Dr Michael Haeck reports on how to maintain the effectiveness of the wastewater treatment process, while at the same time minimising inputs such as energy, labour and raw materials.
Improvements in the accuracy and reliability of sensors, oupled 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.
In combination with Hach Lange sensors, nutrient removal and sludge treatment processes can now be optimised in order to achieve savings in aeration energy and chemical consumption, even on small waste water treatment facilities.
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 the chemical elimination of phosphorus and 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 standalone 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.
Hach Lange has filed a patent application for 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.
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 sufficient dosing is applied to remove the phosphate and 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 feature of the RTC system is the continuous automatic calculation of the β value (overdosing rate), which is required to calculate the right amount of precipitant dosing for open loop control. The calculated β-value takes into account the percentage of phosphate which has to be removed. The less phosphate there is; the more difficult removal becomes and the more precipitant is required to eliminate the same amount. For example, more precipitant is required to lower phosphate concentrations from 4 to 2mg/l than from 6 to 4mg/l.
Wastewater treatment plants operating an open loop real time control system for phosphate removal have demonstrated considerable savings - a UK works has saved approximately 37 per cent of the ferric sulphate cost and 57 per cent of caustic chemical costs and a plant in Italy has shown 50 per cent cost savings in comparison with a constant dosing system, which represents a seven month payback.
If closed loop control is applied, the RTC system requires a measurement of phosphate levels immediately after dosing. As a result, the Phosphate concentration can be held at a fixed desired level and the control performance is monitored.
The results of a trial investigating the benefits of an RTC system on the management of the activated sludge process (ASP) have been published by Thornton, Sunner and Haeck.n
Dr Michael Haeck is with Hach Lange, Salford, Manchester, UK. www.hach-lange.co.uk
