optimised acid phase digester operation · the digestion of sewage sludge to generate biogas for...

7th European Waste Water Management Conference

www.ewwmconference.com

Organised by Aqua Enviro Technology Transfer

OPTIMISED ACID PHASE DIGESTER OPERATION

Grand, A.1, Giuffre, G.2, McIntosh, G.2 and Egarr, D.1

1 MMI Engineering, UK, 2MWH Treatment Ltd, UK

Corresponding Author Tel: 0117 9602212 Email: [email protected]

Abstract

The digestion of sewage sludge to generate biogas for renewable energy is one method

of offsetting carbon dioxide emissions. Recent technological advances have resulted in

the development of Acid Phase Digestion (APD) which gives improved biogas

generation over single stage anaerobic digesters.

MMI Engineering was asked by MHW to assist in the development of two Acid Phase

Digesters of similar design for two sites operated by Severn Trent Water. The digesters are

formed of an inner circular tank (Tank 1) with a concentric outer tank (Tank 2). The tanks

are hydraulically connected by two openings in the inner tank wall and are positioned

on structural safety grounds. The main purpose of the work was to determine, using

Computational Fluid Dynamics (CFD), the amount of fluid that passes between the tanks

during the feed of new sludge.

This paper describes the process and merits of Acid Phase Digestion, the purpose of the

work, the methods implemented in the CFD model and the results of the study.

Keywords

Digestion, Computational Fluid Dynamics, Acid Phase, Gas mixing

Introduction

Digestion encompasses a number of separate processes and reactions. These are

hydrolysis, acidification, acetate formation and methane formation. Hydrolysis is the

process of the breaking down and dissolving the complex organic compounds

(carbohydrates, fats and proteins) of the suspended solids in the digester water. The

sugars, amino acids and fatty acids contained in the dissolved solids react with

fermenting bacteria to created volatile fatty acids (VFAs). Bacteria act on these fatty

acids to convert them to acetic acid (acetate), carbon dioxide and hydrogen in the

acetate formation stage. The final process is the bacterial breakdown of the acetates

into methane. At all stages after hydrolysis other by-products are created that lead to

biogas normally consisting of 30 to 40% carbon dioxide, some hydrogen, hydrogen

sulphide and other gases dependent on the makeup of the feedstock. The single-phase

anaerobic digester incorporates all these stages of digestion in one vessel. Therefore this

digester must provide conditions favourable to all the bacteria and enzymes present in

these processes and some of the stages may have their efficiency compromised (US EPA

2006).

Two-Phase Digestion

A two-phase digestion process works by splitting the digestion in to two separate phases

which take place in the Acid Phase Digester (APD) and the Gas Phase Digester (GPD).

The processes that take place in each are shown in Figure 1. The acid phase digester




provides conditions beneficial to hydrolysis and the fermenting bacteria that produce

VFAs, whilst the gas phase digester provides the ideal conditions for acetate formation

and methane production. Typically the contents of the acid phase digester require

mixing. This is not always true of the gas phase digester.

Figure 1: Processes involved in two phase digestion.

By splitting the overall process, the individual stages can be optimised. The key benefits

of multiphase digestion are as follows (US EPA 2006):

Increased biogas production: By providing the ideal conditions for the methane

producing bacteria yield can be increased.

Improved volatile solids reduction: For a acid and gas phase digestion, typically

an extra 5% volatile solids destruction compared to single stage digestion.

Reduced storage volume as residence times are reduced: Typical APD mean

residence times (MRTs) are 1-2 days, whilst GPDs have MRTs of 12-15 days. For a

single stage digestion system, MRTs of 20 days or more are typical.

Bio-solids control: as the breakdown of solids is better in the two stage process,

both odour and pathogens can be reduced.

Reduced foaming: The acid phase digester has low gas production, low pH and

higher volatile acid concentrations, which are detrimental to foam-causing

micro-organisms.

Reduced short circuiting: Multiphase systems reduce the short circuiting of solids

by separating the digestion phases and optimising the retention time in each

phase.

This last point is considered further in this paper.




Residence Time and Short-circuiting

With respect to the hydrodynamics there are two key factors to the design of an acid

phase digester:

1. Residence Time - In order to achieve the optimal bacterial action within a

digester, sludge has to be present in the tank for an optimum period of time.

2. Effective Mixing - Within the tank the sludge needs to be well mixed to ensure

bacteria are available for the reactions to occur and to ensure homogeneity of

reactive species and temperature.

These are competing factors as a completely mixed tank will convey a proportion of the

new sludge to the outlet immediately after inflow, thereby short-circuiting the tank.

Depending on the design, digesters that use a constant or regular batch feeding system

whilst mixing occurs may not have ideal mixing characteristics in that some of the feed

sludge may short-circuit. The mixing system is likely to have been optimised to achieve

well mixed conditions in a number of hours whilst the residence time of the digester is

likely to be in the order of days.

CFD modelling can be used to determine the residence time of the digester and also

determine whether short-circuiting occurs during the feed of new sludge without the

need for expensive testing.

Digester Design

MWH is undertaking the hydraulic design for acid phase digesters at two Severn Trent

Water sites: Wanlip Sewage Treatment Works (STW) and Clay Mills STW. These acid phase

digesters use a two tank approach. A central circular tank is surrounded by a concentric

outer tank (see

Figure 2). Grit removal is aided by a conical base in the central tank and sloping sumps in

the outer tank. The tanks are of steel construction and are hydraulically connected by

two square openings positioned to ensure structural integrity when the tanks are

emptied. This design is intended to replace a design of similar shape but of concrete

construction where sludge would be pumped between tanks. The change in design

provides savings in both construction and operating costs.

Details of the digesters from the two sites are presented in Error! Reference source not

found..

Digester Site Clay Mills Wanlip

Diameter (Outer Tank), m 14.5 18.7




Table 1: Details of Acid Phase Digesters at Clay Mills and Wanlip STW.

Figure 2: Plan of the Acid Phase Digester design at Clay Mills STW.

The control philosophy applied to the digesters is a batch operation. Feed sludge at 6%

solids by mass is pumped into the central tank and is distributed by the mixing system. This

is followed by removal of older sludge from the outer tank. As the feed sludge is colder

than the digester contents the sludge is heated by two heat exchangers, sited externally.

Sludge is withdrawn from and returned to the inner tank. The heat exchangers ensure the

sludge is maintained at an operating temperature of 42°C.

Mixing is provided by a number of biogas diffusers mounted at the base of the tanks. The

diffusers contain a leaf spring that acts to release large bubbles (with a mean diameter

of approximately 50mm). These bubbles of biogas entrain the sludge on rising therefore

mixing the tank contents.

The energy input of the gas mixing system can be calculated and averaged over the

volume of the tanks and is called the Mixing Energy Level (MEL) (Wu 2010). Typical MELs

for different mixing systems are shown in Table 2 (Wu 2010, Capon and Wahab 2012).

The Clay Mills and Wanlip APDs compare favourably with MELs in the range 0.8 - 2.2

W/m3 dependent on operating level. Hence, the gas mixing system is intended to

generate good mixing with low energy input.

Table 2: Typical Mixing Energy Levels of different types of mixing

Type of Mixing Mixing Energy Level (W/m3)

Volume, m3 1702 3806

Feed Rate, m3/h 140.4 316.8

Duration and Frequency of

feed

20 minutes every 2 hours 30 minutes every 2 hours

Number of gas mixing diffusers 14 18




Pumped circulation 5

Mechanical mixing with draft tube 2.4

Gas mixing 1.5-2.5

Gas mixing (Clay Mills and Wanlip

Digesters)

0.8-2.2

Digester Modelling

CFD models of the acid phase digesters at Clay Mills and Wanlip STW were constructed in

ANSYS CFX 14.0. The key concern was that feed sludge would enter the outer tank, Tank

2, during the period in which new sludge is introduced to Tank 1. The key features of the

CFD model geometry for the digester at Wanlip STW are shown in

Figure 3.

N.B. Part of the Tank 2 wall has been removed for clarity.

Figure 3: CFD Model geometry of the acid phase digester for Wanlip STW.

Case Definition & Objectives

The cases detailed in Error! Reference source not found. were analysed for the two sites.

There were different objectives for each site.

The objectives of Cases 1-3 on the Clay Mills STW APD were to determine:

Whether the quantity of new sludge passing to Tank 2 was acceptable (i.e. a

small percentage) during filling.

Whether filling with or without mixing was better in terms of new sludge passed to

Tank 2.




The objectives of Cases 4-7 on the Wanlip STW APD were to determine:

Whether the quantity of new sludge passing to Tank 2 was acceptable (i.e. a

small percentage) during filling.

The Residence Time behaviour of the tanks and whether the desired mean

residence time was reached in the APD.

Each case was initialised with a steady state flow field to capture the heat exchanger

recycle and gas mixing flow patterns where appropriate. Cases 1 - 5 simplified the system

to consider a fixed operating level with feed of new sludge to the digester and

simultaneous outflow. Case 6 modelled filling only with a moving free surface to

accommodate the additional sludge.

Finally, Case 7 determined the residence time of the two tanks forming the APD and the

residence time of the APD. A fixed operating level was used for this calculation with

continuous feed and outflow assumed with feed and outlet flow averaged over a

fill/empty cycle. Tracer was added for a short period into Tank 1 and Tank 2 (through the

openings) and tracked through the digester over a number of theoretical residence

times until the vast majority of the tracer was recovered.

Table 3: Case Summary

*Initial height ** Only 11.5 mins completed.

Sludge Rheology

The rheology of the sludge in the model is very important as wastewater sludge tends to

exhibit non-Newtonian behaviour. The feed sludge is a mixture of primary and surplus

Digester

Site and

Case No.

Operatin

g Level,

m

Feed

flow,

m3/h

Outlet

flow,

m3/h

Feed

Temp,

°C

HE

Recycl

e, m3/h

HE

Return

Temp, °C

Fill time,

mins

Gas

mixing

?

Clay

Mills

Case 1 5 140.4 140.4 20 172.8 as

extracte

d

20 No

Case 2 10 140.4 140.4 20 172.8 as

extracte

d

20 No

Case 3 5 140.4 140.4 20 172.8 46* 20 Yes

Wanlip

Case 4 13.6 316.8 316.8 7.5 381 46 30 Yes

Case 5 5.6 316.8 316.8 7.5 381 46 14.1 Yes

Case 6 13.0* 316.8 0 7.5 381 46 30** Yes

Case 7 13.6 79.2 79.2 42 381 42 Consta

nt

Yes




activated sludge and is hydrolysed during its time in the APD. Rheology data was

sourced from the BHR Sludge Rheology Database for these different types of sludge and

average properties were chosen for the modelling. The sludge was characterised as the

Herschel–Bulkley fluid. The shear stress relates to the yield stress and shear strain rate by

the following equation:

1

Where is the shear stress, k is the consistency index derived experimentally, is the shear

strain rate and n is the experimentally derived power law exponent. If n<1 the fluid is

shear thinning, and if n>1 it is shear thickening, if n=1 then the fluid is Netwonian. 0 is the

yield stress of the fluid which is another experimentally derived parameter.

As the purpose of the work was to quantify the amount of feed sludge that passes

between the tanks during the feed period, it was assumed that the water-solids mixture

(sludge) would remain reasonably well mixed and so the sludge was assumed to be at a

constant concentration throughout the APD.

Gas Mixing and Buoyancy

The interphase drag law that was implemented was the Grace model (Ansys 2011). The

biogas used for mixing was considered to be 60:40 Methane : Carbon Dioxide by

volume. The mixing equipment supplier confirmed the mean bubble diameter as 50mm

and the flow from each diffuser as 15Nm3/hr (at 1 atm, 0°C). This gas flow was injected at

a temperature of 42°C.

In order to represent the buoyancy effects of the colder feed sludge entering into a

warmer digester the Boussinesq model was used.

Boundary conditions

During the feed of new sludge in to Tank 1 and in all but one case there was

simultaneous outflow from the outlet in Tank 2 at the same rate. This maintained a

constant operating level. Case 6 for the Wanlip STW digester study modelled the filling

process without simultaneous outflow. In this case the computational mesh was

controlled to raise the free surface as sludge was added to the digester. As this method is

computationally expensive, more so than the other cases, the filling period was not

completed.

The free surface was represented as a rigid free slip surface at a given operating level.

The gas phase was removed at the free surface to prevent the accumulation of gas

within the digester.

The transit of the feed sludge through the digester was monitored using a tracer to mark

the feed sludge in the same way as in a dye trace experiment.

Each heat exchanger re-circulates sludge at a constant rate. Sludge is withdrawn from

the base of Tank 1 and reintroduced through return pipes on opposite side of Tank 1.

Tracer drawn into the heat exchanger outlet was returned immediately at the




corresponding inlet. This was considered to be conservative as the residence time

between the APD and the heat exchanger was assumed to be zero.

No heat loss at the walls or heat generation by the bacterial action on the sludge was

modelled. No heat transfer was modelled at the heat exchangers.

Results and Discussion

Clay Mills STW Digester Results

Within the feed time of 20 minutes, it is required to determine the amount of feed sludge

that passes from Tank 1 through to Tank 2. Results are presented for Cases 1 – 3 in Figure

4. The two cases without mixing (Cases 1 and 2) have almost identical curves

independent of operating level where 1.7% of the new sludge passes in to Tank 2 after 20

minutes. With mixing, at the lower operating level, 4% of the new sludge short-circuits

after 20 minutes. The reason for this can be seen by looking at the distribution of the

sludge at the end of the two calculations at the lower operating level (Cases 1 and 3), in

Figure 5. In Case 1, without mixing, the sludge remains at the base of the tank and only

passes through to Tank 2 through the lower opening. In Case 3 by the end of the filling

process, sludge at 1/100th of the initial concentration is present in the vast majority of

Tank 1 and present in almost a quarter of Tank 2. Velocity vectors on a plane through the

inlet are displayed in Figure 6. These show that in Case 1, a density current is formed at

the bottom of Tank 1 due to the colder, denser inlet flow. With mixing however, the inlet

flow is drawn from the base of the tank into a central core of rising fluid created by the

gas mixing system. Hence, the enhanced mixing as a result of the gas injection

distributes the feed sludge therefore resulting in a slightly higher amount of feed sludge

being transported to Tank 2.

Figure 4: Percentage of total mass of new sludge passed to Tank 2 for 5m and 10m

levels without mixing and 5m level with mixing.




Case 1 - No mixing Case 3 - Mixing

Figure 5: Isosurfaces of tracer concentration at 1/100th of the initial concentration in

Case 1 and Case 3 at end of the filling period.

Case 1 - No mixing

Case 3 – Mixing

Figure 6: Velocity vectors on a plane through the inlet for Case 1 and Case 3 at the

end of the filling period.




Wanlip STW Digester Results

The percentage of the total mass of new sludge that passes through to Tank 2 during

filling in Cases 4 and 5 is displayed in Figure 7. Despite the different operating levels and

fill times the results are similar; 3% in Case 4 and 2.5% in Case 5. The mixing system, which

provides a constant mass flow of gas independent of the fill level, is able to disperse the

new sludge more rapidly in Case 5 as the volume of sludge in the digester is 43% of that

in Case 4.

Figure 7: Percentage of total mass of new sludge passed to Tank 2 for the Wanlip

STW Digester at 5.6m and 13.6m levels.

Figure 8: Percentage of total mass of new sludge passed to Tank 2 for the Wanlip

STW Digester at the 13.6m level with and without simultaneous outflow,

Case 4 and Case 6 respectively.




Case 6 analysed filling of the digester without simultaneous outflow, thereby modelling

the free surface rising, with gas mixing on. This is the standard operating philosophy for

this tank. Due to the complexity of modelling a two phase flow with a moving free

surface, the computational run time was significant and just over a third of the

calculation (11.5 minutes) was completed. Figure 8 displays the percentage of the total

mass of new sludge that passes through to Tank 2 in this case. Bounding curves have

been added to estimate the final result based on a similar curve fit for Case 4. In this way

the estimated short-circuiting is 1.5 -2% of the feed sludge. This is 50-67% of that found in

the case with the static operating level (Case 4).

Figure 9: Residence time distribution for the two tanks and whole digester

For Case 7 the calculated mean residence times of the tanks and the whole digester are

displayed in Table 4 with the residence time distribution presented in Figure 9 . The mean

residence times are found to be close to the theoretical residence times of around 24

hours in each tank. The time taken to recover certain percentages of the tracer is also

displayed in Table 4 and corresponding curves are plotted in Figure 10. 10% of the tracer

has passed through Tank 1 in 3 hours and through Tank 2 in under 6 hours. This is due to

the degree of mixing that occurs in the tanks. 50% of the tracer is recovered by 17 hrs in

Tank 1 and 18 hrs in Tank 2. The majority of the tracer, 95%, has passed through the tanks

in under 3 x MRT.

Table 4: Results from the Case 7 RTD Analysis

RTD Analysis Results Tank 1 Tank 2 Digester

Theoretical Mean Residence Time (hr) 24.2 24.3 48.4

RTD Calculated MRT (hr) 23.9 23.3 46.1

t10 (hr) 10% dye trace recovered 3.0 5.8 15.8






The RTD analysis for the digester as a whole is more favourable in terms of short-circuiting

than the individual tanks, demonstrated by 10% of the tracer being recovered after 16

hrs. This is illustrated in Figure 11 by plotting the RTD of the two tanks and the whole

digester against a normalised residence time. For plug flow the peak of the curve would

be about 1 (the mean residence time). This APD would not be expected to have plug

flow but the improvement in performance can be seen by the curve moving significantly

to the right from the curves of the individual tanks.

Figure 10: Dye tracer recovered in the two tanks and the whole digester

Figure 11: Residence time distribution for the two tanks and whole digester with

normalised residence time (mean residence time for the tanks and

digester is at 1)




As the feed flow was applied as a time averaged flow, the CFD calculated RTD for Tank

1 was compared to a theoretical model for an ideal mixer with a batch feed of the same

frequency as the Wanlip STW digester (Figure 12).

The RTDs compare favourably suggesting that the assumption of a constant feed and

outflow in the analysis is reasonable as the Tank 1 flow field is dominated by mixing and is

very close to that of an ideal mixer. The peak in the RTD curve for Tank 1 occurs after 1.4

hrs. As the normalised tracer concentration is so close to 1 at this peak this suggests the

tank can be characterised as a Continuously Stirred Tank Reactor (CSTR). .

Figure 12: Residence time distribution for Tank 1 and a theoretical ideal mixer with

batch inflow with normalised residence time

Conclusions

Digestion is a complex process involving a number of separate chemical and bacterial

reactions. By splitting the overall process into two stages the individual stages can be

optimised. Acid phase digestion followed by gas phase digestion can lead to greater

biogas yield and other benefits. The hydraulic performance of a two tank acid phase

digester design by MWH at two Severn Trent sites has been analysed using CFD. The

results from the studies demonstrated the following about the design of the digesters at

Clay Mills and Wanlip STWs:

During filling no more than 4% of feed sludge passed through to Tank 2. Therefore

the vast majority was retained in Tank 1 during the feed.

Tank 1 in particular is a well-mixed tank and behaves similarly to a perfect mixer.

Therefore the feed sludge will be mixed effectively within the digester.

The effect of the two tanks in series improves the residence time distribution of the

whole digester.




Acknowledgements

Thanks go to the following Severn Trent Water staff: James Morgan and Rob Wild for

allowing details of this work to be presented and Andrew Richards for his involvement in

the Clay Mills project.

References

Ansys, CFX 14.0 User Manual, 2011.

Capon, N. and Wahab, M. (Monsal UK), ‘Best practice design for challenging digester

mixing applications’, 16th European Biosolids and Organic Resources Conference (2012).

Karim, K., Thoma, G. and Al-Dahhan, M., ‘Gas-lift digester configuration effects on mixing

effectiveness’, Water Research 41 (2007): 3051 -3060.

US Environmental Protection Agency, ‘Biosolids Technology Fact Sheet Multi-Stage

Anaerobic Digestion’, EPA 832-F-06-031, September 2006.

Wu, N., ‘CFD simulation of mixing in egg-shaped anaerobic digesters’, Water Research 44

(2010) 1507 -1519.

optimised acid phase digester operation · the digestion of sewage sludge to generate biogas for...

Documents