kg DS T3052, 5 g/kg DS T3052, 15 g/kg DS T3052, 40 g/kg DS Day 0 10 20 30 40 ORP (mV) -350 -300 -250 -200 Original sludge T1052, 1 g/kg DS T1052, 15 g/kg DS T2000, 1 g/kg DS T2000, 15 g/kg DS (a) (b)

Fig. 3. Time evolution of oxidation–reduction potentials dur-ing the anaerobic digestion: (a) original and T3052-ﬂocculated sludge, and (b) original, T1052-, and T2000-ﬂocculated sludge.

Day AT P co n te n t ( g/ l) 0.0 1.0 2.0 3.0 4.0 5.0 Original sludge T3052, 1 g/kg DS T3052, 5 g/kg DS T3052, 15 g/kg DS T3052, 40 g/kg DS Day 0 10 20 30 40 AT P co n te n t ( g/ l) 0.0 1.0 2.0 3.0 4.0 5.0 Original sludge T1052, 1 g/kg DS T1052, 15 g/kg DS T2000, 1 g/kg DS T2000, 15 g/kg DS (a) (b)

Fig. 4. Time evolution of ATP content during the anaerobic digestion: (a) original and T3052-ﬂocculated sludge, and (b) original, T1052-, and T2000-ﬂocculated sludge.

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DS. For the other two polymers, the ﬂocculation could not yield a less negatively charged particle surface.

As Fig. 5(a) and (b) demonstrate, the f-potentials for all sludge samples gradually converged to approximately

steady value of )20 mV regardless of polyelectrolyte

flocculation. For example, the surface charges of origi-nal and of the T3052-flocculated sludge became more negative in the first 6 days. For the T1052-flocculated sludge whose initial surface charge was more negative

than)20 mV, the f-potential became less negative in the

ﬁrst 6 days.

The original sludge samples after sonication

treatment (without digestion) revealed a f-potential of

)19.8 mV. The fragments of the original sludge subject to 20-min sonication exhibited a similar surface charge as that for sludge digested for more than 6 days.

Figs. 6 and 7 demonstrate the microphotographs for the original and the T3052-flocculated sludge (400·) prior to and after 30 days of anaerobic digestion. The photographs of T1052 and T2000-flocculated sludge (at 15 g/kg DS) resembled those of original sludge and were omitted herein. The original sludge exhibited a loose floc structure with the presence of filamentous bacteria (Fig. 6(a)). After 30 days of digestion, partial disintegration of floc structure was noticeable (Fig. 6(b)). However, compared with the original sludge or the T1052 and T2000-flocculated ones, the flocs of T3052-flocculated

p o te nt ia ls (m V ) -30 -20 -10 0 10 20 Original sludge T3052, 1 g/kg DS T3052, 5 g/kg DS T3052, 15 g/kg DS T3052, 40 g/kg DS Day 0 10 20 30 40 potential (mV ) -30 -20 -10 0 10 20 Original sludge T1052, 1 g/kg DS T1052, 15 g/kg DS T2000, 1 g/kg DS T2000, 15 g/kg DS (a) (b)

Fig. 5. The f-potentials of sludge flocs during digestion: (a) original and T3052-flocculated sludges, and (b) original, T1052-, and T2000-flocculated sludges. The dash lines indicate the f-potential of original sludge floc ()19.7 mV) subject to sonica-tion.

Fig. 6. The ﬂoc morphology of sludge during digestion (400·): (a) original sludge prior digestion, (b) original sludge after 30 days of digestion.

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sludge (15 g/kg DS) had an apparently larger size and denser structure (Fig. 7(a)). Although the degradation of flocs was apparent after 30 days of digestion, but the entire floc structure remained almost intact (Fig. 7(b)). Restated, the T3052-conditioned sludge not only ex-hibited a large floc size, but also resisted global struc-tural deterioration during digestion.

4. Discussion

The above-mentioned experimental results clearly reveal that during the ﬁrst stage of digestion (<6 days)

the presence of flocculants could enhance the digestion efficiency, which corresponded to the report of El-Mamouni et al. (1998). In the following stages (>10 days), however, the flocculants would inhibit the pro-gress of digestion process if the sludge were conditioned with cationic polymer to a high dosage. Such an obser-vation correlated with the conclusions drawn by Gossett et al. (1978). This work demonstrated that the role of flocculants could change in different stages of digestion. Restated, it assisted methane production at the initial phase of digestion, but hindered the production at the later phase.

During the ﬁrst stage the SCOD and ATP concen-trations increased accordingly, the ORP dropped, and the f-potentials of particle surface converged to a level

of)20 mV. Hence, the decomposition of ﬂocculant

fa-cilitated the digestion. However, the ORP drops for T3052-ﬂocculated samples at higher dosages were not so apparent as for the other sludges. In the following stage, the digestion was largely inhibited when the sludge was conditioned with T3052 at a high dosage. Based on the digestion tests with polymers (Fig. 1) the dosed polymers had no apparent toxicity to the inoculum. The SCOD, ATP concentrations, ORP, and f-potentials all con-verged to a common value for all tests. These observa-tions could not explain the noted hindered digestion for T3052-conditioned sludge.

Mass transfer resistance was hence proposed to ac-count for the size-dependent digestion efficiency ob-served in Fig. 2(a). During anaerobic digestion the sludge would be first hydrolyzed to small molecules, like acetic acids or other long-chain organic acids (acido-genic stage). Then these short-chain compounds were decomposed to form methane (methanogenic stage). The greater floc size for T3052-flocculated sludge might prohibit efficient species movement within the sludge floc, which hindered the subsequent acidogenic and the methanogenic stages to produce methane. Such an oc-currence also correlated with the smaller ORP drops for these sludge samples. Nonetheless, no direct experi-mental proof supported this hypothesis.

5. Conclusions

This work investigated the effects of polyelectrolyte flocculants on the anaerobic digestion of a wastewater sludge. Three polyelectrolytes at a similar molecular weight but different surface charge (cationic, non-ionic, and anionic) were tested. Only the cationic flocculant could efficiently condition the present sludge.

In the ﬁrst stage of digestion (<6 days) adding polyelectrolyte could enhance methane production, ac-companied with the increases in SCOD and ATP con-centration in the medium, and the drops in the ORP. The corresponding f-potentials of particle surface quickly Fig. 7. The ﬂoc morphology of sludge during digestion (400·).

(a) T3052-ﬂocculated sludge prior digestion, (b) T3052-ﬂoccu-lated sludge after 30 days of digestion.

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adapted themselves to approach a level of )20 mV. During the following stage (>10 days), while all the other conditioned sludge possessed a similar digestion characteristic to that for original sample, application of T3052 (cationic) at high dosage (>15 g/kg DS) could markedly inhibit the digestion efficiency. This may be attributable to its much greater floc size, thereby resist-ing efficient mass exchange within the sludge flocs.

Acknowledgements

DJL wishes to thank Nanyang Technological Uni-versity for appointing him as a visiting professor during June–August, 2002. National Science Council of ROC ﬁnancially supported this work.

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