Decarbonising economies and improving environment can be enhanced through circular economy, energy, and environmental systems integrating electricity, water, and gas utilities. Hydrogen production can facilitate intermittent renewable electricity through reduced curtailment of electricity in periods of over production. Positioning an electrolyser at a wastewater treatment plant with existing sludge digesters offers significant advantages over stand-alone facilities. This paper proposes co-locating electrolysis and biological methanation technologies at a wastewater treatment plant. Electrolysis can produce oxygen for use in pure or enhanced oxygen aeration, offering a 40% reduction in emissions and power demand at the treatment facility. The hydrogen may be used in a novel biological methanation system, upgrading carbon dioxide (CO2) in biogas from sludge digestion, yielding a 54% increase in biomethane production. A 10 MW electrolyser operating at 80% capacity would be capable of supplying the oxygen demand for a 426,400 population equivalent wastewater treatment plant, producing 8,500 tonnes dry solids per annum (tDS/a) of sludge. Digesting the sludge could generate 1,409,000 m3CH4/a and 776,000 m3CO2/a. Upgrading the CO2 to methane would consume 22.2% of the electrolyser generated hydrogen and capture 1.534 ktCO2e/a. Hydrogen and methane are viable advanced transport fuels that can be utilised in decarbonising heavy transport. In the proposed circular economy, energy, and environment system, sufficient fuel would be generated annually for 94 compressed biomethane gas (CBG) heavy goods vehicles (HGV) and 296 compressed hydrogen gas fuel cell (CHG) HGVs. Replacement of the equivalent number of diesel HGVs would offset approximately 16.1 ktCO2e/a.