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Targeting for Total Water Network. 2. Waste Treatment Targeting and Interactions with Water System Elements Denny Kok Sum Ng and Dominic Chwan Yee Foo* School of Chemical and EnVironmental Engineering, UniVersity of Nottingham Malaysia, Broga Road, 43500 Semenyih, Selangor, Malaysia
Raymond R. Tan Chemical Engineering Department, De La Salle UniVersity-Manila, 2401 Taft AVenue, 1004 Manila, Philippines
Part 1 of this series of papers presented graphical and algebraic approaches that are used to identify waste streams in a total water network. In this part of the series, the interaction between waste treatment and water regeneration is explored. Appropriate selection of waste streams for regeneration or waste treatment will lead to the minimum impurity load to be processed in the regeneration and waste treatment units; hence, leads to the reduction of the network capital and operation costs. In addition, a novel wastewater composite curve for targeting the minimum impurity load removal is presented in this work. Targeting for the minimum treatment flow rate and the minimum number of treatment units is presented for the treatment system of the fixed outlet concentration and removal ratio type, respectively. Literature examples are solved to illustrate the proposed approaches. Introduction A total water network consists of the elements of water reuse/ recycle, regeneration (treatment for reuse/recycle), and waste treatment (treatment for final discharge). As noted by Kuo and Smith,1 both treatment systems have a similar function, i.e., to improve the water quality. The only difference between the two is the destination of the treated water, i.e., for further reuse/ recycle or discharge. Furthermore both treatment systems interact with each other in the bigger framework of a total water network. This dictates that the analyses should be conducted simultaneously. For ease of illustration, treatment systems for a total water network (for reuse/recycle and/or for final discharge) shall be called the “total treatment system”. Interaction between the two elements in the total treatment system is visualized with the appropriate stream selection scheme. Wastewater from a network may either be treated for further reuse/recycle or for final discharge. Naturally, treatment for further reuse/recycle (regeneration) will be considered first, because this water will further reduce fresh water and wastewater flow rates in the network. By reusing/recycling regenerated water, new wastewater streams will discharge from the network. As will be shown in the later section, that the selection of streams for regeneration results in the formation of different flow rates and qualities of wastewater which, in turn, affect the final treatment section. This enables the interactions between regeneration and treatment processes to be explored. In rating the total treatment systems, two important variables must be considered: water flow rate and impurity load removal. The size of the water treatment units increases proportionally with the increase in the regeneration flow rate, which leads to higher capital and operation costs.1,2 Furthermore, impurity load removal, which has generally been neglected in many previous * To whom correspondence should be addressed. Tel.: +60-3-89248130. Fax: +60-3-8924-8017. E-mail addresses: Dominic.Foo@ nottingham.edu.my (D.C.Y.F.),
[email protected] (D.K.S.N.),
[email protected] (R.R.T.).
works,1,2 is also equally important, because higher loads removed from a treatment unit lead to higher operational costs of the treatment unit.3 The impurity load that is removed from source i (∆mi) can be determined using the following equation:
∆mi ) Fi(Ci - Cout)
(1)
where Fi is the flow rate of source i, Ci the impurity concentration of source i, and Cout the outlet concentration of the treatment unit. Equation 1 is used to determine the total impurity load removed from the water sources in the total treatment systems, either for further reuse/recycle or discharge. From eq 1, it is known that the impurity load is proportional to the source flow rate as well as its impurity concentration. Thus, the increase of any of these parameters will lead to an increase in the impurity load. As discussed earlier, the operational cost of the treatment units is proportional to the impurity load removed by the total treatment system. Therefore, the overall impurity load removed by the treatment systems gives an indirect indication of the treatment cost. Besides, the cost distribution between the treatment for reuse/recycle (regeneration) and the treatment for discharge is different when different stream selection schemes are used. This point is discussed further in the next section. As mentioned earlier, the interaction between the two elements in the total treatment system is visualized via the selection of appropriate streams for treatment. Previous works have proposed different stream selection schemes for regeneration, which include the sources from pinch concentration4 as well as from the highest concentration level.5 In this work, a new scheme is proposed, where the selection of regeneration source(s) is guided by the waste streams that have already been identified by the waste stream identification technique, presented in Part 1 of the series.6 All three schemes are illustrated as follows. A recent developed ultimate flow rate targeting technique by Ng et al.7 to locate the minimum regeneration flow rate is utilized in this work. The ultimate flow rate targeting technique also sets the minimum freshwater and wastewater flow
10.1021/ie071096+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/28/2007
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9115
Figure 1. Wastewater composite curve.
rates in the water network for a given quality of regenerated water. The main assumption for using this technique will be that the regeneration units are based on the fixed outlet concentration model.7 In the seminal work proposed by Wang and Smith,4 water sources (including freshwater) are reused/recycled to water sinks until the pinch concentration is reached before they are sent for regeneration. However, this scheme was originally developed for fixed-load problems. To extend it for application in fixedflow rate problems, modification is required. The pinch-causing source5 from a water reuse/recycle network will be selected first for regeneration. If the flow rate of the pinch-causing source does not satisfy the regeneration flow rate target (as determined by ultimate flow rate targeting technique),7 the source with the next-higher concentration is sent for regeneration. This practice is continued until the regeneration flow rate target is fulfilled. This approach maximizes the utilization of high-grade water in the higher-concentration region and results in an excess of lowgrade water sources. In the source selection scheme proposed by Foo et al.,5 the water source of the highest concentration is first sent for water regeneration, followed by the source with the second-highest concentration. The practice is repeated until the regeneration flow rate target7 is satisfied or all water sources below the pinch concentration are fully utilized.6 In the newly proposed scheme in this work, the selection of the water source for regeneration is guided by the individual wastewater streams found from the waste streams identification technique proposed in Part 1 of this series.6 Because it is known that the wastewater stream from the pinch concentration is the cleanest among all wastewater streams, this wastewater stream is the first to be regenerated. In cases where the flow rate of the pinch waste stream that is lower than the minimum regeneration flow rate,7 the wastewater stream(s) of the next higher concentration is/are sent for regeneration. This is continued until the regeneration flow rate target is fulfilled, or until all wastewater streams are regenerated. These different regeneration stream selection schemes will be analyzed using case studies in a latter section. However, the comparison among these schemes will not be beneficial without the analysis of the wastewater treatment for discharge, which is discussed in the following section. Waste Treatment Targeting Smith and co-workers proposed that the waste treatment units can be categorized into fixed outlet concentration and load
Figure 2. (a) Minimum treatment flow rate targeting for a single treatment unit of the fixed outlet concentration model. (b) Revised wastewater composite curves after waste treatment. (c) Final treatment network design.
removal ratio models.1,2 The former reduces the wastewater impurity to a specific outlet concentration, whereas the latter removes a fraction of the impurity load, depending on the inlet concentration and flow rate of the inlet stream. In this work, a newly proposed graphical targeting technique is presented to locate the minimum impurity load removal for both types of waste treatment systems. Furthermore, targeting for the minimum treatment flow rate and the minimum number of treatment units are presented for the treatment systems of the fixed outlet concentration and removal ratio types, respectively. Figure 1 shows the newly proposed wastewater composite curve. To construct the composite curve, individual wastewater (WW) streams are first identified through the waste stream identification technique (which was presented in Part 1 of this series6). The composite curve is next constructed by plotting the cumulative impurity load of the wastewater streams (on the y-axis) versus its cumulative flowrate (on the x-axis), in ascending order of the wastewater stream concentration. Because of environmental legislation, there is a maximum allowable impurity concentration that is allowed for wastewater discharge; hence, a discharge locus is plotted, with its slope corresponding
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Figure 3. Minimum flow rate targeting for multiple treatment units of the fixed outlet concentration model.
to the maximum allowable concentration for effluents. The maximum allowable impurity load for discharge is represented vertical distance of the waste discharge locus. The vertical difference between wastewater composite and the waste discharge locus indicates the total impurity load to be removed from the wastewater streams before they may be discharged to the environment. Because of the two main types of waste treatment units (i.e., fixed outlet concentration and load removal ratio models), different techniques were developed for the targeting of the minimum treatment flow rate and the minimum number of treatment units. These are illustrated in the following sections. 1. Treatment Units with a Fixed Outlet Concentration. For treatment systems of the fixed outlet concentration type, the impurity concentration after treatment is determined by the specification of the treatment unit. Examples of these units include filtration and membrane separation. The treatment system reduces the impurity load to a fixed outlet concentration, regardless of the quality of the inlet source. With the newly proposed wastewater composite curve, the minimum treatment flow rate for waste treatment system can be targeted. To comply with environment regulations, a fixed amount of impurity load is to be removed from the wastewater streams before final discharge. Based on eq 1, it is known that, for a
fixed amount of impurity load, it is necessary to maximize the concentration difference between the inlet and outlet streams to minimize the treatment flow rate. Since the outlet concentration is fixed; therefore, the inlet concentration must be maximized. In other words, to achieve the minimum treatment flow rate, the water source of the highest concentration is first to be treated. Figure 2a demonstrates the minimum flow rate targeting for a single fixed outlet concentration treatment unit. To target for the minimum treatment flow rate, a “treatment line” is plotted from the origin, with its slope being the outlet concentration and having the same horizontal distance as the wastewater composite curve. This line indicates that all wastewater streams will be treated prior to discharge. With this treatment approach, a much cleaner wastewater stream is produced, with its concentration being equal to the outlet concentration of the treatment unit. From Figure 2a, it is noted that the total impurity load is also less than the maximum allowable limit. However, discharging wastewater at levels lower than the regulation limits does not bring any economic incentive. To reduce the operating costs of the treatment, the maximum allowable discharge concentration is a more realistic target in treating the wastewater stream. Thus, the treatment line is moved vertically upward in the figure until its discharge point touches the maximum allowable discharge. The horizontal length of the treatment line between the discharge point and the wastewater composite curve in the figure indicates the minimum treatment flow rate. If the wastewater composite curve is reconstructed by rearranging the sequence of the component segments, using the targeted minimum treatment flow rate, with the treatment line lying in the first section of the wastewater composite curve (due to its concentration being lowest among all wastewater streams). The flow rate of the wastewater that bypasses the waste treatment (FBP) is connected to the treatment line to form the wastewater composite curve after treatment, which will take shape as shown in Figure 2b. The final network design for this treatment scheme is shown in Figure 2c. Note, from Figure 2b, that the wastewater streams after treatment have fulfilled the maximum allowable impurity load and concentration for discharge with the minimum treatment flow rate. In some cases, because of topological and other process constraints, a distributed treatment system with multiple treatment units (each with a different outlet concentration) may be considered. Numerous alternative treatment networks may be
Figure 4. Alternative designs for multiple fixed outlet concentration treatment units.
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9117
RR )
Cin - Cout Cin
(3)
For a treatment network with a single treatment unit, the minimum treatment flow rate can be determined by the wastewater composite curve. The impurity load removal by the treatment system is first determined. Equation 2 can be rearranged to relate the impurity loads of the feed stream (mF )FinCin) with that of the impurity load removal (∆mR ) FinCin - FoutCout and Fin ) Fout) by a treatment system of a specific RR value, given by eq 4:
mF ) Figure 5. Minimum flow rate targeting for a single treatment unit of the removal ratio (RR) model.
designed to treat wastewater streams up to the maximum allowable discharge load. Figure 3 illustrates two alternative treatment systems where two (multiple treatment line I) and three (multiple treatment line II) treatment units are used. The reconstructed wastewater composite curves, with their respective final network designs, are shown in Figure 4. However, note that, in this multiple treatment system, the optimal treatment network can only be identified when detailed design or overall costs are included in the analysis. Figures 3 and 4 provide guidelines on how the multiple treatment system can be designed. 2. Treatment Units with a Fixed Removal Ratio. Wang and Smith2 defined the removal ratio (RR) of a treatment unit as the ratio of impurity load removed by the treatment process to the total impurity load in the inlet wastewater, which is given by the following equation:
RR )
FinCin - FoutCout FinCin
(2)
where Fin and Fout are, respectively, the inlet and outlet flow rate of the regeneration unit, and Cin and Cout are the inlet and outlet concentrations, respectively. Because most treatment units have equal inlet and outlet flow rates (i.e., Fin ) Fout), eq 2 may be simplified as follows:2
∆mR RR
(4)
For example, an impurity load removal of ∆mR ) 90 kg/h is determined (via wastewater composite curve) to be removed in a treatment unit with RR ) 0.9; therefore, a total impurity load of mF ) 100 kg/h must be sent for treatment, to ensure that the targeted amount of impurity load (∆mR ) 90 kg/h) is removed in this treatment. This indicates that an additional 10 kg/h (0.1mF, or 0.1 × (100 kg/h)) of impurity load is required in the wastewater streams for treatment. This is shown in Figure 5, where the impurity load of the treatment inlet stream (mF) has a larger flow rate than that of the impurity load removal (∆mR). The minimum treatment flow rate for the treatment unit is located by the horizontal distance between the right-most end of the composite curve with the point on the composite curve with a vertical distance mF from its uppermost tip (refer to Figure 5). When a single treatment unit is unable to remove the targeted impurity load, multiple treatment units in series arrangement should be used. A new equation is proposed in this work to target the minimum number of units for the multiple treatment system, i.e.,
∆mD ) mF(1 - RR)n
(5)
where ∆mD is the maximum allowable discharge load and n is the minimum number of treatment units arranged in series. For multiple treatments system, overall minimum flow rate targeting is not needed, as different treatment configurations and treatment system capacities lead to different flow rates for each treatment unit. Thus, the overall minimum treatment flow
Figure 6. (a) Minimum flow rate targeting for multiple RR treatment units. (b) Network configuration for the treatment method described in Figure 6a.
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Table 1. Limiting Data for Example 1a
a
sink, SKj
flow rate, Fj (t/h)
concentration, Cj (ppm)
1 2 3 4
20 100 40 10
0 50 50 400
Σj Fj
170
source, SRi 1 2 3 4
flow rate, Fi (t/h) 20 100 40 10
Σi Fi
170
concentration, Ci (ppm) 100 100 800 800
Data taken from ref 4.
rate is subject to a specific wastewater network during the detailed design stage. However, the wastewater composite curve can be utilized to generate numerous wastewater network designs and to determine the treatment flow rate when the impurity load removal is specified. One of the design alternatives is presented in Figure 6. As shown, the waste stream of the highest impurity concentration is first treated in the treatment unit TR1 whose flow rate FT1 is determined by the horizontal distance in the figure between the discharge flow rate and the wastewater composite (see Figure 6a). Because the impurity load after the first treatment is still higher than the maximum allowable discharge load ∆mD, a second treatment unit (TR2) is used to reduce the impurity load to the maximum allowable limit (this value is represented by ∆mR2). Moreover, the treatment flow rate for the second treatment unit (FT2) is targeted from the wastewater composite curve. The treatment network design for this case is shown in Figure 6b. Two examples are solved to illustrate the proposed schemes that cover the steps of regeneration streams selection, waste treatment targeting, and the minimum treatment flow rate and minimum number of treatment units targeting for the treatment systems of fixed outlet concentration and removal ratio, respectively. A comparison between the different schemes of regeneration streams selections is presented. Example 1 Table 1 shows the limiting data of Example 1, taken from Wang and Smith.4 In this example, a fixed outlet concentration regeneration unit of 5 ppm and a final treatment system of 10 ppm are used; the maximum allowable discharge concentration (CD) is set as 20 ppm. For this example, Ng et al.7 reported that the minimum regeneration flow rate for a fixed regeneration outlet concentration of 5 ppm is 73.68 t/h, with the ultimate freshwater and wastewater flow rates targeted as 20 t/h. In this work, different selection schemes of the regeneration stream(s) are analyzed. As discussed earlier, each of these schemes will lead to different wastewater streams that will be generated from the water network and, hence, dictate the different wastewater treatment networks to be designed. Comparison may be made among these total treatment schemes and for the selection of an optimum treatment system. Scheme 1: Stream Selection Guided by Waste Streams Identification. In this scheme, the waste stream identification technique presented in Part 1 of this series6 is used as a guideline for stream selection for regeneration. Part 1 of this series of papers6 showed that, for this example, two wastewater streams are generated from the water reuse/recycle network. The first
Figure 7. Minimum treatment flow rate targeting for a single fixed outlet concentration (10 ppm) treatment unit for Example 1 (Scheme 1).
wastewater stream has a flow rate of 44.29 t/h (at the pinch concentration of 100 ppm) and the second stream has a flow rate of 45.71 t/h (800 ppm). In this scheme, it is proposed to first regenerate wastewater streams at the pinch concentration. Therefore, the 44.29 t/h wastewater stream at 100 ppm is first regenerated to 5 ppm (regeneration outlet concentration). Because this wastewater flow rate is less than the regeneration flow rate target of 73.68 t/h, an additional 29.39 t/h (73.68 t/h - 44.29 t/h ) 29.39 t/h) of the waste stream at 800 ppm is regenerated for further reuse/recycle in the network. Table 2 shows the water cascade table (WCT) for water targets after regeneration is used. Note that, with this selection of regeneration stream(s), the flow rate for freshwater and wastewater are observed at 20 t/h, which agree with the ultimate water targets.7 The total impurity loads removed in the regeneration from these streams are determined by eq 1 to be 4.21 kg/h + 23.36 kg/h ) 27.57 kg/h. Next, the waste identification technique is used again to identify wastewater streams after regeneration has occurred. Table 2 shows that, after water regeneration, two pinch concentrations (5 and 100 ppm) emerge in the water network, which separate the water network into three regions. Consistent with the proposed procedure, wastewater streams are emitted from a region that has a concentration higher than the upper pinch concentration of 100 ppm, where excess water source flow rates are found. By performing the WCA targeting in this region of excess water source (after sinks/sources segregation), the minimum pinch flow rate (FPW) is targeted as 5.71 t/h (100 ppm, see Table 3). Because 9.40 t/h of the upper pinch-causing source has been sent to this region (identified in the FC column in Table 2), this means that 9.40 t/h - 5.71 t/h ) 3.69 t/h of the wastewater stream is emitted from the pinch concentration of 100 ppm. Another wastewater stream from the network is determined to be 16.32 t/h, emitted at 800 ppm (identified from the last row of FC column in Table 3). Summing the two wastewater streams leads to the total wastewater stream of 20 t/h, as was reported previously in Table 2. The wastewater composite curve is next constructed to target the minimum impurity load removal from the wastewater streams that was necessary to comply with the environment legislations, as shown in Figure 7. The maximum concentration for environment discharge (CD) is set as 20 ppm in this case;2 therefore, a discharge locus with a slope of 20 ppm is plotted in Figure 7. This figure indicates that the impurity load that
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9119 Table 2. Water Cascade Analysis (WCA) for Water Targeting after Regeneration with Scheme 1 for Example 1 k
Σ Fj
C
1
0
2
Cout ) 5
3
50
Σ Fi - Σ F j
Σ Fi
FRW ) 73.68
4
100
5
400
73.68
75.71
75.71 -10.00
10.00
6
800
20.60
7
1000000
Total
-0.10
53.68
2.42
-86.32
-4.32
-10.60
-3.18
-20.60
-8.24
20.60
0.00
1.80
0.00 170
FFW, k
FC
∆mk
Cum. ∆mk
FFW ) 20.00
-20.00
-140.00
140.00
Cum. ∆mk
0.00
-20.00
20.00
∆mk
FC
-0.10
-20.00
2.32
46.32
-2.00
-20.00
-5.18
-12.95
0.00
0.00
73.68
3.32
-66.32 9.40
-3.32 2.82
-0.60
-0.24
-13.42
-16.78
-11.62
-0.01
FWW ) 20.00
19985.81
FFW,k
FC
∆mk
0.00 (pinch) 3.32 0.00 (pinch) 2.82 2.58 19988.39
170
Table 3. WCA for Waste Stream Identification after Regeneration for Example 1 (Scheme 1) k
Σ Fj
C
Σ Fi - Σ F j
Σ Fi
1
0
0.00
2
100
0.00
3
400
4
800
5
20.60
1000000
20.60
Cum. ∆mk
0.00
0.00
0.00
0.00
-10.00
-4.00
10.60
10595.12
0.00
Cum. ∆mk
FPW ) 5.71
0.00
-10.00
10.00
∆mk
FC
5.71
0.57
5.71
1.71
-4.29
-1.71
16.32
16304.83
0.00 0.00
0.00
-4.00
-5.71
10591.12
10.59
0.57 2.29 0.57 16305.40
Table 4. WCA for Water Targeting after Regeneration for Example 1 (Scheme 2) k
C
1
0
2
Cout ) 5
3
50
4 5 6 7 Total
Σ Fj
Σ Fi - Σ F j -20.00
20.00 FRW ) 73.68
73.68 -140.00
140.00
100 400
Σ Fi
96.32
96.32 -10.00
10.00
800
0.00
1000000
0.00 170
FC
∆m k
Cum. ∆mk
FFW,k
FC
∆mk
Cum. ∆mk
FFW ) 20.00
0.00 -20.00
-0.10
53.68
2.42
-86.32
-4.32
10.00
3.00
0.00
0.00
0.00
0.00
-0.10
-20.00
2.32
46.32
-2.00 1.00 1.00 1.00
-20.00 2.50 1.25 0.00
0.00
0.00
73.68
3.32
-66.32
-3.32
30.00
9.00
20.00
8.00
FWW ) 20.00
19984.01
0.00 (pinch) 3.32 0.00 (pinch) 9.00 17.00 20001.01
170
must be removed from the wastewater stream is determined as 13.02 kg/h. In this example, a 10 ppm fixed outlet concentration treatment model is used. Thus, a treatment line with a slope of 10 ppm is first drawn from the origin, and shifted vertically in the figure until it touches the maximum allowable impurity load of 0.4 kg/h () 20 t/h × 20 ppm/1000). As shown in Figure 7, the minimum treatment flow rate is determined to be 17.79 t/h, which includes 16.32 t/h of the 800 ppm and 1.47 t/h of the 100 ppm wastewater streams. Scheme 2: Stream Selection from a Water Source of the Highest Impurity Concentration. This scheme was adapted from Foo et al.5 for comparison with Scheme 1. As proposed, water sources of the highest impurity concentration (i.e., SR3 and SR4 (800 ppm)) are first regenerated to 5 ppm. Because the minimum regeneration flow rate is targeted as 73.68 t/h,7 the water source at 800 ppm (50 t/h) is fully regenerated, with an additional 23.68 t/h of water source of the second-highest impurity concentration (SR1 or SR2 (100 ppm)). The water flow rate targets after regeneration is presented in the WCT in Table 4. As shown, the freshwater and wastewater flow rates are targeted as 20 t/h, which corresponds to the ultimate flow rate targets that were reported by Ng et al.7 The
total impurity load removed in the regeneration unit is determined, using eq 1, to be 42.00 kg/h, which provided by water sources SR3 and SR4 (39.75 kg/h), as well as SR1 and SR2 (2.25 kg/h). Subsequently, waste targeting is conducted for the water network after regeneration has occurred. As shown in Table 4, two pinch concentrations (5 and 100 ppm) emerge in the water network. In this case, the minimum pinch flow rate is targeted as 10 t/h (not shown). Deducting this value from the upper pinch-causing source flow rate (SR1) that is allocated to this region (30 t/h, Table 4), the wastewater stream flow rate from the pinch concentration is identified as 20 t/h (100 ppm). Note that, because there is only one water sink (SK4) that is located in the region with excess water after water regeneration, no other wastewater emanates from the network after regeneration. Next, the waste composite curve is constructed to target for the minimum impurity load removal from the waste treatment system (Figure 8). Because there is only a single wastewater stream, the wastewater composite is a straight line with a slope of 100 ppm. As shown, the minimum impurity load removal is determined as 1.60 kg/h. Figure 8 shows the waste treatment targeting where a fixed outlet concentration treatment system
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Figure 8. Scheme 2 for minimum treatment flow rate targeting for Example 1.
Figure 9. Scheme 3 for minimum treatment flow rate targeting for Example 1.
of 10 ppm is used for final treatment. As shown, the treatment line locates the minimum treatment flow rate at 17.79 t/h. Scheme 3: Stream Selection Guided by the Pinch-Causing Source. Scheme 3 is adapted from the original work proposed by Wang and Smith4 and Argawal and Shenoy,8 where the pinch-causing source in a water reuse/recycle network is first sent for regeneration. Because a large amount of the water source (120 t/h for both SR1 and SR2) are available at the pinch concentration of 100 ppm (refer Table 1), 73.68 t/h of these sources are sent for regeneration, to satisfy the minimum
regeneration flow rate target. Table 5 shows the WCT that reports the water targets after regeneration in this scheme. As shown, the freshwater and wastewater flow rates after regeneration are targeted as 42.50 t/h, which are much higher than the ultimate water targets that were identified by Ng et al. (i.e., 20 t/h).7 The increase of the minimum water flow rates is mainly due to the reallocation of the pinch concentration to 800 ppm after water regeneration. However, with this selection scheme, the total impurity load that is removed in the regeneration unit is determined to the lowest among all schemes (i.e., 7.00 kg/ h). Because a new pinch concentration is formed at 800 ppm, the only wastewater that emanates from the water network is generated from this pinch concentration (i.e., with a flow rate of 42.50 t/h). Subsequently, the impurity load required to be removed for this wastewater stream is targeted with the wastewater composite. Figure 9 shows the wastewater composite curve for the wastewater stream, where the minimum impurity load removal is reported to be 33.15 kg/h. The minimum treatment flow rate when a fixed outlet concentration treatment of 10 ppm is used is determined to be 41.96 t/h, as shown in Figure 9. Summary. A summary of the results from different stream selection schemes, as well as the base case network (reuse/ recycle only) is presented in Table 6. As shown, when water regeneration is in used, the freshwater consumption and wastewater flow rates are reduced significantly. Both Schemes 1 and 2 achieve the ultimate water flow rate targets that have been determined by Ng et al.7 (i.e., 20 t/h for the freshwater and wastewater flow rates, respectively). Although the same amount of water is regenerated in Scheme 3, the water flow rates are higher than the ultimate water targets. In other words, Scheme 3 generates an additional 22.50 t/h of wastewater, because of its higher freshwater flow rate. In the base case network, where only reuse/recycle is used, two wastewater streams are generated, with concentrations of 100 ppm (44.29 t/h) and 800 ppm (45.71 t/h). To comply with the environmental discharge of 20 ppm, the minimum impurity load removed by the waste treatment for discharge is targeted as 39.20 kg/h, with a minimum treatment flow rate of 80 t/h, for a fixed outlet concentration treatment of 10 ppm. On the other hand, for the network with water regeneration, the impurity load in the water source is removed to allow water for further reuse/recycle. Because of the different stream selection schemes, different impurity loads are removed in each of these schemes. Other than the regeneration unit, the waste treatment unit(s) also has/have an important role in removing the impurity load before wastewater is discharged to the environment. Although both regeneration and waste treatment systems have different treatment objectives, both systems serve
Table 5. WCA for Water Targeting after Regeneration for Example 1 (Scheme 3) k
C
1
0
2
Cout ) 5
3 4 5 6 7 Total
50
Σ Fj
FRW ) 73.68
73.68 -140.00
140.00 46.32
46.32 -10.00
10.00
800
Σ Fi - Σ F j -20.00
20.00
100 400
Σ Fi
50.00
1000000
50.00 0.00
170
170
FC
∆mk
Cum. ∆mk
FFW,k
FC
∆mk
Cum. ∆mk
FFW ) 42.50
0.00 -20.00
-0.10
53.68
2.42
-86.32
-4.32
-40.00
-12.00
-50.00
-20.00
0.00
0.00
-0.10 2.32 -2.00 -14.00 -34.00 -34.00
-20.00 46.32 -20.00 -35.00 -42.50 -0.03
22.50
0.11
96.18
4.33
-43.82 2.50 -7.50 FWW ) 42.50
-2.19 0.75 -3.00 42466.00
0.11 4.44 2.25 3.00 0.00 (pinch) 42466.00
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9121
Figure 10. Total water network for Example 1 (results of Scheme 1). Table 6. Summary of Different Regeneration Stream Selection Schemes (Example 1) Impurity Load Removed (kg/h)
scheme
freshwater flow rate (t/h)
wastewater flow rate (t/h)
regeneration flow rate (t/h)
regeneration unit
treatment unit
total
base case (reuse/recycle)
90
90
nil
nil
39.20
39.20
Scheme 1
20
20
73.68
27.57
13.02
40.59
Scheme 2 Scheme 3
20 42.50
20 42.50
73.68 73.68
42.00 7.00
1.60 33.15
43.60 40.15
the same function of removing impurity load from the wastewater streams. As discussed previously, the cost of treatment is not only a function of the treatment flow rate, but also the impurity load that is removed from the waste stream. As reported in Table 6, Scheme 1 requires the lowest impurity load removal (i.e., 40.59 kg/h) in the total treatment systems among all schemes. Other than that, Schemes 1 and 2 lead to the lowest minimum treatment flow rate (17.79 t/h). Hence, it can be concluded that Scheme 1 is the best case among all schemes for a fixed-load problem, with the lowest total impurity load removal and the lowest treatment flow rate. Figure 10 demonstrates the total water network design for Scheme 1, which was designed using the modified nearestneighbor algorithm.9 As shown, the regenerated water (RW) is sent for further reuse/recycle in the water network. In addition, the wastewater (WW) is treated with a waste treatment system of the fixed concentration model (i.e., 10 ppm) before it is discharged to the environment. Note that the freshwater (FW) and wastewater flow rates (FWW) of the total water network match the ultimate flow rates targeted before detailed design. Example 2 Example 2, which is a fixed-flow rate problem, is solved to analyze the interaction between water regeneration and wastewater treatment via the proposed stream selection scheme. Table 7 shows the limiting data for Example 2.10 In this example, a regeneration unit of fixed concentration (10 ppm) and a final treatment system of RR ) 0.90 are used. Utilizing the targeting
minimum treatment flow rate (t/h)
waste stream WW1 (100 ppm) ) 44.29 t/h WW2 (800 ppm) ) 45.71 t/h WW1 (100 ppm) ) 3.69 t/h WW2 (800 ppm) ) 16.32 t/h WW1 (100 ppm) ) 20.00 t/h WW2 (800 ppm) ) 42.50 t/h
80.00 17.79 17.79 41.96
Table 7. Limiting Data for Example 2a
a
sink, SKj
flow rate, Fj (t/h)
concentration, Cj (ppm)
1 2 3 4 5 6 Σ Fj
120 80 80 140 80 195 695
0 50 50 140 170 240
source, SRi 1 2 3 4 5 6
flow rate, Fi (t/h) 120 80
concentration, Ci (ppm) 100 140
140 80 195
180 230 250
Σ Fi
615
Data taken from ref 5.
technique of Ng et al.,7 the ultimate freshwater and wastewater targets for a regeneration concentration of 10 ppm are determined to be 120 t/h (contributed by water sink SK1, which requires 120 t/h of pure water) and 40 t/h, respectively, with a minimum regeneration flow rate target of 88.89 t/h. Scheme 1: Stream Selection Guided by Waste Streams Identification. As was reported in Part 1 of this series of papers,6 two wastewater streams are identified from this case, which emit water at 180 ppm (35 t/h) and 250 ppm (85 t/h), respectively. Following the proposed procedure, the entire flow
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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007
Table 8. WCA for Water Targeting after Regeneration with Scheme 1 for Example 2 k
Σ Fj
C
1
0
2
10
3
50
4
100
5
140
6
170
7
180
8
230
9
240
10
250
11
1000000
Σ Fi
Σ Fi - Σ F j -120
120 FRW ) 160
88.89
88.89 -160
120
120
80
-60
140
-80
80 105 80
105 80 -195
195 141.11
141.11 0
FC
∆m k
Cum. ∆m k
FFW,k
Cum. ∆mk
FFW ) 120
0 -120
-1.20
-31.11
-1.24
-191.11
-9.56
-71.11
-2.84
-131.11
-3.93
-211.11
-2.11
-106.11
-5.31
-26.11
-0.26
-221.11
-2.21
-80
∆mk
FC
-79980
-1.20
-120.00
-2.44
-48.89
-12.00
-120.00
-14.84
-106.03
-18.78
-110.46
-20.89
-116.05
0
0.00
88.89
3.56
-71.11
-0.33
-91.11
-0.91
-113.89
-26.46
-110.23
93.89
-28.67
-114.67
-80008.67
-80.01
0.00 (pinch)
1.96
-11.11
-26.19
3.56
-3.56
48.89
13.89
0.00 (pinch)
-101.11 FWW ) 40
0.69 0.94
1.96 1.62 0.71 1.41 2.34
-1.01 39990.00
1.33 39991.33
Table 9. WCA for Waste Stream Identification after Regeneration for Example 2 (Scheme 1) k
C
1
100
2
140
3
170
Σ Fj
Σ Fi
Σ F i - Σ Fj 0
140
80
-60 -80
80
4
180
105
105
5
230
80
80
6
240
7
250
8
1000000
-195
195 141.11
141.11 0
∆mk
FC
Cum. ∆mk
FFW,k
∆mk
Cum. ∆mk
FPW ) 40
0 0
0.00
-60
-1.80
-140
-1.40
-35
-1.75
45
0.45
-150
-1.50
-8.89
FC
-8887.78
of 180 ppm wastewater (35 t/h) and 53.89 t/h of the 250 ppm wastewater are sent for regeneration. Table 8 shows the WCT for water targets after regeneration with Scheme 1, where the ultimate water flow rate targets are achieved. Based on eq 1, the total impurity load removed by the regeneration unit is determined to be 18.88 kg/h. Table 9 next locates the minimum pinch flow rate (FPW) of 40 t/h. In addition, the wastewater generated from the upper pinch concentration (100 ppm) after regeneration is determined to be 8.89 t/h, i.e., difference between the flow rate allocated to the higher-concentration region (48.89 t/h), identified from interval between 100 and 140 ppm from FC column in Table 8 and the minimum pinch flow rate (40 t/h): 48.89 t/h - 40 t/h ) 8.89 t/h. Note that Table 9 reports another wastewater stream of 31.11 t/h at 250 ppm. Next, the waste composite curve (Figure 11) is constructed to target a minimum impurity load removal from the wastewater streams. As shown, the minimum impurity load removal from the water treatment unit is determined to be 7.87 kg/h. Based on eq 5, two treatment units of RR ) 0.90 are targeted to treat the wastewater stream before they can be sent for environmental discharge. Scheme 2: Stream Selection from the Water Source of the Highest Impurity Concentration. From the limiting data (Table 7), the highest contaminated source stream is observed as SR6, with an impurity concentration of 250 ppm and a flow rate of 195 t/h. Because the minimum regeneration flow rate is
0.00 -1.80
0.00 -25.71
40
1.60
-20
-0.60
-100
-1.00
-3.20
-40.00
-4.95
-38.08
5
0.25
-4.50
-32.14
85
0.85
-6.00
-40.00
-110
-1.10
-8893.78
-8.89
31.11
31102.22
1.60 1.00 0.00 0.25 1.10 0.00 31102.22
targeted as 88.89 t/h, a portion of water source SR6 is regenerated. As shown in Table 10, the ultimate water flow rates are achieved. Next, following the waste stream identification technique, 8.89 t/h () 48.89 t/h - 40 t/h) of wastewater is generated at upper pinch concentration (100 ppm). Moreover, based on Table 11, another wastewater stream that emits from source SR6 (250 ppm) is determined as 31.11 t/h. According to eq 1, the impurity load removed in the regeneration unit is calculated as 21.33 kg/h. With the regeneration stream selection of Scheme 2, wastewaters generated after regeneration are exactly same as Scheme
Figure 11. Wastewater composite for Schemes 1 and 2 for Example 2.
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9123 Table 10. WCA for Water Targeting after Regeneration for Example 2 (Scheme 2) k
Σ Fj
C
1
0
120
2
10
FRW ) 160
3 4 5 6
Σ Fi
88.89
88.89 -160
100
120 140
170
80
120 -60 -80
80
7
180
140
140
8
230
80
80
9 10 11
240
-195
195
250
106.11
1000000
∆mk
FC
Cum. ∆m k
FFW,k
106.11
-120
-1.20
-31.11
-1.24
-191.11
-9.56
-71.11
-2.84
-131.11
-3.9333
-211.11
-2.11
-71.11
-3.56
8.89
0.09
-186.11
-1.86
-80
0
∆mk
FC
Cum. ∆mk
FFW ) 120
0
-120
50
140
Σ Fi - Σ F j
-1.2 -2.44 -12.00 -14.84 -18.78
-48.89 -120.00 -106.03 -110.46
0
0.00 (pinch)
88.89
3.56
-71.11
-3.56
48.89
1.96
-11.11
-0.33
-91.11
-0.91
-20.89
-116.05
-24.44
-106.28
48.89
2.45
-101.48
128.89
1.29
-66.11
-0.66
-24.36 -26.2164
-79980
0
-120.00
-80006.216
-104.87 -80.01
FWW ) 40
39990
3.56 0.00 (pinch) 1.96 1.62 0.71 3.16 4.45 3.78 39993.78
Table 11. WCA for Waste Stream Identification after Regeneration for Example 2 (Scheme 2) k
Σ Fj
C
1
100
2
140
140
3
170
80
4
240
7
250
8
1000000
-80 140
80
80 -195
195 106.11
∆mk
FC
Cum. ∆mk
FFW,k
106.11
0
0.00
-60
-1.80
-140
-1.40
0
0.00
80
0.80
-115
-1.15
-8.89
0
FC
∆mk
Cum. ∆mk
FPW ) 40
0
-60
80
140
230
6
Σ F i - Σ Fj 0
180
5
Σ Fi
-8887.78
40
1.60
-20
-0.60
-100
-1.00
-24.62
40
2.00
-17.14
120
1.20
-75
-0.75
0.00
0.00
-1.80
-25.71
-3.20 -3.20 -2.40
-40.00
-3.55
-23.67
-8891.33
-8.89
31.11
31102.22
FC
∆mk
1.60 1.00 0.00 2.00 3.20 2.45 31104.67
Table 12. WCA for Water Target after Regeneration with Scheme 3 for Example 2 k
Σ Fj
C
1
0
2
10
3
100
5
140
6
170
7
180
8
230
9
240
11
FRW ) 160
250 1000000
Σ Fi - Σ F j -120
120 88.89
88.89 -160
50
4
10
Σ Fi
140
120
120
80
-60 -80
80 51.11 80
51.11 80 -195
195 195
195 0
∆mk
FC
Cum. ∆mk
FFW,k
Cum. ∆mk
FFW ) 129.75
0 -120
-1.20
-31.11
-1.24
-191.11
-9.56
-71.11
-2.84
-131.11
-3.93
-211.11
-2.11
-160
-8.00
-80
-0.80
-275
-2.75
-80
-79980.00
1, i.e., 8.89 t/h at 100 ppm and 31.11 t/h at 250 ppm. Thus, the wastewater composite curve, the minimum impurity load removal for discharge, and the minimum number of treatment units is the same as that in Scheme 1, as represented by Figure 11. Scheme 3: Stream Selection Guided by the Pinch-Causing Source. In this scheme, the upper pinch-causing source (180
-1.20 -2.44
-120.00
9.75
0.10
-48.89
98.64
3.95
-12.00
-120.00
-14.84
-106.03
-18.78
-110.46
-20.89
-116.05
-28.89
-125.60
-29.69
-123.70
-32.44 -80012.44
-129.75 -80.01
-61.36
-3.07
58.64
2.35
-1.36
-0.04
-81.36
-0.81
-30.25
-1.51
49.75 -145.25 FWW ) 49.75
0.50 -1.45 49742.36
0.10 4.04 0.98 3.32 3.28 2.47 0.95 1.45 0.00 (pinch) 49742.36
ppm) is first selected for regeneration; 88.89 t/h of SR4 is sent for water regeneration. Using eq 1, the impurity load that must be removed from this stream is calculated to be 15.11 kg/h. Table 12 shows the water targets after regeneration, where the freshwater and wastewater targets are 129.75 and 49.75 t/h, respectively, which are higher than the ultimate water targets. Table 12 also shows that, after regeneration has
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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007
Table 13. Summary of Different Regeneration Stream Selection Schemes (Example 2)
scheme
freshwater flow rate (t/h)
wastewater flow rate (t/h)
base case (reuse/recycle)
200
120
Scheme 1
120
Scheme 2
120
Scheme 3
129.75
regeneration flow rate (t/h)
Impurity Load Removed (kg/h) regeneration treatment unit unit total
nil
nil
25.15
25.15
40
88.89
40
88.89
18.88
7.87
26.75
21.33
7.87
29.20
49.75
88.89
15.11
11.44
26.55
occurred, only one pinch concentration is located at 250 ppm (the highest concentration interval). Hence, the region with excess water only contains a single wastewater source, which is the allocated flow rate from the pinch-causing source (SR6) of 49.75 t/h. Next, the wastewater composite is constructed in Figure 12
Figure 12. Wastewater composite for Scheme 3 for Example 2.
Figure 13. Total water network for Example 2 (results of Scheme 1).
waste stream WW1 (180 ppm) ) 35.00 t/h WW2 (250 ppm) ) 85.00 t/h WW1 (100 ppm) ) 8.89 t/h WW2 (250 ppm) ) 31.11 t/h WW1 (100 ppm) ) 8.89 t/h WW2 (250 ppm) ) 31.11 t/h WW2 (250 ppm) ) 49.75 t/h
minimum number of treatment units 2 2 2 2
to target the minimum impurity load removal. The minimum impurity load removal in treatment system is 11.44 kg/h. Following eq 5, two treatment units of RR ) 0.90 (series arrangement) are required to remove the desired impurity load before discharge. Summary. Table 13 shows the summary for all three stream selection schemes, including the base case of reuse/recycle network for Example 2. For an equal amount of regeneration flow rate (88.89 t/h) in all schemes, Schemes 1 and 2 achieve the ultimate freshwater and wastewater flow rate targets of 120 and 40 t/h, respectively, whereas Scheme 3 requires 9.75 t/h more than the ultimate water flow rate targets. Consistent with the proposed approach, the minimum impurity load removed for the base case (reuse/recycle) is targeted as 25.15 kg/h () 35 t/h (180-20) ppm/1000 + 85 t/h (250-20) ppm/1000). In addition, the minimum number of treatment units for the base case is calculated as two with eq 5. Table 13 shows that the minimum impurity load to be removed in the treatment is the base case, because no regeneration is required. However, the water flow rate targets are 80 t/h higher than the ultimate water flow rate targets (120 t/h of freshwater and 40 t/h of wastewater). Because of the different regenerated stream(s) selection pattern in the three schemes, different impurity loads are required to be removed in the regeneration unit and the wastewater treatment units. As shown, Scheme 1 has the lowest
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9125
impurity load removal among all schemes. Moreover, Schemes 1 and 2 generate the same amount of wastewater (with equal quality), whereas Scheme 3 produces a higher wastewater flow rate at higher concentrations. Finally, note that two units of waste treatment at RR ) 0.90 are targeted for all schemes. In conclusion, Scheme 1 is justified as the best stream selection scheme for fixed-flow rate problems, because it achieves both the ultimate water flow rate targets of the network and the lowest impurity load removal in the total treatment system (regeneration and waste treatment). The overall water network for this example is presented in Figure 13. Conclusion Part 2 of the series of papers explores the interaction between waste treatment and water regeneration in a bigger framework of a total water network. Guidelines for stream selection for water regeneration in the network are presented in this work. Three different schemes for regeneration stream selection are proposed, and comparisons among these selection schemes are made. A novel wastewater composite curve is then proposed to target the minimum impurity load removal before the wastewater streams are sent for discharge. Targeting for the minimum treatment flow rate is presented for fixed outlet concentration treatment systems, whereas targeting for the minimum number of treatment units are presented for treatment systems with a fixed contaminant removal ratio. In conclusion, the selection of regeneration streams that are guided by the waste stream identification technique lead to the optimal network that achieves the minimum ultimate water flow rate targets, minimum total impurity load removal, minimum treatment flow rate (treatment system of fixed concentration model), and minimum number of treatment units (treatment system of the removal ratio model). Acknowledgment The financial support from University of Nottingham Research Committee through New Researcher Fund (NRF 3822/ A2RBR9) and Research Studentship is gratefully acknowledged. Scholarship from the World Federation of Scientists (WFS) is appreciated. Feedback from the two anonymous reviewers is gratefully acknowledged. Nomenclature CD ) maximum allowable discharge concentration Ci ) impurity concentration of source i Cin ) outlet concentration into the regeneration/treatment unit Cj ) impurity concentration of sink j Cout ) outlet concentration of the regeneration/treatment unit Cum. ∆m ) cumulative impurity load
FBP ) bypass treatment flow rate FC,k ) cumulative surplus/deficit flow rate of interval k FFW ) freshwater flow rate FFW,k ) interval freshwater flow rate Fi ) flow rate of source i Fin ) inlet flow rate into the regeneration/treatment unit Fj ) flow rate of sink j Fout ) outlet flow rate from the regeneration/treatment unit FRW ) regeneration flow rate FT ) treatment flow rate FWW ) wastewater flow rate i ) index of sources j ) index of sinks ∆mk ) interval impurity load mF ) impurity load of feed stream ∆mD ) maximum allowable discharge load ∆mR ) impurity load removal n ) number of treatment units RR ) ratio of impurity load removed in a treatment unit RW ) regenerated water SKj ) sink j SRi ) source i TR ) treatment unit WW ) wastewater Literature Cited (1) Kuo, W.-C. J.; Smith, R. Effluent Treatment System Design. Chem. Eng. Sci. 1997, 52 (23), 4273-4290. (2) Wang, Y. P.; Smith, R. Design of Distributed Effluent Treatment Systems. Chem. Eng. Sci. 1994, 49 (18), 3127-3145. (3) El-Halwagi, M. M. Process Integration; Elsevier: San Diego, CA, 2006. (4) Wang, Y. P.; Smith, R. Wastewater Minimisation. Chem. Eng. Sci. 1994, 49, 981-1006. (5) Foo, D. C. Y.; Manan, Z. A.; Tan, Y. L. Use Cascade Analysis to Optimize Water Networks. Chem. Eng. Prog. 2006, 102 (7), 45-52. (6) Ng, D. K. S.; Foo, D. C. Y.; Tan, R. R. Targeting for Total Water Network. 1. Waste Stream Identification. Ind. Eng. Chem. Res. 2007, 46, 9107-9113. (7) Ng, D. K. S.; Foo D. C. Y.; Tan, Y. L.; Tan, R. R. Ultimate Flowrate Targeting with Regeneration Placement. Chem. Eng. Res. Des. 2007, 85 (A9), 1253-1267. (8) Agrawal, V.; Shenoy, U. V. Unified Conceptual Approach to Targeting and Design of Water and Hydrogen Networks. AIChE J. 2006, 52 (3), 1071-1082. (9) Prakash, R.; Shenoy, U. V. Targeting and Design of Water Networks for Fixed Flowrate and Fixed Contaminant Load Operations. Chem. Eng. Sci. 2005, 60 (1), 255-268. (10) Sorin, M.; Be´dard, S. The Global Pinch Point in Water Reuse Networks. Trans. Inst. Chem. Eng., Part B 1999, 77, 305-308.
ReceiVed for reView August 10, 2007 Accepted September 20, 2007 IE071096+
