Fugitive Emissions in Wastewater Treatment: A Climate Threat

Fugitive emissions in wastewater treatment are often overlooked in both cities’ and utilities' efforts to reduce their climate impact. Apart from carbon dioxide, the wastewater industry deals heavily with methane and nitrous oxide emissions, which significantly impact global warming. Addressing these emissions alongside point-source emissions is critical, as their reduction significantly contributes to global climate goals.

Understanding Fugitive Emissions in Wastewater Treatment

The term "fugitive emissions" varies slightly across industries but generally refers to unintentional releases of gases and pollutants into the atmosphere. Unlike point-source emissions, which have a clear origin, such as an exhaust stack or a vent, fugitive emissions are challenging to control because they typically escape from multiple, sometimes unexpected, locations. Activities like gas flaring, though they involve emissions coming from a specific and known source, are also often cited as fugitive. This is because the gas is not used for its intended purpose but is released into the atmosphere.

Within wastewater treatment, fugitive emissions typically point to the gases that sporadically escape during the various biological, chemical, and mechanical processes that break down organic matter in sewage and the subsequent handling of these processes' byproducts.

Estimating the volume of fugitive emissions from wastewater treatment plants has been challenging for researchers, given the variability in plant design and operation. Recent studies indicate that they are notably higher than previously thought. For instance, a 2023 Princeton study that assessed over 60 treatment plants in the United States has shown that these facilities' methane emissions are approximately 1.9 times greater than earlier estimates. This finding is significant because the current estimate for the wastewater industry’s contribution to global methane emissions is already 7% of all human-caused emissions. Research has also indicated that smaller-scale, decentralised treatment facilities, sewer systems, and sludge transport are all significant sources of fugitive emissions but that these areas are understudied.

Types, Sources, and Environmental Impact of Fugitive Emissions

Several types of fugitive emissions come from wastewater and sludge treatment processes, each with specific sources within the treatment flow. The three most concerning emissions within the industry are methane, nitrous oxide, and carbon dioxide, all of which are emitted in significant quantities, contributing to the acceleration of global warming.

• Methane

Methane, a greenhouse gas (GHG), has a global warming potential 84 to 87 times greater than carbon dioxide over a 20-year period. This gas has a shorter atmospheric lifetime of 12 years, much shorter than carbon dioxide’s, which can last thousands of years. However, this also means methane is methane is much more destructive than CO₂ during its short lifetime.

In wastewater treatment, methane is the primary byproduct of the anaerobic digestion of organic matter in sewage sludge. Many medium to large treatment plants use anaerobic digestion technology to treat and stabilise sludge before it is landfilled, reused as a soil product, or incinerated.

Anaerobic digestion is crucial for reducing sludge volume and producing biogas (primarily methane and carbon dioxide), which utilities can harness for energy. However, fugitive methane emissions can occur when digesters' gas capture systems are incomplete or poorly maintained or during biogas conversion and upgrading. Methane leaks may also occur from gas pipes and even during dewatered sludge handling.

Methane is also produced when sewage sludge is landfilled and decomposes in low-oxygen environments. If these landfills do not have adequate gas collection systems, methane escapes into the atmosphere.

• Nitrous Oxide

Nitrous oxide, another potent GHG, is primarily released during nitrogen removal processes in wastewater or sewage sludge treatment, particularly during denitrification and, at times, nitrification. Nitrogen removal is an important step in wastewater treatment as high levels of nitrogen in wastewater effluent can cause eutrophication. This gas, though often released in smaller amounts compared to methane and carbon dioxide, has a global warming potential over 270 times that of CO₂ over a 20-year period.

N₂O emissions are particularly common in aerobic digestion processes with nitrogen-rich sludge and can also arise if nitrates are present in anaerobic digesters. Additional N₂O emissions may be generated during sludge dewatering and drying, as ammonia is sometimes oxidised into nitrous oxide under certain conditions.

• Carbon Dioxide

Although CO₂ is a less potent greenhouse gas than methane or nitrous oxide, its sheer volume contributes significantly to the carbon footprint of sludge treatment. Carbon dioxide emissions can originate from both aerobic and anaerobic digestion processes. In aerobic digestion, oxygen is used to stabilise sludge, resulting in CO₂ as a byproduct. Carbon dioxide can also be emitted when biogas (comprising primarily methane and CO₂) is combusted for energy recovery, particularly if not all emissions are captured effectively.

Other fugitive emissions that occur throughout the treatment process include Volatile Organic Compounds (VOCs) such as aldehydes and ketones and odorous compounds such as hydrogen sulphide (H₂S) and ammonia (NH₃). These are often released in facilities with open-air systems and during dewatering, drying, or storage.

Strategies to Mitigate Fugitive Emissions in Wastewater Treatment

There are several ways to reduce fugitive emissions in wastewater treatment. A key action modern facilities can make is to evaluate their sludge treatment and biosolids handling strategies, as these are proven sources of fugitive emissions. Secondary treatment or activated sludge systems (aerobic processes) can also produce carbon dioxide and nitrous oxide. However, anaerobic digestion and sludge handling are key sources of methane and all the previously mentioned gases. Some of the mitigation strategies that can be implemented by wastewater treatment facilities on-site are the following:

• Enhanced Gas Capture and Biogas Utilisation

Investing in robust biogas capture systems can reduce methane emissions significantly. Ensuring that anaerobic digesters, storage tanks, and gas conveyance systems are well-sealed minimises leaks. Furthermore, facilities can utilise captured biogas for energy production, offsetting energy consumption from external sources and reducing CO₂ emissions in the process. Energy recovery systems from biogas, whether it involves biogas upgrading or cogeneration, should be well-designed to minimise biogas flaring.

• Process Optimisation

Optimising sludge treatment processes can reduce the formation of fugitive emissions. For instance, anaerobic digestion can be optimised by controlling feedstock composition and retention times to enhance biogas yield and minimise methane loss. Similarly, improved control over oxygen levels in aerobic systems can limit CO₂ and nitrous oxide emissions. Monitoring sludge temperature, pH, and other factors in real-time helps maintain optimal conditions for gas production and capture.

• Improved Monitoring and Maintenance

Accurate monitoring of fugitive emissions is essential to identify emission sources and quantify their impact. Gas detection sensors and infrared cameras can help detect methane leaks, while regular maintenance ensures that all systems function correctly. Implementing a continuous monitoring system can provide real-time data, allowing facilities to respond promptly to equipment malfunctions or unexpected emissions.

• Use of Covers and Enclosures

Covers on open tanks, drying beds, and other sludge handling units can significantly reduce emissions of VOCs, odours, and methane. Enclosing equipment reduces the release of emissions into the atmosphere and allows for more effective capture and treatment. Additionally, implementing odour control systems, such as scrubbers or activated carbon filters, can further control odorous compound emissions.

Reducing Fugitive Emissions in the Wastewater Industry

For wastewater treatment plants of a specific size, anaerobic digestion will continue to be an essential tool for utilising methane as a renewable energy resource, stabilising sludge, and lowering the environmental impact of biosolids downstream. Using biogas retrieved from anaerobic digestion and optimising its capture and storage is an important step treatment facilities can take to lessen greenhouse gas emissions, including fugitive emissions.

In recent years, researchers have increased efforts to quantify fugitive emissions in urban wastewater facilities and recommend reduction strategies. There is also a growing focus on comparing the greenhouse gas emissions of different sludge treatment processes and disposal methods. This expanding body of research underscores fugitive emissions as a significant environmental concern for the wastewater industry and highlights how their mitigation aligns with net-zero goals.

Want to discover the most effective strategies for your wastewater treatment plant? Visit our literature section for in-depth resources and insights.