Improved nutrient removal using in situ continuous on-line sensors with short response time

Nutrient sensors that can be located directly in the activated sludge processes are gaining in number at wastewater treatment plants. The in situ location of the sensors means that they can be located close to the processes that they aim to control and hence are perfectly suited for automatic process control. Compared to the location of automatic analysers in the effluent from the sedimentation reactors the in situ location means a large reduction in the response time. The settlers typically work as a first-order delay on the signal with a retention time in the range of 4-12 hours depending on the size of the,settlers. Automatic process control of the nitrogen and phosphorus removal processes means that considerable improvements in the performance of aeration, internal recirculation, carbon dosage and phosphate precipitation dosage can be reached by using a simple control structure as well as simple PID controllers. The performance improvements can be seen in decreased energy and chemicals consumption and less variation in effluent concentrations of ammonium, total nitrogen and phosphate. Simple control schemes are demonstrated for the pre-denitrification and the post precipitation,system by means of full-scale plant experiments and model simulations

Exploiting online in-situ ammonium, nitrate and phosphate sensors in full-scale wastewater plant operation

In-situ nutrient sensors are now a proven technology. Having ion membranes eliminates the need for ultrafiltration, and consequently the sensors can be located at suitable places in any of the reactors. This gives the potential for new control structures for the control of nitrification, denitrification, and phosphorus removal. In the paper some examples of such controllers are demonstrated as used in a full-scale wastewater treatment plant. A successful control implementation scheme at full-scale plants includes three steps: monitoring, experimenting and controlling. The benefit of implementing process control based on nutrient sensors is real: by implementing precipitation dosage control a savings of 41 % compared to flow proportional dosage can be reached, while the savings compared to constant dosage is 73 %. An increase in nitrate recirculation shows significant improvement in the nitrogen removal ability at very low cost. Reliable nutrient sensors are not the only prerequisite for a successful control system. The design of actuators, such as drives, compressors and valves, is often overlooked. Furthermore, the lower level controllers have to work properly before the more advanced controllers can work adequately. A collection of practical experience regarding such issues is given in this paper

Benchmarking plant operation and instrumentation, control and automation in the wastewater industry

Benchmarking is an effective tool to compare the performance of full-scale wastewater treatment plant operation. In this study, 29 wastewater treatment plants from eight countries were surveyed with the aim of developing some key performance indicators, which may be used for benchmarking purposes. The level of utilisation of instrumentation, control and automation (ICA) has also been measured. The study of ICA utilisation revealed that on average only 23% of all sensors are used for online control. For most parameters, ICA is not the single dominant factor determining performance, although it is the factor that has the highest potential to improve performance. Eighty percent of the participating plants had implemented new control during the last five years leading to improvements in nitrogen and phosphorous removal, energy efficiency and ease of operation. The survey has quantified how "non-standardised" wastewater treatment plant operational practice is. Some simple key performance indicators are derived that relate the level of removal of ammonium, total nitrogen, phosphorous and suspended solids to the resources needed for their removal, i.e. volume, energy, organic matter and precipitation chemicals. Several indicators are suggested for each substance. The indicators show a great difference from the best to the poorest performance indicating that improvement potentials exist at many plants

Dissolved oxygen controller based on on-line measurements of ammonia combining feed-forward and feedback

As the largest single energy-consuming component in most biological wastewater treatment systems, control of aeration is of great interest seen from an energy savings point of view. This paper suggests a simple way of using on-line ammonium measurements to control aeration in a pre-denitrification plant by controlling the dissolved oxygen setpoint. The controller works primarily by feed-forward based on an ammonium sensor located at the head of the aerobic process part. Using online in-situ sensor measurements directly from the process have the important advantage over effluent measurements that there is no or very short time delay for information. The controller has been implemented in a full-scale wastewater treatment plant for a period of 35 days. During the experiment two identical activated sludge lines were used. The controller was implemented in one line, while the other line worked as a reference for comparison. The preliminary results indicate that the described control strategy leads to energy savings for the aeration in the region of 5-15%, while maintaining approximately the same effluent quality as in the reference line. Even higher energy savings can probably be achieved by optimising the controller. An automatic procedure for updating the controller parameters based on dynamic effluent ammonium measurement has been tested

Benchmarking plant operation and instrumentation, control and automation in the wastewater industry

Benchmarking is an effective tool to compare the performance of full-scale wastewater treatment plant operation. In this study, 29 wastewater treatment plants from eight countries were surveyed with the aim of developing some key performance indicators, which may be used for benchmarking purposes. The level of utilisation of instrumentation, control and automation (ICA) has also been measured. The study of ICA utilisation revealed that on average only 23% of all sensors are used for online control. For most parameters, ICA is not the single dominant factor determining performance, although it is the factor that has the highest potential to improve performance. Eighty percent of the participating plants had implemented new control during the last five years leading to improvements in nitrogen and phosphorous removal, energy efficiency and ease of operation. The survey has quantified how "non-standardised" wastewater treatment plant operational practice is. Some simple key performance indicators are derived that relate the level of removal of ammonium, total nitrogen, phosphorous and suspended solids to the resources needed for their removal, i.e. volume, energy, organic matter and precipitation chemicals. Several indicators are suggested for each substance. The indicators show a great difference from the best to the poorest performance indicating that improvement potentials exist at many plants

A hedging point strategy balancing effluent quality economy and robustness in the control of wastewater treatment plants

An operational space map is an efficient tool to compare a large number of operational strategies to find an optimal choice of setpoints based on a multicriterion. Typically, such a multicriterion includes a weighted sum of cost of operation and effluent quality. Due to the relative high cost of aeration such a definition of optimality result in a relatively high fraction of the effluent total nitrogen in the form of ammonium. Such a strategy may however introduce a risk into operation because a low degree of ammonium removal leads to a low amount of nitrifiers. This in turn leads to a reduced ability to reject event disturbances, such as large variations in the ammonium load, drop in temperature, the presence of toxic/inhibitory compounds in the influent etc. Hedging is a risk minimisation tool, with the aim to "reduce one's risk of loss on a bet or speculation by compensating transactions on the other side" (The Concise Oxford Dictionary (1995)). In wastewater treatment plant operation hedging can be applied by choosing a higher level of ammonium removal to increase the amount of nitrifiers. This is a sensible way to introduce disturbance rejection ability into the multi criterion. In practice, this is done by deciding upon an internal effluent ammonium criterion. In some countries such as Germany, a separate criterion already applies to the level of ammonium in the effluent. However, in most countries the effluent criterion applies to total nitrogen only. In these cases, an internal effluent ammonium criterion should be selected in order to secure proper disturbance rejection ability

Dissolved oxygen controller based on on-line measurements of ammonium combining feed-forward and feedback

As the largest single energy-consuming component in most biological wastewater treatment systems, control of aeration is of great interest seen from an energy savings point of view. This paper suggests a simple way of using on-line ammonium measurements to control aeration in a pre-denitrification plant by controlling the dissolved oxygen setpoint. The controller works primarily by feed-forward based on an ammonium sensor located at the head of the aerobic process part. Using online in-situ sensor measurements directly from the process have the important advantage over effluent measurements that there is no or very short time delay for information. The controller has been implemented in a full-scale wastewater treatment plant for a period of 35 days, During the experiment two identical activated sludge lines were used. The controller was implemented in one line, while the other line worked as a reference for comparison. The preliminary results indicate that the described control strategy leads to energy savings for the aeration in the region of 5-15%, while maintaining approximately the same effluent quality as in the reference line. Even higher energy savings can probably be achieved by optimising the controller. An automatic procedure for updating the controller parameters based on dynamic effluent ammonium measurement has been tested