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Ammoniacal-nitrogen is a key water quality parameter. The determinant can be used to efficiently control
the treatment processes, to demonstrate compliance to imposed discharge consents, to indicate the presence
of untreated sewage at points of abstraction and infer the associated risk from pathogens.
Instrumentation is now extensively installed to monitor the parameter on-line and report data in real- time.
It has therefore become increasing important to select an instrument with a design that minimizes maintenance,
reports accurately and functions reliably.
Principally four methodologies are available for on-line measurements; Ion Selective Ammonia (gas sensing)
electrodes, Ion Selective Ammonium electrodes, Colorimetric and Optical devices. Traditionally two techniques
have predominated for in-situ measurement. Ion Selective Ammonia techniques prevail in applications requiring
continuous analysis such as intake protection and final effluent monitoring. Colorimetric analysis is sometimes
used for intermittent analysis, if acceptable and where sample preparation is possible. (The technique can also
be conveniently packaged for portable test kits designed for grab sample analysis). However, as with most optical
techniques, the sample must be free from suspended solids so an appropriate sample preparation system must be
installed which often adversely affects reliability and increases maintenance.
Pollution and Process Monitoring have optimised the Ion Selective Ammonia (ISE) design especially for
applications containing suspended solids. The ProAm analyser has an integral peristaltic sample pump (capable
of delivery a sample directly from source) which continuously delivers sample through large diameter tubing
to a reaction vessel containing the ISE. The sample is then pH adjusted and agitated using an air-sparging
system to liberate free ammonia for analysis. The internal tubing and the ISE is chemically cleaned using
an automatic acid wash sequence at a user selectable interval (typically daily). This sequence removes any
biological growth and calcification which can otherwise result in blockages. Emphasis has been placed on
designing a measurement system which is simple, compact, robust and repeatable. Above all, reliability of
operation has been the main objective.
System validation is extremely important especially for applications where compliance to an imposed consent
must be demonstrated or where trace level (sub-PPM) concentration must be reliably detected. Automatic calibration,
typically on a daily basis using two known standards, can be used to compensate for any drift within the
measurement system but also validates the reported measurements. The loss of sample, reagent, cleaning solution
and the failure to clean & calibrate are all indicated locally and output to telemetry.
ProAm has been designed to work where other analysers have failed! The sample delivery system has
removed the need to pre-filter sample for applications to 200mg/L suspended solids improving operational
reliability. However, various sample preparation systems are available for more challenging applications
such as monitoring activated sludge, allowing more efficient control of the aeration process. Innovation
has once again confronted traditional weakness. The filter membrane is placed directly in the activated
sludge and the analyser’s integral pump is used to draw filtrate directly from source. This configuration
removes the need to install sample pumps in to harsh environments thus minimizing maintenance and improving
operational performance. |
