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The alarming output of waste activated sludge (WAS) from industries requires proper management routes to minimize its impact on the environment during disposal. Pyrolysis is a feasible way of processing and valorizing WAS into higher-value products of alternate use. Despite extensive research into the potential of WAS through pyrolysis, the technology's long-term viability and environmental impact have yet to be fully revealed. In addition, the environmental effects of utilizing different pyrolysis atmosphere (N 2 or CO 2) has not been studied before, although benefits of CO 2 reactivity during pyrolysis have been discovered. This study evaluates the process's environmental impact, carbon footprint, and bioenergy yield when different pyrolysis atmospheres are used. The global warming potential (GWP) for a functional unit of 1 t of dried WAS is 203.81 kg CO 2 eq. The heat required during pyrolysis contributes the most (63.7%) towards GWP due to high energy usage, followed by the drying process (23.6%). Transportation contributes the most towards toxicity impact (59.3%) through dust, NO x , NH 3 and SO 2 emissions. The initial moisture content of raw WAS (65%) greatly impacts overall energy consumption and environmental impact. Pyrolysis in an N 2 atmosphere will result in a higher overall bioenergy yield (833 kWh/tonne) and a lower carbon footprint (−1.09 kg CO 2 /tonne). However, when CO 2 was used, the specific energy value within the biochar is higher (22.26 MJ/kg) due to enhanced carbonization. The carbon content of gas derived increased due to higher CO yield. From an energy perspective, the current setup will achieve a net positive bioenergy yield of 561 kW (CO 2) and 833 kW (N 2), where end products like biochar, bio-oil and gas can be used for power production. Despite the energy-intensive process, microwave pyrolysis has excellent potential to achieve a negative carbon footprint. The biochar used for soil amendment served as a good carbon sink. The utilization of CO 2 as carrier gases provides a pathway to utilize anthropogenic CO 2 , which helps reduce global warming. This work demonstrates microwave pyrolysis as a negative emission, bioenergy-producing approach for WAS disposal and valorization. [Display omitted] • CO 2 pyrolysis has a lower bioenergy generation potential and higher GWP. • The heat required for drying and pyrolysis contributes toward GWP. • Transportation has a significant impact on AP and HTP. • The carbon sequestration of products can offset the carbon footprint of the process. • Energy sustainability can be achieved by harvesting bioenergy from products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microwave pyrolysis successfully reduce the mass of waste activated sludge by 80 wt%. • CO 2 pyrolysis improves CO generation in the gaseous product (up to ∼ 50 vol%). • CO 2 pyrolysis produce biochar with higher energy density profit (11–14%). • N 2 pyrolysis improves H2 generation in the gaseous product (up to ∼ 29 vol%). • Up to 83.2% of the energy stored within the waste can be retrieved in bioenergy form. Pyrolysis is a green and effective method for converting various waste streams into products with bio-energy potential. Waste activated sludge (WAS) from industries requires post-treatment before disposal and will cause serious pollution if not managed properly. Pyrolysis is a viable method for converting WAS into higher-value bio-products. This is the first study to use a lab-scaled microwave reactor to analyse WAS from a food processing and manufacturing company's wastewater treatment plant. The goal is to compare bioproduct formation under various N 2 and CO 2 atmospheres in order to analyse the WAS waste-to-bioproduct transformation pathway. Result revealed that CO 2 pyrolysis on WAS tends to: 1) increase water formation (∼19 wt%), 2) produce biochar with higher energy density profit (∼14%) and 3) generate gaseous products with a higher CO proportion (∼50 vol%). WAS pyrolysis under N 2 atmosphere showed 1) an overall better energy profit (<83.2 %), while producing 2) a higher gaseous yield (∼32 wt%) with 3) higher H 2 proportion (∼29 vol%) and 4) biochar of larger surface area (22 m2/g). Integrating CO 2 as the pyrolysis medium utilises the excess CO 2 , potentially relieving the carbon burden on the environment. [ABSTRACT FROM AUTHOR]