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Ind. Eng. Chem. Res. 2001, 40, 4506-4512
PROCESS DESIGN AND CONTROL Optimization of Chemical and Physical Pretreatments in a Platform for the Treatment of Liquid Industrial Wastes Paolo Battistoni,*,† Raffaella Boccadoro,† David Bolzonella,‡ and Silvia Pezzoli† Institute of Hydraulics, University of Ancona, Via Brecce Bianche, I-60131 Ancona, Italy, and Department of Science and Technology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italy
The paper presents the results of management of the chemical-physical pretreatment step in a platform for the treatment of liquid wastes. The aim of the work was to optimize the organization of the flow scheme to achieve the best performances in the chemical-physical pretreatment step. Starting from the initial situation (Jan-Apr 1997), the types, amounts, and characteristics of the wastewater and the treatment methodologies were critically discussed. The process management was subsequently (Jul-Aug 1997) modified in order to take advantage of the synergic power among the different liquid wastes to reduce the use of strong oxidative treatments (i.e., the Fenton process) which determined, during the first research period, a large increase of the total dissolved solids and salinity in the effluent, without meaningful chemical oxygen demand reductions. Moreover, the N-NH4 removal was performed by struvite precipitation, using phosphate-rich wastewater or pure chemicals. A good indicator of the reliability of the new flow scheme was the salinity reductionsa loss of 2800 mg/Lsduring the first 2 months. A larger reduction was obtained after a further 3 months (total dissolved solids passed from 17 000 to 5000 mg/L). Therefore, the effluent characteristics became suitable for the following biological process. Introduction Liquid industrial wastes need particular solutions for the treatment, which may differ a lot from the usual treatments of municipal wastewaters. The great variability of the amount and characteristics of treated liquid wastes needs a flexible flow scheme and effective chemical-physical pretreatments.1,2 In particular, the right combination of chemical-physical and biological processes has to be adopted to avoid the inhibitory, toxic, or resistant effect of many organic compounds or heavy metals on the biological activity. In an effective process scheme, the biological treatment should have only a finishing-cleaning task. The typical flow scheme of an industrial wastewater treatment plant foresees the matching of chemical-physical pretreatments with a final biological treatment process, preceded by an equalization step. This flow scheme can be modified, in the daily actual managing, depending on the different characteristics of the conferred liquid wastes.3 Generally, the chemical-physical pretreatments need to reduce the presence of organic and inorganic compounds exerting an inhibitory effect on the following biological step. Therefore, it is of particular importance to optimize the pretreatment step to ensure a finishing role to the biological process. This paper presents the effects of the flow scheme modification in an existing industrial * To whom correspondence should be addressed. Telephone: +39 071 2204530. Fax: +39 071 2204528. E-mail:
[email protected]. † University of Ancona. ‡ University of Verona.
wastewater treatment platform. In particular, some managing tricks and new chemical-physical processes were introduced to improve the characteristics of the effluent, namely, to exploit the interaction among different waste classes (i.e., the neutralization of acidic wastes with basic ones) and to introduce new specific processes in the flow scheme to remove specific pollutants up to an effluent mass loading suitable with the following step of the treatment. Particular attention was paid to the reduction of the salinity in pretreated wastewater. Materials and Methods Wastewater Treatment Plant Layout and Description. The platform for the industrial wastewater treatment received liquid wastes coming from landfills and food, tannery, paint, and galvanic industries. The plant was organized according to the flow scheme in Figure 1. Here, after the starting acceptance and stocking, the different liquid wastes are chemically and physically pretreated before being sent to the biological treatment step. The chemical-physical pretreatment section was organized as shown in Figure 2. The stocking station had a 330 m3 working volume divided into 11 separate tanks. The chemical-physical pretreatment sections were composed of three completely stirred tank reactors, used for neutralization, wet oxidation, flocculation, and clarification, each with a 30 m3 working volume. These were managed in a batch mode to control the reaction times. The chemical sludges were gravitationally settled in a linear settler with a surface
10.1021/ie010006c CCC: $20.00 © 2001 American Chemical Society Published on Web 09/20/2001
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4507 Table 1. Industrial Wastewater Treated in 1996
Figure 1. Flowchart of the platform configuration.
Figure 2. Flowchart of the chemical-physical pretreatments of the industrial platform.
hydraulic loading of 0.7 m3 m-2 h-1. The completely biodegradable industrial wastewater was directly sent to the equalization basin (126 m3 volume) before the biological treatment. This was a two-step biological process with separate biomasses operating in an alternate cycle mode. The effluent of the biological step was collected in a lagoon, 2000 m3 volume, and filtered through activated carbon columns before disinfection and discharge. IndustrialWastewaterCharacteristicsandAmounts. Wastes, conferred by trucks, belong to eight different types of industrial activities; in the examined platform, during 1996 management (Table 1), the main role was exerted by landfill leachate (41%), organic pollutant solutions (12%), and ink wastewaters (10%). Notwithstanding, different situations were observed season by season, because landfill leachate production is not constant. The chemical-physical characteristics of wastewater were determined as described in Standard Methods.4 Typical characteristics of the wastewater are reported in Table 2. These were very variable because of the different industrial processes which originated them; flexible processes had to be adopted. In particular,
wastewater type
amount (ton)
percentage of total treated (%)
phosphate degreasing paint oil emulsions landfill leachate oils and hydrocarbon solutions organic pollutants solutions inks cesspool spurge total in 1996
1423 1200 1000 8100 700 2281 2011 1230 19528
7 6 5 41 4 12 10 6 100
heavy metal had necessarily to be removed by neutralization and flocculation, whereas inks, glues, and landfill leachate needed strong pretreatments to be suitable for the following biological step. Therefore, the Fenton process5 was used because the nonbiodegradable COD had to be broken down. Because of the Fenton process application, the pretreated wastewaters were characterized by a high salinity level owing to chemical reagent utilization: this was a problem for biological treatments. To reach the best performances, a classification of the different conferred waste streams was necessary to easily determine the best pretreatment for each type of wastewater. The main pollutants to be removed before biological treatments were mineral oils, anionic and nonionic surfactants, nonbiodegradable COD, part of the ammonia loading, phenols, heavy metals, and toxic organic compounds. Three main chemical-physical processes were used for the treatment of typical liquid wastes (Table 3, treatments 1-3). The other two treatments (Table 3, treatments 4 and 5) were used for concentrated wastewater streams. Anyway, processes 4 and 5, after dilution, are quite similar to processes 1 and 2, respectively. The typical pretreatments were chemical oxidation, coagulation, reduction, precipitation, flocculation, and/or clarification. These were used in alternative mode, depending on the wastewater characteristics. On the basis of the conferred waste types and characteristics reported in Tables 1 and 2, it is evident that the Fenton process and the acidic and basic clarification processes (treatments 1, 3, and 4) were widely used. Negative aspects related to the application of these processes were a huge recycle flow rate of treated water in order to dilute the concentrated industrial liquid wastes, a large amount of ammonia, and low performance of the Fenton process, which was not able to remove a great part of the nonbiodegradable COD. Results and Discussion First Period Analysis: Jan-Apr 1997. In the first part of the research, a 4-month analysis (Jan-Apr 1997) was carried out to verify the operational conditions of the originally adopted flow scheme. A wide variability in the treated wastewater amounts was observed: the average treated flow was 61 m3 day-1, with a standard deviation of 23 m3 day-1. A weekly analysis allowed one to observe that the largest part of the wastewater was treated from Tuesday to Friday, whereas the treated flows were very low on the weekends because of the reduced industrial activities. The most used pretreatments were the acidification-basification (treatments 3 and 4), which treated 2166 m3 (Table 4), 48% of the total, and the Fenton processes (treatments 2 and 5), which treated 1970 m3 (see Table 4), 44% of the total. Because the treated water recycle was a typical practice to dilute the strong liquid wastes, the number and
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Table 2. Chemical-Physical Characteristics of Treated Wastewater in 1996 min-max wastewater type
COD (mg/L)
NH4-N (mg/L)
total P (mg/L)
phosphate degreasing paint oil emulsions landfill leachate glues inks biological sludge polishers
1000-5000 800-35000 15000-200000 3000-10000 10000-200000 30000-100000 2000-5000 15000-20000
0-50 0-10
50-8000 0-2
400-800
5-15
300-1000 5-15 20-80
2-5 3-10
average mineral oil (mg/L)
surfactants (mg/L)
salinity (mg/L)
0-3000
10000 1000 30000 10000 5000 40000 2000 500
5000-100000 0-5000 3000-10000
phenols (mg/L)
200
Table 3. Treatments for Different Conferred Industrial Wastewaters treatments
wastewater type
1
paints, acidic solutions, sifting
solvents, metals,a chlorides, sulfates, low COD
flocculation-clarification at pH 4.5 and then at pH 9 by soda adding
2
landfill leachate
NH3-N < 200 mg/L
surfactant solutions organic pollutants solutions
chlorides and phenols < 100 mg/L surfactants < 150 mg/L COD < 10000 mg/L
Fenton process at pH 3.5 and then flocculation-clarification at pH 9.5 by soda adding
phosphate degreasing
P < 1000 mg/L
inks organic and inorganic pollutant solutions
metals,a oils, greases high COD
phosphate degreasing
P < 1000 mg/L
oil emulsions inks glues
metals,b oils, greases high COD
landfill leachate
NH3-N > 200 mg/L
organic pollutant solutions
chlorides and phenols > 100 mg/L surfactants > 150 mg/L COD > 10000 mg/L
3
4
5
a
main pollutants and characteristics
treatment processes
flocculation-clarification at pH 2.5-3 by aluminum sulfate adding and then at pH 9 by soda adding
dilution by adding recycled effluent and then flocculation-clarification at pH 2.5-3 by aluminum sulfate adding and then at pH 9 by soda adding
dilution by adding recycled effluent and Fenton process at pH 3.5 and then flocculation-clarification at pH 9.5 by soda adding
Pb, Cd, Cu < 10 mg/L. Al, Fe, Zn < 100 mg/L. b Pb, Cd < 10 mg/L. Cu, Al, Fe, Zn < 100 mg/L.
Table 4. Effluent Water Dilutions for the Different Treatment Processes during Jan-Apr 1997 treatment 1 2+5 3+4 other
no. of recycles wastewater dilution dilution vol. (m3) vol. (m3) (%) min avg max std 337 1970 2166 31
123 1375 2488 88
36 70 115 284
1.0 1.0 1.0 2.1
1.9 6.0 1.5 2.7 29.0 3.1 3.2 9.7 2.2 4.6 6.0 1.9
volume of recycles were observed. This practice can lead also to an increase of salinity. The number of recycles was determined on the basis of eq 1. where VR ) total
no. of recycles ) 1 + VR/VI
(1)
water volume inlet + recycle (m3) and VI ) inlet wastewater volume (m3). The number of recycles and the percentage of recycled volume of wastewater were evaluated. The results are shown in Table 4. As can be seen, the Fenton process needed an average number of recycles of 2.7 ((3.1), whereas the recycled volume was 70% of the treated one. The flocculation and clarification processes (treatments 3 and 4) needed high dilutions, generally over the 100%, and an average recycle number of 3.2 ((2.2).
Performance Analysis of the First Period Study. (a) Fenton Process (Treatments 2-5). Seven different tests were performed on different industrial wastewaters to verify the Fenton process effectiveness. The results are shown in Table 5. The main evidences were that the total suspended solids removal was very effective (>97%), whereas the soluble and total COD removal was changeable. A salinity increase was observed: it averaged 39%. (b) Acidification-Basification (Treatments 1, 3, and 4). Seven acidification-basification tests in the flocculation and clarification processes were carried out. The results of the performed tests are summarized in Table 6. The total suspended solids removal was very effective (average 98%). The total COD removal was in the range 42-84%, whereas the soluble COD was only partially removed. A large salinity increase was observed: +32%. Therefore, also the acidification-basification process showed the same behavior as the Fenton process. Second Period Analysis: Jul-Aug 1997. After the first period of study (Jan-Apr 1997), the approach in the pretreatment step was changed and a second experimental run was carried out between Jul and Aug
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4509
Figure 3. Ink and glue blending and acidification-basification (treatments 1, 3, and 4) during Jul-Aug 1997. Table 5. Chemical-Physical Characteristics of the Treated Wastewater by the Fenton Process and Removal Efficiencies (E %) during the Period Jan-Apr 1997 wastewater + recycled water
TSSin (mg/L)
E% TSS
CODin (mg/L)
E% COD
SCODin (mg/L)
E% SCODin
detergentin (mg/L)
E% detergent
TDSout (mg/L)
salinity increase (%)
landfill leachate phosphate degreasing sifting glues + leachate phosphate degreasing phosphate degreasing glues
1900 3700 6300 17800 6400 9200 6700
98 97 98 99 97 99 97
10 500 12 750 12 500 18 000 8 200 9 250 15 500
44 36 62 72 52 2 79
8750 7000 5250 5750 4500 5250 3250
34 3 17 58 22 29 14
70 190 260 250 115 na 135
93 97 97 96 95 na 63
14 021 17 868 16 224 19 983 17 105 19 689 17 164
+37 +28 +9 +33 +24 +62 +80
Table 6. Chemical-Physical Characteristics of the Wastewater Treated by the Coagulation-Clarification Process (Treatments 1, 3, and 4) and Removal Efficiencies (E %) during the Period Jan-Apr 1997 wastewater + recycled water
TSSin (mg/L)
E% TSS
CODin (mg/L)
E% COD
SCODin (mg/L)
E% SCOD
TDSout (mg/L)
salinity increase (%)
inks + phosphate degreasing inks inks inks inks oil emulsions oil emulsions
19 800 15 200 37 300 21 500 10 200 5 400 5 400
98 99 98 99 97 99 98
21 500 19 750 50 000 30 000 51 000 8 250 9 500
42 65 84 71 50 70 57
10500 6750 9500 9250 32900 2000 2750
13 2 21 17 26 5 -42
na 13 757 19 043 25 210 16 048 9 939 10 585
+38 +16 +46 +34 +42 +8 +41
Table 7. Acidic Sludge Influence on the Fenton Process (Treatments 2 and 5) and Removal Efficiencies (E %) during Jul-Aug 1997 wastewater type
influent
process
E% TS
E% COD
E% SCOD
E% surfactants
TDSout (mg/L)
salinity increase (%)
glues glues glues zinc plating zinc plating zinc plating
acidified acidified acidified acidified acidified acidified
sludge removal Fenton Fenton + sludge removal sludge removal Fenton Fenton + sludge removal
98 98 99 92 94 94
52 71 79 66 68 71
36 50 53 49 45 62
95 97 98 96 100 100
25 010 31 570 25 180
-9 +15 -8
1997. Three strategies were considered in the process reorganization: 1. In the Fenton process two targets were sought: on the one hand, the performances had to be improved and, on the other hand, the Fenton process had to be used as little as possible. Therefore, it was used only for the breakdown of soluble COD, while the particulate COD was removed by settling. This was done to save reagent expenses and to prevent salinity increase problems. 2. The acidification-basification process was used also for wastewater generally treated by the Fenton process by trying to utilize the synergy within different types of wastewaters. 3. Because the ammonia loading was too high to be removed only by a biological process, it had to be partially removed by chemical-physical treatment. The struvite (MgNH4PO4) precipitation process was adopted. After the reorganization, a completely different use of the treatments can be observed: during the period Jul-Aug 1997, 40% of the wastewaters were treated by treatment 1, whereas 33% and 21% were treated by treatments 3 and 4 and treatments 2 and 5, respectively.
Performance Analysis of the Second Period Study. (a) Fenton Process. The Fenton process was used on acidified wastewater or on surfactants when suspended solids were preventively removed by settling with or without filtration. This was performed to investigate the convenience of using a hard process like Fenton in order to break down the soluble COD. Results (Table 7) show that performing only sludge elimination after acidification of glues led to obtain 36% of soluble COD removal, which can be improved to 50-53% using also the Fenton process. In the last case a salinity increase of 15% was also obtained. Analogous results were obtained by treating zinc-plating waste. In both cases surfactant removal was quite total (higher than 95%). Therefore, removal of acidic sludge can be a good alternative to the salinity control. (b) Neutralization Pretreatment. The pretreatment strategies in treatments 1, 3, and 4 were changed in order to avoid a large number of recycles of dilution water (Figure 3). Moreover, all of the ink wastewaters were treated together to give an acidic supernatant to be blended with the glue wastewaters: in this way the
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Table 8. Chemical-Physical Characteristics of the Wastewater Treated by the Coagulation-Clarification Process (Treatments 1, 3, and 4) and Removal Efficiencies (E %) after the Ink Supernatant and Glue Blending wastewater type glues + inks supernatants at pH 3 glues + inks supernatants at pH 3 glues + inks supernatants at pH 3
TSSin (mg/L)
E% TS
CODin (mg/L)
E% COD
SCODin (mg/L)
E% SCOD
surfactantsin (mg/L)
E% surfactants
TDSout (mg/L)
salinity increase (%)
20300
99
22 500
68
4500
7
470
94
17 986
+5
5400
94
8 000
69
3250
35
300
90
13 111
+16
6960
99
12 250
72
2800
29
350
92
14 200
+10
Table 9. Dilution Volumes and Recycle Numbers for the Different Treatments during Jul-Aug 1997 wastewater treatment (m3) 1 2+5 3+4 other
722 361 578 97.2
dilution percentage recycle number water of dilution (m3) (%) min avg max std 285 278 237 46
39.5 77.0 41.0 47.3
1.0 1.0 1.0 1.0
2.0 7.3 1.7 3.8 15.0 4.3 1.6 3.4 0.7 15.1 81.0 32.3
glue acidic sludges can be obtained by saving reagents. This method enabled one to achieve good results in COD, suspended solids, and surfactant removal (Table 8). This performance was similar to the one obtained by the Fenton process application (see Table 7). The soluble COD removal (25%) was lower than the one obtained with the Fenton process (50%); the salinity lightly increased but remained lower than 18 000 mg/ L. Respirometric analysessSOUR, NUR, and AUR6s were carried out to verify the toxic potential of pretreated neutral wastewater and confirmed because the pretreatment effluent was consistent with the biological process. Therefore, the proposed strategy was widely applied to ink and glue wastewaters (Table 9). (c) Ammonia Removal. A number of common solutions could be used in ammonia removal. The stripping process was not considered because of the involved costs and technologies,7 whereas the biological processes have little effect on ammonia removal in industrial wastewater because of inhibitory problems8 and the huge ammonia loading to be treated.9,10 Therefore, a chemical-physical process was applied: the struvite (MgNH4PO4‚6H2O) precipitation. This technology, tested in previous researches on tannery wastewater11 and landfill leachate12 treatment, was chosen because of the availability of phosphorus (phosphate degreasing waste) and ammonia (landfill leachate and inks waste) rich wastewater (see characteristics in Table 10). In particular, the precipitation rather than the crystallization in fluidized-bed reactors13,14 was chosen. The calcium availability in the treated wastewater was checked to satisfy the stoichiometric demand for hydroxyapatite coprecipitation [Ca5(PO4)3OH].15 The ammonia removal batch tests were carried out following two different strategies: on the one hand, the ammonia removal from landfill leachate by using phosphorus and magnesium pure chemicals was checked and, on the other hand, the phosphate degreasing wastewater was used as the phosphorus source, while magnesium was added as the pure reagent. The tests were carried out in a batch mode, in 30 m3 working volume reactors, and the best results are shown in Table 11. Two important aspects were pointed out. The MgO was the best chemical salt for magnesium additions, even though a greater molar ratio was needed: Mg:N ) 2:1 for MgO versus Mg:N ) 1,3:1 for MgCl2. Moreover, the stoichiometric ratio N:P ) 1:1.5 gave the best results in terms of both ammonia removal (as struvite) and hydroxyapatite formation. The last was due to the presence of calcium. When neces-
sary, the phosphorus excess in the effluent was removed by precipitation as AlPO4 or FePO4 at pH 7.5. It is clear that the best strategy was the use of wastewater, as the reagent source, and the pure chemicals only when necessary. The new flow scheme application allowed one to remove the excess ammonia loading (by struvite precipitation) to the biological step. Results Comparison. The impact of the new pretreatment flow scheme was evaluated by the comparison of the results obtained during the two considered periods: Jan-Apr 1997 and Jul-Aug 1997, respectively. The treated loading was quite similar (1490 m3 month-1 in the first period and 1370 m3 month-1 in the second one), but the wastewater characteristics were quite different. In particular, the landfill leachate passed from 320 to 34 m3 month-1, while the paint wastewater passed from 97 to 200 m3 month-1 (see Figure 4). Other wastewater loadings were nearly constant. The effect of the newly introduced chemicalphysical processes was evaluated in terms of treated volumes, number of recycles, and dilution volumes (compare Tables 4 and 9). The treated volume by treatment 1 passed from 7% to 40%; therefore, the dilutions were reduced, and a smaller amount of chemicals was used. Anyway, the average number of recycles was nearly the same: 2.0 recycles instead of 1.9. The wastewater volume treated by strong acidic pretreatments (pH 3 in treatments 3 and 4) passed from 41% to 33%, and the recycle number passed from 3.2 to 1.6. Therefore, a smaller amount of water had to be treated. Finally, the Fenton process was applied to 21% of the treated volume instead of the previous 44%: in fact, the wastewaters which were treated by the Fenton process during the period Jan-Apr 1997 were simply acidified during the period Jul-Aug 1997. Also, the salinity reduction in pretreatment effluent was evaluated. The salinity concentration in the two periods for the different pretreatment techniques was considered. In particular, the treatments 2 and 5 and the treatments 1, 3, and 4 were considered by taking into account the different treated volumes. Equation 2 was used. A 2166 mg/L of dissolved solids decrease was estimated. 2
∆G )
2
(∆SiPi)gen-apr - ∑(∆SiPi)lug-ago ) ∑ i)1 i)1 2166 mg/L (2)
where ∆S ) salinity increase, P ) percentage weight in the considered pretreatment (on volume), i ) 1 for treatments 2 and 5, and i ) 2 for treatments 1, 3, and 4. This target was also confirmed by experimental results which showed a reduction of about 2800 mg/L of dissolved solids in the pretreatment effluent (Figure 5, Jul-Aug). The good results were confirmed also in a
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Figure 4. Treated wastewater volumes (as a percentage of the total) during the two research periods.
Figure 5. Effluent salinity profile during Jul-Dec 1997. Table 10. Chemical-Physical Characteristics of the Wastewater Rich in Phosphorus and Nitrogen wastewater type
pH
alkalinity (mg of CaCO3/L)
landfill leachate inks phosphate degreasing
7-8 7.7-8.5 3.2-6.7
2500-5000 2000-3000 0-2500
P-PO4 (mg/L)
N-NH4 (mg/L)
Ca (mg/L)
400-2000 500-1300
100-300
