Figure 2-29. Package wastewater treatment unit.
b. Contact Stabilization. The other modification that is used in package plants
is called contact stabilization. An overload conventional activated sludge plant can be
upgraded by converting the existing aeration tank to two separate aeration tanks or
basins. For this process, in the first basin, primary effluent and activated sludge are
mixed and aerated for about an hour and a half at a mixed-Iiquor suspended-solids
concentration of about 2500 milligrams per liter. After secondary settling, the return
sludge is aerated again for 4 to 8 hours at a mixed-Iiquor suspended-solids
concentration of about 8000 milligrams per liter before being recycled ahead of the first
aeration chamber. In principle, dissolved organics are adsorbed on the sludge particles
in the first aeration tank, called the contact tank. The second tank, called the
stabilization tank, is where metabolism of the wastes is said to take place. The
advantage of contact stabilization over conventional activated sludge is the reduction in
construction cost. The total volume of the aeration basin is smaller because only the
return sludge is aerated in a stabilization basin.
2-21. EFFLUENT AND SLUDGE
a. Effluent. Package units, Iike conventional treatment plants, are routinely
equipped with chlorination devices to disinfect the effluent prior to its discharge into a
watercourse.
b. Sludge. Sludge from package units may be disposed of by discharge into a
sludge-drying bed, a holding tank, or by pumping directly into a tank truck as with a
septic tank. The method of sludge disposal is determined largely by the amount of land
area available.
MD0161
2-47