Energy, Economic, and Environmental (Triple-E) Analysis of Energy Recovery from Sewage Sludge in Municipal Wastewater Treatment Plants

Wastewater treatment plants (WWTPs) generate large amounts of sewage sludge, and managing this sludge is both energy- and cost-intensive. The organic fraction contains recoverable chemical energy that can be converted into biogas, heat, electricity, or upgraded biomethane. This paper presents a triple-E (energy, economic, environmental) assessment of sludge-to-energy options in municipal WWTPs. The analysis is based on a reference WWTP of 200,000 PE, supported by data from the literature, and includes sludge treatment, anaerobic digestion (AD), combined heat and power (CHP) generation, and optional mono-incineration of solar-dried sludge. Mono-incineration is increasingly adopted in Europe because it is publicly accepted, substantially reduces sludge volume and mass, and enables the recovery of phosphorus from ash.

For a sludge production rate of 50 gDS/(PE·day), the corresponding biogas yield is 300 Nm³/tDS. Biogas-fired CHP systems generate about 15 kWh/(PE·year) of electricity and 20 kWh/(PE·year) of thermal energy, meeting approximately 30% of the electrical demand and up to 80% of the thermal demand in WWTPs. Energy autonomy can be further increased by recovering 100–150 kWh/tDS from the mono-incineration of solar-dried sludge.

The specific investment cost of WWTP energy-recovery systems ranges between 400 and 1,000 EUR/PE, with annual operating costs of 40–75 EUR/PE and energy-related costs of 4–8 EUR/PE. Mono-incineration involves average disposal and processing costs of EUR 350 per tonne of dry solids and can be combined with phosphorus recovery, though this introduces additional costs. Under these conditions, AD+CHP typically achieves a payback period of around 10 years, whereas mono-incineration is more capital-intensive and generally results in longer payback periods.

From an environmental perspective, replacing grid electricity with biogas-derived power reduces greenhouse gases emissions by roughly 60%, assuming an EU-27 average grid intensity of 210 gCO₂eq/kWh. Overall, energy-oriented sludge management improves the triple-E performance of municipal WWTPs, provided that the higher costs of mono-incineration are balanced by the value of recovered phosphorus.