HONOLULU – Sludge. The final, unwanted byproduct of a toilet flush. The semi-solid stuff that even wastewater treatment plants send packing to the landfill. Unseen in the steel pipes snaking high on the exterior of Honolulu’s H-POWER plant, sludge is injected into a massive boiler where it joins the city’s trash in a roaring inferno. From the gravel lot outside, it all seems very antiseptic and smells less than a stroll past the neighborhood dumpster. But the 70-megawatt waste-to-energy facility is a workhorse, processing as much as 2,000 tons of refuse each day from Oahu’s 1 million residents. All told, it generates up to 10 percent of the electricity needed to power this Pacific island.