The City of St. Cloud is located in central
Minnesota a little more than one hour
north of the Minneapolis and St. Paul
metropolitan area. The City of St. Cloud’s
wastewater treatment system consists of
290 miles of sewer main, 36 liftstations,
and a newly rehabilitated, expanded
and upgraded biological nutrient
removal (BNR) facility that can treat 17.9
million gallons a day. The city provides
wastewater treatment services to fi ve
neighboring communities.
The dedicated staff of wastewater
treatment professionals employed by the
City of St. Cloud has developed a culture
of continuous improvement. This dedica-
tion is recognized immediately when you
arrive at the facility. Meticulous maintained
buildings and equipment, well-manicured
grounds, and impressive granite build-
ings that remind visitors of St. Cloud’s
proud heritage. It is easy to be caught off
guard by the facility; it is not what people
normally imagine when they think about a
wastewater treatment facility.
The city just recently completed a
$48M wastewater treatment facility
improvement project. The project included
the rehabilitation of 40 year old structures
and equipment, upgrading the treatment
process from activated sludge to a biologi-
cal nutrient removal process, upgrading
disinfection by using ultraviolet technology
and expansion from 13 million gallons a
day capacity to 17.9 million gallons a day.
A HOLISTIC PLANNING PROCESS
The planning for the improvement project
began over 13 years ago with discussions
with area stakeholders regarding the need
for rehabilitation to provide capacity for
future growth. A key aspect of the discus-
sions and planning was to incorporate St.
Cloud’s culture to not simply meet current
regulatory requirements, but to exceed
them in the short and long term.
To collaborate and plan for the upcom-
ing project, a subcommittee of a joint area
cities planning group was formed. The
subcommittee was named the St. Cloud
Area Wastewater Advisory Commission or
SCAWAC. SCAWAC membership con-
sisted of at least two members from each
community. The goal of SCAWAC is to
promote the equitable and effi cient distri-
bution of wastewater treatment facility cost
and services. SCAWAC’s presence was
crucial to the success of the project. The
City of St. Cloud staff met with SCAWAC
often throughout the planning, design and
construction phases of the improvement
project to ensure success.
The city was able to overcome several
challenges by working with key stakehold-
ers including the fi ve other communities
served by the WWTF and their respective
councils, regulatory agencies, environ-
mental advocacy groups and the public.
City of St. Cloud, MN
Where Continuous Improvement is the
Standard Operating Procedure
PLANT PROFILE
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PLANT PROFILE
PRELIMINARY AND
PRIMARY TREATMENT
Ninety-eight percent of the wastewater
fl ows to the Main Lift Station located
three miles north of the treatment facility.
The Main Lift Station has a barscreen
and wash press. The remaining fl ow
enters through two lift stations that are
directly connected to the 30” forcemain
and from one contract city, which enters
the headworks just prior to another bar
screen and wash press. Wastewater fl ows
through a vortex grit removal system and
a splitter box prior to entering the four
primary clarifi ers.
SECONDARY TREATMENT
The secondary treatment system was
upgraded to a biological nutrient removal
system using a Modifi ed Johannesburg
Recycle system. Although the facility’s
current NPDES permit does not have
a nitrogen limit, the City incorporated
nitrogen removal into the facility design
to provide additional environmental
protection and also to prepare for future
regulatory requirements.
The facility has the fl exibility to operate
exclusively in Bio-P mode (Biological
Phosphorus Removal) or in Full BNR
mode (Nitrogen & Phosphorus Removal).
With four treatment trains in service, the
capacity of the trains to operate in Bio-P
mode is 17.9 MGD. When operating in
full BNR mode, the capacity of the four
trains is 15 MGD.
Wastewater then fl ows through a BNR
basin fl ow splitter box. The fl ow splitter box
is a hexagonal cast-in-place structure. The
splitter box was designed as a hexagon
to distribute fl ow evenly for the future fi fth
BNR treatment train.
The three existing aeration trains and
fi nal clarifi ers were rehabilitated into
new BNR treatment trains and a fourth
BNR treatment train was constructed.
An innovative feature of the project
was the conversion of the existing fi nal
clarifi ers into oxic volume. The existing
wastewater treatment facility had three
90-foot diameter rim-feed fi nal clarifi ers,
with operating depths of 12 feet. The
drive mechanisms and related equipment
had well exceeded the industry standard
for useful life. Instead of demolishing the
fi nal clarifi ers, they were converted to oxic
volume for BNR treatment trains 1-3. By
converting the clarifi ers, the city gained the
required hydraulic capacity to incorporate
nitrogen removal. This reuse of existing
structures saved users of the WWTF
several million dollars in capital cost.
Each BNR treatment train consists of
a pre-anoxic zone, an anaerobic zone,
an anoxic zone and an oxic zone prior to
fi nal clarifi cation. The process engineers
from Black & Veatch incorporated three
separate anoxic zones. The second and
third anoxic zones are called “swing
zones.” If the facility is running in Bio-P
mode, the second and third anoxic zones
are operated as oxic volume. In BNR
mode, the air is shut off in these zones so
there is additional capacity for nitrogen
removal.
The facility has four new 110-foot
diameter fi nal clarifi ers. Wastewater from
the BNR treatment trains fl ows to the fi nal
clarifi ers prior to disinfection.
DISINFECTION AND SOLIDS
PROCESSING
The chlorine and sulfur dioxide gas
disinfection system was replaced with
ultraviolet light disinfection using a Trojan
3000+ UV light system. There are two
channels that can disinfect 17.9 MGD
each.
Solids processing starts with the
primary solids and waste activated
solids being dewatered by a gravity
belt thickener. The city has two primary
anaerobic digesters, one secondary and
one storage digester. The city produces
over 13 million gallons of biosolids a
year. This highly valuable agricultural
product is recycled on approximately
2,000 acres of state approved
application sites. It is a highly sought-
after product.
WASTEWATER TREATMENT PROCESS - PROTECTING OUR WATERWAYS
BNR TREATMENT TRAIN UV LIGHT SYSTEM
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IMPACT OF SOLIDS PROCESSING CHANGE
The Facilities Plan originally included the
conversion of a secondary anaerobic
digester to a primary digester because
solids retention times required to meet
Class B biosolids was getting close to the
minimum 15 days. At that time, the city sent
the primary solids to the anaerobic digester
and the waste activated sludge was going to
dissolved air fl oatation devices. During the
optimization of a new gravity belt thickener
that was installed in late 2003, both primary
solids and waste activated solids were sent
to the gravity belt thickener. This change in
solids processing resulted in increasing SRT’s
from 16 days in 2004 to 30+ days. This
helped the city avoid approximately $12 mil-
lion in capital expenditures.
The solid processing change also elimi-
nated the need for side stream treatment
and the potential for slug loading from the
decanted portion of the biosolids. In the
past, slug loading from decant increased
loading to the system several times above
normal and limited pollutant removal effi -
ciencies due to fi lamentous bacteria growth
and foaming in the activated sludge tanks.
PERFORMANCE
After substantial completion of the con-
struction project, the facility pollutant
removal rates exceeded the design and staff
expectations. A comparison of annual pol-
lutant loading in 2008 (before construction)
versus 2012 (post construction): the facility
has reduced the discharge of nitrogen by
over 46,000 pounds and phosphorus by
over 19,000 pounds.
An additional benefi t to the improve-
ment project was the increased removal
effi ciency for total suspended solids and
biochemical oxygen demand. Prior to
project completion, the effl uent averaged
6 mg/L for BOD and TSS; now the effl uent
concentration averages less than 2 mg/L.
The majority of the results are below the
laboratory method detection limit. In 2012,
the facility discharged 118,769 fewer
pounds of TSS and 135,605 fewer pounds
of BOD than it did in 2008.
CONCLUSIONS
The performance results and success of the
recent project is a direct refl ection of the
culture of continuous improvement mindset
of the staff. Signifi cant effort and ingenuity is
always on the to-do list with protecting the
receiving water as a primary daily objective.
OXICFINAL CLARIFIER MECHANISM
Primary Anaerobic Digestion
Solids Retention Time (SRT)
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
SRT (days)
40
30
20
10
0
Minimum SRT to meet Class B Biosolids Regulations
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As the project reaches fi nal completion, city staff continues
to strive for improvement. The city was recently notifi ed by the
National Biosolids Partnership that the Biosolids Environmental
Management System (EMS) Program has achieved Gold Level
certifi cation. Gold Level certifi cation is a result of substantial
effort by the facility’s EMS Team and the ongoing commitment to
continuous improvement.
Facility staff are now focusing on providing cost-effective
services by researching potential energy resource and recovery
options that can be employed at the facility. The goal is to always
go above and beyond.
PLANT PROFILE
TOTAL ANNUAL POUNDS OF PHOSPHORUS IN EFFLUENT
TOTAL ANNUAL POUNDS OF AMMONIA IN EFFLUENT
TOTAL ANNUAL POUNDS OF
TSS & BOD IN EFFLUENT
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