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H2S Monitoring/Datalogging
in Manholes and Lift Stations

by Paul Szymanski. Clark County, NV, Sanitation District (CCSD)
Reprinted from Industrial Hygiene News

The Problem:  In Clark County, Nevada, as in other parts of the country, there is a problem with hydrogen sulfide (H2S) forming in sewer lines. This can cause decay of the underground piping. Often, privately operated lift stations have difficulty maintaining acceptable levels of H2S as established by the sanitation district.
     The monitoring of H2S is accomplished by the pretreatment department of the sanitation district. The usual method is to schedule multiple day monitoring of manholes using a steel frame insert positioned just beneath the manhole cover and a watertight H2S datalogging instrument. If excessive levels of H2S exist, a vilation is noted and a non-compliance letter is sent to the operator.
     Currently, many detectors are available for real time monitoring of H2S and some include datalogging. The typical H2S sensor is an electrochemical cell.
     The cell, though quite accurate, has a life span of 12 to 18 months under ideal conditions. In the humid and sometimes high level H2S conditions experienced in the sewer line, this life span can end in a matter of hours. This is a costly situation when you consider the labor associated with a multi-day monitoring event, the cost of a new sensor and the time lost waiting for the instrument to be repaired and calibrated.

The IQ-1000 with data logging (shown in photo at right) is placed into a protective enclosure which provides free sample flow and a 12V DC battery power supply. The complete assembly is then placed inside of a manhole for a predetermined amount of time and is then retrieved to analyze the data.

The Solution:  Recently, the pretreatment department of the CCSD requested that I locate a replacement for the aging H2S instruments. I felt that a paperless recorder was the best way to save the data, but the electrochemical sensor cells displeased me. After some research, I located International Sensor Technology (IST), a manufacturer of solid state sensor-based instruments.
     After speaking with several IST representatives, I was certain that we should use an H2S datalogger based on their solid state sensor technology. The IST model IQ-1000 appeared to fit the requirement for a portable gas detector that could store months of data. In comparison testing, the accuracy and response time was similar to that of electrochemical cells. The real difference was in the field.
     In field conditions that previously rendered our instruments useless, the solid state sensors survived even after hours of exposure to H2S levels exceeding 100 ppm. The only maintenance required after 6 months of field conditions was the normal monthly calibration performed locally with an available kit.
     A feature that our operators find very useful is the trigger on delta. This allows the recording of changes in gas levels based on concentration. The ability to download and analyze data using a laptop computer saves time and keeps the instrument in the field.
     IST's solid state sensor has provided us with the solution that we needed for our H2S monitoring program.

     For additional information on this application, contact Mr. Szymanski at 702-434-6600, ext. 6502.

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