Wastewater Efficiency and Treatment Act of 2021 This bill requires the Department of Energy (DOE) to establish a Water and Energy Efficiency Program. Under the program, DOE must award grants to certain small public wastewater treatment facilities that serve disadvantaged communities or populations that do not exceed 10,000. The facilities must use the grants to conduct energy efficiency audits of the facilities and update equipment based on the audits. In addition, DOE must provide loan guarantees to eligible municipalities and Indian tribes for (1) projects that convert waste in the treatment process of wastewater treatment facilities into renewable fuels, biosolids, or other byproducts; and (2) projects that will assist in transitioning facilities towards the use of energy-efficient technologies. In carrying out the program, DOE must establish a small-scale extension services program to provide assistance to rural communities through grants, outreach, training, and technical assistance regarding energy-efficient technologies at wastewater treatment facilities in eligible municipalities and tribes.