TM 5-814-3/AFM 88-11, Volume III
UPGRADING EXISTING TREATMENT FACILITIES
Upgrading of wastewater treatment plants may be required to handle increased hydraulic and organic loadings
to meet existing effluent quality or to meet higher treatment requirements. Any of these situations requires
optimization of existing facilities before consideration of additional treatment facilities. It is necessary that
a distinction be made between upgrading to accommodate higher hydraulic and organic loads, and upgrading
to meet stricter treatment requirements. Existing facilities can be made to handle higher hydraulic and organic
loads by process modifications, whereas meeting higher treatment requirements usually requires significant
expansion and/or modification of existing facilities. Additional information may be found in the EPA Manual
Process Design for Upgrading Existing Wastewater Treatment Plants.
8-2. Techniques for upgrading existing sedimentation facilities.
Improved solids separation in primary and secondary settling tanks or clarifiers results in concurrent reduction
of biochemical oxygen demand levels in the tank effluent. Solids separation can be enhanced by adding more
clarification capacity, either by chemical treatment of the wastewater or by use of more efficient settling
devices. Upgrading the primary clarifier has the advantage of decreasing the organic loading and thus
reducing the amount of secondary sludge produced in the biological treatment units. Furthermore, more
solids will be selected and removed with the primary sludge, which is thicker and more amenable to
dewatering than secondary sludge. The secondary clarifier in a biological treatment process determines the
overall plant efficiency because it removes the organic matter that is converted from a soluble to an insoluble
form in the biological units.
a. Addition of chemicals. Chemicals may be added to the primary settling tank as a means of upgrading
performance and relieving organic overloading in wastewater treatment plants; but it is not normal practice
to add chemicals to secondary clarifiers since aerobic biological sludge flocculates and settles readily when
normal growth conditions are maintained, i.e., sludge retention times are optimized. Lime addition to an
activated sludge secondary clarifier may not be feasible because of the potential adverse effect of recirculated
lime sludge on the microbiological characteristics of mixed liquor. Moreover, lime addition generally requires
pH adjustment of the effluent before further treatment or discharge to the receiving waters. When considering
the addition of chemicals to primary clarifiers, it is important to examine the effect of increased primary
clarifier efficiency on subsequent treatment units. If the biochemical oxygen demand load in the biological
aeration basin falls below 0.25 to 0.35 pound biochemical oxygen demand per pound of mixed-liquor, volatile
suspended solids per day because of increased primary clarifier efficiency, nitrification conditions may prevail.
This can reduce the total oxygen demand of the effluent but, at the same time, it will increase the oxygen
demand on the aeration facilities. A decrease in loadings in the aeration basin will also require more careful
sludge handling to ensure stable conditions. Chemical addition to the primary clarifier may produce more
sludge; therefore, sludge handling facilities must be evaluated to ensure proper capacities and loadings.
b. Tube settlers. Shallow-depth settling systems such as tube settlers have been used, to a limited extent,
in primary and secondary clarifiers to improve performance as well as to increase throughput in existing
tanks. These settlers capture settleable solids at higher than normal overflow rates. However, they do not
improve the efficiency of primary clarifiers that are already achieving high degrees of suspended solids
removal (40 to 60 percent). Moreover, they will neither remove colloidal solids that remain in suspension nor
induce additional coagulation to bring about additional particle removal. The design of shallow-depth settling
systems will be based on pilot studies of the waste liquid for which solids settling is required. Approval for
their design criteria and use must be obtained from HQDA (CEEC-EB) WASH DC 20314-1000 for Army
projects and HQ USAF/LEEE WASH DC 20332 for Air Force projects. For more information regarding tube
settlers, refer to the EPA Process Design Manual for Suspended Solids Removal.