optimised acid phase digester operation · the digestion of sewage sludge to generate biogas for...
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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
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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.
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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
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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
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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.
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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
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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
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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.
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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.
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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.
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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
t50 (hr) 50% dye trace recovered 16.9 17.6 40.2
t95 (hr) 95% dye trace recovered 70.3 61.5 101.4
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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)
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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.
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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.