The Nigg Wastewater Treatment Works faced several challenges before its recent upgrades. Biogas production was inefficient, producing only 250 cubic meters of gas per tonne of dry solids.

The facility’s old compressor mixing system frequently caused pipe blockages, leading to a loss of 45% of digester capacity due to grit and compacted sludge. Energy consumption was high, with standard mixing systems requiring 17 watts per cubic meter.

Combined Heat & Power (CHP) units were underutilized, operating at only 56% efficiency. Additionally, the facility had limited resilience for processing extra sludge and lacked the ability to co-digest organic materials efficiently.

To overcome these issues, Scottish Water implemented several targeted upgrades. The Cambi Thermal Hydrolysis Process (THP) was modernized to improve sludge treatment efficiency, and a new externally-mounted digester mixing system from Landia was installed. The Landia system provided robust, reliable mixing while requiring significantly less energy, running for less than 12 hours daily and consuming only 7 watts per cubic meter.

Mixing times were optimized to 28 minutes per hour for maximum gas yield. Additionally, Nigg conducted a trial to co-digest organic residues from local distilleries and breweries, which increased methane levels and showcased the potential for sustainable, circular processes. The upgrades were supported by ongoing collaboration between Scottish Water, Cambi, and Landia to ensure performance optimization.

The upgrades delivered remarkable improvements. Biogas production increased to 400–420 cubic meters per tonne of dry solids, a significant boost of over 30%. Energy efficiency improved dramatically, with CHP utilization rising from 56% to 73%, allowing the facility to save £0.5 million annually on fuel oil and export surplus energy to the National Grid.

CO2 emissions were reduced by more than half, and the digester mixing system now operates reliably without blockages or maintenance issues. The successful co-digestion trial resulted in methane levels increasing from 60% to 64% and saved 58 tonnes of carbon. Based on Nigg’s success, Scottish Water is now planning six additional sludge-to-energy facilities, taking a major step toward achieving net-zero carbon emissions by 2040.