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Comparative study of phenol and cyanide containing wastewater in CSTR and SBR


Bioresource Technology 100 (2009) 31–37

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Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

Comparative study of phenol and cyanide containing wastewater in CSTR and SBR activated sludge reactors
C.A. Papadimitriou a,*, P. Samaras b, G.P. Sakellaropoulos a
a b

Chemical Process Engineering Research Institute and Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, 54006 Thessaloniki, Greece Department of Pollution Control Technologies, Technological Educational Institute of W. Macedonia, 50100 Kozani, Greece

a r t i c l e

i n f o

a b s t r a c t
The objectives of this work were the examination of the performance of two bench scale activated sludge systems, a conventional Continuous Stirring Tank Reactor (CSTR) and a Sequential Batch Reactor (SBR), for the treatment of wastewaters containing phenol and cyanides and the assessment of the toxicity reduction potential by bioassays. The operation of the reactors was monitored by physicochemical analyses, while detoxi?cation potential of the systems was monitored by two bioassays, the marine photobacterium Vibrio ?scheri and the ciliate protozoan Tetrahymena thermophila. The reactors in?uent was highly toxic to both organisms, while activated sludge treatment resulted in the reduction of toxicity of the in?uent. An increased toxicity removal was observed in the SBR; however CSTR system presented a lower ability for toxicity reduction of in?uent. The performance of both systems was enhanced by the addition of powdered activated carbon in the aeration tank; activated carbon upgraded the performance of the systems due to the simultaneous biological removal of pollutants and to carbon adsorption process; almost negligible values of phenol and cyanides were measured in the ef?uents, while further toxicity reduction was observed in both systems. ? 2008 Elsevier Ltd. All rights reserved.

Article history: Received 23 August 2006 Received in revised form 31 May 2008 Accepted 5 June 2008 Available online 22 July 2008 Keywords: Activated sludge Toxicity reduction Activated carbon Phenol Cyanide

1. Introduction High strength wastewaters are currently produced from various industrial plants such as petrochemical industries, coke-processing plants, metal ?nishing units etc. Wastewaters generated from these processes contain a large number of pollutants at high concentrations and have adverse environmental impacts. A certain type of wastewaters, such as those produced from coke oven plants, contain toxic xenobiotics, phenols and their derivatives (pyrocatechol, quinone, pyrogallol), as well as ammonia, rhodanate and cyanide; typical pollutants concentration may reach up to 1000 mg/L of phenol and 300 mg/L of cyanides respectively (Wild et al., 1994). A typical composition of coke oven wastewaters is shown in Table 1. The introduction of such in?uents to an activated sludge system may result in loss of activated sludge viability and may change sludge community structure. Stringent limits for coke oven ef?uents have been set in countries where these wastewaters represent a major problem, such as Germany, and the corresponding guidelines presented in Table 2 (Morper and Jell, 2000). These guidelines include the requirements for coke oven ef?uents before being discharged to a water receiver and prior to mixing with other ef?uents. As shown very low concentrations are required for cer-

* Corresponding author. Tel.: +30 2310 498254; fax: +30 2310 498255. E-mail address: papadim@cperi.certh.gr (C.A. Papadimitriou). 0960-8524/$ - see front matter ? 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.06.004

tain pollutants like benzenes, sulphites, PAH, phenols and cyanides. The removal of pollutants from coke oven wastewaters is a signi?cant issue due to the environmental impact of these compounds. Especially cyanides and thiocyanites present an important problem in wastewater treatment plants, due to their toxic properties, including reduction of enzymic activity of unicellular organisms, such as the typical bacteria inhabiting in the activated sludge (Raef et al., 1977). Furthermore, due to incomplete biodegradation, ef?uents may contain high concentrations of compounds such as phenols, cyanides and thiocyanates exerting significant toxic effects to the organisms of an aquatic ecosystem (Uygur and Kargi, 2003; Suschka et al., 1994); phenol may reduce the enzyme activity, at relatively low concentrations, ranging from 5 to 25 mg/L (Suschka et al., 1994). The potential mechanisms of degradation of cyanides and phenols during activated sludge treatment include volatilization, biological metabolism, adsorption onto biomass and chemical reaction; in an ef?cient biological detoxi?cation system, cyanides are converted into thiocyanate ions, exhibiting toxic properties only at high concentrations (Rayback, 1992; Knowles and Bunch, 1986; Aronstein et al., 1994). Signi?cant removal of cyanides has been found under the presence of ?occulating organisms, such as Zooglea sp. suggesting that the extracellular characteristics of the cells attribute to the removal of cyanide species (Raef et al., 1977), through the direct adsorption of cyanides to the

32 Table 1 Typical coke oven wastewater composition Parameter, mg/L BOD5 COD SS TKN NH4–N P Phenol SCN? CN?

C.A. Papadimitriou et al. / Bioresource Technology 100 (2009) 31–37

Coke oven ef?uents 1600–2600 4000–6500 2–10 300–500 50–150 <1 400–1200 200–500 4–15

Table 2 Requirements for treatment of coke oven ef?uents in Germany (Morper, 2000) Parameter At the discharge point BOD5 NH4–N + NO2–N + NO3–N Total N (TNb) COD reduction Prior to mixing with other ef?uents Benzene and derivatives Sulphide Polynuclear aromatics Volatile phenols Cyanide (free) Fish toxicity Value 69 mg/l 69 mg/l 612 mg/l P90% 0.03 mg/l 0.03 mg/l 0.015 mg/l 0.15 mg/l 0.03 2

extracellular polymeric substances. The cellular forms and the characteristics of the activated sludge bacterial and protistan microfauna are directly related to the operational parameters of a wastewater treatment unit such as retention time, in?uent rate, organic loading and sedimentation time. Different operational conditions result in the formation of various cellular structures, posing a stress factor for the adaptation of activated sludge microfauna. Various operational conditions and activated sludge reactor types have been employed for the removal of phenol from ef?uents (Uygur and Kargi, 2003; Suschka et al., 1994). A high phenol removal ef?ciency, up to 70%, has been reported in systems fed by phenol in?uent concentration up to 600 mg/L, containing a well acclimatized sludge (Uygur and Kargi, 2003; Suschka et al., 1994). However, the increase of in?uent phenol to concentration of 600 mg/L, resulted in operational problems and in reduced treatment ef?ciency (Uygur and Kargi, 2003). Several studies have been conducted for cyanide removal through adsorption mechanisms onto activated carbon (Monser and Adhoum, 2002; Seke et al., 2000), while biodegradation mechanisms have been investigated in sequential batch reactors and sequential bio?lm batch reactor systems for in?uent concentrations up to 20 mg/L (White and Schnabel, 1998). Early work showed that the addition of powdered activated carbon in an activated sludge unit, a process known as Powdered Activated Carbon Technique (PACT), may have an important bene?t associated to improved organic removal (Roberataccio, 1973). Such a process may be attributed to the adsorption on the activated carbon particles and to the development of certain interactions between the carbon and biomass (Diamadopoulos et al., 1997). The ef?ciency of the treatment systems is usually evaluated by chemical based analytical methods. However, conventional chemical based analytical methods have been found to be inadequate for the evaluation of the toxic properties of wastewaters: municipal and industrial wastewaters contain a large number of compounds and analytical scans are very expensive and time consuming providing a circumstantial evidence to the inhibitory nature of the waste. Furthermore, chemical tests alone do not give information

concerning potential synergism of the chemical substances existing in wastewaters. Finally, data on the effects of individual compounds do not account for the interactions among pollutants that may occur in a complex mixture of toxicants, that comprise the industrial and municipal wastewaters (Reemtsma et al. 1999). As a result, other methods using living organisms, have been proposed for the assessment of in?uent and ef?uent toxicity; living organisms have the ability to respond to actual disruptions in a range of effects (Blaise et al., 2000). Toxicity tests using Daphnia magna and Vibrio ?scheri, protozoa and those involving respirometric techniques are widely used for industrial samples (Kong et al., 1996; Reemtsma et al. 1999; Baudo, 2001; Sponza 2003; Dalzell et al., 2002). The implementation of toxicity tests for the evaluation of discharges to surface wasters has been recently included in the legislation of several countries (Baudo, 2001; Sponza, 2003). Although the treatment of phenol or cyanide in various reactors has been examined in several studies, the investigation of their simultaneous removal in assimilated coke oven wastewater in SBRs is limited (Staib and Lant, 2007; Maranon et al., 2008). In other studies the biodegradation and/or toxicity of these compounds has been investigated in pure microbial cultures (Ballesteros-Martín et al., 2008; Park et al., 2008) however the toxicity reduction through activated sludge treatment has not been thoroughly examined. In addition, the relation of toxicity reduction of high strength in?uents to the type of the activated sludge reactors has not been conducted. The objectives of this work were the examination of the performance of two bench scale activated sludge systems, a conventional Continuous Stirring Tank Reactor (CSTR) and a Sequential Batch Reactor (SBR), for the treatment of wastewaters containing phenol and cyanides and the assessment of the toxicity reduction potential by bioassays. 2. Methods Two bench scale reactors were used in this study: a conventional continuous system and a sequential batch reactor unit. The conventional system consisted of a 12 L plexiglas unit with two compartments, an aeration tank and a sedimentation tank. Aeration was supplied by two ceramic diffusers, while internal sludge recirculation rate was controlled by an adjustable vertical baf?e. Sludge recirculation was not directly measured but it was adjusted in order to maintain a constant MLSS content in the aeration compartment. A peristaltic pump (Watson Marlow) was used for continuous feeding of about 0.6 L/h in?uent. The SBR system consisted of a 5 L cylindrical plexiglas reactor, operated in 12 h cycles including ?ve sequential stages: ?ll, react, settle, draw and idle. Each cycle consisted of 2 h mixed ?ll, 6 h react – aeration with 1 h anoxic stage, 2 h sedimentation, 1.5 h draw of the supernatant and 0.5 h idle (EPA 832 – R – 92-002). The operation of the bench scale systems started by the addition of about 1 L of recirculated sludge from a full-scale municipal wastewater treatment plant, followed by a sludge acclimatization period where municipal wastewater was introduced to both systems. Municipal wastewater was gradually replaced by synthetic high strength in?uent, followed by the addition of phenol and cyanides. Real coke oven wastewater could not be used, as there is not a coke oven plant in Greece. As a result synthetic wastewater was used, simulating coke oven ef?uents and especially the most toxic compounds, phenol and cyanides. Synthetic wastewater used in this work, was prepared using the following analytical grade substances: 833 mg/L glucose, 2000 mg/L sodium acetate, 100 mg/L NaCl, 50 mg/L CaCl2 * 2H2O, 20 mg/L KCl, 600 mg/L NH4Cl and 333 mg/L K2HPO4 * 3 H2O (Uygur and Kargi, 2003). All chemicals were of analytical grade supplied by Merck. Additional constituents such as phenol, thiocyanide and cyanide (Merck) were added to the in?uent in concentrations up to 1400, 270 and 100 mg/L, respectively.

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The whole operation time of the reactors for the simultaneous removal of phenol and cyanides lasted about one year, 260 days the systems were operated as a single activated sludge process, while the other 140 days powdered activated carbon was added to the reactors. In?uent and ef?uent samples were collected from both systems and were analysed for the measurement of the following parameters according to standard methods of analysis (APHA, 1989): COD, BOD5, Suspended Solids (SS), ammonia nitrogen, nitrate nitrogen, phenol and cyanide. Furthermore, samples were withdrawn from the aeration tank of each system and the MLSS content was measured. Phenol concentration was determined colorimetrically at 507 nm by the 4-aminoantipyrine method (APHA, 1989). The absorbance was measured using an Aqualytic spectrophotometer. The total cyanide ef?uent concentration was determined by the reaction of cyanide with acidic ferric ion solution that formed an intensive purple colour followed by colorimetric determination at 585 nm (APHA, 1989). A commercial Powdered Activated Carbon (SAE 2, Norit) was added to both systems in concentration up to 1000 mg/L, in order to evaluate its effectiveness for the removal of speci?c compounds. This type of carbon is a steam-activated peat/wood carbon with a BET surface area of 928 m2/g, pore volume of 0.88 cm3/g at p/po = 0.99 and average pore size 2.6 nm. In order to maintain a constant concentration of carbon in the aeration tank throughout the operation period, powdered activated carbon was added to compensate for PAC losses in the ef?uent suspended solids. Hydraulic retention time in both systems was 30 h, no sludge was wasted and the solids residence time (SRT) was quite high. The estimation of an average SRT, based on solid loss in the ef?uent gave a value of around 26 days. MLSS content was maintained constant through the loss of suspended solids in the ef?uent. Sludge Volume Index (SVI) was periodically measured. In addition to physicochemical analysis, samples were collected from in?uents and ef?uents of both reactor systems and their toxicity was evaluated using two bioassays: the inhibition to the bioluminescence of V. ?scheri bacteria and the growth inhibition of protozoan Tetrahymena thermophila. These organisms were selected based on the applicability of the bioassays and the use of these organisms as standard indicator species (Dalzell et al., 2002). V. ?scheri is a widely used bioassay, incorporated in national environmental legislations for the monitoring of ef?uent toxicity and of toxicity of surface waters. T. thermophila is a common bacterivorous activated sludge protistan, able to simulate the effect of in?uent to activated sludge microfauna. (a) The bioluminescence bacteria V. ?scheri (Microtox test) were in freeze-dried form (SDI, USA) and were activated prior to use by a reconstitution solution. Since V. ?scheri is a marine organism, adjustment of the osmotic pressure of the samples was applied up to 2% salinity, using a concentrated salt solution (solution containing 22% NaCl in deionized water). The light emitted from the test organisms, obtained by their direct contact to the samples, was measured using the Microtox 500 analyzer (SDI) within a short exposure time of 15 min. The data processing was performed using the Microtox Omni software (SDI). (b) The protozoan T. thermophila was in the form of cells in suspension in the Prototox Kit F (stock solution by Microbiotests?). Feed solutions were prepared with the appropriate feeding substrates provided in the tests kit. The organisms were placed into 5 cm photometer cuvettes containing 2 mL of the sample and 40 lL of the feeding solution. The initial optical density of each cuvette was measured by a Shimadzu UV–Vis spectrophotometer at 440 nm. The cuvettes were then placed in an incubator in darkness at 30 oC for 24 h; at the end of this period the optical density values

were measured. The decrease in optical density was related to the consumption of food substrate and thus, to the viability of the organisms. Growth inhibition of the organisms was calculated against a blank sample, prepared by deionized water. The toxicity reduction was calculated as the percentage reduction between the in?uent and ef?uent toxic effects, according to the following equation:

TR ?

TI ? TE ? 100 TE

where TR, toxicity reduction, %; TI, in?uent toxicity units; TE, ef?uent toxicity units. 3. Results and discussion In?uent and ef?uent phenol, cyanides, COD and BOD5 concentrations from both reactors, during the period of simultaneous biodegradation of phenol and cyanides, are shown in Fig. 1. Both systems presented an ef?cient operation for pollutant removal; however, the performance of the SBR system was better than the continuous one: higher concentrations of substances were measured in the ef?uent form the latter system than the corresponding samples from the intermittent unit. The average COD concentration in the ef?uent from the SBR system was 360 mg/L corresponding to a removal capacity reaching up to 93%, while COD reduction achieved by the continuous system varied between 63 an 92%, giving an ef?uent with average COD values exceeding 600 mg/L. Similar results were obtained for the BOD5, nitrogen, phenol and cyanide removal rates, ef?uent phenol concentrations in the SBR system reached up to 6 mg/L, while the corresponding values in the continuous system reached up to 60 mg/L. The ability of the SBR system for increased removal of organic loading, nutrients and speci?c constituents has been reported by several researchers (Diamadopoulos et al., 1997; Vidal et al., 2004; Obaja et al., 2005). Furthermore, the increased sludge sedimentation capacity in SBR units, and the production of a clari?ed ef?uent with a low suspended solids content, has been well documented as a competitive advantage over conventional treatment plants (Uygur and Kargi, 2003; Kargi et al., 2005). NH4–N ef?uent concentrations in the SBR reactor varied from 2 mg/L to 23 mg/L, corresponding to removal ef?ciencies from 93% to 99%. Higher ef?uent values were observed in the CSTR unit, varied from 25 mg/L to 91 mg/L, with corresponding removal ef?ciencies from 83% to 95%. The nitri?cation process was not affected by the addition of phenol and cyanides, while at extended operation time, SBR showed a more ef?cient performance than the CSTR system, resulting in lower ammonia ef?uent values. Nitri?cation capacity was further assessed by measuring the nitrate–nitrogen ef?uent content: nitrates reached up to 9 mg/L in the SBR system, while the respective concentration for the CSTR system was between 1 mg/L and 12 mg/L. The effect of pollutants addition on activated sludge characteristics was depicted by the estimation of SVI as a function of the operation time. SVI remained almost constant at about 45 mL/g in the SBR unit. However, the SVI values in the CSTR plant presented higher ?uctuations; at phenol concentrations above 800 mg/L, SVI values were increased to about 100 mL/g while at higher phenol content (850 mg/L), SVI values increased to 150 mL/g. It seems that, activated sludge organisms developed during SBR operational mode were not adversely affected by the phenol addition, even at high concentrations, resulting in good sedimentation characteristics. However, in the CSTR unit, at phenol concentrations above 800 mg/L, inactivation and disintegration of organisms possibly occurred, having as a result an increase in SVI values along with poor sedimentation characteristics and high

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Fig. 1. In?uent and ef?uent concentrations as function of operation time in the bench scale activated sludge reactors: (A) phenol, (B) cyanides, (C) COD, (D) BOD5.

content of ef?uent suspended solids. Ef?uent SS content in the CSTR unit exceeded 500 mg/L at high in?uent pollutants concentration, while the corresponding values in the SBR system maintained below 100 mg/L. The increase of in?uent phenol content to 800 mg/L, severely affected the performance of the CSTR reactor, resulting in poor phenol removal capacities and in the following operation problems:  decrease in the removal capacity of the organic substances (BOD5, COD);  decrease in nitri?cation ef?ciency;  limited phenol degradation, resulting to ef?uent phenol concentrations ranging from 10 to 60 mg/L;  decrease in sedimentation capacity. Legislative measures for cyanides are very stringent, setting ef?uent with concentration lower than 1 mg/L (Morper and Jell, 2000). In both reactors, cyanide ef?uent concentrations were above the desired limit and thus, an additional treatment step should follow the activated sludge process. The addition of powdered carbon in the aeration tank of both systems resulted in enhanced system performance and the corresponding results are shown in Fig. 2. In?uent COD was about 4600 mg/L, while ef?uent COD values from the SBR unit varied from 153 mg/L to 476 mg/L corresponding to removal ef?ciencies ranging from 93% to 99%. However, lower removal capacities were observed in the CSTR unit ranging from 77% to 95%, corresponding to ef?uent concentrations between 305 mg/L and 1360 mg/L. Negligible phenol and cyanide concentrations were observed in the SBR system in the presence of activated carbon while the respective values in the CSTR ef?uent reached up to 12.5 and 1.7 mg/L, respectively. Addition of the activated carbon resulted in increased phenol removal, enabling the adsorption of phenol as complementary to biodegradation processes.

In general activated carbon resulted in a better performance of the SBR system and in an ef?uent with a low organic matter content and negligible content of phenols and cyanides. The continuous system performance was slightly improved by the addition of activated carbon, but ef?uent quality still remained lower than the corresponding ef?uent from the SBR system. Legislative measures for cyanides are very stringent, setting ef?uent limits lower than 1 mg/L (Morper and Jell, 2000). Cyanide ef?uent concentrations in both reactors were above the required limit prior to carbon addition; however the presence of powdered carbon enhanced the SBR performance and the treated ef?uent cyanide concentration was within the required limits. The addition of carbon in the CSTR system had signi?cantly reduced the ef?uent concentrations of cyanide, and increased the removal ef?ciency from 76% to 99%; however CSTR ef?uent values still did not comply with the required limits. NH4–N ef?uent concentrations in the SBR reactor varied from 1 mg/L to 29 mg/L, corresponding to removal ef?ciencies from 90% to 99%. Higher ef?uent values were observed in the CSTR unit, varying from 7 mg/L to 78 mg/L, with corresponding removal ef?ciencies from 73% to 98%. Toxicity of in?uent and ef?uent samples from both systems, with and without activated carbon, are presented in Fig. 3 for the T. thermophila bioassay and in Fig. 4 for the V. ?scheri bioassay, respectively. In Figs. 1 and 2 the characteristic parameters for the samples obtained for toxicity testing are presented. Samples from the activated sludge process were collected from both reactors from the 410th to the 500th day of operation. The concentration of cyanide ranged from 2 mg/L to 3 mg/L, and from 3 mg/L to 14 mg/L for the SBR and the CSTR reactor respectively. Phenol concentrations ranged from 2 mg/L to 4 mg/L, and from 25 mg/L to 38 mg/L for the SBR and the CSTR reactor, respectively. Samples from both reactors operating under combined activated carbon/activated sludge process were obtained from 585th to 675th days of operation. The concentration of cyanide ranged from 0.4 mg/L to 0.7 mg/L,

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Fig. 3. Growth inhibition of Tetrahymena thermophila during contact to in?uent and ef?uent samples from the activated sludge units with and without activated carbon ((N) in?uent; (h), (j) SBR unit; (e), (r) continuous system; closed symbols: without activated carbon; open symbols: with activated carbon).

Fig. 4. Bioluminescence inhibition of Vibrio ?scheri during contact to in?uent and ef?uent samples from the activated sludge units with and without activated carbon ((N) in?uent; (h), (j) SBR unit; (e), (r) continuous system; closed symbols: without activated carbon; open symbols: with activated carbon).

and from 2 mg/L to 4 mg/L for the SBR and the CSTR reactor, respectively. Phenol concentrations in SBR reactors were in most cases below the detectable limit (0.1 mg/L), while the respective values for the CSTR reactor ranged from 1 mg/L to 2 mg/L. In?uent samples were highly toxic to both organisms, resulted in almost complete bioluminescence inhibition of V. ?scheri and growth inhibition of the protozoan populations, respectively. How-

ever the treatment of wastewaters by the activated sludge process resulted in the reduction of the in?uent toxicity. Enhanced toxicity reduction was observed in the SBR system and ef?uents presented negligible toxicity. The bioluminescence inhibition and the growth inhibition reached up to 10 and 7%, respectively, without the addition of activated carbon, corresponding to toxicity reduction values exceeding 90%. Additional toxicity reduction, especially in the case

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of V. ?scheri, was observed in samples collected during the period of combined activated sludge – activated carbon treatment: growth inhibition reached up to 10%, while V. ?scheri bioassay presented slightly negative bioluminescence inhibition values, indicating the development of hormesis effects i.e. stimulation response in the presence of toxicant at sub-inhibitory levels (Stebbing, 1982; Christo? et al., 2002). Similar hormesis effects have been observed for phenol concentrations up to 100 mg/L (Christo? et al., 2002). Almost complete detoxi?cation of wastewaters was measured in this period, and was attributed to the optimum SBR system performance and the complete removal of phenols and cyanides. Increased ef?uent toxicity was observed in the continuous system, corresponding to a limited toxicity reduction capacity: in the absence of activated carbon, ef?uent samples presented about 70% bioluminescence inhibition and 60% growth inhibition, corresponding to a detoxi?cation capacity of about 30–40%. The addition of activated carbon resulted in enhanced pollutant removal capacity and in decreased ef?uent toxicity values, reaching up to 40% for both growth inhibition of T. thermophila and bioluminescence inhibition of V. ?scheri, corresponding to a toxicity reduction capacity of about 60%. Consequently, bioassays by T. thermophila and V. ?scheri indicated that the continuous aerobic activated sludge treatment system was able to partially remove the high concentrations of in?uent phenol and cyanide; thus the detoxi?cation capacity was limited, reaching up to about 40%. It is possible that during this treatment process, insuf?ciently metabolised toxic compounds were produced, which resulted in the loss of mixed liquor suspended solids and in microfauna deterioration, generating sludge with poor settling characteristics (Cybis and Horan, 1997). The relatively high sludge loading ratio, F/M = 0.3 kg BOD5/kg MLVSS/day, compared to typical values of 0.06–0.24, initiated the predominance of ?lamentous organisms in the system, resulting in bulking sludge (Cybis and Horan, 1997). Such ?lamentous organisms were observed in samples collected from the continuous system. However, the results from T. thermophila and V. ?scheri bioassays were particularly encouraging for the utilization of the SBR process as an ef?cient treatment method of high strength toxic industrial wastewaters. Moreover, protozoan species belonging to Tetrahymena family are key organisms in activated sludge processes and have been identi?ed in several municipal and industrial activated sludge plants (Madoni, 1994). The SBR processes utilize an intermittent in?uent feeding mode, creating a period of high loading followed by a starvation period (often referred to as feast-famine); such conditions impose a selective force for the development of a multispecies activated sludge microfauna, able to assimilate readily biodegraded substances, ensuring their viability (Martins et al., 2003). This process is conducted by deploying mechanisms, in order to form thicker cell walls with increased assimilation and storage capacities, enhancing in?uent puri?cation (Martins et al., 2003; Obaja et al., 2005; Kargi et al., 2005). In addition, thicker cell walls may enable better ?oc formation and increased settling capacities (Martins et al., 2003). Thus, the SBR activated sludge microfauna has been proved more ef?cient than the corresponding continuous system sludge population, resulting in almost complete toxicity reduction. The addition of powdered activated carbon enhanced the removal ef?ciency of organic matter, phenol and cyanide, in both systems, due to the combined action of biodegradation and adsorption processes. According to the technical speci?cations of the particular carbon the adsorption of phenol was 100 mg/g, accounting for almost 10–20% of the total phenol removal from coke oven wastewater (Norit Technical Bulletin, 2005), suggesting that a well acclimatized activated sludge biological community may be able to biodegrade up to 800 mg/L phenol. Previous studies with phenol containing wastewater treatment in an SBR exhibited that inhibi-

tion of organic load and nutrient removal occurred when in?uent phenol concentration exceeded 400 mg/L (Uygur and Kargi, 2003). The high surface of activated carbon might partially supply the systems with additional ‘‘spaces” for the formation of activated sludge ?ocs (Orshansky and Narkis, 1997; White and Schnabel, 1998; Ebbs, 2004), enhancing the growth of activated sludge microorganisms. Microscopical observations of the activated sludge microfauna that were conducted throughout the operation of the reactors, exhibited an increase of the protozoan populations, especially of the sessile species, in the presence of activated carbon (Papadimitriou et al., 2007). Although a signi?cant reduction of phenol and cyanide ef?uent concentration was observed in the continuous system, the toxicity reduction capacity was still limited to about 60%, and a complete detoxi?cation was not achieved. Thus, the ef?uent from the continuous unit might cause a signi?cant environmental impact in a water receiver. Furthermore, the addition of such an in?uent to a conventional activated sludge plant, during the co-treatment of municipal and industrial wastewaters, might have a severe effect on the system performance, deteriorating the growth of activated sludge microfauna. A better performance was observed in the combined activated carbon-activated sludge SBR system; this process characterized by enhanced pollutant removal ability, enabling the production of ef?uents with low toxicity, imposing minimal environmental risks. 4. Conclusions The operation of two bench scale activated sludge reactor units (a continuous system and an SBR unit) was investigated in this work for the treatment of high strength wastewater, in order to evaluate their toxicity reduction capacity in the presence of phenol and cyanides. An ef?cient pollutant removal capacity was observed in both systems; however, the performance of the SBR system, was better than the continuous one. Higher pollutants removal capacities and lower ef?uent concentrations were observed in the former system, COD removal rates up to 93% were measured, corresponding to COD ef?uent values lower than 400 mg/L, while for the continuous system COD ef?uent values were higher than 600 mg/L. Similar results were estimated for parameters such as BOD5, suspended solids, nitrogen, phenols and cyanide content. The addition of powdered activated carbon in the aeration tanks resulted in the optimisation of both systems performance; however, lower pollutants values were observed in the SBR system ef?uent than the continuous unit, indicating a better ef?ciency of this system. The evaluation of the toxicity of in?uents and ef?uents was performed by using two bioassays: the marine photobacterium V. ?scheri and the ciliate protozoan T. thermophila. In?uent samples proved to be high toxic to both test species resulting in complete inhibition of their activities. A different toxicity reduction was achieved by the two activated sludge units: the enhanced pollutant removal capacity observed in the SBR system was associated to a high toxicity reduction, reaching up to 90%. The addition of activated carbon improved the system ef?ciency resulting in ef?uents of negligible toxicity, permitting their discharge in environmentally sensitive areas. However, ef?uents from the continuous system presented a signi?cant toxicity to both test organisms: 70% of bioluminescence inhibition and 60% growth inhibition; the addition of activated carbon resulted in the reduction of ef?uent toxicity to inhibition values of about 30–40%. Acknowledgements This study was supported by the RFCS project ‘‘Minimization of coke – oven emissions”, Contract No: RFCS 7220-PR139.

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