Suspended and attached growth processes

Biological wastewater treatment uses micro-organisms (mainly bacteria) to convert dissolved and colloidal organic matter into carbon dioxide, water, new cells and stable end products. The organisms may be kept in suspension in the liquid or grown on a fixed surface.

Suspended growth process

The micro-organisms are kept in suspension in the wastewater by mixing or aeration, forming flocs. The mixed liquor is later settled and part of the biomass is recycled.

• Examples: activated sludge process (ASP), aerated lagoons, oxidation ditch, sequencing batch reactor (SBR), anaerobic digesters.
• Needs aeration/mixing energy, a clarifier and sludge return; more sensitive to shock loads; higher operating skill.
• Biomass is measured as MLSS/MLVSS.

Attached (fixed-film) growth process

The micro-organisms grow as a biofilm on a fixed medium (stones, plastic media, discs) over which wastewater flows.

• Examples: trickling filter, rotating biological contactor (RBC), submerged biofilters.
• Simple, low energy, stable with variable loads; needs a medium and a final clarifier to remove sloughed film; larger area.

Principles of biological treatment

1. Food and organisms: organic matter in sewage is food (BOD) for bacteria. They need oxygen (aerobic), nutrients (N, P) and suitable pH (6.5-8.5) and temperature (20-35°C).
2. Aerobic oxidation (energy): $\text{organics} + O_2 \xrightarrow{\text{bacteria}} CO_2 + H_2O + \text{energy}$
3. Synthesis (growth): $\text{organics} + O_2 + N \rightarrow \text{new cells}$
4. Endogenous respiration: when food is short, cells oxidise their own mass: $\text{cells} + O_2 \rightarrow CO_2 + H_2O + NH_3$
5. Colloidal and suspended matter is first adsorbed on the floc/film, then hydrolysed and metabolised.
6. The cells are removed by settling as sludge, which leaves a clear effluent of low BOD.
7. Anaerobic treatment (no oxygen) converts organics to methane and CO₂, used for sludge digestion and strong wastes.
8. The process is controlled by the food-to-micro-organism ratio ($F/M$), mean cell residence time, oxygen supply and temperature.