anaerobic co-digestion of mechanically biologically treated municipal waste with primary sewage...

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Bioresource Technology 99 (2008) 3354–3364

Anaerobic co-digestion of mechanically biologically treatedmunicipal waste with primary sewage sludge – A feasibility study

Ole Pahl a,*, Anna Firth b, Iain MacLeod a, Jim Baird a

a Caledonian Environment Centre, School of the Built and Natural Environment, Glasgow Caledonian University, Glasgow G4 0BA, UKb Scottish Water, 55 Buckstone Terrace, Edinburgh EH10 6XH, UK

Received 18 May 2005; received in revised form 23 May 2007; accepted 14 August 2007Available online 27 September 2007

Abstract

This bench scale study investigated the suitability of MBT material for treatment by anaerobic digestion and the impacts of co-diges-tion of these wastes with sewage sludge. The results suggest that MBT material is amenable to anaerobic digestion with sewage sludge.The main problems for scale-up are related to the physical composition of the MBT material, the accumulation of heavy metals andother inert contaminants and the impact of both of these factors on final sludge quality. Full-scale trials would be required to assessthe long-term impacts of MBT waste on anaerobic digestion, if this form of co-digestion were to be pursued. The material contaminationthat presents a barrier to the direct recycling of MBT material in land-applications is also a major hurdle in commercial co-digestion.Better quality input material would be likely to result in higher methane yields and fewer restrictions on the utilisation of the productin recycling.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion; Biowaste; Mechanical biological treatment (MBT); Co-digestion; Sewage

1. Introduction

The Landfill Directive (1999/31/EC; EU, 1999), theScottish National Waste Plan and corresponding AreaWaste Plans have fundamental implications for the devel-opment of integrated waste management strategiesthroughout Scotland. Of particular significance is the stat-utory requirement derived from the Landfill Directive toachieve a number of targets including quantified andphased diversion of biodegradable waste from landfill dis-posal by 2020, and pre-treatment of waste prior to final dis-posal by landfill. The specified quantities of biodegradablemunicipal waste to be diverted from landfill are based onrecorded 1995 waste arisings and are phased over the fol-lowing timescales:

0960-8524/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2007.08.027

* Corresponding author. Tel.: +44 141 331 3572; fax: +44 141 331 3570.E-mail address: [email protected] (O. Pahl).

• reduction to 75% of baseline by 2010;• reduction to 50% of baseline by 2013;• reduction to 35% of baseline by 2020.

These targets were supplemented by a 2006 Scottishdomestic target of a reduction to 85% of baseline quanti-ties, equivalent to 1.5 million tonnes of biodegradablemunicipal solid waste (MSW) being landfilled, and a com-bined recycling and composting rate of 25%. Whilst thisinitial domestic target was achieved, the increasingly strin-gent Landfill Directive targets may pose a real problem forthe Scottish waste management sector. Strategies designedto maximise the diversion and treatment of biodegradablewaste from the municipal waste stream have the potentialto deliver these targets, and generally are based on 60%biodegradable waste content in a typical MSW sample.

Against this background, a number of local authoritieshave been committed to mixed waste composting technol-ogies via mechanical biological treatment (MBT). This cov-ers a range of technologies designed to process mixed

mailto:[email protected]

Table 1Concentrations tested using rapid test method (ml)

# MBT product slurrya Primary sludge Digested sludge

1 0 400 4002 100 300 4003 200 200 4004 300 100 4005 400 0 4006 800 0 0

a 1.5 kg of dried MBT product mixed with 1000 ml tap water.

O. Pahl et al. / Bioresource Technology 99 (2008) 3354–3364 3355

wastes by composting; the primary purpose is to reduceand stabilise the organic fraction within the municipal solidwaste (MSW). Specifications for the use of biostabilisedwaste have been the subject of various reviews in recentyears (Brinton, 2000; EU, 2001; GET, 2003; SEPA,2004), and an important distinction is drawn betweensource-segregated ‘‘green-wastes’’ and residual or mixedwaste which is subject to MBT. One particular feature ofMBT systems in place in Scotland is a short but intensein-vessel composting phase followed by longer-term matu-ration in open windrows. As a consequence, the materialretains a considerable amount of biodegradable materialafter the initial intensive aerobic treatment. Alternativesto this long-term aerobic maturation could be possibledepending on the residual biodegradability.

Whilst aerobic treatment via composting or MBT isaimed mainly at reduction in biomass and concentrationand enhancement of fertiliser/soil improver value, there isalso use of anaerobic digestion (AD) in the treatment oforganic (fraction of) wastes. The benefit of AD is that someof the energetic value of the removed biomass can be uti-lised in the form of biogas (Rao and Singh, 2004). AD alsoallows for cost-effective control of pathogens (Held et al.,2002; Termorshuizen et al., 2003) through the use of thegenerated biogas as a fuel for pasteurisation. Given thesepotential benefits, the use of anaerobic digestion shouldnot be overlooked in the development of Scottish waste-treatment strategies. However, the current emphasis ininfrastructure development is clearly on the compostingof green waste and the mechanical biological treatment ofMSW so that a short- or medium-term change-over toAD systems is unlikely. At the same time, there are cur-rently more outlets and treatment options in place for dig-estate such as sewage sludge than there are for MBTmaterial. Consequently, this study was carried out, as partof a suite of research programmes to identify the best wayforward, in order to determine the suitability of MBTmaterial for treatment by anaerobic digestion and theimpact of co-digestion of sewage sludge with this material.In light of a potential future phasing out of the compostingelement of the MBT treatment, two types of material weretested in co-digestion, namely the mechanically pre-sortedMSW before aerobic treatment (denoted ‘MBT educt’),and the material after aerobic treatment prior to long-termmaturation in open windrows (denoted ‘MBT product’).

2. Methods

This study consisted of a series of tests including rapidtreatability tests and bench scale co-digestion of waste withsewage sludge.

2.1. Origin and preparation of organic material

The sewage sludges used for the tests were collectedfrom Scottish Water’s wastewater treatment plant atBo’ness, West Lothian. Primary sedimentation sludge and

return sludge from the aeration basins was used in therapid treatability tests (Table 1) whereas the bench scaletests used return sludge only for seeding (3:1 return andprimary sludge) followed by additions of primary sludge.The sludges were refrigerated prior to use.

The MBT material was collected from Levenseat landfillsite, where MSW is subjected to a dry screening processemploying a bag splitter, 60 mm trommel, over-band mag-net and Eddy current. The remaining fraction (denotedMBT educt) is then composted in a bunker system forabout 2 weeks (at this stage denoted MBT product) andsubsequently moved to large, open windrows of around3 m height, where it matures for 6 months. The maturedMBT product at Levenseat undergoes a final screening pro-cess prior to further use but, for the purpose of this study,any inert contaminants larger than approximately 5 mmwere removed by hand. In preparation for particle sizereduction and subsequent production of a slurry of a con-sistency that could be handled by the laboratory set-up, thematerial was partially dried at 35 �C in a fan-assisted ovenin layers of no more than 20 mm thickness for 24 h andreduced in particle size to pass a sieve of 2 mm mesh size(Retsch cutting mill SM100) prior to being stored; the dry-ing temperature was limited to 35 �C in order to avoid vol-atilisation of fatty acids. Stored material was made into aslurry with water at a ratio of 1.5 kg of partially driedmaterial mixed with 1000 ml of tap water. The mixtureswere made up in batches in sufficient volume for the rapidtreatability tests and the bench scale trials, and frozen untilneeded. This was in order to limit variability in the feed-stock during preparation. The feedstock was defrostedand incubated in a water bath at 35 �C before use.

2.2. Sampling routine

Analysis for determinants was carried out on feedstock-,interim- (half-way through the 3 HRT digestion period)and final sludge-samples. All analyses were conducted inaccordance with the guidance on methods of samplingand analysis for determining the quality of environmentalmatrices given by the UK Environment Agency’s NationalLaboratory Service (‘Blue Books’; EA, 2007):

• heavy metals – cadmium, copper, chromium, nickel,lead, zinc (nitric acid/atomic absorption spectrophotom-etry; Blue Book 49) and mercury (flameless atomicabsorption spectrophotometry; Blue Book 10);

Fig. 1. Schematic of bench scale digester set-up.

3356 O. Pahl et al. / Bioresource Technology 99 (2008) 3354–3364

• ammonia (Nessler/spectrophotometry; Blue Book 48);• total solids (gravimetric; Blue Book 83 and 105);• volatile solids (gravimetric; Blue Book 83);• volatile fatty acids (gas chromatography; Blue Book 21);• pH (electrode; Blue Book 149).

Wasted sludge was analysed twice a week in a similarfashion but excluding heavy metal analysis.

Manual particle size analysis using soil sieves of variousgraduations was carried out on both MBT product andeduct. Biogas production rate (ml/h) and quality (CH4

and CO2 content) was measured twice/three times perweek. Daily measurements of temperature were also takento ensure optimum operating conditions of the digesters.After completion of the digestion period, samples of theresidual sludge were collected for analysis of inert material.

2.3. Rapid treatability tests

An initial set of rapid anaerobic digestion tests was car-ried out in order to assess the suitability of the MBT prod-uct for anaerobic co-digestion with sewage sludge.Suitability was assessed by comparison of gas productionrates with those of sewage sludge only. A number of con-centrations of MBT product were assessed against the con-trols (Table 1) in two consecutive runs.

Anaerobic digestion was carried out in 1 l plastic bottles.Each bottle was filled to 800 ml with a mixture of biowasteand sewage sludge, as detailed above. Digested sludge wasused in the mixture to provide a quicker start-up periodthan primary sludge alone. Each bottle was shaken tomix the contents thoroughly and then fitted with an airtightlid and a tube to carry biogas to a collecting tube. The vol-ume of gas collected was measured by the displacement ofwater from the collecting tube. The test vessels were incu-bated in a water bath at approximately 35 �C for 5 h andthe volume of gas produced recorded every hour, aftergently shaking each bottle to release any gas trapped inthe sludge.

At this initial stage of the project (rapid treatabilitytests) detailed analysis of the feed material was notundertaken.

2.4. Bench scale trials

The bench scale trials were carried out using 5 l Quickfitvessels with 4 port lids. Each vessel was fitted with a stain-less steel stirrer, which was powered by a motor and stirredcontinuously at 60 rpm, a thermometer and a gas collectionsystem. The fourth port was used to add and removesludge. Due to its thick consistency, sludge additions wereadministered manually through the port, with a flexibletube and clamp acting as gas lock. Heating tapes wrappedaround the outside of each vessel were used to maintain thetemperature of the digesting sludge at 35 �C ± 3 �C. Fig. 1shows the experimental set-up, which is similar to thatdescribed by Wilkie et al. (2004).

Initially, all the digesters were filled with 4 l of sewagesludge (3:1 digested and primary sludge). This was followedby a 2-week stabilisation period, during which time 500 mlof sludge was first removed and a further 500 ml of freshprimary sludge added every 2 days. This allowed time forthe experimental set-up to be checked, i.e. to ensure gasseals were intact and temperature controls were correct,and for the microbiological activity to stabilise.

Following the stabilisation phase, biowaste slurry wasmixed with primary sludge at different concentrationsbefore the mixed sludge was added to each digester: con-centrations of 0%, 12.5% and 25% MBT material weretested in duplicate, plus an extra two controls using pri-mary sludge.

Every 2 days, 500 ml of sludge was removed from eachdigester, before a further 500 ml of fresh sludge/sludge-bio-waste mix was added. Using the feeding pattern of 500 mlfeed every 2 days (excluding Saturdays and Sundays), thehydraulic retention time (HRT) of the sludge was 18.7days. The trials were run for 3 HRTs in total.

Following analysis of interim results, taken after 1.5HRTs, it was thought that the control digesters were notperforming as well as anticipated and that this was dueto low-solids content of the primary sludge. Therefore,from day 12 of the second HRT onwards, feedstock mixeswere made up using ‘thicker’ primary sludge from alterna-tive sources (Dalderse and Galashiels wastewater treatmentworks).

At the end of the first HRT, due to an accumulation ofvolatile fatty acids and increasing acidity in the digesters(setpoint: pH 6), 2 M Na2CO3 was added to each digesterin sufficient quantity to adjust the pH to within the range6.8–7.8 and to increase buffering capacity. Subsequently,the pH of the feed to be added was checked before additionand corrected to within this range if necessary, also using2 M Na2CO3.

At the end of the final HRT the resultant sludge wasanalysed. Once the stirrers had been turned off, the


contents were left to settle overnight. The supernatant wasthen removed and discarded, and the remaining sludge wasmixed before composite samples from each set werecollected.

2.5. Gas measurements

Gas production rates were measured daily via the vol-ume of water displaced in an inverted graduated tube filledwith water (Fig. 1; amended from Blue Book #5, EA,2007). The tube was filled under vacuum and the volumeof gas collected in the tube measured after 1 h.

The main purpose of the gas measurement, for this pro-ject, was to identify whether there were any differences ingas production rates for different feed make-ups, ratherthan to assess absolute gas production rates. Given thatthe process was operated continuously, measurement ofan hourly rate was deemed sufficient to compare betweendifferent set-ups. Similarly, the effect of biogas solution inthe liquid phase was considered negligible as it affectedall vessels.

Following gas volume measurement, the collecting tubeswere detached from the digesters and gas collecting bagsattached. Gas was collected overnight and analysed formethane and carbon dioxide content the following dayusing a Geotechnical Instruments GA45 Plus Infrared gasanalyser. Such gas analysis was carried out 2–3 times perweek.

3. Results and discussion

3.1. Rapid tests

In both test runs, the control digesters produced thehighest volume of biogas (Table 2). The second test runshowed a better correlation between reduction in gas pro-duction and increase in biowaste concentration. On thebasis of these results it was decided that bench scale testswould be carried out at 12.5% and 25% concentrations of

Table 2Results of rapid test – biogas production

Feedstock MBTproduct content (%)

Biogas produced (ml/h)

1sthour

2ndhour

3rdhour

4thhour

5thhour

Total(ml/5 h)

Control: 0.0 50 30 0 10 10 10012.5 10 25 30 10 10 8525.0 0 30 15 10 5 6038.0 20 20 10 20 5 7550.0 5 0 0 0 5 10100.0 15 15 0 0 5 35

Control: 0.0 65 30 30 25 30 18012.5 35 10 10 5 5 6525.0 30 15 15 5 10 7538.0 20 10 5 5 0 4050.0 15 10 5 10 15 55100.0 15 10 5 0 0 30

biowaste. Higher concentrations of biowaste were deemedunlikely to be suitable for co-digestion, not least due tothe increasingly paste-like consistency of the mixed slurry.

3.2. Bench scale trials

3.2.1. Feedstock analysis

Following initial preparation of feedstock materials,analyses were carried out for volatile solids content andheavy metal concentration of the feedstock.

3.2.1.1. Volatile solids (VS) content. Assessment of volatilesolids loading rate during anaerobic digestion is importantto maintain a stable digester, as an overload of organicmaterial can lead to excessive acid production, which inhib-its the activity of methanogenic bacteria. It was shown(Table 3) that the MBT waste feedstocks (both MBT prod-uct and educt) contained higher concentrations of volatilesolids than the primary sludge, despite the presence of agreater proportion of total solids present as VS in primarysludge. This was due to the low concentration of total sol-ids in the primary sludge in comparison with the sludge/MBT mixtures. It is interesting to note that there was lessdifference in the volatile solids content between MBT prod-uct and educt mixed slurries than was expected after theintense aerobic pre-treatment during the MBT process.

The VS content and loading rates utilised during thisproject (Table 3) ranged from 36% to 71% TS and 1.9 to3.8 g l�1 d�1, respectively, and were comparable to thosereported in related studies on the co-digestion of theorganic or source-segregated organic fraction of municipalsolid waste: 50–95% TS and 0.7–4.5 g VS l�1 d�1 (Davids-son et al., 2007; Angelidaki et al., 2006; Gomez et al.,2006; Hartmann and Ahring, 2005; Krupp et al., 2005;Sosnowski et al., 2003; Vogt et al., 2002).

3.2.1.2. Heavy metal concentration. For all metals exceptcopper, metal concentrations in the mixtures containingMBT material exceeded the levels in the primary sludgebut were of similar magnitude to those reported as averagefor the European Union (Table 4). Copper in both the12.5% and 25% mixes of MBT was less than in primarysludge. The relatively high copper level in the primarysludge may have been due to copper pipes used for watersupply and this is not an unusual level for sewage sludge.

Table 3Volatile solids contents of feedstocks

Feedstock % TS % VS(of TS)

VS content(g VS l�1)

VS loadingrate(g l�1 d�1)

Control (primarysludge)

5.0 72.4 35.9 1.9

12.5% MBT educt 9.6 59.1 56.7 3.025.0% MBT educt 13.9 50.6 70.2 3.812.5% MBT product 10.6 50.5 53.7 2.925.0% MBT product 16.6 42.6 70.7 3.8

Table 4Heavy metal contents of feedstocks (all in mg/kg TS)

Feedstock type Cd Cr Cu Pb Ni Zn

Control (primary sludge) 0.8 29 197 69 18 33112.5% MBT educt 0.6 59 127 94 35 36925.0% MBT educt 0.7 57 163 98 43 37712.5% MBT product 0.8 54 536 188 68 53325.0% MBT product 0.8 40 253 184 79 558EU sewage sludge (AROMIS, 2005) 2.0 73 330 36 104 811


In all cases heavy metal levels in the mixes containing MBTproduct were higher than in those containing MBT educt.A particularly high result of 536 mg/kg was obtained forcopper concentration in the 12.5% MBT product. As thebiowaste contained large quantities of solid material, thismay have been due to a slug of copper present in the par-ticulate matter.

3.2.1.3. Particle size analysis. Particle size analysis of theMBT product and educt (Table 5) showed that the MBTproduct contained a higher proportion of physical contam-inants (75%) than the MBT educt (64%), and that around athird of physical contaminants of the MBT product wereretained on a 20 mm sieve, compared with almost half ofthe physical contaminants for the MBT educt. It isassumed that the relatively higher content of physical con-taminants in the MBT product was due to a concentrationeffect arising from the loss of organic material during theMBT process. The largest proportion, by weight, of thephysical contaminants retained in the MBT product wasglass (52% of the total sample tested). However, in the

Table 5Result of fractionation analysis

Sieve size (mm) Retained – absolute (g) Type of physical con

Sample Organica Physical Glass Metal

MBT product

37.5 24.8 0.4 24.4 0.0 2.720.0 203.7 16.7 187.0 141.0 6.210.0 228.7 24.7 203.6 171.4 2.45.0 124.9 57.6 67.1 58.6 02.36c 68.8 34.4 34.4 – –Panc 129.3 64.7 64.7

Total 780.2 198.5 581.2 371.0 11.3Total (%) 25.4 74.5 47.6 1.4

MBT educt

37.5 22.9 16.8 5.8 0.0 0.020.0 278.1 62.5 215.5 97.1 54.110.0 211.8 79.0 132.0 38.0 3.95.0 100.2 48.1 52.0 15.7 16.82.36c 46.3 23.2 23.2 – –Panc 49.6 24.8 24.8

Total 708.9 254.4 453.3 150.8 74.8Total (%) 35.9 63.9 21.3 10.6

a Paper, wood, compost and dirt.b Also contains ceramics and other consolidated mineral contaminants.c Due to the fine nature of this material an assumption is made that it was

MBT educt, the largest proportions were glass and stones(21% and 22%, respectively).

3.2.2. Gas production and gas quality

The production of biogas (Table 6) was variablethroughout the duration of the trial and this may indicatethat the gas collection periods should have been longer.However, average absolute gas production rates wereshown to be higher for mixed substrate than for the controlin all cases, except for the 25% MBT product. Here, despitecontaining higher levels of volatile solids, gas productionwas less than for the control. However, when expressedas a ratio of gas produced per mass unit of volatile solidsentering the digesters (Table 6) it is clear that the controlachieved the highest levels of gas production. The additionsof MBT material at 12.5% resulted in a reduction in gasproduction of similar proportions for both MBT productand educt, and a further drop in gas production rate wasobserved at 25% addition of MBT material, which wasmore pronounced for the MBT product. The observeddrop in gas production in line with increased MBT concen-tration could be explained by the resultant increase in TSand VS content; Rao and Singh (2004) observed a similardrop with a comparable increase in solids concentration.However, it also has to be noted that the increasing concen-tration of MBT material (both product and educt) used inthis project led to a significant introduction of heavy metalsinto the digesters (Section 3.2.4), which may have contrib-uted to the reduced gas production rates.

Overall, the gas production rates achieved (Table 6) ran-ged from 0.13 m3 kg�1 VS (25% MBT product) to

taminants retained – absolute (g) Retained – accumulative (%)

Plastic Stoneb Organica Physical

1.1 20.6 0.2 4.23.9 35.9 8.6 36.4

19.4 10.4 21.1 71.42 6.5 50.1 82.9

– – 67.4 88.9100.0 100.0

26.4 73.43.4 9.4

0.2 5.6 6.6 1.315.2 49.1 31.2 48.810.7 79.4 62.2 77.90.6 18.9 81.1 89.4

– – 90.3 94.5100.0 100.0

26.7 1533.8 21.6

50% organic.

Table 6Biogas production rates and quality during bench scale trials

Average production of biogas Biogasrecoveryb

l h�1

(StdDev)m3 kg�1 VS % CH4

(StdDev)% v/v(StdDev)

Control (primarysludge)

0.115 (81) 0.29 35 (17) 86 (13)

12.5% MBTeduct

0.147 (120) 0.23 40 (20) 88 (9)

25.0% MBTeducta

0.148 (108) 0.19 42 (17) 96 (6)

12.5% MBTproduct

0.146 (93) 0.24 47 (11) 89 (10)

25.0% MBTproduct

0.100 (101) 0.13 43 (17) 92 (9)

a One of the replicates failed at an early stage in the trials. The reasonsfor this are not established and the data have been excluded from theresults presented here.

b Sum of concentration (%) of carbon dioxide and methane as an indi-cation of analytical capture of main biogas constituents.


0.29 m3 kg�1 VS (control) and are therefore comparable tosimilar data reported elsewhere, which range from 0.1 to0.7 m3 kg�1 VS (Davidsson et al., 2007; Hartmann andAhring, 2005; Rao and Singh, 2004; Sosnowski et al.,2003). It should be noted that the data reported here relateto anaerobic digestion of aerobically pre-treated materialand utilised mechanical sorting where reference data wasobtained using source-separated organic fraction only ofMSW (OF-MSW), and this may explain why gas produc-tion rates observed here were at the lower end of the rangeobserved elsewhere.

However, the methane content of the biogas was vari-able and low in all cases: 40–50% methane in biogas com-pared to 55–70% reported elsewhere (Davidsson et al.,2007; Hartmann and Ahring, 2005; Rao and Singh, 2004;Sosnowski et al., 2003). The methanogenic bacteria, whichconvert acetate and carbon dioxide to methane, are themost sensitive group of bacteria in the process. The resultsseem to suggest that the high concentrations of heavy met-als (Section 3.2.4) may have caused the poor gas qualitydue to the effect on methanogenic bacteria, while the rela-tively low level of overall gas production may be explainedby the high solids loading and the pre-digested make-up ofthe feedstock.

3.2.3. Sludge quality – volatile fatty acidsReduced activity of the methanogenic bacteria can lead

to an increase in the levels of volatile fatty acids (VFAs) aswell as a reduction in methane production. Although aver-age fatty acid levels varied considerably (Fig. 2), a generaltrend was apparent in all digesters as concentrations ofVFAs increased from low levels to higher levels throughthe middle period of the trials and decreased again to lowerlevels towards the end of the period. The reduction in VFAconcentration from the mid period onwards was slightlyoffset by the addition of Na2CO3, and resultant increase

in both buffering capacity and pH, from the end of the firstHRT onwards. Average concentrations of VFAs in a full-scale, healthy sewage-sludge digestion process are expectedto be less than 500 mg l�1 (MacBrayne, 2004); VFA con-centrations observed during these trials were on averageup to 10 times this value, or more. However, VFA levelsin the digestion of the organic fraction of MSW have beenreported in the region of several thousand mg l�1 (Angeli-daki et al., 2006; Gallert et al., 2003; Rao and Singh, 2004).There are conflicting reports on the causes and effects ofelevated levels of VFA; Burton and Turner (2003) reportthat VFA levels up to 10,000 mg l�1 have little inhibitoryeffect on methanogenic activity at neutral pH levels, possi-bly with the exception of propionate at levels above1000 mg l�1 (McCarty and McKinney, 1961). Gallert andWinter (1999) attribute inhibitory effects to elevated levelsof acetate, in addition to the effect of elevated hydrogenconcentrations on syntropic metabolisms. However, hydro-gen inhibition of fermentative bacteria appears not to haveoccurred during this project because the formation of ace-tic acid was similar to that of the controls. Some build-upof propionic acid was observed towards the end of the tri-als, so that a long-term inhibition of fermentative bacteriacould not be excluded. Overall, it appears most likely thatthe observed accumulation of VFAs and the sub-normalmethane concentrations in the biogas indicate a reductionin the activity of methanogens, most likely caused by heavymetal inhibition (Section 3.2.4).

3.2.4. Sludge quality – heavy metals

An accumulation of zinc, lead and nickel in the digesterswas observed as the trials progressed (Table 7). This effectwas most pronounced in the digesters containing MBTproduct (�70, �250 and >220% increase, respectively)and it is suggested that metals were concentrated in theMBT product due to loss of organic material. A similartrend is also apparent for cadmium, although less pro-nounced. Relatively high levels of copper were detectedin the initial samples and this is assumed to have beendue to high concentrations present in the sewage sludge fol-lowing initial set-up. Copper concentrations in the digesterscontaining MBT product were higher than the control inthe interim and final samples, again suggesting a concentra-tion effect in the MBT product. Accumulation of mercuryand chromium did not follow a similar pattern to that ofthe other metals tested. In the case of mercury, interim con-centrations were lowest and initial concentrations largerthan those in both interim and final samples. High initialconcentrations may suggest that most of the mercury pres-ent was contributed by the primary sludge rather than bythe biowaste. For chromium, evidence of accumulation isshown as, in all cases, there was an increase in content frominitial to interim and final samples.

Relative toxicities of heavy metals for anaerobic diges-tion have been reported (Codina et al., 1998) asZn > Cr > Cu > Cd > Ni > Pb and absolute EC50 values(50% inhibition of microbiological activity), respectively,

Control

0

2000

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6000

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10000

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

0 10 20 30 40 50 60

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Aci

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12.5% MBT educt

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d co

nc. [

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25% MBT educt

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12.5% MBT product

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25% MBT product

02000400060008000

10000

Time [days]

Aci

d co

nc. [

mg/

l]

Total VFA acetic acid propionic acid

Fig. 2. Average VFA levels observed in the digesters.


as 50, 50, 100, 200 and 350 mg/l (Pb not reported). Withthe exception of cadmium and nickel, these EC50 valueswere exceeded in all digesters. Zinc has the lowest EC50and was present at high levels in all digesters. This mayhave inhibited the methanogenic bacteria, which are themost sensitive group of bacteria in the process, and may

be the reason for the poor gas quality results reportedabove.

3.2.5. Sludge quality – particle size analysisAnalysis of the sediment collected over the duration of

the trials (Table 8) showed total solids contents in the

Table 7Accumulation of heavy metals during bench scale digestion (all concentrations are provided in mg kg�1 TS)

Zinc Lead Nickel Cadmium Copper Mercury Chromium

S I E D(%)

S I E D(%)

S I E D(%)

S I E D(%)

S I E D(%)

S I E D(%)

S I E D(%)

Control 566 487 595 5 103 130 180 75 27.8 28.5 33.9 22 1.10 0.98 1.05 �5 269 233 334 24 1.51 1.02 1.07 �30 46.3 51.1 69.2 4912.5% MBT

educt568 577 641 13 111 176 199 80 27.2 53.2 66.5 144 1.10 1.10 1.30 18 290 229 202 �30 1.58 1.05 1.04 �34 46.3 83.8 115.0 148

25.0% MBTeducta

563 500 671 19 109 144 215 98 31.8 48.6 73.4 131 1.00 0.85 1.30 30 314 176 191 �39 1.74 0.81 1.45 �16 50.9 66.5 117.0 130

12.5% MBTproduct

512 821 883 73 96 288 337 253 32.6 78.4 106.5 227 1.05 1.40 1.65 57 240 377 402 67 1.09 0.83 0.90 �18 48.5 62.8 95.0 96

25.0% MBTproduct

532 766 912 71 99 260 343 246 30.8 83.9 116.0 277 1.20 1.05 1.60 33 220 374 426 93 1.57 0.68 1.13 �28 49.8 48.8 104.1 109

Concentration of heavy metals as measured at: S = start, I = interim, E = end date; D = accumulation of heavy metals compared to start date.a End value based on single measurement only.

Table 8Total solids, organic matter (OM) and OM-normalised heavy metal contents of digester sediment

Feedstock type Total solids(%)

Organic matter(% TS (StdDev))

Cadmium(mg kg�1 OM)

Chromium(mg kg�1 OM)

Copper(mg kg�1 OM)

Lead(mg kg�1 OM)

Mercury(mg kg�1 OM)

Nickel(mg kg�1 OM)

Zinc(mg kg�1 OM)

Control 7.2 56.7 (0.12) 1.5 96 463 249 1.5 47 82412.5% MBT

educt15.3 41.1 (2.12) 1.3 117 206 203 1.1 68 653

25.0% MBTeduct

16.2 53.5 (12.1) 1.7 151 246 277 1.9 95 866

12.5% MBTproduct

23.1 30.0 (2.75) 1.4 81 345 289 0.8 91 757

25.0% MBTproduct

24.3 39.0 (2.00) 1.6 102 419 337 1.1 114 897

SEPA Limit <3 <400 <200 <200 <1 <100 <1000

Note: heavy metal limits normalised to 40% organic matter content.

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MBT digesters of between 2 and 3 times that of the controldigesters, for MBT educt and product, respectively. Thereappeared to be little effect of the mixture MBT content(12% or 25%) on the total solids concentration in the finalsediment, whereas there was a marked difference in organicmatter content of total solids between the different mix-tures. Surprisingly, lower levels of organic content wererecorded for the 12.5% mixtures than for the respective25% mixtures, and this may indicate more effective diges-tion of organic matter in the more dilute mixtures. Overall,up to 70% of total solids were found to be made up of inor-ganic material, and the highest inorganic content was pres-ent in the sediment of mixes containing MBT product, withless elevated levels of inorganic content found in the sedi-ment of mixes containing MBT educt. The occurrence ofphysical contaminants was higher in all digesters containingbiowaste than in the control digesters. This was apparent,also, from visual inspection of the accumulated sediment.

3.3. Summary of results and discussion

3.3.1. Stabilisation and biogas productionDuring these trials there appeared to be an increase in

the absolute volume of biogas produced after the additionof MBT material, with the exception of the larger additionof MBT product at 25%. However, the addition of 12.5%MBT product and educt reduced the biogas productionrate, normalised to VS-added, by about 20%, while theaddition of 25% MBT material resulted in significantlyreduced gas production rates: down to less than 50% ofthe control in the case of 25% MBT product.

The gas quality data was limited, but there appeared tobe no improvement or detriment to the methane contentresulting from the addition of biowaste. In general, meth-ane content was lower than that usually produced byfull-size sewage-sludge digesters, which would be expectedto be around 65% (MacBrayne, 2004), and also lower thanthat reported in other studies using OFMSW.

In combination, the results (Table 6) show that compar-atively poor further stabilisation of the MBT organic mat-ter was achieved during the anaerobic digestion process,which could be expected given that the material hadalready been put through an aerobic treatment process.Interestingly, the observed reduction in gas productionratios (m3 kg�1 VS) is equivalent to the percentage additionof MBT material (except for 25% MBT product, where it iseven lower). In terms of absolute gas production, theresults show that the co-digestion of MBT material withsewage sludge resulted in a higher absolute biogas yieldper available digester volume for all but the 25% MBTproduct mixes, although feed conversion and methane pro-duction were poor compared to the control.

3.3.2. Total solids and heavy metalsDuring the short duration of these trials two major

issues were encountered: solids content and contaminationwith heavy metals.

An estimated 10% of the available operating capacity inthe digester containing 25% MBT product was lost due tosediment build-up over the duration of the trial (51 days).The other digesters with co-digestion also showed signs ofexcessive sediment build-up, despite the fact that up to 50%of inorganic material in the MBT feedstock was removableby means of a 20 mm screen. Significant improvement inthe removal of this inert material prior to anaerobic diges-tion would be required to avoid its excessive accumulation,and wear and tear on pumps. This could be achieved viascreening to 10 mm but this screen size may be too smallto be feasible for application at full-scale.

There was evidence of the accumulation of heavy metalsin the digesters. This occurred most notably in the digestersreceiving MBT product and may lead to increasing long-term toxicity to the digestion process. In particular, zincand chromium, which are reported to have the highest lev-els of toxicity, were present in the MBT slurries at levelsclose to or in excess of reported EC50 values. There mayalso be concerns about final sludge quality in terms ofheavy metals, depending on the anticipated end-use ofthe sludge. The Scottish Environment Protection Agencyposition on the recycling of waste in land-application pro-vides maximum limits for the heavy metal content of suchmaterial (SEPA, 2004). These values are normalised to 30%organic matter content in the waste material. MBT mate-rial has a lower organic matter content than sewage sludgeand, ironically, this somewhat reduces the ‘normalised con-centration’ of heavy metals (Table 8) in cases where muchof the heavy metal load originates from the sewage sludge,e.g. copper, even though this effect decreases with increas-ing ratio of MBT addition. For lead and nickel the addi-tion of MBT material potentially increases thenormalised concentration of heavy metals beyond the lim-its set by the regulator, and this means that land-applica-tion of the final sludge is further restricted.

3.3.3. Feasibility of this form of co-digestion

From the results described so far emerges an argumentagainst the co-digestion of MBT material with sewagesludge in full-scale, commercially operated anaerobicdigesters, as they are typically operated by water treatmentworks.

The MBT material would need to be made into a slurrywith water and/or sewage sludge, thus allowing the mate-rial to be conveyed in existing pipework and pumps. Still,the increasingly paste-like nature of the mixed slurry, inaddition to the extra infrastructure requirement, wouldpresent a mechanical problem and, in combination withthe danger of sediment build-up in the digesters, representsa major hurdle to the feasibility of this form of co-digestion.

The extra capital and running costs for the co-digestionof MBT material may be offset by gate fees and increases inabsolute biogas yield achievable for a given digester vol-ume. However, apart from these operational consider-ations, the results of this project indicate that the main


hurdle to the feasibility of this form of co-digestion lies inthe accumulation of heavy metals in the sediment. Follow-ing implementation of the Landfill Directive, this materialis likely to be applied to land in areas where facilities foradvanced treatment such as incineration or gasificationare not available (currently about 50% of Scottish sewagesludge falls into this category). The elevated level of heavymetals would effectively rule out this form of disposal orrecycling and would therefore present a considerable prob-lem for sewage treatment operators, who would have tofind other outlets for their final sludge in an environmentwhere the pre-existing concentrations of heavy metalsalready limit recycling.

So, while it may be informative to collect further data onthe digestibility of MBT material in co-treatment with sew-age sludge and other organic waste streams, it wouldappear that the high level of heavy metals in the MBTmaterial, together with the high load of inorganic material,would render commercial utilisation of this form of co-digestion not feasible in practice.

Significant reductions in heavy metal concentrations arerequired before further investigation of this form of co-digestion should be undertaken. Then, trials could includea comparison of the viability of anaerobic digestion ofMBT material with that of pre-screened and pulped mate-rial, which has proved very successful in full-scale operat-ing plants in Denmark (Evans, 2002). This wouldhighlight the benefits of screening processes on final prod-uct quality and could tie in with other work on the designof pilot scale digesters that are suitable for the developmentof treatment schemes for future translation into full-scaletreatment (Gallert et al., 2003). Until then, there is anurgent need to improve the quality of MBT material interms of its heavy metal as well as inorganic content, forexample via improved collection schemes, source segrega-tion or improved mechanical treatment. However, this willtake time and it can be argued that this improved MBTmaterial should be treated directly either in compostingor anaerobic digestion facilities, depending on desiredend-use and material characteristics (e.g. green gardenwaste for composting or kitchen waste for anaerobic diges-tion). Thus, the co-digestion of MBT material with sewagesludge appears not to be feasible as an interim, short-termsolution towards achieving the Scottish landfill diversiontargets.

4. Conclusions

The results of this trial suggest that MBT waste is amena-ble to anaerobic digestion with sewage sludge at bench scale.

The trial was of short duration (51 days) and the MBTmaterial used may not have been truly representative ofmaterial which would be used at full-scale – i.e. large con-taminants had to be removed by hand and the remainingmaterial milled to reduce particle size before use.

The concentrations of heavy metals and inert materialpresent a considerable barrier to commercial, full-scale

co-digestion of MBT material with sewage sludge in exist-ing water industry installations. Improvements in collec-tion or pre-treatment may reduce these barriers but theywould also negate the need for co-digestion as an outletroute for poor quality MBT material. This form of co-digestion is therefore unlikely to provide an interim solu-tion towards achieving the Scottish landfill diversiontargets.

Acknowledgements

This research was initiated and funded by the ScottishExecutive, via the Scottish REMADE project.

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