Ammonia Is Sticky So Measuring It Is Tricky

Wastewater treatment plants (WWTPs) are a major source of greenhouse gas (GHG) emissions, including methane (CH₄) and nitrous oxide (N₂O).

Many studies have looked at emissions of potent greenhouse gases from treatment plants, however little attention has been paid to ammonia (NH₃).

Ammonia that ends up in the atmosphere comes mainly from nitrogen losses from the soil following fertiliser application and from animal waste.

Ammonia emissions from sewage treatment plants are often assumed to be negligible because the domestic wastewater going into treatment plants has a low ammonia concentration (20-50 milligrams of nitrogen per litre of wastewater).

However, what happens during the treatment process can change this concentration, and these treatment plants are a potentially large source of ammonia emissions.

Ammonia gas is a secondary greenhouse gas. It doesn’t directly cause warming, but can be transformed into another nitrogen-based gas, nitrous oxide, a greenhouse gas 265 times more potent than carbon dioxide over its 100-year life span.

Atmospheric ammonia can also be absorbed by plants or crops in a process called ‘dry deposition’, or it can be ‘scavenged’ by water droplets in the air (in rainfall or fog) and deposited on landscapes as particulate matter (PM2.5 and PM10).

These ammonium-containing aerosols can impact human health (they are a known factor in lung cancer and eye diseases), and hurt ecosystems by forming acid soils, polluting waterways and reducing biodiversity.

Finally, the deposited ammonia can be transformed in the soil and reemitted to the atmosphere as nitrous oxide.

Sludge drying pans (SDPs) are a common intervention used for sludge dewatering, particularly in regions with ample land, favourable climatic conditions and in some developing countries. Their popularity stems from the cost-effectiveness, simplicity and low energy needs.

A number of Australian sewage treatment facilities use sludge drying pans (SDPs) as the main way to dewater and treat residual sludge.

The sludge that comes out of anaerobic digesters is transferred to open-air SDPs. In these shallow, open-air pans, sludge is spread out on an impermeable base so that water is lost primarily through evaporation.

Free liquid is drained away from the pan for further treatment, leaving a semi-solid material that is dried by the sun and wind for easier storage, re-use or disposal (these nutrient rich ‘biosolids’ are highly sought by some sectors like agriculture).

During this process, some of the nitrogen in the sludge can be turned into ammonia and released as a gas.

However, the magnitude of these emissions over time and how the operation of the drying pans – like how often the sludge is mixed and what other materials are added – impacts these emissions, was unclear.

This is because ammonia is difficult to accurately measure. Ammonia is ‘sticky’ – it is highly reactive and readily absorbs to any surface, and often the concentration (as a gas) will change between collection and measurement.

Existing techniques, like pumping air through an absorbent collector, are labour-intensive and not practical in the field over extended periods of time.

In our recent study, published in Nature Water, we developed a micrometeorological technique that combines a series of simple measurements, including gas concentrations from upwind and downwind of the source, meteorological wind information and a map of the source and equipment locations to monitor ammonia from a sludge drying pan.

This collaboration was led by the University of Melbourne, University of Queensland and Melbourne Water.

A device, called an open-path Fourier transform infrared spectrometer, sends a beam to a reflector on the other side of the pan and then uses the returned beam to simultaneously measure a suite of trace gases, including ammonia, nitrous oxide, methane, carbon dioxide, carbon monoxide and water vapour.

This technique does not require complex setups like pumps and heated sample inlets that are impractical in the field.

This study found substantial ammonia emissions during the sludge drying process.

This is important because sludge drying pans are widely used in many countries due to their cost-effectiveness and simplicity.

While not as high as the ammonia emissions from agricultural fertilisers, the ammonia emissions measured in the drying pans were notably higher than other known sources of ammonia, like livestock waste and plant-based emissions.

This suggests that the contribution of wastewater treatment plants and especially sludge drying pans to global ammonia emissions may be substantial and largely overlooked.

As all parts of the economy work towards not only achieving net zero emissions, but also maintaining our human and environmental health, wastewater treatment facilities should take actions to reduce the ammonia losses to the atmosphere.

These actions could include switching to alternative sludge dewatering approaches, like centrifugation, belt filter presses and thermal drying.

Additionally, treatment plants could install ‘post aerobic treatment units’ that remove the ammonia from the sludge. These units also offer additional benefits, including enhanced pathogen removal, heavy metal leaching and breakdown of micropollutants.

These alternatives can be costly and have their own environmental impacts, so proactively understanding the full range of impacts supports ongoing operations, decision-making and design.

Understanding the scope of these emissions is the first step – the next is developing low-cost, innovative measures to reduce ammonia emissions from wastewater treatment plants.

This study was a collaboration between the University of Melbourne, the University of Queensland (UQ) and Melbourne Water. Contributors included Dr Dilini Seneviratne, James Lloyd and Dr Pieter De Jong from Melbourne Water Corporation, Dr Mei Bai and Professor Deli Chen from the University of Melbourne, and Dr Zhiyao Wang and Professor Liu Ye from UQ.