Compressed All Sub-Surface Aeration

Author:

Stephen Laberis

This document examines the mechanical principles and industrial application of sub-surface aeration systems in wastewater treatment, specifically highlighting the use of static mixers to enhance oxygen transfer efficiency. It details a case study from a newsprint mill that implemented an extended aeration process, outlining the specific design criteria and operational adjustments required to meet strict environmental discharge limits. The content further explores the relationship between basin geometry, power levels, and the management of secondary solids to maintain consistent effluent quality throughout seasonal temperature variations.

Key Learnings

• Aeration Efficiency Factors: Oxygen transfer in wastewater treatment is primarily driven by three variables: surface area (bubble size), turbulence (mixing), and contact time (path length).
• Static Mixer Advantages: Sub-surface static aerators use fixed helical elements to shear air into fine bubbles and create a winding path; this increases oxygen transfer without the clogging issues associated with porous ceramic or cloth diffusers.
• Four-Step Oxygen Transfer: Transfer occurs during initial bubble formation at the orifice, intense contacting within the mixing device, the rising of the air-water column to the surface, and the final turbulence created as the column breaks the water surface.
• Operational Temperature Sensitivity: Seasonal changes significantly impact biological activity; higher summer temperatures may require lower MLVSS (Mixed Liquor Volatile Suspended Solids) concentrations (e.g., 1100–1200 mg/liter) to prevent oxygen depletion and odor issues.
• Nutrient Management: Successful secondary treatment requires maintaining a specific nutrient balance, typically targeted at a ΒΟD₅ : N : P ratio near 100 : 5 : 1, though actual consumption may vary based on the specific waste stream.
• Mixing Characteristics: The degree of basin mixing (completely mixed vs. plug flow) is a function of the basin's length-to-width geometry and the power input per gallon of water.
• Sludge Handling: Secondary solids management, including dissolved air flotation (DAF) and centrifugation, is critical for maintaining system equilibrium and ensuring total suspended solids (TSS) remain within permit limitations.