24-4 WATER AND WASTEWATER ENGINEERING
These media offer larger surface areas for slime growths (typically 90 square meters of surface area
per cubic meter of bulk volume, as compared to 40 to 60 square meters per cubic meter for 75 mm
rocks) and greatly increase void ratios for increased air flow. The materials are also much lighter
than rock (by a factor of about 30), so the trickling filters can be much taller without facing struc-
tural problems. While the rock in filters is usually not more than 3 m deep, synthetic media depths
may reach 12 m. This reduces the overall space requirem
ents for the trickling filter portion of the
treatment plant.
O xygen is typically supplied to the rock filters by natural draft ventilation of air. Deeper
synthetic media filters may be provided with forced draft ventilation (called a biological aerated
filter or BAF). They may be designed with an aerobic zone at the top and an anoxic zone at the
bottom to promote denitrifi
cation.
A 10-year side-by-side study of the performance of full scale conventional activated sludge
and a biological aerated filter found the following (Hansen et al., 2007):
• Both systems are operationally reliable and easy to handle and maintain.
• The BAF is less sensitive to both hydraulic and BOD load fluctuations.
• The BAF maintains better and more stable nitrification at both normal and low temperatures.
• Activated sludge unit operating costs are less than BAFs.
Biofilters
This process is not used to treat wastewater. It is used to treat odors from wastewater treatment
processes. Biofilters are packed bed filters. The packing material is often one of the following
materials: peat, compost, or a m ixture of compost and other materials such as perlite, oyster
shells , and limes
tone. As the od orous gas passes through the biofilter, two processes occu r
simultaneously: absorption/adsorption and bioconvers ion. The gases are absorbed into the moist
surface biofilm layer. Microorganisms, principally bacteria, actinomycetes, and fungi that are
attached to the media, oxidize the absorbe
d/adsorbed gases. Moisture content and temperature
are important environmental conditions that must be maintained.
Over 50 papers on the performance of biofilters have been written in the last two decades.
Iranpour et al. (2005) presents a literature review of the effec tiveness of these units for air
pollution control. Many articles on modeling and d
esign appear in the Journal of the Air & Waste
Management Association.
24-3 ATTACHED GROWTH DESIGN PRINCIPLES
The growth conditions and the hydrodynamics of the trickling filter control the thickness of the
biofilm. The thickness may range from 100 m to 10 mm. A stagnant liquid layer called the
diffusion layer separates the biofilm from the bulk liquid wastewater ( Figure 24-2a) . Substrate,
oxy gen, and nutrients diffuse a
cross the stagnant liqu id layer to the biofilm, and products of
biodegradation diffuse from the biofilm to the bulk liquid layer.
A s shown in Figure 24-2a , the substrate concentration, S
s
, decreases with biofilm depth as
it is consumed and diffuses into the film. As a result, the process is said to be di ffusion limited.
The rate of mass transfer is termed the substrate surface flux. It is expressed as mass per unit area