Fixed film biological wastewater treatment basics are the same from trickling filters to MBBRs

One of the oldest wastewater treatment methods, trickling filters, works on the same biological principals as one of the latest treatment technologies – the MBBR. All systems relying of attached biofilms can be termed fixed-film systems. At the core, you have microbial colonies attached to a support structure. The film tends to have three distinct ecosystems inside the biofilm depending upon the location inside the biopolymer matrix supporting the colony.

• Aerobic
• Anoxic
• Anaerobic

The biofilm acts to protect the microbes from toxic influent compounds, pH swings, hydraulic washout, and other factors that would kill individual microbial cells. Much like floc in suspending growth systems, the fixed-film biomass consists of living bacteria, inorganic materials, extracellular polymers (the glue), and adsorbed organics. The percent of the mass that is truly living microbes depends upon influent makeup, organic concentration, and biofilm sloughing rates.
The fixed film system overcomes solids seperation problems often seen in suspended growth system’s secondary clarifiers. In effect, the fixed film allows for a higher MLSS (up to 3,000 mg/L higher) than a comparable suspended growth system. This directly translates to:

1. Increased treatment capacity with a given foot-print
2. Ability to meet permits by retro-fitting an existing plant
3. Less susceptiblity to shock loadings
4. Can allow for better nutrient removal with respect to both nitrogen & phosphorus

What are the downsides?

1. Initial expense for installing the system
2. Maintenance of aeration system more difficult with media
3. In mobilized media systems the screens must be maintained to prevent media loss
4. All are susceptible to fouling – especially from oil & grease that encapsulates the biofilm