United States	Office of Water
Environmental Protection	EPA 820-F-20-004
Agency	January 2021
Energy Efficiency Measures Provide Opportunities for Nutrient Reduction
At many publicly owned treatment works (POTWs),
operators experimenting with cost-saving energy
efficiency find their plants also benefit from improved
nitrogen removal. These successes provide staff with
confidence to implement low-cost modifications
and operational changes to further reduce effluent
nutrient discharges. EPA's National Study of Nutrient
Removal and Secondary Technologies investigates
optimization efforts across the country, and this fact
sheet highlights achievements at the Harriman POTW
in Tennessee.
In 2011, the Tennessee Water and Wastewater Energy
Efficiency Partnership (TWEEP) was formed between
many associations, including EPA and the Tennessee
Department of Environment and Conservation (TDEC).
The partnership supplied Tennessee wastewater
utilities with energy efficiency tools and expertise
to support operators in reducing energy costs and
pollution. This included providing in-person technical
assistance to staffs across Tennessee, including
Harriman POTW in 2014.
Harriman POTW
Harriman POTW has a design capacity flow of 1.5
million gallons per day (MGD) and an average daily
flow of 0.5 MGD. The plant has two equalization
basins, two oxidation ditches, two secondary clarifiers,
chlorine disinfection, and two aerobic digesters. Each
ditch has two fixed-speed rotors, and no chemicals are
added for phosphorus removal.
Prior to the partnership's visit, aeration for Harriman
POTWs oxidation ditches and digesters consumed
Harriman Staff: Donnie Fitzhugh and Ray Freeman
43% of the plant's total energy use. The four ditch
rotors ran continuously and the digester blowers ran 16
hours each night during the week and continuously on
Harriman POTW's staff started by following the
partnership's suggestion to cycle the four rotors 1
hour on/1 hour off, which decreased aeration energy
use by 50%. They noticed a drop in effluent Total
Nitrogen (TN), although concentrations were still
high, averaging over 20 mg/L. Inspired to realize
greater energy savings, staff continued to refine the
plant's aeration cycling on their own, resulting in a TN
concentration consistently under 10 mg/L beginning in
In July 2018, Ray Freeman took over as Chief Plant
Operator, and, assisted by Operator Donnie Fitzhugh,
the two began a quest to drive effluent TN as low as
possible. They experimented by ratcheting down rotor
National Study of Nutrient Removal and Secondary Technologies
Nutrient removal through optimizing plant operations

run times in small increments and alternating the
rotors' operation. The plant now operates 1 rotor per
ditch, cycling 1 hour on/2 hours off in the summer and
1 hour on/3 hours off in the winter.
"I started by taking baby steps to reduce power
consumption. In that process, I could see the
reduction in nitrogen. I just kept altering DO
levels and equipment run times until I could no
longer reduce TN without negatively effecting
other parameters, such as BOD." -Ray Freeman
Dissolved oxygen (DO) readings are obtained with a
hand-held probe near the influent inlet on the aft side
of the first rotor. The DO upper set point averages 1.75
mg/L on the aft side of the rotor, with the lower set
point targeted to 0.18 mg/L or less. The plant does
have a limited SCADA system that incorporates some
timers for the digesters, but the two operators closely
monitor and manage all aeration cycling in the ditches
by hand. Beginning in 2020, the average effluent TN
concentration was an impressive low of 2 mg/L.
Ray also adjusted the digester valves so only one
blower is needed to aerate both digesters for six hours
each night, further reducing plant energy costs. These
aeration strategies save the plant $30,000/year in
energy costs, achieving a total reduction in aeration
energy use nearing 85%.
Ray and Donnie have now turned their attention
to reducing total phosphorus (TP) effluent
concentrations and improving the plant's biological
phosphorus removal. Over the summer, they began
interrupting the 1 hour on/2 hours off schedule twice
each day to let the rotors run for 2 hours to drive
DO up to 2 mg/L. This was followed by 2 hours off
before resuming the 1 hour on/2 hours off schedule.
When the plant transitioned to the winter 1 hour on/3
hours off schedule, the 2 hours on/2 hours off cycle
was introduced only once per day. Harriman POTW's
average effluent TP concentration has already been
reduced 25% by these rotor cycling changes over the
course of the year.
Harriman Daily Maximum Monitoring Data
Optimization Opportunities and Benefits
Optimizing existing treatment
systems not only effectively reduces
nutrient discharges from POTWs,
but it can also result in significant ^ 4
energy and cost savings for utilities,
Support from regulatory agencies,
onsite consulting, and, most importantly,
operator ambition and enthusiasm enabled these
Tennessee POTW operators to reach both their
nutrient reduction and energy efficiency goals.

Effluent TN
(mg/L as N)
Effluent TP
(mg/L as P)
01 - 04 2017*
01 - 04 2020
Percent Removal
*Monitoring data from the first phases of optimization
(2014-2016) are not available.
Nutrient monitoring data were collected from EPA's Integrated Compliance and Information System-National
Pollutant Discharge Elimination System (ICIS-NPDES) and internal plant records. Energy savings are also
from internal plant records. TDEC and the TWEEP Partnership Team aided POTWs in Tennessee in improving
their energy efficiency and, in some cases, nitrogen discharges. Grant Weaver of CleanWaterOps has
supported Harriman staff with improving biological phosphorus removal.