Fixed-Film and Lagoon Biological Systems
What you must know
Attached-growth systems include trickling filters, rotating biological contactors (RBCs), moving bed biofilm reactors (MBBR), and submerged aerated filters. Media provide surface area for biofilm; performance depends on organic loading rate (kg BOD/m³·d or kg BOD/m²·d), hydraulic loading, and media specific surface area. Trickling filters stratify biomass by depth; nitrification occurs lower in the bed. Clogging, ponding, and media loss are common issues. MBBR offers high surface area and resilience to shock loads, useful for industrial streams.
Lagoons include aerobic, facultative, and anaerobic types. Facultative lagoons rely on algae-bacteria symbiosis; algae produce oxygen, bacteria oxidize organics. Loading is expressed as kg BOD/ha·d. Anaerobic lagoons handle high-strength waste but generate odors and require sludge management. Cold weather reduces biological activity; short-circuiting and stratification reduce performance.
Traps in questions
- Confusing organic loading (mass/area or volume) with hydraulic loading (volume/area).
- Assuming lagoons are maintenance-free; sludge accumulation and odor control matter.
- Forgetting that algae contribute BOD and TSS in lagoon effluents.
- Overlooking media clogging in trickling filters.
Worked example / rule of thumb
A facultative lagoon of 2 ha receiving 1000 kg BOD/d has loading = 500 kg BOD/ha·d - high for facultative (typical 20–60 kg BOD/ha·d). Rule of thumb: check loading units carefully; lagoons are area-loaded, biofilm reactors are surface-area or volume loaded.