SECONDARY WASTEWATER TREATMENT: Fixed film biological reactors

In wastewater treatment plants, after primary sedimentation we have removed 25 to 35% of the BOD (in some plants up to 50%if there is a larger settleable BOD fraction) and 40 to 60% of the suspended solids. We still need to reduce these constituents to a lower, acceptable level. The "sugars, starches, and carbohydrates" are basically soluble, and/or colloidal in nature in the water…and will NOT settle out in a basin. We therefore convert these soluble, non-settleable, materials into microorganisms by allowing the microbes to consume them, reproduce their kind; and then we settle the microbes out in a basin. Fixed film reactors have microorganisms attached to their media. The microorganisms, form a zoogleal film, that is a film of many different species, depending on the flow rate of the water, the amount and type of organic materials being converted, temperature of the air and water, etc. The film sloughs off in small portions when it becomes thick, and in many cases the microbes next to the media have died. These small portions are then settled out in a secondary clarifier, which effectively removes the microbes and the "food" still attached to the film.

TRICKLING FILTERS, BIOFILTERS, FIXED FILM REACTORS, and ROTATING BIOLOGICAL CONTACTORS ....are all terms that are used to describe this "fixed film" type of secondary treatment process unit.

A) TRICKLING FILTERS, BIOFILTERS use slag, rocks, wood racks, to grow that zoogleal film of microorganisms on.

B) ROUGHING FILTERS are special biofilters that are used to reduce heavy organic loadings to activated sludge units, or other similar secondary treatment process unit. Most new roughing filter installations are now utilizing a plastic type media on which to grow their zoogleal film. These units are installed to substantially reduce the organic loading to a downstream secondary process, NOT eliminate it!

C) ROTATING BIOLOGICAL CONTACTORS are plastic disc units that rotate in the passing wastewater stream, partially submerged, allowing for contact with the wastewater stream being treated. This allows for a transfer of oxygen in the atmosphere to the wastewater, and hence, to the zoogleal film microorganisms. The discs are rotated by motors, or by air diffusion - rising air bubbles contact one side of the disc and rotate it upward.

D) ACTIVATED BIOFILTERS use activated sludge that is returned to flow over the media with the incoming influent wastewater process stream.

E) TRICKLING FILTER-SOLIDS CONTACT employs short term aeration of the biofilter effluent with some return activated sludge before secondary clarification. This increases the solids capture in the downstream secondary clarifier.
 

FIXED FILM FILTER DESIGN PARAMETERS:

Trickling filters, also called biofilters:

Standard Rate Filters:  hydraulic loading 25-100 gal/day/sqft
organic loading 5-25lbs/day/1000 cuft
media depth 6-8 feet
BOD removal 80-85%
Growth sloughs off at intervals, esp. spring & fall. Growth heavy. Effluent BODs as low as 20-25mg/l.

High Rate Filters: hydraulic loading 100-1000 gal/day/sqft
organic loading 25-300 lbs/day/1000 cuft
media depth 3-5 feet rock OR 5-40 feet plastic
BOD removal 65-85%
High Rate: Result of trying to reduce costs of standard filters, and increased organic loads; Loadings may be higher for synthetic media with almost all using recirculation modes. Uniform sloughing, due to higher flows, but humus slough is lighter than std rate.

roughing hydraulic loading 1000-4000 gal/day/sqft
organic loading over 300lbs/day/1000 ft3
media depth 3-20 feet
BOD removal 40-65%
A high rate with higher organic loading. Has higher reductions per unit volume, but lower overall reductions.

Rotating Biological Contactors, also called RBC’s:
hydraulic loading rate: 1.5 to 6 gpd/sqft

organic loading rate: 2.5 to 4 lb BOD/1000 ft3
BOD removal: 80 to 95%

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