SA Water receives a double-benefit from on-site generation underpinned by biogas – it buys less electricity and gas and sells to the grid when power prices spike.
The utility's Bolivar wastewater treatment plant now meets about 87% of its electricity needs using biogas, according to its manager of energy strategy and water trading, Andrew Jackson.
Bolivar is South Australia's largest treatment plant and processes about 60% of Adelaide's wastewater.
Thanks to three 2.4MW reciprocating engines at the plant, which were installed in 2015 at a cost of $26 million, Bolivar last year generated 27GWh from biogas (87% of on-site power requirements) and another 6GWh from natural gas, while importing 4GWh of electricity.
That totals 37GWh and the site only used 31GWh, with the excess of 6GWh comprising on-site generation that SA Water sold back into the grid when price spikes occurred.
The ability to sell into the grid at times when power prices are high is critical, as the utility buys electricity and gas on wholesale energy markets.
That leaves it exposed to price changes that can fluctuate from negative $1,000 per MWh, when there is low electricity demand and high wind generation, up to as much as $14,000 at times of extreme peak demand.
"We manage Bolivar to take advantage of those events," Jackson told Footprint.
When power prices are high, it increases on-site generation using natural gas and exports the electricity to partially offset utility-wide electricity costs.
Similarly, when the grid price is very low "we might choose to import electricity at those times as opposed to using the engines to generate electricity", something it can do because it has some capacity to store biogas for future use.
SA Water also keeps a close eye on power prices when timing energy-intensive activities – particularly pumping to its reservoirs, which accounts for between 60% and 70% of its total energy use.
As Jackson points out, SA Water has "a lot of control over when that pumping occurs" and can generally restrict the activity to times when electricity prices are low.
Co-digesting with trade waste
Jackson added that biogas production is likely to soon become more efficient at Bolivar because the utility plans to co-digest sewage at the plant with high-strength fats and greases.
The move has been approved internally, and now needs external clearances, Jackson said.
Dosing sewage with the right mix of other organic waste streams can significantly boost biogas production (see background), something that SA Water already knows first-hand from its Glenelg treatment plant, where it adds dairy effluent and trade waste to sewage sludge.
