Environmental Engineering Reference
In-Depth Information
8.9.3 ph
Activated sludge microorganisms can be injured or destroyed by
wide variations in pH. The pH of the aeration basin will normally be
in the range of 6.5 to 9.0. Gradual variations within this range will not
cause any major problems; however, rapid changes of one or more pH
units can have a significant impact on performance. Industrial waste
discharges, septic wastes, or significant amounts of stormwater flows
may produce wide variations in pH. pH should be monitored as part of
the routine process control testing schedule. Sudden changes or abnor-
mal pH values may indicate an industrial discharge of strongly acidic
or alkaline wastes. Because these wastes can upset the environmen-
tal balance of the activated sludge, the presence of wide pH variations
can result in poor performance. Processes undergoing nitrification may
show a significant decrease in effluent pH.
8.9.4 Mixed liquor suspended solids, Mixed liquor volatile
suspended solids, Mixed liquor Total suspended solids
Mixed liquor suspended solids (MLSS) and mixed liquor volatile sus-
pended solids (MLVSS) can be used to represent the activated sludge or
microorganisms present in the process. Process control indicators, such
as sludge age and sludge volume index, cannot be calculated unless the
MLSS is determined. Adjust the MLSS and MLVSS by increasing or decreas-
ing the waste sludge rates. The level of mixed liquor total suspended solids
(MLTSS) is an important activated sludge control parameter. To increase the
MLTSS, for example, the operator must decrease the waste rate or increase
the MCRT. The MCRT must be decreased to prevent the MLTSS from chang-
ing when the number of aeration tanks in service is reduced.
Note:
When performing the Gould sludge age test, assume that the
source of the MLTSS in the aeration tank is influent solids.
8.9.5 return activated sludge rate and Concentration
The sludge rate is a critical control variable. The operator must
maintain a continuous return of activated sludge to the aeration tank or
the process will show a drastic decrease in performance. If the rate is too
low, solids remain in the settling tank, resulting in solids loss and a sep-
tic return. If the rate is too high, the aeration tank can become hydrauli-
cally overloaded, causing reduced aeration time and poor performance.
The return concentration is also important because it may be used to
determine the return rate required to maintain the desired MLSS.
8.9.6 Waste activated sludge flow rate
Because the activated sludge contains living organisms that grow,
reproduce, and produce waste matter, the amount of activated sludge is
continuously increasing. If the activated sludge is allowed to remain in
the system too long, the performance of the process will decrease. If too
