improving biological wastewater treatment process control through ...

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IMPROVING BIOLOGICAL WASTEWATER TREATMENT PROCESS CONTROL THROUGH ATP-BASED PARAMETERS P. A. Whalen*, P. J. Whalen, D. R. Tracey LuminUltra Technologies Ltd. 440 King Street, King Tower, Suite 630 Fredericton, New Brunswick Canada, E3B 5H8 [email protected] ABSTRACT Biological wastewater treatment processes harness the ability of microorganisms to break down and assimilate organic compounds that, if left untreated, cause detriment to the environment. Although this process occurs naturally, engineered systems are used to accelerate the process by optimizing bioreactor conditions to promote higher-than-typical biomass concentrations, thus increasing the rate of removal of biodegradable substances. However, in part due to the lack of an effective monitoring parameter for living biomass, these systems are rarely able to maximize efficiency and can be prone to debilitating upsets. Through years of research, LuminUltra Technologies Ltd. has developed test kits based on the measurement of Adenosine Triphosphate (ATP) for rapid and accurate monitoring of the concentration and health of living biomass at any location in a biological wastewater treatment system. ATP-based control parameters include: Cellular ATP (cATPTM), a measure of living biomass concentration or energy level; Biomass Stress Index (BSITM), a measure of living biomass relative health; and Active Biomass Ratio (ABRTM), a measure of the proportion of bioreactor inventory that is living. This paper will describe the advantages of operating biological wastewater treatment processes based on ATP control and discuss some conceptual control schemes that exploit optimization opportunities, such as Food-to-Microorganism ratios, supplement feed rates, mixing efficiency, load balancing, and more. KEYWORDS ATP, adenosine triphosphate, activated sludge, wastewater treatment, process control, biomass, dissolved ATP, total ATP, cellular ATP, biomass stress index, LuminUltraTM, tATPTM, dATPTM, cATPTM, BSITM, ABRTM, biological monitoring, aerobic, anaerobic, active. INTRODUCTION As early as thirty-five years ago, the value of monitoring ATP (adenosine triphosphate) in biological waste treatment was recognized (Paterson et al., 1970). More recently, Archibald et al (2001), in a study using a suite of respirometric tests on mixed liquor from paper mill activated

Copyright ©2006 Water Environment Foundation. All Rights Reserved

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sludge processes, concluded that ATP measurements provided a useful monitor of the proportion of viable cells and a toxicity indicator in an activated sludge process. The continuing scientific interest in ATP monitoring of biological waste treatment processes is not surprising. As the keystone of metabolic activity (Lehninger, 1982), most of the energy within microorganisms is stored and transmitted via ATP. ATP is produced as microbial food and is subsequently utilized for cell maintenance and the synthesis of new cells and biochemicals. Furthermore, ATP can be easily measured with high specificity by the firefly luciferase assay. The reaction is as follows: Mg + + luciferase

ATP + O 2 + luciferin ⎯⎯⎯⎯→ AMP + PPi + oxyluciferin + light Where, ATP = Adenosine triphosphate AMP = Adenosine monophosphate PPi = pyrophosphate Mg++ = Magnesium ion The chemical energy produced from the breakdown of ATP is converted into light energy. Each molecule of ATP consumed in the reaction produces one photon of light. This light output can be quantified using a luminometer within a matter of seconds. Although ATP is vital to all wastewater treatment microorganisms and the measurement process described is simple, ATP has not been routinely adopted as a process parameter in operating wastewater treatment plants. Possible reasons for lack of routine use include the following: • • • •

Instability of reagents; Ineffective or cumbersome ATP extraction techniques for wastewater treatment samples; Lack of test protocols optimized for wastewater treatment applications; Insufficient monitoring guidelines.

Furthermore, it is frequently assumed that ATP is only found within living cells. Typically, during ATP analyses, samples from waste treatment plants are immersed into an extraction agent such as boiling buffer (Paterson et al, 1971 ), organic solvents (Lefebvre, 1988), proprietary surfactant solutions, or acid solvents (Archibald, 2001) with no separation of the microorganisms from the liquid portion of the sample. Thus, if the sample included any extracellular ATP, it would not be distinguished from ATP contained within the living cells (i.e. intracellular ATP). Archibald et al. (2001) note that pulp and paper mill wastewaters contain many non-biological solids that are poorly or non-biodegradable, which can accumulate in the floc of a biological waste treatment process. Doubtless, this occurs in other wastewater treatment systems.


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Therefore, conventional measurements such as mixed-liquor suspended solids (MLSS) or mixedliquor volatile suspended solids (MLVSS) can provide misleading information about the amount of viable biomass in the reactors. Furthermore, these measurements do not distinguish between living and dead cells. Because ATP is produced only by living cells, its measurement can overcome these difficulties and provide an opportunity for superior control of such fundamental operating issues such as food to microorganism ratio, sludge age, and nutrient feed. In 1999, our organization began a program to optimize the ATP assay application for monitoring biological wastewater treatment processes. Based on prior experience with biocide treatment of contaminated industrial water systems, we were aware that in environments that are lethal to microorganisms, significant amounts of extracellular or “dissolved” ATP can be created and maintained for a period of time. To accurately quantify the intracellular ATP component (herein referred to as ‘Cellular ATP’, or cATPTM), the extracellular component must be accounted for in the analytical process. The accurate quantification of extracellular ATP is a cornerstone of the technology discussed herein. METHODOLOGY For the most part, tests were conducted on samples from full-scale biological wastewater treatment operations of various designs and industries. Such processes were monitored by collecting grab samples at various time intervals and locations throughout the plant. ATP analyses were conducted on sub-samples removed from the sample containers, usually as soon as the samples had been brought to the plant laboratory. tATPTM (Total ATP – intracellular ATP plus extracellular ATP content) analyses were performed by adding a sub-sample of wastewater to an ATP-releasing agent and mixing. The mixture was then diluted and assayed for ATP using the bioluminescent firefly luciferase test. dATPTM (Dissolved ATP – extracellular ATP only) was measured by adding a sub-sample of wastewater to an ATP-stabilizing reagent. The diluted sub-sample was then assayed for ATP. The results of these analyses directly yield two critical control parameters: Cellular ATP (cATPTM), a measure of living biomass concentration or energy level; and Biomass Stress Index (BSITM), a measure of living biomass relative health. These control parameters are calculated as follows: cATP ( ng / mL ) BSI (%)

=

=

dATP

tATP − dATP tATP

The measurement of Cellular ATP can be considered to be an indication of potential energy available to the living biomass in a given sample. Because potential energy represents the capacity of biomass to do work, Cellular ATP can thus be taken as a representation of living biomass concentration. A direct measurement of living biomass provides superior information for control versus conventional suspended solids measurements, such as Mixed Liquor Suspended Solids (MLSS) or Mixed Liquor Volatile Suspended Solids (MLVSS). Copyright ©2006 Water Environment Foundation. All Rights Reserved

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Independent research has shown that the fraction of living biomass contained in the total suspended solids of a bioreactor is generally in the vicinity of 15 to 40 percent (Levin et al., 1975) but is highly variable depending on bioreactor conditions and substrate characteristics. Hence, there is substantial interference contained in the conventional measurements. Conversion factors exist to approximate the actual living biomass concentration based on the measurement of Cellular ATP. Independent research has shown that, on average, a biomass population contains 250 parts of biomass carbon per 1 part of intracellular ATP (Holm-Hansen, 1970). It is also given that biomass is approximately 50% carbon on a dry basis (Tchobanoglous et al., 2003). Thus, these factors can be combined to achieve the following conversion for Mixed Liquor Active Volatile Suspended Solids (MLAVSS): MLAVSS ( mg / L )

=

0.5 × cATP ( ng / mL )

The MLAVSS can be considered to be a direct indication of living biomass concentration. Having this parameter, along with the conventional MLSS measurement, facilitates the computation of a third new control parameter, the Active Biomass Ratio (ABRTM): ABR (%)

=

MLAVSS ( mg / L )

MLSS ( mg / L )

ATP-based parameters have been routinely used by a collection of North American sites since 2001. The facilities utilizing ATP-based parameters represent various industries and unit operations. Grab samples from various process locations at each site were analyzed for tATPTM and dATPTM levels, including aerobic, anoxic, and anaerobic bioreactors, as well as influents, effluents, and return solids streams. All ATP analyses were performed using reagents designed and optimized for the wastewater treatment application, manufactured by LuminUltraTM Technologies Ltd. The light produced in the luciferase reaction was measured in a luminometer (either Turner Designs Model 20e or Kikkoman Lumitester C-100). Using statistical process analysis concepts, LuminUltra has worked with operators to develop meaningful cause and effect relationships between these parameters and conventional operating parameters to identify opportunities for improvement. RESULTS AND DISCUSSION ATP monitoring has been recognized by industry as a superior tool for quantifying living biomass in wastewater treatment operations (Degremont, 1991); however, until now, there have not been sufficient methods or guidelines for its application. Through the technology developed by LuminUltra, it is now possible to measure and apply ATP-based parameters to develop meaningful cause-and-effect relationships in these processes to achieve a new level of operational efficiency with minimal investment. The opportunities for process optimization in a typical biological wastewater treatment process are many. Because it is the engine driving the process, monitoring the living biomass is critical


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to maintain acceptable performance. Figure 1 shows a conventional activated sludge process and identifies some of the bioreactor control opportunities offered using ATP-based parameters. Figure 1 – Activated Sludge Process Control Using ATP Macronutrients

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By having a superior indication of biomass concentration and health, operators can more effectively pace such critical process inputs as oxygen concentration, macronutrient or biostimulant delivery, and activated sludge return rate, among other opportunities. The specific advantages of each control parameter versus conventional measurements have been covered in other papers: cATPTM and ABRTM (Whalen et al., 2006) and BSITM (Cairns et al, 2005). Using ATP Control Parameters to Troubleshoot Process Upsets The most common issue encountered during the course of operating a biological wastewater treatment process is ‘upsets’. In general, an upset is an event that results in effluent compliance being compromised, whether it be elevated Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), Ammonia, or the failure of a toxicity evaluation. Upsets may also be defined as events that compromise the fundamental operation of the wastewater treatment process, such as settlability issues. Such events are problematic because of the environmental effects and typically the resulting scrutiny and fines levied from enforcement agencies. Often, these upsets occur without warning and without historical precedent. That is, there is no prior indication of an impending upset event, which results in the need to react rather than taking proactive measures to prevent the event from occurring. The cryptic nature of upset occurrence is probably the single largest contributing factor in the perception of biological wastewater treatment reactors as ‘black boxes’. A survey of 49 trout toxicity occurrences between 1996 and 2003 in Canadian pulp and paper effluents revealed that 81% of the events were due to failures of the biotreatment process to maintain stable operation (Kovacs et al., 2004). It is highly likely that a similar trend could be established for other industries as well. Thus, it can be said that having a better mechanism with which to control and therefore stabilize biomass in biological wastewater treatment processes would help to dramatically reduce the number of toxicity occurrences.


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Our experience has been consistent with the findings that most upsets are a result of biomass failures. For example, Figure 2 demonstrates the direct relationship between elevated effluent ammonia concentration and elevated biomass stress in the aerated basin of a system treating food processing wastewater.

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In this situation, a controlled increase in temperature caused inhibition of the nitrifying segment of the aerobic population, which effectively ceased ammonia degradation, further elevating stress on the remaining population. The temperature effect is shown in Figure 3. Figure 3 – Relationship Between Temperature and Effluent Ammonia for a Food Industry Plant 900

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It is accepted that nitrifying organisms are sensitive to a variety of stresses, including temperature. It is also known that many heterotrophic organisms operate more efficiently and with more vigor at higher temperatures. The reason for the temperature increase was that plant operators had noticed greater process performance while operating at high temperatures, but for some unknown reason the population would crash after a sustained period at high temperature. ATP monitoring revealed the exact chain of events leading to the upset, defining a maximum sustainable temperature for bioreactor operation, and thus stabilizing the process. In another example, a pulp and paper mill experienced a variety of instances over the course of a month where effluent ammonia concentrations exceeded regulation specifications. In nearly all instances, the elevated ammonia concentrations corresponded with drops in living biomass concentration (Cellular ATP), as demonstrated in Figure 4.

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Figure 4 – Relationship of cATPTM with Effluent Ammonia for a Pulp and Paper Plant

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Because pulp and paper effluents are typically deficient of critical nutrients (i.e. nitrogen and phosphorous), operators at this plant supplement the levels based on the amount of COD entering the bioreactor. Without factoring in the living biomass concentration, however, the operators were unknowingly adding excess ammonia-nitrogen that was traveling through the treatment system undegraded because of the loss of living biomass. In one case, the cause was a dramatic pH drop: Figure 5 – Cause and Effect: pH versus cATPTM


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When the pH changed, the cATPTM immediately changed with it, while MLSS changed much more gradually. A more efficient method for pacing supplemental nutrients is to base the amount on the living biomass concentration rather than the available food, which will help to elevate cost efficiency (i.e. eliminate waste) and reduce the occurrence of permit violations. It has also been shown that solids settlability issues are often preceded by a physiological change in the population, indicated by a dramatic change in the BSITM. Figure 6 presents evidence that spikes in the Sludge Volume Index (SVI) are preceded by spikes in BSITM. Figure 6 – Relationship between BSITM and SVI for a Municipal Sewage Plant 700

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This is indicative of a stress and subsequent adaptation of the biomass to new conditions, as is the case with bulking events. These results indicate that BSITM has the potential to provide early-warning in mitigating bulking events. Figure 7 shows the strength of the relationship between BSITM and SVI over a prolonged period: Figure 7 – Relationship between BSITM and SVI for a Pulp and Paper Mill 80%

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The flexibility of ATP monitoring, which allows biomass quantification at any point in the biological wastewater treatment process, can facilitate indication of the source of solids settlability issues. For example, in some cases, there may be question as to whether some influent constituent is the cause of bulking tendencies, or if it is a process (i.e. supplementation) deficiency. Figure 8 shows the relationship between biomass stress in the influent and bulking in the bioreactor, further to Figure 7. Figure 8 – Relationship between BSITM and SVI for a Pulp and Paper Mill


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The trends of influent stress versus bulking tendencies appear to coincide, with a slight lag in the SVI trend. This would seem to indicate that some influent constituent is primarily responsible for bulking tendencies in the bioreactor. Such a finding could lead the process operator to evaluate the components of the influent and determine the source of this issue. ATP monitoring can be very useful for influent screening. Similar to resolving settlability issues, comparing BSITM in the raw influent to BSITM in the bioreactor can help to distinguish between stresses arising from influent composition versus stresses arising from operational failures (i.e. loss of aeration, temperature changes, etc). Figure 9 shows a comparison between influent stress and bioreactor stress: Figure 9 – Relationship between Influent and Bioreactor BSITM for a Municipal Sewage Plant 70%

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In this case, the bioreactor stress is directly related to the stress experienced by the indigenous organisms contained in the raw sewage. Knowing this, the operator can evaluate the impact of individual component streams on plant biomass through the Biomass Growth Index (BSITM) test developed by LuminUltra. In short, the test involves incubating plant biomass in mixtures containing elevated amount of each individual components and monitoring their response over a short period. This is shown graphically in Figures 10a and 10b. Figure 10a – Example Process Flow Diagram

Figure 10b – Example Incubation Layout for BGITM

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Comparing the biomass response in the component incubations versus the control will identify the source of toxicity, thereby allowing operators to take measures to reduce stress effects (e.g. by diversion, pretreatment, or other alternatives). Using ATP Control Parameters for Continuous Process Improvement In addition to troubleshooting upset conditions in a biological wastewater treatment system, there are a number of opportunities for continuous process improvement through ATP-based control. Because ATP-based parameters relate directly to process performance, they can play a central role in establishing cause-and-effect relationships: Figure 11 – Comparison of cATPTM to Process Performance for an Aerobic Bioreactor

In general, there are 3 steps to implementing a continuous improvement program using ATPbased control parameters:


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1. Conduct ATP analyses throughout the process daily; 2. Trend ATP parameters with independent (operating) variables that affect biomass; 3. Use Statistical Process Analysis tools to uncover important cause & effect relationships. The most fundamental process output in a biological wastewater treatment facility is the Foodto-Microorganism (F/M) ratio. All recycle-based unit operations – the most common being the activated sludge process – require control of F/M to ensure reliable process operation. When F/M deviates from optimal conditions, solids settlability issues can result, such as: •

F/M too high Æ Pin-floc formation results, which can cause heavy secondary clarifier solids overflow; F/M too low Æ Bulking results, which results in reduced clarification and treatment capacity.

•

The ‘F’ component is easily quantified using COD or BOD measurements. The ‘M’ component has been more troubling to estimate in the past, however, due to the conventional measurements of Mixed Liquor Suspended Solids (MLSS) or Mixed Liquor Volatile Suspended Solids (MLVSS) overestimating the amount of living biomass and not being sensitive to frequent process changes common in BWWT operation. For example, Figure 12 demonstrates the variability of living biomass proportion in MLSS measurements as defined by the Active Biomass Ratio: Figure 12 – ABRTM Averages of Sites by Industry for Activated Sludge Bioreactors 60% 56.0% 52.4% 50.9%

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Having the ability to measure Cellular ATP, a direct indication of living biomass, facilitates substantially improved control of F/M ratios. The advantages of controlling based on actual living biomass include:


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•

Reduce aerobic digestion in the bioreactor. Digestion of dead solids requires supplementation (i.e. aeration, biostimulant additives) above and beyond that required to consume influent BOD, thereby reducing cost efficiency. True F/M stability. Rather than basing control on MLSS or MLVSS measurements that do not distinguish the living from dead or inert solids, the F/M can be maintained at a truly optimal level on a routine basis. Eliminate the requirement for excess biosolids wasting. Operating with the lowest possible solids inventory (through maximizing the living component) facilitates a reduction in waste biosolids production while maintaining the same solids retention time. Optimize aeration & macronutrient requirements. By optimizing the process to reduce dead or inert solid retention, mass transfer of metabolic components will be increased, reducing the need for over-supplementation and reducing excess costs. Maximize the capacity of existing infrastructure. Having better information facilitates better optimization practices, ensuring that plants work at the highest possible level, thus delaying or eliminating the need for expansion and capital investment. Load Balancing and Mixing Efficiency. True F/M control can help to better distribute influent loads and bioreactor conditions in parallel reactors.

• • • • •

Such approaches have been proven by other researchers (Levin et al., 1975) using inferior ATP measurement techniques and without the interpretation guidelines developed by our organization in recent times. For example, Figure 13 compares F/M ratios with plant performance for one pulp and paper effluent treatment facility:

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Figure 13 – Relationship of Plant Performance and F/M Ratio for a Pulp and Paper Mill

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Maximizing the proportion of living biomass contained in the bioreactor inventory also has the benefit of improving and maintaining satisfactory solids settlability. For example, a comparison of the ABRTM for three similar processes (activated sludge) treating similar wastewater (pulp & paper) in Figure 14 demonstrates how striving to maintain a higher fraction of living biomass corresponds to improved solids settlability: Figure 14 – Comparing ABRTM to SVI for Three Activated Sludge Pulp & Paper Facilities 250

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Utilizing cATPTM as a control basis for optimization can also help to alleviate restrictions on biomass performance. Figure 15 shows an opportunity for pulp & paper effluent treatment improvement via ATP monitoring for control. Figure 15 – Relating cATPTM to Macronutrient Availability 0.090

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Specifically, it shows the relationship between the cATPTM and influent nutrient (i.e. Nitrogen and Phosphorous)-to-COD ratio for an aerated bioreactor treating pulp & paper mill effluent. The correlation between these parameters strengthened during the final three months of monitoring because of a severe drop in nutrient supply, causing a limitation on biomass growth. It is apparent that there is an opportunity for improved control by pacing the nutrient flow according to cATPTM, rather than conventional ‘proportional’ pacing to available food. Conversely, the use of ATP-based control can help to reduce occurrences of costly oversupplementation. One of the largest costs in the operation of biological wastewater treatment is aeration. In one example, a facility removed an aeration restriction, but overshot and provided a large excess: Figure 16 – Relating cATPTM to Oxygen Restrictions 20

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Bioreactor DO

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Essentially, the utilization of BSITM to directly determine the optimum DO level for a given cATPTM value would enable aeration horsepower savings, loading to direct cost savings. CONCLUSIONS Accurate assessment of living biomass characteristics is critical to biological wastewater treatment operations. The measurement of Total and Dissolved ATP in a biological wastewater treatment sample can tell within minutes the potential energy and health of the living biomass contained in the sample. The potential energy, as defined by Cellular ATP, can be further converted to an actual living biomass concentration, and through a ratio with the Mixed Liquor Suspended Solids, can define the percentage of living biomass contained in the total solids. Having this information allows the operator to take measures to optimize the concentration, health, and viability of the bioreactor through statistical process analyses on the data generated from such a technology, facilitating easy upset troubleshooting and continual process improvement. REFERENCES Patterson, J.W.; Brezonik, P.L.; Putnam, H.D. (1970) Measurement and significance of adenosine triphosphate in activated sludge. Environ. Sci. Technol. 4(7) 569-575. Levin, G. V.; Schrot, J. R.; Hess, W. C. (1975) Methodology for application of adenosine triphosphate determination in wastewater treatment. Environ. Sci. Technol. 9(10), 961– 965. Lehninger A. L. (1982) Principles of Biochemistry, Part II, 1011 pp. Worth Pubs. Inc., New York, USA. Lefebrve, Y.; Coulture, P.; Couillard, D. (1988). An analytical procedure for the measurement of ATP extracted from activated sludge. Can. J. Microbiol. 34, 1275-1279. Archibald, F.; Me´thot, M.; Young, F.; Paice, M. G. (2001). A Simple System to Rapidly Monitor Activated Sludge health and performance. Wat. Res. 35 (10) 2543 –2553.


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Holm-Hansen, O. (1970). ATP Levels in Algal Cells as Influenced by Environmental Conditions. Plant and Cell Physiology. Tchobanoglous, G.; Burton, F.; Stensel, H.; Wastewater Engineering: Treatment & Reuse. Metcalf & Eddy, 2003, P. 558. Cairns, J. E.; Whalen, P. A.; Whalen P. J.; Tracey, D. R.; Palo, R. E. (2005). Dissolved ATP – A New Process Control Parameter for Biological Wastewater Treatment. WEFTEC 2005, Session 21. Whalen, P. A.; Whalen P. J.; Tracey, D. R. (2006). Cellular ATP – A superior measure of active biomass for biological wastewater treatment processes. WEFTEC 2006, Session 39. Kavos, T.; Gibbons, S.; O’Connor, B.; Martel, P.; Paice, M.; Naish, V; Voss, R.; (2004). Summary of Case Studies Investigating the Causes of Pulp & Paper Mill Effluent Regulatory Toxicity. Water Quality Research Journal of Canada, Volume 39, No. 2, Pp. 93-102. Degremont Water Treatment Handbook, 6th Ed., 1991, Volume 1, P. 285.


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