Sludge, a byproduct of wastewater treatment, doesn't contain the liquid wastewater from toilets (which is sewage), but rather the concentrated solids, organic matter, and microorganisms separated from all wastewater, including sewage; it also doesn't contain significant amounts of potassium, which is mostly removed during treatment, focusing on nitrogen and phosphorus. So, sludge is the solid residue, not the flowing toilet water itself, and is different from sullage, which is greywater from sinks and showers.
The sludge does not contain waste water from
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