Encyclopedia of Environmental Science and Engineering, Volume I and II

146 BIOLOGICAL TREATMENT OF WASTEWATER

The available information strongly indicates that immo-

bilized biological systems are less sensitive to toxicity and

have a higher efficiency in degrading toxic and hazardous

materials.^34 Fixed-film wastewater treatment processes are

regarded to be more stable than suspended growth processes

because of the higher biomass concentration and greater

mass transfer resistance from bulk solution into the biofilm

in fixed-films.^35 The mass transfer limitation effectively

shields the microorganisms from higher concentrations of

toxins or inhibitors during short-term shock loads because

the concentrations in biofilms change more slowly than in

the bulk solution. Also, since the microorganisms are physi-

cally retained in the reactor, washout is prevented if the

growth rate of microorganisms is reduced. 34,35 The biofilm

systems are especially well suited for the treatment of slowly

biodegradable compounds due to their high biomass concen-

tration and their ability to immobilize compounds by adsorp-

tion for subsequent biodegradation and detoxification.^34

Trickling Filters

Wastewater is applied intermittently or continuously to a

fixed bed of stones or other natural synthetic media resulting

in a growth of microbial slime or biomass on the surface of

this media. Wastewater is sprayed or otherwise distributed so

that it slowly trickles through while in contact with the air.

For maximum efficiency, food should be supplied continu-

ously by recirculating, if necessary, the treated wastewater or

settled sludge or both. Oxygen is provided by the dissolved

oxygen in influent wastewater, recirculated water from the

air circulating through the interstices between the media to

maintain aerobic conditions.

Active microbial film, biomass, consisting primarily

of bacteria, protozoa, and fungi, coats the surface of filter

media. The activity in biological film is aerobic, with move-

ment of oxygen, food and end-products in and out of it as

shown in Figure 13. However, as the thickness of the film

increases, the zone next to the filter medium becomes anaero-

bic. Increased anaerobic activity near the surface may liquify

the film in contact with the medium, resulting in sloughing

or falling down of the old film and growth of a new film.

The sloughed solids are separated in a secondary settling

tank and a part of these may be recirculated in the system.

Two types of trickling filters are recognized, primarily on

the basis of their loading rates and method of operation, as

shown in Table 1. In low-rate trickling filter, the wastewater

passes through only once and the effluent is then settled prior

to disposal. In high-rate trickling filter, wastewater applied

FIGURE 12 Typical wastewater treatment sequence.

Raw

Wastewater

Pretreatment Primary

Treatment

Secondary

Treatment

(Biological)

Sedimentation Te r t i a r y

Treatment

Final

Effluent

1. Screening and
   Grit Removal


2. Oil Separation

Disposal Sludge

Digestion

1. Flotation


2. Sedimentation
   
   
   1. Activated Sludge
   
   
   2. Trickling Filters
   
   
   3. Anaerobic Lagoons
   
   
   4. Aerated Lagoons
   
   
   5. Stabilization Ponds
   
   
   6. RBC
   
   
   
   

MICROBIAL

FILM

WASTE

WATER

NUTRIENTS

OXYGEN

END PRODUCTS

AIR

FILTER

MEDIUM

ANAEROBICAEROBIC

FIGURE 13 Process of BOD removal in trickling filters.

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