Page 111 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 87
BIOLOGICAL TREATMENT. In the presence of the ordinary bacteria found in
water, many organic materials will oxidize to form carbon dioxide, water,
sulfates, and similar materials. This process consumes the oxygen dissolved in
the water and may cause a depletion of dissolved oxygen. A measure of the
ability of a waste component to consume the oxygen dissolved in water is known
as the biochemical oxygen demand. The biochemical oxygen demand (BOD) of a
waste stream is often the primary factor that determines its importance as a
pollutant. The biochemical oxygen demand of sewage, sewage effluents, polluted
waters, or industrial wastes is the oxygen, reported as parts per million,
consumed during a set period of time by bacterial action on the decomposable
organic matter.
One of the more common biological wastewater treatment procedures
today involves the use of concentrated masses of microorganisms to break down
organic matter, resulting in the stabilization of organic wastes. These organisms
are broadly classified as aerobic, anaerobic, and aerobic-anaerobic facultative.
Aerobic organisms. require molecular oxygen for metabolism, anaerobic organ-
isms derive energy from organic compounds and function in the absence of
oxygen, while facultative organisms may function in either an aerobic or anaero-
bic environment.
Basically, the aerobic biological processes involve either the activated
sludge process or the fixed-film process. The activated sludge process is a
continuous system in which aerobic biological growths are mixed with wastewa-
ter and the resulting excess flocculated suspension separated by gravity clarifi-
cation or air flotation. The predominant fixed-film process has, in the past, been
the conventional trickling filter. In this ;‘process, wastewater trickles over a
biological film fixed to an inert medium. Bacterial action in the presence of
oxygen breaks down the organic pollutants in the wastewater. (An attempt to
improve on the biological efficiency of the fixed-film process has resulted in the
development of a useful rotating disk biological contactor.) A comparison of
biological loading and area requirements for various aerobic biological pro-
cesses is shown in Table 10. ’
Many organic industrial wastes (including those from food processing,
meat packing, pulp and paper, refining, leather tanning, textiles, organic chemi-
cals, and petrochemicals) are amenable to biological treatment; however, a fair
number may prove to be refractory, i.e., nonbiodegradable. Thus, although the
BOD removal may be excellent, the removal of the chemical oxygen demand
(COD) may be low. Process evaluation -prior to system design should center on
characterization of the waste stream, particularly to determine the presence of
inhibitory or toxic components relative to biological treatment, and the estab-
lishment of pollutant removal rates, oxygen requirements, nutrient requirements
(nitrogen and phosphorus), sludge production, and solids settleability.
Anaerobic treatment is important in the disposal of municipal wastes but
has not been widely used on industrial wastes. It has found some use in
reducing highly concentrated BOD wastes, particularly in the food and beverage
industries as a pretreatment or roughing technique. However, compared to