THE NEXT GENERATION OF cTHP Abstract

European Biosolids and Organic Resources Conference

15-16 November, Edinburgh, Scotland

www.european-biosolids.com

Organised by Aqua Enviro

THE NEXT GENERATION OF cTHP

M. van de Ven1, A. Hol1, L. Luning1 and D. Traksel1

1Sustec BV, The Netherlands

[email protected]

Abstract

The thermal hydrolysis process (THP) has proven to be a valuable process to optimize anaerobic

digestion by increasing the biogas production from waste activated sludge (WAS) and by substantially

increasing the capacity of digester plants. The main drawback of the widely applied batch operation

systems are the high operation costs, which are mainly related to pre-dewatering and energy

consumption of the process. As an alternative, the continuous THP TurboTec® (cTHP) is available,

which recovers its heat via heat exchangers and/or a mixing separation step. As cTHP allows lower

pre-dewatering concentrations and steam consumptions, the costs are significantly reduced. To

reduce the costs even further, a new cTHP version has been developed. In this concept, the final

dewatering is added to the THP to separate the solids directly from the liquid fraction. This separation

makes it possible to increase the total solids content from the solid fraction up to 45% with limited

polymer (PE) use, while the liquid fraction contains most of the solubilized components. This liquid

fraction can, depending on the obtained total suspended solids concentration, be treated in a UASB

reactor to produce biogas. Nutrients (N & P) can be recovered from the UASB effluent with the

NutriTec® process.

Keywords

Continuous THP, direct dewatering, pilot experiment, TurboTec®, UASB.

Introduction

Wastewater is a potential source for renewable energy and raw materials. For the water authorities,

this is nowadays an important topic as can be seen from several initiatives, like in the Netherlands the

“WWTP as energy and raw materials factory” initiative and the European “Full-scale demonstration of

energy positive sewage treatment plant concepts towards market penetration, (Powerstep)” project.

Next to improvements in the waterline, specifically the sludge line has great potential for recovery of

renewable energy, nutrients and potential costs reduction.

Already for a long time mesophilic anaerobic digestion (MAD) is applied at Waste Water Treatment

Plants (WWTP) to convert biosolids into biogas. The performance of the digester can be strongly

enhanced by applying a thermal hydrolysis process (THP) as a pre-treatment prior to digestion. In a

THP pre-treatment the sludge is treated at a high temperature (140 – 160 °C) and high pressure (4 –

6 bar). The advantages of applying THP are more biogas production, stronger volatile solids (VS)

reduction, shorter digestion retention times and an increased total solids (TS) content of the final

sludge cake. By THP it is also possible to apply higher loadings to the digester, resulting in more

capacity in the same volume. These effects result in substantial lower costs for the sludge treatment,

while at the same time more renewable energy is produced.

The potential of the THP-process was recognised almost a decade ago by the Dutch company Sustec

BV, when it decided to develop a practical process of its own. The activities led from lab experiments

to determine overall process parameters, to pilot plant development and operation and finally to full-

scale design, construction and operation.

http://www.european-biosolids.com/





In developing the technology, it was decided to take continuous operation as a starting point, as an

alternative to the batch wise operation commonly applied at that time. The arguments for the

preference of continuous operation were amongst others:

Appropriateness: as the production of sludge is a continuous all-round the year process, its

treatment is best done in a similar way.

Efficiency: continuous operation does not require different process-steps to be performed in a

sequential order resulting in periods where part of the equipment is not active. Because of the

efficient utilisation of process- and auxiliary equipment, including pumps and boilers for steam

production, units of smaller capacity can be chosen. Overall this results in a smaller footprint,

lower investment costs and increased benefits of scale.

Reduction of the heat demand, specifically for steam. In batch-wise systems heat is

recovered by regeneration of the injected steam (flash-steam). The flash steam is released

when allowing the hydrolysed sludge to depressurise and is used to heat the incoming sludge.

This way of operation has a fundamental limitation in that it can only cool the hydrolysed

sludge to just over 100 °C. Even then not all the available heat between the hydrolysis

temperature and 100 °C can be utilised. In continuous operation, the application of

appropriate heat exchangers would overcome this limitation, making the process more

efficient.

Developing the basic ideas into a working process has led to the typical set-up of the TurboTec® 1.0

process as illustrated in Figure 1. In this process, the heat is recovered in two steps by applying heat

exchangers and/or mixing/separation. For heat exchange via mixing/separation the difference in

physical properties of the raw sludge and hydrolysed sludge is used in his most optimal way. In the

Netherlands two full-scale references at WWTP Venlo and Apeldoorn are nowadays operational and

working according to this principle.

Figure 1: TurboTec® 1.0 process






The next generation of cTHP

Based upon the positive experiences with the operation of these two full-scale plants, further

improvements have been sought to optimise the concept of cTHP. As thermal hydrolysis is a process

in which organic materials are hydrolysed under the influence of increased pressure and temperature,

the typical rate-limiting step in the anaerobic digestion process, being exactly this hydrolysis, can be

overcome. This provides the main reason for the present success of the application of the THP

concept (Whitlock, 2014).

However, the benefits of thermal hydrolysis are not limited to the improvement of the anaerobic

digestion process. The main financial benefits with cTHP can be achieved by increasing the total

solids content of the sludge cake with limited polymer (PE) use (sludge flocculant to improve

dewatering).

In the framework of its development of the TurboTec® cTHP, Sustec has paid considerable attention

on the effect of thermal hydrolysis on the dewatering of biosolids on a laboratory scale. The main

findings of these experiments were:

The degree of dewatering that can be achieved is higher for hydrolysed sludge than for

hydrolysed digested sludge. Apparently, part of the effect of the thermal hydrolysis treatment

is lost in the anaerobic digestion process.

An increase in the centrifugation temperature of the sludge showed a positive effect on the

final total solids content of the sludge cake (pellet).

This increase in the total solids content of the pellet also caused an increase of the total

Chemical Oxygen Demand (COD) content in the liquid part (supernatant) of the sludge

sample.

With the experimental results above, Sustec came up with the concept of dewatering on a high

temperature directly downstream of the thermal hydrolysis process. The process diagram of this next

generation of cTHP principle is illustrated in Figure 2.

WAS

THP

*

|+

Belt press Centrifuge UASB NutriTec WWTP

N & PFiltrate Cake

Figure 2: Process diagram of the next generation of cTHP

The concept in Figure 2, with the inclusion of the dewatering in the process, makes a further

optimization in the design of the anaerobic conversion process possible. Instead of the application of

conventional sludge digesters in which no retention of anaerobic biomass is achieved, treating a liquid

fraction with high COD-contents and limited amounts of suspended solids allows for the use of more

high-grade reactor technologies such as Upflow Anaerobic Sludge Bed (UASB). In these systems,

hydraulic retention times can be much shorter than in conventional sludge digesters saving space and

investment costs. In addition, compared with a sludge digester, an UASB requires no sludge

recirculation and/or no mechanical mixing because of the dynamic behaviour of the water phase in an

UASB reactor (Bal, 2001).






The (economical) feasibility of the concept in Figure 2 is related to the achievable TS content of the

sludge cake after dewatering in combination with the amount of PE needed to obtain a centrate

quality applicable for biogas production in an UASB. To investigate this feasibility, Sustec decided to

do a pilot research with direct dewatering after THP at their TurboTec® 1.0 full-scale installation at the

WWTP in Venlo. The method and materials used for this pilot experiment are described below.

Methodology

For the dewatering (centrifugation) experiment, a side stream of hydrolysed sludge, produced by the

TurboTec® 1.0, was used. This side stream was fed to a 1 m3/h decanter (supplier Hiller) and

operated at a rotation speed of 4,200 rpm (revolutions per minute) with a differential speed around 1.0

rpm.

The hydrolysed sludge from the hydrolysis tank was cooled down from 140 °C to circa 70 °C by using

the heat exchangers from the full-scale installation. This hydrolysed sludge was pumped then to the

Hiller pilot centrifuge where the sludge was divided in a centrate and a sludge cake part. These

streams were collected together with the centrifuge feed to determine the mass balance. This

experiment was repeated with different polymer additions (kg active/ton TS) to obtain a centrifugation

curve with polymer addition as the only variable. The pilot setup is shown in Figure 3.

Figure 3: Pilot experiment setup with Hiller centrifuge on WWTP Venlo

The collected feed, centrate and sludge cake samples were analyzed in the Sustec laboratory.

Analyses were done on total solids (TS), volatile solids (VS), total suspended solids (TSS), COD (total

and soluble), total phosphate (total P) and total nitrogen (total N). The COD, total P and total N

concentrations were determined in mg/l with the corresponding cuvette tests from Hach Lange. The

other methods used are described below:

Total solids (TS): the total solids content of the samples was determined by drying the sludge

samples overnight at 105 °C in an oven. The TS content [%] can be calculated according to

the following formula:






𝑇𝑆 (%) =𝑀𝑑 − 𝑀𝑖

𝑀𝑓 − 𝑀𝑖 ∗ 100

Where Md is the total oven dry mass, Mi the initial weight of the sample cups and Mf the

weight of the filled samples before drying. These values (in grams) are multiplied by 100 to

determine the total solids content as a percentage of the total weight.

Volatile solids (VS): the percentage of organic material in the sludge samples was determined

by combustion of the dried sludge from the drying oven (105 °C) in an oven at 550 °C, for at

least two hours. The VS content can be calculated according to the following formula:

𝑉𝑆 (%) = 1 −𝑀𝑎 − 𝑀𝑖

𝑀𝑓 − 𝑀𝑖 ∗ 100

Where Ma is the total mass of the ash including the cup after combustion, Mi the initial weight

of the sample cups and Mf the weight of the filled cup with dry sample before combustion.

These values (in grams) are multiplied by 100 to determine the volatile solids content as a

percentage of the total weight.

Total suspended solids (TSS) were determined as solids which are not able to pass a

Whatman 589/2 filter paper (4-7 µm pore size). The material remaining on the filter is dried in

an oven at 105 °C for one night. The TSS concentration in g/l can be calculated according to

the following formula.

𝑇𝑆𝑆 (𝑔/𝑙) =𝑀𝑑 − 𝑀𝑖

𝑀𝑐 ∗ 1000

Where Md is the total mass of the used filter including the tin after drying, Mi the initial weight

of the dried filter including the tin and Mc is the weight of the amount of centrate added to the

filter. These values (in grams) are multiplied by 1000 to determine the TSS in grams per liter

centrate.

The experiments to determine the biogas potential were conducted in duplicate in 1 L bottles with a

total liquid volume of 200 mL. Blanks with only the inoculum were included in the experiment to be

able to correct for the biogas production from the inoculum. All bottles were added with a phosphate

buffer to a final concentration of 10 mM. In addition, a macro elements solution (6.0 mL/L liquid

volume) and a trace elements solution (0.6 mL/L liquid volume) were added. The tests were done at a

temperature of 38 °C under continuous mixing on a shaking plate (100 rpm). The gas production was

monitored by a pressure measurement system during the experimental period of 28 days. The biogas

composition was analyzed (gas chromatography) at the end of the test.

Results

The centrifugation curve of hydrolysed sludge is shown in Figure 4. The hydrolysed sludge fed to the

centrifuge had a TS content of ~8%. In Figure 4, the TS concentration of the obtained sludge cake

and centrate is plotted against the amount of PE used to operate the centrifuge.






Figure 4: TS curve of the sludge cake and centrate in relation to the PE dosage.

Figure 4 illustrates the potential to achieve a sludge cake with a TS content up to 45% through direct

dewatering of hydrolyzed WAS. An optimum in PE dosage in relation with the obtained TS content of

the sludge cake is observed at ~4 kg active/ton TS. Above this value, an overdose of PE results in a

decrease of the sludge cake TS content. As the TS content of the sludge cake increases the quality of

the centrate increases as well (less TS in solution). At a PE dosage of > 0.5 kg active per ton TS, the

fraction of TSS in the wastewater is low enough (<6 g/l, (Metcalf, Wastewater engineering: treatment,

disposal, and reuse)) to be fed to a UASB reactor.

The percentage of total P, total N and total COD in the centrate are illustrated in Figure 5. These

percentages are related to the total P, N and COD load in the hydrolysed sludge fed to the pilot

centrifuge.

0

2

4

6

8

10

0

10

20

30

40

50

0,0 1,0 2,0 3,0 4,0 5,0 6,0

TSce

ntra

te [%

]

TS s

ludg

e ca

ke [%

]

PE [kg active/ton TS]

Sludge cake

Centrate






Figure 5: P/N/COD curve for the obtained centrate in relation to the PE dosage.

Figure 5 shows more or less the same trend for all different parameters. Without addition of PE, the

percentage of total P, N and COD in the centrate is the highest. This can be explained by the high TS

content in the centrate as can be seen in Figure 4. With addition of PE the average part of total P and

total COD in the centrate is circa 30%, while the amount of total N is roughly 50% of the incoming

hydrolyzed sludge.

An extra test was performed at a polymer dosage of 1.0 kg active/ton TS fed and a slightly lower

differential speed to investigate the biodegradability of the feed, sludge cake and centrate. This

resulted in a sludge cake of 38% and a centrate containing 3.2% TS. In this set-up, ~60% of the

feeding TS content was captured in the sludge cake. The remaining ~40% ended up in the centrate

stream. The related VS division between the sludge cake and centrate was respectively ~54% over

~46%. The samples were collected and subjected to a digestion test of 28 days. The results of the

digestion experiment are shown in Figure 6.

0

10

20

30

40

50

60

70

80

0,0 1,0 2,0 3,0 4,0 5,0 6,0

P/N

/CO

D in

cen

trat

e [%

]

PE [kg active/ton TS]

P (Total)

N (Total)

COD (Total)






Figure 6: Biogas production curves at 38 °C.

As shown in Figure 6, the feeding towards the centrifuge (hydrolyzed WAS) resulted in a biogas

production of 411 mL/g-VS. Direct centrifugation after THP resulted in 550 mL/g-VS and 323 mL/g-VS

for the centrate and sludge cake respectively. This indicates that the centrate ends up with a high

concentration of biodegradable material, but that the sludge cake contains part of the biodegradable

material as well. Based upon the data from Figure 6 the conversion efficiencies shown in Table 1 can

be calculated.

Table 1: Conversion efficiencies of the feed, sludge cake and centrate.

Sample TS [g/L] VS [g/L] Biogas [mL/g-VS] VS reduction [%] TS reduction [%]

Feed 78.5 55.1 411 41.4 29.0

Sludge cake 57.3 35.2 323 31.4 19.3

Centrate 31.9 27.3 550 61.6 52.7

Table 1 confirms that for the centrate a higher conversion efficiency is reached. With almost 62% VS

reduction, its biodegradation is much higher than for the feeding and sludge cake.

0

100

200

300

400

500

600

0 5 10 15 20 25 30

Bio

ga

s [m

l/g

VS

]

Time [days]

Feed

Centrate

Sludge cake






Discussion

Table 2 shows an overview of the comparison between the current process route and the

implementation of a centrifuge to determine the feasibility of direct centrifugation after THP.

Table 2: Overview of the comparison between TurboTec® 1.0 and direct dewatering after THP based on VS-destruction in batch-biogas tests

Parameter Unit TurboTec® 1.0 Direct dewatering after THP

Feeding TS Ton TS 1.0 1.0

Feeding VS Ton VS 0.70 0.70

VS reduction Ton VS 0.29 0.20

COD reduction Ton COD 0.45 0.30

Methane production Nm3 157 107

Sludge cake % 30 38

Sludge cake production Ton 2.37 2.11

PE consumption Kg active/ton TS 15 1

Sludge cake disposal € 153.70 137.30

PE costs € 53.20 5.00

Value methane production € -37.70 -25.60

Total costs 1 ton TS € 169.30 116.70

Values used: sludge cake disposal costs 65 €/ton, PE costs 5 €/kg active, methane proceeds €0.24 per Nm3

Table 2 shows a lower VS reduction for direct dewatering after THP than for TurboTec® 1.0 (0.20 ton

VS versus 0.29 ton VS). This can be explained by the fact that in TurboTec® 1.0 all the thermal

hydrolysed sludge is fed to the digester with 41.4% VS conversion to biogas (Table 1), while with

direct dewatering after THP only 46% of the total organic material (VS) will be digested (centrate part

after separation) with a VS conversion to biogas of 61.6% (Table 1). According to the difference in

methane production in Table 2, 32% of the biodegradable VS ends up in the sludge cake. A number

of actions can be considered to improve the amount of VS in the centrate (solubilization) after direct

dewatering:

An increase of the thermal hydrolysis temperature, which results in a higher soluble COD

content of the treated sludge (Wilson, 2009). This centrifuge experiment was performed after

thermal hydrolysis on a temperature of 140 °C.

The addition of alkali agents like NaOH, KOH, Mg(OH)2 or Ca(OH)2 before the thermal

dewatering. This treatment allows an enhanced solubilization of the sludge, but also causes a

pH increase together with the formation of refractory compounds (Penaud, 1999). A lot of

experience is available with this type of treatment, specifically for lime addition.

Introduction of a purification step, which makes it possible to wash out the VS from the sludge

cake to the centrate. This potential solubilization technique requires additional research

before application.






Although the final biogas production is lower for direct dewatering after THP, the main advantages of

this new concept can be found in the increase of the TS content of the produced sludge cake in

combination with the strong reduction of the PE consumption required for the dewatering.

The strong difference in required PE consumption between dewatering of digested sludge (TurboTec®

1.0) and direct dewatering after THP is illustrated in Table 2. Because of the low PE consumption

needed for the direct dewatering after THP, an additional test (with the same PE) was conducted on

the dewaterability of the digested sludge from the WWTP Venlo. However, the PE used was not able

to have the same effect on this digested sludge, which resulted in PE concentrations above 15 kg

active/ton TS instead of only 1 kg active PE/ton TS for direct dewatering after THP. This clear

difference in PE binding between digested sludge and direct dewatering after THP indicates that the

material significantly changes in the digester. It is known that exopolysaccharides (EPS) and other

exopolymers in sludge contain a negative surface charge. EPS, with its strong polarity, are basically

responsible for the polymer demand of sludge. This EPS is broken down at high temperatures in

combination with a high pressure, which results in an improved dewatering directly after THP without

the requirement of significant amounts of PE (Julia B. Kopp, 2016). While anaerobic digestion results

in a deterioration of sludge dewatering properties measured by capillary suction time (CST), which

increases the need of PE addition (Ye, 2014).

The financial benefit of direct dewatering after THP is shown in Table 2. This concept results in a total

costs reduction of €52.60 per ton treated TS (= €169.30 – €116.70). When a full-scale installation

treats for example 10,000 ton TS per year, the obtained cost reduction would result in a yearly saving

of more than €500,000 (half a million). This implies that if biogas production is not the key driver, THP

followed by direct dewatering is a more interesting option to install. The substantial financial reduction

results in opportunities to invest in installations to recover N & P (fertilizers) from waste water.

Materials recovery possesses a higher rank in the bio-based economy’s value pyramid than the

recovery of energy as illustrated in Figure 7.

Figure 7: Bio-based economy’s value pyramid (Betaprocess, 2012).

This valuable recovery of nutrients (Figure 7) can be combined perfectly with the examined thermal

dewatering after THP. It was determined that during the experiments roughly 50% of the total N and

30% of the total P ended up in the centrate stream which can be fed to a UASB. The NutriTec®






installation from Sustec BV is a high potential technology to recover nutrients (N & P) from the

centrate formed after direct dewatering. The NutriTec-N® membrane stripping process diffuses

dissolved ammonia gas from the centrate water (at pH > 9.5) across the pores of a microporous

hydrophobic membrane where sulfuric acid (H2SO4) is introduced to produce the fertilizer ammonium

sulfate ((NH4)2SO4). Phosphate can be recovered in the NutriTec-P® installation in which struvite

(NH4MgPO4·6H2O) is formed (L. Luning, 2015).

The VS reduction of the feed of 41.4% (Table 1) is relatively low compared to the VS results obtained

at the WWTP Venlo in the past. On a yearly average, the VS reduction is in the range of 45 to 50%.

The most likely explanation for this relatively low VS reduction value were the weather conditions

during the experimental period. At the time of the experiment there was a long period of drought. This

results in a lower ratio of easy degradable primary sludge in the thermal hydrolysis installation at the

WWTP. This primary sludge always has a higher degradability than WAS which would have a positive

effect on the biogas production (Wilson, 2009). However, in this case the comparison between

TurboTec® 1.0 and direct dewatering after THP in Table 2 was executed with the obtained VS

reduction of 41.4% in the biogas batch test. That is why the overall result of the comparison would

stay the same with an increase of the VS reduction.

The direct dewatering after THP results in a lower biogas production compared with the TurboTec®

1.0 technology. After direct dewatering, more VS ends up in the produced sludge cake. However, in

combination with the high TS content of ~40%, a positive effect is expected for subsequent processes.

Especially the higher VS content in the treated sludge is an improvement for incineration of the sludge

cake (better combustible), the composting process and for agricultural application (soil stabilizer)

(Mantovi, 2005). The enhanced TS content decreases transportation costs of the sludge cake.

Conclusion

The conclusions which can be drawn based on this paper are the following:

1. The thermal hydrolysis process (THP) has proven to be a valuable process to optimize

anaerobic digestion by increasing the biogas production from waste activated sludges and by

substantially increasing the capacity of digester plants (Whitlock, 2014).

2. Based on the pilot experiment it is possible to obtain a sludge cake for WAS with a TS content

up to 45% with limited use of PE for THP followed by direct dewatering.

3. The centrate TS content after centrifugation is about 3.0%. This corresponds with a TSS

content < 6 g/l, which makes it applicable to feed the centrate to a UASB reactor.

4. With direct dewatering after THP, about 50% of the total VS ends up in produced centrate. Of

this fraction > 60% can be converted in a UASB.

5. Part of the biogas potential is lost when THP followed by direct dewatering is applied.

Depending on the key drivers of the business case, most important cost savings are obtained

from the PE reduction and reduced sludge cake production.

6. The direct dewatering concept results in a total costs reduction of €52.60 per ton treated TS.

When a full-scale installation treats for example 10,000 ton TS per year, the obtained cost

reduction would result in a yearly saving of more than €500,000 (half a million), which makes

THP followed by direct dewatering a very interesting and competitive technology to the

technologies currently applied.

7. Direct dewatering after THP results in a lower biogas production compared with the

TurboTec® 1.0 technology because more VS ends up in the produced sludge cake. However,

in combination with the high TS content of the sludge cake, this can have positive effects for

subsequent processes as incineration, composition and agricultural application of the

obtained sludge cake.






Acknowledgements

The Dutch contractor GMB have contributed to the work by giving the permission to run the pilot

dewatering experiment at the WWTP in Venlo (Water board ‘Waterschapsbedrijf Limburg’), which they

are operating.

References

(2012). Retrieved from Betaprocess: http://www.betaprocess.eu/the-value-pyramid.php

Bal, A. S. (2001). Upflow Anaerobic Sludge Blanket Reactor A Review. Indian Journal of

Environmental Health, 43(2), 1-82.

Julia B. Kopp, O. K. (2016). Impact of Phosphate on Sludge Dewatering and Improvement.

SludgeTech 2016, University of Surrey, UK.

L. Luning, L. v. (2015). Results from pilot- tests for ammonia-stripping by Membrane-modules.

Stickstoffrückbelastung - Stand der Technik 2015 -. Leipzig: Grömping, Schäpers.

Mantovi, P. B. (2005). Reuse of liquid, dewatered, and composted sewage sludge on agricultural land:

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Metcalf, L. E. (Wastewater engineering: treatment, disposal, and reuse). 1991. New York: McGraw-

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wastewater sludge by thermal hydrolytic pretreatment. Water research, 43(18), 4489-4498.

Ye, F. L. (2014). Extracellular polymeric substances and dewaterability of waste activated sludge

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THE NEXT GENERATION OF cTHP Abstract

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