Page 206 - Industrial Ventilation Design Guidebook

I 68 CHAPTER 4 PHYSICAL FUNDAMENTALS

suspended living microorganism. This type of system is suited for low-strength
1
waste, typically on the order of 50-200 mg L" BOD.
To enhance the purification process and increase the degree of purifica-
tion, powdered activated carbon (PAC) may be added directly to the aeration
tank, or the biologically treated wastewater may be filtered through granu-
lated activated carbon (GAC) for posttreatment.
Pre- or post-treatment with ozone of wastewater may also be applied. Pre-
treatment with ozone takes place in the presence of biorefractory compounds,
as ozone increases the BOD/COD ratio.

Anaerobic Treatment. Typical of this method is the upflow anaerobic
sludge blanket. This consists of a corrosion-resistant tank complete with sepa-
rators. The flow network enters the reactor base without short-circuiting, en-
suring the proper formation of the granular sludge. New bacterial cells are
formed in the reactor and aggregate into tiny granules, which have good set-
tling characteristics.
Biogas is produced by the bacteria in the form of small bubbles; these
float upward through the sludge bed/blanket, providing a good mixing ac-
tion. When the biogas reaches the top of the reactor, it is collected and
used as a fuel.

Design Considerations
In determining the best method of treatment, the following factors have to
be considered:

Owning and Operating Costs

• Initial cost
• Maintenance
• Energy costs
• Water treatment
• Corrosion costs
• Odor treatment
• Abrasion problems
• Slurry pumping problems
• The maximum temperature the drains and the water sinks can accept
(bearing in mind thermal stresses and corrosion in discharge pipelines,
and algae growth and oxygen depletion in the watercourses)

Properties of Liquid Used
• Vapor pressure
• Temperature of decomposition
• Viscosity
• Density