Final answer:
Anaerobic secondary wastewater treatment produces a gaseous product that can be used as fuel for a wastewater treatment plant.
Step-by-step explanation:
The process that produces a gaseous product that can be used as fuel to provide heat or power for a wastewater treatment plant is anaerobic secondary wastewater treatment.
In anaerobic secondary wastewater treatment, organic material in the wastewater is decomposed by anaerobic bacteria in the absence of oxygen. This process produces methane gas, which is a gaseous product that can be used as fuel.
For example, municipal sewage and garbage, livestock manure, kitchen, and garden scraps can be used as organic waste to produce methane gas through anaerobic fermentation, and this gas can be used as an energy source for the wastewater treatment plant or supplied to homes for heating.