United States Office of Water EPA-821-R-13-001
Environmental Protection Agency Mail Code 4303T March 2013
Report on the Performance of Secondary Treatment Technology
The EPA established secondary treatment standards for publicly owned treatment works (POTWs)
at 40 CFR 133 based on the measured performance of a set of municipal wastewater treatment
plants generally utilizing primary treatment (simple settling) followed by suspended growth aeration
processes (activated sludge), which are designed and operated to treat soluble organic matter and
settle suspended solids. These technology-based regulations have since been applied to POTWs via
effluent limitations for total suspended solids (TSS) and either 5-day biochemical oxygen demand
(BODS) or 5-day carbonaceous-BOD (cBODS).
The Agency examined recent existing data sources to compile information on the degree of effluent
reduction being attained in practice through the application of secondary treatment via activated
sludge unit operations (this is the technology most commonly associated with secondary treatment).
The Agency examined effluent monitoring data reported to EPA's Permit Compliance System (PCS)
and Integrated Compliance Information System for the National Pollutant Discharge Elimination
System (ICIS-NPDES) for municipal treatment plants relative to their requirements for BODS,
cBODS, and TSS effluent discharge limitations. The secondary treatment standards require effluent
discharge requirements (at a minimum) for most general municipal discharges to meet 30-day
average effluent concentration limits of 30 mg/L for BODS and TSS. Alternative 30-day average
limits for cBODS of 25 mg/L also apply to many facilities nationally in lieu of BODS limits.
Although the secondary treatment standards also provide for special considerations regarding
combined sewers, industrial wastes, facilities considered equivalent to secondary treatment (waste
stabilization ponds or trickling filters), and less concentrated influent wastewater for combined and
separate sewers, these aspects have not been examined herein.
In order to determine the secondary treatment technology (activated sludge) facilities evaluated in
this analysis, the Agency examined the Clean Watersheds Needs Survey ("Needs Survey") for 2008,
which required POTWs with flows of 10 million gallons per day (MGD) and greater to report
detailed unit operations for their treatment plants. These detailed unit operations provided
sufficient detail to distinguish between activated sludge treatment systems, those with alternative
systems such as attached growth operations, lagoons and ponds, polishing filter operations, and
those with advanced/tertiary treatment trains such as ammonia, nitrogen, and/or phosphorus
removal operations. The Agency compiled statistics on the treatment unit operations reported via
the Needs Survey and also utilized publicly available information (such as publications/information
from municipalities and satellite imagery) to verify or refute treatment train information. The Agency
is not aware of other compiled data sources that provide sufficient detail on treatment technologies
in use at POTWs in order to augment the data set compiled. While data collection for this study
only includes a small portion of the POTWs in the nation (approximately 5 percent), the combined
flow of those POTWs with flows of 10 MGD and greater represents approximately 70 percent of
the total wastewater treated at all POTWs nationwide.
Table 1 summarizes information on the numbers and types of treatment technologies at POTWs
with design flows of 10 MGD and greater. The Agency compiled and independently verified this
information from the 2008 Needs Survey.
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Table 1. Unit Operations at POTWs with Design Flows of 10 MGD and Greater
(Needs Survey 2008)
POTWs with Design Flows of 10 MGD and Greater 653
Reporting Detailed Treatment Unit Operations for 2008 556
with Secondary Treatment Only (Activated Sludge) 116
with Both Nitrogen and Phosphorus Removal Treatment 59
with Nitrogen Removal (no Phosphorus Treatment) 65
with Phosphorus Removal (no Nitrogen Treatment) 122
with Ammonia Removal (no Nitrogen and no Phosphorus Treatment) 107
with other Treatment (eg, Lagoons, Filter Units, etc.) 84
with Trickling Filters (any media type) only 32
Table 2 summarizes information on effluent discharges from the 116 Secondary Treatment facilities
utilizing activated sludge treatment technologies with design flows of 10 MGD and greater. Many
POTWs have effluent limits that are more stringent than the minimum requirements established by
40 CFR 133. Table 2 summarizes only the effluent monitoring data for those Secondary Treatment
facilities meeting the applicable requirements of part 133 (that is, those meeting or out-performing
30 mg/L TSS and 30 mg/L BODS or 25 mg/L cBODS).
The data contained in the PCS and ICIS-NPDES databases included data from POTWs served by
combined sewer systems (CSS). A subset of the POTWs with secondary treatment included in the
data set reported combined sewer overflow (CSO) outfalls ranging from one to dozens. There were
noticeable impacts on the discharge concentrations reported by POTWs as the number of CSO
outfalls increased. While this effect may not be a direct result of the number of the CSO outfalls,
EPA decided to limit the number of CSO outfalls included in this data summary. In order to
identify sufficient data to establish the capability of secondary treatment and to limit the potential
influence of combined sewers and CSOs; EPA has excluded all POTWs where the ICIS or PCS
database identified greater than five CSO outfalls from the data summarized below in Table 2.
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Table 2. Monitoring Summary for POTWs with Design Flow of 10 MGD and Greater (2008)
POTWs with Present Secondary Treatment (Activated Sludge) 116
with Secondary Treatment and 30 mg/L TSS limits 82
with Secondary Treatment and TSS limits < 30 mg/L \ 9
with Secondary Treatment, 30 mg/L TSS, and fewer than 6 CSO Outfalls 68
with Secondary Treatment, TSS limits < 30 mg/L, and fewer than 6 CSO Outfalls \ 6
Median TSS Monthly Average for POTWs with 30 mg/L TSS Limits and less than 6 CSO Outfalls 8.0 mg/L
95th percentile for POTWs with TSS Monthly Average with 30 mg/L TSS Limits and less than 6 CSO 20.0 mg/L
Outfalls
mg/L — Median TSS Monthly Average with TSS Limits < or = 30 mg/L and less than 6 CSO Outfalls 7.5
mg/L -- 95th percentile TSS Monthly Average with TSS Limits < or = 30 mg/L and less than 6 CSO \ 9.0
Outfalls
Number of Measurements with TSS Limits < or = 30 mg/L and less than 6 ('ISO Outfalls 854
POTWs with Secondary Treatment and 30 mg/L BODS Limits 40
with Secondary Treatment, 30 mg/L BODS Limits, and less than 6 CSO Outfalls 33
with Secondary Treatment and BODS Limits < 30 mg/L 9
with Secondary Treatment, BODS Limits < 30 mg/L, and less than 6 CSO Outfalls 8
mg/L -- Median BODS Monthly Average with 30 mg/L BODS Limits and less than 6 CSO Outfalls 9.2
mg/L -- 95th percentile BODS Monthly Average with 30 mg/L BODS Limits and less than 6 CSO 24.0
Outfalls
XumherofMeasurements with 30 mg/L BOD5 Limits and less than 6 CSO Outfalls ^(rf
mg/L -- Median BODS Monthly Average BODS Limits < or = 30 mg/L and less than 6 CSO Outfalls 9.1
mg/L -- 95th percentile BODS Monthly Average BODS Limits < or = 30 mg/L and less than 6 CSO 23.0
POTWs with Secondary Treatment and 25 mg/L cBODS Limits 50
with Secondary Treatment, 25 mg/L cBODS Limits, and less than 6 CSO Outfalls 42
with Secondary Treatment and cBODS Limits < 25 mg/L 25
with Secondary Treatment, cBODS Limits < 25 mg/L, and less than 6 CSO Outfalls 22
mg/L — Median cBODS Monthly Average with 25 mg/L cBODS Limits and less than 6 CSO Outfalls 5.2
mg/L -- 95th percentile cBODS Monthly Average with 25 mg/L cBODS Limits and less than 6 CSO \ 5.Q
Outfalls
\umber of Measurements with 25 mg/LcBOD5 Limits and less than 6 ("ISO Outfalls 452
mg/L — Median cBODS Monthly Average with cBODS Limits < or = 25 mg/L and less than 6 CSO 4.Q
Outfalls
mg/L -- 95th %tile cBODS Monthly Avg with cBODS Limits < or = 25 mg/L and less than 6 CSO 13.0
Outfalls
Number of Measurements with cBOD5 Limits < or = 25 mg/L and less than 6 CSO Outfalls ()SS
In developing the secondary treatment standards, the Agency included a small number of municipal
facilities employing trickling filter (attached growth) treatment in lieu of suspended growth
operations in the set of facilities forming the basis of the calculated standards. EPA examined the
degree to which current POTWs implementing trickling filter as their secondary treatment unit
operations have recently performed relative to the 30 mg/L TSS effluent limitations. Of the 21
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POTWs implementing trickling filter units (only) with 30 mg/L TSS effluent limitations the median
effluent 30-day average was 11.1 mg/L and the 95th percentile of 298 measurements was 20.0 mg/L.
Because POTWs differ between BODS and cBODS requirements, insufficient data sets exist to
support statistical calculations for biochemical oxygen demand performance at the trickling filter
plants. If the Agency integrated the trickling filter performance along with the activated sludge data
presented in Table 2, the median TSS 30-day average would be 9.0 mg/L and the 95th percentile
would not change from 20.0 mg/L.
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