Desalination 249 (2009) 337–342

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Treatment of tannery wastewater through the combination of a conventionalactivated sludge process and reverse osmosis with a plane membrane

Sabino De Gisi a,1, Maurizio Galasso b,2, Giovanni De Feo a,⁎a Department of Civil Engineering, University of Salerno, via Ponte don Melillo, 1-84084 Fisciano (SA), Italyb Bierre Chimica Srl, Via Canfora, 59/61-84084 Fisciano (SA), Italy

⁎ Corresponding author. Tel.: +39 089 964113; fax: +E-mail addresses: sdegisi@unisa.it (S. De Gisi), galas

g.defeo@unisa.it (G. De Feo).1 Tel.: +39 089 964113; fax: +39 089 964045.2 Tel./fax: +39 089 8201469.

0011-9164/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.desal.2009.03.014

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 25 March 2009Available online 3 October 2009

Keywords:Activated sludgeReverse osmosisTannery wastewaterWastewater reuse

Tannery wastewater contains high concentrations of organic matter (COD) with a significant percentage ofrefractory organic compounds, ammonium substances, salts (i.e. chloride and sulphate) as well as sulphur.Contaminants have to be removed in order to avoid significant environmental impacts. This paper presentsthe results obtained from a pilot scale study developed in the tannery district of Solofra in Southern Italy. Itwas aimed at evaluating the reuse of wastewater produced in the retanning process. The treatment processconsisted of a biological treatment, as a pre-treatment, followed by a physico-chemical process (with apolymer as a coagulant) and reverse osmosis with a plane membrane. The biological pre-treatment was ableto remove approx. 67% of COD, while the membrane system completed the purification process with theremoval of the refractory organic compounds (chloride and sulphate). In the test carried out, thecombination of a biological pre-treatment with a plane membrane system showed satisfactory results interms of wastewater recovery and reuse in the tannery production cycle.

39 089 964045.so2@interfree.it (M. Galasso),

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Leather tanning is among one of the most important industrialactivities carried out in Mediterranean Europe. There is an importanttannery industrial district in the Campania Region in Southern Italy.The Campania tannery district is in the Province of Avellino andincludes the towns of Solofra, Montoro Inferiore, Montoro Superioreand Serino. In total, it covers an area of 60 km2, with a population of35,000 inhabitants. It is the third largest tannery centre in Italy withabout 400 tanneries, 130 being medium-sized while all the others canbe considered small. There are also approximately 100 workshopsmaking leather items as well as 20 chemical product plants. Thetanneries in Southern Italy were important at the beginning of theprevious century, however, significant development occurred duringthe postwar period, with subsequent increasing impacts on theenvironment. Two wastewater treatment plants (WWTP) wererealized in order to treat the wastewater produced in the tannerydistrict [1]: the WWTP in Solofra which is a chemical–physical andbiological pretreatment plant, and the WWTP in Mercato SanSeverino, a conventional activated sludge treatment plant. Planners

initially preferred to realize centralized rather than on-siteWWTP. Thebiggest tanneries subsequently realized decentralized pre-treatmentplants thus reducing the pressure on theWWTP in Solofrawhich treatsan average daily flow of approx. 10,000 m3/d, with 75–80% of waste-water coming from the tannery district and 20–25% from the nearbyurban areas. In the light of the sustainable development paradigm aswell as the global water scarcity, local authorities and entrepreneursdecided to develop an environmental policy oriented at rationalizingwatermanagement and, in particular,minimizingwater usewhich is afundamental goal for tanneries.

The main aim of the study was to evaluate the reuse of treatedtannery wastewater at sustainable costs. The test was developed in atannery in the town of Solofra, which employs about 100workers. Thestudy was carried out on a pilot scale and was based on thecombination of an activated sludge process and reverse osmosiswith a plane membrane. Tannery wastewater is difficult to treat dueto the high load of organic matter as well as high concentrations ofsulphate, sulphide, chloride and chromium (for a chromium tannery)[2,3]. Moreover, syntans which have not been purified by primary andsecondary treatments inhibit microorganism activity depending onthe size of the molecules, the solubility in water as well as thefunctional groups nature [4,5].

Due to the high chloride concentration (pickling and tanningeffluents) a reverse osmosis (RO) process is necessary in order toreuse tannery wastewater [6]. Moreover, the high organic mattercontent in the tannery effluent requires an exhaustive pre-treatment

Fig. 1. The flow sheet of the treatment system.

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before an RO process [6]. In accordance to the results of theMembraneBio Reactor (MBR) experiments carried out in the centralizedwastewater treatment plant of San Miniato [7,8], in “Santa Crocesull'Arno” tanning district, in Central Italy, it was preferable toeliminate the upstream chemical–physical treatment and sendwastewater directly into the biological activated sludge process,having preventively controlled the pH. The treatment efficacy of thebiological activated sludge process (realized with a low F/M ratio) aspre-treatment at an RO process was therefore tested. The previouslydescribed treatment scheme was also adopted due to the availabilityin the tannery studied of a basin which allowed a biological activatedsludge treatment with a low F/M ratio to be created (equivalent, inthis particular case, to an HRT of about 30 h).

The treatment process scheme had the following phases: barscreens, equalization basin, correction of wastewater pH with soda,biological activated sludge with a hydraulic retention time of 30 h,secondary sedimentation assisted by the addition of polyelectrolyte,sand filtration and reverse osmosis process with a plane membrane(Fig. 1). It is worth noting that this treatment scheme is different from

Fig. 2. The pilot plant of the complete mix aeration tank.

the traditionally used scheme and essentially consists of an upstreamchemical–physical process as a pre-treatment (with a significantincrease in costs) and a biological activated sludge process completelymixed with a low F/M ratio [9]. In the process scheme adopted, thebiological activated sludge assumed the principal role of removing thecontent of organic matter present in the inlet wastewater, while theremoval of ammonium compounds was assured by the RO process. Itis well known that some substances in tannery wastewater (i.e. total

Fig. 3. The pilot plant of the reverse osmosis (RO) system.

Table 1Parameters monitored (⁎) in the conventional activated sludge experiments.

Measured parameters Influent Oxidation tank Effluent

COD ⁎ ⁎

N–NH4+ ⁎ ⁎

N–NO3− ⁎ ⁎

N–NO2− ⁎ ⁎

Conductivity ⁎ ⁎ ⁎

Cl− ⁎ ⁎

SO42− ⁎ ⁎

Non ionic surfactants ⁎ ⁎

TSS ⁎ ⁎ ⁎

MLVSS ⁎

pH ⁎ ⁎ ⁎

DO ⁎ ⁎

Temperature ⁎ ⁎

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chromium) may have an inhibition effect towards denitrification andnitrification reactions. This adverse effect occurs in correspondence toa concentration threshold that in the case of total chrome is approx.120 mg/L [9]. The tannery wastewater studied had a concentration oftotal chromium greater than the threshold of 120 mg/L.

2. Materials and methods

2.1. Pilot plant characteristics

The research activity was carried out on a pilot plant forconventional activated sludge treatment with the following parts:feed tank, feed pump, 0.78m3 completemix aeration tank and settlingtank. The aeration system adopted in the aeration tank was realizedwith fine bubble tubular diffusers (Fig. 2). While, in relation to the ROsystem, tests were carried out with a specific pilot plant (RO 120), asshown in Fig. 3. This plant was equipped with a Disc Tube Module(DT) in polyamide with an effective membrane area of 7.5 m2.

2.2. The phases of experimentation

The study was developed in the following phases: wastewatercharacterization, conventional activated sludge start up, activatedsludge experimentation in a steady state condition and, finally, thedevelopment of reverse osmosis tests. Both exogenous activatedsludge (coming from theWWTP of the Campania tannery district) andselected bacteriumwere used as an inoculum for the start up phase ofthe biological treatment process. Moreover, powdered activatedcarbon and nutrients for controlling foams were used during thetests in the biological phase. In relation to the process conditions, themain parameters of the activated sludge treatment process weremonitored and controlled as shown in Table 1. In the fourth and last

Table 2Influent wastewater characterization.

Parameter Unit Range of values

COD mg/L 2000–7600N–NH4

+ mg/L 80–160N–NO3

− mg/L 200–510N–NO2

− mg/L 0Conductivity µs/cm 3000–10,500Cl− mg/L 1200–2060SO4

2− mg/L 900–2270Non ionic surfactants mg/L 35–80Hardness °F 50–60TSS mg/L 500–800pH mg/L 2.5–5.5S2− mg/L 2Cr3+ mg/L 120

phase, after sedimentation, the wastewater was treated in thefiltration unit (sand filter) and subsequently used in the RO pilot plant.

2.3. Analytical methods and wastewater characteristics

The inlet wastewater to the conventional activated sludge processwas taken from the equalization basin present in the oldWWTP of thetannery industry. Before being biologically treated, the tannerywastewater was processed in the following units: bar screens,equalization basin and wastewater pH correction through theaddition of soda. Raw wastewater was characterized in relation tothe standard methods [10] and the results obtained are shown inTable 2. It is worth noting that the concentration of COD and saltswere highly variable with typical values of raw wastewater of low–

middle size tanning industries (Table 2).

2.4. Operating conditions

In relation to the conventional activated sludge treatment, eachtest included the following phases: aerobic reaction loading with inletwastewater after a pH correction, wastewater aeration in a completemix aeration tank, accumulation and sedimentation of treatedwastewater after the biological treatment. The inlet flowrate to thebiological reactor was equal to 26 L/h, corresponding to a hydraulicretention time of 30 h. The DO concentration in the aeration tank was5 mg/L, while the concentration of mixed liquor volatile suspendedsolids (MLVSS) was maintained at approx. 8000 mg/L. The sludgerecirculation was realized by adding a known quantity of secondarysludge coming from the sedimentation tank (determined on the basisof biological tests previously carried out). Finally, the wastewatertemperature in the biological reaction ranged between 24 and 27 °C.

In relation to the reverse osmosis (RO) with a plane membrane,tests were carried out using three different values of transmembranepressure (TMP), respectively of 80, 85 and 90 bar, a feed flowrate of950 L/h, a temperature of 25 °C and a pH of 7.5.

The TMP values were used due to the need to maximize thepercentage of the permeate, evaluated on the basis of the rawwastewater entering the RO system, in order to reduce as much aspossible the flowrate to discharge into the sewerage system and,consequently, disposal costs. The greater quantity of energy requiredfor the application of the adopted TMP values did not constitute aproblem due to the quantity of energy required for the depurationbeing rather small when compared to the energy required for thewhole tannery production cycle.

Finally, the permeate and concentrate streams were recycled in asuitable feed, while, during the tests, the permeate flux (JP) was

Fig. 4. Activated sludge inlet and outlet COD.

Table 3Results of the conventional activated sludge treatment process.

Number ofexperiments

CODin

[mg/L]COD removal[%]

CODremoved

[mg/L]F/M ratio[kg COD/kg MLVSS d]

HRT[h]

Conductivity[µS/cm]

T [°C] DO[mg/L]

pH Feedflowrate [L/h]

1 3700.0 60 2220.0 0.37 30 7340 24.3 4.87 8.10 262 3812.0 68 2,592.2 0.38 30 7500 25.0 4.73 8.34 263 3840.0 69 2649.6 0.38 30 6945 25.0 4.71 8.25 264 3960.0 61 2415.6 0.40 30 7020 25.1 4.71 8.02 265 4130.0 78 3221.4 0.41 30 7300 25.1 4.70 8.30 266 4580.0 63 2885.4 0.46 30 7900 25.2 4.67 8.50 267 4689.0 67 3141.6 0.47 30 7600 25.2 4.65 8.50 268 5800.0 70 4060.0 0.58 30 7401 26.2 4.32 8.50 269 7223.0 68 4911.6 0.72 30 8780 26.5 4.03 8.45 2610 7341.0 69 5065.3 0.73 30 8900 26.5 4.02 8.36 2611 7475.0 67 5008.3 0.75 30 9340 26.6 4.00 8.46 2612 7560.0 72 5443.2 0.76 30 9453 26.8 3.98 8.01 2613 4490.0 65 2918.5 0.45 30 10,440 25.2 4.69 8.27 2614 4506.0 54 2433.2 0.45 30 8000 25.2 4.68 8.00 2615 6807.0 68 4628.8 0.68 30 10,110 26.4 4.05 8.24 2616 4300.0 72 3096.0 0.43 30 7680 25.1 4.70 8.20 2617 4390.0 70 3073.0 0.44 30 8030 25.1 4.70 8.23 2618 6340.0 70 4438.0 0.63 30 9567 26.3 4.21 8.30 2619 5300.0 66 3498.0 0.53 30 8430 26.0 4.48 8.10 2620 5450.0 65 3542.5 0.55 30 8300 26.1 4.40 8.04 2621 5967.0 60 3580.2 0.60 30 9500 26.2 4.31 8.00 2622 3781.0 69 2608.9 0.38 30 8741 24.8 4.80 8.20 2623 5265.0 65 3422.3 0.53 30 8300 25.3 4.53 8.00 2624 6729.0 70 4710.3 0.67 30 10,040 26.3 4.05 8.58 2625 6460.0 69 4457.4 0.65 30 10,340 26.3 4.18 8.45 2626 5730.0 68 3896.4 0.57 30 8356 26.2 4.32 8.20 2627 7000.0 65 4550.0 0.70 30 9320 26.4 4.05 8.50 2628 7160.0 65 4654.0 0.72 30 8428 26.4 4.04 8.12 2629 6573.0 71 4666.8 0.66 30 10,200 26.3 4.10 8.21 2630 5946.0 74 4400.0 0.59 30 9340 26.2 4.31 8.61 2631 4693.0 59 2768.9 0.47 30 8933 25.3 4.61 8.24 2632 6200.0 68 4216.0 0.62 30 8410 26.3 4.30 8.40 2633 5563.0 64 3560.3 0.56 30 8424 26.2 4.33 8.07 26Average 5538.2 67 3719.2 0.55 30 8617 25.8 4.40 8.27 26Dev. St. 1243.6 5 921.0 0.12 0 1003 0.7 0.29 0.19 0

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measured periodically. As soon as the tests were finished, thepermeate and concentrate samples were analyzed in the laboratory.

3. Results and discussion

3.1. Conventional activated sludge treatment

The results of the activated sludge treatment tests are presentedand discussed in this paragraph, in relation to a steady state condition.The following parameters were monitored: CODinfluent, COD removal(in percentage), hydraulic retention time (HRT), conductivity,temperature, dissolved oxygen concentration and pH.

Fig. 5. Applied and removed loads of COD.

The biological reactorwas fedwith 26 L/h ofwastewater, with a HRTof 30 h. Table 3 reports the activated sludge treatment results. On thewhole, the outlet COD averaged approx. 1819 mg/L (Fig. 4). Fig. 5reports that the COD percentage removal was 67%, while Fig. 6highlights that there is no relationship between COD percentageremoval and COD inlet into the reactor. Table 3 does not show theresults in terms of ammonia compounds (organic ammonia, ammonia,nitrate and nitrite). The activity carried out showed a low percentageremoval of ammonia compounds, being mainly due to a value of totalchrome in the influent wastewater greater than 120 mg/L [9]. Finally,Fig. 7 shows the relationship between DO concentration in theoxidation reactor and F/M ratio, while Fig. 8 reports the relationshipbetween DO concentration and average mixed liquor temperature.

Fig. 6. Percentage COD removal vs applied COD load.

Fig. 8. DO concentration vs temperature in the aeration tank.

Fig. 7. DO concentration vs the F/M ratio in the aeration tank.

341S. De Gisi et al. / Desalination 249 (2009) 337–342

3.2. RO experiments

Table 4 shows the characterization of the feed, the permeate andconcentrate streams for the RO experiments, carried out in the socalled RO 120 pilot plant. The removal percentage of all theparameters was very high and, therefore, the permeate streamcould be reused in the tannery production cycle. In fact, the CODremoval percentage for a TMP of 90 bar was 97.4%. Ammoniumsubstances removal (not sufficiently removed by the activated sludgetreatment) was 96.1% and 98.5%, respectively for ammonia andnitrate. While, in relation to salts, chloride and sulphate removal was98.8% and 99.8%, respectively.

In order to reuse a permeate stream in the tannery productioncycle, the hardness value is very important. A hard permeate can

Table 4Feed, permeate and concentrate characteristics in RO experiments.

Sample Soluble COD[mg/L]

N–NH4+

[mg/L]N–NO3

−

[mg/L]N–NO2

−

[mg/L]Conductivity[µS/cm]

Feed 1174.0 86.3 221.0 0 6310.0Permeate80 bar

40.2 3.4 5.1 0 66.0

Permeate85 bar

35.0 3.5 4.1 0 53.5

Permeate90 bar

30.8 3.3 3.2 0 41.9

Concentrate80 bar

2227.0 128.4 419.2 0 11,970.1

Concentrate85 bar

2783.8 157.9 467.5 0 13,770.0

Concentrate90 bar

3028.8 169.6 474.5 0 14,250.0

create encrustation in pipes as well as other equipment (i.e. boilers).For the three tests with as many different values of transmembranepressure (TMP), a permeate with an almost inexistent hardness wasobserved (Table 4), confirming that the permeate stream obtainedcould be directly reused for vapour production.

The results obtained in relation to the chemical property of theconcentrate (Table 4) confirm the possibility for a direct dischargeinto the sewerage industrial system, according to establishedcompliance limits.

The variation of the permeate fluxes with time for the threeadopted transmembrane pressures can be observed in Fig. 9. For thethree TMP values, the steady state condition was reached after a briefperiod of approx. 20 min. In particular, at 90 bar, the permeate fluxwas about 94 L/h m2, in a steady state regime. The recovery ratepercentages (permeate flowrate/feedwater flowrate ratio) were61.0%, 70.3% and 74.7%, respectively for 80, 85 and 90 transmembranepressure. Scaling and fouling problems cannot be observed in thetests. Even though the tests results were considered appropriate,longer tests should be carried out in order to study in greater detailthe behaviour of the membranes.

4. Conclusion

The main aim of the tests carried out on a pilot scale was toevaluate the technical feasibility of the reuse of treated tannerywastewater in the industrial production cycle, thus reducinggroundwater consumption. The process studied was based on aconventional activated sludge process as pre-treatment to a subse-quent reverse osmosis system. The use of reverse osmosis (RO) with aplane membrane was necessary due to the high salt content in thetreated tannery wastewater, produced by the tannery industriesbased either on vegetable or chromium processes. During the testperiod, a valuable pre-treatment capability of the activated sludgeprocess was observed in terms of COD removal (around 67%). Thehydraulic retention time in the aeration tank was 30 h, while thedissolved oxygen content was about 5 mg/L. A total chromiumconcentration greater than 120 mg/L did not allow nitrificationreactions to develop. Therefore, only reverse osmosis was allocated tothe ammonium substances removal. Moreover, after the biologicaltreatment process, COD averaged approx. 1819 mg/L and its removal,as well as for ammonium substances, was carried out through reverseosmosis. The high quality of the permeate produced by the RO systemwith a plane membrane allowed a reuse in the tannery productioncycle, thus reducing groundwater consumption. While, in relation toconcentrate substances, the results obtained highlighted the feasibil-ity of the direct discharge into the sewerage system. Even though thetest results were considered appropriate, longer tests should becarried out in order to study in greater detail the behaviour of the

Chloride[mg/L]

Sulphate[mg/L]

Non ionic surfactant[mg/L]

Hardness[°F]

TSS[mg/L]

pH

1917.0 1166.0 9.15 50.0 610 7.1437.9 3.0 0.25 0.9 0 7.05

30.5 1.4 0.18 0.7 0 7.01

23.0 1.2 0.16 0.6 0 6.88

3636.5 2211.9 15.05 0.0 0 8.16

4240.0 2646.2 37.50 0.0 0 8.21

4856.0 2748.4 38.75 0.0 0 8.23

Fig. 9. Evolution of permeatefluxeswith operating time at three different transmembranepressure in RO experiments.

342 S. De Gisi et al. / Desalination 249 (2009) 337–342

membranes. Finally, in the pilot plant study, the membrane systemcoupled with a conventional biological treatment process, as a pre-treatment, appeared useful in order to directly recover and reusetreated wastewater in the same tannery production cycle.

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