An updated proposal for including further detail in the BSM2 PE calculation

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1 An updated proposal for including further detail in the BSM2 PE calculation Krist V. Gernaey *, Ingmar Nopens, Darko Vrecko, Jens Alex and Jeremy Dudley * Center for Biochemical Engineering, Dept. of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark kvg@kt.dtu.dk 23 June 2006 It was suggested that the pumping energy (PE) power consumption calculation for the BSM2 could be done more detailed by differentiating between the different pumped flows. Presently, the same pumping cost, 0.04 kwh/m 3, is applied to all pumped flows, similar to BSM1. lntroduction The actual pumping energy consumption (= a measure for the pump operating cost) for maintaining different flows in a WWTP is related to the headloss. For a straight pipe, the pumping power required to transport a liquid through the pipe can be calculated as: P = ρ g Q h (1) Where: ρ = liquid density (kg/m 3 ) g = gravity constant (m/s 2 ) Q = flow rate (m 3 /s) h = headloss (m) Two contributions contribute to the headloss in a straight pipe: static head ( h s ), related to the required height the liquid has to be lifted by the pump, and friction headloss ( h f ). The static head corresponds to the difference between the initial and the final water level for every pumped flow. Its quantification necessitates that a detailed layout of the plant is available. The friction headloss in a straight pipe can be calculated using the Darcy-Weisbach equation: 2 L u h f = f ( ) ( ) (2) D 2 g Where: f = friction coefficient (-) L = pipe length (m) D = pipe diameter (m) u = average liquid velocity (m/s) For the calculation of the friction headloss, it is again a necessity that a detailed plant layout is available, which will allow to determine the length of the different pipes for pumped flows. The value of the friction coefficient f depends on the pipe wall material and roughness. In addition to the static head and the friction headloss, there are additional factors contributing to the pumping energy required to pump a liquid through a pipe: bends, pipe fittings, inlet and outlet structures will all require extra pumping energy. These extra losses are called minor losses, although they can be quite high relative to pipe length, e.g. for short pipes. These minor losses can all be represented as equivalent lengths of pipe, which still allows 1

2 calculating the friction headloss using the Darcy-Weisbach equation (Eq. 2). Again, the contribution of bends, pipe fittings etc. to the overall headloss can only be quantified if a detailed assessment of the pipeline length and structure is available. A general equation for head loss due to minor losses in pipes with the same diameter and velocity both upstream and downstream of the non-uniformity in the pipe is: u h L = K ( ) (3) 2 g Where: K = empirical minor loss coefficient. Expressing the headloss due to minor losses in pipes as the equivalent pipe length (L/D) is the approach adopted here. For a 90 degree elbow L/D is assumed to be 30, and for a 45 degree elbow a value of 20 is applied ( Moreover, in the calculations summarized in this report it will be assumed that outflow of a pipe in a tank corresponds to minor losses equivalent to a static head of 0.5 m. Similarly, when two pumped flows are joined together in one pipe, that is a pipe junction, it will be assumed that the resulting minor losses also correspond to a static head of 0.5 m. From the previous, it can be concluded that the requirement to provide a more detailed PE calculation necessitates the availability of a conceptual layout for the BSM2 plant. In the remainder of this report the BSM2 layout will first be described, a detailed set of PE calculations will be presented for all pumped streams in the BSM2, and in the conclusions a set of PE weighting factors will be proposed for the BSM2 PE calculations. 2 The BSM2 layout Considering only straight pipes for the headloss calculation allows differentiating between the energy required for pumping different flows, but resulted in a pumping energy requirement that was 10 times lower compared to applying the standard BSM1 pumping energy consumption factor of 0.04 kwh/m 3 (Vrecko, personal communication). It was therefore decided to extend the first analysis (released on 1 September 2005, related to the Benchmark meeting in Lyngby) with a more detailed calculation of the headlosses. This necessitates a more detailed layout of the BSM2, compared to the rough sketch of the plant layout that has been used until now (Figure 1). The proposed BSM2 layout is shown in Figure 2. When designing the plant layout, each unit process was originally assumed to take place in 1 tank. However, in a real plant of a size similar to the BSM2 activated sludge plant (approximately PE), some of the unit processes, for example the activated sludge tanks or the sedimentation in the secondary settler, would typically be constructed as a set of 2 or more parallel units, to facilitate for example maintenance of the system. Dimensions of all the tanks are also provided in Figure 2. It is assumed that the influent to the BSM2 has been pumped up to a level which allows it to flow gravitationally to the outlet of the activated sludge plant. No control strategy imposed on the BSM2 can change this, and thus this pumping energy contribution is not to be considered in the PE calculations. Furthermore, the following flows are assumed to be gravitational (indicated by red text in Figure 2): 2

3 1) digested sludge from digester to dewatering, including the additional assumption that the sludge takeoff is near the base of the digester. Depending upon the dewatering equipment there may be additional power demands which are also neglected here. 2) overflow thickener to inlet primary clarifier 3) bypass of plant (before primary clarifier) 4) bypass of activated sludge plant (after primary clarifier) Bypass Influent wastewater Primary clarifier V = 900 m 3 Activated sludge reactors Secondary clarifier Effluent water TSS Å3% Thickener Gas TSS 7% ASM/ADM interfaces Anaerobic digester Θ H Å 20d ADM/ASM interface Dewatering TSS 28% Sludge removal Additional loads Figure 1. BSM2 layout (Jeppsson et al., 2006) Controllable flow rate Valve The following flows are assumed to require pumping: 1) Q int, internal recirculation flow rate 2) Q r, return activated sludge flow rate 3) Q w, waste sludge flow rate 4) Q pu, primary clarifier underflow flow rate 5) Q tu, thickener underflow flow rate 6) Q do, dewatering overflow flow rate (reject water) The overflow of the thickener, which flows to the inlet of the primary clarifier, is assumed to be gravitational. This assumption is valid because the BSM2 plant design (see below) includes a thickener that has its basis (bottom) at ground level, whereas the overflow of the thickener starts 4 meter above ground level. Note also that according to the original BSM2 design the reject water can be dosed in the inlet to the primary clarifier and in the inlet to the activated sludge plant. For simplicity, it was decided not to differentiate between PE calculations for these two cases. The longest pipe will be used for the PE calculations. In addition to that, it was decided to only come up with one constant PE coefficient for each pumped flow, despite the fact that we know that the energy needed for pumping will depend non-linearly on the flow rate that is pumped through a pipe (see Eq. 2). Detailed calculations of the pumping energy requirement for each pumped flow will be presented below. 3

4 Figure 2. Proposed BSM2 layout 4

5 Detailed PE calculations Starting from design flow rates, which can be assumed available for the BSM2 (see table 1) the following rules of thumb can be applied to come to the pipe diameters presented in Table 1: 1) Liquid pipelines are commonly designed for a liquid flow velocity of around 1 m/s (Sinnott et al., 1999) 2) 100 mm is usually applied as a minimum diameter for pipes in wastewater systems, to avoid clogging of the pipes (Jeremy Dudley, personal communication) 3) Only the PE consumption factor calculated for design flow rates will be used in the BSM2 calculations. We are aware of the fact that the energy consumption (in terms of kwh/m 3 ) will depend on the actual flow rate, but we have chosen to neglect this effect. It was assumed that design flow rates were continuous flow rates, that is no possibility for onoff pumping was considered. On-off pumping, for example of the waste sludge, could result in quite a high design flow rate that is applied over a short period of time. As a consequence of making the layout in Figure 2, the pipe length is approximately known for the pumped flows in BSM2. Applying the above equations for water, we can assume f = 0.03 for water and a turbulent flow regime. Furthermore, for mixed liquor and return sludge (+ waste sludge) (WRc report TR 185, 1983, How to design sewage sludge pumping systems ) and also for reject water the liquid properties (viscosity) can be assumed close to that of water, and thus f = 0.03 is applied. For the primary settler underflow flow rate, f = 0.06 is applied. For concentrated sludge flows (underflow thickener), a common rule of thumb is to multiply the water values by a factor of However, it seems more realistic to assume that the higher viscosity pushes us into laminar flow, for which f = 0.80 seems a reasonable value. Finally, it was taken into account that the conversion of electrical energy in the pump does not have a 100% efficiency. An efficiency of was assumed, meaning that all calculated PE consumption factors were multiplied with 1.5 to convert the theoretical PE consumption values into more realistic PE consumption values. In practice, values of pumping efficiency ranging between 60 and 75 % are reported (Christian Rosen, personal communication). The calculations summarized in Table 1 focus on the 6 flow rates that are currently considered to require pumping in the BSM2 performance evaluation. Calculations for each pumped flow summarized in Table 1 will be commented in more detail. In appendix, a separate figure is provided for each pumped flow, to illustrate the numbers in Table 1 (e.g. selection of number of 45 degrees elbows): 1) For the internal recirculation flow rate (Q int, Figure A.1), the design flow rate is m 3 /h, corresponding to m 3 /h per activated sludge tank system. Considering that the average BSM2 influent flow rate is around m 3 /d (including the effect of rain events on the influent flow rate), it seems reasonable to assume that the design value of Q int corresponds to about 4 times the influent flow rate. If the Q int design values are chosen lower, it would mean that a high Q int, e.g. imposed by a controller, would be very expensive w.r.t. energy consumption. In that case the piping would be too narrow, and energy consumption for pumping increases proportional to u 2 (Eq. 2). The static head is assumed to be 0.25 m. The minor losses factor contains a contribution from the 90 degree elbows (0.14 m), and a contribution from the outflow 5

6 structure. The total theoretical energy consumption for Q int pumping is about kwh/m 3. Taking into account that the pump efficiency is 0.667, it is suggested to use a weight factor of kwh/m 3 for Q int pumping in BSM2. 2) The return sludge flow rate (Q r, Figure A.2) needs to be pumped from the bottom of the secondary clarifier to the mixing box. In the mixing box, the pre-settled wastewater (effluent primary clarifier) is mixed with the return sludge. It is assumed that the liquid surface level in the mixing box is one meter higher than in the secondary clarifier, i.e. static head = 1 meter. The return sludge flow is pumped from about five meters below the liquid surface level of the secondary clarifier (five meters corresponds to the depth of the secondary clarifier, including the sludge hopper). In the design, it is assumed that the return sludge from each secondary clarifier is pumped to the mixing box in a separate pipe. The reason for not including a pipe junction in the return sludge line is that this configuration with two separate pipes provides the highest flexibility (while performing maintenance or reparations on one pipe, the other half of the activated sludge plant can operate normally). The minor losses factor contains a contribution from the 45 and 90 degree elbows (0.18 m), and a contribution from the outflow structure (0.5 m). The total theoretical energy consumption for Q r pumping is about kwh/m 3. Taking into account that the pump efficiency is 0.667, it is suggested to use a weight factor of kwh/m 3 for Q r pumping in BSM2. 3) The waste sludge (Q w, Figure A.3) needs to be pumped from the bottom of the secondary clarifiers (5 m deep) to the inlet to the thickener, where the liquid level in the thickener is assumed to be 4 m above ground level. In other words, the bottom of the thickener (not including the sludge hopper) is at ground level. The resulting static head is 4 m, since the liquid level in the settler is assumed to be at ground level. The design flow rate is set to 50 m 3 /h, corresponding to a daily flow rate of 1200 m 3 /d. This apparently high number is selected due to the fact that sludge wasting is often done discontinuously on an activated sludge WWTP, e.g. during working hours only (8 hours per day). Thus, the design flow rate should be considerably higher than the average Q w flow rate, and has here been selected as about 3 times the BSM1 default waste sludge flow rate. For simplicity, the calculations were done assuming that the design flow rate needs to be pumped through one pipe. The pipe length in this calculation corresponds to the distance between the sludge outflow in the thickener and the sludge hopper of the secondary clarifier farthest away from the thickener (see Figure 2). The calculated total theoretical energy consumption for Q w pumping is kwh/m 3, which is an order of magnitude larger than the values obtained for Q int and Q r. It is suggested to use a weight factor of 0.05 kwh/m 3 for Q w pumping in BSM2, which is in the same order of magnitude as the pumping weight factor used in the BSM1. 4) The underflow of the primary clarifier (Q pu, Figure A.4) (5 m deep, including the sludge hopper) needs to be pumped to the top of the digester. Assuming a liquid volume of 3400 m 3 and an aspect ratio (H/D) close to 1 (an aspect ratio of 1 to 1.5 is used as a rule of thumb in classical digester design - not applicable for egg-shaped digesters - resulting in a liquid height of about 17 m for a digester diameter of 16 m), a static head of 17 m is obtained. Fresh sludge is introduced at the bottom of the digester, which is at ground level. Headlosses due to distribution structures are not considered in detail. As a design flow rate, 10 m 3 /h is selected. The 1 m/s design average flow velocity does not apply. Instead, a pipe with the minimum diameter of 0.1 meter is selected. Consequently, the average flow velocity is very low (0.35 m/s). 6

7 The friction headloss is low (0.33 m). The calculated total theoretical energy consumption for Q pu pumping is kwh/m 3. It is therefore suggested to use a weight factor of kwh/m 3 for Q pu pumping in BSM2, taking into account that the pump efficiency is ) The underflow of the thickener (Q tu, Figure A.5) (assumed to be at ground level) needs to be pumped to the anaerobic digester, which has a liquid level of 17 m. The resulting static head is 13 m (subtracting the liquid level in the thickener). It is assumed that fresh sludge is introduced at the bottom of the digester. Sludge flow distribution structures in the digester are not considered. As a design flow rate, 5 m 3 /h is selected. The 1 m/s design average flow velocity does not apply here, since fulfilling that condition would mean introducing a very narrow pipe. Instead, a pipe with the minimum diameter of 0.1 meter is selected. Consequently, the average flow velocity is very low (0.18 m/s), and the friction headloss is low as well (0.32 m), despite the fact that we assume laminar flow for this concentrated sludge flow (f = 0.80). The calculated total theoretical energy consumption for Q tu pumping is kwh/m 3. It is suggested to use a weight factor of kwh/m 3 for Q tu pumping in BSM2, which is quite similar to the weight factor used for Q w pumping energy consumption calculations. 6) The overflow of the dewatering (Q do, Figure A.6) needs to be pumped to the inlet of the plant, before the raw wastewater enters the primary clarifiers. In the detailed BSM2 layout, the possibility exists to also dose this flow in the pre-settled wastewater. However, this last possibility is not considered in the calculations. It is assumed that the inlet and outlet of this pipe are at ground level. The design flow rate is again rather low (15 m 3 /h), and therefore the minimum pipe diameter of 0.1 m applies. For the PE calculations, a weight factor of kwh/m 3 is proposed, similar to the Q int pumping energy calculations. Conclusions Considering that the design flow rates used in Table 1 are based on the BSM2 flow rates, rather simple calculations allow distinguishing between PE requirements for pumping different flows. In general, calculated pumping energy requirements are lower than proposed for the BSM1 (0.04 kwh/m 3 ). Based on the calculations summarized in this report, the following PE factors are suggested: PE_Q int : kwh/m 3 PE_Q r : kwh/m 3 PE_Q w : kwh/m 3 PE_Q pu : kwh/m 3 PE_Q tu : kwh/m 3 PE_Q do kwh/m 3 As a consequence, PE consumption calculations for the BSM2 can be done using the following equation: t = 609days 1 PE _ Qint Qint + PE _ Qr Qr + PE _ Qw Qw + PE = dt T = PE _ Q + + t days pu Qpu PE _ Qtu Qtu PE _ Qdo Q 245 do Or: 7

8 1 PE = T t = 609days ( Qint Qr Qw Qpu Qtu Qdo ) t = 245days In the above equations, T is the length of the evaluation period in days. References Jeppsson U., Rosen C., Alex J., Copp J., Gernaey K.V., Pons M.-N. and Vanrolleghem P.A. (2006) Towards a benchmark simulation model for plant-wide control strategy performance evaluation of WWTPs. Water Science and Technology, 53(1), Sinnott R.K., Coulson J.M. and Richardson J.F. (1999) Coulson & Richardson s chemical Engineering: Chemical Engineering Design Volume 6 (3 rd Edition). Elsevier Science. dt 8

9 Table 1. Summary of PE calculation design assumptions, and power consumption for design flow rates Pumped flow Qint Qr Qw Qpu Qtu Qdo From AS 1 tank 5 Underflow SC Underflow SC 2 Underflow PC 3 Underflow Dewatering thickener To Inlet AS tank 1 Inlet AS tank 1 Inlet thickener Inlet digester Inlet digester Inlet PC Design flow rate Q (m 3 /h) L (m) D (m) ρ (kg/m 3 ) u (m/s) f (m) degree elbows degree elbows Number of pipe junctions Number of outflow structures hs (m) hf (m) hl (m) Power for pumping due to hs (W) Power for pumping due to hf (W) Power for pumping due to hl (W) Total power (W) Theoretical PE consumption (kwh/m 3 ) Pump efficiency (-) PE consumption (kwh/m 3 ) Proposed energy consumption (kwh/m 3 ) Activated sludge Secondary clarifier Primary clarifier 9

10 Appendix. Details layout Figure A1. Detail Q int Figure A2. Detail Q r 10

11 Figure A3. Detail Q w 11

12 Figure A4. Detail Q pu 12

13 Figure A5. Detail Q tu Figure A6. Detail Q do 13

BSM2 Plant-Wide Wastewater Treatment Plant Modelling

IWA TG on Benchmarking of Control Strategies for WWTPs BSM2 Plant-Wide Wastewater Treatment Plant Modelling 6 May 2007 Washington, DC, USA Dr Ulf Jeppsson IEA, Lund University Sweden Outline Background