Instrumentation and Process Control

Topics Covered

Critical Monitoring Leachate collection system performanceMoisture measurement In-place specific weight

Critical Monitoring - Gas

Production Rate Gas Composition (methane, carbon dioxide,

oxygen)

Critical Monitoring - Leachate

Head on liner Quantity COD, BOD Temperature pH VOA

Critical Monitoring - Waste

Temperature Settlement Specific weight Volatile solids Biochemical Methane PotentialMoisture Content pH

Instrumentation to be Discussed

In situ temperature measurement, Leachate collection system performance

assessment, In situ moisture content measurement, andWaste specific weight measurement.

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

Technologies

Thermocouples Thermistors Liquid expansion Resistance detectors Infrared radiators Bimetallic thermometers

Thermocouples

Photo Courtesy Burns Engineering

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 areslightly less accurate.

Type J thermocouples are most commonly used inlandfills.

Damage to the sensor wiring cable is one of themost common causes of sensor failure.

Thermocouple profile well near anchor trench

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Temperature (degrees F)

Dep

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dfill

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TP3 TP2

TP4 TP1

TP4 TP5

New River Regional LandfillTemperature Profiles

Example of T-Profile Well Results

Thermistors

Thermistors

Measure temperature as a function of the change inelectrical 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.

Measure Temperature

T < 140 °F

Temperature may be below optimum. Investigate and revise operations as desired. Not a safety hazard.

Process Control –Temperatrue

T < 140 °F

T > 140 °F T < 160 °F

Temperature may be below optimum. Investigate and revise operations as desired. Not a safety hazard.

Optimum conditions. Operate and monitor as scheduled.

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.

Leachate Collection System Performance

Performance Monitoring

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.

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

Pressure Transducers

Use a calibrated strain gage mounted on a flexible,silicon diaphragm protected by a membrane.

Transducers used to measure absolute pressurerequire a vent tube be run with the wiring cable.

Landfill applications require either high-gradestainless steel or titanium materials to preventcorrosion

Reference measurements of landfill gas pressuresrequired

Monitoring System Installation

Monitoring System Installation

Head on the Liner Measurements –New River Regional Landfill

LCS Surface

Pressure Transducer

Geomembrane

Geofabric

Drainage Layer

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Easting

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Leachate Collection pipe

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Head on Liner Experiment

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Central Leachate Collection Point

Slope

Slope Slope

NRRL Cell 3

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03-Jul 03-Aug 03-Sep 04-Oct 04-Nov 05-Dec 06-Jan 06-Feb 12-Mar 16-Apr 17-MayTime (days)

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Rainfall vs. Time Avg. Head on Liner vs. TimeHELP Model

o

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 thesensor nose cone

Inability to isolate from influence by atmosphericpressure changes

Process Control – Head on the Liner

Maintain a properly designed leachatecollection system, store or dispose ofleachate outside of the landfill, Remove leachate at sufficient rates Adjust moisture addition rates

Measurement of Moisture

Water balance Collect waste samples and measure

gravimetrically In situ methods

Water Balance

Track moisture into and out of landfill Storage

In

Out

Time

CumVol

LeachateRecirculation

LeachateCollected

Collection of Waste Samples

Great data Expensive Infrequent

In Situ Moisture Measurement

Time domain reflectometry, Time domain transmissometry Neutron density Electrical conductivity

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.

Time Domain Reflectometry

ADVANTAGES Provides an in situ

method of moisture determination.

The technology is non-toxic 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

Time Domain Transmissometry

measures the time that a fast pulse takes to travel along a section of transmission line

time measured is a one-way propagation time

TDR Device Usedin Florida

Neutron Probe

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

Neutron Probe

Hydroprobe, the electronic scaler of a neutron probe assembly

Neutron Probe

Access Tube

Access Tube

Housing

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

Electrical Resistance

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

ADVANTAGES Sensors are

inexpensive. The technology is non-

hazardous 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.

Resistivity Probe For Moisture Measurements

Moisture Distribution

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50 ft

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Lower moisture

Higher moisture

Moisture content obtained from resistance values

Response of MTG

Day 1 = 01/01/03

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Start of Leachate Recirculation

Moisture content from sensors

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Start of leachate recirculation

New Moisture Content Technologies

Gas tracer method Fiber optics Geophysical, noninvasive techniques

electrical sounding, electrical 2-D imaging, electromagnetic mapping, and radar profiling

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

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?

In-Place Specific Weight

Settlement Plates

Global Positioning System

Roctest Total Pressure Cell

Total Pressure Cell Installation

Pressure Measurements Obtained Using the Total Pressure Cell

Average measured specific weights vary between 300-1700 lb/ft3

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time (days)

PSF

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PSFWaste Height

Process Control Using Total Pressure Cells/Surface Surveys

Evaluate in-place specific weight Evaluate stresses on the liner and leachate

collection system