Wastewater Treatment Optimisation Provides Cost Savings
The operators of wastewater treatment plants constantly seek new opportunities to improve plant efficiency and environmental
performance. In order to achieve this they need to be able to maintain the effectiveness of the treatment process, producing a consistent discharge within consent limits, whilst minimising inputs such as energy, labour and raw materials.
As technology advances new opportunities materialise and this article will outline the cons - iderable benefits that can be obtained from the latest sensors coupled with a new breed of real- time controllers. Improvements in the accuracy and reliability of sensors, coupled with a new facility providing
information
about the sensors' performance, in addition to the measurement itself, means that real-time control
(RTC) has become very reliable which means that it has become an attractive option in a large number of applications.
HACH LANGE has developed a set of standardised control modules, enabling the application of processes improvements and optimisation strategies without the need for complex programming and expensive customisation.
In combination with HACH LANGE sensors, Nutrient Removal and
Sludge Treatment Processes can now be easily optimised in order to achieve savings in aeration energy and chemical consumption, even on small waste water treatment facilities.
RTC Opportunities
Stand-alone wastewater treatment optimisation solutions (WTOS) control modules are now available to optimise individual treatment processes at treatment plants. These can be easily integrated into an existing plant structure and currently include (1) the chemical elimination of phosphorus and (2) dissolved oxygen adjustment according to the actual NH4-N load in an aeration tank. Control modules for sludge management as sludge retention time controller or desludging controller will be added in the near future.
In addition to the stand-alone modules mentioned above, it is also possible to combine different RTC modules to optimise an entire plant, as outlined in the trial below. Termed an 'enterprise solution' this activity involves a review of the plant as a whole and the creation of customised specifications for the application of different control modules for nitrification, sludge retention time, methanol dosing, and/or chemical phosphate removal to achieve the best overall performance.
Sensor Technology
In recent years, improvements in sensor technology have focused on greater resolution and accuracy in combination with longer intervals
between calibration or service. However, in order for an RTC system to operate effectively it is also necessary for sensors and analysers to be able to provide information on the quality of the signal and the service status.
HACH LANGE has filed a patent application for this facility under the brand name 'PROGNOSYS'. This provides the RTC control modules with a continuous indication of a sensor's status so that if pre-determined conditions occur (sensor failure, outside calibration, service overdue, drift etc) the RTC automatically adopts an alternative control strategy, which might be a typical weekly and diurnal flow profile that has been stored in the system's memory.
Stand-Alone RTC Example: Chemical Phosphate Removal
As outlined above, the measurement technology for phosphate has advanced considerably in recent years in tandem with a reduction in capital and operational costs. As a result, an easy to integrate RTC module in the phosphate removal process can deliver pay back periods of less than one year.
The measurement of phosphate levels in combination with an RTC system can be utilised to manage the dosing of precipitant salts. This precipitates the phosphate and facilitates sedimentation and removal. Accurate continuous monitoring is necessary to ensure that (a) sufficient dosing is applied to remove the phosphate and (b) excessive dosing does not take place. Over-dosing would be undesirable on three counts; firstly, from an environmental perspective the objective is to minimise the amount of iron being added that could remain in the effluent; secondly, ferric sulphate is expensive and excessive dosing would be costly; thirdly the amount of precipitation sludge should be kept to a minimum because sludge disposal can represent a significant cost.
A unique feature of the RTC system is the continuous automatic calculation of the 'ß' value (overdosing rate), which is required to calculate the right amount of precipitant dosing for open loop control. The calculated ß-value takes into account the percentage of
Figure 1: Example for Stand Alone P-RTC performance
RTC Control and display unit
IET
Annual Buyers Guide 2010
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