Case Study SA-JW1
AERATION BLOWER EFFICIENCY
AERATION BLOWER EFFICIENCY
Ref / Case Study SA-JW1 / Responseinformation,descriptionandremarks1 / Location: / Bushkoppies Wastewater Treatment Works (near Soweto, Gauteng), Johannesburg Water.
High density urban population.
2 / Sector: / Wastewater
3 / WorksOwnerorOperator: / The plant is owned by the City of Johannesburg (CoJ) Metropolitan and operated by Johannesburg Water on behalf of CoJ. The final effluent from the plant is regulated in terms of the discharge license issued to the plant by the Department of Water Affairs.
4 / Size: / The plant has a design capacity of 200 ML/d, but is being upgraded to 250ML/d. The existing activated sludge system is equipped with fine bubble aerators, but these are due for replacement. The option of replacing the existing fine bubble diffusers with a higher density of diffusers has been chosen.
The plant is situated in the south western area of Johannesburg and so the majority of the sewage is domestic, but there is a yeast factory which discharges to the works. The yeast factory effluent assists in fermentation, but there are also intractables present which result in some treatment problems.
5 / EnergyProvider: / Power for the blowers which produce the air is provided by the national electricity agency, ESKOM, The plant requires 2400kW/h.
At present the plant has 3 electrical blowers rated at 2400 kW in total (800 kW each) and 1 diesel blower. The upgrade will include the installation of an additional electrical blower (also 800 kW), but the new blower has an 85% efficiency rating as opposed to 70% for the old blowers. The new blower will be larger and it will therefore be used more in order to further increase the energy savings.
6 / Process: / This is an activated sludge process, so air diffusion is a physical procedure but the process is biological. The first stage of the project involves the installation of a larger, more energy efficient blower, which will provide around half of the air requirement. The existing blowers will provide the balance and will also serve as standby blowers.
7 / Component: / This case study involves only the activated sludge process.
8 / Motivation for the case study: / The energy problem addressed in this case study deals with the energy savings achievable through using a more energy efficient blower andoptimising the use of this blower.
9 / Process/Plantchanges: / The plant changes are mainly mechanical, involving replacement of fine bubble diffusers and some additional pipework. Bushkoppies activated sludge plant is 140 000m3 in capacity and the retention time is 13 to 14 hours treating 200Ml/d. The current fine bubble diffusers need replacing and they are being replaced with new, more efficient Teflon coated synthetic rubber discs with micro slots at a higher density than is currently the case. This will allow for the treatment of an additional 50 ML/d without the need for extending the activated sludge basin. For the purposes of this case study however, only the changes to the blowers are being considered, so this involves only a mechanical change (new blower), since the installed kilowatts at the plant are adequate for the replacement system.
10 / Civil/PhysicalChanges: / No civil or physical changes will be required as the new blower will fit into the existing blower house.
11 / OperationalChanges: / There will be no substantial changes to the operation or the skills levels etc. Obviously the plant operational staff will have to optimize the system once it has been commissioned, but otherwise, operation is expected to be very similar to that at present.
12 / RisksandDependencies: / The risk in not implementing these changes is that the existing plant will fail, the effluent will not conform to the standards as determined in the license and the plant will run out of capacity.
The risk in terms of the technology being used is low, since it is tried and tested and fine bubble aeration has been used at the plant for some years already.
13 / Implementation: / This was a consulting appointment, so involved preparation of tender documents and award of a contract for the installation of the new diffusers. There was no operational contract, since the current staff is capable of operating the upgraded system.
14 / EnergyEfficiencygains: / There is a significant cost savings in terms of civils, since the replacement of the old diffusers with a higher density of new diffusers has avoided the need for costly civil upgrades. The capacity of the plant is being increased by 25% without the need for extending the activated sludge basin. However, the savings in terms of energy used by the blowers will bearound 7,5% for the first stage of this project. The existing blowers were recently refurbished and therefore in order to recoup the cost of the refurbishment, replacement of the blowers is being phased in. For the first stage, a new, larger blower is being installed and will provide around half of the air requirement, the balance being provided by the existing smaller blowers. The new blower has an efficiency rating of 85%, while that of the existing blowers is 70%, so the first stage will see an energy saving of 7%, but once all the blowers have been replaced, this will increase to 21%. It has been estimated that the initial new blower will allow an energy savings of 1 050MWh/annum (4.2kWh/m3) and once all the blowers have been replaced the potential energy saving will be 3154MWh/annum (12.6kWh/m3) .
15 / Cost/Benefitanalysis: / The payback period has been estimated to be 10 years. However the project is saving on costly civil extensions. It would not have been possible to achieve treatment in an activated sludge basin of this size using surface aerators without increasing the size of the basin.
16 / Projectreview: / It would not be possible to improve on this within the current constraints, since the latest technology is already being used.
However it should be noted that this project trades off efficiency of oxygen transfer against capital costs, since as the oxygen concentration in the activated sludge increases, so the α factor decreases (i.e. transfer efficiency of oxygen into the mixed liquor decreases).
17 / Confidencegrade: / HIGH. This project is still in the planning stage, but the technology has been used before and so there is a high level of confidence in this project.
18 / Reference/Information source / David J Nozaic,