MBR performance: Operational problems in industry

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Filtration+Separation November/December 2011

A major study undertaken by a number of institutions and a leading

engineering consultancy has investigated factors affecting the

performance in operation of membrane bioreactors in a variety of

different industrial applications. José Antonio Gil summarises the findings.

One disadvantage not often reported in the field of industrial membrane bioreactors (MBRs), is related to operational problems affecting the process. Operational problems affecting MBRs are normally linked either to the quality of the sludge, problems related to the equipment or to the incoming wastewater derived from the industrial process. According to Le-Clech et al. [1], although membrane fouling captures most attention, the loss of membrane integrity, selection of pre-screen devices and production of biosolids are much bigger concerns for MBR users.

One of the most common problems affecting the operation of industrial MBRs appears when the dissolved oxygen (DO) drops under a certain threshold (normally 1 mg L-1) for long periods. Bad design of the aeration system (air blowers, conductions and diffusers) can result in undesirable consequences. Low DO concentrations are responsible for deflocculation and particle release processes [2]. These processes liberate sub-micron particles (< 1μm) to the mixed liquor, which is detrimental to filterability. Low DO profiles also create favourable conditions for excessive growth of filamentous microorganisms [3]. Filamentous species attach to and stabilise air bubbles to become thick, stable, persistent and scum-like foam. Stable foams disturb the biological process interfering with level sensors, whereas spillages of foam from aeration tanks to walkways with the consequent loss of biomass, cause safety problems.

Filterability measurements offer a powerful tool for operators and can provide useful information to monitor MBR performance and seek causes in case of a problem in operation. In this study, filterability measurements and the monitoring of different activated sludge parameters have been completed with operator’s feedback in order to investigate the causes leading to bad filterabilities in industrial scenarios.

Eleven full-scale industrial MBRs were investigated for a period of eight months (April-November 2009). The MBRs were located in the following industrial applications in the Netherlands, Germany and Belgium: A – chemical; B – rendering; C – malt processing; D – waste treatment; E – malt processing; F – food (wheat and corn); G – brewing; H – tank cleaning; I – tank cleaning (food grade); J – tank cleaning (food grade); and K – food storage/juice production.

In order to determine the filterability of the activated sludge, the delft filtration characterization method (DFCm) was employed. The DFCm consists of a standardised small-scale membrane Filtration Characterization unit (FCu) operated on the basis of a standardised measuring protocol. The DFCm allows the measurement and characterisation of different samples of activated sludge under identical conditions, facilitating unequivocal comparison of the filterability quality of activated sludge samples collected under different circumstances or from different installations.

The ΔR20 value is used for easy comparison between tests. This value refers to the resistance increase after a specific permeate production of 20 L m-2. The classification of the filterability (see Table 1) is based on a scale adapted from Geilvoet [4] and Van den Broeck [5].

SCOD removal and MBR operators

The typical range for soluble chemical oxygen demand (SCOD) in the effluent from municipal wastewater treatment plants, is 30-50 mg SCOD L-1, which constitutes the inert SCOD component of municipal waste [6]. Despite operational problems that some MBR faced, Figure 1 shows that on average, chemical oxygen demand (COD) values (191 ± 438 mgCOD L-1) were below the discharge limit (200 mgCOD L-1), indicating the robustness of the MBR technology. However, this limit varies according

to municipal regulations, point of discharge and characteristics of the inflow, such as in location D where the discharge permit increases up to 3,270 mgCOD L-1 due to the highly toxic and non-biodegradable inflow.

Within this industrial sample, a high number of MBR operators did not have a background in wastewater treatment and on occasions some employees were assigned to take care of the process with no previous training. Industrial MBRs are normally fully automated. However when problems arise, well-trained and experienced personnel are required to restore the situation within a short period of time.

Filterability values related to reactor

perturbations

Figure 2 shows an overview of the ΔR20 values corresponding to those MBRs facing problems along with the mixed liquor suspended solids (MLSS) concentration. Of the eleven industrial MBRs analysed in this study only three did not present any problems at all, showing good filterabilities. On the other hand, in some cases, the filterability was so bad that a limit value for representation of 5×1012 m-1 was established.

Operational problems affecting

filterability

Figure 3 shows the impact of COD on the supernatant (CODsup), dissolved organic carbon (TOCsol) and counts in the range 0.4-5μm on

Table 1: ΔR20

and corresponding

filterability qualification

ΔR20 [*1012 m-1] Qualification

< 0.1 Good

0.1-1.0 Moderate

1.0-3.0 Poor

> 3.0 Bad

MBR performance:

Operational problems in industry

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filterability for locations A, B and F, which presented different problems.

The poor filterability monitored in location A (Chemical industry) could be related to the high sludge retention time (SRT) – 160±10 days – as a result of the shutdown of one division of the company. The progressive improvement observed on filterability did not show any correlation with the counts or with TOCsol, but with CODsup. This suggests that the colloidal pollutants contained in the supernatant produced either at high SRTs and/or derived from the photo-chemical process are more relevant in terms of fouling.

To remove phosphorus and achieve discharge concentrations below 1-2 mg L-1, FeCl3 has been used as a coagulant for several years in location B (Rendering). Visual observation revealed a brown pigmentation on the membrane straws in this location suggesting the solubilisation of metals by acids coming from the nitrification-denitrification process, as well as the acidity of FeCl3. Since the beginning of November

2009, Al(OH)3 has replaced FeCl3. The reasons to change the coagulant were:

• The acidity of Al(OH)3 is lower than that of FeCl3.

• The price of Al(OH)3 is around half of FeCl3.

• Fe and other metals could precipitate onto the straws causing inorganic fouling.

Figure 3 shows no correlation between colloidal and soluble pollutants with the filterability. In other words, in the rendering industry, the particles derived from the industrial process are to some extent more responsible for changes in filterability than the pollutants. However, changes in the operation can lead to serious problem with filterability, adding to the negative effects of normal components of industrial activated sludge. In this location the change of coagulant increased the value of ΔR20 by a factor of around 60, resulting in a clear deterioration of the filterability.

After many FCu measurements, location D (waste treatment) recorded the worst filterability measured (see Figure 2). The cause of such bad filterability is attributed to high polluted inflow and infinite SRT. This high retention time leads to the absence of flocs, leaving the microfauna dispersed in the sludge. Together with a really high MLSS concentration (53±14 g L-1) the activated sludge at this location was categorised as ‘unfilterable’. In this location, one of the highest number of particles was observed (34 ± 2×106 mL-1), probably due to both the poor characteristics of the sludge and the applied shear force. However, due to the high crossflow velocity imposed (> 3.5 m s-1), no cake layer is formed and the particles have less chance to deposit onto the membrane. Of course this has economic repercussions, such as the high energy cost (7.3 kWh m-3) necessary to maintain such an operation.

In location F (wheat and corn processing) a spillage occurred at the beginning of March due to a DO limitation. As a consequence, a new start-up was required. The start-up of an MBR

Figure 1: COD and discharge permits for industries. Figure 2: Filterability along with MLSS values for those MBRs presenting operational problems.

Figure 3: ΔR20

vs TOCsol

, CODsup

and counts for locations A, B and F.

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is a complex process, in which, depending on the strategy selected, different performances can be expected. It was decided that location F would start-up at the same flux as in steady-state conditions (22 L m-2 h-1) with a low MLSS concentration (~3 mg L-1). At this first stage of the process, the biomass is not yet acclimatised, and the flocs are in the process of being formed, gaining in size and strength. The improvement observed on the filterability with the increase of the MLSS concentration during the start-up (see Figure 2), suggests that at this first stage the sludge is not capable of coping with the high strength industrial wastewater derived from the process of converting wheat and corn into starches. Both CODsup and colloidal particles followed the same tendency to decrease as ΔR20 during the start-up, as can be seen in Figure 3. This suggests that the higher the MLSS concentration and the size of the flocs, the more capable the activated sludge becomes to degrade pollutants and trap particles [7]. This is also reflected by the COD removal efficiency shown in Figure 1. Possible considerations when starting-up industrial MBRs handling high strength industrial wastewater could be high MLSS concentrations, the use of coagulants, or methods to accelerate the floc-growing process.

Low DO and filamentous bulking

Low DO concentrations were observed in locations C, F, G and K. In locations C, F and K, the aeration capacity was undersized, while malfunctioning problems were reported in the blowers from location G. Figure 4 shows the DO concentration profile for locations C, F and G for the studied period.

Location C (malt processing) presented DO values below 1 mg L-1 in February and in the period May-October. As can be seen in Figure 5, foams were observed in the aerobic compartment coinciding with the highest values of ΔR20, 4.7×1012 and 1.3×1012 m-1 for June and October respectively. Foaming periods can be an indicator of an excess of filamentous microorganisms. The MLSS concentration also dropped, which coincided with the periods where foams were present (see Figure 2).

For location C a slight increase in all analysed parameters matches the problems with filterability observed in the last sampling period. In the June sampling, none of the parameters analysed in Figure 6 are able to explain the bad filterability observed. An excess of filamentous bacteria seems to be the only explanation for such bad filterability (ΔR20 = 4.7×1012 m-1).

Images corresponding to the structure of the activated sludge and classified according to the Jenkins scale [8] are shown in Figure 7. As a result of the low DO conditions, location C showed poor filterability in October (ΔR20 = 1.3×1012 m-1) coupled with a very common (4) classification of filamentous plus dispersed materials i.e. cell material resulting from cell lysis and deflocculation processes.

Location B presented a common (3) number of filaments showing the change of coagulant as the main reason for the bad filterability observed.

Low DO concentrations had led to a spillage in location F. However, during the final sampling, operating again under steady state conditions, the activated sludge showed poor filterability (ΔR20 = 2.9×1012 m-1), as a consequence of another low DO episode in the September-October period. Low and prolonged DO concentrations are related to deflocculation processes [2]. Indeed, a high number of counts (28×106 mL-1) was found in the last sampling coupled with an excess of filamentous microorganisms. In fact, in Figure 7 an abundant (5) number of these microorganisms can be seen in the outline of the flocs in particular. These results suggest that both the particles released to the supernatant plus the excess of filamentous derived from a deficient DO concentration resulted in a loss of filterability.

In location G (brewing), poor filterabilities can be related to the low MLSS working concentration imposed by a deficiency in the settlers. The first filterability value (ΔR20 = 1.1×1012 m-1) was obtained under normal circumstances, reflecting a sludge with a poor/

moderate filterability. In the second sampling, a bad filterability (ΔR20 = 4.5×1012 m-1) was observed. Alternative aerators broke down consecutively and, coupled with the high temperatures of the summer time, led to a drop of the DO for the June-October period (see Figure 4). Figure 6 indicates a stronger impact of CODsup than that of TOCsol, and a big impact of counts on filterability. Gil et al. [9] suggested that the soluble fraction contributes less to the fouling process due to its ability to break through the membrane. Figure 7 shows the composition of the sludge for the final sampling with an abundant (5) number of

Figure 4: DO profiles for locations C, F and G. (Data from location K not provided).

Figure 5: The aerobic basin from location C on the same day the filterability was analysed.

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filamentous microorganisms coupled with a poor filterability (ΔR20 = 2.2×1012 m-1). As in location F, particle release processes coupled with an excess of filamentous bacteria, led to such bad filterabilities.

Location J (tank cleaning) exhibited good filterabilities for all the period analysed except July. The substances cleaned from trucks in this tank cleaning location, for example juices, syrup, chocolate and lecithin, are categorised as ‘food degree’. Different foaming episodes have been noticed by operators due to these substances. Figure 7 illustrates the structure of the sludge in July, indicating an excessive (6) growth of filamentous bacteria (Thiotrix I, II and Type 021N). It seems that foaming agents derived from food transportation, coupled with an excess of filamentous bacteria, led to the bad filterability observed in July (ΔR20 = 3.4×1012 m-1). In contrast Figure 7 shows the composition of the last sample taken in November with a common (3) number of filamentous bacteria forming the base of the floc. As a result, the activated sludge exhibited a good filterability (ΔR20 = 0.06×1012 m-1).

Finally, in location K (juice production), bad filterability was observed in June (ΔR20 > 5×1012 m-1). In this location, the oxygen necessary for the biological process was underestimated. Another problem observed in this first sampling was related to the excess of polymer used to dewater the sludge. The polymer was introduced to the system with the filtrate stream coming from the belt filter. In Figure 7 the excess of polymer (shiny flocs) attached to the surface of the flocs and slimy clusters of bacteria can be observed for the sampling undertaken in June. As a consequence, the hydrophobicity increases which hampers the transfer of oxygen into the floc, resulting in deflocculation processes. Currently the polymer is added between the

belt filter and the bioreactor, which increases the contact time between polymer and sludge. Even though low DO and bad filterability were observed, filamentous bacteria were present in a common (3) number in June and November. In June S. Natans, Thiotrix I, and Nocardia sp were detected, with Nocardia sp being the predominant one. The last problem reported at this location was the malfunction of the belt filter in the period September-October, increasing both the MLSS concentration and the SRT up to 23 g L-1 and 600 d respectively. None of these problems affected the filterability of the sludge (ΔR20 = 0.02×1012 m-1), but the filtration performance in terms of clogging. A new cleaning strategy was adopted by the operators, cleaning the membranes with pressurised water to remove the excess of sludge. Hence, excluding the excess of filamentous bacteria, the bad filterability observed in June was due to a combination of two factors: low DO conditions and an excess of polymer attached to the flocs. As in previous locations, these two factors led to deflocculation processes, increasing the number of counts up to 38×106 mL-1. As a result of the measures taken, (DO set-point restored and increased contact time between

polymer and sludge), excellent filterabilities were observed for the rest of the analysed period. Figure 7 shows the floc structure of the final sample. Well defined and consistent flocs with a

Figure 6: ΔR20

versus TOCsol

, CODsup

and counts for locations C, G and J.

Figure 7: Images of the activated sludge for locations B, C, F, G, J and K - ×100 (Ph1).

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common (3) number of filamentous bacteria led to a good quality of the activated sludge in terms of filterability (ΔR20 = 0.02 × 1012 m-1).

Filamentous bacteria and counts

versus filterability

It has been demonstrated that low DO conditions can lead to both excessive growth of filamentous bacteria and particle release, negatively affecting the filterability. Secondary effects derived from low DO levels, for example defloculation processes and excessive growth of filamentous bacteria, cause a deterioration of the filterability of the activated sludge. In Figure 8 ΔR20 values are plotted along with the amount of filamentous bacteria observed. Figure 8a shows how a common (3) number of filamentous bacteria is related to good filterabilities. A tendency to increase can be observed between ΔR20 and the content of the filaments. In Figure 8b those locations reporting high number of counts due to low DO conditions are plotted.

The average value of counts in this research was 14.8 ± 12.1×106 mL-1, and only 0.54 ± 1.2% of the counts represented a size of > 1μm for the range analysed (0.4-5μm). Location C showed moderate filterability in May. This is in accordance with the findings reported by Gil et al. [9] – low influence of counts on filterability for tank cleaning and malt industries. The rest of locations showed poor filterabilities (ΔR20 > 1×1012 m-1) coupled with a high number of counts (29.7 ± 5.6×106 mL-1). Therefore, for those industrial MBRs affected by low DO concentrations, both the negative impact of excessive growth of filamentous bacteria and an excess of fine particles was detrimental to filterability. •Acknowledgements

This study was financially supported by the research project MBR-TRAIN contract

No.: MEST-CT-2005-021050. Marie Curie Host Fellowship for Early Stage Researchers Training and Witteveen + Bos.

This work has been also supported in part by projects OT/10/035, OPTEC (Center-of-Excellence Optimization in Engineering) PFV/10/002 and SCORES4CHEM KP/09/005 of the Katholieke Universiteit Leuven, and by the IWT-O&O-MEMFICS-70406 project and the Belgian Program on Interuniversity Poles of Attraction. In addition, the cooperation of engineers, MBR operators and the management of the eleven locations included in this research.

References

[1] P. Le-Clech, A. Fane, G. Leslie, MBR focus: the operators' perspective, Filtr. Separat., 42 (2005) 20-23.

[2] B.-M. Wileá, K. Keiding, P.H. Nielsen, Anaerobic deflocculation and aerobic reflocculation of activated sludge, Water Res., 34 (2000) 3933-3942.

[3] A.M.P. Martins, K. Pagilla, J.J. Heijnen, M.C.M. van Loosdrecht, Filamentous bulking sludge—a critical review, Water Res., 38 (2004) 793-817.

[4] S.P. Geilvoet, The Delft Filtration Characterisation method – assessing membrane bioreactor activated sludge filterability. PhD thesis, in, Delft University of Technology, Delft, The Nerderlands, 2010.

[5] R. Van den Broeck, P. Krzeminski, J. Van Dierdonck, G. Gins, M. Lousada-Ferreira, J.F.M. Van Impe, J.H.J.M. van der Graaf, I.Y. Smets, J.B. van Lier, Activated sludge characteristics affecting sludge filterability in municipal and industrial MBRs: unraveling correlations using multi-component regression analysis, J. Membr. Sci., 378 (2011) 330-338.

[6] IAWQ Activated Sludge Model No. 2 IAWQ Scientific and Technical Report No. 3, (1995).

[7] X. Tu, S. Zhang, L. Xu, M. Zhang, J. Zhu, Performance and fouling characteristics in a membrane sequence batch reactor (MSBR) system coupled with aerobic granular sludge, Desalination, 261 (2010) 191-196.

[8] D. Jenkins, M.G. Richard, G.T. Daigger, Manual on the causes and control of activated sludge bulking and foaming, (1986).

[9] J.A. Gil, P. Krzeminski, J.B. van Lier, J.H.J.M. van der Graaf, T. Wijffels, D. Prats, Analysis of the filterability in industrial MBRs. Influence of activated sludge parameters and constituents on filterability, J. Membr. Sci., 385-386 (2011) 96-109.

Contributors/Authors:

J.A. Gil, P. Krzeminski and J.B. van LierDepartment of Water Management, Section Sanitary Engineering, Delft University of Technology, The NetherlandsTel: +49 241 750 82 17E-mail: [email protected]/[email protected]/[email protected]

J.H.J.M. van der Graaf and T. WijffelsWitteveen + Bos, The NetherlandsE-mail: [email protected]/[email protected]

R. Van den Broeck, I.Y. Smets and J.F.M. Van Impe BioTeC – Chemical and Biochemical Process Technology and Control, Chemical Engineering Department, Katholieke Universiteit Leuven, BelgiumE-mail: [email protected]/[email protected]/[email protected]

D. PratsUniversity Institute of Water and Environmental Sciences, University of Alicante, SpainE-mail: [email protected]

Figure 8:(a) Relation between filterability and the presence of filamentous bacteria; (b) Relation between filterability and the number of counts for locations with low DO.

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MBR performance: Operational problems in industry

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