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WERF Research Synopsis (continued)

On-Line Nitrogen Monitoring and Control Strategies

Summer 2006

A survey 24 wastewater treatment facilities showed that 20 facilities utilized online nitrogen instrumentation.  Of these 20 facilities, only two reported using the analyzers as part of a automatic nitrogen control strategy.  Data are reported on the types of instruments in use, their capital and annual maintenance costs, and how effective the user considered the instrumentation to be.  The survey also compiled information on nitrogen control strategies used by these facilities.

 

Of the fifteen survey respondents to report nitrogen control strategy effectiveness, only five (33%) reported that their control strategies were extremely effective.  Of these five facilities, four utilize manual control and only one treatment facility reported utilizing automatic control in conjunction with their online nitrogen instrument.  This facility was therefore identified as a case study. These five treatment facilities primarily control dissolved oxygen concentration, RAS and WAS flows.  None of the surveyed treatment facilities reported their nitrogen control strategy as “Not Effective.”   In summary, one treatment facility noted that if they could start over with nitrogen monitoring knowing what they know now, they would do the following things:

 

1. Use only one type of instrument,

2. Choose the most reliable instrument that requires the least maintenance, and

3. Use at least one instrument for each battery (process train).

 

Plant Surveys

 

Literature Search and Case Studies

The survey was supported with a review of the technical literature to document case histories in which automated nitrogen control strategies were practiced.  A variety of control strategies are identified and described based on this literature review.  These are summarized in Table 2.

The literature information was augmented with case histories developed based on interviews and information provided by six agencies (one industrial and five municipal) operating 12 treatment plants.  Of these 12 facilities, six utilized manual nitrogen control procedures while four had online nitrogen instrumentation that is used for monitoring purposes with limited control.  In most cases, these agencies installed online nitrogen analyzers and implemented a nitrogen control strategy to comply with regulatory limits placed on the discharge of ammonia-nitrogen, nitrate or total nitrogen from their facilities.

 

In summary, treatment plant case study investigations revealed that most of the treatment plants installed on-line nitrogen instruments and implemented their nitrogen control strategy to comply with regulatory discharge permit limit requirements of ammonia-nitrogen, nitrate and total nitrogen and to optimize the performance of their nutrient removal treatment processes.  Furthermore, most of the treatment plants utilized a dissolved oxygen set point to control aeration (either by manual or automatic adjustments of blower airflow) and used effluent nitrogen instrumentation (measuring either ammonia or nitrate or a combination of nitrogen compounds) to determine the need for dissolved oxygen concentration blower airflow adjustments that optimized nitrogen removal to permit requirements.  A WERF project 99-WWF-4, Sensing and Control Systems: A Review of Municipal and Industrial Experiences revealed that “an instrumentation and control survey designed to assess the current state of automation showed that most respondents justified installing automation systems because of cost savings, though less than 10% of the facilities had data demonstrating these savings.”

 

 

Field Test

Based on the technology review and case studies, on-line, in-situ nitrate analyzers were selected for field testing to determine the performance of this instrumentation in an actual wastewater treatment plant monitoring application.  Eight manufacturers were requested to participate in the field test and five agreed.  The nitrogen instruments included in the field test are listed in Table 3.

The field test collected 12 weeks of data on instrument performance as well as subjective information of the design of the equipment can be important to the user in selecting the appropriate equipment for a particular application.

Table 4 compares the performance of the five nitrate test instruments.  Considerable differences were evident in the performance of the five test instruments in terms of accuracy compared to the off-line laboratory measurements and the number of calibration and maintenance events required during the test period.

Note (1) :  On 10-31-05, the Hach Evita underwent a maintenance event by the factory representative, following which it underwent a complete recalibration.  This event is considered to be a maintenance event only in the above summary.

Table 2. Nitrogen Control Strategies Categorized by Process

Treatment Process

Type of Nitrogen Control Strategy

Activated Sludge

Mean Cell Residence Time (MCRT) or Solids Retention Time (SRT) Control

Respiration Rate (Oxygen Uptake Rate) Control

RAS Flow Control

WAS Flow Control

Blower Control (aeration control)

Dissolved Oxygen Levels Control (aeration control)

Sludge Blanket Level

Nitrification

pH levels

Alkalinity levels

Automated MCRT or SRT Control

Aeration Control based on Dissolved Oxygen (DO) Concentration Set Point

Aeration Control based on Ammonium Concentration Set Point

Aeration Control based on DO and Ammonium

Phase length (intermittent aeration) Control

Respiration Rate (Oxygen Uptake Rate) Control

Redox or ORP Control of Aeration

Denitrification

Control of external carbon source (i.e. methanol) based on nitrate concentration

Control of recirculation rate based on nitrate concentration

Phase length (intermittent aeration) Control

Redox or ORP Control

Table 3Nitrogen Instruments Field Tested

Manufacturer

Model/Sensor

Technology

Endress+Hauser

Stamo-Sens CNM750/CNS70

In-situ UV Absorption

HACH

Evita Insitu 5100 (Danfoss)

In-situ UV Absorption

HACH

NITRATAX plus sc

In-situ UV Absorption

s::can Messtechnik GmbH as distributed by Royce Technologies

82 N Nitrolyser sensor / 8580 Constat analyzer

In-situ UV Absorption

Wedgewood Analytical/E+H

STIP-scan

In-situ UV Absorption

Table 4.  Comparative Analysis Summary

Parameter

Endress + Hauser

Stamosens

Hach

Evita

Hach

Nitratax

Royce S::can

Messtechnik

Wedgewood

Stip Scan

Mean Deviation from Laboratory Conformance Test

-0.08

2.88

0.68

0.26

0.55

Correlation to Laboratory Conformance Test (R2)

0.92

0.02

0.97

0.91

0.62

Percent of Values within +/- 1.0 mg/L of Laboratory Conformance Test

95%

36%

82%

69%

54%

Number of Out-of-Compliance Events

0

4

1

4

4

Number of Recalibrations

0

0

0

3

5

Number of Maintenance Events

0

3 (1)

1

0

1

Support Required from Supplier during Test

No

Yes

Yes

No

Yes