Chapter 20

Wastewater and Solid Waste Management

Aerobic Wastewater Treatment

Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O).

Aerobic treatment utilizes biological treatment processes, in which microorganisms convert nonsettleable solids to settleable solids. Sedimentation typically follows, allowing the settleable solids to settle out. Three options include:

Activated Sludge Process

In the activated sludge process, the wastewater flows into an aerated and agitated tank that is primed with activated sludge. This complex mixture containing bacteria, fungi, protozoans, and other microorganisms is referred to collectively as the biomass. In this process, the suspension of aerobic microorganisms in the aeration tank, are mixed vigorously by aeration devices which also supply oxygen to the biological suspension.

Attached Growth (Biofilm) Process

The second type of aerobic biological treatment system is called "Attached Growth (Biofilm) Process" and deals with microorganisms that are fixed in place on a solid surface. This "attached growth type" aerobic biological treatment process creates an environment that supports the growth of microorganisms that prefer to remain attached to a solid material.

Trickling Filter Process

In the trickling filter process, the wastewater is sprayed over the surface of a bed of rough solids (such as gravel, rock, or plastic) and is allowed to "trickle down" through the microorganism-covered media.

Biofiltration Towers

A variation of a trickling filtration process is the biofiltration tower or otherwise known as the biotower. The biotower is packed with plastic or redwood media containing the attached microbial growth.

Rotating Biological Contactor Process

The rotating biological contactor process consists of a series of plastic discs attached to a common shaft.


These are slow, cheap, and relatively inefficient, but can be used for various types of wastewater. They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).

Sludge Treatment and Disposal

In general, aerobic treatment systems like the activated sludge system produce relatively large quantities of sludge which requires disposal. The sludge can undergo a dewatering treatment either by reconsolidated centrifugation, vacuum filtration or in a pressure filter.

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