Energy use goes up when you upgrade a wastewater treatment plant, right?
That's been our experience to date, and yet the Energy Efficiency Team has been watching the monthly energy use at Burwood Beach Wastewater Treatment Works (WWTW) trend down as a result of the upgrade work that is reaching completion under the Alliance.
First some statistics:
- Burwood Beach WWTW is our largest treatment plant both in terms of volumes treated and energy use.
- It is second only to Tomago Water Pump Station in the amount of energy it consumes - 5400 megawatt hours (MWh) in 2009/2010.
- The Alliance Upgrade at the plant was driven partly by growth but mainly to improve operating stability, to reliably achieve its operating licence requirement. Both these actions would normally drive up energy use, but Hunter Water and the Alliance were able to capitalise on the energy saving opportunities at the plant and make good 'whole of life' decisions so that the energy use actually went down.
There are three energy stories to be told about Burwood Beach WWTW:
Secondary Pump Station
The Secondary Pump Station (SPS) takes wastewater from the primary plant to the top of an activated biofilter tower, which is the first stage of secondary treatment at Burwood Beach WWTW. Until the upgrade, the station's three pumps had separate speed controls, one for each pump, and due to a lack of coordination between the pump controls, it was common for one or two of the pumps to be running but not producing flow.
The Alliance Project included increasing capacity of the SPS with three new pumps and new control hardware to improve the reliability of the station. A new control method was designed from scratch and meets the demanding requirements on the pump station whilst using the least amount of energy. The chosen scheme uses an average of 80kW to 100kW less power than the old control scheme. This is leading to a saving of 830 MWh of energy consumption each year, equating to $102,000 or 740 tonnes of greenhouse gas (GHG) emissions.
Improved aeration was identified as key to the reliable function of the plant. Prior to the upgrade, the plant did not have sufficient capacity to control the dissolved oxygen levels in the aeration tanks making it impossible to tune the biological processes of the plant. The Alliance Upgrade included a significant increase in the aeration capacity to allow operators to regain control which would decrease energy usage at the plant.
When highly efficient turbo blowers were considered to replace the existing blowers, the smaller footprint allowed them to meet the extra airflow without the need for a new blower building. This cost saving, along with the reduced electricity usage, meant the lowest whole of life option was to meet the air demand with four new turbo blowers. The energy saving from the new blowers is estimated to be 175 MWh of energy per annum over the alternative positive displacement blowers. The control scheme for the new blowers has been designed to optimise energy performance and is expected to save 50 MWh each year compared to a conventional control scheme.
In total both aeration options will save $28,000 or 200 tonnes of GHG emissions each year and have resulted in aeration treatment at Burwood Beach achieving world's best practice in energy efficiency.
Primary Pump Station
The Primary Pump Station (PPS) is responsible for raising all the inflow to the plant from the deep inlet pipe to the level of the screen house, a lift of around 16 metres. The control scheme of the PPS, similar to the SPS, was wasting energy by running pumps at speeds that did not actually produce any flow. As a stop gap measure the energy efficiency team worked with plant operator Ray Davis to re-tune the settings in the pump control scheme to minimise the wasted energy. By monitoring the overall plant power we were able to measure the energy difference our changes had made. The data showed that the energy usage at the plant dropped by an average of 670 kWh per day. Over a year this amounts to 245 MWh or $31,000 or 218 tonnes of GHG emissions. A full overhaul of the control scheme, as per the Secondary Pump Station, is planned for this year which is expected to yield similar savings again.
Burwood Beach WWTW is an excellent example of how upgrades present a rare opportunity to fix energy inefficiencies of the past. It is often uneconomic and wasteful to change out existing equipment in order to achieve an energy saving. Asset upgrades are the best and often the only opportunity to lock in energy efficiency and so we need to keep our eyes open to the low energy options. Hunter Water's ever improving management of energy is helping us to recognise and seize those opportunities.