Artigo 13 - Coagulação/floculação

SAVE OFFLINE

Chemical Engineering Journal 254 (2014) 283–292

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Optimisation of coagulation/flocculation for pre-treatment of high strength and saline wastewater: Performance analysis with different coagulant doses G. Di Bella ⇑, M.G. Giustra, G. Freni Facoltà di Ingegneria e Architettura – Università degli Studi di Enna ‘‘Kore’’, Cittadella Universitaria, 94100 Enna, Italy

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 24 February 2014 Received in revised form 29 April 2014 Accepted 27 May 2014 Available online 4 June 2014 Keywords: Cost analysis Jar test Inorganic salts Organic matter Slops

Performance [%]

Performance [%]

12 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12

FeCl3 50 mg·L-1

30

IN NTU

Residual NTU

min

max

Al2(SO4)3 50 mg·L-1

40

20

10 0

30 20 10 0

0

2 4 6 8 Flocculant dose [mgA57·L-1]

10

0

2 4 6 8 Flocculant dose [mgA57·L-1]

10

a b s t r a c t In this study, the coagulation and flocculation processes were evaluated in the treatment of slops with the aim of reducing the organic concentration in the pre-treated influent, as measured by Chemical Oxygen Demand (COD), Total Organic Carbon (TOC) and Total Petroleum Hydrocarbons (TPH). The coagulation was optimised for the wastewaters sampled from a floating tank of an oil costal deposit in the Augusta Harbour (Sicily-Italy). The jar test experiments were run at 200 rpm for 1 min, 30 rpm for 20 min and settling for 180 min. Two different trivalent salts were used as the coagulant: FeCl3 and Al2(SO4)3  18H2O. To limit the residual concentration of Al and Fe after coagulation, a low coagulant dose was used. Furthermore, both anionic and cationic polyelectrolytes were used as flocculating agents. In general, the application of the coagulation process with low coagulant doses was effective in the pre-treatment of the slops. In fact, the percentages of removal, although rarely exceeding 70–80% (in term of TOC), reduced the organic load resulting from recalcitrant or poorly biodegradable substances. In particular a pre-treatment, with aluminium sulphate is more versatile for the removal of the main contaminants, that can hinder the performance of subsequent biological processes. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Slops are wastewaters generated by washing oil tanks that can create a serious disposal problem because of the accumulation and ⇑ Corresponding author. Tel.: +39 0935 536350. E-mail addresses: [email protected], [email protected], [email protected] (G. Di Bella). http://dx.doi.org/10.1016/j.cej.2014.05.115 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

residual TOC

100 90 80 70 60 50 40 30 20 10 0

TOC after test [mg·L-1]

TOC removal

residual TC 60 55 50 45 40 35 30 25 20 15 10 5 0

40

Turbidity [NTU]

pollutant (TPH, TC, TOC, turbidity, COD).  Good efficiency of TPH removal (70%) was achieved.  The optimisation was based on coagulation pre-treatment, before a MBR.  The cost-benefit analysis has shown the advantage of the use of aluminium salts.

TC residual 100 90 80 70 60 50 40 30 20 10 0

TC after test [mg·L-1]

 The jar tests were applied to the slops.  Analysis reports data on many

Turbidity [NTU]

h i g h l i g h t s

persistence of xenobiotic compounds in the environment [1]. They are characterised by high salinity and high content of organic matter (more or less biodegradable). The achievement of discharge limits in the treatment of such waters can be challenging with respect to many of the relevant parameters, such as dissolved organic matter, hydrocarbons and heavy metals [2]. Slops can be treated using a Conventional Activated Sludge (CAS) plant, but a proper pre-treatment of these oily wastewaters

284

G. Di Bella et al. / Chemical Engineering Journal 254 (2014) 283–292

is critical in the design of the entire process [3]. Several pre-treatment options have been proven useful for the separation of oil from water (e.g., oil extraction, filtration, flotation), though they differ in their effectiveness, management and cost. Generally, after the treatment with specific oil separators (for example, with an American Petroleum Institute separator – API), the slops may undergo a chemical process that removes pollutants from oily wastewaters. In particular, coagulation coupled with a flocculation process displays high removal efficiencies of varied parameters, mainly Chemical Oxygen Demand (COD), Total Organic Carbon (TOC) and suspended solids. Coagulation is an important step in reducing the suspended and colloidal materials responsible for both turbidity and the organic matters in wastewater, which contributes to the pollutant content of the wastewater [4]. Particularly, the effectiveness of this process can be properly assessed by means of specific contaminants removal performances (hydrocarbons, heavy metals, etc.). In fact, the reduction of turbidity defines a global effect on the colloids removal (often qualitative) and, consequently, the value of residual turbidity is not fully useful in the case of the analysis of pollutant removal from heterogeneous mixtures. Coagulation is extensively used in large scale water treatment facilities and have been found to be preferable in the treatment of wastewaters from petroleum refineries, olive oil mills and slaughterhouses [5–7]. In full-scale applications, because natural organic coagulants are costly compared to other common coagulants, the coagulation is mainly induced by metal salts. In particular, the process usually employs Al(III) or Fe(III) salts alone or in combination with flocculants. These coagulants promote particle agglomeration by reducing the electrostatic particle surface charges. The coagulant dose required for wastewater treatment varies across a wide range [8]: low dosage < 150 mg L1, medium dosage 150–600 mg L1and high dosage > 600 mg L1. The coagulants that are usually employed in the treatment of domestic and industrial wastewaters are Al2(SO4)318H2O or FeCl3 [9,10]. However, aluminium and iron in coagulated wastewater effluents have been considered a human and environmental health concern [8]. Further, the presence of residual concentrations of Al and Fe may cause phenomena that negatively affect the subsequent treatment of coagulated wastewater, for example, the ‘‘scaling’’ of ultrafiltration membranes in advanced filtration systems [11]. In the present study, the treatment of slops was performed by coagulation and flocculation, and the pre-treatment efficiency was assessed in terms of the COD, TOC, turbidity and residual concentration of hydrocarbons. This chemical process was applied to investigate its potential for effective pre-treatment of slops. In particular, because this study has been framed within a project funded by the European Community (SibSac – An integrated system for sediments remediation and high salinity marine wastewaters treatment), which provides for the combined treatment of slops and liquid discharge by sediment washing with a Membrane Bioreactor (MBR) [11] the coagulation process was investigated to minimise the coagulant dose (with reference to the conventional coagulant dosage in wastewater treatment) to control the risk of scaling in membrane fouling. 2. Materials and methods 2.1. Jar test To identify the appropriate type of coagulant, the aluminium sulphate (Al2(SO4)3) and ferric chloride (FeCl3) were tested in specific jar tests. After coagulation/flocculation, the suspension was poured into a graduated cylinder for sedimentation. Supernatant samples were taken beneath the liquid surface for turbidity

measurements. Multiple jar tests were performed on the slop samples, tripling the tests in the most cases. The jar tests were run at 200 rpm for 1 min, 30 rpm for 20 min and settling for 180 min. To identify the best coagulant type and dose, only the specific coagulant dose was analysed, setting the parameter values in accordance with the values reported in literature [12]. The doses of FeCl3 and Al2(SO4)3  18H2O were varied in the range between 50 and 90 mg L1. On the other hand, both the anionic and cationic polyelectrolytes were used as flocculating agents with a dose ranging from 1 to 10 mg L1. More specifically, the experimental protocol provided an initial dosage of coagulant to the beginning of mixing ‘‘rapid’’ phase (200 rpm) and subsequent a dosage of flocculant to the beginning of mixing ‘‘slow’’ phase (30 rpm, 30 min later). The ‘‘jar test battery’’ was composed of 6 batch tests conducted in parallel: in the first one, only the coagulation was planned without the addition of flocculant; subsequently, during mixing ‘‘slow’’ phase, the flocculant was dosed in the other batch tests, varying the dosage in accordance with the previous observations. Despite the coagulation and flocculation processes have been widely investigated for the treatment of municipal wastewater as well as fresh waters, the case of slops presents still some uncertainty which need to be solved. In this context, in order to conduct a logical numbers of repetitions, each jar test was replicated (at least) 3 times. At a later time, and only for certain parameters and dosages, the repetitions have been increased (TPH and COD). A preliminary (and bibliographic) analysis showed that the optimum pH range was found close to a neutral pH. For this reason, in accordance with the previous observations, the pH was not controlled a priori because all tests were ‘‘spontaneously’’ performed in the range reported above. Furthermore, a neutral environment is preferable when the coagulation-flocculation process is only a preliminary step before a biological processes (in accordance with SibSac project). 2.2. Chemicals As discussed, two different trivalent salts were used as a coagulant: aluminium sulphate and ferric chloride. Furthermore, both anionic and cationic polyelectrolytes were used as the flocculating agent: anionic polymer type ‘‘A57’’ and cationic polymer type ‘‘VP584’’ (only with FeCl3, see results and discussion paragraph). The wastewater (slop) was sampled from a floating tank (Fig. 1) of an oil costal deposit in the Augusta Harbour (Sicily-Italy). The slops were derived from the washing of tanks containing naval derivatives (mineral oil without diesel). A preliminary and simple gravity separation (2 h) was performed in the laboratory to reduce the high floating oil content. Subsequently, approximately 300 L of the clarified wastewater was refrigerated at 4 °C to inhibit biological activity. Table 1 shows the wastewater characteristics after oil separation. As shown in the table, the organic matter (COD) and hydrocarbons (TPH) are the main components of pollution in the collected wastewater. Concerning the presence of heavy metals in the wastewater, the data show that there is only a low concentration of iron and boron. However, these metals do not represent a pollution of interest. Finally, the chlorides concentration and conductivity are high (compared with conventional wastewater). Notably, the preliminary separation of the oil significantly reduced the turbidity of the sample (because oil is predominantly an emulsifier). In Table 2 are summarised the different ‘‘Test code’’ used for each Jar Test. In particular, the coagulant and anionic flocculant doses are also reported. Of further note, the cationic flocculant (VP584) was used only in the first phase of FeCl3 addition because the first tests have not shown improvements in the removal efficiency (see the

G. Di Bella et al. / Chemical Engineering Journal 254 (2014) 283–292

285

Fig. 1. Sampling operation at Augusta Harbour.

Table 1 The characteristics of the slop after gravity separation. Parameter

Symbol Value

Legal limit Units (D.Lgv. 152/2006)

Chemical Oxygen Demand Total Carbon Inorganic Carbon Total Organic Carbon Total Petrolium Hydroc.

COD TC IC TOC TPH

1088 ± 251 6160 57.4 ± 4.6 38.2 ± 4.82 19.1 ± 3.3 232.5 ± 28.5 65

mg L1 mg L1 mg L1 mg L1 mg L1

Suspended solids Turbidity

SS

352.4 ± 84.1 680 25.0 ± 5.4 --

mg L1

Aluminium Arsenic Boron Cadmium Chrome Iron Manganese Nickel Copper Selenium Lead Zinc

Al As Bo Cd Cr Fe Mn Ni Cu Se Pb Zn