Dual anaerobic co-digestion of sewage sludge and confectionerywaste
S. La®tte-Trouqu�e, C.F. Forster*
School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Received 21 December 1998; received in revised form 24 February 1999; accepted 9 March 1999
Abstract
Three con®gurations for a dual digestion system were examined. The units were based on three 5 l completely stirred tank re-
actors (CSTR). A ®rst-stage thermophilic digester was used to provide the feed to each of the two second-stage mesophilic (35°C)
digesters. Using a mixture of sewage sludge and strong confectionery waste, the thermophilic digester was operated at 55°C with a
hydraulic retention time of 4 h. The mesophilic digesters were operated at hydraulic retention times of 8, 12 and 15 days. In terms of
the reduction of volatile solids (VS), the three dual digestion con®gurations were similar but were more e�ective than the single-stage
reactor which was used as a control. However, based on the speci®c methane yield (m3 CH4/kg VS removed), the con®guration with
a ®rst stage operating at 55°C and a secondary digester at 35°C with a hydraulic retention time of 12 days was the most e�ective.
This con®guration also maintained a more stable pH, irrespective of the quality of the feed sludge. Ó 1999 Elsevier Science Ltd. All
rights reserved.
Keywords: Anaerobic digestion; Hydraulic retention time; Thermophilic/mesophilic dual digestion; Performance
1. Introduction
The biological treatment of sewage and industrialwastewaters will invariably generate a surplus sludgewhich must be processed and disposed of in an envi-ronmentally acceptable manner. The Municipal Waste-water Treatment Directive prohibits the disposal ofsludge to sea. Thus, alternative disposal routes will needto be established. The Directive also requires that agreater degree of treatment is a�orded to many otherdischarges. This will generate further quantities ofsludge. If disposal to agricultural land is considered,these sludges will have to be stabilised both chemicallyand biologically (Matthews, 1992; Wright, 1992). It islikely that, in the future, the degree of biological stabi-lisation will be extended and that pasteurisation ofsludges will be required. Mesophilic anaerobic digestion,which currently is the accepted way of achieving sludgestabilisation, does not achieve pasteurisation. Pasteu-rising conditions have been de®ned as 70°C/30 min or55°C/4 h (Anon, 1989). Thermophilic digestion would
achieve this, provided the retention time was long en-ough.
Industrial organic wastes are also an integral part ofa developed society. Many of these are readily biode-gradable and, in recent years, considerable e�ort hasgone into examining the treatment of these wastes byanaerobic digestion (Bull et al., 1984; Garcia et al.,1991). Provided that digester capacity was available, itwould seem logical to consider the co-digestion ofmany of these industrial wastes in existing sewagesludge digesters. The concept of centralised co-diges-tion is not new (Forster and Jones, 1976; Converti etal., 1997) and, indeed, Thames Waste Management(UK) are actively pursuing a co-digestion policy forsewage sludge and the putrescible fraction of householdwaste. If co-digestion of organic wastes and sewagesludge is to be developed, the process would have toconform to any requirements for the digestion of sew-age sludge by itself and this could, in the future, meanpasteurisation.
As well as stabilising the sludge microbiologically,any digestion process must optimise the production ofgas in relation to the size of the digester and the de-struction of volatile solids (VS). Coupling these factswith the need for a greater microbial stabilisation has
Bioresource Technology 71 (2000) 77±82
* Corresponding author. Tel.: +44 0121 414 5069; fax: +44 0121 414
5051; e-mail: [email protected]
0960-8524/00/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 4 3 - 7
led to the concept of dual digestion. That is to say, atwo-stage system with one stage being operated in thethermophilic range and one in the mesophilic range. Anumber of workers have evaluated dual digestion. Forexample, Oles et al. (1997) have used a thermophilic (55±60°C) ®rst-stage digester for acidogenesis coupled with amesophilic second-stage. The results demonstrated thatthere was a signi®cant reduction in the time required toachieve a speci®ed degradation of organic matter,compared with a single-stage digester. However, boththese earlier studies used retention times in the ther-mophilic reactor of >1 day. Working with a singlesubstrate, sewage sludge, Roberts et al. (1999a, b) haveshown that a dual thermophilic±mesophilic digestioncould operate e�ectively with the thermophilic ®rst-stagehaving a retention time of 4 h. The authors are notaware of any studies into the use of the dual digestionconcept for co-digestion.
This paper, therefore, compares the results obtainedfrom the dual digestion systems which were operatedwith di�erent hydraulic retention times in the second-stage units. The feed for the units was a mixture of ac-tivated sludge and a strong confectionery waste. Thepaper also compares the results with those obtainedfrom a single-stage mesophilic digester with a hydraulicretention time of 20 days which was used as a control.
2. Methods
2.1. Wastes
For ease of operation, the feed for the digestion sys-tems was waste activated-sludge. This was obtainedfrom a full-scale activated sludge plant operated bySevern Trent Water and was stored in a refridgerated(5°C) tank (300 l) until use. Typically, this had a VSconcentration of 4±5 g/l. The loading rates applied to thedigesters are given in Table 1. In the case of the dualdigestion systems, the loading rates were calculated onthe basis of the overall retention times.
The confectionery waste, consisting mainly of sugarsyrups, was obtained from a local factory on a weeklybasis and was stored at 5°C until required.
2.2. Digesters
The two dual digestion systems consisted of threeidentical continuously-stirred tank reactors, which con-sisted of a purpose-built ¯anged glass tank with a sideport such that the working volume was 5 l (Fig. 1). Oneof these, designated TAND-55, was operated at 55°Cand acted as a ®rst-stage unit which was common toboth second-stage digesters. The feed was pumped to thebase of the reactor by a peristaltic pump (WatsonMarlow, Model 302S) at a rate of 30 l/d giving a re-tention time of 4 h. The stirrer speed was controlled at70 rpm (Electrolab, Bredon, Glos.) and the temperaturewas controlled at 55°C by a heating pad/thermistersystem (Electrolab, Bredon, Glos.). The ¯ow from thisdigester was collected in a stirred splitter-box fromwhere it was pumped to the two second-stage digesters,any excess going to waste. The second-stage digesterswere identical in design to the thermophilic unit butwere operated at 35°C. Initially, (Phase 1) one was op-erated at a hydraulic retention time of 8 days and theother at 12 days. These were designated DUAL-55/8 andDUAL-55/12. After 70 days Phase 2 was started and thehydraulic retention time of DUAL-55/8 was increased to15 days and the system was re-designated as DUAL-55/15. From day 86, the COD of the confectionery wastewas lowered to 7000 mg/l to counteract a foamingproblem. The feed pumps (Watson Marlow, Model302S) were controlled by a timer and were operated oncea day.
Table 1
Sludge loading rate applied to the di�erent digesters
Regime Sludge loading rates (kg VS/m3d)
Max. Min. Mean (�SD)
SS-20 0.403 0.234 0.333 (�0.039)
DUAL-55/8 0.859 0.432 0.631 (�0.125)
DUAL-55/12 0.560 0.280 0.422 (�0.068
DUAL-55/15 0.492 0.288 0.408 (�0.064)
Fig. 1. Schematic diagram of the dual digestion unit showing splitter
box (A) the thermophilic anaerobic digester (TAND-55) and the two
mesophilic anaerobic digesters (MAD-1 and MAD-2).
78 S. La®tte-Trouqu�e, C.F. Forster / Bioresource Technology 71 (2000) 77±82
The single-stage digester (SS-20) was constructedfrom a pyrex bottle (10 l) which had been sealed with asilicone rubber stopper drilled to allow a stirrer and aninlet/exit line to be ®tted. The stirrer (Model RW16,IKA Labortechnik) operated at 200 rpm and was driventhrough a guide shaft which ended 15 cm below theliquid level to avoid gas losses. The inlet/exit line alsostarted at this level. The temperature was maintained at35°C by standing the digester in a water bath. Sludgewas pumped (Model 7554, Cole Parmer Instrument) outof the digester on a daily basis and was replaced by theactivated sludge/confectionery waste mixture. The sol-ids' retention time of 20 days was regulated by theamount of sludge removed each day. The gas from allthe digesters was collected by the downward displace-ment of acidi®ed (0.5M H2SO4) water and measured atSTP.
2.3. Analytical methods
Total and VS were measured by the standard gravi-metric methods (Greenberg et al., 1992). Alkalinity wasmeasured by titration with 0.05M H2SO4 and the pH byusing a standard electrode/meter (Mettler Toledo,Model 320). Volatile fatty acids (VFAs) were measuredwith a gas chromatograph (GC) (Cambridge Ai GC94)which had been calibrated with dilutions of a standardmixture of acids (acetic� 1000 mg/l, propionic to cap-roic� 500 mg/l). A megabore column (D-BFFAP,30 m ´ 0.536 mm ID) was used and the carrier gas washelium (3.2 ml/min). The initial column temperature of105°C was increased at the rate of 30° per minute until atemperature of 145°C had been reached and then at arate of 15° per minute until a temperature of 190°C wasachieved. Samples were centrifuged (6000 rpm/30 min),and then ®ltered through a 0.2 lm nitro-cellulosemembrane. Before analysis, the sample (1.0 ml) wasacidi®ed with formic acid (10 ll) and a sample volume of0.2 ll was injected into the GC. The composition of thegas was also measured by gas chromatography (Pye,Model 104) using a glass column (1.63 m ´ 3 mm ID)with a Poropack (mesh size 80±100) support. The col-umn temperature was 50°C, helium was used as thecarrier gas (40 ml/min) and the sample size was 1 ml.
3. Results and discussion
One of the most serious problems which occurredduring this study was caused by the high strength of theindustrial waste. The COD added to the storage tankwas around 13,000 mg/l and, although the temperaturewas kept at 5°C, the tank contents acidi®ed very rapidlyand it was not easy to stabilize the acidity/alkalinitybalance in the thermophilic digester. As can be seen
from Fig. 2, it produced pH values which, in the main,were between 3 and 4. Such a situation could well occurat a centralised digestion facility where wastes werestored before being passed to the digesters. In addition,the alkalinity concentrations were low, ranging from 0to 500 mg/l. Fig. 2 also shows the e�ect that these lowpH values had on the second stage digesters. Althoughinitially both secondary digesters operated at a more orless neutral pH, after about 40 days, the pH in the re-actor with the shorter retention time (HRT� 8 days) fellto <6. This could have been due to the low pH in thesludge coming from the thermophilic reactor or, morelikely, it was due to a decline in the methanogenicpopulation. Methanogens are known to have long meangeneration times, for example, Methanothrix soehngeniihas a doubling time of 3.5±9 days (Wilkie and Colleran,1988), therefore, the 8 days hydraulic retention time(and solids retention time) would appear to be insu�-cient to support a stable methanogenic population. Thereactor with the longer retention time (HRT� 12 days)had a better bu�ering capacity and had the capacity todevelop a stable methanogenic population. Typically, itspH values ranged from 6.40 to 7.25 and its alkalinityconcentrations from 1000 to 2100 mg/l. In the secondphase of the work, in which the retention times in thesecondary digesters were 12 and 15 days, there was littlechange in the pH of either the thermophilic digester orthe DUAL-55/12. However, although the pH of DUAL-55/15 was low initially, it became stable at a value of 7after about 35 days (data not shown). The alkalinity inthe digester also stabilised at a concentration of 1000±1500 mg/l, a value similar to that in the 12-day digester.Table 2 presents the distributions of the main VFAs inthe treated sludges and shows that there was little dif-ference between the digesters as far as acetate was con-cerned. However, there were di�erences in the
Fig. 2. Variation in the pH values in the di�erent digestion systems.
S. La®tte-Trouqu�e, C.F. Forster / Bioresource Technology 71 (2000) 77±82 79
concentrations of propionate and butyrate althoughthey did not explain the di�erences in performance.
The impact of the thermophilic digesters can also beseen from the data in Fig. 3. This shows that there was ahigh VFA production in the thermophilic digester dur-ing the period when the pH of the 8 day secondary di-gester fell. It also shows that the majority of this acidwas acetate. Taken together, these results suggest thatfor co-digestion with a strong industrial waste to operatesuccessfully, it is necessary to operate the methanogenicdigester with a retention time capable of bu�ering highpH surges. The results suggest that a retention time of atleast 12 days is necessary.
These results need to be viewed in relation to thoseproduced previously with similar dual systems. Robertset al. (1999a) have reported the results of the dual di-gestion of sewage sludge with a 4 h/12 day thermophilic/mesophilic system and have not noted any pH or alka-linity problems. However, the thermophilic digester usedby these workers was receiving an organic load of only30 kg VS/m3d compared with a loading rate in the initialphase of this current work of 72 kg VS/m3d. Roberts etal. (1999b) have also reported the results of a dual di-gestion study with a 4 h/10 day thermophilic/mesophilicsequence in which the loading rate being applied tothe primary, thermophilic digester was even higher
(164 kg VS/m3d) than that currently being reported.However, it did not experience any drop in pH althoughit was producing a total VFA concentration of 2500 mg/l. Obviously, this means that it is not merely the mag-nitude of the applied load which is critical in determin-ing the pH of the primary digester.
One way of assessing the performance of a digester isto examine the e�ciency of the VS reduction. Figs. 4 and5 show the day by day variations in the VS concentra-tions for Phase 1 and 2, respectively and Table 3 showsthe mean values for the VS reductions which wereachieved when the retention times were taken into ac-count together with their maximum and minimum val-ues. Table 3 also compares the reduction achieved by thedual digestion systems, both with the reductionsachieved by the single-stage system and the reductionswhich have been produced by other dual digestion sys-tems. Although it might be argued that the DUAL-55/12system showed the best performance, in reality, the re-sults show that there was little di�erence between theperformances of the dual digestion con®gurations whichhave been the focus of the current study. Earlier work,which focused on the dual thermophilic±mesophilic di-gestion of sludge on its own (Roberts et al., 1999a, b),showed VS reductions of a similar magnitude to those
Fig. 3. Variation in the VFA concentrations in the treated sludge from
the thermophilic digester.
Fig. 4. Variation in the VS concentrations for the di�erent digestion
systems operated in Phase 1.
Table 2
VFA concentrations (mg/l) in the treated sludges
Regime Acetate Propionate Butyrate
Max. Min. Mean Max. Min. Mean Max. Min. Mean
SS-20 2598 11 642 3689 0 1149 829 0 177
DUAL-55/8 1717 38 491 1782 214 611 2114 0 437
DUAL-55/12 2273 3 436 3897 0 301 788 0 89
DUAL-55/15 2114 31 981 2277 0 381 2372 0 713
80 S. La®tte-Trouqu�e, C.F. Forster / Bioresource Technology 71 (2000) 77±82
reported in this current study. All these data originatedfrom dual digestion systems which were operated withshort (4 h) retention times in the primary digester andthese can be said to give, in general, an e�ciency of 47±50%. The reductions reported by Oles et al. (1997) arefrom a dual digestion system using a longer retentiontime in the primary digester and are greater. Obviously,the e�ect of varying the primary tank retention time willneed to be investigated further. The single-stage me-sophilic digester did not give as great a reduction of VS inPhase 1, but in Phase 2 it did produce comparable results(50%). All these results need to be viewed in relation tothe reduction in VS which is achieved by a full-scale di-gester treating sewage sludge. Typically, this is about50%. Brade and Noone (1981) have reported a value of49% as the average obtained in a 6 month trial with adigester operating with a retention time of 15 days.
Table 4 shows the mean values of the speci®c meth-ane yield, together with their maximum and minimumvalues, which were calculated with the retention timesbeing taken into account. Table 4 also compares theyield values achieved by the dual digestion systems withboth the values obtained by the single-stage system andthose which have been produced in other dual digestionstudies using sewage sludge alone. The data presentedshow that, although there were quite wide ¯uctuationsin the daily data, the overall mean values are compa-rable with previous results for unthickened sewagesludge in dual digestions with 4 h retention times in thethermophilic ®rst-stage (Roberts et al., 1999a). The re-sults are also comparable with those for the full-scaledigestion of sewage sludge which was reported to give anaverage yield of 0.34 m3/kg VS added (Brade andNoone, 1981). As with the VS reductions, the results areslightly di�erent from those reported for the dual di-gestion with the longer ®rst-stage retention times. Ta-ble 4 also includes the gas composition data and showsthat, in general, the gases being produced by the various
Table 4
Speci®c methane yields achieved by the dual digestion systems a
Mode of digestion Gas composition (% methane) Methane yield (m3/kg VS applied)
Max. Min. Mean Max. Min. Mean
This study
DUAL-55/8 ± Phase 1 68 82 77 0.34 0.04 0.12
DUAL-55/12 ± Phase 1 83 61 76 0.56 0.03 0.34
DUAL-55/12 ± Phase 2 87 66 82 0.39 0.11 0.30
DUAL-55/15 ± Phase 2 85 44 72 0.36 0.02 0.31
SS-20 ± Phase 1 77 45 66 0.60 0.05 0.36
SS-20 ± Phase 2 84 62 76 0.64 0.02 0.28
Previous work
4 h/12 days thermophilic/mesophilic (Roberts et al., 1999a) ÿ ÿ c. 85 ÿ ÿ 0.28
4 h/15 days thermophilic/mesophilic (Roberts et al., 1999a) ÿ ÿ c. 85 ÿ ÿ 0.26
Thermophilic/ mesophilic (Oles et al., 1997) ÿ ÿ ÿ ÿ ÿ 0.39
a Mean for re-stabilised region.
Fig. 5. Variation in the VS concentrations for the di�erent digestion
systems operated in Phase 2.
Table 3
Volatile solids (VS) reduction by the dual digestion systems
Mode of digestion VS reduction (%)
Max. Min. Mean
This study
DUAL-55/8 ± Phase 1 69 29 51
DUAL-55/12 ± Phase 1 70 31 52
DUAL-55/12 ± Phase 2 74 28 51
DUAL-55/15 ± Phase 2 65 20 41
SS-20 ± Phase 1 58 7 28
SS-20 ± Phase 2 60 36 50
Previous work
4 h/12 days thermophilic/mesophilic
(Roberts et al., 1999a)
ÿ ÿ 45
4 h/15 days thermophilic/mesophilic
(Roberts et al., 1999a)
ÿ ÿ 44
Thermophilic/mesophilic
(Oles et al., 1997)
ÿ ÿ 56
S. La®tte-Trouqu�e, C.F. Forster / Bioresource Technology 71 (2000) 77±82 81
dual digestion modes were very similar and were richerin methane than would normally be expected.
Overall, the results show that dual thermophilic/me-sophilic digestion systems can operate successfully withthe sewage sludge/confectionery waste mixture and that,the concept of a centralised multi-waste digestion facilitycould be a possibility.
4. Conclusions
The results have shown that:· an unthickened sewage sludge can be supplemented
with a strong industrial wastewater and produce per-formances in a dual thermophilic/mesophilic diges-tion system which were comparable with thoseobtained for sewage sludge alone;
· a hydraulic retention time of 8 days in the second-stage digester was not able to assimilate high volatileacid concentrations and low pH values from the ®rst-stage digester. This was probably due to the retentiontime being too short to maintain a strong met-hanogenic population;
· a hydraulic retention time of 12 days in the second-stage digester appeared to give the best performancein terms of stability, VS reductions and speci®c meth-ane yield.
Acknowledgements
The authors would like to acknowledge the ®nancialsupport given by the Technology Development Team ofNorth West Water Ltd.
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