PLANNING THE REHABILITATION OF
CAMPERDOWN’S SEWERAGE PIPELINE
SYSTEM
Jeff Haydon Peter Wilson Max Anderson -
Montgomery Watson Australia
South West Water
Montgomery Watson Australia
Abstract:
The rehabilitation of sewerage pipelines is carried out for either hydraulic, structural or operational
reasons. The planning and investigation work completed to date for the town of Camperdown in
South West Water is presented as a good example of using logical processes of low cost hydraulic
planning to minimise high cost field investigations. Through the use of a calibrated hydraulic sewer
model the effects of applying repair, lining or replacement solutions to reduce the quantity of wet
weather inflow and infiltration have been modelled. The office based modelling of various
combinations of the above works, together with supplementary flow management solutions, such as
offline detention storage, has provided South West Water with valuable knowledge. The Authority
has used this knowledge to evaluate a range of treatment and disposal options some of which
involved I/I abatement and flow containment.
Keywords:
Planning, hydraulic modelling, flow monitoring, I/I abatement, pipeline rehabilitation, flow
management, hydraulic levels of service, time series analysis.
Background
Camperdown is located in South West Victoria and has a population of 3,150 (ABS 1996) people
serviced by 33 kilometers of reticulation sewer and a conventional trickling filter plant. Refer
Figure 1. Discharge of treated effluent is to Lake Colongulac. The existing lake discharge does not
comply with the requirement of the State Environment Protection Policy and South West Water is
required to bring the plant into compliance by December 2001. The treatment plant includes a
maturation lagoon and sludge lagoon and the required effluent discharge standards cannot be
achieved without substantial augmentation to the plant. The entry of effluent into the treatment
plant is currently restricted resulting in excess wet weather flows bypassing the treatment plant. In
addition to the overflows at the treatment plant, high ingress of stormwater inflow and groundwater
infiltration is causing hydraulic overloading of the reticulation system. Sewage spills have occurred
in the town.
South West Water has embarked on a program of investigation to determine the best option or
combination of options to treat and dispose of wastewater from Camperdown. The Authority has
identified the significant options as the:
1.
2.
Provision of post treatment storage to utilise treated wastewater for irrigation; or
Upgrading of the treatment plant’s capacity and treatment facilities to achieve the required
discharge quality to continue discharging to the Lake; or
Insert to be added: Figure 1
3
Piping of pre treated sewage to an existing treatment plant at a neighboring town and
upgrade that treatment plant’s capacity to cater for additional flows.
Permutations of these options were considered which accounted for the inclusion or exclusion of
the following actions:


Reduction in wet weather inflow and infiltration through pipeline system repair, lining and
replacement; and
Retention and management of high volumes of pre treated wet weather flow to reduce the
downstream costs and to eliminate sewage overflows.
Treatment plant inflow records and system operator concerns had provided South West Water with
an impression of very high wet weather ingress into the sewerage system during and following
rainfall events. An investigation in 1992 concluded that the peak wet weather flow was close to 10
times the average dry weather flow during wet weather.
South West Water believed that by reducing the ingress of rain dependent inflow and infiltration to
about half the existing amount the loading to the treatment plant could be significantly reduced over
any one year. This reduction in flow would also reduce storage facilities and the extent of treatment
plant upgrade works.
Before undertaking the modelling and cost versus benefit analysis outlined in this paper the
Authority had set aside a preliminary budget of over $4 million. This paper presents the planning
work undertaken to ascertain the expected benefits from implementing an I/I source detection and
abatement program. The results of the modelling of various flow containment upgrading and offline detention storage works are also discussed.
Outline of the Investigation Methodology
Previous studies of this type have focused on isolating small sub-catchments where the highest
component of wet weather ingress is occurring. Source detection of I/I and sewer rehabilitation
then follows to reduce the quantities of inflow and infiltration. The planning approach for
Camperdown was to develop a calibrated model which would simulate both dry and wet weather
flows. The recorded flow monitoring data and rain gauge recorded data would be used to achieve
an accurate calibration of the model.
The development and calibration of the model would have two benefits, the quantification of rain
dependent inflow, rain dependent infiltration and base groundwater infiltration between the subcatchments. Secondly on quantification of these ingress components the model could then be used
to assess the impact or effect of various pipeline rehabilitation strategies. By modelling percentage
reductions in inflow and infiltration the volume of sewage flow and sewage overflow could be
calculated by the model. In this way the cost of upgrading works, as defined by the extent of
rehabilitation and flow management works, could be compared to the expected volume reductions
in contained flow and overflow. The outline of the methodology is shown in Figure 2.
To assess the impact or value of rehabilitation and containment works over the period of a year a
number of years of actual rainfall would be simulated through the model with the improvement
works in place. This time series assessment would help quantify the extent of pre and post
treatment storage and the level of upgrading of the treatment plant.
Quantifying the Existing Hydraulic Performance of the Camperdown Sewerage System
Hydraulic modelling identified several locations of hydraulic deficiency within Camperdown.
Wastewater overflows to Lake Colongulac and the town storm water drainage system prior to
treatment were simulated during the recorded wet weather events.
Define Study
Objective
Capture & Validate
Sew er Data
Select Flow
Monitoring Sites
Build Hydraulic
Model
Calibrate Hydraulic
Model
Assess Existing
Service Levels
Undertake Flow
Monitoring
Model Solutions
Undertake Cost vs
Benef its Analysis
Plan Flow
Management
Works
Identif y SubCatchments f or I/I
Abatement
CCTV & Other
Plan Repair Lining
& Replacement
Works
Plan Treatment
Plant Upgrade
Implement Works
Figure 2
Investigation Methodology
A total of three recorded storms were used to calibrate the detailed hydraulic model. The hydraulic
analysis results during dry and synthetic wet weather events are shown in Table 1. The total
overflow volume is the sum of spill volumes predicted from the seven emergency relief overflow
structures constructed throughout the catchment.
Event
Weekday Dry
Weather Flow
(3 day simulation)
1 in 1 year ARI –
24hr duration
(3 day simulation)
1 in 5 year ARI –
24hr duration
(3 day simulation)
Table 1
Rainfall
(mm)
Total
Discharge
Volume (m3)
Total
Overflow
Volume (m3)
0
3120
0
39
11100
3650
65
11300
5600
Summary of Modelled Hydraulic Performance
The existing hydraulic performance or level of service of the catchment failed to meet a minimum
1 in 1 year ARI design storm.
Wet Weather Response by Sub-Catchment
The wet weather response in the sewer is the additional flow due to rain and ground water entering
the pipeline system. The three major sources of inflow and infiltration ingress into the sewer
system were identified as rain dependent inflow, rain dependent infiltration and wet catchment
infiltration. The hydraulic model was invaluable in separating these different components as the
calibration process requires developing individual fast and slow responses for each sub area
upstream of a flow monitoring site.
For a 1-in-5 year ARI 24 hour duration storm the I/I volume was calculated by subtracting the dry
weather flow from the wet weather flow over the length of sewer for each of the sub-catchments.
Figure 3 shows the results of these calculations for each sub-catchment.
Camperdown 1 in 5 year event
Simulated wet weather ingress by sub-catchment
500
Rainfall dependant Inflow
Rainfall dependant Infiltration
Wet Catchment Infiltration
Total Inflow / Infiltration
450
Volume of Ingress (m3/km)
400
350
300
250
200
150
100
50
0
1
2
3
4
5
6
7
8
Catchment Number
Figure 3
Summary of Wet Weather Response by Sub-Catchment
The most dominate source of wet weather ingress identified was ‘wet catchment infiltration’. It
was recorded entering the sewers up to two days after a significant rainfall event whilst the rain
dependent inflow was proportionately low in all sub-catchments.
The worst performing sewers with respect to inflow and infiltration are those contained within subcatchments 3, 4, 5 and 8, whilst the best performing sub-catchments were 1 and 7.
Confirming the Required Level of Service
The Victorian water authorities have not had to license their sewage overflows to this point in time.
South West Water is examining the implications of adopting the 1 in 5 year level of service for flow
containment for Camperdown as this has been the accepted standard applied by the EPA elsewhere
in Victoria. This level of service means that the Authority must develop a flow abatement strategy
and a flow management strategy to prevent overflows occurring for any wet weather event up to
and including the 1 in 5 year event.
Summary of the Methodology for Options Analysis
The search for solutions to Camperdown’s problems of excessive wet weather sewage flow and
inadequate quality of treatment plant effluent have focused on the total catchment management
approach. The extent of the improvement works on the reticulation system are dependent on
whether treatment plant upgrading works proceed or whether the alternative treatment/disposal
option is adopted. The solutions trialled in the planning phase have been carried out quickly and
efficiently with the hydraulic model.
In summary the options available to South West Water were to either:




Reduce I/I flow volumes through pipeline rehabilitation;
Manage the wet weather flow by providing detention storage facilities;
Eliminate throttles and overflows by augmentation and diversion works; and
Upgrade the treatment plant.
Figure 4 outlines the procedure followed to develop the optimum solution or solutions for
Camperdown.
Existing Sewer System
Modified
Treatment
Plant
Existing
Treatment
Plant
I/I Abatement
Works
Repair
Relining
Renewal
Flow Management Works
Economic Analysis
Catchment Development Plan
Figure 4
Methodology to Analyse Options
Pipeline Inflow and Infiltration Abatement Works
Montgomery Watson and South West Water considered previous inflow and infiltration abatement
experience in getting achievable reductions in the volume of wet weather ingress that could be
prevented from entering the sewer system for the different methods of rehabilitation. The three
types of sewer rehabilitation examined and the areas of I/I abatement they target are shown in
Table 2.
Method of I/I Reduction
Repair
Relining
Renewal
Table 2
Expected Result
High percentage removal of rain dependent inflow, but a
low percentage reduction of infiltration due to partially
repairing the system and the system still exhibiting weak
points.
Modest to low percentage removal of rain dependent
inflow and modest reduction of infiltration due to
property branch connection and relining joint infiltration.
High percentage removal of infiltration ingress, whilst
modest to low reduction in rain dependent inflow.
Expected Benefit of Inflow/Infiltration Abatement Methods
The hydraulic modelling of the I/I abatement scenarios involved applying the percentage reductions
in inflow response and infiltration response for the particular rehabilitation technique trialed. As
the work was targeted to specific sub catchments the cost for each rehabilitation option was
determined. Relationships between the cost of the option and the modelled reduction in wet
weather ingress were prepared.
It was determined that the most cost effective sewers to look at rehabilitation through repair,
relining and renewal were found in sub-catchments 3, 4 and 5.
The three methods of I/I abatement in the Camperdown catchment exhibit the relationship between
cost and volume reduction as shown in Figure 5. The pattern of diminishing returns is evident for
any one of the inflow and infiltration abatement works modelled. The relationship derived assumes
application of either of the three rehabilitation techniques.
Camperdown 1 in 5 year event
Modelled flow reduction benefit from I/I Abatement
7000
6000
3
Volume Reduction (m )
5000
4000
3000
2000
1000
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Cost ($ millions)
Repair
Figure 5
Relining
Replacement
Flow Reduction Benefits
Treatment Plant Upgrade
The recommended solution for the future of the treatment plant is augmentation of the existing
facilities to treat the current and expected peak flows generated from the catchment. A separate
detailed investigation on the performance of the existing facility and future requirements of the
treatment plant was also undertaken concurrently with the planning study. In summary the study
concluded that it was more cost effective to upgrade the existing treatment plant to meet the
expected flow volumes from the adopted catchment development solution rather than rebuild the
treatment plant to a higher level of treatment.
Flow Management Through Modelling Detention Storage
The rehabilitation solutions developed for I/I abatement, even taken to the extreme of rehabilitating
most of the reticulation pipeline system, will still not reduce wet weather ingress to levels to prevent
system overflows during the 1 in 5 year event.
The hydraulic model was utilised with each of the I/I abatement options to determine the location
and preliminary sizing of wet weather detention storage facilities.
Localised Sewer Augmentation and Flow Diversion Works
A number of sewers require augmentation either in the form of upsizing the diameter of the existing
sewer or constructing an additional sewer in parallel to increase the hydraulic capacity. Additional
sewer augmentations specific to each option have been determined and costed to meet the specific
catchment requirements.
Cost Comparisons of Options
Figure 6 illustrates the value of the hydraulic modelling to determine the lowest cost solution and
achieve the maximum benefit in overflow reduction. The cost of undertaking I/I abatement work to
the three worst catchment along with treatment plant upgrade is compared with the development of
a complete flow management solution to meet the required level of service.
Figure 6: Cost Comparison of Options
South West Water’s Preferred Adopted Solution
The hydraulic analysis concluded that reduction in I/I abatement through repair of the three worst
catchments coupled with treatment plant upgrading works was the most cost effective option. This
option satisfies the 1 in 5 year containment requirement and produced an effluent quality that
complies with the existing requirements of discharge to the Lake or for storage in readiness for
irrigation. The quality of the effluent does not meet the discharge standards that would apply if the
Lake discharge was to continue beyond 2001 (tertiary treatment would be needed).
South West Water has also been investigating the option of complete disposal of effluent to land
through irrigation of farm land in the region. The works include the construction of pumping
stations and rising mains together with a facility to store the winter flows until the weather is
favourable for irrigation. Storage site limitations have made the cost of the storage very sensitive to
the size needed to accommodate the winter flows. Accordingly, minimising the volume to be stored
through more extensive I/I abatement works occurring in the town, results in a lower storage cost.
The storage costs range from $2.3 million to $3.8 million.
To determine the storage size, the monthly volumes generated from the catchment needed to be
determined for a number of different ‘wetness’ years. This was simulated through time series
analysis using the hydraulic model. Actual rainfall data was used for a number of years that were
selected according to the annual rainfall measured. The cost of the resulting storage could then be
determined for the specific I/I abatement option being considered.
It was concluded that the lowest cost and best option was the disposal of effluent to land coupled
with the I/I abatement option that involves the renewal of sewers in the three worst catchments.
Thus when considering the whole sewerage cycle of collection, conveyance, treatment and disposal,
the higher cost I/I abatement option of sewer renewal has been demonstrated to be cost effective in
this instance.
South West Water has adopted a program of works that will see the 1 in 5 year containment
standard of the EPA met together with the potential renewal of all sewers in the three worst
catchments.
Current Works
CCTV inspection of the three worst catchments is currently underway (involving 13 km of sewers)
to determine if renewal of all the sewers is necessary. It may be possible to avoid the renewal of
some of the sewers depending on their condition and the nature of inflow or infiltration.
Further modelling of the sewer system was carried out by Montgomery Watson to determine if
reduction in the rain dependant inflow (direct storm water connections) would make a substantial
difference to the size of the detention storages or the augmentation works needed to prevent sewage
overflows. The modelling demonstrated that significant savings in capital costs were possible if the
anticipated reductions in this component of the inflow was achieved. Accordingly, South West
Water is proceeding with a smoke testing program of the four catchments that were the worst
performers in this area; three of which are those that are up for renewal.
Implementing the Solution
It is proposed to stage the augmentation, detention storage and I/I abatement works over the next
two years. Monitoring of the treatment plant inflow and a number of the emergency relief
structures will provide additional data upon which to base the detailed design and provide data to
further validate the model accuracy. The calibrated model becomes a useful tool in the detailed
design of all the elements within the works.
The storage and land disposal works are proposed to be completed and operational by 2001. The
discharge to the Lake can then cease and the treated effluent be put to a beneficial use.
Conclusions
The Camperdown study has identified the value of planning to develop optimum solutions for
system upgrading and pipeline rehabilitation and to save costs on implementing the solutions. The
analysis has theoretically quantified the reductions in wet weather ingress and system overflows
through the application of various flow management and I/I abatement solutions throughout the
catchment. Whilst assumptions about the degree of wet weather ingress reduction have had to be
made the first message of this paper is that hydraulic modelling in combination with flow
monitoring is the most accurate analysis available to quantify the expected benefits from any I/I
work. This is due to the ability of the model to simulate the observed differences between the
ingress components of rain dependent inflow, rain dependent infiltration and wet catchment
infiltration.
Flow measurement on its own in the hands of an experienced data analyst can quantify the various
inflow and infiltration components. However it is very difficult to extrapolate the likely ingress
expected from the theoretical level of service storm or from historical events which were larger than
those recorded. Most storms recorded during a short term period of flow monitoring will be less
than the 1 in 1 year event. Furthermore, without modelling, the data analyst would not be able to
assess the impact on system performance and overflows throughout the catchment as a consequence
of the wet weather ingress.
Most I/I abatement strategies focus on reducing the number of stormwater drain cross connections
to the sewer system. Large scale smoke testing; dye testing and CCTV surveys are carried out to
find the sources of wet weather ingress. Camperdown has illustrated that it is the high infiltration
through pipeline joints and cracks as the groundwater table rises following a prolonged period of
rainfall which is the principal problem for this system. It is acknowledged that this is not always
the case and that each system will be unique. The field work mentioned would do little to identify
the infiltration problem, particularly if carried out in dryer periods of the year. The second message
of the paper therefore is that until the problem is quantified and until the benefits are established
through a cost versus benefit study very little money should be going into field investigations and or
pipeline rehabilitation.
The final message is that the assumed benefits of I/I abatement must be conservative. That is, the
expected percentage reductions of inflow volume and infiltration volume for the different
rehabilitation works of repair, lining and replacement must not be too high. For commercial
confidentiality reasons it has not been possible to disclose the values used in the Camperdown
study. It can be disclosed however that the final values were derived through a combination of
research and negotiation between South West Water and Montgomery Watson.
Acknowledgements:
South West Water
Thiess Environmental Service
EPA Victoria
Bureau of Meteorology