A cost-effective IoT strategy for remote deployment of soft sensors – a case study on implementing a soft sensor in a multistage MBBR plant

The rise of digital water, driven by a rapid increase in information technology, has created an impetus to develop smarter and more efficient monitoring tools in wastewater treatment plants. Soft sensors have gained importance as a viable alternative to expensive online sensors (Olsson et al. 2014; Wang et al. 2019). Soft sensors are computer codes, which use a process model together with available online measurements from treatment plants to estimate additional wastewater quality parameters (Haimi et al. 2015). These soft sensor scripts are executed in a programmable logic controller (PLC), which has access to real-time data from the online sensors. Soft-sensors can also be implemented in a remote device if the plant's supervisory control and data acquisition (SCADA) has remote monitoring capabilities.

Most commercial SCADA vendors use proprietary software for programming their PLCs. Therefore, implementing the soft sensor in a PLC requires the code to be written in its proprietary programming suite, which limits the possibility of running soft sensor scripts written in commonly used scientific/academic programming languages such as Matlab or Python in industrial PLCs. The alternative is to develop a communication layer interface between a client (with the soft sensor script) and a server that has access to data from the online sensors. Open Platform Communication (OPC) has recently emerged as a global standard for communication between various sensors, PLCs and SCADA providers (González et al. 2017). Most SCADA vendors provide OPC servers with remote access options. These remote services do, however, come at an additional cost.

This work discusses an alternative, cost-effective IoT strategy, which provides flexible and secure remote access to the PLC. The new IoT architecture is implemented at a multistage moving bed biofilm reactor (MBBR) pilot plant to access the data from the online sensors remotely. Access to real-time data from the pilot enables remote deployment of an Extended Kalman Filter (EKF) to estimate additional water quality parameters in the plant.

Figure 1 presents the operational schematic and monitoring station of a multistage MBBR pilot plant for municipal wastewater treatment (Saltnes et al. 2017). The reactor has four anaerobic chambers, followed by six aerobic chambers. The plant is retrofitted with dissolved oxygen sensors from chamber 5 to 10 and an ammonia sensor in the effluent line. All online sensors are connected to a PLC provided by Beijer Electronics (https://www.beijerelectronics.no/). The current monitoring system is enhanced by deploying an array of soft sensors for monitoring additional water quality parameters. The aim of the soft-sensor array is to estimate the ammonia concentration in the influent raw wastewater as well as in each stage of the pilot plant. The soft sensor also provides estimates on additional parameters (such as the biomass activity of the nitrifiers) that cannot be measured directly using a physical sensor. Real-time data from soft sensors enable plant operators to adjust carrier recycle and aeration rate (manually or through control valves) to achieve optimal operation. The estimation of ammonia concentration in each aerobic chamber can enable the possibility of implementing ammonia-based aeration control (Rieger et al. 2013), which has proven to be a superior control strategy compared to a constant dissolved oxygen (DO) setpoint control (Vrečko et al. 2011).

T_S is the sampling rate, k is the time subscript, and τ is the residence time in each chamber. The term represents the ammonia concentration and represents the dissolved oxygen concentration in the i^th chamber. The model-predicted value of the ammonia concentration in the effluent is denoted as y. The values of kinetic parameters and are taken from Stare et al. (2006).

In the equations above, x_k is the state vector and z_k is the measurement vector at a time instance k. The process and measurement noise covariance matrices are represented as Q and R respectively. I is the identity matrix, is the a priori estimate of the state, the covariance of a priori estimation error and the covariance matrix of the a posteriori estimation error. The sequential order of executing the EKF equations is presented as a flowchart in Figure 3(b).

Before implementing the soft-sensor in the pilot plant, the mathematical model (Equations (1)–(6)) and the EKF (Equations (7)–(13)) were implemented on a simulator platform to assess the system observability and identify the minimum number of online sensors required to estimate the ammonia concentration in each chamber. The simulator-based testing strategy implemented in Nair et al. (2019) was used to assess the performance of EKF. The values of the tuning parameters of EKF; Q, R, P₀x₀ are obtained using the tuning methods described in Haugen et al. (2014).

Figure 2 presents two possible schematics for remote deployment of the soft sensor code. Option 1 presents the default system provided by the SCADA vendor, which consists of an OPC server accessible through a secure VPN connection. The EKF is written as a script in Matlab and is executed remotely on a PC. The OPC Client toolbox in Matlab is used to receive real-time sensor data from the OPC servers. The second option includes an IoT gateway, which forms an interface between the PLC and a remote MySQL server, which is made accessible using Dynamic DNS (DDNS). The soft sensor algorithm is written as a Python script, which is then deployed on a single board computer – Raspberry Pi (https://www.raspberrypi.org/).

The network architecture for the remote deployment of the soft sensor is presented in Figure 3(a). The IoT gateway at the pilot plant collects data from online sensors and inserts them as a new row in a table created in the remote MySQL server. The client running the EKF code (in the Raspberry Pi) begins the cycle by reading the last row of the table. This table consists of the estimated states x_k, the covariance matrix of estimation error P_k, measurements from online sensor z_k and u_k. The EKF algorithm then calculates new values of the estimated state x_k+1 and covariance matrix of estimation error P_k+1. The updated values are inserted as a new row in the MySQL table. The Python script for implementing an EKF on a non-linear state-space model, the communication code that enables read and write of data from the remote SQL database, and the associated configuration files are provided in the Supplementary Material. The code has a modular structure and can be easily adapted to a new system by anyone with basic knowledge of programming in Python.

The estimation results for a period of 1 month (28th January 2019–12th March 2019) are presented in the following figures. Figure 4 shows the comparison between the influent NH₄-N concentration estimated by soft sensor and the biweekly measurements obtained by standardized laboratory tests (Hach-Lange kits). The R² values presented in the graph show that the values predicted by the estimator are close to the ammonia measurements from the laboratory. This demonstrates the effectiveness of soft sensors in providing real-time values of influent NH₄-N concentration.

The estimation results for NH₄-N concentrations at various stages in the pilot plant are presented in Figure 5. The time span for ammonia concentration in the individual chamber is shortened to one week for better visualisation of data.

Figure 6 presents the value of the nitrification rate r estimated by the soft sensor. A simplified nitrification model, such as the one used in this work, is derived from the comprehensive activated sludge model by considering certain assumptions. One such assumption involves excluding the kinetics of biomass growth and death and considering a constant biomass concentration (Julien et al. 1999). However, a time-varying estimate of the nitrification rate in the pilot could be a result of variations in biomass activity. Therefore, the real-time estimation of the time-varying parameter r could provide significant insight on variations in the concentration of active nitrifiers in the pilot plant. A qualitative assessment of the trends (presented in Figure 6) indicates a positive correlation between the nitrification rate and the ammonia concentration of influent wastewater. The nitrification rate (r) showed a steady decline on February 13th, a few hours after the influent ammonia concentration dropped. The pilot plant showed a lower nitrification rate from February 15th until March 6th, when the influent ammonia concentration was low. The nitrification rate started to increase after March 6th, when the influent ammonia concentration started to rise. This observation corresponds to the usual behavior of nitrifiers present in a biological wastewater treatment plant. However, a detailed analysis of the estimated results and cross-validation against laboratory measurements should be performed to substantiate the claim and fine-tune the estimator for soft sensing the concentration of autotrophs in the pilot plant.

A cost comparison between the default option provided by the SCADA vendor (Option 1) and two different versions (Option 2 and Option 3) of the new IoT system is presented in Table 1. Table 1 also presents the cost incurred while using a physical ammonia sensor (ion selective electrode) for measuring ammonia concentration in all six aerobic chambers. It can be inferred from the overall cost that using a soft-sensor can result in significant savings.

Among the options for soft-sensors, the default option provided by the SCADA provider has an average cost of about 1,580 € compared to the new system, which costs about 155 € (with SSL certificates). There is the possibility of running the setup without SSL, which would bring the cost down to 35 €, but this is not recommended due to security concerns (Buchanan et al. 2017). Recently there are providers that supply free SSL certificates (Aertsen et al. 2017), but with a slightly lower security level than the commercial options due to limited support. This could be a valid option for security if the system is not critical and other operational security measures like limited access control levels and network monitoring are taken into consideration. For more critical water infrastructure, additional layers of security can be added to the communication network by providing a VPN or an intermediate API link. The open source VPN software OpenVPN is relatively easy to implement, but API links require significant design considerations and can become inflexible. The new system discussed in this work provides a cost-effective option where the only investment is in the hardware (Raspberry Pi) and the cost associated with the purchase of commercial services for establishing a secure data connection.

This work illustrates a functioning example of a cost-effective and flexible alternative for non-intrusive remote monitoring in a wastewater treatment plant. The soft-sensor array deployed in the pilot plant proves the ability to bolster the monitoring system in a treatment process. The ability to monitor additional wastewater treatment parameters that are otherwise difficult to monitor via conventional online sensors would enable the implementation of multi-parameter based optimal control strategies. The case study presented in this work also illustrates the possibility of integrating scripts written in commonly used scientific programming languages such as Matlab or Python into any commercial SCADA. The cost comparison between the new IoT system and the options available in the market today shows that remote deployment of soft sensors can be achieved at much lower costs. This novel cost-effective option also opens up new possibilities to develop more comprehensive soft-sensing algorithms for estimating water quality parameters, which are otherwise difficult to monitor by a physical sensor.

We acknowledge the financial support provided by Norwegian Research Council (RECOVER project) and the technical support from the research team at Norwegian University of Science and Technology (NTNU).

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.067.