Effect of Hypochloride on Microbial Ecology of Bulking and Foaming ...

Journal of Environmental Science and Health Part A, 41:2163–2174, 2006 C Taylor & Francis Group, LLC Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520600867813

Effect of Hypochloride on Microbial Ecology of Bulking and Foaming Activated Sludge Treatment for Tannery Wastewater 1 ¨ ¨ 2 Selda Murat,3 and Derin Orhon1 Suleyman Ovez, Canan Ors, ¨ 1 Istanbul Technical University, Faculty of Civil Engineering, Department of Environmental Engineering, Maslak, Istanbul, Turkey 2 Istanbul Organized Leather Tanning Industrial District Wastewater Treatment Plant, Aydinli-Orhanli Mevki, Tuzla, Istanbul, Turkey 3 TUBITAK Marmara Research Center, Gebze, Kocaeli, Turkey

This study investigates the effect of hypochloride application for controlling bulking and foaming on the microbial ecology of an activated sludge system treating tannery wastewater. Detailed characterization of the wastewater treatment influent and effluent is also reported for the study period. During the study, bulking and foaming are first monitored with a sudden burst in the sludge volume index over 250 mL g−1 , creating a significant deterioration of the effluent quality. The corresponding upset in the microbial ecology is the combined excessive proliferation of M. parvicella, N. limicola II and Gordona (Nocardia) spp., but mainly triggered by Gordona contamination of the floc structure and the rapid outward growth of this filamentous microorganism extending to adjacent flocs. Chlorine application at an average rate of 3 g Cl−1 (kg MLSS.day)−1 for 12 days provide an effective solution for bulking and foaming, restoring the effluent quality. It destroys filamentous texture between the flocs, leaving only a lot of loose and chopped filament fragments and, totally removes the Gordona spp. from solution which retrieves back into the flocs. Therefore, chlorine remediation of bulking and foaming, although temporarily effective, is only superficial as the Gordona seeding inside the floc remains intact and potentially available for excessive growth in the next favorable conditions. Key Words: Activated sludge; Bulking and foaming; Chlorination; Gordona; Microthrix parvicella; Nostocoida limicola II; Sludge volume index; Tannery wastewater.

Received 6 April, 2006. ¨ ¨ Address correspondence to Suleyman Ovez, Environmental Engineering Department, Civil Engineering Faculty, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey; E-mail: [email protected]

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INTRODUCTION Bulking and foaming is perhaps the most serious chronic problem encountered in the operation of activated sludge systems. Bulking is always associated with excessive growth and dominance of filamentous bacteria in the aeration basin. During settling, the filamentous biomass stays largely in suspension causing significant escape of particulate solids from the settling tank. Moreover, filamentous growth when supported by the presence of surface active agents, synthetic or microbial, often leads to bacterial foaming. These foams accumulate in the aeration basin, sometimes entirely covering the surface or spilling out, and cause serious operation difficulties. These problems, when started, are often chronic in nature and tend to occur periodically. Remedial actions against bulking and foaming are mostly defined on a trial- and-error basis as site-specific applications. Among the large number of different filamentous microorganisms reported in the literature only a few such as Microthrix parvicella, Nostocoida limicola, Nocardia spp., Type 021N and Type 0041 of the Eikelboom species have been repeatedly cited with bulking and foaming problems. Wanner et al.[1] identified five dominant species-Microthrix parvicella, Type 0092, Nostocoida limicola, Type 0803 and Type 0041-regardless of the characteristics of the activated sludge systems. Different parameters exert controlling effect on these microorganisms. Low temperature and long-chain fatty acids are two of the most relevant parameters for tannery wastewaters as they support the preferential growth of filamentous microorganisms such as Microthrix parvicella at the surface, on accumulated fats and lipids. Higher temperatures trigger breakdown of the slowly biodegradable chemical oxygen demand (COD), which then becomes available to all other microorganisms, gradually eliminating filamentous growth.[2–3] Extensive research effort has so far been devoted to characterization, biodegradation and treatment of tannery wastewater[4–7] with no emphasis however on bulking and foaming problems. Only a recent study[8] revealed a striking feature of an activated sludge system treating tannery wastewater indicating that two of the important filamentous microorganisms, Microthrix parvicella and Nostocoida limicola, were always present in the floc structure, periodically at excessive levels, without causing however noticeable settling problems. Bulking and foaming were always associated with Gordona (Nocardia), which first contaminated the inner floc and continued proliferating, branching out and extending through adjacent flocs. At this stage, Microthrix parvicella and Nostocoida limicola significantly contributed to bulking and foaming, completely covering the bulk solution and decreasing the floc density. Chlorination is one of the most popular applications for the rapid and effective control of activated sludge bulking and foaming. Chlorine usually

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disrupts the cell wall structure and this way seriously affects the metabolic functions associated with the cell wall. Chlorination, while focusing on the destruction of filamentous bacteria should also make sure that the viability of floc formers is not drastically affected. In other words, the decay rate of filaments during chlorination should be higher than that of floc formers, with the presumption the floc structure hinders the inward diffusion of chlorine so that the pronounced effect stays on the outer part of the floc.[9] Chlorination is usually started with a dosage lower than 5.0 mg/L and gradually reaches 35 mg/L.[9] The most effective chlorine level is in the range of 1–15 mg/L according to Jenkins et al.[10] The effect of chlorine dose and period has been investigated in many studies. Ramirez et al.[11] studied the effect of chlorine dosage in the range of 4 to 8 g/kg VSS. day on Type 021N, observing significant SVI reduction after 10 days and a 90% decay of the same microorganism after 22 days. The objective of this study was to investigate the effect of hypo-chloride application for controlling bulking and foaming on the microbial ecology of an activated sludge system treating tannery wastewater. In this context, microscopic observations were carried out to visualize the impact of chlorine application on the floc structure and different filamentous microorganisms during a period of bulking and foaming, together with SVI measurements indicating gradual improvement in settling conditions of activated sludge.

MATERIALS AND METHODS Facility Characteristics Wastewater treatment at the Istanbul Organized Leather Tanning Industrial District basically involves an activated sludge system preceded by preliminary treatment (homogenization, screening, grid and oil and grease removal) and plain settling. The activated sludge unit receives combined tannery wastewater, without prior segregation and pretreatment of sulfide and chromium streams and therefore, it is often susceptible to conditions and compounds upsetting the stability of the activated sludge. The unit has a plug flow configuration with a total aeration volume of 27,500 m3 . The characteristics of the activated sludge influent (plain settled wastewater) generated in the District is well documented in a number of studies conducted on the same site for different purposes.[7,14,15]

Chemical Analyses Daily routine chemical analyses and the control parameters were completed in the District Laboratory. The samples were taken from the inlet of the aeration tank and the outlet of the final clarifier. The chemical analyses

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were performed according to Standard Methods 1998.[12] The analysis of total suspended solid (TSS) and volatile suspended solids (VSS) were measured by using Whatman GF/C glass fiber filter with an effective pore size of 1.3 µm the same approach and also used in soluble (filtered) COD analysis.

Microbiological Analyses Samples for microscopic examination were taken from the aeration basin three times a week and everyday during the bulking and chlorination period. The original samples were immediately observed after sampling under bright field and phase-contrast microscopy by preparing wet-mounts slides. These viable original samples were prepared using 0.5 mL (about one drop) of activated sludge on a glass slide. Two air-dried smear slides were also prepared for Gram and Neisser staining. The stained slides were also prepared spreading 0.5 mL activated sludge sample on glass smear slides and allowing them air dried in a room temperature. The Gram and Neisser staining methods were applied according to modifications of Jenkins et al.[10] The wet mounts slides were examined by using 40X, 100X, and 200X magnifications of phase contrast and bright field microscope and Gram and Neisser stained slides were examined under bright field microscopy by using 40X, 100X, 400X, and 1000X magnifications. The dimensions of the filamentous bacteria were measured on taken microphotos by using software, called the Spot program. The calibrations of measurements were corrected by using an ocular and an objective micrometer. The microscopic analyses were performed using an Olympus BX50 Model research microscope with attachments of phase contrast microscopy, an Ikegami camera and Spot analysis computer program. The filamentous bacteria were identified by using schemas, pictures and microbiological characteristics that were defined by Jenkins et al.[10] The abundance of each filamentous bacterium was counted using the subjective scoring method developed by also Jenkins et al.[10] Macro structures of the activated sludge flocs were examined according to Sezgin et al.[13]

RESULTS AND DISCUSSION Operation Characteristics of the Activated Sludge System The analytical data collected during the 1 month onsite survey at the activated sludge unit, where the operational problems due to sludge bulking and foaming have been observed and remedied, agreed with the previously reported results and revealed as shown in Table 1a typical strong wastewater with an average total COD of 2300 mg/L, total suspended solids of 790 mg/L, total Kjeldahl nitrogen of 226 mg/L, total sulfide of 59 mg/L and total chromium of 36 mg/L. During the survey, the average wastewater flow was


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Table 1: Characteristics of the biological treatment influent and effluent during the survey.

Parameter

Mean

Total COD, mg L−1 Soluble COD, mg L−1 TSS, mg L−1 VSS,% pH TKN, mg L−1 NH3 -N, mg L−1 Sulfide, mg L−1 Total Chromium, mg L−1 Oil & Grease, mg L−1

2331 1308 790 60 7.8 226 140 59 36 515

∗ Except

Wastewater characteristics Influent Effluent Range Mean Range 1680–2450 820–1370 450–1400 — 6.5–8.5 110–310 35–205 4–58 15–65 280–1800

256 172 75 — 7.7 114 104 0 2.2 20

180–430 100–250 40–145 — 7.1–8.0 67–140 76–120 — 0.8–4.0 15–30∗

under bulking conditions.

11,400 m3 day−1 and highly fluctuated in the range of 4900–21,400 m3 day−1 . An average mixed liquor suspended solid (MLSS) concentration of 5600 mg L−1 was sustained in the aeration tank, which secured a system operation with a food to microorganism (F/M) ratio of 0.20–0.50 kg COD (kg COD.day)−1 and an average sludge age of around 15 days.

Effect of Bulking and Foaming on Treatment Performance The experimental results provided a clear indication that activated sludge is an efficient process for the treatment of tannery wastewater, if bulking and foaming are effectively controlled. The effluent quality expressed in terms of major polluting parameters is summarized in Table 1, characterizing the whole survey aside from the period where bulking and foaming was experienced and remedied by chlorine application. As shown in this table, an average soluble COD of around 170 mg L−1 for the effluent may be interpreted to reflect complete removal of biodegradable organics as it corresponds to initial inert (non-biodegradable) COD in the process influent and soluble inert microbial products generated during biological treatment.[16,17] Similarly, average values of 256 mg L−1 for total COD, 75 mg L−1 for suspended solids 2.2 mg L−1 for total chromium and total removal of sulfide support effective treatment by activated sludge. Nitrogen was only partially removed due to selected process configuration and operation mode, as a more effective N removal is not a requirement for tannery wastewater. During the survey, the gradual development of bulking and foaming was observed through (i) high sludge volume index values and (ii) a deterioration of the settling characteristics of the activated sludge which resulted, for a few

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days, in a significant carry-over of particulate matter from the settling tanks and an outburst of related parameters in the effluent, i.e., SS, COD, total chromium, TKN, etc. Figure 1 illustrates the detrimental effect of bulking and foaming on effluent quality, increasing the SS concentration from 80 mg L−1 to 530 mg L−1 . Consequently, total COD was observed to increase from 270 mg L−1 to 850 mg L−1 , TKN from 125 mg L−1 to 160 mg L−1 and total chromium from 4 mg L−1 to 15 mg L−1 . The dominance of filamentous microorganisms in the biomass also resulted in a retarding effect on the biodegradation of the organic carbon possibly due to slower kinetics involved, as evidenced by the increase in the effluent soluble COD. The sludge volume index (SVI) profile in Figure 1 also shows that SVI, although still widely used as the main parameter of bulking, does not provide meaningful and reliable indication for predicting expected bulking and foaming problems, this way underlining the merit of microscopic observations for possible changes in the activated sludge ecology.

Chlorine Application Chlorine application was started to control bulking and foaming conditions, the day excessive biomass escape was observed from secondary settling tanks, causing serious deterioration of the effluent quality and continued for six days (Fig.1). The first day, an initial shock dose of 5100 kg of commercial chlorine with 15% hypochloride content was added to the sludge return flow during the first four hours and chlorine application was continued with an additional 5020 kg the same day in a sequence of 4-hour dosing and 2houridle periods. The following days, a total daily chlorine dose of 5000 kg was applied in the same sequence, as schematically indicated in Figure 2, corresponding to an average of 30 mg Cl−1 per liter of wastewater treated or 3.0 g Cl−1 per kg of mixed liquor suspended solids per day. Similar values for the first day were calculated as 207 mg Cl−1 L−1 and 11.8 g Cl−1 (kg MLSS.day)−1 during the initial shock loading and as 68 mg Cl−1 L−1 and 4.0 g Cl−1 (kg MLSS.day)−1 the rest of the day. The overall effect of chlorine application was quite noticeable in terms of improving the effluent quality beginning from the second day, inducing significant reductions in all measured parameters as shown in Figure 1. Initial effluent characteristics were fully restored at the end of the chlorination period.

Effect of Chlorination on the Ecology of Activated Sludge The core of the study was mainly composed of the microscopic examination of the activated sludge ecology at different stages, namely, normal conditions before bulking, during bulking and foaming and subsequent chlorine


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Figure 1: Concentration profiles of major parameters before, during and after bulking and foaming, (a) SVI and SS; (b) total COD, soluble COD and TKN.

application, and after the restoration and healing process, mainly to observe and evaluate the impact of chlorination on the fate of different filamentous microorganisms. Microbial properties of bulking and foaming in the activated sludge ¨ treatment of tannery wastewater were first observed by Ovez and Orhon,[8] who reported the presence of the Gordona spp. in the foam structure and within the flocs at the start of each problematic period; bulking developed through rapid multiplication of Gordona spp., branching out and combining with M. parvicella and N. limicola II to join with adjacent flocs. These two filamentous microorganisms were always present in the floc structure, unlike

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Figure 2: Schematic representation of chlorine application for (a) the first day; (b) the following days.

however in many other studies, only contributed to bulking when combined with excessive outward growth of Gordona spp. In this study, microscopic investigation of the floc structure during normal conditions before bulking and foaming started confirmed previous observations revealing a compact and healthy zoogleal type of floc, also involving as shown in Figure 3a. M. parvicella and N. limicola II filaments with slight Gordona contamination inside the floc(dark blue rod cells in the center of the floc). Observations during bulking and foaming were also almost identical with ¨ those by Ovez and Orhon,[8] showing disturbance of the floc structure, mainly triggered by excessive growth of Gordona spp. from inside the floc, rapidly branching and extending out in a way to reach adjacent flocs (Fig. 4a). As the outward branching growth of Gordona spp. proceeds, M. parvicella and N. limicola II provide a significant role in establishing bulking conditions by

Figure 3: Healthy floc structure: (a) before bulking and foaming; (b) after the healing process (Gram staining, bright field microscopy (1000X).


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Figure 4: The evolution of bulking and foaming (a) multiplication and branching out of Gordona spp.,(b) spreading of M. parvicella and N. limicola II to combine flocs (Gram staining, bright field microscopy (1000X).

extending between and connecting flocs (Fig. 4b). Consequently, the volume and the surface area of the activated sludge rapidly increases resulting in a much lower floc density and sludge volume index values over 150 mg g−1 , usually evaluated as a threshold limit for bulking and foaming. Microscopic observations during the chlorination period indicated, as illustrated in Figure 5, the clear destructive impact of chlorine as filaments became drastically disconnected, with a lot of loose and chopped filament fragments increasing the turbidity of the bulk solution. The most pronounced effect was on M. parvicella and Gordona spp., the latter being practically wiped out from solution, “hiding back” inside the flocs (Fig. 5a). The fragmentation was somewhat lower for Nostocoida limicola II filaments but their number was significantly reduced (Fig. 5b). After the chlorination period, the floc

Figure 5: The effect of chorine on filamentous microorganisms: (a) chopped and loose filaments in the bulk; (b) filament fragments of Nostocoida limicola II. (Gram staining, bright field microscopy (1000X).

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structure before bulking and foaming conditions was basically restored, but with substantial Gordona seeding still present within the floc (Fig. 3b). This indicates the inherent drawback of the chlorination process, which is adjusted to exert an effect on excessive filamentous growth outside the flocs to sustain the viability of the flocs after chlorine application. However, bulking is triggered by Gordona seed within the floc and its outward growth for tannery wastewater. Therefore, chlorine remediation of bulking and foaming is only temporary and superficial as the Gordona seeding inside the floc remains intact and potentially available for excessive growth in the next favorable conditions.

CONCLUSIONS The study provided a scientific insight into the microbial ecology of activated sludge during the practical application of chlorine as a remedial action against bulking and foaming in a biological treatment system for tannery wastewater. During the study, the overall effect of bulking and foaming was experienced with a sudden burst in the sludge volume index over 250 mL g−1 , creating a significant deterioration of the effluent quality in terms of all measured parameters. The disturbance was observed to be triggered mainly by Gordona (Nocardia) contamination of the floc structure and the rapid outward growth of this filamentous microorganism extending to adjacent flocs. This process was stimulated by similar proliferation and connecting action of mainly M. parvicella and N. limicola II. Chlorine application at an average rate of 3 g Cl−1 (kg MLSS.day)−1 for 12 days provided an effective solution for bulking and foaming, restoring the effluent quality. It was observed to exert a total destruction of the filamentous texture between the flocs, leaving only a lot of loose and chopped filament fragments with resulting turbidity in the bulk solution. Gordona spp. the main microbiological agent of bulking and foaming was totally removed from solution and retrieved back into the flocs. Therefore, based on evidence provided by microbiological observations, it can be concluded that chlorine remediation of bulking and foaming, although apparently effective, is bound to be temporary and superficial for tannery wastewater, as Gordona spp. still remains intact inside the floc and maintains its potential for excessive growth. Potential control strategies for the excessive growth of this species within the activated sludge floc still remain to be investigated.

ACKNOWLEDGMENTS This research was supported by two different research grants, one from the ITU Research Foundation and Development Center and the other from


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the Scientific and Technical Research Council of Turkey (TUBITAK-Project No:100Y045). The corresponding author acted as the principal investigator in both research projects. The contribution of Derin Orhon was partly sponsored by the Turkish Academy of Sciences. The support of the Istanbul Organized Leather Tanning Industrial District I activated sludge treatment plant management, engineers and technicians is gratefully acknowledged.

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