Applications of Computational
Fluid Dynamics Modelling in Water Treatment
Report No 1075/1/05
March 2005
EXECUTIVE SUMMARY
Almost all water and wastewater treatment equipment rely on continuous
through flow of water. Some equipment requires this flow to be
well-mixed, whereas other equipment requires plug-flow. Examples of
well-mixed systems are activated sludge plants, chemical dosing zones
and anaerobic digesters while sand filters (in both filtration and
back-wash modes), clarifiers, adsorption columns (ozone, activated
carbon and ion exchange) and dissolved air flotation cells are examples
of the plug-flow systems. Some processes such as nutrient
removal activated sludge plants require the combination of both plug
flow and completely mixed reaction zones.
The laboratory-scale experiments that are used to obtain design data
for a plant are usually operated under ideal flow conditions;
unfortunately it is usually not feasible to carry this through to
full-scale plants, due to the greater difficulties and expense of
achieving similar ideal conditions on a large scale. The complexity of
the flow patterns, and the uncertainties about how they affect the
relevant performance indicators for the process involved have led
designers of equipment to use safety factors based on experience to
ensure that the process achieves its required objectives. This means
that equipment that is installed is often larger and more expensive
that it needs to be.
Computational fluid dynamics (CFD) is a numerical procedure to
calculate the properties of moving fluid. Most water treatment
processes involve the movement of water. This motion is often complex
and difficult or very expensive to observe. The prediction of the flow
patterns and other properties of flowing fluids would provide insight
into processes which otherwise would not have been possible. A previous
WRC project (No 648) indicated the value of CFD modelling of clarifiers
and an anaerobic compartment. It was able to both logically explain the
unexpected behaviour of the clarifier and in designing features to
modify the undesirable flow pattern.
Apart from its use in design of water treatment equipment, CFD
modelling can also assist in research into water treatment
processes. The project on which this report was based was
unusual in that it was initiated to provide a CFD modelling service to
assist water researchers who felt that it could enhance their
investigations. As a result the project did not have a specific
research focus of its own, but adapted to the objectives of each
research project that it became involved with. Furthermore,
not all the collaborations that were started were fruitful. The main
content of the report is a series of case studies, each corresponding
to a different investigation. To give the report some thematic
consistency, it has been compiled with a view to illustrating the kinds
of situations where CFD modelling is useful. To this end, the case
studies that were selected for inclusion in the report are those which
best fitted this purpose, i.e. they each involved an appropriate
application of CFD, and yielded some useful conclusions. Various
investigations which for one reason or another did not fit these
criteria have been left out. The investigations which are presented
have also been cleaned up to reflect a logical development which did
not always take place in reality; i.e. the various misapplications,
misconceptions and dead ends which occurred along the way have been
removed from the narratives.
1 Project Objectives
- To provide a service to water researchers by undertaking
modelling exercises on proposed and existing equipment so that they may
design more efficient experimental equipment and to better understand
their experimental results.
- To promote the use of computational fluid mechanics by
water authorities, consultants and water researchers.
- To model wastewater treatment secondary settling tanks.
- To assist in the training of academics and students in the
practical use of computational fluid mechanics.
2 Overall Course of the
Project
At the inaugural meeting in 1999, a list of water-related projects with
potential for CFD input was put forward. Seven activities were
originally placed in the work programme for 1999, which would have
involved collaborations with researchers from around South
Africa. These were:
- Ozone Contactor
- Anaerobic baffled reactor (ABR)
- Secondary settling tanks
- Passive mine water treatment
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- Filter back washing
- Capillary membrane modules
- Ultrafiltration manifolds
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Not all of these evolved into fruitful collaborations; the direction
and extent of progress depended on unpredictable factors which emerged.
During the course of the project, reasons for making progress, or
failing to do so, largely revolved around personnel capacity to
undertake the work, though the logistics of collaborating with remote
groups also played a part in some cases. No postgraduate
students were directly associated with the project during 2000,
although some work was done by undergraduate students taking vacational
employment.
Between December 2000 and January 2001 modelling or the secondary
settling tanks at Durban's Northern Wastewater Wastewater Treatment
Works was carried out to see if it was possible to achieve a
significant increase in capacity without extensive redesign.
Unfortunately this exercise was inconclusive. Promising
results with were initially obtained later proved to be a result of
incorrect flow rate data supplied by the Metro. When these
were corrected, the improvements that could be achieved using simple
baffles were found to be inadequate for the requirements of the
treatment works. In view of the lack of a conclusive outcome,
this investigation has not been included in the report.
At the beginning of 2001 Tzu-Hua Huang, a chemical engineering graduate
of the University of Natal, enrolled for a MScEng, taking the study of
the ozone contactor as her thesis topic. Chapter 3 of this report is
based on her work. Additionally, Emilie Pastre, a chemical engineering
graduate from ENSIGC in Toulouse, undertook her MScEng studies in
Durban, in terms of an exchange agreement between ENSIGC and the
University of Natal. The topic that she chose involved modelling the
final product water reservoirs at the Wiggins Water Treatment Plant, in
order to develop an effective control strategy for chlorine dosing. The
primary focus of her work was the control, with the CFD model
contributing only to the conceptual development of a control
model. This work will be reported in another WRC report, so
has not been included here. The During 2001 the pilot plant Anaerobic
Baffled Reactor (ABR) was installed at the Umbilo Wastewater Treatment
Works and was operated to gather data for the design of system for
treating wastewater from informal settlements. The CFD support that was
provided to this investigation was never a central issue; rather the
CFD modelling was used to help deciding on structural details such as
the shape and placement of baffles, by allowing visualisation of the
consequences of different options. Operating experience led to the
conclusion that the concerns which motivated the CFD modelling were not
of crucial importance, and that the CFD work had not made a significant
contribution to the investigation. This work has also been omitted from
this report as it will be part of the ABR report.
In July of 2001 Ms. Huang visited Institut National des Sciences et
Appliquée (INSA) de Toulouse, for an month. There,
with the help of Prof. A. Liné, she undertook some two phase
modelling of the ozone contactor. The investigation formed the basis of
a paper presented at the IWA
Conference on Water and Wastewater Management for Developing Countries,
Kuala Lumpur, Malaysia, 29-31 October 2001, which was subsequently
accepted for publication in Water
Science and Technology. Ms. Huang subsequently applied
successfully to have her MScEng registration changed to PhD.
Also during 2001, an investigation into the backwashing of sand filters
at the Faure Water Treatment Plant operated by the Cape Metropolitan
Council. The motivation for the investigation was that the efficiency
of back washing the sand filters was not uniform over their whole area,
and that excessive quantities of backwash water were required to get
some areas of the filter clean. A number of other treatment works under
the control of Cape Metro have filters of similar design, with similar
problems, so a solution to the problem would be widely applicable. This
study is the basis of chapter 4.
In 2002 an extension to the project was granted, in order to complete
the study on the ozone contactor at the Wiggins Water Treatment Plant.
A series of studies was carried out to monitor the contactor at the
Wiggins Waterworks, to obtain data to be used in partial validation of
the computational fluid dynamic model.
The studies reported in chapters 5 and 6 resulted from suggestions made
by members of the steering committee. During 2002 CFD
modelling was undertaken to support the design of modifications to a
potable water clarifier at the Hazelmere Water Treatment Works north of
Durban, operated by Umgeni Water. This was the same clarifier
that had been reported in WRC Report No 648/1/02 The Application of Computation
Fluid Dynamics to Water and Wastewater Treatment Plants,
and the upgrade was a direct sequel to the previous investigation. The
final investigation was prompted by a suggestion that a CFD analysis of
batch settling tests might be useful for developing a strategy to
advance the CFD modelling of secondary settling tanks.
3 Literature Survey: the
use of CFD in water treatment
A survey was carried out on the literature relating to the
use of CFD
modelling in water and wastewater treatment. This covered recent papers
on CFD in water research, CFD in the water and wastewater
industry, and CFD in environmental studies. It was found that, although
CFD has a very extensive literature, very little of this is related to
water treatment. In a search conducted through the ISI Web of Knowledge
site, the keyword CFD found 2196 references in the 12 months preceding
September 2003, but CFD water treatment found only 8, and of these only
3 referred to water treatment as understood in this report.
4 Case Studies
The case studies carried involved a range of aspects that
covered many
of the broad issues found in the literature.
4.1 The Ozone Contactor at the
Wiggins water treatment works
Ozonation is used in drinking water treatment primarily to oxidise iron
and manganese, to remove odour- or taste-causing compounds, and to
destroy micro-organisms. The peripheral benefits include
possible reduction in coagulant demand, enhancement of algae removal
and the colour removal. Ozonation of water is typically
carried out by dispersing gas containing ozone into the liquid phase.
The contact between the two phases accompanied by an ozone mass
transfer takes place in ozone contactors.
The pre-ozonation system at Wiggins Waterworks, operated by Umgeni
Water in Durban, consists of four contactors. Each of the contactors is
preceded by a static mixer such that every chamber can operate
individually or in parallel with another contactor. An ozone-oxygen gas
mixture is injected as a side-stream through the static mixer which is
employed to achieve high mass transfer of ozone to water. The
Wiggins pre-ozonation system has an unusual configuration, as it was
adapted from an existing structure, which had originally been designed
for a different purpose. Water enters from the static mixer
at the bottom, and passes through three horizontal compartments before
it exits over the weir at the top.
The objectives of the investigation were:
- To determine the actual liquid residence time distribution
as a function of flow conditions through the contactors
- To optimise the disinfection efficiency of the contactors
by selecting the most appropriate position for
monitoring residual ozone concentration achieving the most
efficient use of the ozone dosed to the system.
In outline, the phases of the investigation were approximately as
follows (various mistaken or misguided excursions have been excised
from the sequence):
- A single phase (water only) CFD model was set up to provide
an initial understanding of the flow patterns in the contactor.
- Tracer tests using lithium chloride were carried out to
compare with the model. These were conducted with and without
gas injection into the static mixer. Although the gas injection did
cause some noticeable difference in the measured outlet concentrations,
the effect on the overall residence time distribution was very
small. After some adjustment to the model, it was concluded
that a single phase (i.e. water only) model would give an adequate
representation of the RTD for modelling the ozone reactions.
- An ozone reaction scheme was added to the model, using
kinetic data obtained from the literature. From this it was evident
that the ozone consumption is very dependent on the local
characteristics of the water, which need to be determined
experimentally.
- Sampling lines were installed on the contactor which
allowed ozone concentrations to be measured at various
points. The positions of these were chosen with reference to
the CFD model results. Consideration of both the model results and the
measurements suggested that the best point for monitoring the ozone
concentration for control purposes was located between the 2nd and 3rd
compartments, rather than at the outlet of the 3rd compartment as at
present.
- A laboratory study was initiated to obtain rate constants
for the reaction scheme. At the time of writing, this was in progress.
- To check the validity of neglecting the effect of gas
injection, some two phase modelling (gas bubbles in liquid) was carried
out. First a two dimensional model was tried, and when this proved
successful, a full three dimensional model was implemented. However
satisfactory results were not achieved, due to grid resolution and
convergence difficulties. These might well have been resolved with more
powerful computers than those available.
The main conclusions related to the objectives of the case study were:
- The location of the residual ozone monitor used in the
control of the ozone dose should be moved from its current position
close to the outlet weir to a point at the end of the middle
compartment, level with the position of the experimental sampling point
4. However, although sampling point 4 was located on the right side of
the contactor, the model suggested that a more consistent and reliable
signal would be obtained if the sensor were located on the left.
- The disinfection efficiency of the system is very
sensitive to the level of ozone consuming substances in the raw water
feed. Since this factor can be expected to vary seasonally and with
weather conditions, operational procedures should be developed to take
it into account in determining the ozonation control strategy.
The investigation was aimed at improving the
operating rules for the contactor rather than changing any aspects of
its design, however the mass of detailed information provided
by the models did indicate aspects of the design which could be
improved.
- The Residence Time Distribution exhibited by the
contactor (both simulated and actual) is somewhat disappointing given
the its structure. The US EPA Disinfection
Benchmarking and Profiling Guidance Manual presents a
broad classification of contactors in terms of their general
configuration, the Wiggins contactor might be placed under the category
of being provided with "serpentine intra-basin baffles". According to
this classification the "baffling condition" should fall somewhere
between "average" and "superior" The measured residence time
distribution fell between "average" and "poor" The model
results showed that this degradation in performance was mainly due to
left-right asymmetry in the flow distribution, and recirculating
vortices in the bottom compartment. The latter occur because
the feed is concentrated at one point rather than being distributed
evenly across the width of the compartment. CFD modelling could be used
to design modifications (such as strategically placed baffles) to
reduce these non-idealities and therefore improve the contactor
performance.
Since the study was by far the most comprehensive
undertaken during the project, it provided the broadest illustration of
the use of CFD in research into water treatment processes, and some of
its strengths and weaknesses.
- The large
size and
geometrical complexity of the contactor: on the one hand
this meant that the CFD model provided insight into details of the
processes within the reactor that would be almost impossible to obtain
experimentally, on the other hand it led to a very large model with
convergence difficulties and long solution times.
- The hydraulic
sub-model: the water-only hydraulic model gave
very good agreement with experimental data, with only minimal
calibration (i.e. the adjustment of the turbulent intensity of the feed
from the static mixer). Together with similar experiences in
other investigations, this indicates that CFD predictions of
straightforward flow patterns and RTDs can be usually be used with a
high degree of confidence.
- The two phase
(gas-liquid) sub-model: two phase
modelling was undertaken, but the conclusion that could be drawn were
limited. Difficulties associated with grid resolution and
convergence are very much greater than with the water-only
model. These were be overcome successfully in the 2-D case by
making the computational grid fine enough, but the 2-D model proved to
be an inadequate representation of the system. In the 3-D model, making
the grid fine enough for stability resulted in a model that was too
large for the
- computers available to handle. Experimental
verification
would have also presented much greater difficulties than in the single
phase case.
- The reaction
sub-model : in
this study, adding the reaction sub-model did not involve much extra
difficulty as far as the modelling was concerned. This
fortunate result was due to the relatively slow reaction rates, with
time constants of the same order as the hydraulic residence time of the
system. The main difficulty associated with the reaction
modelling is due to the lack of detailed knowledge of the reaction
mechanisms and the kinetic parameters involved. This meant
that model predictions could not be trusted without experimental
verification and calibration. It also meant that
extrapolation of the model to operating conditions different from the
calibration set was unreliable. In an attempt to improve the situation,
a laboratory study of the reaction kinetics has been initiated, but is
incomplete at the time of writing.
These conclusions can be generalised to an extent by noting that CFD
modelling is very successful where the underlying physics of the
process are very well understood, but becomes less useful and reliable
when sub-models are added which involve approximations and
uncertainties.
4.2 The sand filter
backwash system at the Faure Water Treatment Works
A CFD model was set up to model the water-only phase of the
backwash cycle one of the sand filters operating at the Faure treatment
works. The objective of the investigation was to determine
why parts of the filter take much longer to clean than others, and to
propose modifications that would lead to improved operation. The
modelling was accordingly divided into two phases: modelling of the
existing configuration and modelling of the proposed improvement.
The model of the existing configuration showed that the pressure in the
underdrain tends to increase towards the far end from the feed, due the
general deceleration of the flow. Because of this increase in pressure,
the flow through the nozzle slabs also tends to increase towards the
far end, where the model predicted that the flow would be about 30%
higher than the average for the filter. This means that the
parts of the filter close to the feed end get less than their fair
share of the flow, which explains why they take longer to clean.
Having satisfactorily explained the reason for the operational problem,
a CFD model of a proposed solution to the problem was set up. This was
to install a flow distributor down the centre of the under drain, which
ensures an even supply of water to each section. This would
take the form of a pipe laid along the length of the under drain, with
holes on each side. The diameters and spacing of these holes
would need to be carefully gradated down the length of the pipe to
deliver a uniform volumetric flow per unit length in spite of the
pressure rise.
The model was constructed in two parts, one for the flow inside the
distributor, and one for outside the distributor. The model predicted
that the variation over the filter surface should be reduced to less
than 1%.
Although the distributor appeared to be relatively inexpensive to
install, at the time of writing it had not been installed so
its efficacy had not been verified.
4.3 The
clarifier upgrade at the Hazelmere Water Treatment Works
In this case study, a series of CFD models were generated to support
the design work for modifications to a clarifier which needed to have
its performance upgraded. The peripheral feed arrangement for this
clarifier was particularly unusual, and caused it to be plagued by poor
feed distribution resulting in severe short-circuiting. An
investigation into the maldistribution of flow occurring in this
clarifier using tracer testing and a CFD model was reported in WRC
Report No 648/1/02 The
Application of Computation Fluid Dynamics to
Water and Wastewater Treatment Plants. The conclusion of
that
investigation had been that converting the clarifier to a central feed
arrangement was the only way to obtain a significant improvement in its
performance.
In June 2000 Umgeni Water reviewed the existing design and made
recommendations on proposed improvements to Clarifier 1 and Clarifier
2. The working group tabled the following design proposal:
- to convert the existing peripheral inlet system
to a central
inlet, which required construction of the inlet pipe below the existing
floor;
- to install baffles on the central inlet port
imparted a
rotational component to the flow in the flocculation zone to enhance
passive flocculation;
- To provide a 10 m diameter flocculator zone in
the centre of
the clarifier providing, a flocculation time of 30 minutes;
- To install two paddle flocculators within the new
flocculator
chamber.
It was expected that these modifications would increase the clarifier
capacity from about 9 ML/d to 15 ML/day at an up flow velocity of 1.2
m/h (within the design guideline value of
1.5 m/h overflow rate for this type of clarifier). While the design
work was being carried out, CFD modelling was undertaken to help
evaluate various design options. This interaction led to a
number of changes to the design:
- a single central sludge discharge hopper was designed in
place of the hopper originally located within the sedimentation zone.
- the floor within the flocculation zone was sloped at 1:12
towards the centre, to aid transport of concentrated sludge into the
central hopper against the outward flow of water.
- at the point where the water flow passed under the skirt
between the flocculation zone and the settling zone a step was made for
the sludge to flow over, to prevent it being re-entrained.
- the dimensions of the flow area immediately beneath the
flocculation skirt were decided after evaluating several CFD models.
The modelling was based on data supplied by the Umgeni Water design
team as the design work was proceeding, and the configuration was
continually being changed while the modelling exercise was in progress.
The design modifications were implemented on the No. 2 clarifier at
Hazelmere, which was re-commissioned in August 2002. A
comparative performance test between clarifiers No. 2 and No. 3, which
also has a central feed configuration but none of the other
CFD-designed features, was carried out during August 2003.
% Cases where turbidity is
exceeded
Turbidities measured at
each clarifier during the comparative test
The graph shows the inlet and outlet turbidities measured
during the
test, plotted first against the elapsed time of the test, and also on a
percentile basis. During the test period the feed water
turbidity was extremely low, so that flocculant dosage increased the
turbidity significantly, which is how the turbidity from No. 3 comes to
be higher than the water feeding it at times. The superior
performance of the No. 2 is clearly evident, which vindicates the use
of CFD in its design.
4.4 Batch settling of
secondary sewage sludge
The modelling of solids settleability is essential for
modelling
settling tanks in water and wastewater treatment. Until the
advent of hydrodynamic models, the focus of modelling solids
settleability was on describing the behaviour of the solids in the
water while the water itself was considered a stationary or ideally
moving medium in which the solids settled. Hydrodynamic models now
allow the behaviour of the water in the settling tank to be
modelled. While the modelling of the water flow has made
extraordinary advances in the past 20 years, modelling the
settleability of the solids has not improved much over the this
time. In fact, the weakest part of hydrodynamic models of
settling tanks may be the modelling of settleability of the
solids. This investigation explored methods for measuring and
modelling solids settleability with the view of improving these for
hydrodynamic models of settling tanks.
The design and operation of secondary clarifiers is commonly based on
the solid flux theory. The basic data required for the application of
this theory can be obtained from multiple batch tests by which the
stirred zone settling velocities over a range of sludge concentrations
are measured (dilution experiments).
Many CFD modellers of settling tanks have used the Takács
equation to describe the settling velocity of the solids, however the
equation is not well formulated for experimental calibration. It
contains 4 constants that require measurement to calibrate
it. Only 2 of these constants are readily measurable from
laboratory scale tests, the remaining 2 usually have to be inferred
from measured values of the suspended solids in the effluent from the
full-scale clarifier. This is unsatisfactory, in that the clarifier
cannot be properly modelled without using its own operating data.
The strategy of this investigation was to incorporate the
Takács settling model into the simulation of batch settling
tests, in an attempt to identify characteristics which might be
amenable to experimental measurement, and which might allow the
complete set of Takács parameters to be estimated.
From the batch settling simulations, two characteristic settling
behaviours were identified, dependent on the initial concentration of
sludge in the settling test. The features of Type I settling, which
occurred at higher sludge concentrations were:
The notable features of this result are:
- The sharp interface between "clear" liquid and sludge
settling at the initial concentration.
- The residual concentration of non-settling sludge in the
"clear" liquid.
- The smooth build-up of a concentrated layer of settled
sludge
on the floor of the column.
Type I settling occurs for initial sludge
concentrations higher than a critical value Cm for which the sludge
settling velocity is a maximum. For tests starting from
concentrations below Cm the a qualitatively different Type II settling
behaviour takes over. Under these conditions, the interface with the
"clear" liquid is diffuse, whereas the interface with the
settled sludge at the bottom of the column is sharp. These
qualitative features correspond well to experimental observations.
Although the simulation results indicated there was no direct way to
determine all the Takács equation parameters from a single
batch settling test (which confirms practical experience), a series of
experiments with different starting concentrations could be conducted
to determine the value of the critical concentration Cm at which the
settling behaviour switches between Type I and Type II. This value,
together with the settling velocity vm of the sludge at this
concentration, could then be used to infer the remaining
Takács equation parameters.
The character of this case-study was somewhat different to the others
undertaken during the project, in that the CFD model was used to
suggest a direction for further research, rather than to interpret or
extrapolate research results. An experimental investigation needs to be
undertaken to verify the suggested protocol.
4.5 Conclusions and
recommendations
This section presents the more general conclusions which
arise from
considering the project as a whole.
4.5.1 The scope for
application of CFD modelling in water treatment
It is interesting that most of the broad issues identified the
literature were touched on in one form or another during the
investigations undertaken during this project. The Wiggins ozone
contactor started with simple hydraulic modelling and prediction of the
residence time distribution, and progressed to more complex physical
modelling of reaction kinetics, disinfection performance and 2-phase
flow. The Faure filter backwashing investigation looked at simple
hydraulic modelling of flow distribution in the context of an equipment
re-design exercise, concentrating entirely on the one specific issue
for the design, and ignoring or approximating all other aspects of the
system. The Hazelmere clarifier investigation was similarly a re-design
exercise, but this time it involved two-phase modelling. It also
provided experience of working interactively in the design team, with
the concomitant time and budget constraints, requiring strict focus on
the specific design objectives, at the expense of realism and
unnecessary detail. Finally the batch settling investigation
again involved two phase modelling, but this time addressed a purely
theoretical question. Thus the experience gained allows a reasonably
comprehensive assessment of the role that CFD can play in water and
wastewater treatment.
The very fundamental nature of the CFD approach has the advantage of
being able to represent appropriate systems (see below) in great detail
with minimal requirements for empirical data, but the disadvantages of
complexity and difficulty in solving the resulting systems of
equations. These practical difficulties prevent CFD from
being a universally appropriate approach to all problems involving
fluid flow, in spite of its fundamental basis. Generally, CFD is most
useful for systems which are well-connected, that is, where all the
boundary conditions have relatively strong influences on all parts of
the flow field. This applies to many systems found in water treatment,
such as reservoirs, contact chambers, sedimentation basins, ponds and
even lakes and lagoons. However there are also many systems
in water treatment where CFD does not provide an effective approach,
for example a set of equipment connected by a pipe network, or a long
reach of a river. In such cases the CFD model would expend enormous
computational effort on calculating the practically negligible effects
of remote boundary conditions.
The simplest CFD models consider only the hydraulic aspects of a
system. Frequently these models are used to predict the
residence time distribution (RTD), which often provides a link to more
direct performance indicators through empirical rules based on
experience (e.g. the disinfection CT rule). As CFD modelling has become
more established, more detailed models are appearing which attempt
direct representations the physical and chemical processes taking place
in the treatment processes, such as sedimentation, flocculation,
inter-phase mass transfer and chemical reaction. In all cases
these more complex models need to be supported by experimental studies
to establish the parameters for the physical and chemical parts of the
models. The CFD modelling thus has a role in both the
interpretation of results from experimental apparatus, and in
extrapolating research results to the design of full-scale processes.
The relationship between tracer testing and CFD modelling to determine
the RTD of a system is worth mentioning. The ozone contactor
study demonstrated the use of tracer testing to verify the CFD model,
and concluded that CFD modelling is often able to predict the RTD very
accurately. However, if the RTD is all that is required, the tracer
test may be quicker and less expensive to perform than to develop a CFD
model. However this depends on the size of the system: for
many water treatment systems the size is such that a very large dose of
tracer is required, together with an elaborate and expensive sampling
and chemical analysis programme, and the time required to complete the
test is so long that it is not feasible to maintain conditions steady
for long enough.. Nevertheless, tracer testing should always
be considered as a possible alternative to a CFD study, as long as the
RTD is adequate to address the required purpose. A less tangible factor
that should be borne in mind is the extra insight that the CFD model is
able to bring to the investigator.
4.5.2 The costs involved
in CFD modelling
The literature does not reflect a widespread acceptance of
CFD
modelling in water and wastewater treatment, and this is mirrored in
the South African water industry. The cost involved in undertaking such
modelling is undoubtedly one of the factors contributing to this
situation. To some extent this is a matter of perception, but the
reality is that the overall cost of a CFD investigation is likely to be
fairly high.
To start with, the skills required are relatively rare, and take some
time to develop. CFD has not yet found a place in undergraduate
curricula, so postgraduate training is involved. The
underlying mathematics is complex and not easy for practising engineers
to master on their own. It is true that the CFD software now available
takes care of almost all the mathematical complexities, but
paradoxically this may make the problem worse rather than better,
because it makes it so easy to obtain plausible results which one does
not really understand, increasing the potential for making serious
errors. The large international water treatment firms such as
Veolia and Thames Water have a small number of CFD specialists who act
as internal consultants for equipment design. During this project, the
example of the Hazelmere clarifier illustrated how such an arrangement
might work..
CFD modelling does require more than usually powerful computing
hardware, and the requirements escalate rapidly when modelling the more
complex physical processes, as illustrated by the difficulties
encountered with the gas phase in the ozone contactor. However the
hardware cost is less and less significant as a cost factor, as it
continues to decline steadily, and since the skilled personnel costs
involved in such advanced modelling are likely to be very
significant.
The cost of CFD software has come down steadily during the duration of
the project, but is still high, even with a discounted academic
licence. A non-academic licence would have been much more
expensive. The development of the software seems to be driven
by much higher cost applications that water treatment - aerospace,
chemical manufacture, power generation, automotive design etc., and the
pricing appears to reflect this kind of market. Many of the
models, for instance combustion or solidification, that are available
in the software are not relevant to water treatment - it could be that
more limited package could be marketed to the water and wastewater
industry.
The wide range of problems which might be tackled with CFD makes it
very difficult to make any general statement about the cost of
undertaking CFD modelling. However, some idea can be obtained by
considering a relatively straightforward investigation, such as the
Hazelmere clarifier (chapter 5). The time involved was about
40 h (excluding report-writing), so the personnel cost would be of the
order of R6 000 (2003 rand values). The software licence cost for
commercial use of the Fluent software was about R16 000 per
month. Since the minimum period made available by Fluent was
1 month, it depended whether other jobs were available to share the
cost, so the software cost would be between R4 000 and R16 000, and the
overall cost would be between R10 000 and R22 000, to which would be
added the costs for computer time: if done with a suitable personal
computer (1GHz pentium with 500Mb RAM) this would amount to about R100.
This cost might be considered reasonable for a large clarifier, but
appear excessive for a small unit. The problem is that the cost would
probably be about the same irrespective of the size of the
installation. For small units the costs could be effectively reduced by
developing standard designs which could be reused a number of
times. The extra design cost should be recoverable through
lower operating cost, however this will probably be difficult to
quantify beforehand.
4.6 Recommendations
The recommendations are divided into those that relate to the
individual case studies, and the more general ones that relate to the
application of CFD to water and wastewater treatment.
4.6.1 The Wiggins ozone
contactor
- The position of the ozone sensor monitoring the residual
ozone
should be moved to the position identified in the investigation.
- A new strategy for the control of the ozonation should be
developed
that takes into account the ozone demand of the raw water and the
disinfection efficiency of the contactor.
- The cost-benefit balance of the
ozonation in the overall water purification process is not easy to
quantify; it could even be the case that the benefits do not justify
the cost. The model that has been developed for the contactor
could be very useful as a component in a wider investigation of the
role of ozonation in the overall treatment process.
4.6.2 The Faure filter
backwash system
A backwash distributor should be installed on a trial basis on one of
the filters at the Faure Water Treatment Works. If
this proves as successful as predicted, the system could then be
installed on the other filters at the works, and on other filters of
similar design.
4.6.3 The Hazelmere
clarifier
The design modifications which proved so successful should
be
implemented on the remaining peripherally fed clarifier at the
Hazelmere Water Treatment Works when appropriate. The design should
also be considered for new clarifiers of a similar size.
4.6.4 The batch settling
test for sewage sludge
An experimental project should be undertaken to test the protocol
suggested by the modelling results. This would involve carrying out
laboratory settling tests to determine the sludge settling parameters,
using these parameters in a CFD model of a full-scale clarifier, and
testing the model predictions experimentally on the full-scale unit.
4.7 General
recommendations
The South African water industry still needs to develop an
adequate
pool of CFD expertise that can be called upon when appropriate. This
might involve:
- Introducing CFD in a limited way in undergraduate
university
courses, to promote awareness of the technique.
- Postgraduate university training to build up the CFD skills
base in the country.
- Courses, symposia and workshops designed to promote
awareness
of the benefits of CFD among water industry managers and policy-makers.
- Centres of expertise which can undertake research or
consultancy work to provide CFD support to water researchers and
operators, and promote CFD by providing the academic and industrial
training mentioned above.
- CFD specialists employed in the design teams of the larger
water authorities, municipalities and engineering consultants.
- A way should be sought to engage the developers of CFD
software in order to make appropriate CFD packages available at costs
which are appropriate to the water and wastewater treatment industry.