This study was done a couple of years fago or a project that ended up going vacuum. It was a high water table flat area. A few things that weren't highlighted were the reduction in methane emissions by using a vacuum system. Also the reduction in flow characteristics by using vacuum or pressure due to the closed system. In a gravity system infiltration rates would apply increasingthe size of the pipework, pump stations and treatment plant, both in construction costs and ongoing energy.
Greenhouse Gas Emissions for Sewerage Reticulation Options
A Comparison between Vacuum Sewerage, Gravity Sewerage and Grinder Pumps
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FLOVAC SYSTEMS PTY LIMITED Telephone: (61 2) 9417 1966
19/390 Eastern Valley Way
Chatswood 2067
CONTENTS
1.0 SCOPE
2.0 BACKGROUND
3.0 APPROACH AND ASSUMPTIONS
3.1 General
3.2 Gravity Option Description
3.3 Vacuum Option Description
3.4 Fossil Fuel Used in Construction
3.5 Electricity Used During the Operation of the Assets
3.5.1 Gravity and Vacuum Option
3.5.2 Grinder Pump Option
4.0 SUMMARY OF RESULTS
4.1 Fossil Fuel Used in Construction
4.2 Electricity Used During the Operation of the Assets
4.3 Summary of Carbon Emissions
5.0 CONCLUSIONS
APPENDICES
A CALCULATIONS
B SCHEME DRAWINGS
1.0 SCOPE
The purpose of this report is to assess the greenhouse gas emissions created during construction and operation for three options of a reticulated sewerage scheme. The three options to be considered are:
1. A gravity sewerage scheme to Water Corporation Standards
2. A vacuum sewerage scheme to Water Corporation Standards 3. Grinder pumps at each house - To a lesser service Standard
The report is based upon the Garden of Eaton development report which was a detailed analysis to compare a gravity scheme with a vacuum scheme for both capital costs and a 50 year NPV analysis of power consumption and operational costs. This area was selected to minimise the effort in calculating the inputs for determination of greenhouse gasses. The extent of this area is shown on the plan in Appendix B.
Key Points of the Analysis are as follows.
· A planning study
· Preparing estimates of options to give an indicative comparison
· Assumptions need to be consistent between different options and be usable for other projects
2.0 BACKGROUND
This report is based on The Ardross Estate “Garden of Eaton” report. This report has used the information on the gravity and vacuum options explained in the Garden of Eaton report as a basis for the calculations of greenhouse gas emissions created by each of the options.
The greenhouse gas emissions are found using the formulas derived in “Methodology on greenhouse gas emissions used by Turkey” by Feliz Onder and Kismet Akcasoy and based on the United Nations Framework Convention on Climate Change (UNFCC).
3.0 APPROACH AND ASSUMPTIONS
3.1 General
This report looks at greenhouse gas emissions from two sources
· Fossil fuel used in the construction of the Assets
· Electricity used during the operation of the Assets
The scheme is shown on the planning drawing attached at Appendix D, and the following data summarises the scheme.
Ultimate number of lots 1684
Approximate Area of Scheme 159.75 ha
Approx length and width of scheme 2.7 km long 1.4 km wide
Assumed number of persons per lot 3.5 persons
Assumed flow per person per day 220 l/person/day
Assumed peak factor for pump station design 2.0
Ultimate peak flow for scheme 35.26 l/s
Ultimate Population Approx 5900 persons
3.2 Gravity Option Description
A Gravity planning study has already been carried out for this area by the Water Corporation and there is accepted flows and catchment boundaries for this option. Documented planning for this catchment shows 5 gravity pump stations.
The Gravity option would be very difficult to construct due to a number of constraints which are quite common for this type of development and relate to high groundwater and difficulty in constructing deep sewers.
The overall catchment boundary for the Gravity scheme is identical to the overall catchment boundary for the Vacuum scheme.
3.3 Vacuum Option Description
The Vacuum scheme would only require a single pump station, and the siting of this pump station is quite flexible. It has been assumed that the station will be sited as shown on the attached drawing, however any site located relatively centrally would be hydraulically possible. The impact of a different site on the NPV would only be minor and would be related to the length and hence cost of the pressure main and the electricity costs for pumping the liquid. The current siting of the vacuum pump station is the same as the larger gravity pump station which means that the costs of the pressure main and electricity to discharge the sewage is identical in both options.
3.4 Grinder Pump Option Description
A Grinder Pump Scheme would not only require a single pump at each house which discharge into common pressure mains, but in order to compare this option with the other two an additional transfer pump station would need to be constructed which would discharge all of the sewage from the development off site. This is already incorporated into the other two options in that they both discharge through about a 2 km discharge main. This also means that the additional electricity to discharge through this main must also be calculated.
It should be pointed out that both the vacuum and gravity options are totally owned and operated by the Authority and all of the assets are in crown land. In addition all of the power for the system is provided from an Authority meter and the system has alarms and standbys all managed by the Authority. If there is a fault on either the gravity or vacuum system the Authority will be notified by its alarms and will rectify the fault without intervention from the householder.
In contrast to this the grinder pump system has no standby pumps in the grinder pump stations, these grinder stations are located on the householders property and power is supplied from the houses. In addition if there is a fault with a grinder pump station the system relies on the householder noticing a flashing light on their pump station and calling either the Authority or a third party operator to replace the faulty pump.
It is clear then that the Grinder Pump System offers a lower level of service to the Gravity and Vacuum Systems.
3.5 Fossil Fuel Used in Construction
In determining the fossil fuel used for construction, the Caterpillar Handbook was used to determine the production of a typical machine that would be used to excavate and backfill the pipelaying trenches. The approximate volume of soil to be excavated and then backfilled was calculated for all three options and then the time taken determined based on the volume of soil and production of the machine. Once the time was calculated this was then converted to approximate litres of Diesel used.
Both the vacuum and grinder option had quite shallow pipes and the volume of soil to be excavated for the vacuum mains and grinder pump pressure mains was essentially the same. The small difference occurred because the volume of soil to be excavated was also calculated for the vacuum pits and grinder pump stations. Again the volume of each of these was essentially the same but there are far more grinder pump stations than vacuum pits.
For all options the volume of soil to be excavated was calculated. In order to determine the total soil to be moved this volume was doubled to get the total. Whilst it is true that less volume of soil would be replaced than removed, there is a requirement to carefully replace the soil and compact it as it is backfilled. So this was considered a reasonable assumption. It is also possible that soil of a higher quality would need to be imported for the gravity option which would greatly increase to diesel used. This however was not considered.
It should be pointed out that the approach provided only rough values for Diesel used. It did not quantify all of the plant that would be on site to support the excavators for example compactors and personnel transport. This would underestimate the fuel used for the gravity option because the trenches are so much deeper, that option would take much longer to excavate and so use more fuel in additional plant. It is considered however that the approach used gives a reasonable comparison of the approximate greenhouse gas implication of the three options. The results are tabulated under Section 4.0 and the calculations are shown in Appendix A.
3.6.1 Gravity and Vacuum Option
As stated previously all of the power demands were determined in the report that looked at the whole of life costs for the “Ardross Estate Garden of Eaton” project.
Generally power demands were determined from first principles by using the formula
P=rQh/e
where
P = power required
r= specific weight of water
Q = Flow – obtained from the planning estimates
h = total dynamic head (m) – calculated based on elevation difference and friction losses
e = pump efficiency – assumed to be 50% for all pumps
The power for each pump was determined and the annual power usage was based on an assumption that the pumps were to operate at their ultimate capacity. Since the peaking factor was 2 times average flow the pumps will pump one half of the day which is 12 hours per day. This approach was used for both vacuum and gravity sewage transfer pumps. Vacuum pump power usage was based on the kW size of the required vacuum pump at ultimate flow rate and the design air to liquid ratio of 6:1. This meant that the vacuum pumps were essentially operating for the same proportion of the day as the sewage pumps at ultimate flow.
The analysis a method of calculation of the greenhouse gas emissions was based on the power consumption of the option. This was then used to approximate the green house gas emissions for the option.
All power assumptions are based on the maximum flow rates for the catchment area. That is after the area has been completely developed. The original report looked at the growth in connections and increased the power demand gradually over the life of the scheme. To provide an indicative value this report looks only at ultimate flows
3.6.2 Grinder Pump Option
The power consumption for the grinder pump option is relatively difficult to estimate. Each house has its own pump and this pump will pump when it has the correct amount of sewage in its wet well. This means that at peak times there are more pumps pumping than at other times during the day and this means that the head that the pumps will discharge against will be higher.
In addition to this suppliers of grinder pump systems are quite protective of the actual power usage data for their systems. We have obtained a pump curve for an E-one pump from the internet and have based the calculation of power consumption on this. This curve is attached at Appendix B. The curve shows pump performance in terms of flow and head, it has no information about power usage we have used the same approach as detailed above for determining the power consumption for the gravity sewage pumps. This assumes a 50% efficiency for the pumps just as we have for the other two options. It is possible that both the gravity/vacuum scheme sewage pumps and the grinder pumps are more efficient than this but it is an assumption that is the same for all three options. This approach has provided a range of power demands for the grinder pumps and clearly the grinder pumps will all discharge against differing heads depending on the flow in the pipe and their location in the scheme. The pump curve clearly shows an operating range for the pumps and so we have tabulated this range of power consumptions and also an average. In the absence of a full design the average value is the most reasonable for this option.
The green house gas emissions created by the construction and maintenance of the Vacuum scheme is outlined in the summary with the calculations in Appendix A.
The greenhouse gas emissions created by the construction and maintenance of the Low-pressure scheme is outlined in the summary with the calculations in Appendix A.
4.0 SUMMARY OF RESULTS
4.1 Fossil Fuel Used in Construction
Item | Gravity Scheme | Vacuum Scheme | Grinder Scheme | Max difference |
Volume of Soil Excavated (m3) | 45091 | 23621 | 26792 | 21470 |
Volume of Soil Backfilled (m3) | 45091 | 23621 | 26792 | 21470 |
Total Volume of Soil (m3) | 90182 | 47241 | 53584 | 42941 |
Total Diesel Used (litres) | 12073 | 6324 | 7173 | 5748 |
Carbon Discharged (Tonne) | 461 | 241 | 274 | 219 |
4.2 Electricity Used During the Operation of the Assets
Grinder Pump Power Usage |
Basis |
|
|
|
| Average |
Per Pump |
|
|
|
|
|
Flow/pump l/s | 0.91 | 0.76 | 0.69 | 0.63 |
|
Head/pump m | 3.45 | 20.68 | 27.58 | 35.85 |
|
Power/pump kW (50% efficiency) | 0.62 | 3.07 | 3.75 | 4.43 |
|
Time to pump/day hours | 0.23 | 0.28 | 0.31 | 0.34 |
|
kWh/day/pump | 0.14 | 0.87 | 1.16 | 1.50 |
|
kWh/year/pump | 53 | 317 | 422 | 549 |
|
kWh/year for 1683 lots | 88779 | 532673 | 710231 | 923301 | 563746 |
Transfer station power kWh/year | 63619 | 63619 | 63619 | 63619 | 63619 |
Total grinder pump option power kWh/year | 152398 | 596292 | 773850 | 986920 | 627365 |
Annual Electricity Used During the Operation of the Assets |
Item | Gravity Scheme | Vacuum Scheme | Grinder Scheme | Max difference |
Annual Power Consumption kWh | 154,174 | 96,799 | 627,365 | 530,566 |
Ratios to lowest | 1.6 | 1 | 6.5 |
|
4.3 Summary of Carbon Emissions
Analysis Detail | Gravity Scheme | Vacuum Scheme | Grinder Scheme | Highest/Lowest % |
Carbon Dioxide emissions from construction (tonnes) | 451 | 241 | 274 | 53% |
Yearly Carbon Dioxide emissions created by a coal power station for the scheme (tonnes) | 54.0 | 33.9 | 219.9 | 15% |
Yearly Carbon Dioxide emissions created by a lignite power station for the scheme (tonnes) | 57.8 | 36.3 | 235.2 | 15% |
Yearly Carbon Dioxide emissions created by an oil power station for the scheme (tonnes) | 42.3 | 26.6 | 172.2 | 15% |
Yearly Carbon Dioxide emissions created by a natural gas power station for the scheme (tonnes) | 32.5 | 20.4 | 132.4 | 15% |
Average Annual Carbon Dioxide | 46.7 | 29.3 | 189.9 | 15% |
Years of operation for construction | 9.7 | 8.2 | 1.4 | 8.2 years |
Carbon Dioxide emissions over 50 year life (tonnes) | 2,334 | 1,465 | 9,496 | 15% |
5.0 CONCLUSIONS
The analysis shows that in the construction of the sewerage system, the vacuum option creates the least amount of greenhouse gasses closely followed by the low-pressure scheme. These two schemes create nearly half the total amount of greenhouse gasses that a gravity scheme would create. We feel that this would be a conservative estimate but that the amount would vary from scheme to scheme mainly dependent on the topography and ground conditions.
The analysis also shows that the low-pressure system, by far releases the largest amount of greenhouse gasses into the atmosphere in its routine operation. This is a common misconception because the power usage for each pump station is quite small. We should not forget however that there are a lot of these pump stations. In our view an option that uses 6.5 times the power of the other options should not be implemented unless the other options cannot be used for technical reasons. It is not a valid argument to simply shift this cost onto the householder.
It is also clear that the construction carbon emissions equate to between 8 and 10 years of operational emissions for gravity and vacuum but only 1.4 years for the grinder pumps system. This means that any construction savings are quickly lost for the grinder pump option.
Key points from this study:-
· Vacuum is about half the cost in carbon to construct for this catchment than gravity and slightly cheaper than grinder pumps.
· Grinder pumps are 6.5 times more costly in annual power consumption and hence greenhouse gas generation than Vacuum.
· Gravity is 1.6 times more costly in annual power consumption and hence greenhouse gas generation than Vacuum.
This area has already been constructed in vacuum but the decision was made on financial grounds. Vacuum was also the only option which preserved the embankments and flora along the creek in the middle of the development. This was an environmental consideration but also gave much more amenity to the new residents of the development.
Due to this unreliable aspect of the analysis it has been concluded that the vacuum option is most suitable. Since it has the lowest greenhouse gas emissions in the construction and second lowest in the powering of the scheme.
We feel that more analyses of this type should be carried out for developments.
APPENDIX A
Calculations
Power Consumption
Gravity
Maximum power cost occurs in 2010 and is $34887
Therefore power used if it costs 16c/kw-hr =34887/0.16
=218043.75 kw-hrs
Therefore the power created by the power station = 1.05*218043.75
=228945.94 kw-hrs
Note: the 1.05 is due to a power lose of around 5% in the wires due to transportation
Vacuum
Maximum power cost occurs in 2010 and is $21991
Therefore power used if it costs 16c/kw-hr =21991/0.16
=137443.75 kw-hrs
Therefore the power created by the power station = 1.05*137443.75
=144315.94 kw-hrs
Note: the 1.05 is due to a power lose of around 5% in the wires due to transportation
Low pressure
Power consumption per pump = 4.9 kw-hrs
Therefore total power = 4.9*1600
=7840 kw-hrs
Where 1600 is the number of lots.
Therefore power created at power station =1.05*7840
=8232 kw-hrs
Note: the 1.05 is due to a power lose of around 5% in the wires due to transportation.
Note: any power relating to the operation of a pump station, if one was required has been ignored
Carbon dioxide Emissions
The carbon dioxide emissions are found using the equation
CO2 emissions = FC*EF*FOC*CO2/C*CC
Where CO2=Carbon Dioxide emissions (Gg)
FC= Fuel Consumption (toe)
EF=Emission Factor (tonnes of carbon/TJ)
Fraction of carbon Oxidised
CO2/C= 44/12
CC= Conversion coefficient (41.868 toe/1000 TJ)
Hard coal, Petroleum coke, Asphalt Power Stations
Rose street report
Emission factor = 25.8 tonnes of carbon/TJ
Fraction of carbon oxidised = 0.98
Gravity
Fuel consumption = 228945.94 kw-hrs = 19.68 toe
Therefore Carbon dioxide emissions = 19.68*25.8*0.98*(44/12)*41.868/1000
= 76000 tonnes
Vacuum
Fuel consumption = 144315.94 kw-hrs = 12.41 toe
Therefore Carbon dioxide emissions = 12.41*25.8*0.98*(44/12)*41.868/1000
= 48170 tonnes
Low Pressure
Fuel consumption = 8232 kw-hrs = 0.71 toe
Therefore Carbon dioxide emissions = 0.71*25.8*0.98*(44/12)*41.868/1000
= 2750 tonnes
Lignite
Emission factor = 27.6 tonnes of carbon/TJ
Fraction of carbon oxidised = 0.98
Gravity
Fuel consumption = 228945.94 kw-hrs = 19.68 toe
Therefore Carbon dioxide emissions = 19.68*27.6*0.98*(44/12)*41.868/1000
= 81720 tonnes
Vacuum
Fuel consumption = 144315.94 kw-hrs = 12.41 toe
Therefore Carbon dioxide emissions = 12.41*27.6*0.98*(44/12)*41.868/1000
= 51530 tonnes
Low Pressure
Fuel consumption = 8232 kw-hrs = 0.71 toe
Therefore Carbon dioxide emissions = 0.71*27.6*0.98*(44/12)*41.868/1000
= 2950 tonnes
Oil
Emission factor = 20 tonnes of carbon/TJ
Fraction of carbon oxidised = 0.99
Gravity
Fuel consumption = 228945.94 kw-hrs = 19.68 toe
Therefore Carbon dioxide emissions = 19.68*20*0.99*(44/12)*41.868/1000
= 59820 tonnes
Vacuum
Fuel consumption = 144315.94 kw-hrs = 12.41 toe
Therefore Carbon dioxide emissions = 12.41*20*0.99*(44/12)*41.868/1000
= 37720 tonnes
Low Pressure
Fuel consumption = 8232 kw-hrs = 0.71 toe
Therefore Carbon dioxide emissions = 0.71*20*0.99*(44/12)*41.868/1000
= 2160 tonnes
Natural Gas
Emission factor = 15.3 tonnes of carbon/TJ
Fraction of carbon oxidised = 0.995
Gravity
Fuel consumption = 228945.94 kw-hrs = 19.68 toe
Therefore Carbon dioxide emissions=19.68*15.3*0.995*(44/12)*41.868/1000
= 46000 tonnes
Vacuum
Fuel consumption = 144315.94 kw-hrs = 12.41 toe
Therefore Carbon dioxide emissions=12.41*15.3*0.995*(44/12)*41.868/1000
= 29000 tonnes
Low Pressure
Fuel consumption = 8232 kw-hrs = 0.71 toe
Therefore Carbon dioxide emissions = 0.71*15.3*0.995*(44/12)*41.868/1000
= 1660 tonnes
Carbon dioxide emissions due to construction
Assumptions
50 min hour
Bucket payload =1.5 m^3
150 cycles per hour
Removal rate = 1.5*150*0.83 = 186.75 m^3/hr
Fuel consumption rate = 25 L/hr
Fraction of carbon converted = 10514.73 g/L
Emission factor = 0.99
Gravity
Volume of soil removed = 44095.5 m^3
Total fuel consumption = 44095.5/186.75*25 = 5903 L
Carbon Dioxide emissions = 5903*0.99*10514.73*(44/12) = 225.3 Tonnes
Vacuum
Volume of soil removed = 23620.63 m^3
Total fuel consumption = 23620.63/186.75*25 = 3162.1 L
Carbon Dioxide emissions = 3162.1*0.99*10514.73*(44/12) = 120.7 Tonnes
Low Pressure
Volume of soil removed = 26791.89 m^3
Total fuel consumption = 26791.89/186.75*25 = 3586.6 L
Carbon Dioxide Emissions = 3586.6*0.99*10514.73*(44/12) = 136.9 Tonnes
