Start-up strategies of thermophilic anaerobic digestion of sewage sludge

Q IWA Publishing 2009 Water Science & Technology—WST | 59.9 | 2009

1777

Start-up strategies of thermophilic anaerobic digestion of sewage sludge J. Palatsi, A. Gimenez-Lorang, I. Ferrer and X. Flotats

ABSTRACT The aim of the present work was to evaluate two different strategies to convert a continuous mesophilic anaerobic reactor into thermophilic, treating sewage sludge. The compared strategies were a single direct temperature change, from 358 to 558C, and a multi-step temperature change (358 –438 –508– 558C). The single direct step strategy was shown to be as effective as the more conservative multi-step strategy, recovering the system efficiency after 20 days in terms of methane yield, although there was a higher volatile fatty acids accumulation and higher propionate/acetate ratio values during the temperature transition. From the results of the multi-step strategy it could be stated that the critical process performance during temperature transition occurred between 43 and 508C, which should be taken into account to define more accurate future transition strategies. Sewage sludge thermophilic vs mesophilic biodegradability was assessed by means of anaerobic biodegradability tests, and no significant statistical

J. Palatsi I. Ferrer X. Flotats GIRO Technological Centre, Rambla Pompeu Fabra 1, Mollet del Valle`s, Barcelona E-08100, Spain E-mail: [email protected]; [email protected]; [email protected] A. Gimenez-Lorang Department of Environment and Soil Science, University of Lleida, Avda. Rovira Roure, 191. 25198 Lleida, Spain E-mail: [email protected]

differences in biodegradability or methane yields were found. Key words

| anaerobic digestion, mesophilic, sewage sludge, thermophilic

INTRODUCTION Sewage sludge, obtained after biological wastewater treat-

enhance dewatering post-treatment in order to reduce the

ment, contains very low total solids (TS) concentration

final volume of sludge and to reduce transport or disposal

(between 2 – 5%), 70% of which is organic matter, nutrients

costs. Considering the stability of the process and the low

(i.e. nitrogen and phosphorus), microorganisms (including

energy needs, mesophilic digestion is a suitable technique

pathogens), and variable amounts of organic and/or

for this purpose. When increasing organic loading rates,

inorganic pollutants, depending on the origin of the

hygienization of sludge becomes an important requirement

wastewater treated. Due to its potential impact on the

(Za´branska´ et al. 2000). According to the EU (Carrington

environment and public health, sewage sludge requires an

2001) and US (US EPA 1999) policies, thermophilic

appropriate treatment and management. Some alternatives

digestion appears as an interesting alterative to mesophilic

(like anaerobic digestion and/or composting) are aimed to

anaerobic digestion.

obtain a more stabilized product, which can be reused as

The growth rates of thermophilic bacteria are higher

an organic fertiliser. Anaerobic digestion is a treatment

than those of mesophilic bacteria, and thermophilic

technology that stabilizes the organic matter yielding a

anaerobic digestion is more efficient in terms of organic

biogas with high methane content, which is a renewable

matter removal and methane production than the meso-

energy source.

philic process (Za´branska´ et al. 2000; Ahring et al. 2002;

Traditionally, anaerobic digestion of sewage sludge has

Gavala et al. 2003). In addition, digestion in the thermo-

been carried out at mesophilic temperatures (, 358C). The

philic range (558C) achieves a considerable pathogen

main objective of anaerobic digestion of sewage sludge is to

reduction, faster reaction rates and higher resistance to

doi: 10.2166/wst.2009.180

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J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

Water Science & Technology—WST | 59.9 | 2009

foaming, although it might result in a less stable process

straight increase (or single-step) of the temperature from

(Za´branska´ et al. 2002) and higher steady state volatile fatty

35 to 558C, together with an initial drop of the OLR,

acids (VFA) concentration (Kim et al. 2002). Despite the

followed by a gradual OLR increase as process performance

major energy needs to reach hygienization requirements,

improves (Ahring et al. 2002; Bolzonella et al. 2003). The

thermophilic digestion is a good option if a heat recovery

second type of strategy is based on a slow and gradual

system from the effluent is used (Zupancˇicˇ & Rosˇ 2003).

increase of the temperature in the digester at constant

The start-up of an anaerobic digester is a slow and critical

OLR, in order to maintain the process efficiency but

step, as it affects subsequent process operation. The start-up

requiring much longer transition times (Garber et al. 1975;

depends on a variety of factors, such as the inoculum source

Rimkis et al. 1982; Za´branska´ et al. 2000). A third type

and characteristics, the organic loading rate (OLR), the

of strategy consists of a staged temperature increase, or

volume and digester design, or the start-up strategy. These

multi-step, rather than a continuous temperature increase,

aspects are particularly critical in many areas where

minimizing the time transition between critical tempera-

anaerobic thermophilic inoculum is not available, and

tures in which neither the mesophilic nor the thermophilic

mesophilic inoculum is used instead. In some cases anaero-

population are favoured (Bouskova´ et al. 2005; De la

bic digesters are seeded with activated sludge with good

Rubia et al. 2005).

results (Kim & Speece 2002). In any case, a transition period

De la Rubia et al. (2005) studied a gradual increase

in which the mesophilic populations from the digester are

between four different intermediate temperatures (5 steps:

replaced by thermophilic ones takes place. The key factor

358– 438 –458 – 508 – 528– 558C) modifying the OLR at every

during this transition is how to favour the development of the

step until an adapted system was achieved in a continuous

initially minor thermophilic populations (Van Lier et al.

stirred tank reactor (CSTR). The experimental time for all

1993). At industrial scale, or WWTP facilities, it is necessary

steps took more than 25 weeks. Bouskova´ et al. (2005)

to modify the operation from mesophilic to thermophilic

compared a direct temperature change strategy from 378 to

range with the minimum economical cost, in terms of

558 with a step-wise strategy defined by three intermediate

treatment efficiency (minimizing process disturbances) or

temperatures (4 steps: 378– 428 –478 – 518 – 558C), achieving

in terms of time expended (minimizing it), while maintaining

a stable thermophilic operation after 30 day and 70 days

the usual OLR of the plant.

respectively, with a constant OLR of 1.38 kg VS·m23·d21.

In general, methanogenic activity is more sensitive to

In the work of De la Rubia et al. (2005), the unstable periods

temperature changes than the acidogenic step (Van Lier

were in the transitions from 438 to 458 and from 458 to 508C,

et al. 1993). Hence, during a temperature transition period,

and in the work of Bouskova´ et al. (2005) the one-step

accumulation of VFA tends to occur (Angelidaki & Ahring

temperature increase caused severe disturbance process

1997). This phenomenon is particularly remarkable for

parameters, reaching the system a stable operation after

propionic acid, as propionate oxidation is the most sensitive

few days. After these works, the question that arises is

to a sudden temperature change (Van Lier et al. 1990),

whether a decrease on the number of steps to 3, using as

which can be expressed as an increase in the ratio

intermediate temperatures 438 and 508, could decrease the

propionate/acetate (HPr/HAc). Nielsen et al. (2007) also

impact of disturbances compared with the one step

suggest HPr as key parameter for regulation and optimiz-

transition (Bouskova´ et al. 2005), or could decrease the

ation of biogas process, as during the recovery of a system

transition time compared with the described studies.

after a disturbance process, the return of HPr back to

The objective of the present work was to compare the

steady-state level was the slowest than any other VFA.

best strategy obtained by Bouskova´ et al. (2005) with a

Accumulation of VFA results in a rapid decrease of methane

three-step strategy, using the critical temperature range

production (Van Lier et al. 1993).

transition values obtained by De la Rubia et al. (2005) as

Different start-up strategies in thermophilic anaerobic

intermediate temperatures (35 –43 – 50 –558C), maintaining

digesters using a mesophilic inoculum have been reported

in both strategies a constant OLR in a CSTR. Anaerobic

in the literature. The first type of strategy consists of a

biodegradability tests (ABT) were performed, in order to

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J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

Water Science & Technology—WST | 59.9 | 2009

evaluate the possible differences between mesophilic and

sludge (50:50). Both inoculum used in ABT were intro-

thermophilic sewage sludge biodegradability, which could

duced at a constant concentration of 5 g VSS l21. A reducing

produce results interference in further start-up strategies

solution was finally added (5 ml of 10 g Na2S l21). The flasks

experiments.

were stirred and bubbled with N2/CO2 (80:20 v/v) gas in order to remove O2 before closing them with rubber stopper. The flasks were incubated at 358C and 558C, respectively, and shaken by hand once a day. The time-

METHODS

course methane production and VFA concentration were

A mixture of primary and secondary sludge (50:50) from

monitored by gas chromatography until biogas production

Lleida (Spain) municipal wastewater treatment plant

ceased. Biodegradability index were calculated as follow:

(WWTP) was used as digestion substrate.

Acidogenesis ð%A ¼ ðCODCH4 þ CODVFA Þ=CODin Þ, Methanogenesis ð%M ¼ CODCH4 =CODin Þ and Biodegradability (%BD ¼ %A þ %Cel), calculated with an estimation of

Analytical methods

cell growth index (%Cel) from coefficients of biomass

Total solids (TS), volatile solids (VS), total suspended solids

yield of acidogenic and methanogenic bacteria, according

(TSS), volatile suspended solids (VSS), total dissolved solids

to Field et al. (1998). Every experiment was conducted

(TDS), total chemical oxygen demand (COD), total Kjeld-

in triplicate.

hal nitrogen (TKN) and ammonia nitrogen

ðNHþ 4 -NÞ,

pH

and alkalinity were determined according to Standard Methods (APHA 1995). Methane content in the flasks (ABT), or in the CSTR headspace, and volatile fatty acids (VFA), acetate (HAc), propionate (HPr), iso-butyrate (iso-HBut), n-butyrate (n-HBut), iso-valerate (iso-HVal) and n-valerate (n-HVal), were determined by gas chromatography (TCD and FID respectively) following methods described by Campos et al. (2008).

Temperature transition tests in CSTR Two 5 l volume CSTR were used to evaluate the temperature transition from 35 to 558C at 22 days HRT (OLR 1.29– 21 1.73 kg VS l21 ). Two strategies were studied: (1) single CSTR d

direct increase in reactor R1 (from 358C to 558C), and (2) multiple steps or staged increase in reactor R2 (from 358C to 438C, then to 508C, and finally up to 558C). Average methane productions at every temperature

Anaerobic biodegradability tests (ABT)

period were compared using the Duncan’s Multiple Range test (MRT) with a significance level a ¼ 0.05 (Sheskin 2000)

ABT were performed following Field et al. (1998). Glass

in order to elucidate whether the different conditions

vials of 115 ml, with a media working volume of 50 ml, were

imposed produced significant differences among system

filled with the sludge mixture up to a final concentration

responses.

of 5 g COD l21, and were supplemented with macro and micronutrient solution (following Soto et al. 1993). An alkaline solution was also added (1 g NaHCO3 g21 COD) and the pH was adjusted to neutrality (HCl or NaOH 0.1 N). Digested sewage sludge from Lleida (Spain) WWTP was

RESULTS AND DISCUSSION Substrate and inoculum composition

used as mesophilic inoculum, while 558C adapted sludge

The composition of the substrate used in ABT and CSTR

from a lab-scale continuously stirred tank reactors

assays and the composition of the inoculum used in ABT

(CSTR), operated for 80 days at 20 days hydraulic retention

are shown in Table 1. Digested sludge from digesters of

time (HRT) at thermophilic conditions, was used as

Lleida WWTP was used as mesophilic inoculum for the

thermophilic inoculum. Full scale and lab scale reactors

mesophilic ABT, and previously adapted digested sludge

were fed with same mixture of primary and secondary

from a thermophilic lab-CSTR was used as thermophilic

J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

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Table 1

|

Water Science & Technology—WST | 59.9 | 2009

Mesophilic and thermophilic anaerobic

Composition of substrate and inocula in mesophilic and thermophilic ABT

biodegradability tests (ABT) Parameter

Substrate

358C inoculum

558C inoculum

TS (g/kg)

36.74 ^ 0.64

37.60 ^ 0.23 26.00 ^ 0.31

The results of the ABT are summarized in Table 2.

VS (g/kg)

26.69 ^ 0.69

18.59 ^ 0.30 15.71 ^ 0.27

Anaerobic biodegradability index (%BD) and final methane

VS/TS (%)

72.7%

49.4%

yields (l CH4 g21 VSin and l CH4 g21 CODin) were slightly

60.4%

TSS (g/kg)

30.89 ^ 0.53 23.89 ^ 0.39

higher for the thermophilic assays. However, differences at

VSS (g/kg)

23.74 ^ 0.19 15.34 ^ 0.35

the end of the experiments (day 39) were not statistically

TDS (g/kg)

3.68 ^ 0.15

significant.

COD (g/kg)

57.53 ^ 0.70

4.49 ^ 0.34

28.37 ^ 0.10 24.55 ^ 0.71

Results of ABT at day 7 show that methanogenic (%M)

TNK (mg/kg)

2,840 ^ 22

2,762 ^ 45

2,958 ^ 346

and acidogenic (%A) indexes were already similar in the

NHþ 4 -N

849 ^ 16

1,540 ^ 28

1,750 ^ 20

mesophilic assay (Table 2). In the thermophilic assay, %A

5.78

7.90

8.25

index was higher than %M index, indicating certain

Alk (g CaCO3/kg) 3.30

5.93

6.79

accumulation of VFA (specially HPr) and a higher ratio

VFA (mg Ac/l)

7,433.1

2.8

88.5

HPr/AHc, over 4.0 (Figure 1). These results suggest an

HAc (mg/l)

2,198.7 ^ 145.0 2.2 ^ 3.9

53.0 ^ 3.0

initial instability (Hill et al. 1987) probably due to a

HPr (mg/l)

1,105.0 ^ 75.1

0.2 ^ 0.3

13.2 ^ 0.8

non-completely adapted thermophilic inoculum, although

Iso-HBut (mg/l)

304.6 ^ 17.9

0.1 ^ 0.1

2.3 ^ 0.5

the low VFA level of the thermophilic reactor from

n-HBut (mg/l)

1,353.7 ^ 90.1

–

1.8 ^ 0.3

where the inoculum was taken (Table 1). Nevertheless,

Iso-HVal (mg/l)

557.9 ^ 36.1

–

5.8 ^ 1.0

the final biodegradability indexes were almost the same

n-HVal (mg/l)

288.8 ^ 19.4

0.1 ^ 0.1

0.2 ^ 0.3

(Table 2), and no differences between mesophilic and

(mg/kg)

pH

thermophilic stages were expected due to the sludge biodegradability potential.

seed. A very significant difference in VSS and VFA content between mesophilic and thermophilic inoculum was found. Differences in VSS between both inoculums were

Mesophilic to thermophilic transition

taken into account in the design of the ABT experiments, and same concentration was used (5 g VSS l

21

). The

21 Methane production (l CH4 l21 ) during transition CSTR d

lower VFA content from the mesophilic sludge corre-

period is shown in Figure 2. For each strategy, different

sponded with a low OLR and stable process of the industrial

stages could be differentiated (mean values for each

WWTP, compared with the thermophilic lab-CSTR oper-

temperature stage are represented in Figure 2 by solid

ated for providing inoculum, although the VFA levels of

lines) and summarized in Table 3, where the statistical

this reactor indicated a stable and an adapted process

differences according to the Multiple Range Test performed

(Kim et al. 2002).

(a ¼ 0.05) are indicated.

Table 2

|

Results (averages and standard deviations) of mesophilic and thermophilic ABT indexes and methane production yields

358C

558C

Indexes and yields

day 7

Final

day 7

Final

%M

31.8 ^ 3.6

44.1 ^ 4.1

28.7 ^ 2.1

48.1 ^ 1.0

%A

32.8 ^ 3.9

44.1 ^ 4.3

34.3 ^ 5.2

48.3 ^ 1.2

%BD

38.0 ^ 5.0

52.4 ^ 5.5

39.8 ^ 6.4

57.8 ^ 1.5

L CH4/kg VSin

332.4 ^ 31.2

363.0 ^ 7.3

L CH4/kg CODin

154.2 ^ 14.5

168.4 ^ 3.4

%M: Methanogenesis index; %A: Acidogenesis index; %BD: Biodegradability index.

J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

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Figure 1

|

Water Science & Technology—WST | 59.9 | 2009

Propionate (HPr) and propionate/acetate ratio (HPr/HAc) on 358C and 558C ABT.

Along the experiment, the HRT was maintained with no

(day 21) in R2. The production was increased after the final

significant differences among periods, but the OLR was not,

temperature increase from 50 to 558C, until a complete

due to small differences in VS feeding concentration.

recovery at day 29.

Nevertheless, volumetric methane production and methane

An increase on methane production was detected on

yields obtained showed a well defined behaviour. These

period III in R1 or in period IV in R2, that can be related

yields showed significant statistical differences at the

with the degradation of accumulated undigested substrate

35 –558C period in strategy 1 (R1) and in the 43– 508C period

during the previous stage, but with a final production on

in strategy 2 (R2) only, while were maintained in the same

thermophilic stage at both reactors similar (Table 3) to the

range of values before and after these transient periods.

initial mesophilic production, according to the ABT results

It can be stated (Figure 2) that initial methane

(Table 2).

production was the same in both reactors operating at

Figure 3 shows a comparison between strategies in

358C (before day 0). In R1 there was an immediate drop of

terms of methane yield (l CH4 g21 VSin). It is important to

volumetric methane production after temperature change

notice the similar behaviour of both strategies after the

from 35 to 558C, with a recovery period of 15 days. In R2

temperature increase from 35 to 558C in R1 and from 43 to

methane production was maintained after temperature

508C in R2, with a methane yield time recovery of 20 days.

increase from 35 to 438C, suggesting the maintenance

In Figure 3, time scale from R1 and R2 has been overlaid, in

of

Choorit

order to compare the one step strategy with the critical step

& Wisarnwan (2007). A similar methane production drop

(43 to 508C) in the gradual change strategy. The fact that

to that of R1 was observed after temperature increase

methane yields are the same before and after temperature

from 43 to 508C (day 7) with a recovery period of 14 days

transition (as in ABT test) could be attributed to the

mesophilic

Figure 2

|

populations,

as

reported

by

Volumetric methane production (l CH4/lCSTR day) in the transition assay study. Solid lines represents global mean in each temperature period.

J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

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Table 3

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Water Science & Technology—WST | 59.9 | 2009

Mean values of HRT, OLR and biogas yields in the stages of the transition procedure

HRT

OLR

Methane

Methane yield

D

G VS/l d

l CH4/l d

l CH4/g VSin d

l CH4/g CODin d

I: (358C)

19.6 a

1.58 ab

0.572 a

0.385 a

0.231 a

Reactor/period

R1

R2

II: (35 – 558C)

21.8 a

1.43 a

0.127 b

0.087 b

0.051 b

III: (558C)

21.4 a

1.73 b

0.616 a

0.371 a

0.228 a

I: (358C)

22.3 a

1.40 ab

0.570 a

0.422 a

0.256 a

II: (35 – 438C)

23.0 a

1.29 a

0.607 a

0.481 a

0.272 a

III:(43 – 508C)

22.8 a

1.51 bc

0.120 b

0.080 b

0.050 b

IV: (50 – 558C)

21.5 a

1.72 c

0.578 a

0.340 a

0.212 a

V: (558C)

23.4 a

1.56 bc

0.613 a

0.399 a

0.235 a

Different letters in columns indicate statistical significant differences between periods (a ¼ 0.05).

relatively high HRT used, since it is reported in the

ratio, also represented the imbalance caused by the

literature that differences between mesophilic and thermo-

temperature shift, but did not show differences between

philic operations, in biogas production and organic matter

strategies, recovering HBu low values after day 30 in both

degradation, become more evident at lower HRT or more

strategies. The role of HPr as indicator of process imbalance

high rate systems (Ahn & Forster 2000; Gavala et al. 2003;

in mesophilic-thermophilic transition process was also

Ferrer et al. 2006).

suggested by Lindorfer et al. (2008). Bouskova´ et al. (2005)

Differences in VFA accumulation were also detected,

reported higher HAc and similar HPr values on mesophilic

with similar results to those obtained in the ABT. The

to thermophilic transition experiments, concluding that

comparison clearly shows that the critical temperature

one-step temperature increase was proved to be better

change occurs between 43 and 508C. From undetectable

(faster). However, present results suggest that method (2) is

VFA levels under mesophilic conditions, acids accumulated

as fast as (1), comparing with the transient period from 438

rapidly after temperature increase. After transition, the HAc

to 508, but with less disturbances. There are no differences

concentration was similar in both reactors, whereas HPr

between strategies in terms of velocity, as both reactors

was much higher in R1, consequently HPr/HAc ratio was

efficiency in terms of methane yield was recovered in 20

always higher in R1 (Figure 3). Other indicators of

days after the critical temperature change (from 35 to 558C

instability, like butyric acid (HBu) or acidity/alkalinity

in R1 and from 43 to 508C in R2). Since it is possible to

Figure 3

|

Evolution of methane yield (l CH4/g VSin) and HPr/HAc ratio in transient CSTRs. The time scale, in daysp, overlays the temperature change from 35 –558C in R1 and from 43 –508C in R2.

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J. Palatsi et al. | Start-up strategies of thermophilic anaerobic digestion

maintain a good performance in the digester with temperatures up to 438C and over 508C, due to the activity of mesophilic and thermophilic populations respectively, second strategy could be suggested as better if less HPr/ HAc ratios values are desired, although with slightly higher expended time in the transition process. Further works will enclose the temperature range (43 – 508C) in which thermophilic populations became dominant to design more optimized transition strategies.

CONCLUSIONS The biodegradability test of sewage sludge showed no significant differences between anaerobic biodegradability and methane yield in mesophilic and thermophilic ranges. Process performance following the temperature change from 358C to 558C (1) and from 438C to 508C (2) was very similar and the system efficiency was recovered after 20 days in both cases. Total time expended with the second method is higher due to the transitions from 358 to 438 and from 508 to 558C, although without significant reactor performance changes during these periods. According to the results, the temperature range between 43 to 508C was considered critical for the methanogenic activity in mesophilic to thermophilic transition. In terms of time expended, the one-step strategy is better than the multi-step strategy, but in the three-step strategy studied in the present work the propionic acid concentration and the HPr/HAc ratio were always lower, without a significant delay time in methane production recovery respect to the one-step method, providing an alternative strategy for minimizing reactor disturbances.

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