8.7 What are the environmental risks and mitigation measures?

Sludge burial and disposal present several potential environmental impacts, including effects on land use, air quality, public health, aesthetics, and wildlife. Operational considerations play a crucial role in mitigating these impacts, focusing on key aspects such as leachate, run-off, and emissions. The severity of these factors is closely tied to the level of pre-treatment the sludge undergoes before disposal.

Runoff

Runoff is rainwater or other liquid that drains over the land and runs off the land surface, contaminating nearby surface waters. Runoff from an active sludge burial site might be contaminated. If a contamination risk exists, it must be collected and disposed of.

Leachate

Leachate is fluid from excess moisture in biosolids or rainwater percolating through the disposed biosolids. Leachate from a burial site may transport nitrates, metals, organics, and pathogens to the groundwater. If sludge is insufficiently treated prior to disposal, the disposal site requires a liner system.

Lining and a sand barrier at the bottom of the pit or trench are recommended when contaminated leachate is expected, or the site conditions are not matching the recommendations.

Emissions

Emissions cannot be avoided as long as sludge decomposes. This anaerobic process produces gases (methane CH₄, carbon dioxide CO₂, hydrogen sulphide H₂S, etc.) that must be collected to avoid its uncontrolled release into the air.

The level of emissions will depend on the preceding treatment that the sludge has undergone. Ideally, the sludge undergoes controlled anaerobic digestion (in a biodigester) that enables us to collect and use the gases that will be produced.