Fixed Film Systems - Trickling Filters

Often when dealing with questions on a system, I am given a trade name for a system which is thought to be unique. Well, in most cases the trademarked or patented system is just a variation on one of the time tested formats that have proven effective for years. In the past decade, many fixed film type systems have been installed - no matter if you call them fluidized bed, IFAS, FAST, RBC (Rotating Biological Contact), or even a tricking filter - all of these systems work on the principal of a biofilm attaching to a surface to support a high population of microbes that are not as susceptible to washout as a suspended growth (standard activated sludge or lagoon) system. We are now seeing retrofits where media is being added to increase aeration basin capacity by increasing the M in the F/M ratio. Today, I want to break down the various types of fixed film systems so comparisons of various "new" technologies is more easily accomplished.

All fixed film system rely on a "host" surface for a microbial colony that forms a biofilm. The standard film is represented in the graph below and is found in all variations of fixed film systems.



I am going to do a series of posts on each of the fixed film system technologies with the goal being to give a general overview of the technology and most importantly the pros and cons of each. Today I am going to start with the oldest of the fixed film technologies - the trickling filter.

Trickling Filters
The oldest of the fixed film system is the trickling filter.  The most basic systems spray influent over rocks or other media contained in a tower.  Treatment capacity and efficiency can be mproved by adding blowers to provide additional oxygen to the lower levels of the tower. Existing trickling filters can also be improved by utilizing newer, higher surface area plastic media rather than rock or other traditional media. Other mostly industrial trickling filters, recycle biologial solids from the secondary clarifier to "re-seed" biomass to the front of the system.

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Source: gif·web.deu.edu.tr

Pros of Trickling Filters

• Simple, durable system
• Good efficiency at BOD and Ammonia removal
• Low power use
  
  

Cons of Trickling Filters

• Limited flexibility
• Not well buffered for shock loadings or spills
  
• Plugging or clogging of media
• Potential media fouling by substances such as oils & grease
• Odors
• Nuisance flies
  

Expected Treatment Efficiency

                                            Loading (kg BOD/100 m3/day)                  BOD Removal %
Low Rate Filters                           40                                                              80 - 90%
Intermediate Rate                        64                                                             65 - 85%
High Rate                                    160                                                            50 - 75%
Roughing Filters                           480                                                           40 - 65%