should we keep do for nitrification...

Presented at the 2012 ISA Water / Wastewater and Automatic Controls Symposium

Holiday Inn Castle Resort, Orlando, Florida, USA – Aug 7-9, 2012 – www.isawwsymposium.org

SHOULD WE KEEP DO FOR NITRIFICATION CONTROL?

THE PROOF IS IN THE AMMONIUM ELECTRODES

Robert Lagrange1*, Sue Baert

2, Amanda Poole

3, Dave Green

3 and Nick Camin

4

1Lagrange Consulting, 4310 Royal Mustang Way, Snellville, Georgia, USA, 30039

(*correspondence: [email protected]) 2Wheaton Sanitary District, 1S640 Shaffner Road, Wheaton, Illinois, USA, 60189

3Baxter & Woodman Inc., 39 South LaSalle Street, Suite 816, Chicago, Illinois, USA, 60603

4Endress+Hauser Inc., 2350 Endress Place, Greenwood, Indiana, USA, 46143

KEYWORDS

Process Control, Optimization, Nitrification, Ammonium, Dissolved Oxygen, Energy Consumption,

Compliance, Instrumentation

ABSTRACT

Aeration often is the process that consumes the most energy in a wastewater plant. At the same time it

is an essential process to maintain compliance for BOD and Ammonia limits. It thus deserves a lot of

attention.

The use of dissolved oxygen measurement to control aeration is widely accepted. The literature has

many examples of excellent return on investment in wastewater plants that have implemented this

control. There are even theories that a concentration of 2 mg/l is the perfect value. But a DO sensor

only tells how much oxygen has not been used in the process and at times this measurement may not

bring the expected results. The effluent may end up out of compliance or too much energy can be

spent. Examples are available.

As one of the main objectives of the aeration tank is nitrification the measurement of the concentration

of ammonium or ammonia provides much better information than dissolved oxygen. Ion Selective

Electrodes (ISE) installed directly in the tank eliminate the need for sampling line and ensure an easier

integration. Their selectivity, accuracy and lower maintenance make them an attractive solution. As

demonstrated at Wheaton and Colorado Springs, additional energy savings and compliance are

possible when using either ammonia in a cascade loop with DO control or in direct control of the air

flow. Direct control either in feed-forward or feed-back provide faster and better response when spikes

are present.

The return on investment is very good even on smaller plants and compliance is achieved.

Dissolved oxygen concentration remains an important factor as it affects the quality of the sludge. The

control strategy has to maintain it within limits.

Lagrange, Baert, Poole, Green & Camin 2



INTRODUCTION

Many wastewater plants around the world are working hard to reduce their energy consumption and

the amount of nutrients discharged in their effluents.

As both carbon removal and nitrification consume oxygen it is important to insure that oxygen is

present in the right amount in the aeration tank. Dissolved Oxygen Instruments (DO) are typically used

for this application. But they are only an indication of the amount of free oxygen available at the

measuring point. The measurement of the amount of air sent to the tank provides the additional data to

determine the oxygen uptake rate.

In this paper the data from Wheaton Sanitary District (Illinois) provides the base for the discussion and

data from other sources will be added to confirm the findings: the installation of instruments for

automatic control is justified for small to large plants.

THE GOOD AND THE BAD OF DISSOLVED OXYGEN MEASUREMENT

The good

The literature abounds in examples of plants using DO to control the airflow to the aeration tank. The

return on investment is good even for small plants.

Eagle Lake Sewer Utility District in Wisconsin treats only 0.26 MGD and was able to justify the

installation of an optical DO instrument with the additional benefit of reduced maintenance cost

(Kohlmann, 2010).

As water and wastewater consume about 4% of the energy produced in the USA, power companies

have shown a big interest in promoting DO control associated with blowers with Variable Frequency

Drive (VFD). The Tennessee Valley Authority (TVA) sponsored demonstrations in real life at existing

wastewater plants. A result is shown in Figure 1.




Figure 1 TVA demonstration

The Installation of DO sensors and VFD’s provided a 13% reduction in consumption and most

importantly a 39% reduction in peaks with the corresponding rate change..

More recent data shows that reduction in consumption of up to 50 to 60 % can be achieved and 80%

when new diffusers are used (Poole, 2011)

The bad

As we said earlier DO only shows the residual amount of oxygen available in the tank. It certainly does

not define the elimination of carbon nor does it tell precisely what is happening with nitrification or

denitrification.

The Hanover Area Regional WWTP (Pennsylvania) installed Ion Selective Electrodes instruments in

their oxidation ditch. For the initial period the data, Ammonium and Nitrate concentrations, were

recorded but not available to the operators. They had the information from a flowmeter, a DO and a

NADH (Fluorescence of nicotinamide adenine dinucleotide) to operate the plant. The surface aerators

had only 2 speeds. (Lagrange, 2009)

Figure 2 shows the location of those instruments at about 1/3rd

of the tank.




Figure 2: Location of the instruments at Hanover

Figure 3 Hanover Data

Figure 3 shows the data for almost a week of operation. In the initial part the DO value does not

change while both the flow and the ammonium concentration (scaled 0 to 15 mg/l) change. As a result

the effluent was out of compliance. Then the aerator speed is finally adjusted between high and low to

follow the diurnal flow variations. The ammonium concentration is drastically reduced. Too much in

fact as at 1/3rd

of the ditch it is already below 2 mg/l. The plant was expecting to see some

denitrification taking place at low aerator speed (nitrate concentration scaled 5 to 25 mg/l). Again it is

not happening. The plant was using far too much energy.




EXPERIENCE AT WHEATON SANITARY DISTRICT

The Wheaton Sanitary District is also looking at ways to maintain or improve the quality of its effluent

while reducing the consumption of energy. A new turbo blower was installed in 2011 to replace an

existing multistage centrifugal blower. Then a joint project was started with Baxter & Woodman to

find the best control strategy for that specific plant. Endress+Hauser Inc and WTW (now Xylem)

provided instrumentation assistance for the project.

The plant is designed to treat a dry weather flow of 8.9 million gallons per day (MGD), a design

maximum flow of 19.1 MGD and a peak wet weather flow of 45 MGD. The effluent is discharged in

Spring Brook with very low flow that dictates the willingness of the District to discharge effluents with

a quality well above their discharge permit.

The treatment consists of screens, sand, grease and fat removal, primary clarification, trickling filter,

aeration, secondary clarification, filtration and disinfection. The District has 5 aeration tanks in

parallel. Each equipped with one Air Flow measurement and one DO measurement. For the test one

Ion Selective Electrode (ISE) based measurement for ammonium was installed toward the start of tank

2 and another at the end of tank 3

Figure 4: Wheaton aeration showing the location of the instruments




Step 1 Air Flow Control

After the installation of the new blower the first step was to maintain a constant air flow to each tank.

With this in place the DO concentration keeps changing reverse tracking the ammonium concentration

measured close to the inlet in tank 2 as expected. During that time the ammonium concentration in

tank 3, measured close to the end of the tank, remains constant around 0.4 mg/l very close to the limit

of detection of the instrument. This confirms that nitrification is complete. In the final effluent, after

some post aeration, the ammonia concentration is at a non detectable level in laboratory samples

(<0.05 mg/l)

Figure 5: Tank2 in airflow control mode

A 28% energy consumption saving was achieved, 10% of which can be attributed to the control itself

Step 2 DO Control

In that phase the control objective is to maintain a constant DO. With the correct DO set point there

should be enough air to keep the solids in suspension and prevent settling in the aeration tank.




In that period, as can be seen in figure 6 there were some ammonium spikes the largest and with

highest rate of increase was on January 14. The control reacts well with that spike. As the DO

measurement is located toward the end of the tank, some 20 feet, the control had difficulty to keep up

with the fast change resulting in a very small blip on the effluent ammonium measurement. The change

is less than 0.2 mg/l increase.

There was a 9% increase in energy consumption compared with the period with airflow control.

Figure 6 DO Control

Step 3 Ammonium Control

Figure 7 compares tank 2 in ammonium control with tank 3 in airflow control. The variations in DO

concentration are drastically reduced and in tank 3 the process remains under control with the big spike




of ammonium on January 28/29. At the same time with the same wastewater in tank 3 the oxygen is

completely depleted and the nitrification is not complete.

The use of ammonium in feed-forward provides a better control, more stable process and insures that

the concentration of ammonia in the effluent remains at the low level desired by Wheaton.

Figure 7 Feed forward NH4 control

ISE A BETTER TOOL

When normalizing for changes in flow and BOD load the results favor ammonium control

• Step 2 consumes 5% more energy

• Step 3 consumes 6% less energy.

From Wheaton’s point of view achieving a stable process while maintaining their very low ammonia

discharge justified the investment to install one ISE instrument in the splitter box ahead of the 5

aerations tanks and modify the control strategy to feed-forward control. The decommissioning of the




trickling filters will bring a higher nutrient load to the aeration tank allowing for a more efficient

control as the low limit on air flow from the blower will be prevented. As we can see in figure 7 there

is margin to reduce the energy consumption.

Colorado Springs Utilities

Colorado Springs Utilities (Lagrange, 2011; Camin, 2011) with a different set of problems took a

different approach. That plant faces very high daily ammonia peak while trying to run only their small

compressor. Designed for 20 MGD the actual flow was only 8 MGD at the time of the study, While on

DO control (those DO units were installed at construction time) the plant faced some high ammonia

concentration in the effluent or at other times low pH as nitrification consume alkalinity. The decision

was then made to install ISE measurement for both ammonium and nitrate as some denitrification was

implemented to increase alkalinity. The locations are described in figure 9.

Figure 8 Concentration of influent ammonia in green and power consumption at CSU




Figure 9 Installation point for the instruments at CSU

To increase the speed of response of the control loop the air flow is directly controlled based on the

ammonium concentration. The control takes into account the DO concentration to maintain it within

high and low limits.

With that control strategy CSU was able to reduce the cost of energy while the flow increased as

shown in figure 10 for the months of November 2009 and November 2010.




Figure 10 CSU flow and cost

The consumption of energy per MGD per pound of ammonia nitrified was reduced by 12% compared

with standard DO control. In the future the Ammonium sensor after the primary will be relocated to the

swing zone to combine feed-forward and feedback.

Others

The city of Peoria, Arizona, operated their Beardsley Road Water Reclamation Facility, a 4 MGD

plant, with a cascade control where the output of the Ammonium controller adjusts the set point of the

DO control loop. They were able to achieve a 3 months return on investment and improve the sludge

quality. (Dabkowski, 2011)

Rieger (2012) also demonstrated in three wastewater plants with average flows of 3.5, 10.5 and 54

MGD in Switzerland that the use of ammonium control provides benefits on other processes such as a

reduction in the consumption of chemicals for phosphorus removal as more volume can be used in

anaerobic phase.




SUMMARY

Wheaton Utilities District decided to use a control strategy based on feed-forward based on ammonium

concentration as it provides them with the best results for their objectives. Colorado Springs used a

partial feedback and achieved excellent results. Peoria included the ISE in a cascade control for a high

return on investment.

Further reading is available. (Grievson, 2012)

We would like to leave the final comments to Leiv Rieger

“Ammonia controlled aeration in an activated sludge system leads to a significant reduction of the total

energy consumption in conjunction with decreased effluent concentrations of total nitrogen…..The

best solution will be the simplest concept that still yields a significant benefit in comparison with the

annual costs.”

LIST OF ACRONYMS:

ISE ................. Ion Selective Electrode

DO .................. Dissolved Oxygen

NADH ........... nicotinamide adenine dinucleotide (fluorescence of)

CSU................ Colorado Springs Utilities

MGD .............. Million Gallons per Day




REFERENCES

Camin & al. Ammonia Based Aeration Control Workshop 105 WEFTEC 2011

Dabkowski & al. Ammonia Based Aeration Control Workshop 105 WEFTEC 2011

Kohlmann. Saving costs by continuous monitoring of D.O. in plant Application Note Endress+Hauser

Inc. 2010

Grievson Aeration Control in wastewater activated sludge plants using mixed liquor, organic loading

and ammonia analysis Water Industry Process Automation & Control Group, March 2012

https://www.onlinefilefolder.com/2s7lVnXAEb7zZW

Lagrange & al. The revival of the Ion Selective Electrode Applications in Wastewater Plants ISA Expo

2009

Lagrange & al. A change in control strategy reduces power consumption at Colorado Springs Utilities

Energy & Water 2011

Poole & al. Aeration System Automation: Control Strategies to Maximize Energy Savings at Low

Capital Cost WaterCon 2012

Rieger & al. Improving Nutrient Removal While Reducing Energy Use at Three Swiss WWTPs Using

Advanced Control Water Environment Research, Volume 84, 2012




About the Authors:

Robert Lagrange, PhD is a Doctor in Physics from the University of Grenoble, France. After 11 years

with Endress+Hauser as Business Manager Water and Wastewater Robert is now working part time as

a consultant. Robert presented multiple times at the ISA WWACS and is a member of WEF and

AWWA instrumentation and control committees

Sue Baert, B.Sc. is the Plant Superintendent at the Wheaton Sanitary District where she started as a

chemist became the Lab Manager and wrote the Quality Assurance Project Plan for the stream water

quality project, collected, analyzed and submitted the data. She has a degree in Biology/Chemistry

from the UW-Lacrosse. Sue is involved with the DuPage Salt Creek watershed group as Vice-

President. She is an active member of WEF through the local MA Central States Water Environment

Association (CSWEA). She was President of the Illinois section of Central States for a one year term,

and then Illinois Trustee to the Executive staff for two years. In all, her career in the water/wastewater

section has spanned over 22 years.

Amanda Poole, M.Eng. is an environmental engineer at Baxter & Woodman, where she focuses on

energy reduction and generation measures at wastewater treatment plants. Amanda has been involved

in numerous wastewater treatment plant energy audits and aeration energy reduction projects

throughout the Chicago area. Amanda received her B.S. and M.S. in Environmental Engineering from

the University of Illinois in Urbana – Champaign. She has been working in the water/wastewater sector

for the past 3 years and awaits her P.E. licensure in December 2012. She is an active member of the

Central States Water Environment Association.

David Green, AAS EET joined Baxter & Woodman in 2008 as a Senior Systems Integrator. Dave’s

passion for automation and innovation led him to become the Automation Technical Director. He

brings over 16 years of experience designing and providing unique solutions to Automated Systems

Integration projects. He has quickly become a technical leader in the organization, and has a hunger

for learning new technology and finding the best solutions for automation problems.

Nick Camin, BSEE, MBA has worked in the instrumentation field for over thirteen years specializing

in environmental industry applications for over ten of them. Nick is a graduate of Purdue University

with a bachelor of sciences degree in electrical engineering and he also holds a master of business

administration degree from Indiana Wesleyan University. With Endress+Hauser he has held the

positions of application engineer, project manager, municipal business manager, regional sales

manager and currently he is marketing manager for the environmental industry. Nick is a member of

WEF and AWWA instrumentation and control committees.

should we keep do for nitrification...

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