What is a Biosolids?
Biosolids: Treated sewage sludge
Biosolids are the nutrient-rich organic solids left after sewage sludge has been treated to reduce pathogens and odor, usually by anaerobic or aerobic digestion, lime stabilisation or composting. A wastewater treatment plant produces them continuously: everything settled in the clarifiers and wasted from the biological process ends up as sludge, and once stabilised it can be applied to farmland, composted, landfilled or incinerated. Klarifi tracks 1,548 US plants with anaerobic digesters, the main way large plants stabilise their solids, and 341 biosolids and dewatering projects in published capital improvement plans.
From sludge to biosolids
Raw sludge is 96 to 99 percent water. It is thickened (gravity thickeners, dissolved air flotation or rotary drums), stabilised (digestion, lime or composting) and dewatered (belt filter presses, centrifuges or screw presses) to a cake of 15 to 30 percent solids. Dryers can take it further to a granular product. Each step is a separate market for equipment, polymers and hauling.
The EPA’s Part 503 rule sets two classes. Class A biosolids have pathogens reduced to below detectable levels and can be used without restriction; Class B biosolids have reduced pathogens and are land-applied with site and crop restrictions. Most digested and dewatered sludge is Class B; drying, composting and thermal hydrolysis produce Class A.
Why biosolids are a growing capital cost
Land application is the cheapest outlet, but PFAS found in biosolids has led Maine to ban it and other states to restrict it, pushing plants towards drying, incineration and landfill, all of which cost more. Hauling and tipping fees rise every year, so reducing volume through better dewatering pays back quickly.
In the capital plans Klarifi has extracted, 341 line items concern biosolids, sludge and dewatering, and a further 136 concern digesters and biogas. They range from a replacement belt press to full solids-handling buildings.
Find plants by solids-handling technology
The facility directory lists which US plants have anaerobic digesters or gas holders, from Klarifi’s aerial imagery review, and which report mechanical dewatering, thickening or drying as a unit process. Together with the capital plans, that shows where biosolids equipment will be bought next.