Luminescent Dissolved Oxygen (LDO) Measurements for Wastewater Applications Edward C. Craig, Ph.D.* Cary B. Jackson, Ph.D. Christopher P. Fair Hach Company Loveland, CO Ed Craig – R & D Chemist ecraig@hach.com Cary Jackson – Regulatory Chemist cjackson@hach.com Chris Fair – Applications Chemist Specializing in Wastewater Applications cfair@hach.com

Purpose of this Talk… Discuss our effort to add Luminescence (LDO) to the List of EPA Approved Methods for Dissolved Oxygen Measurements Proposed EPA Method 360.3

Specifically… Discuss our In-House Preparation for a External Nationwide Validation Study run as Part of a Tier 3 EPA Approval Process for the Luminescence Method Also share some of the results from the External Validation Study

Current EPA Approved Methods for Measuring Dissolved Oxygen Conc. Winkler Titration (1888) EPA Method 360.2 Membrane Electrodes (1959) EPA Method 360.1

Winkler Titration L.W. Winkler, “The Determination of Dissolved Oxygen in Water”, Ber. Deut. Chem. Ges., 21, 2843 (1888) EPA “Reference Method” for Dissolved Oxygen Measurement This is a note

Winkler Titration – Reactions Based on the quantitative oxidation of Mn(II) to Mn(III) in alkaline solution and the subsequent oxidation of I- by Mn(III) in acid solution. The iodine that is liberated is then titrated with Sodium thiosulfate. Depends on careful control of pH and [I-] Precisions better than +/- 0.1 mg/L of dissolved oxygen can be realized

Winkler Method – Floc Formation

Winkler Method - Titration

Winkler Titration Pros Cons Direct DO Measurement Easy Clean-up Samples must be Analyzed in a Laboratory Subject to Several Chemical Interferences Very Technique Sensitive Direct DO Measurement  No Salinity Correction Needed Easy Clean-up  Pour the BOD bottles out and they’re ready for use next time Analyzed in Lab  No Field Measurements / Requires Sample Collection and Transport Chemical Interferences  Any oxidizing or reducing agents in the sample will interfer with the REDOX titration Notable Interferents: Chlorine, Iron (?) Technique Sensitive  Trained Laboratory Technicians Required for Analysis

Membrane Electrode L.C. Clark “Clark Cell” Patent Nov. 19, 1959 Sometimes called “MPODs” for Membrane covered Polarographic Oxygen Detectors

O2 + 2 H2O + 4 e-  4 OH- Clark Cell – Edit3 Cathode Highlighted Yellow/Gold (site of reduction) O2 + 2 H2O + 4 e-  4 OH-

Clark Cell – Edit 4 Anode Highlighted (Red)

Clark Cell – Edit 5 Electrolyte Solution Highlighted (Blue)

Clark Cell – Edit 6 Membrane Highlighted (Green)

Membrane Electrode Pros Cons In-situ Measurements Possible Easy to Make Measurements Cons Frequent Maintenance Required Must be Polarized Before Use Must be Calibrated Before Use Consumes Oxygen Very Sensitive to Sample Flow In-situ Measurements  No Sample Collection Necessary Frequent Mainenance Required Membranes tear very easily and require replacement Membranes must be kept hydrated at all times (must be hydrated before initial use) Electrolyte needs periodic replacement – it may become poisoned by metal ions etc. Electrodes need to be cleaned/polished periodically The quality of the probe response varies with the condition of the electrode surfaces. Polarization – Depending on the particular brand of instrument it can take anywhere from 10 min. to half an hour for full polarization Note – the more electrode surface you have the longer it takes to polarize the electrode Side Note – Larger electrodes generally mean more stable responses and longer periods between electrode cleaning/polishing but they also require longer polarization times. Calibration – The calibration is actually a simple process. Typically the probe is inserted into a Water Saturated Air environment and the response vs. Dissolved Oxygen is set. Two things cause the calibration to be difficult. First is the polarization, if the electrode isn’t fully polarized when the instrument is calibrated then it will appear to drift over time. Second is the electrode surface, as the electrode operates oxides tend to build-up on the surface of the electrode. This changes the characteristics of the electrode surface and (more importantly) the current flow through the electrode. The result is the response tends to drift over time. The instrument eventually needs to be recalibrated to compensate for gradual changes in the electrode surface. Consumes Oxygen – the Clark Cell actually consumes oxygen. It is the consumption oxygen that generates the electrical current that is measured. The difficulty is the longer the probe sits in the sample the more oxygen it consumes. If you have a large sample this generally isn’t a problem. If you have a small sample (e.g., BOD bottle with very low DO Conc.) then it may be a very significant problem. Sensitive to Sample Flow – Because the Clark Cell is “Consumptive” it must replinish the oxygen consumed at the same rate as it is consumed otherwise the DO response drifts downward over time. To offset this effect the sample must be stirred very rapidly. This is even more significant at higher DO concentrations because it becomes progressively more difficult to replinish the oxygen that is consumed. (Mass Transport Limited Process)

Luminescent Dissolve Oxygen (LDO) Completely New Technology Measures the Quenching of a Luminescent Reaction caused by Oxygen

Luminescent Dissolved Oxygen (LDO) - Probe

Luminescent Dissolved Oxygen (LDO) Probe Components

Luminescent Dissolved Oxygen (LDO) Probe LED Photo Diode LED

Luminescent Dissolved Oxygen (LDO) Probe Sensor LED Photo Diode LED

Luminescent Dissolved Oxygen (LDO) LED Photo Diode Probe Sensor Oxygen Luminescent Indicator Molecules Gas Permeable Polymer Matrix Clear, Gas Impermeable Substrate

Luminescent Dissolved Oxygen (LDO) Pros No Maintenance Field Measurements Possible No Polarization Insensitive to Sample Flow Rugged “Essentially” No Maintenance – beyond changing out sensor caps and wiping them occasionally with a rag to clean them there’s no maintenance (no membranes / no electrolyte / no electrode surfaces)

Luminescent Dissolved Oxygen (LDO) Pros (cont.) Accurate Precise Very Stable Calibrations No Chemical Interferrents Lower Day-to-Day Cost than Winkler Titration

Luminescent Dissolved Oxygen (LDO) Cons Higher Initial Cost than Winkler Titration Not Currently EPA Approved Higher Initial Cost than Winkler Titration – You have to buy a meter and a probe but once you have the probe and meter your day-to-day cost is significantly less than that of running Winkler Titrations Initial Cost are comparable to that for the Membrane electrodes It isn’t EPA Approved yet but we expect to have full EPA approval within 1 to 1 ½ years and we expect that we’ll be granted provisional approval within a few months (verify this with Cary)

Process LDO… Cost Savings John Anderson Electricity, Labor, and Chemicals John Anderson City of Longmont, WWTP Superintendent Process LDO has been out for a little more than a year longer than Lab unit. One person to talk to about the Process LDO is John Anderson (WWTP Superintendent, City of Longmont) whose been using them for over a year now. John has documented savings in Labor and Chemicals. The Electricity savings are estimated from reduced blower usage times.

Process LDO – Longmont WWTP Savings Over 3 Month Period Electricity ($ 28,000) Labor ($ 15,000) Chemicals ($ 36,000) Total Savings ($ 70,000) Displayed values were given to me by Devin Standard at Hach company. John Anderson says the Electricity savings are estimated from blower usage times. Labor and Chemical savings are directly documented.

External Validation Study… 12 Wastewater Laboratories Nationwide Accuracy & Precision (IPR) Method Detection Limit (MDL) Determn. 5 Day BOD Side-by-side Comparisons with Other DO Methods A total of 12 wasterwater (including us as one) laboratories from all over the nation were involved. Accuracy & Precision (at 2 ppm and ASW) --- “IPR’s” Initial Recovery & Precision Samples [we sent them the 2 ppm std.s and the protocol to prepare ASW] MDL – the data from each lab was used to determn. The MDL for that Lab and the resulting MDL’s were then pooled to obtain a “nationwide” MDL BOD5 – Each lab did a full 5-day BOD determination with 2 Effluent and 2 Influent samples plus controls (GGA, blanks, & multiple dilutions) Side-by-side Method Comparisons – Each lab was tasked to make comparison measurements using Winkler Titrations and Membrane Measurements.

In-House Preparation for External Validation Study… Preparation of Replicate DO Standards Shelf-Life Experiment Shipping Experiment Ruggedness Testing Use Model Comparisons Accuracy & Precision Determinations MDL Determination BOD5 Demonstration Side-by-side Comparison with Other Methods Preparation of Replicate DO Standards – We had to actually develop all new technology to allow us to prepare and ship replicate DO standards Shelf-Life Experiment – We ran experiments to verify that the prepared DO samples would be stable long enough to ship them to the external validation laboratories and long enough for those laboratories to make the required DO measurements. Shipping Experiment – verified the prepared samples would survive shippment to the external validation laboratories Ruggedness Testing – demonstrated that the LDO instrument will produce reliable DO measurements on a day-to-day basis over a period of several days. Use Model Comparisons – we compared different Use Models to determine which one was most appropriate for BOD applications in particular and Wastewater App.s in general Accuracy & Precision Determinations – We looked at two DO conc.s in particular (nominal 2 ppm and ASW) as part of the In-House preparation but I will share data from across the whole range of DO Conc.s In-house MDL Determination – Complete Dress Rehearsal of the External MDL (Method Detection Limit) Determination. To get a “best case” determination of the MDL. BOD5 Demonstration – A complete In-house BOD5 determination with Influent and Effluent samples Side-by-side Comparisons w/ Other EPA Approved Methods for DO Measurements Clearly I don’t have time to tell you about everything we did so I decided to focus on a few of the more significant experiments – Namely the ones I’ve highlighted in Yellow.

Accuracy & Precision Determinations

5 LDO Probes Replicate Measurements w/ dry sensor caps (data collected on probe test stand with NIST traceble gas mixtures) Key Points – Linear Response across the entire concentration range. Calibration Curve passes through zero. LDO Response is exactly correlated with the Theoretical DO Conc. in ASW Additional Point Precision actually improves at lower end of the concentration range (this is because the S/N is higher at the lower concentrations because the signal is actually quenched “less”)

Same LDO data as in previous slide. I’ve superimposed side-by-side data collected with an electrochemical probe and by Winkler Titration Winkler – is linear and for the most part scattered around the calibration curve. Response from Membrane Electrode matches the Theoretical values prettly well up to Air-saturated Water ( ~ 7 ppm) and then it starts to drop off and get more erratic at higher concentrations. This happens because the membrane electrode becomes more sensitive to the sample flow rate. It essentially can’t replenish the O2 from the bulk solution at a faster rate than it consumes it.

Percent Recovery at 1.69 mg/L 8 replicates in BOD bottles for each of the three methods LDO, Clark Cell (YSI), and Winkler Titration The error bars correspond to 1 standard deviation about the mean.

Percent Recovery in Air-saturated Water 8 replicates in BOD bottles for each of the three methods LDO, Clark Cell, and Winkler Titration The error bars correspond to 1 standard deviation about the mean.

LDO – Histogram Air-saturated Water Replicate DO Samples in BOD Bottles (a total of 48 replicates, 4 each measured with 12 different LDO meter/probe/sensor combinations calibrated vs. WSA)

MDL Determination

LDO – In-House Method Detection Limit 49 Replicated DO Samples (Theoretical DO Conc. 0.07 mg/L) 7 LDO Instruments (calibrated vs. WSA and “preconditioned” prior to measurements) 7 Replicates with each instrument (actually there were 8 replicates for LDO #4) 2 operators Avg. = 0.070 (+/- 0.007) mg/L MDL ~ 3 x Std. Dev. = 3 x 0.007 = 0.023 mg/L Source: LDO EPA Approval / DataSummary.xls / Expt 4 - Analysis -- In-House MDL Determination

Pooled MDL and ML Results (mg/L) Data Source Pooled MDL ML In-house Study 0.02 0.07 Inter-laboratory Study 0.06 0.18 Pooled 0.05 0.16 Method Detection Limit (MDL is essentially 3 x the Std. Dev. at zero conc. --- The Std. Dev. is actually measured at a concentration that is 3 to 5 times that of the estimated MDL The Method Limit (ML) is more of a practical limit representing 3 x the MDL (or ~ 9 X Std. Dev. at zero conc.)

BOD5 Demonstration

BOD5 Comparison This slide summarizes the BOD data obtained for replicate samples. A total of 3 dilutions was used for each of the G&GA’s (Glucose and Glutamic Acid), Effluent and Influent Wastewater samples. After correcting for dilution, the G&GA BOD’s should be 198 mg/L

Method Comparison – DO Measurements Influent and Effluent Samples from 12 Different Wastewater Facilities This data is small part of the data that was collected from the External EPA Approval. What is shown is the combined influent and effluent DO measurements made at 12 different wastewater facilities nationwide. Each facility measured parallel influent and effluent samples using each of the different methods for measuring DO (Winkler Titrations, Membrane (Clark Cell) Electrodes, and LDO). What is plotted is the measured DO Conc. from either the Winkler Titration or Membrane Electrode measurement vs. the Measured DO concentration obtained for a parallel sample with the LDO instrument. Note: the correlation between the LDO, and the other methods is good throughout the concentration range (from ~ 0 to 9.5 mg/L) The other thing that came out of this study was that the LDO was completely insensitive to the type of sample matrix. That ultimately means you can use the same quality control criteria for LDO for both influent and effluent samples.

What’s Next? Report for External Validation Study has been submitted to EPA Next the results are opened to the public for Review and Comments (~ 3 mo.) Expect Full EPA Approval in 12 to 18 months Currently Partitioning EPA for Provisional Approval (6 to 12 mo.) Soliciting/Helping WW Labs seeking Tier 1 Approvals

Contacts… Edward C. Craig, Ph.D. Cary B. Jackson, Ph.D. Research & Development Chemist ecraig@hach.com Cary B. Jackson, Ph.D. Regulatory Chemist cjackson@hach.com Christopher Fair Wastewater Applications Chemist cfair@hach.com