Appendix B. Automatic Water Quality Monitoring
B1. General considerations
The analytical methods employed with automatic water quality monitors (or on-line instruments) are, in the main, fundamentally the same as those used in the laboratory. The main difference between laboratory instrumentation and on-line instrumentation is to do with the robustness of construction and the addition of automatic systems for sample preparation, instrument/sample line cleaning and instrument calibration.
In an ideal world, an on-line chemical analyser would employ low cost non-invasive measurement techniques, produce highly accurate results and never need servicing. In reality a target of achieving results of acceptable accuracy at an acceptable cost, with a service requirement not greater than once per week is likely to be more appropriate. To achieve this, the main features required in an automatic water quality monitor are:
- appropriate location of sampling point;
- purpose-designed robust construction, both in terms of the physical protection provided by the instrument housing and the robustness of the operational methodology;
- tolerance to the extremes of temperature likely to be encountered;
- resistance to the ingress of dust and water;
- tolerance of electromagnetic fields, electrical transients and power supply disturbances;
- minimum supervision and maintenance requirements;
- designed for easy access and fault-finding when maintenance is required;
- purpose-designed sample transport and conditioning system.
The two main applications are for monitoring or control. In general, monitoring applications require predictable, long-term analytical performance in terms of accuracy and reproducibility to ensure comparability of the data. On the other hand, fast response time and high sample throughput rates are not usually an issue. In contrast, analysers used in process control applications often have to respond rapidly and reproducibly to small changes in the composition of the process fluid, whereas absolute accuracy is often of lesser importance.
The choice of analysis method and hence instrument has to be made with due regard for the use to which the resulting data will be put. For example, instruments based on well documented colorimetric methods can provide data of predictable and consistent quality. However, depending on the inherent delay in the chemistry involved, they tend to have fairly long response times from the sample entering the analyser to the output of the result. Instruments based on such methods may, therefore, be less than ideal in control applications requiring a fast response, but are suited to monitoring applications. In contrast, electrochemical analysers are less predictable in their performance, thus requiring frequent recalibration. However, their relatively rapid response to changes in the sample stream composition is often an important consideration in process control applications.
The degree of complexity inherent in any given analyser installation is dependent on both the complexity of the measurement technique and the nature of the sample. As an example, the on-line measurement of conductivity can be easily and reliably performed on a wide range of sample types using a non-contact measurement technique. In contrast, the measurement of determinants such as phenol, on a treated or partially treated waste effluent, is fraught with difficulty and requires a high level of operator input.
The basic measuring techniques which are in general on-line use are physical, electrochemical and photometric. Examples of the range of determinants for which on-line analysers based on these techniques are available, are listed below.
- Physical: colour, turbidity, suspended solids, conductivity, pressure, depth, level, density, temperature, flow rate, volumetric flow.
- Electrochemical: pH, ammonia, nitrate, bromide, calcium, carbon dioxide, chloride, chlorine, metals, cyanide, fluoride, REDOX, dissolved oxygen.
- Colorimetric: ammonia, nitrate, nitrite, phosphate, chloride, fluoride, sulphate, metals, manganese, phenols.
- Other measurement techniques which are available on-line include: high temperature and low temperature methods for organic carbon measurement; respirometry for BOD and toxicity; and gas chromatography and HPLC for phenols and organics.
Dedicated analysers are available for many of these determinants, but in situations where this is not the case, then a user configurable analyser can be used. Such analysers, often referred to as 'process titrators' or 'process analysers', are available from a number of manufacturers. These instruments consist of a programmable controller and a selection of valves, pumps, sample-conditioning devices and sensor options. The flexibility offered by these analysers enables laboratory methods based on titrimetric, colorimetric and electrochemical techniques to be operated on-line.
There are three basic types of process analyser configuration, two of these are continuous flow systems and the third is a batch process in which measured volumes of sample are processed in a series of discrete steps on a continuous basis. The time interval between each analysis is usually user selectable, with a minimum value which is a function of the design of the instrument and the method of analysis. These analysers are usually constructed in a modular form to facilitate simple adaptation to a wide range of analytical methods.
The selection and installation of an on-line analyser should be approached in a similar way to that employed in the selection of appropriate laboratory methodology/instrumentation. The application should be identified in terms of the:
- determinant to be measured;
- reason for making the measurement;
- required frequency of the measurements;
- consequences of analyser failure;
- composition of the sample;
- accessibility of a suitable sampling point; and,
- availability of suitable locations for the analyser installation.
Using this information, the analyser performance requirements should be defined and the sample conditions identified. Points to be considered include:
- The performance required i.e. systematic error, random error, specificity, limit of detection and response time;
- The environment in which the analyser will be installed and hence the degree of environmental protection required. If an appropriately protected analyser is not available then additional protection may have to be provided;
- The electrical environment in which the analyser will be operated. A poor quality electrical supply, the close proximity of heavy electrical plant or sources of electromagnetic radiation may necessitate the installation of power supply conditioning equipment or additional shielding;
- The requirements for sample transport and conditioning prior to analysis. Limitations on the acceptable range of sample composition at the input to the analyser may necessitate additional sample conditioning to be undertaken. The delays which are likely to occur within the proposed sampling and analysis system should be estimated and compared with the identified measurement response time requirements to ensure that the installation is capable of meeting the requirements.
Sampling systems play a very necessary and vital role in the successful operation of on-line analysers. Unless the sensor is located directly into the waterbody there is a requirement to convey the sample to the analyser. Even in the case of sensors inserted directly into the waterbody the location of the sensor is crucial in obtaining representative results.
There is a temptation to view the sampling system as simply a method of transporting the sample from the waterbody to the analyser, without considering all the potential implications. This can lead to a number of problems occurring:
- significant changes in the composition of the sample within the sampling system giving rise to unrepresentative results;
- insufficient sample flow or long delays between the sample being extracted from the waterbody and delivered to the analyser;
- the sample line becoming blocked;
- failure of the instrument to live up to expectations or being considered to be unreliable and hence gradually falling into disuse.
It is clear that the sampling system is an integral part of the installation which must be taken into account early in the design stage if the overall objectives are to be met.
The choice of which system to apply will depend on a number of factors such as:
- the separation between the analyser and the sampling point;
- the system response time requirements;
- the consequences of changes occurring in the sample; and,
- the nature and composition of the sample.
The design of the sampling system should encompass the design objectives listed below. The priority assigned to each of these objectives will depend on the details of the specific application.
- Representative sampling: The sample that is delivered to the instrument should be representative of the process stream with respect to the determinants being measured.
- Compatibility: The sample should be presented to the analyser in a state which is compatible with the measurement technique used by the analyser.
- Sample transport delay: The design of the sample system should take account of the inherent time lag, between the sample being taken from the waterbody and delivered to the inlet to the analyser, so as to ensure the overall response time objectives can be met.
- Reliability: The sampling system should be reliable and require the minimum of maintenance. If necessary automatic back flushing and/or air purge can be employed, along with a duty and standby system.
- Safety: The sampling system must be safe to operate and maintain.
- Validation: The system should be designed with grab sample tapping points at suitable locations to facilitate system validation both at the commissioning stage and routinely during its operational life.
B2. Maintenance Requirements for Automatic Water
The majority of on-line analysers require direct contact with the water to be sampled. Wherever this is the case, there is the potential for fouling of the sampling system or sensor to occur, thus affecting the overall performance of the installation. The affect of fouling on the analyser may result in the collection of misleading and unreliable data or the failure of an automatic control system.
The types of fouling encountered in sampling natural waters are usually biofouling, (growth of bacterial/fungal films) and particulate fouling by particles present in the effluent.
More often than not the types of fouling outlined above will form as a combination of different types and form a complex fouling layer. This presents problems in how to predict what type of fouling will occur at a particular stage of the treatment process and the rate at which it will occur.
If sensor fouling is likely to occur there are three main approaches to reducing the affect of fouling. These are manual cleaning, preventive techniques and automatic cleaning.
One method of overcoming sensor fouling is to instigate a rigorous manual cleaning schedule. A procedure needs adopting where the sensor is removed from the waterbody and cleaned manually at an interval which is sufficient to keep the analyser operating within its operational requirements. This may be undesirable because resources may be limited, costs may be excessive, or safety may be an issue.
If manually cleaning on a frequent basis is undesirable then choosing a sensor which is less prone to fouling or which incorporates some form of automatic cleaning is an alternative option.
If the fouling cannot be prevented or reduced to an acceptable level, some form of automatic cleaning may be needed. Many analysers are available with automatic cleaning options.
Fouling may be reduced by filtering or separating out particulate matter from the sample fluid before it reaches the sensor. This technique can only be used on analysers that are not affected by the removal of the particulate matter from the sample. Any separation device that is used should be inherently self-cleaning, non-fouling or require infrequent manual cleaning. Suitable devices include hydrocyclones for the removal of larger particles or cross flow filtration devices that are available in a range of sizes.
B3. Portable Monitors
Portable monitoring equipment used both in conjunction with automatic monitors and for measuring determinants in situ which cannot be reliably measured by the time the sample has been returned to the laboratory (such as pH, DO and conductivity) are in routine use in many countries.