ww_t.am_09.10_dewitt.pdf

7/27/2019 WW_T.am_09.10_DeWitt.pdf

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NITRIFICATION AND ACTIVATED SLUDGE FOAMING -

RELATIONSHIPS AND CONTROL STRATEGIES

Darrell DeWitt, Charlotte-Mecklenburg Utilities*

David Wagoner, P.E., CDM

ABSTRACT

Regulatory limits for nitrogen and phosphorus in wastewater treatment plant effluents are becomingprogressively more stringent. As a result, wastewater treatment plants (WWTP) are being challengedmore than ever as operating margins grow tighter. Operators are being challenged to achieve greaterand/or more consistent performance to meet NPDES discharge limits with existing facilities.

One of the key biological processes that is critical for nitrogen discharge control is nitrification. Nitrifyingmicroorganisms (nitrifiers) compose a group of microbes specialized in converting ammonia nitrogen tonitrite and nitrate nitrogen. Nitrifiers are slower growing, are maintained at much lower populations, and

are much more sensitive to operating conditions (temperature, alkalinity, pH, solids retention time) thanmore common floc forming heterotrophic microorganisms. In controlling the balance of these operatingconditions to maintain nitrification under various diurnal and seasonal conditions, activated sludgefoaming can become a problem. Nocardia sp. filamentous microorganisms may over-populate the mixedliquor suspended solids (MLSS) creating excessive foaming in the aeration basins and scumaccumulation in downstream treatment units. Under excessive foaming conditions, the nitrificationprocess can be impacted. The process control relationships between biological foaming and nitrificationcan be counterintuitive and require careful operator attention in order to maintain process control.

For the Charlotte-Mecklenburg Utilities (CMU) Mallard Creek Water Reclamation Facility (WRF), foamingconditions have posed operational challenges for maintaining nitrification performance to meet effluentdischarge limits. While the basis for achieving nitrification is consistent, each wastewater treatment facilityis unique and must develop its own set of conditions and control measures for addressing process issues.

This paper discusses the relationships of nitrification and foaming, presents process data and controlapproaches used to enhance performance reliability and maintain permit compliance, and providesinsight to others that may be experiencing similar process challenges.

KEYWORDS

Activated Sludge, Foam, MLSS, Nitrification, Nocardia sp.

INTRODUCTION

One of the key biological processes that is critical for nitrogen discharge control is nitrification. Nitrifyingmicroorganisms (nitrifiers) compose a group of microbes specialized in converting ammonia nitrogen tonitrite and nitrate nitrogen. Nitrifiers are slower growing, are maintained at much lower populations, and

are much more sensitive to operating conditions (temperature, alkalinity, pH, solids retention time) thanfloc forming heterotrophic microorganisms. In controlling the balance of these operating conditions tomaintain nitrification under various diurnal and seasonal conditions, activated sludge foaming canbecome a problem. Nocardia sp. filamentous microorganisms may over-populate the mixed liquorsuspended solids (MLSS) creating foaming in the aeration basins and scum accumulation in downstreamtreatment units. Under excessive foaming conditions, the nitrification process can be impacted. Theprocess control relationships between biological foaming and nitrification can be counterintuitive andrequire careful operator attention in order to maintain process control.

For the Charlotte-Mecklenburg Utilities (CMU) Mallard Creek Water Reclamation Facility (WRF), foamingconditions have posed operational challenges for process control and maintaining nitrificationperformance. While the basis for achieving nitrification is consistent, every wastewater treatment facility is


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unique and must develop its own set of conditions and control measures for addressing relative processissues. The work effort described and the resulting information presented in this paper is part of aprogram to promote continuous improvement at the WRF - a key initiative throughout CMU.

The WRF treatment system includes flow equalization, primary clarifiers, Modified Ludzack-Ettinger(MLE) activated sludge system, tertiary sand filters, and anaerobic digesters. Historically, the WRF hashad issues with foaming conditions which caused issues in the aeration basins and in downstream unit

processes - conveyance foaming, secondary clarifier excessive scum, excessive blinding of final filters,and impacts to water reuse program. Figures 1 and 2 present typical nocardial foam and scumconditions.

Figure 1 Nocardial foam on aeration basin surface

Figure 2 Nocardial scum on secondary clarifier surface

Compliance with ammonia nitrogen limits is challenged at times by periodic ammonia pass-through of theaeration basins particularly during winter operating conditions.

This paper discusses the relationships of nitrification and foaming, presents the results of process dataevaluations, presents control approaches used to control foaming and to enhance nitrification reliabilityand overall plant operations, and provides insight to others that may be experiencing similar processchallenges.


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METHODOLOGY

An integrated, hands-on operations approach was used to evaluate the issues of foaming and nitrification

performance at the WRF. The steps included:

Defining the situation at the facility with regard to foaming and nitrification issues

Understanding background information - assimilating process data and information resources

Assessing the information, and

Developing recommendations to address the issues

Background data used for the evaluation were historical operations and process data from operator

sampling, and archived and real-time data from SCADA via on-line analyzers. Key on-line monitoring

includes ammonia, pH and dissolved oxygen (DO). As the evaluation program progressed, additional on-

line analyzers for ammonia (primary effluent and aeration basins) and TSS (aeration basins and

secondary effluent) were installed for process/performance monitoring and data trending. To fully

understand the components of the foam to help establish the root causes, microscopic analyses of MLSS

and foam were conducted.

The initial review of historical data from periods of ammonia pass-through of the aeration basins was

somewhat puzzling as no critical process factors seemed to be askew DO, MLSS/MCRT, alkalinity, and

temperature all appeared to be in good range for providing complete nitrification based on the flows and

loading conditions to the WRF. Short circuiting through the aeration basins was not a factor. From this

review, sample collection points were examined and additional MLSS sampling points along with

additional auto samplers for TSS (MLSS) were implemented to help with understanding the MLSS

concentration and profiles in the aeration basins and in aeration basin effluent. Grab samples from the

aeration basin were collected and analyzed for ammonia nitrogen to develop a nitrification profile along

the length of the aeration basins. From this work, additional ammonia on-line analyzers were installed to

track ammonia levels in the influent to and through the aeration basins, see Figure 3.

Figure 3 On-line ammonia analyzer for secondary influent monitoring

RESULTS

The microscopic analyses of the MLSS and associated foam identified Nocardia sp. bacteria as the

agents promoting the foaming conditions. Figure 4 presents an example micrograph of Nocardia sp.

bacteria. Nocardial filaments cause foaming due to their hydrophobic nature. Their growth is promoted by

several conditions generally in combination including elevated mean cell residence time (MCRT), higher

levels of fats/oils/grease/fatty acids in the wastestream, acidic range pH levels, and foam trapping


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conditions. Excessive nocardial growth creates a persistent foam and scum conditions in treatment units

which can overcome the handling capacity for treatment systems. Foam on the aeration basins contains

the same organisms as in the MLSS except in more concentrated form. Activated sludge bacteria residing

in the foam are removed from the active mixture and not as available as those bacteria within the

liquid/mixed MLSS for stabilization of the wastewater constituents including ammonia nitrogen.

Figure 4 Nocardial filaments

Evaluation of MLSS data initially from grab samples and later from online analyzers indicated that there

was significant variability in MLSS concentrations across the length aeration basins, the severity of which

was dependent upon the severity of foaming conditions. The MLSS profile data indicated a progressive

decline in the liquid concentration of solids in the MLSS in the aeration basins (samples collected from the

liquid beneath the foam layer). This liquid MLSS concentration was given the term effective MLSS as it

represents the MLSS that is most actively engaged in the aeration basin mixture. The term total MLSS

was given to the mixture of liquid and foam. Samples of this mixture was historically collected

downstream of the aeration basins and used for process control decisions. Figure 5 presents an example

of a trend of the change in Total and Effective MLSS as foaming conditions increase.

Figure 5 Total Vs Effective MLSS Trend with Increasing Foaming

The ammonia nitrogen data from the aeration basin profiles indicated that incomplete nitrification is linked

to elevated foaming conditions causing lower effective MLSS concentrations.


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DISCUSSION

Nocardia sp. is a type of branched filamentous bacteria that is part of the population of microorganisms in

a healthy activated sludge system. However, excessive/disproportionate nocardial levels will lead to

increasing levels of foaming and scum production. To help minimize foaming in systems that are prone to

Nocardia, aeration systems should be carefully controlled so over-aeration does not occur. Excessiveaeration creates excessive foaming in elevated Nocardia conditions. The WRF has installed automated

DO control systems that help to minimize excessive aeration conditions. Key food sources for Nocardia

are fats/oils/grease and fatty acids. Limiting these foods will help to control nocardial populations.

Maintaining a near neutral pH in the aeration system is also a benefit to help control Nocardia and to

support nitrification performance. MCRT perhaps is the most important process parameter for Nocardia

control and for nitrification performance. Generally, nitrifiers require an older sludge or higher MCRT.

However, higher MCRT levels will promote Nocardia growth. Herein rests the critical balance -

maintaining high enough MCRT to support nitrification performance and low enough MCRT to minimize

Nocardia growth.

For the WRF, the MCRT operating range to control this balance is very narrow and presents a challenge

to maintain adequate MCRT to achieve complete nitrification while controlling foaming. Historically,

winter operations have presented the greatest challenge. During winter operations when nitrification rates

decrease due to lower aeration basin temperatures, to maintain adequate nitrification performance, the

MCRT generally needs to be raised in the effort to increase the population of nitrifiers in the MLSS. In

this effort, WAS is systematically reduced to raise the MLSS (increases MCRT). However, in systems

prone to nocardial foaming this increase in MCRT can lead to negative results. The increasing MCRT

promotes Nocardia, increases foaming, decreases effective MLSS, and decreases active nitrifiers

resulting in diminished nitrification performance. When nitrification performance starts to decrease, the

normal impulse is to preserve/build nitrifiers by reducing wasting. However, this creates more selective

forces for increasing Nocardia which creates further detrimental conditions for nitrification. Figure 6

presents a chronological plot showing the impact of decreasing wasting, increasing foaming, drop in

effective MLSS, and decrease in nitrification performance.

Figure 6 Chronological Plot of Effective and Total MLSS, and Nitrification with Decreasing WAS

Additionally, even as temperatures increased during this period, nitrification performance did not improve.

Given the understanding of these relationships, the WRF instituted careful controls for MCRT, aeration

intensity, and alkalinity/pH to minimize Nocardia foam and scum in order to maintain more consistent


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nitrification performance. The WRF performance has become more consistent and with experience, the

plant staff has become keenly aware of how quickly the system can react to changes in WAS to promote

foam and scum increases.

Given this sensitivity which has been seen to be more pronounced in winter operations, additional means

to help reduce ammonia nitrogen pass-through during periods of higher foaming conditions were

investigated. Data from the on-line ammonia analyzer monitoring secondary influent provided information

that ammonia nitrogen concentrations in the influent after flow equalization varied significantly during a 24

hour period. Historically, the WRF has operated on the basis of equalized diurnal flow. This system is

automated through SCADA programming for control of flow equalization basins and pumping systems.

With the ammonia nitrogen concentration variability, a constant flow approach created a wide range of

actual ammonia nitrogen loading to the activated sludge system. Using the SCADA system, online

ammonia analyzer, and existing equalization system capabilities, an ammonia nitrogen load control

system was developed. Figure 7 provides a screen shot of the nitrogen load control switch which is now

used to target the ammonia load to the activated sludge system.

Figure 7 SCADA load control screen shot

The ammonia loading control switch is located near the center of screen. As shown, the targeted

ammonia loading rate is 2,600 lb/day. The switch window also includes the actual current loading value.

Implementation of this control system to enable more equalized ammonia nitrogen loading provided

immediate positive impact to the WRF including:

Reduced MCRT required to achieve nitrification (no need to maintain higher MCRT for the higher

loading periods)

Reduced/eliminated nocardial foaming and scum minimal Effective and Total MLSS differential

Stable aeration system demands - more consistent loading creates aeration supply consistency

and lower peak power requirements; lower aeration intensity lower foaming tendencies

Stable alkalinity demands

Stable anoxic selector operation


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Reduced scum on secondary clarifiers and improved clarifier performance

Improved final filter performance and reduced backwashing

Figure 8 and 9 present SCADA screen shots of typical conditions before and after the ammonia nitrogen

load control program was implemented.

Figure 8 Typical flows and load conditions prior to ammonia nitrogen load control

Chart presents the conditions of variable ammonia load with near constant flow. Ammonia loading rate

range under these equalized flow conditions was 1,900 4,100 lb/day.

Figure 9 Typical flows and load conditions after ammonia nitrogen load control

Flow to aeration

Ammonia load

Flow to aeration

Ammonia load


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This chart presents an example of targeted consistent ammonia load by variable control of flow rates

relative to varying ammonia nitrogen concentrations. The ammonia loading rate under these conditions

was 1,300 2,740 lb/day. Note that the minimum ammonia load value is affected by a dip in flow that

occurs when the flow EQ basins reach minimum low level.

CONCLUSIONS

To date, the WRF evaluation to control foaming and enhance nitrification stability has provided good

insight into the relationships between the variables that are involved in maintaining a balance in the

microbiological community in the WRF. Each wastewater treatment plant is unique and the responses to

process control approaches and adjustments need to be tuned to the specific conditions for each facility.

For this effort, key process data is necessary. On-line analyzers tied into SCADA provide an advantage to

the operations team to make informed process control decisions and to see the responses of the system

to these calculated adjustments.

While great improvements have been realized in the performance of the WRF through this evaluation and

progressive implementation of operational changes and control equipment, the WRF continues to adjust

the operation to optimize conditions. The approaching winter season will provide the first opportunity to

operate using the equalized ammonia nitrogen loading control system. The WRF staff awaits thechallenge of these conditions with greater understanding, more tools and higher confidence that improved

control of Nocardia foam and scum, and more consistent nitrification performance will be achieved.

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