What are some of the most eco-friendly ways of wastewater sludge disposal that has the least detriment on natural environment?

natural drying.................................................................................

Sludge treatment using constrcuted wetland

Beneficial Reuse of Biosolids has been advocated by the United States EPA since the 1980's. Lime Stabiliztion of the Sludge to pH 12.5 and then dewatering with a belt press is probably the most efficient and cost effective means to prepare the Biosolids for Land Application. One has to ensure the Class A quality of the sludge thru appropriate Priority Pollutant analyses. Aerobic Digestion is energy intensive and requires 30 days of treatment to acheive Stabilization... 

One is the conversion of the sludge ( domestic wastewater) to fertilizers through vermiculture.

My friend; If the treatment process stream is going well and the extracted sludge processing "dewatering, digesting of burning" has done effectively, that makes the produced sludge matched with EPA classes of sludge; then, it can be used as a fertilizer for corps and no need to find a disposal way, But ownestly, the mentioned processes of sludge can not be applied to the small STP's as the Financial benefits are not guaranteed.

Here in the Philippines we treat the sludge via dewatering process , either mechanically or drying beds. Once the sludge moisture has significantly reduced, it will be sold to a company utilizing sludge as a component for their vermicomposting.

Aerobic digestion is faster and cost economical,composting dewatering(by centrifuges) & thickening are the main Eco-friendly methods.Reusing composted sludge as soil conditioner or organic manure are the latest trends in GREEN TECHNOLOGY.