Instrumentation and Process Control

Transcription

1 Instrumentation and Process Control

2 Topics Covered Critical Monitoring Leachate collection system performance Moisture measurement In-place specific weight

3 Critical Monitoring - Gas Production Rate Gas Composition (methane, carbon dioxide, oxygen)

4 Critical Monitoring - Leachate Head on liner Quantity COD, BOD Temperature ph VOA

5 Critical Monitoring - Waste Temperature Settlement Specific weight Volatile solids Biochemical Methane Potential Moisture Content ph

6 Instrumentation to be Discussed In situ temperature measurement, Leachate collection system performance assessment, In situ moisture content measurement, and Waste specific weight measurement.

7 Temperature Measurement In anaerobic bioreactors, temperature is controlled by the introduction of moisture. In aerobic bioreactors temperature is controlled by moisture content and airflow. High temperatures have been associated with spontaneous combustion and landfill fires. Temperature does not significantly impact the interface shear strength geomembranes

8 Technologies Thermocouples Thermistors Liquid expansion Resistance detectors Infrared radiators Bimetallic thermometers

9 Thermocouples Photo Courtesy Burns Engineering

10 Thermocouples Technology most frequently applied to landfills Thermocouples are pairs of dissimilar metal wires joined at one end. Thermocouples tend to be more rugged and less expensive than thermistors, however they are slightly less accurate. Type J thermocouples are most commonly used in landfills. Damage to the sensor wiring cable is one of the most common causes of sensor failure.

11 Thermocouple profile well near anchor trench 0 New River Regional Landfill Temperature Profiles Depth from Top of Landfill (ft) TP3 TP2 60 TP4 TP1 TP4 TP Temperature (degrees F) Example of T-Profile Well Results

12 Thermistors

13 Thermistors Measure temperature as a function of the change in electrical resistance of materials Composed of inexpensive semiconductor materials. They are accurate over a smaller range than thermocouples, They are quite stable and accurate over temperature ranges expected in bioreactor landfills.

14 Process Control Temperatrue Measure Temperature T < 140 F Temperature may be below optimum. Investigate and revise operations as desired. Not a safety hazard.

15 T < 140 F Temperature may be below optimum. Investigate and revise operations as desired. Not a safety hazard. T > 140 F T < 160 F Optimum conditions. Operate and monitor as scheduled.

16 T > 160 F T < 170 F T > 170 F Possible concern. Check gas concentrations. If anaerobic Check moisture content monitors. Add leachate or other moisture If aerobic Reduce air flow rate. Track the rate of temperature increase of this area. If a rate of 1 F per day (weekly average) is exceeded, stop air injection and add leachate/water. Condition of concern. Stop air injection. Add leachate or other moisture.

17 Leachate Collection System Performance

18 Performance Monitoring

19 Sump Measurements Head losses must be calculated with little knowledge of the condition of piping and drainage layers. Without an internal means of measuring hydraulic parameters the actual head on the liner cannot be determined easily.

20 Piezometers Limited measurement area, May put the liner at risk, May clog, May interfere with landfill operation, and Vertical wells may be impacted by gas production

21 Pressure Transducers Use a calibrated strain gage mounted on a flexible, silicon diaphragm protected by a membrane. Transducers used to measure absolute pressure require a vent tube be run with the wiring cable. Landfill applications require either high-grade stainless steel or titanium materials to prevent corrosion Reference measurements of landfill gas pressures required

22 Monitoring System Installation

23 Monitoring System Installation

24 Head on the Liner Measurements New River Regional Landfill Geomembrane LCS Surface Drainage Layer Geofabric Pressure Transducer

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28 Liquid Depth, in. 0 Cell 2 Head on Liner

29 Head on Liner Experiment Northing Slope Leachate Collection pipe Easting

30 NRRL Cell Slope Slope Northing Slope Central Leachate Collection Point Easting

31 Rainfall (inch) Rainfall vs. Time HELP Model o Avg. Head on Liner vs. Time Avg. Head on Liner (inch) Jul 03-Aug 03-Sep 04-Oct 04-Nov 05-Dec 06-Jan 06-Feb 12-Mar 16-Apr 17-May Time (days)

32 Failure Causes Lightning strike at data station Overburden pressure due to waste placed above sensors Marine grease applied to sensors to prohibit biological growth from occurring inside of the sensor nose cone Inability to isolate from influence by atmospheric pressure changes

33 Process Control Head on the Liner Maintain a properly designed leachate collection system, store or dispose of leachate outside of the landfill, Remove leachate at sufficient rates Adjust moisture addition rates

34 Measurement of Moisture Water balance Collect waste samples and measure gravimetrically In situ methods

35 Water Balance In Track moisture into and out of landfill Storage Cum Vol Leachate Recirculation Leachate Collected Out Time

36 Collection of Waste Samples Great data Expensive Infrequent

37 In Situ Moisture Measurement Time domain reflectometry, Time domain transmissometry Neutron density Electrical conductivity

38 Time Domain Reflectometry TDR theory states that the time for a transmitted electromagnetic pulse to be reflected is dependent on the dielectric constant of the medium.

39 Time Domain Reflectometry ADVANTAGES Provides an in situ method of moisture determination. The technology is nontoxic and harmless. Unaffected by organic carbon content changes. DISADVANTAGES Is affected by changes in specific conductivity. Heterogeneity of landfills may lead to erroneous readings. Must be calibrated to solid waste matrix

40 Time Domain Transmissometry measures the time that a fast pulse takes to travel along a section of transmission line time measured is a oneway propagation time

41 TDR Device Used in Florida

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43 Instrument emits neutrons which are thermalized in proportion to moisture content. Neutron Probe

44 Neutron Probe Hydroprobe, the electronic scaler of a neutron probe assembly

45 Neutron Probe Access Tube Housing Access Tube

46 Neutron Probe Advantages and Disadvantages Advantages: Average moisture contents over depths Readings are related directly to soil moisture Non-destructive moisture measurements It has a large radius of influence between 150 mm in wet soil and 700 mm in dry soil Disadvantages Cannot measure absolute moisture content safety hazards The presence of non-water hydrogen and changes in density may reduce the accuracy

47 This technology is based on the principle that the moisture of a medium can be determined from the value of the measured electrical resistance. Electrical Resistance

48 Electrical Resistance ADVANTAGES Sensors are inexpensive. The technology is nonhazardous permitting in situ monitoring of moisture content. DISADVANTAGES Gypsum sensors suffer from hysteresis unless complete wetting is achieved before each drying cycle. Each sensor may require its own individual calibration.

49 Resistivity Probe For Moisture Measurements

50 Moisture Distribution 05/30/03 10/01/03 11/01/03 Higher moisture ft ft ft Moisture content obtained from resistance values Lower moisture

51 Response of MTG Resistance (KOHMS) Start of Leachate Recirculation L6E-Middle L6E-Lower L6E-Upper Days Day 1 = 01/01/03

52 Moisture content from sensors (%) Gravimetric Moisture Content Start of leachate recirculation TDR 11 MI5-Middle Days

53 New Moisture Content Technologies Gas tracer method Fiber optics Geophysical, noninvasive techniques electrical sounding, electrical 2-D imaging, electromagnetic mapping, and radar profiling

54 Process Control Moisture Content Evaluate rate of moisture addition Evaluate compaction Evaluate alternative moisture introduction techniques Identify dry areas of the landfill requiring longer time to stability

55 Remaining Questions In Situ Moisture Content Measurement How long will they work? Once wetted, do they stay wet? Does installation create preferential flow paths? Can true moisture content be measured, or only relative moisture content? Are they economical? Can wireless measurement be implemented?

56 In-Place Specific Weight

57 Settlement Plates

58 Global Positioning System

59 Roctest Total Pressure Cell

60 Total Pressure Cell Installation

61 Pressure Measurements Obtained Using the Total Pressure Cell Average measured specific weights vary between lb/ft PSF Waste Height PSF Height (ft) /18/00 10/26/00 2/3/01 5/14/01 8/22/01 11/30/01 3/10/02 6/18/02 9/26/02 1/4/03 4/14/03 7/23/03 10/31/03 2/8/04 5/18/04 time (days)

62 Process Control Using Total Pressure Cells/Surface Surveys Evaluate in-place specific weight Evaluate stresses on the liner and leachate collection system