In the continuation of the blog on the resistance to the effective use of instrumentation Oliver Grievson, manager of the Water Industry Process Automation and Control (WIPAC) group on LinkedIn, is going to examine what an instrument and more importantly what an instrumentation system can achieve. In this analysis I can going to make a few basic assumptions, namely:
- The correct instrumentation has been installed
- The instrumentation work and is maintained
- The instrumentation is connected to a automation & control system
- The data from the instrumentation is used and is converted to information for the use of the company.
A good instrumentation system at a treatment works (or network of treatment works) are the eyes and ears of any treatment process, they are a useful tool to the operators of a works insofar as they analyse 24 hours a day 365 days a year. With the correct integration and data acquisition system they allow an operator to see how the treatment process is performing and they allow someone at a remote location to check on the progress, all of this in almost an instant at a accuracy that is required and certainly quicker that sending a sample to an offsite location for analysis. THESE ARE THE BASICS OF WHAT INSTRUMENTATION CAN ACHIEVE.
There are several areas that instrumentation can provide a greater efficiency and effectively pay for themselves, not all inclusively, these are:
- Process Safety Control – With a control system some aspect of safeguarding the process
- Process Optimization – With the right controller an control systems instruments are fundamental in controlling the dissolved oxygen in an ASP or controlling the chlorine dose at a water treatment plant
- Value and Process Management – Often used in the potable side of the water industry but not in the wastewater side (in the UK at least) but analysis of the relatively efficiencies of the treatment works or indeed the treatment process.
- Asset Management – The modern instrument and control system can provide significantly more data than just the value its measuring with the correct control system a vibration monitor can provide data on the performance of the process unit or protect a unit from destroying itself.
In demonstrating these different areas I hope you will excuse me from using an activated sludge plant as an example as this is my area of expertise.
To demonstrate Process Safety Control I’ll use the example of a final settlement tank. This example is commonly used in the wastewater industry in the UK (and probably elsewhere). In the final settlement tank you settle the mixed liquors from the activated sludge process. The danger is that the mixed liquors will build up and solids will pass through to the effluent. The instruments that are useful here are a sludge blanket detector or a turbidity monitor on the effluent, the part of the process being controlled is the bellmouth (if actuated). or desludge pump on the tank Either detection of the sludge blanket or high solids in the effluent would trigger the bellmouth level to drop or the desludge pump to start. This is basic control saving the water company from polluting if there is an instrument in place.
Simple but effective
If the same scenario is taken however the bellmouth or desludge pump has an idea of the flow entering the process and the flow through the bellmouth then the return activated sludge rate can be controlled to a rate that is necessary for the process to work and will fact improve the performance by ensuring good floc loading in the anoxic zone (if present) of the activated sludge plant. This is a good example of Process Optimization. On the Water Industry Process Automation & Control group site at the moment is another example with a discussion of the use of ammonia sensors to control the air input into the process, its proving to be contentious but can save significant amounts of operational expenditure.
This brings us into an area which is often ignored on the wastewater side and that is using instrumentation to discover how efficient that the process is operating. Take the example of the air blowers that are controlled by the ammonia sensors. On a medium to large site the air going into the process from the blowers if often measured in terms of its air mass (kg O2), the blowers themselves will often have a variable speed drive which as part of it has power consumption, as an alternative an electrical sub-meter can be installed. The two bits of data can be combined to give an approximation in the efficiency of supply in kg O2/kWH. This gives the efficiency of the aeration system and is often used in design but not as commonly used in the operation of wastewater treatment works (at least in the UK). These elements seem basic but are often not used. A drop the efficiency can be used to measure when is the best economical time to drop the activated sludge plant lane and clean it. This demonstrates the use of instrumentation in Value and Process Management.
Another example is the use of power consumed or cost per unit product treated. Often used for chemical and power consumption in the potable water side but hardly ever in the wastewater side, again this is Value and Process Management
Finally the blowers that feed that air are monitored and their stops and starts measured. They will have recommended service periods just as any mechanical item such as a car has. An oil change at 500 hours or a full service at 1,000 hours of operation, however maintenance is scheduled in terms of calendar time. The instrument that measures the stop and the start gives a totalised time of operation to the controller. So instead of a maintenance engineer who comes every six calendar months and services a blower whether it has run for six hours, six week or six months can actually be ordered by the instrument when it the blower actually needs it (i.e. after 500 hours of running) rather than because its that time of the year.
The more complex alternative is Condition Based Monitoring where the information from the sensors on the asset are used to monitor the units performance. A blower in this case but could quite easily be a pump. These are examples of instruments (and sensors) in Asset Management.
These are several examples where much more can be achieved from the instruments and instrumentation systems that are present at our treatment works that help operator, engineer or manager to more effectively operate. There are many more examples and I am sure I am not giving any trade secrets away by scribing them here. The question is why are even these basics in this article not routinely applied in the water industry.
To peruse or join the Water Industry Process Automation and Control (WIPAC) group please return to the homepage and click on the correct link under Blog Roll in the far right column below the picture.