IntroductionA variety of pretreatments have been investigated toimprove the digestive efficiency of waste activatedsludge (WAS) by solubilizing decomposable organicsubstances in it. (Pinnekamp, 1989; Shimizu et al.,1993; Wang et al., 1995). Anaerobic digestion of WASmay be markedly affected by the specific characteristicsof feed substance, operational parameters such ashydraulic retention time (HRT), and environmentalfactors such as pH, temperature, reactor configuration,redox potential and available trace minerals. An impor-tant operational variable, which can be easily manipu-lated, is the HRT. It is possible to feed sufficientsubstrate by decreasing HRT and increasing organicsludge load. Although a tremendous effort has been putto research the effect of HRT on anaerobic systems,(Grobicki and Stuckey, 1991; Li and Noike, 1992),sufficient information is still not available on this topic.

On the other hand, methanogenic digestion process issensitive to changes in hydraulic and organic loadingsand recovers slowly once it is upset. Indicators for theinhibition of this process should be capable of signallingimpending upsets before they occur. Common indica-tors, such as volatile fatty acids (VFAs), gas composi-tion and pH are useful for monitoring gradual changes,but do not directly reflect current metabolic status ofthe active organisms in the system. They are generallyuseful for detecting process upsets once they areunderway but, in most cases, are not adequate enoughto avoid system failure due to difficulties such as gradual

organic and hydraulic overloads. Thus, it is necessary toexamine a satisfactory indicator which can check distur-bances caused by organic overload.

In this paper, influence of HRT on methane generationand the relationship between VFAs and pH duringanaerobic digestion were investigated, utilizing ther-mally pretreated and untreated WAS. Further, animportant indicator to estimate the organic load fed inthe digester was also examined.

Materials and methodsThe seed sludge and WAS were collected from amesophilic anaerobic digester and floatation thickeningtank in Hiagari Sewage Disposal Plant, Kitakyushu city,Japan.

A 4–1 stainless steel vessel containing WAS, was held at set temperatures for 30 mins, then cooled andkept at 4°C. This WAS was used for one series ofexperiment.

In four 1.5–1 anaerobic digesters, having workingvolume of 1–1 and equipped with gas and sludgesampling ports, fixed quantity of pretreated anduntreated WAS were poured. Temperature wascontrolled at 36 ± 1°C after replacing air by helium for5 min. HRT were prescribed as 3, 4, 6, 8 and 10 days,when organic loads of 10.23, 7.75, 5.17, 3.88 and 3.10kg-VS/l day were fed, respectively. Total gas generatedwas measured volumetrically. Gas composition and

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Biotechnology Techniques, Vol 11, No 2, February 1997, pp. 105–108

© 1997 Chapman & Hall Biotechnology Techniques · Vol 11 · No 2 · 1997 105

Influence of hydraulic retention time onanaerobic digestion of pretreated sludgeQ. Wang, C.K. Noguchi, M. Kuninobu1, Y. Hara, K. Kakimoto, H.I. Ogawaand Y. Kato*Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1–1, Sensui-cho, Tobata-ku, Kitakyushu, Fukuoka 804, Japan. 1Kawasaki Giken Engineering and Construction Company Limited, 1-Chome 22–11, Minami-ku, Fukuoka 815, Japan.

Continuous anaerobic digestion of waste activated sludge pretreated at low temperatures below 100°C increasedmethane generation by 30%. pH values of the digestion mixture increased, approximately from 0.3 to 0.55 bypretreatment, although its volatile fatty acids concentration was greater than the control. An abrupt increase inpropionate : acetate ratio in digestion stage (e.g. from less than 1:1 to over 3.5 :1), provided a reliable indicator for impending failure.

24 pts min base to base from Key words to line 1 of text

VFAs (C2–C6) concentration were measured by thermalconductively detector and flame ionization detector gaschromatography, respectively.

Results and discussionAverage values of methane generated from the control(untreated) and pretreated WAS in the steady stateperiod at different HRT are illustrated in Fig. 1.Thermal pretreatment resulted in significant increase inmethane yield at every HRT, however, methane gener-ation did not show a remarkable difference among WASpretreated at 60, 80 and 100°C. Thus, temperature ofwaste hot water from a boiler was not strictly necessaryto be controlled, but had to be maintained in a rangeof 60–100°C during thermal pretreatment

Further, the magnitude of methane generation wassignificantly affected by HRT. Difference in methanegeneration became remarkable between the control andpretreated WAS, as the HRT was decreased, exceptwhen HRT was 3 days. When HRT was 4 days, thecontrol digestion was still unstable and latter resulted

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Figure 1 Effect of hydraulic retention time on methanegeneration in anaerobic digestion (average value). d,control; n, pretreated at 60°C; s, at 80°C; u, at 100°C.

Figure 2 Changes in concentration of total volatile fatty acid and methane generation after waste activated sludge wasfed. (a), (b) control, (c), (d) pretreated at 60°C. Hydraulic retention times (days): d, 3; m, 4; s, 6; n, 8; u, 10.

in a failure. Therefore, the maximum organic load ofthe control fed in the digester was approximately 5.0kg-VS/m3 day, when the HRT was extended to 6 days.However, the digestions of WAS pretreated at60–100°C brought about maximum methane genera-tion when the HRT was 4 days, and the maximumorganic load was approximately 8.0 kg-VS/m3 day.Thus, it was apparently possible to enhance organic loadand reduce HRT necessary for anaerobic digestion byusing thermally pretreated WAS.

Fig. 2 shows the results of short-term monitoring oftotal VFA (t-VFA) and methane generation during 24h digestion, with respect to different HRTs. WASpretreated at 60°C gave higher concentration of t-VFAin the initial digestion stage at every HRT as comparedto the corresponding control runs. The overall t-VFAin both cases was extremely high when HRT was 3days. When the HRTs were extended to 4 and 6 days,the maxima of t-VFA concentration in pretreated WASwere 1856 and 1179 mg/l, respectively, while those inthe controls were only 950 and 647 mg/l, respectively(Fig. 2-a,c). But after 24 h, t-VFA in pretreated WASconsiderably decreased at the HRTs from 4 to 10 daysindicating that VFAs were consumed, whereas in controlt-VFA concentration decreased slowly. When HRT was3 days, even after 24 h , t-VFA in pretreated WAS andthe control retained in high amounts (Fig. 2-a,c),resulting in extreme slowdown in methane generation(Fig. 2-b,d). The cause of these failures could be due toWAS overloading. The results obtained from WASpretreated at 80°C and 100°C were essentially similarto those at 60°C (data not shown). Consequently, Fig.2 indicated that drastic increase in methane generation

occurred with increase in t-VFA, within the initial 3to 6 h. These results suggest that VFAs are, more orless, dominant substrate for methanogenic bacteria.

Since the digestion mixtures, after 3 h, retained highconcentration of t-VFA , acidogenic process seemed tobe considerably in progress. All the digestion systems,after 24 h, exhibited trace to very low concentration oft-VFA, except when HRT was 3 days. Therefore, sludgesamples collected from 3 and 24 h mixtures wereexpressed as acidogenic stage and digestion stage,respectively. Fig. 3-a,b illustrates change in t-VFA withrespective change in pH. t-VFA and pH were seen tobe inversely related. Amount of t-VFA increased whileit’s pH decreased, with decrease in HRT (Fig. 3-a,b).When HRT was 3 days, both stages showed maximumaccumulation of t-VFA and lowest pH value. This wasapparently due to organic overloading, which couldcause failure in digestion.

In case of pretreated WAS, pH value of the digestionmixture approximately increased by 0.3 to 0.55, whenHRT was 4 to 10 days, compared to correspondingcontrol, although t-VFA concentration of the former was greater than the later. This could be due to specificcharacteristic differences in the feed, which could have denatured due to heat or changes in microfloralactivities.

Changes in VFAs, including acetate and propionate havebeen monitored for all digestion mixtures. Influence ofHRT on the ratio of propionate : acetate was shown inFig. 4. In both stages of pretreated WAS, propionate :acetate ratios were lower than 1:1, when HRT was

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Biotechnology Techniques · Vol 11 · No 2 · 1997 107

Figure 3 Effect of hydraulic retention time on total volatile fatty acid and pH of the control and pretreated waste activatedsludge. (a) acidogenic stage, (b) digestion stage, d, total volatile fatty acid (80°C); m, pH (80°C); s, total volatile fatty acid(control); n, pH (control).

longer than 6 days. On decreasing HRT to 4 days, theratios increased over 1.5 :1 in acidogenic stage and over4 :1 in digestion stage.

In case of control, the propionate : acetate ratios werealso less than 1:1 when HRTs were over 8 days. Butthe ratios were considerably increased to over 1.5 :1 inacidogenic and 3.5 :1 in digestion stages, on reductionof HRT to 6 day.

In digestion stage, when ratios of propionate : acetatewere over 3.5 :1, the digestion was not a failure in spiteof the presence of bacterial stress (i.e. in case of controlHRT was 6 days, while in pretreated WAS, HRT was4 days). But when organic load was further increased,it resulted in digestion failure (i.e. in case of control

HRT was 4 days, while in pretreated WAS, HRT was3 days). These observations consequently suggested thata drastic increase in propionate : acetate ratio (e.g., fromless than 1:1 increased to over 3.5 :1, Fig.4-b) could beused as reliable warning for impending failure.

ReferencesGrobicki, A, Stuckey, D.C. (1991), Biotechnol. Bioeng., 37:

344–355.Li, Y.Y., Noike, T. (1992), Wat. Sci. Tech., 26: 857–866.Pinnekamp, J. (1989), Wat. Sci. Tech., 21: 97–108.Shimizu, T., Kudo, K., Nasu, Y. (1993), Biotechnol. Bioeng, 41:

1082–1091.Wang, Q.H., Chen, J.C., Kakimoto, K., Ogawa, H.I. and Kato,

Y. (1995), Journal of Japan Society on Water Environment, 18:875–882.

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Figure 4 Effect of hydraulic retention time on the ratio of propionate : acetate in acidogenic and digestion stages. (a)acidogenic stage, (b) digestion stage. d, control; n, pretreated at 60°C; s, at 80°C; u, at 100°C.

Received as Revised 14 January 1997