New Zealand is committed to reducing emissions and the Climate Change Response (Zero Carbon) Amendment Bill has the potential to drive a change in the thinking and economy by setting a target of net zero emissions.
It’s a period of great change for industries that have traditionally not had to move at pace, and the wastewater industry is no exception. Reducing carbon, lowering energy consumption and adapting to climate change are front-of-mind challenges for the organisations I work with.
However, we’re potentially discharging more than we’re saving in reduction strategies. I say potentially because we simply don’t know – currently we’re only estimating, not measuring.
To start, we need to understand the sources of the emissions. For wastewater there are three types:
- Embodied - concrete steel and the installation of assets
- Operational - electricity, fuels
- Fugitive - waste gases such as methane and nitrous oxide.
The cycle of humans eating food, burning energy and leaving waste as a result is a natural cycle that’s in equilibrium. As such, carbon dioxide produced directly from a wastewater treatment system is a biogenic source.
However, there are many other contributions from the wastewater systems that aren’t natural, including emission gases such as nitrous oxide and methane.
In every wastewater system there is biological transformation of organic material and nitrogen. Under some conditions this results in methane and nitrous oxide – these are the conditions we need to focus on.
Some of our reduction strategy could also be contributing to emissions and so-called “green technologies” may not be all that sustainable.
For example, we’re seeing a lot of interest in generating energy from waste. This is a great use of otherwise waste material and we produce lots of methane from it, and then power. Most of the gas is captured in the main reactor tank and taken and burnt, either to heat the process or to generate power. Using this power to offset the power from the grid makes great financial sense, but is it saving carbon? The average New Zealand grid is 0.11 kgCO2/kwh (2018) whereas burning biogas may be more with values of 0.05 to 0.45kgCO2/kwh being reported. Typically, this is around three times more than our grid power.
Also consider that the microscopic methane-generating bacteria carry on producing methane after they leave the tank – are we considering what happens to that methane and other sources?
- a gas engine or boiler may have around 0.5 -1% methane in the exhaust
- sludge dewatering releases methane to atmosphere
- tanks and pipes leak
- bacteria continue treating in the sludge stock pile, which goes to land (or landfill) where more methane is produced.
- All are lost emissions to atmosphere.
Being conservative, this is another 10%+ losses of methane. It’s an issue because methane has 84 times the global warming potential of CO2 over 20 years and 1 kg of methane has the carbon equivalent of 84 kg of CO2. That’s the equivalent impact of using over 764 kwh of NZ grid power.
Even the humble septic tank needs to be considered. Over 500,000 New Zealanders use septic tanks, a massive number of small anaerobic systems. Using IPCC guidelines, an estimate of emissions for this number gives around 9 tonnes of methane every day, 3,285 tonnes per year, over 275,000 tonnes of CO2-equivalent per year. As a technology its very low in energy so has low operational emissions, but is very high in fugitive emissions.
Nitrous oxide (N2O) is another gas that’s not fully understood. Even a little N2O has a significant effect – it has 268 times the global warming potential of CO2 and will persist for over 120 years.
There are processes being sold as carbon friendly on the premise they reduce power, but they also produce conditions that favour N2O production.
No laughing matter
Low oxygen in an aerobic treatment plant can produce N2O, so can a denitrifying plant with low carbon. Simultaneous denitrification (a short cut where conditions do not permit full oxidation of ammonia to nitrate and then convert the intermediate nitrite to nitrogen gas,) saves on aeration energy, but the lower oxygen conditions favour N2O production. Data is scarce but what’s available shows this can be substantial.
A recent comparison of a conventional and simultaneous denitrification plant using published N2O emissions showed that a 12% reduction in energy from the latter was offset by around six times the impact from N2O emissions. Essentially, what was sold as “carbon friendly” was slightly cheaper to run but had four times the total emissions.
Another challenge is that we want wastewater to go to land. However, one study reports that 25% of all nitrogen in wastewater put to pasture was converted to N2O in the soil and another US study showed 50% of nitrogen in a wetland was converted to N2O.
Planting trees is a good thing for reduction in carbon dioxide, but it does absolutely nothing to combat N2O levels in the atmosphere. Trees don’t absorb N2O and molecules can take decades or centuries to be naturally removed. Our emissions will be around for our great, great, great, great, great grandchildren and we need facts to determine the best approach to take.
Currently New Zealand isn’t measuring its methane or N20 emissions in the water industry, so how can we be making informed, sustainable decisions?
Andrew Springer, Technical Principal Wastewater Engineer, has over 30 years in the water industry and a wide range of experience as scientist, engineer, client, contractor and designer.
The views expressed are the opinions of subject matter experts and do not necessarily reflect those of WSP.