For chemically enhanced primary treatment (CEPT) with microsieving, a feedback proportional integral controller combined with a feedforward compensator was used in large pilot scale to control effluent water turbidity to desired set points. The effluent water turbidity from the microsieve was maintained at various set points in the range 12–80 NTU basically independent for a number of studied variations in influent flow rate and influent wastewater compositions. Effluent turbidity was highly correlated with effluent chemical oxygen demand (COD). Thus, for CEPT based on microsieving, controlling the removal of COD was possible. Thereby incoming carbon can be optimally distributed between biological nitrogen removal and anaerobic digestion for biogas production. The presented method is based on common automation and control strategies; therefore fine tuning and optimization for specific requirements are simplified compared to model-based dosing control.
Municipal wastewater treatment plants (WWTPs) have been forced to become more efficient as a result of economic incentives and regulatory requirements (Olsson et al. 2014). One method that has gained interest in recent years is optimized primary treatment. It has been demonstrated that energy neutrality can be realized by maximizing the removal of carbon-utilizing chemically enhanced primary treatment (CEPT) (Remy et al. 2014). With maximized carbon removal, the possibility of conducting biological nitrogen removal with the carbon remaining can be limited and external carbon addition might be necessary. A controlled removal of the incoming carbon in the raw wastewater is therefore of interest for the performance and economics of WWTP.
Suspended solids (SS) and chemical oxygen demand (COD) removal of around 50 and 30% respectively are normally achieved with primary settling. To improve this, CEPT can be introduced to obtain SS removal in the range 60–90% (Kristensen et al. 1992; Ødegaard 1992). Common practice for the control of the chemical dose with CEPT and settling is dosing proportional to the flow (Morrissey & Harleman 1992; Ødegaard 1992; Ratnaweera et al. 1994). In CEPT based on settling, common proportional integral (PI) feedback control strategies have not been entirely successful. The problems are related to the retention time for settling, that is, introducing a long lag time (L) and a long time constant (T) in the range of 2–6 h, negatively affecting the stability by lowering the phase margin of the open loop system (Seborg et al. 2004). In addition to this, the variation in influent water composition during the same period increases the complexity (Ratnaweera et al. 1994). Other control strategies have been suggested, for example model predictive-based feedforward control relying on historical data or continuous laboratory scale flocculation experiments (Zhang et al. 1990; Ratnaweera et al. 1994; Huang & Liu 1996; Bello et al. 2014).
Microsieving as a separation method can, however, circumvent the problem related to retention time, and thus controlling the chemical dosing using feedback control can be applicable. Microsieving, here referred to as particle separation via physical straining processes (EPA 1975), in municipal wastewater treatment is mostly used for tertiary treatment (Wilén et al. 2012) but microsieving in primary treatment has gained more attention recently, also in combination with CEPT (Rusten & Ødegaard 2006; Ljunggren et al. 2007). The total retention time in the coagulation/flocculation and sieving process is around 5–20 minutes (Ewing 1976; Remy et al. 2014; Väänänen 2014). Moreover, particle separation via the sieving mechanisms in disc and drum microsieves seems predictable (Väänänen et al. 2016) and these aspects are favorable when implementing feedback control. For possible improvement in performance on occasions were a faster response is needed, feedback control can be combined with a feedforward compensator, which could be of interest for CEPT and microsieving.
In adition to flow-induced variation in lag time and time constant for CEPT and microsieving, the removal is nonlinear in relation to the applied chemical dose (Väänänen et al. 2016). To improve control performance some modifications of a standard PI controller might be necessary. Scheduling of the proportional (P) and integral (I) control parameters, the proportional gain (Kp) and integration time (Ti) (or integral gain Ki) can be implemented. The parameters are then adjusted depending on process conditions (Seborg et al. 2004). These methods have been successful for example in automotive air/fuel mixture or aircraft autopilot control (Rugh & Shamma 2000) but also at WWTPs for ammonium feedback control (Åmand 2014). Thus, scheduling of the control parameters could be applicable to control the chemical dosing for controlled COD removal in CEPT with microsieving as well. Online measurement of the carbon content, directly or indirectly, is required and turbidity measurements have shown to correlate with sufficient accuracy with the particulate COD content (Mels et al. 2004). COD to turbidity correlations for wastewater have also been reported by others (Métadier & Bertrand-krajewski 2012; Nguyen et al. 2014).
Implementing controlled COD removal in wastewater treatment can vastly improve the performance of WWTPs (Bachis et al. 2015; Arnell 2016). The objective of this work was to obtain a controlled removal in primary treatment by maintaining a preset effluent water turbidity (NTU units) independent of load or flow variations, this in the broadest range possible and to show proof of concept in full scale. The method was developed and verified at a treatment capacity corresponding to 3,200 person equivalents utilizing coagulation, flocculation and discfiltration.
MATERIALS AND METHODS
Test site, pilot plant and analysis
The Källby WWTP in Lund, Sweden, treats, on average, 27,000 m3/day from a combined sewer system. The WWTP consists of conventional primary sedimentation, activated sludge, including biological phosphorous and nitrogen removal, and a post precipitation process. A microsieve pilot plant with controlled coagulation/flocculation and discfiltration was installed, treating water after grit and sand removal. A schematic illustration of the pilot plant and a block diagram of the control structure are shown in Figure 1(a) and 1(b).
In the pilot plant a centrifugal pump controlled with a variable frequency drive with an operational window of 15–40 m3/h was used to supply water to the coagulation/flocculation and the microsieve. Flow was monitored with a flowmeter from Siemens (Sitrans F M MAG 5100 W, Germany). The coagulation (1.7 m3) and flocculation (2.4 m3) tanks were circular and made of stainless steel with baffles and top-mounted frequency-controlled stirrers. The mixing intensity was estimated to be G ≈ 100 s−1 in both the coagulation and the flocculation tanks. A microsieve (Hydrotech HSF 2202/1-1H discfilter) equipped with 100 μm woven polyester filter panels with a total surface area of 5.6 m2 was used. Polymer preparation was done in a Tomal® Polyrex 0.6 automatic polymer station. The coagulant (polyaluminiumchloride, PACl, 9.3% Al3+) was supplied by Kemira Kemi AB, Sweden. The polymer was a high molecular weight, medium-charged cationic powder polymer (100% active) from Veolia Water STI/Hydrex, France, and was found to be the most suitable after pre-screening in the laboratory. Coagulant and polymer dosing pumps were Alldos DDA and Alldos DME (Grundfos, Denmark), respectively. Online influent and effluent turbidities were measured with Hach Lange SC1000 controller and Hach Lange Solitax® sc sensors with automatic cleaning with wipers, wiping the glass of the sensor every 10 minutes. The Hach Lange SC1000 controller was calibrated for an operational window between 0 and 700 NTU for both influent and effluent turbidities. For the control of the process (P), influent turbidity (d) was used for the feedforward compensator and the effluent turbidity (y) was measuring the output and was used for feedback (Figure 1(b)). The influent turbidity sensor was positioned just after grit and sand removal, close to the influent pump. The effluent sensor was positioned in a 1 m3 IBC equalization tank after the discfilter. The control system consisted of a Beijer Electronics iXT12B softcontrol PLC, and PLC programming was done in CoDeSys/IXdeveloper (Beijer Electronics AB, Sweden). Chemical parameters were analyzed from grab samples using colorimetric methods and Dr Lange cuvette test kits (COD, LCK 114/314; total phosphorus, LCK 348/349/350). SS were analyzed according to APHA/AWWA/WEF (2005). Additionally, turbidity was also measured with a portable turbidity meter (Hach Lange 2100P portable turbidity meter). The initial experimental phase consisted of general performance data acquisition (without chemical pretreatment) and performance evaluation with chemical pretreatment by adjusting the chemical dose manually. Verifying experiments for the evaluation of the control strategy were conducted with a duration ranging from 17 to 96 h depending on set point. The time on site was between April and November 2015.
PI controller with feedforward compensator
The PI controller was to adjust the feedback polymer dose (ufb) in the interval 0–4 mg polymer/L. The feedforward polymer dose (uff) was turbidity limited to 2.4 mg/L. The effluent set point was limited between 0 and 140 NTU. The coagulant (uc) was dosed in proportion applying a factor (Cf) to the total polymer dose (u). The coagulant (uc) was dosed proportional to the polymer dose (u) as follows: uc(t)=Cf·u(t) where the proportional factor is chosen according to the operational set point.
Integral gain (Ki) scheduling
By scheduling Ki accordingly, sluggish or even unstable behavior is then avoided. The constant k was obtained from open loop experiments at 30 m3/h (Qin). A lag time (L) of 300 s was recorded and a corresponding appropriate integration time (Ti) of 900 s was selected according to the Ziegler–Nichols step response method. From the definition of Ki, the constant k was calculated to ≈27,000 and was thereafter used to schedule the integral gain (Ki) according to Qin.
Proportional gain (Kp) scheduling
The scheduling factors for the proportional gain (Kp) 0.3, 0.06 and 0.2 were derived from trial-and-error closed loop experiments with the evaluation criterion of having an overshoot ≤25%. The factor 0.3 was derived from initial experimental conditions of Kp = 1, r= 90 NTU, Qin = 30 m3/h and dosing polymer only (Cf = 0). This factor was kept for the following experiments. To scale Kp depending on the coagulant factor (Cf), similar closed loop experiments were performed. At set point of r = 90 NTU, Cf =1, Qin = 30 m3/h and starting from 0.1, the factor 0.2 was identified as the most appropriate. Kp was also scaled in relation to the operational regime, with a similar closed loop experiment. An effluent set point of r = 60 NTU was used. The experiment was initiated with a factor of 0.02 and moving upwards. The factor of 0.06 was shown to most closely fall within the evaluation criterion. All experiments started with the pilot plant operating without chemical dosing and with an influent turbidity in the range 200–400 NTU. For comparison the proportional gain of 0.3 is in the range of a calculated Kp of 0.45 obtained from step response analysis at Qin = 30 m3/h (L = 300 s; T = 600 s) and applying tuning rules according to the Ziegler–Nichols step response method described in Åström et al. (1993).
For situations when RR was below 30%, then the polymer dose from feedforward compensation (uff) was 0.
RESULTS AND DISCUSSION
Treatment results without chemical pretreatment
COD and turbidity correlation
Set point 40 NTU for low effluent carbon content
Set point 80–90 NTU for high effluent carbon content and dynamic inflow
Set point step test for very low carbon content (12 NTU)
In Figure 7(a) it is shown that, from around 04:48–11:00 during the 20, 40, 80 NTU experiment, feedback control was only in operation. Again, feedback control alone was shown to control effluent water turbidity to the desired set point. During this stage the set point was also changed and consequently the proportional gain (Kp), without a loss in control performance, indicating that scheduling of the Kp was an appropriate method.
Summary of the experimental results
It is demonstrated that control of effluent water turbidity from CEPT with microsieving offers the opportunity to successfully implement common PI control with scheduling combined with a feedforward compensator. This is due to a short lag time and a predictive particle separation. Scheduling of the proportional gain (Kp) and integral gain (Ki) was implemented for stability reasons and was shown to be an appropriate method to get the system to operate within the entire operational window with sufficient stability and response quickness. Experimental results showed that with influent turbidities <200 NTU, effluent turbidity was maintained at the desired set point only with scheduled feedback control. Further experiments are needed to verify if this is also valid for the whole influent turbidity range and then feedforward compensation could be omitted, simplifying the control system further. Step response methodology was used to obtain the scheduled P and I control parameters. Hence, general PI tuning methodology was sufficient. Moreover as the results showed that the control system was able to operate in a broad operational window, this can be useful in for example overflow situations where maximum removal is necessary, simply by changing the set point. A correlation between effluent COD and turbidity was demonstrated, which is in accordance with other studies. The removal of COD in CEPT with microsieving was controllable, meaning that the biogas production at the treatment plant can potentially be increased without reducing nitrogen removal performance.
It was shown that during storm water events the removal effect of the applied chemical dosing is increased. Therefore, one could implement a proportional gain (Kp) damping function for these events in order to increase stability. The control program also had a fixed coagulant dose related to the polymer dose by a factor (Cf). It was shown that depending on the set point and influent water turbidity, polymer dosing only was sufficient, and thus, the coagulant factor (Cf) could be varied accordingly, in order to optimize chemical dosing further.
It was demonstrated that it is possible to use common PI control with scheduling combined with a feedforward compensator to control the effluent water turbidity to a desired set point by CEPT with microsieving. By implementing process-dependent scheduled P and I control parameters the system can operate within the entire operational window with sufficient stability. The method is based on scheduled feedback supported by feedforward control algorithms and online turbidity and flow measurements. With the proposed system, changes in effluent requirements for such reasons as to optimize the carbon content for biological nitrogen removal and biogas production or to maximize treatment results in case of storm water events are possible simply by changing the set point of the control system. Thereby performance of a municipal WWTP can be improved. The method and the design of the control system are based on traditional PI control, flow and turbidity measurements. The results also indicate that only scheduled feedback control can be sufficient and accurate enough to maintain the desired set point, simplifying the control system even further.
This work was financially supported by VA-teknik Södra/Svenskt Vatten, Project: The Warm and Clean City/VINNOVA and Veolia Water Technologies AB.