Considerations for the Use of ORP in Wastewater Treatment Applications

Transcription

1 Considerations for the Use of ORP in Wastewater Applications Enos C. Inniss, Ph.D. Department of Civil and Environmental Engineering University of Texas at San Antonio

2 Abstract Additional parameters like ORP may be used to improve understanding of the wastewater treatment process performance. We will explore some of the current understanding of oxidation-reduction potential, its limitations, and its potential for use in wastewater treatment.

3 Background of Technology Oxidation-Reduction Potential a.k.a. ORP or redox potential A measure of the potential for reactions to take place The redox potential of a media is related to the overall availability of electrons in the media, specifically the ratio of positive and negative ions in the solution. Note that redox measurements vary significantly with changing ph. (

4 Background of Technology Exchange of electrons defines the relative oxidation potentials of a particular material. The arbitrary standard for the potentials is the hydrogen electrode. The state of the reaction is then measured by the potential developed between an inert, noble metal electrode and a reference electrode. (

5 Background of Technology The measuring electrode for ORP is usually gold or platinum. The noble metal donates and accepts electrons. The electrode acquires the electrochemical potential of the electrons, relative to the strongest redox equilibrium of the solution being measured. The electrode develops a voltage relative to the state of the reaction. The reference electrode is the same electrode that is used for ph measurement. The ORP measurement becomes dependent on ph when the reaction involves hydrogen ions. (

6 Background of Technology Anaerobic Anoxic Aerobic Nitrification Activated Sludge Methanogenesis Denitrification -200 mv 0 mv +200 mv Figure 1: Redox potential ranges (mv) for typical wastewater processes (Modified from Stumm and Morgan 1996 and Byl and Williams 2000)

7 Opportunities for Use in WWT Detection of sulfide conditions in collection system Detection on non-municipal wastes/toxic conditions Determination of activated sludge aeration tank conditions

8 Opportunities for Use in WWT Determination of nutrient removal conditions Determination of disinfection conditions Determination of conditions in sludge digestion tanks

9 Expectations Advanced Relatively High Slightly Higher Still Preliminary Primary Secondary Final Low (anoxic/ anaerobic) Slight increase Depends on digestion process Sludge Land Application Figure 1: Wastewater treatment illustration with corresponding ORP expectations Illustration taken from

10 Actual Results found at Various WWT Facilities Raw (influent) Primary effluent Secondary effluent Final effluent ~ -200 mv (dependent on strength of ww) ~ -150 mv (change due to loss of settleable solids) ~ +100 mv (dependent on strength of ww) ~ +150 mv (dependent on strength of ww)

11 What We (ORP Researchers) Have Done Recently, ORP has become popular for wastewater disinfection control. ORP is based on the concept that it is the oxidative potential derived from the residual that kills the microorganisms and not the concentration of the residual. In practice, a set-point ORP value (measured in millivolts) is maintained using an ORP controller. (Harp, 2000) In anaerobic systems, sulfide control can be accomplished by oxygen dosing with the help of ORP. (Khanal, 2002) The relationship between the sludge production and ORP (i.e., how sludge production can be optimized with the help of ORP measurements. (Lee, 2003)

12 What We (ORP Researchers) Have Done The application of ORP in micro aerated processes such as simultaneous nitrification and denitrification. (Holman, 2000) Typical ORP Vs time profile showing nitrate breakpoint(holamn 2000) ORP (mv/30)/ NO3-N(mg/L) Nitrate Breakpoint time(hrs)

13 Survey of AB Facility Advanced -75 mv -46 mv Preliminary Primary Secondary Final -166 mv -120 mv -112 mv Depends on digestion process Sludge Land Application Figure 1: Wastewater treatment illustration with corresponding ORP expectations Illustration taken from

14 Survey of OD Facility Advanced Preliminary Primary -157 mv Secondary -129 mv Final -79 mv -250 mv Not present Depends on digestion process Sludge Land Application Figure 1: Wastewater treatment illustration with corresponding ORP expectations Illustration taken from

15 What We (ORP Researchers) Have Done It was observed that DO and ORP correlation was valid throughout the treatment plant. Higher ORP in aeration basin can be attributed to the availability of oxygen in the aeration basin than oxidation ditch. Addition of RAS (i.e., increase in mixed liquor) caused decrease in DO and ORP at the influent of the secondary treatment units. ORP values for the effluent of the final clarifier increased. This increase is due to the removal of solids and organics in the clarifier

16 What We (ORP Researchers) Have Done ORP is a relative measurement (no absolutes). Two electrodes cannot be compared directly. Differences in metal bands Differences in leak rates through reference junction Can determine differences between electrodes in same solution

17 Considerations for Future Research More clearly correlate ORP trends with other parameters such as ph, nitrate, ammonia, and COD. Determine changes in ORP profiles at times when the treatment facility is not operating under normal conditions.

18 Considerations for Future Research Develop and compare the ORP profiles for different facilities in and around Bexar county. Create a Database of ORP Responses to Various Scenarios