1200 URBAN RUNOFF
TABLE 26
Comparison of physical treatment systemsPhysical Unit ProcessSuspended
Solids BOD 5 CODSettleable
SolidsTotal
PhosphorusTotal Kjeldahl
NitrogenSedimentation
Without chemicals 20–60 30 34 30–90 20 38
Chemical assisted 68 68 45 — — —
Swirl Regulator/
Concentrator 40–60 25–60 — 50–90 — —Screening
Microstrainers 50–95 10–50 35 — 20 30
Drum screens 30–55 10–40 25 60 10 17
Rotary screens 20–35 1–30 15 70–95 12 10
Disc screens 10–45 5–20 15 — — —
Static screens 5–25 0–20 13 10–60 10 8Dissolved Air Flotation* 45–85 30–80 55 93 † 55 35High Rate Filtration‡ 50–80 20–55 40 55–95 50 21High Gradient Magnetic
Separation§ 92–98 90–98 75 99 — —
*^ Process efficiencies include both prescreening and dissolved all flotation with chemical addition.
† From pilot-plant analysis EPA-600/8-7-014.
‡^ Includes prescreening and chemical addition.
§ From bench scale pilot-plant operation, 1–4 L/min (0.26–1.06 gal/min).the full-scale application of screening/DAF for the treatment
of CSO. Table 26 represents a tabulated summary of various
physical treatment systems, including the swirl regulator/
concentrator previously described.5 BIOLOGICAL TREATMENTBiological treatment is a means for stabilizing dissolved organic
matter and removing nonsettleable colloidal solids. This can be
accomplished either aerobically or anaerobically. Several bio-
logical processes have been applied to CSO treatment. These
include contact stabilization, trickling filters, rotating biologi-
cal contractors (RBC), and treatment lagoons. Descriptions of
these processes and typical combined sewage treatment instal-
lations are provided in Tables 27 and 28. Biological treatment
processes are generally classified as secondary treatment pro-
cesses, capable of removing 70–95% of the BOD 5 and SS from
waste flows at dry-weather design flowrates and loadings.
When biological treatment processes are used for combined
sewage treatment, removal efficiencies are lower (the percent
organic matter is smaller for CSO solids than for dry-weather
solids) and are controlled to a large degree by hydraulic and
organic loading rates. Most biological systems are suscep-
tible to overloading conditions and shock loads as comparedto physical treatment processes. However, RBC have achieved
high removals at flows 8–10 times dry-weather design flows.
Typical pollutant removals for contact stabilization,
trickling filters, and RBCs (wet-weather loading conditions)
are presented in Table 29. These processes include primary
(except contact stabilization) and final clarification. Final
clarification greatly influences the overall performance of
the system by preventing the carry-over of biological solids
produced by the processes. Pollutant removal efficiencies by
treatment lagoons have varied from highs of 85–95% to nega-
tive values due to excessive algae production and carry-over.
In addition to the type of lagoon and the number of cells in
series (stages), several major factors that influence removal
efficiencies include: (1) detention time, (2) source of oxygen
supply, (3) mixing, (4) organic and hydraulic loading rates,
and (5) algae removal mechanisms. A single-cell storage/
oxidation lagoon in Springfield, Illinois averaged 27%
BOD 5 removal and 20% SS removal; however, fish kills in
the receiving water were greatly reduced as compared to that
prior to the construction of the facility. Multiple-cell facilities
with algae control systems constructed at Mount Cleimens,
Michigan and Shelbyville, Illinois provide 75–90% SS and
BOD 5 removal efficiencies during wet-weather conditions.
An operational problem common to all stormwater bio-
logical systems is that of maintaining a viable biomass to treatC021_002_r03.indd 1200C021_002_r03.indd 1200 11/23/2005 9:45:48 AM11/23/2005 9: