E L S E V I E R Desalination 128 (2000) 257-267

DESALINATION

www.elsevier.com/Iocate/desal

Treatment of wastewater effluent from an industrial park for agricultural irrigation

• a* S.H. Lln , H.Y. Chan a, H.G. Leu b

aDepartment of Chemical Engineering, Yuan Ze University, Chungli, Taoyuan 320, Taiwan Fax: +886 (3) 455-9373; email: ceshlin@saturn.yzu.edu.tw

;'County Environmental Protection Bureau, Taoyuan 330, Taiwan

Received 2 November 1999; accepted 3 January 2000

Abstract

Treatment of wastewater effluent from an industrial park for possible agricultural irrigation was investigated. The treatment method consisted of chemical coagulation, Fenton oxidation and ion exchange. The effectiveness of the combined physical and chemical methods in treating the industrial wastewater effluent was evaluated in terms of the water quality requirements for agricultural irrigation. Experimental tests were conducted to examine the effects of various operating variables of each treatment unit on the water quality and the optimum operating ranges of those variables were identified. The water quality of wastewater effluent after the combined chemical treatments was found to be very good, exceeding the agricultural irrigation standards. A preliminary economic evaluation of the treatment cost is also provided.

Keywords." Industrial park wastewater effluent; Chemical coagulation; Fenton oxidation; Ion exchange; Agricultural irrigation

1. I n t r o d u c t i o n

During the past decades, rapid industrial developments in Taiwan have been putting an increasing strain on the water resource require- ments in this island country. The demand for quality water resources in the industrial and agricultural sectors will be difficult to meet in the

*Corresponding author.

foreseeable future because o f dwindling supply. The imbalance in the demand and supply o f water resources will become a common major issue confronting many countries around the world as well in the next few decades [1,2].

An important user o f water resources in Taiwan is the industrial park that has been formed in various locations around the island for the last two decades. An industrial park consists

0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 0 ) 0 0 0 4 0 - 0

258 S.H. Lin et al. / Desalination 128 (2000) 257-267

of various types of manufacturing plants clustered in a well defined area. The industrial park in Chungli, a medium size city of 360,000 population in northern Taiwan, is typical. Located in this industrial park are over 450 manufacturing plants, including auto-making, steel milling, heavy machinery, pharmaceutical, food, plastic, textile, paper-making, heavy electric, electronic, computer peripheral, fine chemicals.

Currently the daily wastewater generation from the manufacturing facilities is over 30,000 tons. The bulk of these wastewaters is collectively treated by a common activated sludge wastewater treatment plant. This treatment is primarily intended to lower the pollutant concentration of the wastewater effluent to the discharge standards required by the government. Currently, the wastewater effluent from the activated sludge wastewater treatment plant barely meets the discharge standards and thus direct discharge is permitted. However, due to dwindling supply and increasing demand for quality water resources in the agricultural sector, a better alternative to direct discharge of the wastewater effluent is to elevate its water quality further to an appropriate level for possible agricultural irrigation. Thus far little attention has been paid to this aspect.

The objective of this study is to treat the wastewater effluent from the activated sludge wastewater treatment plant of the industrial park and elevate its water quality to the reuse standards for agricultural irrigation. The treatment method employed includes chemical coagulation, Fenton oxidation and ion exchange. Chemical coagulation using polyaluminum chloride (PAC) and polymer (coagulation aid) was found effective as a pretreatment means for partial removal of suspended solids (SS), color and chemical oxygen demand (COD) of many industrial wastewaters [3,4]. On the other hand, Fenton oxidation process offered efficient removal of organic compounds in various types

of wastewater [5-11]. The residual inorganic (salt) and metal ions that are primarily respon- sible for high conductivity and total dissolved solids (TDS) were effectively lowered by ion exchange [11,12]. Batch and continuous experi- mental tests were conducted to evaluate the effectiveness of each treatment unit and to identify the optimal operating conditions. Based on the test results, preliminary economic evaluation was performed to estimate the system cost of the combined treatment method.

2. Materials and methods

The present experimental investigation consisted of three major parts: chemical coagu- lation, Fenton oxidation and ion exchange, as shown in Fig. l(a). The three treatment units could be operated in batch or continuous mode. The batch operation was convenient for determining the operating conditions of each unit. In the chemical coagulation, PAC and polymer (coagulant aid) were used. The treatment was performed in an apparatus shown in Fig. l(b). The PAC and polymer tanks had volumes of 5 and 101, respectively, and both tanks were equipped with a variable mixer. The wastewater was fed at a constant rate of 0.251/min to maintain residence times of 20 and 40 min for PAC and polymer treatments, respectively. The mixer speeds were set at 100 and 30 rpm in these two tanks, respectively. While the amount of PAC addition varied, the relative amount of PAC to polymer was kept at 100:1 for good treatment results according to past experience [3,4]. After chemical coagulation, the wastewater was allowed to settle for 1 h in the sedimentation tank. Samples were then taken from the clear waste- water effluent from the tank for water quality measurements.

The Fenton reactor, as shown in Fig. l(c), consisted of rectangular tank which was divided into four sections. Each section had a size of

S.H. Lin et al. /Desalination 128 (2000) 257-267 259

(a)

Wastewater

I.nfluent

Chemical

Coagulation ~ Fenton ~ Ion

] - Reactor Exchange

Treated .=

Wastewater

(b)

_ J Waslewaler

pH Control PAC

Che,nical

Pol,'mer

Coagulalion Sedimentation

(c)

Waslewa{er

pH Control

H20: FeSO4

Fenton Reactor

Fig. 1. Experimental schematics of the overall and individual treatment systems.

25 cm (H) x 25 cm (W) x 20cm (L) for a total reactor volume of 50,000 cm 3. During the oxida- tion reaction, a constant mixing speed of 100 rpm was maintained in each section. The ratio of hydrogen peroxide (H202) to ferrous sulfate (FeSO4) was kept at 1:1 according to past experience [5-11], but the optimum amount of H202 remained to be determined.

In a test run, the wastewater effluent from chemical coagulation was fed by a micro-pump at a rate of 830 cm 3 for pH adjustment. It then went

to the Fenton reactor with proper amounts of H2O z and FeSO4 added in the first section. After approximately 1 h of treatment in the reactor, the wastewater effluent was allowed to settle for 1 h in the sedimentation tank. Samples were taken after sedimentation for water quality measure- ments.

Ion exchange was conducted in a column apparatus, shown in Fig. 2. Each resin column had a diameter of 2 cm and height of 30 cm. The ion exchange resins employed were the H-type

260 S.H. Lin et al. / Desalination 128 (2000) 257-267

Pump

Wastewater

k.~ N\"~ ~,.~ H-Type

OH- Type Resin

Fig. 2. Experimental schematic of ion exchange.

Amberjet 1500 and the OH-type Amberjet 4000, both obtained from Rohm and Haas Co. (Philadelphia, PA, USA). These two resins were used primarily for removing the inorganic and metal ions and for lowering the TDS and conductivity. These resins were pretreated with acetone to remove potential impurities on the resin surfaces. They were then immersed in n-hexane for 1 h and washed several times with deionized water. They were finally dried in an electric oven at 50°C overnight. The amounts of dry H- and OH-type Amberjet resin packed in the column was varied using four different ratios of 10:10, 20:20, 10:20 and 15:30 (g weight ratio). The wastewater effluent from the Fenton oxida- tion process was pumped through the ion exchange columns at a constant flow rate of 5cm3/min. The water quality of wastewater effluent exiting the ion exchange column was determined.

The important water quality parameters measured for all samples included COD, color (ADMI), NTU, TDS, surfactant (ABS), conduct- ivity and inorganic and metal ion concentrations. The COD, ADMI, TDS, ABS and NTU were determined using standard methods [13]. The conductivity was measured using a Suntex SC- 12 conductivity meter (Suntex Industrial Co., Taiwan). The metal ion concentrations were measured using a GBC 932 atomic absorption spectrophotometer (GBC Scientific Equipment, Victoria, Australia). The inorganic anions (SO4, NO~ and Cl-, etc.) were analyzed with a Dionex 2000 ion chromatography (Dionex Corp., Cali- fornia, USA).

3. Results and discussion

3.1. Characteristics of wastewater effluent and irrigational water quality requirements

During the experimental test period of the present study, the water quality of the wastewater effluent from the industrial park wastewater treatment plant was monitored for over 11 months. Fig. 3 shows the monthly variations of COD concentration and transparency. The transparency was measured by a device similar to the one for measuring the Jackson unit [13]. It consists of a 4-cm-ID glass cylinder 40cm high sitting on a piece of white plastic with a black cross mark and against a 5-cm-wide black plastic strip on the side. Near the bottom of the glass cylinder a wastewater outlet is provided, and on the cylinder surface is a graduated scale (in centimeters) starting from the center of the bottom wastewater outlet.

The wastewater transparency is measured on spot and in the broad light by filling the glass cylinder. The wastewater in the cylinder was let out until the bottom cross mark is visible. The reading on the surface scale of the cylinder is the wastewater transparency. Apparently the transparency represents a measurement lumping

S.H. Lin et al. / Desalination 128 (2000) 257-267 261

200 180 30

160 ~ ~ 6;

O 140 ~ d , N C~ 42

120t 1 / , , 7 7 , , ~ t '15~

100 ~ 10 0 1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 7 8 9 10 11

Month

Fig. 4. Monthly variations of dissolved oxygen and ammonia nitrogen concentrations ofwastewater effluent.

Month

Fig. 3. Monthly variations of COD concentration and transparency of wastewater effluent.

10

• o

<

4

the SS, color, turbidity, etc., into a single parameter. It is easy to measure and quite effective to represent an important aspect of water quality. The required transparency of wastewater effluent for direct discharge in Taiwan is 30 cm. Apparently the present waste- water effluent monitored was not up to this standard. However, the COD concentration shown in Fig. 3 was better than the required 200 mg/l standard for direct discharge.

The monthly variations of the dissolved oxygen (DO) and ammonia nitrogen (NH3-N) concentrations are demonstrated in Fig. 4. The DO and NH3-N concentrations of natural (unpolluted) stream stipulated by the government in Taiwan are 6.5 and 0.5 mg/l, respectively. On both accounts, the wastewater effluent was below the standards by a significant margin. The monthly variations of sulfate (SO4) and chloride (CI ') concentrations and conductivity of the wastewater effluent are displayed in Fig. 5. Although these three parameters are not regulated, they are far higher than those usually found in a natural stream. The overall water quality of the wastewater effluent from the

4= 8 I

..a.,

0 0

o 6

2,

4 r.-,

o r._)

800

C l "

2 E I 1 1 I l [ ! 2

, % 600

400 3

" 4

O 200

0 3 4 5 6 7 8 9 t0 11

Month

Fig. 5. Monthly variations of conductivity and sulfate and chloride concentrations of wastewater effluent.

wastewater treatment plant is far from the standards for agricultural irrigation or any purpose, and hence further treatment of this wastewater effluent by the combined treatment method proposed in the present study is in order.

262 S.H. Lin et al. / Desalination 128 (2000) 257-267

Table l Water quality requirements for agricultural irrigation in Taiwan

Item Long-term Short-term usage a usage b

pH 6-9 5.5-9 Conductivity,/~mho/cm 750 1200 SS, mg/l 100 250 Chloride, mg/l 175 280 Sulfate, mg/l 200 300 Total N, mg/l 1 10 Surfactant, mg/l 5 l 0 Oil/grease, mg/l 5 10 AI, mg/l 5 20 Cr, mg/l 0.1 0.5 Cd, mg/l 0.01 0.02 Cu, mg/l 0.2 0.4 Pb, mg/l 0.1 10 Mn, mg/l 2 2 Hg, mg/l 0.005 0.005 Ni, mg/l 0.5 0.5 Zn, rag/1 2 4 Temp., °C 35 35

aContinuously used for all types of soil. bUsed for no more than 15% of total irrigation water.

The water quality requirements for agricul- tural irrigation are not as rigorous as these for drinking water. Also there are no unified standards for this purpose, and often these standards vary greatly from country to country [ 14,15]. In addition, the water quality standards for agricultural irrigation are usually not strongly enforced by the government. Table 1 lists the revised water quality standards adopted in Taiwan [16]. The list is not an official one and is usually used as a reference.

From this table it is apparent that there exist some deficiencies in the listed standards. For example, the COD concentration and turbidity (in terms of NTU or transparency) are not stipulated.

For practical reason, these two parameters are believed to be rather important for agricultural irrigation. Hence in the present study, the COD concentration and NTU need to be reduced to as low levels as possible.

3.2. Effects o f operating variables on the water quality

The two important operating parameters to be determined in chemical coagulation are the amount of PAC required and pH for good treatment performances. Fig. 6 demonstrates the changes in COD and color removal and NTU as a function of the amount of PAC, noting that the amount of polymer is kept as 1% of PAC, as mentioned earlier. The figure clearly reveals that 100 mg/l of PAC yield quite satisfactory results. The COD and color removal achieved are 47% and 76%, respectively. At this removal level, the color of wastewater effluent was turned faint, only slightly visible to the naked eye, from the original red, blue, brown or grey. Due to the low NTU below 3 achieved in the treatment, the wastewater effluent became clear.

The treatment results of chemical coagulation are very satisfactory indeed. The effect o fpH on chemical coagulation is shown in Fig. 7. Apparently the pH effect on the water quality of the treated wastewater effluent is marginal and hence pH adjustment before chemical coagulation is not needed.

In the Fenton oxidation process, the reactor was initially filled with wastewater effluent before treatment started. Hence a start-up transient was expected in the measured water quality at the reactor outlet. Fig. 8 displays the COD transients for two wastewater flow rates. The COD removal was improving steadily with the elapsed time until a pseudo-steady state was reached. The time to reach the pseudo-steady state is seen to decrease with a decrease in the wastewater flow rate, varying from about 120 min for 870 ml/min to 90 min for 580 ml/min.

S.lt. Lin et al. / Desalination 128 (2000) 257-267 263

80 6 100

>. ©

E

60 / ~ C o l o r

+- COD

2

1

0 ] I I i t I 1 , I L / 0 20 40 60 80 I00 120

PAC, mg/l

Fig. 6. COD, color rernoval and NTU change as a function of the PAC concentration with initial pH 7.1, COD 121 rag/l, ADMI 36 and 100:1 PAC/polymer ratio.

80

60 > O

E 40

i

2O

0 ~

6

+- Color A 5 4 _ - ~

+-- COD

NTU --~ ~- i_ , i i 5 6 7 8

pH

3 ~ Z

)2

Fig. 7. COD, color removal and NTU change as a function ofpH with initial COD 143 mg/l, ADM146 and 100 mg/l PAC.

80

ct3 > O

©

Flow Rate:580 mWmin

--i-- Flow Rate:870 ml/min

60

i 40 -

0 °

0 40 80 120 160 200

Time, min

Fig. 8. COD removal transients for two wastewater effluent flow rates of continuous Fenton treatment with inlet pH 3, COD 69 rag/l, H202 and FeSO 4 100 mg/l each.

According to the previous investigations [3-10], a 1:1 ratio of H202 to FeSO 4 would

provide good treatment results of the Fenton oxidation process. Furthermore, an optimum pH around 3-4 was also observed for Fenton oxidation. The optimum HzO2/FeSO4 ratio and pH were adopted in the present study. However, the amount of H202 or FeSO4 remained to be determined experimentally. Fig. 9 shows the COD removal as a function of the elapsed time for two different H2O 2 concentrations. The COD removal transients approached a pseudo-steady state in approximately 120 min. A significantly higher pseudo-steady-state COD removal was achieved with 100 mg/l H202 than with 70 mg/1. Hence a 100mg/l dosage of HzO 2 is a better choice for Fenton oxidation. A HzO 2 dosage higher than 100 mg/l was not considered because the Fenton oxidation process seldom achieved 90% COD removal [5-11 ], and thus there was relatively little room for further improvement of COD removal above the 81% obtained here.

Table 2 lists the water quality changes before and after the Fenton oxidation treatment for three typical test runs. It is apparent from this table that removal of organic compounds and oil/grease by

264 S.H. L ine t al. /< Desalination 128 (2000) 257 267

I00

80 --- . . . . . . . . . . . . . . ~ , ~ h ~ - ~ i , - - - - - ' - -~:~- - ©- " " I~ '~ " -

~ .

~ 60 > o

40 Hi o mg/l © r..)

2O

0 40 80 120 160 200

Time, rain

Fig. 9. COD removal transients for two H202 concen-

t rat ions of continuous Fenton treatment with inlet pH 3, COD 57 mg/I and 1:1 H202/FeSO 4 ratio.

the Fenton process is quite good. This oxidation process had little effect on the inorganic ion removal . The SO~ and C1- concentrat ions were

seen to be quite high, far higher than the agricultural irrigation standards. In fact, the

Fenton process caused notable increases in SOL TDS and conductivi ty due to the FeSO 4 added in this process. To lower the TDS and conduct ivi ty and remove inorganic ions, ion exchange offers a good remedy.

Much like that o f the Fenton process, the

relative amounts o f the H- and OH- type Amber je t resins required for good t reatment results need to

be determined in the experimental tests. Hence in the column test runs, the relative amounts o f these resins were chosen as 10:10, 20:20, 10:20

to 15:30 (g H- type:g OH-type) . Fig. 10 displays

the changes o f conductivi ty and pH o f the wastewater effluent exiting the ion exchange

Table 2 Water quality changes before and after the Fenton treatment (all units in mg/l except for conductivity in #mho/cm)

ADMI COD TDS SO; C1 NO 3 NH3-N Conductivity ABS

Before 21 80 2167 658 376 21 4.6 3691 0.15 After 8 20 2330 822 382 23 3.3 4490 0.0l Before 17 131 1810 479 385 18 2.1 3290 0.17 After 4 62 2048 678 389 17 1.3 3002 0.01 Before 35 120 2422 527 390 25 5.6 4041 0.31 A~er 12 60 2570 701 392 24 3.9 4660 0.08

Table 3 Water quality changes of ion exchange (all units in mg/I except for conductivity in #mho/cm)

pH TDS NH3-N Cond. SO 4 C1 NO 3 Cu Zn Pb Fe

Influent 7.21 10:10 3.66 20:20 3.71 10:20 6.07 15:30 6.32

2345 3.28 3830 521 221 31 1.29 1.24 0.19 0.59 17 0.53 87 1.2 1.7 ND 0.04 0.15 0.03 0.05 36 0.51 55 0.9 1.4 ND 0.06 0.19 0.02 0.03 21 1.13 72 0.3 1.2 ND 0.03 0.61 ND 0.01 ll 0.56 81 0.8 1.1 ND 0.03 0.13 ND 0.01

4000 ~_

E 3000

2, "~ 2000 " , ~

c -

G

1000

S.H. Lin et al. / Desalination 128 (2000) 2 5 ~ 2 6 7

. . . . . . . . . . . . . . . . . . . . . _ m 2

0 k ' ~ , a! =,.~4---~ t ~ ''m t ~ 0 0 60 120 180 240 300 360

Time, min

265

Fig. 10. Conductivity and pH changes as a function of the

elapsed time of ion exchange with initial pH 5.2l ,

conductivity 3830 ~ho/cm, 5 ml/min wastewater flow rate

and 20 g each of the H- and OH-type ion-exchange resins.

column for a 20g:20g ratio of the Amberjet resins. With a wastewater flow rate of 5 ml/min, the residence time of the wastewater effluent passing through the column was about 15 min, which was found sufficient for ion exchange in the several batch test runs. The figure reveals that both conductivity and pH of the wastewater effluent reached essentially a constant level before the resins became saturated. At that point there appeared a rapid decrease ill pH and a sharp rise in conductivity. The horizontal dashed line corresponds to the agricultural irrigation standard for conductivity (750~ho/cm). At that conduct- ivity level, the corresponding pit of the waste- water effluent was low at 2.45, which is not acceptable.

The effects of various resin ratios on the water quality of treated wastewater effluent are demonstrated in Table 3. The water quality was obtained from the wastewater effluent exiting the ion exchange column at the time before the rapid increase in conductivity (i.e., the saturation point). Apparently ion exchange in any weight

ratio employed here is highly effective in removing the inorganic and metal ions and lowering TDS and conductivity. However, the pH of the wastewater effluent is seen to be strongly weight ratio dependent. With equal amounts of ion exchange resins, the wastewater effluent was turned into acidic. But with the 1:2 ratio, the wastewater effluent pH became much closer to neutrality. In view of this and economic considerations, the 10g:20g ratio of ion- exchange resins is considered as the best choice for the present tests. The final water quality of the wastewater effluent exiting the ion-exchange process is very good, exceeding the agricultural irrigation standards.

3.3. Preliminary economic evaluation

To estimate the treatment cost of the com- bined treatment method, it is necessary to consider the three treatment units separately. Chemical coagulation consumed 100mg/1 PAC and 1 mg/l polymer. PAC was supplied by the supplier in 11% w/w concentration and cost $0.188/kg (all cost figures in US dollars). Hence, the PAC required for treating the wastewater effluent cost $0.173 per ton of the wastewater effluent treated. Although the unit price of polymer was over five times higher than that of PAC, its small amount used in the treatment adds only 5% to the PAC cost and hence the polymer cost was negligible.

For optimum treatment, the Fenton oxidation process required 100mg/l each of H202 and FeSO4. Industrial grade H202 was supplied in 35% w/w concentration and cost $0.375/kg. Industrial grade FeSO4 had a 98% w/w concen- tration and cost $0.313/kg. Based on the amounts required and the unit prices, the Fenton treatment was estimated to cost $0.139/ton.

To estimate the treatment cost of ion exchange, 10g H-type and 20g OH-type resins were considered as the optimum requirement. According to the test results, these resins needed

266 S.H. Lin et al. / Desalination 128 (2000) 257-267

regeneration in 185 rain, and the wastewater effluent treated in this period totaled 950 ml. According to the manufacturer, the Amberjet resins could last 3 years, and the unit prices for both resins were the same at $25/kg. Based on the operating conditions and the unit prices, the linearly amortized cost of ion exchange Was found to be $0. 169/ton on the basis of 200 days of operation per year. Assuming the regeneration cost of exhausted ion-exchange resins to be 20% of the amortized one, the final cost of ion exchange was $0.203/ton.

The total treatment cost of wastewater effluent by the present combined treatment method came to 0.515 per ton of the wastewater effluent treated. This treatment cost was reason- able in view of the high pollutant contents of the wastewater effluent adopted for the present study.

4. Conclusions

Wastewater effluent from the secondary wastewater treatment plant of a large industrial park in northern Taiwan was treated by combined chemical treatment lnethods for possible agri- cultural irrigation. The treatment process consists of chemical coagulation, Fenton oxidation and ion exchange. Chemical coagulation provides good pretreatlnent of the wastewater effluent by reducing over 50% of COD and color. The Fenton oxidation treatment is effective in removing the majority of the remaining COD and color. Ion exchange offers an attractive means for removing the residual inorganic and metal ions and for lowering the conductivity and TDS.

Experimental test runs were conducted to examine the performance characteristics of the treatment units and identify the optimum operating conditions. Based on the test results, the following conclusions can be drawn:

1. The wastewater effluent from the second- ary wastewater treatment plant of the industrial

park had a COD concentration between 150 and 200 mg/l which meets the direct charge standard permitted by the government. However, the rest of water quality parameters did not comply with the discharge requirements.

2. The optimum operating conditions identi- fied in the experimental test are: 100mg/l PAC and 1 mg/l polymer for chemical coagulation, 100 mg/1 for both H202 and FeSO4 for the Fenton oxidation treatment and 10g H-type Amberjet 1500 and 20g OH-type Amberjet 9000 ion- exchange resins. Under these operating condi- tions, the water quality of the treated wastewater effluent was excellent, amply meeting the standards for agricultural irrigation.

3. Based on the test results obtained in the test runs, a preliminary cost of the combined treatment method was attempted and found to be $0.515 per ton of the wastewater effluent treated. The total treatment cost is deemed reasonable considering the high pollutant contents of the wastewater effluent entering the treatment system.

Acknowledgments

The authors are grateful to the County Environmental Protection Bureau, Taoyuan, Taiwan, ROC, for the financial support of this project.

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