Solids Handling
Sludge Treatment
Sludge handling is a major share of plant operating cost. Understanding the chain — thickening, stabilisation, dewatering — is key to an efficient upgrade.
From Waste Activated Sludge to Final Product
Secondary treatment continuously produces waste activated sludge (WAS). Managing it well matters because sludge handling is one of the largest operating costs at a treatment plant, and because anaerobic digestion turns part of that sludge into biogas — an on-site energy resource.
Thickening
Removes free water to raise solids concentration modestly (e.g. from under 1% to several percent), shrinking downstream tank volumes. Gravity thickeners, flotation or mechanical thickeners are common.
Stabilisation
Most often anaerobic digestion, which reduces volatile solids, controls pathogens and odour, and generates biogas that can be recovered for energy. Aerobic digestion is used at smaller plants.
Dewatering
Removes much more water to produce a handleable cake (often 15–30% dry solids) using centrifuges, belt or screw presses, cutting transport and disposal cost.
Final use / disposal
Depending on quality and local rules: agricultural use, incineration or landfill. Phosphorus recovery from sludge streams is increasingly of interest.
Key Levers
Dewatered Cake
Typical dry-solids range
Digestion Energy
Recoverable to power & heat
Sludge Age Effect
Higher SRT, less waste sludge
Engineering Note
Frequently Asked Questions
Secondary treatment produces waste activated sludge (WAS). It is typically thickened, stabilised (often by anaerobic digestion, which also generates biogas), and then dewatered before final use or disposal such as agricultural use, incineration or landfill. Good sludge management is essential because sludge handling is a major share of plant operating cost.
Thickening removes free water to raise solids concentration modestly (e.g. from under 1% to several percent), reducing downstream tank volumes. Dewatering removes much more water to produce a handleable cake (often 15–30% dry solids) using centrifuges, belt or screw presses. Both reduce volume and transport cost.
Longer sludge age generally produces less waste sludge per unit of BOD removed, because more of the organic matter is oxidised (endogenous respiration), but it requires more aeration and tank volume and is essential for nitrification. Sludge age is therefore a central design lever balancing effluent quality, aeration energy and sludge quantity.
Engineering Assessment
Not sure what your plant needs?
Run a Secondary Treatment Engineering Assessment to identify process limitations and develop practical upgrade pathways.
Start Engineering Assessment