Enabling wastewater treatment process automation: leveraging innovations in real-time sensing, data analysis, and online controls

Wenjin Zhang *a, Nicholas B. Tooker b and Amy V. Mueller a
aDepartment of Civil and Environmental Engineering, Northeastern University, 360 Huntington Ave., Boston, MA 02115, USA. E-mail: zhang.wenji@northeastern.edu; Fax: +617 373 4419; Tel: +617 373 2444
bDepartment of Civil and Environmental Engineering, University of Massachusetts Amherst, 130 Natural Resources Road, Amherst, MA 01003, USA

Received 26th April 2020 , Accepted 10th August 2020

First published on 7th September 2020

The primary mandate of wastewater treatment facilities is the limitation of pollutant discharges, however both continued tightening of permit limits and unique challenges associated with improving sustainability (i.e., resource recovery) demand innovation. Enabling increasingly sophisticated treatment processes in a cost-effective and energy-efficient way requires expanding capabilities for rapid, accurate real-time quantification of a broadened range of wastewater constituents as well as envisioning novel feedback control strategies based on these signals. This manuscript quantitatively compares results of early adoption of instrumentation and process upgrades at operating wastewater treatment facilities and proof-of-concept research results, with a focus on leveraging real-time sensing of wastewater chemistry for process monitoring and control. Up to 10% improvement in nutrient removal and energy savings are already being achieved, yet shortfalls in hardware readiness, lack of field-relevant context of research results, and a widening gap between the training of environmental engineers and the skillsets required to develop and maintain sensor-driven solutions present challenges. A forward-looking roadmap highlights opportunities for accelerating innovation, including (1) ensuring research results are published in units and context that allow operators to make an informed cost–benefit analysis with explicit comparison to existing operational baselines, (2) promoting integrated design of hardware and software to generate novel approaches for improved sensing of target analytes, (3) strengthening partnerships nationally, including for data sharing, field testing of new hardware, and expanding educational curricula, and (4) building forums for sharing of expertise and data among plant operators to enable smaller facilities to more cost-effectively collect information required to design and evaluate upgrades.

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Wenjin Zhang

Wenjin Zhang is currently a Ph.D. candidate in Interdisciplinary Engineering in the Department of Civil & Environmental Engineering at the Northeastern University. Her research focuses on informatics for decision and control of civil and environmental infrastructure systems, including machine learning applied to wastewater treatment sensing and process control.

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Nick Tooker

Nick Tooker is a Professor of Practice in the Department of Civil & Environmental Engineering at the University of Massachusetts Amherst. He has experience in design and research of nutrient removal and recovery systems for water resource recovery facilities. In 2019, he received the Ralph Fuhrman Medal for Outstanding Water Quality Academic-Practice Collaboration.

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Amy Mueller

Amy Mueller received the S.B. (′02) and M.Eng. (′03) degrees in electrical engineering and computer science and the Ph.D. (′12) degree in environmental chemistry from the Massachusetts Institute of Technology, Cambridge, MA, USA. She is currently an Assistant Professor jointly appointed in the Colleges of Engineering and Science at Northeastern University, Boston, MA, USA. Her primarily field of research is sensor and control system development for study and optimization of biogeochemistry of natural systems and urban infrastructure.

Water impact

Stringent pollutant regulation and a vision of future water resource recovery demand innovation in wastewater treatment. Implementation of novel sensor-driven processes shows 10% improvement in nutrient removal and energy savings, but broadened stakeholder partnerships (e.g., data sharing, field testing of hardware, education design) are critically needed to translate adoption of such processes at a wider range of facilities.

1 Introduction

The primary mandate of wastewater treatment facilities is the limitation of pollutant discharges1 (i.e., primarily nutrients, organic carbon substrates, and metals) in accordance with legislation like the Clean Water Act2 and the European Union Water Framework Directive (WFD).3 Both the continued tightening of permit limits (e.g., the 2018 revision to the National Pollutant Discharge Elimination System (NPDES)4) and the unique challenges associated with improving the sustainability aspects of water treatment (i.e., resource recovery and reuse) are driving innovation in this field.5,6 For instance, novel microbial consortia (e.g., anammox7,8 and phosphate accumulating organisms (PAOs)9) are already providing alternatives, or complementary methods, to the traditional nitrification–denitrification scheme; such strategies can decrease aeration costs (which can represent up to 50% of total plant energy use10,11), decrease use of metals for phosphate removal, and/or increase opportunities for nutrient recovery.12–14 Recently, the potential for wastewater treatment facilities to play a role in moving toward a closed-loop approach on water – and the constituents of wastewater, such as nutrients and energy – is also gaining recognition, with implementation of strategies such as nutrient recovery and direct (or indirect) potable water reuse exemplifying how wastewater treatment plants are being re-envisioned as wastewater resource recovery facilities (WRRF).15

Enabling these increasingly sophisticated treatment processes in a cost-effective and energy-efficient way demands innovation in online, real-time techniques for process monitoring and automated process control – as does the automation necessary for improving cost-effectiveness and sophistication of processes at small or decentralized treatment facilities.16 Achieving these goals requires expanding capabilities for rapid, accurate quantification of wastewater constituents as well as innovating in feedback control strategies based on either traditional or novel sensing modalities,17 while at the same time balancing tradeoffs between complexity and management of potential failure modes.

One strategy for accelerating progress in these efforts is through increased visibility of existing tools and successes. For instance, recent reviews provide a summary of the current state of nutrient monitoring methods (nitrogen and phosphorus)18 as well as insight into the potential for use of biosensors19 and fluorescence spectroscopy20 in wastewater monitoring. Wastewater process control technologies21 and methods22,23 have also been discussed, increasingly promoting the use of big data and statistical process control to address unique characteristics in wastewater treatment.24,25 Different teams have focused on the wide range of options available in leveraging big data, e.g., regression models for biological process variable prediction26,27 or resource recovery,28 plant-wide process modeling and monitoring,29 and development of multiple sensor-driven inference models.30

Largely, however, these efforts have focused primarily on emerging research and the long-range future of the discipline, which has created a gap wherein limited actionable information is available for plant operators to assess which technologies are ready to be adopted, what the outstanding challenges are, and what timelines might be feasible for integrating these innovations into operational strategy. This manuscript seeks to bridge this gap by considering the practitioner needs and identifying imminent roadblocks preventing the flow of innovation into use at operating wastewater treatment facilities, updating similar efforts undertaken as the instrumentation boom began in the early 2000s.31 This is achieved through an integrated analysis of case studies for which quantitative results are reported, both in operating plants and in research that leverages pilot plants or uses operational data from wastewater treatment facilities. The results are evaluated in the context of user needs (focusing primarily on wastewater chemistry of secondary treatment processes, where instrumentation is driving impactful progress toward WRR goals) to identify gaps and recommend strategies for near-term critical research and for better leveraging expertise of all stakeholders, from plant operators and engineers to researchers to commercial instrumentation providers, to collectively work toward future solutions.

Briefly, the manuscript is laid out as follows. Section 2 provides an overview of wastewater treatment processes and introduces relevant terminology as context for the reader. Section 3 describes strategies used and results achieved at operating treatment facilities using sensors and real-time controllers primarily for improved nutrient removal and energy efficiency. Section 4 highlights current and ongoing research on novel sensors, data analytics methods, and control schemes for use in treatment facilities, highlighting studies for which practically interpretable numerical results available. Section 5 proposes targeted areas for continued research to support innovation in process design and control, seeks to understand how to improve transfer of innovations from academic research to operations, and suggests opportunities for progress built around collaborative efforts. Finally Section 6 concludes with a summary of key insights.

2 Wastewater treatment: context and terminology

2.1 Wastewater process overview

Wastewaters can be roughly divided into two categories based on chemical makeup: industrial and municipal.1 Industrial wastewaters are highly variable depending on the source process, can include a wide range of potentially toxic materials (e.g., lead, silver, mercury, copper, nickel, chromium, zinc, cadmium, or tin32), can have widely varying pH (e.g., as a consequence of use of strong acids or bases for cleaning17), and/or can have high biochemical oxygen demand (BOD).33 Because of the variability in treatment needs, industrial discharges are generally mandated to be actively managed on site (e.g., through pre-treatment) before discharge into municipal sewers. In contrast municipal wastewaters, i.e., the collected waste streams of residences, non-industrial workplaces, and pretreated industrial effluents, are typically high in organic content and nutrients which are therefore the primary targets of municipal wastewater facilities. Because of the relative similarity in aims at most municipal plants, the more variable range of industrial treatment purposes, and the relative lack of available publications on industrial treatment processes (which tend to be managed by private commercial entities), the case studies in this manuscript will focus primarily on municipal treatment processes.

In municipal wastewater facilities, the secondary treatment stage is where carbon and nutrient removal are achieved, most frequently by using biological processes.5 Because of the need to maintain a stable microbial population and due to the high cost of aeration (as a fraction of plant operating cost), optimization efforts both in deployment of sensors and in control strategies have focused mostly on secondary treatment, and innovation continues to focus on this process stage as the most impactful area for improvement.

2.2 Terminology

Typically innovations in wastewater biological treatment processes go through one or more development or test phases at smaller scales before being implemented in operating plants. Lab-scale reactors (small, e.g., size of ≤5 liters) may be operated at municipal facilities or within academic labs, often on synthetic wastewater (i.e., stable conditions that allow controlled testing and evaluation during development). Pilot-scale reactors (e.g., hundreds to thousands of liters) are typically operated as a sidestream at municipal facilities to evaluate usability on real wastewater streams. Pilot-scale systems often represent collaborations between municipal, academic, and/or engineering consulting (e.g., sensor or controller design) partners, may represent a scale-up of lab-scale work, and are generally a good test for stability in the face of real wastewater variability. Full-scale generally refers to any process implemented in the main treatment train at a municipal utility, however it is important to note that the size range for such facilities is extremely wide (i.e., from <1 to >100 million gallons per day (MGD) processed waste flow or, alternatively, serving <2000 to hundreds of thousands of people).

Usage of terminology related to sensor and data approaches can vary; this manuscript will follow published references (e.g., ref. 34–36), briefly summarized here. A sensor is a type of instrument that responds to a property of the environment, reporting in real time as an analog or digitally readable signal (e.g., a voltage or ASCII data stream). An analyzer is an instrument that measures some property of a sample after completing some chemical alteration, e.g., mixing with a reagent, and results typically have a time delay (often 5–10 minutes but up to 30 minutes when chemical digestions are required). Real-time refers to data that are available very shortly (typically <1 min) after requesting a reading, while online refers to an approach that digitizes data instantaneously to make it available for post-processing or use (noting this does not necessitate it be web-linked, just that the data be available to a local processing unit or controller). Logged data refers to the data recorded over a period of time by a computer system or by lab staff which can later be analyzed both for retrospective investigations or prospective testing of new approaches. Chemometrics is the umbrella term capturing the application of mathematical, statistical, and computation methods to address questions related to system chemistry. Soft sensor refers to a system that utilizes a tuned algorithm to estimate the concentration of a target analyte from readings from multiple individual sensing devices or, alternatively, to estimate from a single sensor one or more outputs which are not measured directly by the sensor being used.37–39 In contrast with traditional sensors, which (ideally) respond only to and directly to a single property of the environment, soft sensors leverage responsiveness to multiple environmental properties, indirect relationships, and/or expert knowledge embedded in models to improve quality of the estimate of the target analyte. As a consequence the associated algorithms are generally more complex (have more tuning parameters) than a standard calibration curve.

3 Current state of sensing and control strategies in treatment facilities

Many wastewater treatment plants already utilize sensors for pH, dissolved oxygen (DO), oxidation–reduction potential (ORP), and/or electrical conductivity (EC) for monitoring and, in some cases, automated process control,40,41e.g., maintaining DO levels through control of blowers in response to real-time DO sensor readings.42 In the past decade, a major focus of operational interest has been direct measurements of nutrients and integration of these data into control strategies. The following sections provide an overview of current capabilities through a lens of case studies from a number of facilities implementing novel uses of sensors in conjunction with innovation in data processing and controls.

3.1 Innovative sensing approaches in operating wastewater treatment facilities

As expanded options for real-time sensing technologies become increasingly available, many treatment districts are developing pilot studies (some in collaboration with academic researchers) or upgrading operational facilities43 to (1) improve nutrient sensing for process characterization and (2) optimize oxygen delivery (i.e., achieve energy efficiency gains); Table 1 provides an overview of projects discussed in this section.
Table 1 Examples of real-time sensing applied in operating WWTPs. * indicates sensor type not reported. Abbreviations listed in the table follow. CSTR: continuously stirred tank reactor; MLE: modified Ludzack–Ettinger; PFR: plug-flow reactor; SBR: sequencing batch reactor; TSS: total suspended solids
Treatment plant Location Type of process Scale Year sensor-driven strategy implemented Highlighted sensing strategy Reported results
Källby WWTP44 Lund, Sweden Pre-denitrification Full-scale 2001 DO (optical), NH4–N (ISE), nitrate (ISE) 13% energy savings annually
City of Layton, WWTP45 Layton, FL Nitrification/denitrification (SBR) Full-scale 2009 DO*, ORP (electrode), TSS* TSS:
Pre-upgrade: 89.3 mg L−1
Post-upgrade: 64.1 mg L−1
Pre-upgrade: 7.88 mg L−1
Post-upgrade: 3.33 mg L−1
Strass WWTP46 Strass im Zillertal, Austria Nitritation/denitritation (SBR) + sidestream anammox for deammonification Full-scale 2011 NH4–N (ISE) Over 30% energy savings
HRSD WWTP47 Virginia Beach, VA Nitritation/denitritation (CSTR sidestream) Pilot-scale 2012 DO (optical), NO2/NO3 (UV/vis), NH4–N (ISE) Acetate savings of 60% compared to conventional nitrification–denitrification
WSHD48 Rotterdam, Netherlands Partial nitritation–anammox (PFR) Pilot-scale 2012–2013 Automated analyzer for NO3 10% energy savings
Wildcat Hill WWTP45 Flagstaff, AZ Nitrification/denitrification (MLE) Full-scale 2013 Combined NO3 and NH4+ (ISE) Nitrogen (TN) effluent:
Pre-upgrade: 14 mg L−1
Post-upgrade: 8.5 mg L−1
Ejby Mølle WWTP49 Odense, Denmark Nitrification/denitrification (CSTR sidestream) Full-scale 2014 DO (optical), NO2, NO3, NH4+ (UV/vis) 18% energy savings, 56% decreased N2O emissions
Municipal WWTP of Manresa50 Barcelona, Spain Nitrification stage of multi-stage process (SBR) Pilot-scale 2017 DO (optical), combined NO3 and NH4+ (ISE) 25% reduced operating costs

In particular, ammonium and nitrate sensors (ion selective electrodes (ISEs)) marketed for use in wastewaters have greatly improved in recent years as companies have created multi-ISE soft sensor packages that improve accuracy of nutrient measurements by internally accounting for sensor interferences (e.g., ref. 51–53). These devices have already been leveraged for a range of purposes in wastewater process control. For instance, nitrate sensors have been used to (1) stabilize nitrate concentrations in the anoxic zone of a nitrification–denitrification process (municipal WWTP in Sluisjesdijk, Rotterdam48 and Ejby Mølle WWTP in Denmark49), (2) monitor sludge recycle (City of Layton WWTP, Florida45), and (3) allow continuous monitoring of both primary and secondary effluents (Wildcat Hill Wastewater Treatment Plant in Flagstaff, Arizona45). Ammonia sensors have been used to (1) inform DO controller rates in a pilot-scale deammonification process (Blue Plains Advanced Wastewater Treatment Plant (AWTP) in Washington, DC54) and a pilot-scale nitritation process in Municipal WWTP of Manresa, Spain,55 (2) optimize anammox processes (Hampton Roads Sanitation District in Surry, Virginia47,56,57), and (3) underpin ammonia-based aeration control (ABAC) (e.g., at Hampton Roads58 and Inland Empire Utilities Agency in Chino, California,59,60 and Källby WWTP, Sweden44). In the case of ABAC, an aeration strategy attracting a great deal of attention recently,61 major cost savings have so far been demonstrated. Hampton Roads reported an overall decrease of 10% in energy consumption, of 53% in supplemental carbon demand for denitrification, and of 40% reduction of final effluent ammonia (monthly aggregated quantities),58 while Inland Empire reported achieving comparable energy savings (∼10%).60 Innovation beyond ABAC has been considered by using real-time ammonia, nitrate, and nitrite sensors together to control aeration in an ammonia verses nitrate (AVN) configuration, with preliminary results suggesting improvements possible relative to ABAC (e.g., 8.6% reduction in oxygen demand and 17.5% increase in nitrogen removal62).

In contrast, reported efforts related to phosphorous removal have primarily taken advantage of high resolution data from online analyzers to inform manual process parameter adjustment rather than real-time controls, e.g., at the Upper Blackstone Water Pollution Abatement District in Worcester, Massachusetts63 and the Westfield (MA) Water Recovery Facility (WRF).64

3.2 Data processing and controls in WWTPs

To ingest and respond to data streaming from commercial sensing technologies, treatment plants are adopting increasingly sophisticated automated real-time data analysis and advanced feedback control systems. In some cases, complex data analytics such as machine learning approaches are being tested to help wastewater treatment facilities improve decision making and online process characterization,65,66 however adoption remains at an early stage and/or proprietary to the implementing companies, making its adoption poorly represented in the published literature.

In contrast, novel and tested control schemes have been more widely published for this application. Due to the highly nonlinear nature of wastewater treatment processes and component responses (e.g., butterfly valves, pumps, biological metabolisms), advanced controllers are in many cases better suited to accounting for component characteristics, time lags, or hysteresis in system response than simple set-point controls.67 Such methods, like PID (proportional integral derivative) control which has been widely used in industrial processes,68 including wastewater treatment plant applications, for decades,69,70 smooth performance by taking into account the magnitude of deviation from the target value and projected rates of change, and can be especially effective when coupled with upgraded hardware such as variable frequency drives (VFD) for pump speed control.

For treatment process control this has been particularly successful for the stabilization of dissolved oxygen (DO) in secondary treatment bioreactors71 and overall management of complex processes (e.g., activated sludge plants) by using multiple PID control loops (e.g., for independent control of recycle pumps, DO blowers, etc.).72 Ammonia-based aeration control (ABAC), introduced above, uses in situ ammonia measurements to calculate deviation from a target setpoint, and a PID controller that takes into account the process dynamics (i.e., O2 required to achieve desired change in ammonia) is typically implemented to achieve the setpoint.

3.3 Lessons learned and remaining gaps

The above studies provide some insight for operators in expanding instrumentation beyond well-established approaches leveraging vetted technologies such as pH, DO, and ORP probes. In some cases online sensors supplement measurements otherwise done in the lab, e.g., TSS sensors can provide a surrogate for laboratory mixed liquor suspended solids (MLSS) analysis. The major success of previous decades, however, is leveraging sensors for ammonium and nitrate to achieve more consistent effluent total nitrogen (TN) concentrations and significantly decreased oxygen consumption. Well-detailed examples of approaches particularly for both mainstream nitrification/denitrification and sidestream anammox are available as a starting point to roadmap potential improvements in other facilities, supported by significant improvements to commercially-available options for sensing nitrate and ammonium from multiple different companies.
Gaps. Despite these successes, however, many operating plants – particularly smaller facilities – still rely on manual measurements of process chemistry and efficiency to inform manual operator control/updating of pump and blower settings, particularly in cases where sensors have failed to meet reliability requirement or exceed available operational budgets.10 Given the heavy human resource overhead, need for toxic chemical reagents, and long time delays associated with lab analysis of organic carbon (minutes for total organic carbon (TOC), hours for chemical oxygen demand (COD), and days for BOD),73–76 there remains great interest in use of organic carbons sensors both for process monitoring (e.g., recovery of BOD for sustainability improvement,77 COD for efficient process control,78 TOC for monitoring impending breakthroughs and upsets/recoveries79) and for compliance with regulatory reporting requirements, yet deployment of emerging commercial options in operating plants remains limited (further details provided below). The lack of commercial sensors for phosphorous despite ongoing efforts (e.g., ref. 80 and 81) is a major gap, with achievable resolution from instrumentation (i.e., ∼30 minutes for total phosphorous (TP)) insufficient to support process control.
Looking forward. As regulations continue to evolve, another potential need in coming years is detecting metals (many of which are already covered by existing regulations) and emerging contaminants in wastewater, e.g., copper, lead, nickel, zinc, mercury, antibiotics, pharmaceuticals, and personal care products.82 In addition, some forward-looking facilities are starting to raise concerns regarding the contribution of treatment processes to global warming, e.g., as N2O emission have been correlated with process parameters such as BOD and NH3–N load.83,84 Preliminary survey of 12 plants across the US over a two year period has suggested ways to meet discharge regulations while minimizing N2O emission,85 and therefore measurements that support monitoring of N2O as well as enabling these processes may be of upcoming interest.

4 Research in sensing and control strategies for WWTPs

Reviews published in the early 2000s suggested the major focus of research to support application of online sensing to wastewater treatment should be (1) adaptation for working in the complex wastewater environment and (2) improving ability to monitor organic matter.86,87 Although some progress has been made on these points, TOC, COD, and BOD remain prime targets for sensor research, with goals of decreasing costs and improving generalized application (reducing need for site-by-site calibrations). In addition phosphorous has been added as a major parameter of interest for online monitoring in the past decades.81,88,89

Other motivations for research on sensors, including those in competition with existing commercial products, include improving accuracy, improving selectivity, decreasing upfront costs, increasing service lifetime, and decreasing maintenance requirements (and therefore associated O&M costs). In particular, both purchase and recurring costs remain major constraints, relative to capital improvement and payroll budgets, resulting in mainly larger treatment facilities at the forefront of adding or upgrading online sensing systems.45 Therefore, decreases in any sensor-related costs may provide critical opportunities for upgrading treatment processes at smaller facilities.

In this context the following sections present emerging research on both hardware and software solutions that represent potential avenues for overcoming challenges currently faced in the design and implementation of novel treatment processes, highlighting options that may be usable in the relatively near future.

4.1 Research in sensing technologies

Active research in sensor technologies applied specifically in the wastewater context includes novel approaches to real-time measurement of a number of parameters, including nitrate/nitrite and ammonia/ammonium, TOC/COD/BOD, volatile fatty acids (VFAs), emerging contaminants, and metals, among others. Promising options already being demonstrated in the wastewater context fall broadly into three categories: optical sensors, ion selective electrodes (ISEs), and biosensors (see Table 2).
Table 2 Sensor types being explored for expanding capabilities in online sensing for wastewater treatment process control and monitoring. Note that advantages, limitations, and highlighted research apply generally to the sensor type
Sensor type Target parameters Advantages Limitations Highlighted research
Optical sensors DO Durability, stability Variable (some high) (Saif et al., 2019)99
Nitrate and nitrite Low maintenance Upfront cost (Shenashen et al., 2013)100
BOD, COD Fast response Variable maintenance costs (Sivchenko et al., 2018)101
Images (e.g., flocs)
ISEs Nitrate, ammonium Fast response Poor specificity (Bakker, 2019)103
Metals Low upfront cost Sensor drift (Kim et al., 2019)109
Low power demand Limited lifetime (Elbalkiny et al., 2019)108
Biosensors BOD, COD Low upfront cost Short lifetime (Adekunle et al., 2019)118
Metals High selectivity Poor stability (Zhao et al., 2018)120
High maintenance costs (Saberi et al., 2019)126

Optical sensors. Optical sensors based upon principles of absorbance or fluorescence at one or more wavelengths, are typically relatively durable and require minimal maintenance (aside from cleaning); this has resulted in optical DO sensors becoming commonplace in process monitoring and exploration of these principles for other sensing purposes. In particular, optical sensors are a promising option for the measurement of dissolved organic matter,90 an approach studied since the 1990's91,92 but recently becoming more cost-effective and accurate as prices of electronics have dropped and increasingly complex calibrations utilizing multiple wavelengths have been realized.93,94 Real-time sensors for BOD and COD based on spectral absorbance are already commercially available (e.g., ref. 95–97) and have some proven successes, e.g., a sensor leveraging only two wavelengths achieved COD prediction (R2 = 0.9407) in wastewater from a dairy plant in Minnesota.98

Current research on fluorescence-based optical sensors focuses primarily on expanding the number of properties they can measure, e.g., to include metals included in most permit limits. Preliminary successes have been achieved, such as (1) detection of chromium ion down to 3.8 × 10−9 mol L−1 in real wastewater samples (using a nano-phosphor (EBZO) sensor)99 and (2) detection of multiple metals (Cd2+, Hg2+, Cu2+ and Co2+) down to 49.8–50.5 ppb with 60 second response time (using a silica nanoparticle-based sensor),100 however as yet these innovations are not yet commercialized. As electronics prices have dropped, novel uses of cameras for process monitoring are also being explored. For instance an online sensor prototype based on textural image analysis has already been demonstrated for floc monitoring (RMSE = 0.297 mL s−1 and R2 = 0.76), informing dose calculations for a coagulation process in Skiphelle WWTP (Norway).101

Ion selective electrodes (ISEs). Ion selective electrodes (ISEs) are potentiometric sensors that use selective membranes to detect charged species in water samples. ISE technology is promising due to the fast response times (<1 minute), relatively low equipment cost ($100–$1000), low power consumption (mW), and wide measurement concentration range (5 or more orders of magnitude in linear range).102 In contrast, the poor selectivity of most membranes is problematic in wastewaters, and signal drift may require frequent recalibration.103,104 Commercial products using ISEs for nutrient measurement overcome selectivity issues by designing these probes as soft sensors, explicitly measuring and correcting for interfering analytes onboard before reporting data to the user. Research continues to hone this approach to enable more sophisticated methods to correct for interferences even for minority constituents in wastewaters. e.g., for detection of ammonium at concentrations of 10s of micromolar,105 to provide early warning systems for pollutants/hazards (e.g., diesel fuels, antibiotics such as sulfonamides and tetracycline, pharmaceuticals such as ibuprofen and carbamazepine) at the inlet of sewage systems.106–108 It is notable here that these sensor advancements achieved are primarily due to advanced data analytics approaches used (see section 4.2) rather than innovation in the materials or hardware of the sensors. Innovations on that front are also being pursued, e.g., to improve selectivity, reduce equilibration time, minimize sensor drift, extend service lifetime, expand the range of analytes that can be detected.109–111 For example, 30% shorter equilibration time was achieved with novel ion-selective polymer-coated carbon electrodes, demonstrated in a pilot-scale wastewater reactor where these online signals enabled up to a 20% higher NO3 removal efficiency in the final effluent.109 ISEs have also been demonstrated for monitoring of diclofenac, an emerging contaminant, at concentrations down to 0.1 μM and with less interference (∼50% lower selectivity coefficients) than traditional membrane-based ISEs.108 These novel electrode types are in general not, however, in the commercialization process and may therefore benefit from more testing and validation in conditions representative of wastewater treatment processes.
Biosensors. Biosensors leverage living (micro)organisms or biological molecules to detect the presence of chemicals. Potential benefits of biosensors are sensitivity, specificity, low upfront cost, and fast response time (e.g., enzyme-based biosensor response time can be 3–30 seconds (ref. 112)). In wastewater treatment plants, biosensors have been studied to measure specific characteristics of activated sludge processes (e.g., biological materials such as nitrogenous matter113,114 and electrical current produced during the oxidation of hydrogen peroxide115). Currently only very few biosensors are commercially available, specifically for organic matter monitoring in wastewater treatment,116 however published reports of deployments in operating plants are lacking. Despite the limited commercial successes to date, the potential for fast response times and high specificity compared to other technologies on the market continues to encourage active biosensor research, particularly for organic targets such as BOD and COD. A key challenge is expanding the detection range for these analytes.117 Real-time measurement of BOD (range of 0–250 mg L−1, R2 > 0.9) in a wastewater aeration process has been demonstrated using a bioelectrochemical open-type in situ biosensor, while real-time monitoring of COD (range 100–500 mg L−1, R2 = 0.97) in brewery wastewater was achieved using a biosensor based on a microbial fuel cell.118 Research is also expanding the number of targets that can be detected using biosensor sensors, including for (1) nitrogen compounds (e.g., monitoring of NO2via a biosensor based on bacterial reduction of NO2 to N2O, achieving detection limit 1 μM in wastewater with response time 0.5–3 minutes (ref. 119)) and (2) metals (e.g., Cr(VI) monitoring using a biosensor based on a sediment microbial fuel cell, achieving R2 = 0.9935 for concentration range 0.2–0.7 mg L−1 and showing no interference from Cu2+, Zn2+, or Pb2+ in industrial wastewater treatment albeit with ∼18 min response time120), and (3) emerging contaminants (pharmaceutical compound diclofenac measured over range (10–50 mM) in wastewater in real time with a genetically modified yeast-populated foil-based microfluidic flow cell sensor121). Despite this progress, limitations of biosensors can include responses to light,122 a need for high electrical conductivity in sample waters,118 high maintenance needs,123 and the need for a large surface area to achieve usable response magnitudes.115 The need for corrosion resistance, especially in wastewater applications, is also a practical driver of ongoing research.124
Looking forward. For widespread adoption in wastewater treatment processes, sensors should ideally accurately quantify important chemical targets and be user-friendly, durable, accurate and cost-efficient, though in some cases sensors may still be leveraged without achieving all of these characteristics (e.g., if low drift can be guaranteed, accuracy may not be needed to identify changes between operational regimes). For spectrometer-type optical sensors the main issue remains cost, either of the instrumentation itself or the site-specific calibrations required; innovations leveraging only a few wavelengths or low-cost cameras are, however, pushing the envelope and have potential for practical use in the very near term if combined in a scalable way with data analytics (next section). ISE type sensors are already being leveraged in treatment plants, however these also typically require site-specific calibrations and detection limits preclude application to all types of processes or analytes of interest. In this case, in contrast to optical sensors, the inexpensive nature of individual sensor membranes suggests systems with more sensors (also combined with advanced data analytics) have the potential to vastly expand use in wastewaters within the next few years. Relatively speaking, biosensors, while providing potential for a much wider range of targets, remain farther from commercialization and deployment, given outstanding issues such as short reported lifetimes and poor long-term stability.125

4.2 Research in data analytics and controls

In biological treatment processes, microbially mediated reactions coupled with environmental interactions highly nonlinear dynamic systems where traditional modeling approaches may be reaching limits of tradeoffs in accuracy vs. complexity or in ability to accurately calibrate the many parameters. In response a growing number of studies are applying machine learning and statistical methods to wastewater treatment measurement, monitoring, and automation using groups of sensors.34,127,128 Such arrays of sensors, in conjunction with multivariate data analysis (i.e., soft sensors) form a powerful and promising approach for obtaining both quantitative and qualitative information from otherwise complex or cross-interfering data sources,129 particularly given that this approach has already been demonstrated for operationalizing ISE sensors despite outstanding issues of poor selectivity. The most critical challenges in the soft sensor approach are (1) appropriately pairing hardware and software into integrated solutions, (2) identifying data from which to generate calibrations, and (3) designing an accurate method for extraction of useful information from disparate data streams (termed data fusion) – all of which is ideally invisible to the end user who interacts with the system as they would for any out-of-the-box sensor.

Approaches to data fusion that provide an alternative to mechanistic models must be calibrated (“trained”) using data collected to characterize the target application (e.g., biological process),80,113 and therefore transferability of results between sites/applications must always be assessed. However by learning from system data this approach can offer a unique ability to extract information about patterns that may have been previously unidentified or on factors for which no sensor is specifically available.130Table 3 and the following sections highlight several popular data methods as applied to treatment processes to help meet tightening discharge limits131 and to enable real-time provision of a greater number of process parameters.132 While many other applications of these methods exist, the subset selected for discussion were prioritized based on (1) publication in peer reviewed literature and (2) the relative completeness of results (i.e., reporting of coefficient of determination (R2), root mean square, mean square, or mean percent error (RMSE, MSE, MPE), and a comparison with baseline status quo approaches). It is important to note that, while these studies are intended for and in most cases validated with application to wastewater treatment processes, many utilize data which are not yet available via real-time sensing and therefore highlight gains that may be achieved as improved sensor options become available commercially.

Table 3 Recent studies applying machine learning to wastewater treatment challenges, with reported error/predictability. Improvement is defined as the quantitatively improved R2 or decreased RMSE compared to the base case (without the applied model). Definitions of parameters and abbreviations listed in the table follow. ARX: auto-regressive representations with exogenous inputs; CRB: carbamazepine; dose: coagulant dosage; E2: 17β-estradiol; EE2: ethinylestradiol; EF-flow: gaseous emission factor flow; HRT: hydraulic retention time; IBU ibuprofen; MLR: multiple linear regression; morphological parameters: total filamentous bacteria number, filamentous bacteria mean length, total filamentous bacteria length; OLR: organic loading rate; PHI: inlet pH; PHO: pH after coagulant dosage; Qgas: gas flow; QIN: inlet wastewater flow; SVI: sludge volume index; SMX: sulfamethoxazole; TUI: inlet turbidity; TMP: wastewater temperature; TUO: outlet turbidity
Study Sensor type Input Model Output [range] RMSE (unit) R 2 Improvement
(Amaral et al., 2005)133 None Morphological parameters PLS TSS [0.5–4.5 g dm−3] Not reported 0.906 Not reported
SVI [200–620 cm3 g−1] 0.885
(Quintelas et al., 2019)134 Near infrared spectroscopy (NIR) Wavelengths values of IBU, SMX, E2, EE2, CRB PLS IBU [0.05–1.99 mg L−1] 9.17% 0.943 Not reported
SMX [0.31–7.60 mg L−1] 10.20% 0.923
E2 [0–2.75 mg L−1] 10.72% 0.951
EE2 [0.44–6.93 mg L−1] 17.50% 0.858
CRB [0.56–8.12 mg L−1] 8.79% 0.963
(Sivchenko et al., 2018)101 Floc sensor Signals (QIN, PHI, TUI, TMP, dose, PHO, TUO), textural feature of flocs image PLS Coagulant dosages [2.4–7.3 mL s−1] 0.297 mg L−1 0.78 Not reported
(Qin et al., 2012)135 UV/vis spectrometer and a turbidimeter COD, TSS, O&G PLS COD [0–2500 m L−1] 141 mg L−1 0.945 Not reported
TSS [0–450 m L−1] 30.2 mg L−1 0.965
O&G [0–550 m L−1] 34 mg L−1 0.945
(Lee et al., 2008)136 Multi-sensor system Temperature, ORP, pH, DO, flow rate, turbidity, conductivity PCA + ARX + ANN TN [normalized 0–1] Not reported 0.952 RMSE decreased 75% with ARX for COD [0–40 mg L−1]
TP [normalized to 0–1] 0.934
TCOD [normalized to 0–1] 0.921
(Liu et al., 2019)137 pH, ORP, temperature sensor, gas flowmeter pH, ORP, Ogas, CO2, CH4, H2 PCA + LSSVM COD [0–500 mg L−1] 3.08 mg L−1 0.975 Not reported
(Yoo et al., 2003)138 Not reported Flow rate, cyanide, CODin, DO, T, pH PCA + fuzzy regression CODremoved [mean = 605.4 mg L−1] 0.5898 mg L−1 0.9 RMSE decreased 12.85%
(Zounemat-Kermani et al., 2019)139 Odalog logger L2 instrument Q, T, TS, pH, BOD PCA + ANN EF-flow [0–500 mg m−3] 15.26 mg m−3 0.95 RMSE decreased 50% compare to MLR
(da Silva Ribeiro et al., 2019)140 Monopolar electrodes pH, treatment time, current density ANN Boron removal [0–100%] Not reported 0.973 Not reported
(Antwi et al., 2018)141 Spectrophotometer for NH4+–N Influent COD, pH, NH4+–N wavelength, VFA, OLR, biogas yield ANN COD removal [0–100%] 0.104% 0.83 Not reported
(Zaghloul et al., 2018)142 HACH DR for COD, laser particle size analysis system Influent NH4+–N, PO43−, pH, OLR, superficial air velocity, TMP, HRT, setting time ANN COD [0–5000 mg L−1] 0.5198 mg L−1 0.9998 R 2 increased 11%, RMSE decreased 85.7% compare to (Gong, 2018)143 for COD removal prediction
NH4+–N [0–150 mg L−1] 0.2828 mg L−1 0.9987
PO43− [0–60 mg L−1] 0.0383 mg L−1 0.9987

Partial least squares (PLS) regression. Partial least squares (PLS) regression is the most common linear calibration approach in chemometrics,144 used to realign information contained in signals (e.g., sensor signals) with a target set of axes (e.g., chemical concentrations). PLS is particularly useful when traditional one-to-one calibrations do not apply, e.g., when sensors have known interferences or where multi-wavelength or multi-pixel data are collected. Nonlinear PLS is more frequently used in wastewater applications, to capture process changes and dynamics in addition to dealing with mixed sensor signals.145,146 To date this approach has been successfully used to estimate (1) TSS from microscope images of aeration basin samples (R2 = 0.906),133 (2) coagulant dosage in wastewater using a camera-based optical floc sensor prototype (RMSE = 0.297 mg s−1 and R2 = 0.76),101 (3) COD, TSS, and oil & grease (O&G) concentrations in samples of restaurant wastewater from UV/vis spectrometry (R2 of 0.872, 0.928 and 0.931 respectively),135 and (4) pharmaceutical concentrations (e.g., ibuprofen, sulfamethoxazole, 17β-estradiol and carbamazepine) in wastewaters from FT-NIR spectroscopy (R2 = 0.95).134
Principal component analysis (PCA). Principal component analysis (PCA) reduces dimensionality of data by extracting a small number of explanatory factors that describe the major trends in the original data set,147,148 making it a powerful tool for data compression and information extraction in applications that may have many sensor signals.149–151 However the extracted information is not necessarily aligned with target axes, and therefore PCA is most often used as a pre-processing stage for other numerical methods that may require or simply perform better on uncorrelated inputs. The use of PCA in two-stage approaches is a common practice in wastewater process monitoring research, where sensor inputs are often expected to be correlated in some way, and provides another approach for reducing impacts of interferences or complexity of multi-wavelength sensor outputs. For example, real-time wastewater effluent COD, nitrogen, and phosphorus concentrations (R2 of 0.921, 0.952, and 0.934 respectively) were estimated at a small plant in Gyeongbuk, Korea from a 6-sensor suite (pH, DO, ORP, temperature, turbidity, conductivity) by combining PCA (extracting 4 uncorrelated components explaining 82.5% of variability from the 6 interrelated sensor signals) with artificial neural networks (ANN).136 Similarly, COD was estimated (RMSE = 3.08 mg L−1 over range 0–500 mg L−1, R2 = 0.9752) in a lab-scale aerobic wastewater treatment system from 6 data inputs (online monitored gas flow, pH, ORP, H2, CH4, and CO2) by combining PCA (identified 3 components explaining 92.34% of variability) with a least square support vector machine (LSSVM) model.137 A different set of 6 data parameters (sensors for temperature, flow rate, and pH and lab measurements of TSS, BOD, and total dissolved sulfides) was used to estimate the gaseous emission factor flow (EF-flow) for four different WWTPs operated by one collection system in Louisiana by combining PCA and NARX (nonlinear autoregressive with exogenous inputs neural networks), achieving average RMSE = 15.26 mg m−3 (range 4.95–34.11 mg m−3 for 4 plants) and R2 = 0.96 (range 0.962–0.993 for 4 plants).139 Finally, 10% error reduction in estimation of COD removal (CODin–CODout) was achieved in a full-scale plant from 11 inputs (including DO, pH, measured volatile suspended solids, temperature, COD, cyanide, and flow rate from two locations, etc.) by combining PCA with fuzzy regression138 Although less commonly used in wastewater processes, PCA can also be leveraged to better understand complex process correlations, with or without integration with other algorithms. One good example of this is use of PCA with parallel factor analysis (PFA) to characterize cyclic changes in a multistage biological lagoon treatment system in Melbourne, Australia.152
Artificial neural networks (ANNs). Artificial neural networks (ANNs) are powerful tools for complex system modeling and pattern recognition in various fields,153 capable of representing multivariate relationships in highly nonlinear and dynamic systems without requiring any explicit formulation of the mathematical model of the underlying physics. ANNs have already been explored for modeling bioprocesses154,155 and wastewater treatment plants,156 though to date results in the literature represent primarily research studies rather than evaluation for plant operations. In research applications, this approach is pushing the envelope for measuring parameters of interest identified at the start of section 4. For instance, effluent characterization (COD, NH3–N and PO43−) for a lab-scaled aerobic granular reactor processing synthetic wastewater was conducted using an ANN based on 8 input variables (pH, temperature, air velocity, influent NH3–N and PO43−, organic loading rate, HRT and settling time of the sequencing batch reactor); this system achieved RMSE < 5% and R2 > 0.99 for all parameters.142 Finally, ANNs have been used to estimate COD removal from real wastewater samples from an upflow anaerobic sludge blanket reactor using 6 inputs (pH, spectrophotometer measured NH4+, and lab measured COD, effluent VFA, organic loading rate (OLR), and biogas), achieving good agreement with lab measurements (R2 = 0.87 for COD range 42.18–97.93 mg L−1).142
Other approaches. Although the above represent common approaches, a number of other data analytics techniques have also been applied to characterization of wastewater treatment processes in recent years. This includes random forests (e.g., for hourly influent flow rate prediction from historical weather information and flow data for a wastewater treatment plant in Canada, achieving R2 = 0.925 by using time-lag information157) and topological analysis, a variety of graph theory that can be used on large datasets (e.g., for continuous real-time measurement of ammonium and nitrate at the outlet of an aeration plant based on 23 ISE sensors158). Fuzzy logic applications are also being expanded, both for sensing and for controller design (next section). For process characterization, fuzzy logic simulation was evaluated to improve the sequential treatment of wastewater collected from a paper and pulp industry (Rawalpindi, Pakistan) by predicting 4 outputs (total dissolved solids (TDS), TSS, COD, and BOD) from 7 inputs (treatment time, pH, EC, TDS, TSS, BOD, and COD before treatment), achieving R2 of 0.94, 0.89, 0.86, 0.85 respectively.159
Looking forward. The opportunity to expand sensing capabilities in wastewater treatment processes by leveraging advanced data analytics is already clear, with numerous examples above showing application in operating plants and a pipeline of opportunities to replicate those successes as novel hardware becomes commercially available. The essential shared factor across all of these methods is data availability, i.e., this approach relies on historical process data, and therefore the representativeness and completeness of those datasets are critical to ensuring that the model will produce accurate results across all (or a sufficiently large fraction of) possible operational modes or conditions of the facility. In many cases, multiple algorithmic approaches could be applied to the same dataset, but some advantages of each can be identified. The main strength of PCA is as a pre-processing tool to identify (and remove) correlations between signals, to identify relationships between sensor signals and other available information (e.g., lab measurements), or to flag outliers (e.g., extreme events); conversely PCA has little utility if the sensor signals are known to be relatively independent. Both NL-PLS (nonlinear-partial least square) and ANN can be used to approximate complex systems without a need to define underlying physics or controlling parameters; of these two, PLS can provide more specific insight on a mathematical relationship between predictors (i.e., sensors) and outputs (e.g., nutrient concentration) while the structure of an ANN model is less directly interpretable but therein potentially more flexible in what relationships can be represented. ANN models can be particularly useful for large datasets, especially for multi-dimensional data such as images (e.g., photographs of flocs) and non-numerical (categorical) data (e.g., certain weather conditions). A major shortcoming of most academic publications is the lack of comparability between results achieved using novel models and a traditional counterpart (or results achieved using competing model architectures like PLS and ANN); this makes a priori recommendations for model selection in the design of a new project more difficult. In general, it should be noted that the effort required for model development (software creation) is greatly decreased compared to design and programming of a more traditional physics and process-based model, however the need for collection of training data (in some cases up to one or more years of data, if weather conditions vary greatly) may offset this benefit.
Controllers. Innovation in controller algorithms is another promising tool for stabilizing novel plant process architectures. Here fuzzy logic is also highlighted as a method for modeling the complexity and residual uncertainty of biological wastewater treatment processes.160 Many potential gains are demonstrated by evaluating algorithms on logged data from real plants. These include a projected 10% decreased in energy usage (by using fuzzy logic compared to setpoint control) for aeration control based on influent flow rate, COD, and nitrogen at Taradell Wastewater Treatment Plant in Barcelona, Spain,160 a projected increased TN removal efficiency (from 83.7% to 92.5%) and up to 10% energy saving for the biological nutrient removal (BNR) process at Chelas and Castelo Branco WWTP in Portugal,161 and a projected increased phosphorous recovery (29.6% compared to 13.7%) with an elutriation process controlled by fuzzy logic in a lab-scale reactor fed with the thickener supernatant from Calahorra WWTP in La Rioja, Spain.162 In contrast, some successes have already been demonstrated. For instance, a Phaseplit European research program already succeeded in fully automating an entire pilot wastewater treatment facility using fuzzy logic rules, finding that fuzzy logic was more suitable than PID to deal with the variability in the input waste stream and the long setting times in the anaerobic process.163 This required, however, increasing the complexity of the control mechanisms, which was achieved through significant collaboration between plant and research staff – an excellent example of how to successfully approach development of next-generation solutions. More traditional controller architectures (e.g., feed-forward/feedback) are also being leveraged to enable novel processes like ammonia-based aeration control in operating plants and can serve as excellent models for collaborative design processes.164
Looking forward. Application of new techniques for data analytics and online control to wastewater treatment process is already producing real opportunities for energy savings and improved effluent water quality. In particular machine learning algorithms (ANNs, among others) show promise in improving predictability through learning patterns from historical data. However, as one of the characteristics of machine learning algorithms is a requirement for large volumes of representative training data, most studies reported have developed and tested algorithms only on simulated, lab-measured, or (at best) WWTP-logged data. In many of these studies, data used as inputs to the algorithms may not actually be available in real (or near-real) time, making it difficult to envision application in an operating facility. Furthermore publications rarely include a sensitivity analysis that provides insight into the value of each input signal, which would be required to do a cost/benefit analysis for design of required sensors/instruments. A similar issue arises in use of multi-stage algorithms, e.g., cases where PCA is applied as a pre-processing step; because results are rarely provided for models with and without the pre-processing stage, it is difficult to evaluate the trade-offs between complexity and accuracy. On the whole, however, results of preliminary studies are promising and ready for on-site testing. A critical next step in these research arenas will be for researchers to develop more collaborations with operating facilities to evaluate the robustness of these approaches when applied to real-time streaming data in operating facilities with variable inputs and complex dynamics.

5 Discussion

The lack of suitable and robust online sensors for key variables at appropriate detection limits and temporal resolution (e.g., for nutrients) is a major roadblock to the implementation of more advanced bioprocesses and more energy-efficient control schemes. At the same time, there is an increasing desire for high resolution data describing parameters that characterize plant efficacy or are covered in permits (e.g., BOD, COD, N2O, etc.). Further despite progress reported above, plant operators (consulted through a series of stakeholder workshops run by the authors over 2019–2020 and at instrumentation sessions at regional conferences) continue to identify cost (both capital and maintenance) of instrumentation as a significant factor in design and implementation of treatment systems, particularly for small and medium-sized facilities (e.g., those operating on only a single shift which may not have sufficient staff time or expertise to maintain instrumentation). Data reliability remains a concern at facilities of all sizes, and even in facilities where sensors are already deployed, there is a desire for improved operationalizing of the data.

There is therefore a critical role to be played through continued innovation in sensor hardware and commercialization of new technologies – a role that may be shared by academia and industry or potentially led by forward-looking hardware companies or treatment facilities. For all sensor types, there is a clear need for continued improvements toward faster response times, long-term stability, and accuracy under operating conditions (i.e., lower detection limits, improved selectivity, improved stability).More immediately, there may be opportunities for improved operationalizing of the data through use of more advanced data processing approaches, e.g., machine learning or statistical techniques like those discussed above, while hardware innovation progresses.

Taken together, however, these issues point to several more general and fundamental ways in which stakeholders (collectively: academic researchers, practitioners, operators, consulting engineers, educators, etc.) can overcome roadblocks and accelerate innovation in this field.

5.1 Innovation transfer: ensuring research results are relevant to stakeholders

The reality is that innovative process results are exciting because they remain the exception rather than the rule. There may be a number of reasons for the outstanding gap between real-time sensing techniques studied or demonstrated in research labs and those in use in operating treatment facilities. In some facilities, there may be a preference to keep control systems relatively simple or a lack of operational staff trained for this role. In many cases sensors may not yet be sufficiently reliable for deployment in wastewater systems – or there may be a perception that this is the case due to experiences with prior products. Costs may be (or may be perceived as) prohibitive, particularly given the uncertainties surrounding sensor functionality in situ.

A less acknowledged issue is the lack of information in scientific publications that allows operators to make an informed cost–benefit analysis on new technologies. Designing experiments and studies such that one is able to report results as improvement relative to the current approach is critical – this allows operators to directly calculate cost savings or understand potential effluent quality improvements. A related issue is that of lack of consistent or clear reporting metrics across the scientific literature. Among the publications cited in this manuscript, many metrics (MSE, RMSE, percent error, R2, etc.) were reported without units or characterization of the underlying data (e.g., range), making it difficult (or impossible) to evaluate utility in a real application. Studies with wide concentration ranges would be more informative if accuracy and precision metrics were reported both for the overall fit as well as for the range of relevance to plant operations, particularly as the trend to report results only for a multiple-orders-of-magnitude range can artificially inflate the perceived utility if only the lowest end of the concentration range is of relevance to real wastewaters.

Finally, in many studies leveraging logged plant data, there is a tendency to utilize variables that have highest likelihood to be correlated to the target – regardless of whether these are actually measurable online. While the results of such studies may not be usable by operating plants for near-term solutions, they do present an excellent roadmap for directing sensor development goals to support future automation (see the following section).

5.2 Partnerships, data sharing, and accelerating innovation

Development of strong and collaborative partnerships between operators and academic researchers has the potential to be invaluable in testing new sensors and data methods under realistic operating conditions, e.g., as demonstrated already by research partnerships being cultivated at facilities like Hampton Roads,165 the University of Arizona Water & Energy Sustainable Technology (WEST) Center developed in collaboration with Pima County Wastewater Reclamation,166 the NSF ReNUWIt Engineering Research Center Testbeds that allow researchers to demonstrate technologies in pilot-scale plants,167 and the University–Utility Collaborative Partnerships initiated by the Water Environment Federation (WEF).168 The need for stronger partnerships is actually a theme, in our experience, repeated by operators consistently in focus groups and conference sessions, regardless of the specific topic being discussed.

Unfortunately currently such initiatives are rare, regional, and frequently missing representatives from small treatment facilities whose challenges may be very different from large and well-funded districts. Ad hoc collaborative projects (e.g., collection of samples to support a student thesis project) may be more frequent but are less likely to lead to deep or long-term collaborations. This review highlights several instances of data sharing that have enabled development of novel analytical approaches and, importantly, evaluation of these approaches with respect to the status quo. In similar ways, even exploratory data sharing agreements between plants and researchers may be a fruitful avenue for identifying patterns in process chemistry or biology, even with a starting goal of taking fuller advantage of equipment that is already installed at operating facilities (i.e., low cost with potentially large benefit).

Another set of partnerships that may bear fruit is between software and hardware teams, as alluded above. Evidence shows that development of products that integrate hardware and software into soft sensors is likely to become the norm, however currently research on sensor hardware and algorithm development for measurement of nutrients remain relatively separated. Integrated efforts, for instance where results of algorithm development studies (like those presented in section 4.2) suggest priority targets for sensor development or where hardware limitations suggest opportunities for algorithm development, have the potential to vastly accelerate commercialization of high-impact monitoring tools for wastewater applications.

Finally, encouraging adoption of soft sensor instruments – especially those based on complex machine learning architectures – is going to require close collaborations between developers and practitioners to ensure the robustness of the algorithms across the highly variable conditions experienced in individual treatment facilities. Ultimately, as for any machine learning application, this will come down to an issue of data representativeness, and researchers can best understand whether (or to what extent) data are representative through strong and communicative connections with the operators who are familiar with the full range of plant conditions (good and bad) over the course of years. Collaboration throughout the process will enable all stakeholders and partners to share responsibility for assessing the trade-offs between increased model complexity (improved monitoring/prediction accuracy) versus increased number of failure modes, potentially in the limit beyond what can be exhaustively tested. This also highlights the practical need to evaluate how gracefully algorithms fail and build in an ability for self-diagnosis (e.g., recognizing a malfunctioning sensor or influent conditions out of the range for which algorithm is well tuned). Currently the academic literature fails for the most part to address these issues, however collaborative teams have the tools and knowledge required to bridge this gap.

5.3 Expanding innovative solutions to all treatment facilities

In addition to the collaboration building advocated above, it is equally important to highlight the role of partnerships between plant operators, e.g., forums for sharing experiences with specific sensor hardware, ways of interpreting and operationalizing data streams, context and data surrounding infrequent failure modes, and using visualization tools for assessing information from real-time and lab-generated data in an integrated way. In particular, sharing of data, which can only achieve statistically relevant predictive power when aggregated, is a critical component of accelerating adaptation in not just the large and well-funded facilities but also to those facilities serving smaller and potentially less wealthy communities. Some regional conferences, e.g. New England Water Environmental Association (NEWEA) annual and regional meetings, have already begun this process, but round-table discussions and tutorial-style workshops are critically needed to effect meaningful information transfer.

Another potentially rich resource would be a shared data portal, wherein researchers and operators can access historical data from multiple different plants – those using similar processes but at different scales, for example. This would allow for improved identification of patterns (e.g., precursor conditions to common failures) and robustness testing for newly developed soft sensor or control algorithms. While not without challenges (e.g., those of data ownership or privacy concerns, question of who would manage the cyberinfrastructure), such solutions are not without precedent (e.g., for medical diagnostics on rare conditions) and therefore should be closely considered, particularly by professional societies and research organizations supporting this discipline (e.g., the Water Environment Federation, Water Research Foundation).

5.4 Wastewater workforce of tomorrow

Finally, as a growing number of facilities are adopting advanced instrumentation, and retirements will require replacement of nearly 10% of the existing workforce between 2016–2026,169 there is a great need for – as well as an opportunity in – re-envisioning the fundamental training of environmental engineers. Repeatedly, in regional conferences attended by the authors, workshops hosted by the authors, and through global organizations,170,171 operators are stating a growing need for a deeper knowledge of statistics, data processing, data visualization, and data management on their teams. These skills are critical to leverage information from an increasing number of sensors for assessing plant health and automating plant operations. Yet these topics remain minimally addressed in current ABET curriculum requirements172 and therefore most undergraduate programs. Creative adaptation of modules from other disciplines to teach these skills in the context of environmental engineering applications is critical, and design of new curriculum modules or student projects in collaboration with plant operators provides an opportunity to give students insight into the challenges associated with real data. Development of associated practical short-courses, e.g., to be offered through regional professional societies, is also an excellent option for supplementing on-the-job training with a more rigorous and in-depth discussion of these topics.

6 Conclusions

To achieve mandated discharge limits and improve process efficiencies online sensing and automated control are increasingly being implemented for wastewater treatment processes. These needs are amplified by demands related to next-generation processes and environmental sustainability challenges. Real successes have already been shown in improving process management through new sensor-based control schemes such as ABAC (∼10% reduction in energy demand), but highly instrumented WWTPs remain the exception. Academic exploration of novel use of sensor data, e.g., applying machine learning methods, shows the potential for accurate quantification of key analytes (e.g., COD and BOD) at lowered cost and improved accuracy, and development of new control strategies may further improve process results (e.g., nutrient removal and energy efficiencies). Ongoing research in development of novel sensor hardware is pushing the envelope from another direction, for measurement of organic carbon, metals, and even emerging contaminants in wastewaters, though many are not yet ready for commercialization. Major gaps also remain in affordable real-time sensors for phosphate and improved sensors for nitrate/nitrite and ammonia/ammonium (i.e., lower detection limits, better selectivity), as well as generally adoptable sensors for BOD, COD, and TOC (i.e., lower cost, less site-specific calibration). To accelerate overcoming these outstanding challenges, and migration of innovative research from the lab to operating plants, a case is made for building stronger collaborations between operating wastewater treatment facilities and academic research labs, including supporting technology development and technology transfer, ensuring data integrity by design, and working toward an improved environmental engineering curriculum that adequately prepares engineers for this changing landscape in plant design and operations.

Conflicts of interest

There are no conflicts to declare.


We would like to express our gratitude to Mickey Nowak (Massachusetts Water Environment Association) who assisted with planning and logistics for roundtable workshops and to the following individuals who shared with us their experiences to support development of this manuscript: Bob Amaral (Woodard & Curran), Marc Drainville (GHD Group), Brad Furlon (Hoosac Water Quality District), Jeff Gamelli (City of Westfield Water Pollution Control Plant), Tim Hutchins (HACH), Jeff Kalmes (Billerica Water Resource Recovery Facility), Jim Legg (Uxbridge Public Works Department), Tim Loftus (Upper Blackstone Water Pollution Abatement District), Jeff Murawski (City of Fitchburg Water/Wastewater Commission), Jim Nyberg (Sewer Department, Town of North Brookfield), Zack Ritzer (Chicopee Water Department), Edris Taher (Upper Blackstone Water Pollution Abatement District), Liz Tagleiri (Charles River Pollution Control District), Carl Williams (Easthampton Wastewater Treatment Plant), Mike Williams (Holyoke Water Works).

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