Desalination 199 (2006) 319–321
Membrane bioreactor performances: comparison between continuous and sequencing systems Jorge Lobos*, Christelle Wisniewski, Marc Heran, Alain Grasmick Laboratoire Génie des Procédés d’Elaboration des Bioproduits UMR-CIRAD 016, Université Montpellier II- CC005, 34095, Montpellier Cedex 05, France email:
[email protected] Received 18 October 2005; accepted 2 March 2006
1. Introduction Nowadays, membrane bioreactor performances are well known in term of quality of pollutant removal from wastewater including possibility of treated water reuse. As regards the excess sludge production, the strict levels of environmental directive and the limiting disposal options, imposed to develop strategies on the reduction of biomass production in wastewater treatment [1]. A type of strategy consists in limiting the amount of substrate supplied to the biomass, i.e. the food to microorganisms (F/M) ratio. In this substratelimited condition the allocation of the substrate is preferentially turned to maintenance energy requirements [2,3]. However, to ensure a capability of the system to treat acceptable organic volumetric loadings, development of reactors with high biomass concentration is essential. On this way, fixed cultures and specially membrane bioreactors (MBR) appears as adequate solutions. Indeed, according to Pirt theory [3], it seems possible, for a given F/M ratio, to reach a steady state biomass concentration in a continuous system without any sludge extraction and
consequently, a MBR system should be able to achieve a stable biomass concentration in the reactor avoiding any excess sludge production. As well Ref. [4] showed that the initial substrate/biomass ratio is a critical parameter in batch experiments: when cell growth is limited by carbon source, low initial substrate/biomass ratios, the bacterial metabolism imposed storage compounds synthesis. This phenomenon — synthesis and consumption — leads to biomass conversion yields lower than a biomass replication process which is favourable at high substrate/biomass ratios [5]. This result may be an important point when comparing continuous and sequencing bioreactor functioning at the same organic loading. Indeed a continuous feeding of the reactor corresponds to a continuous low F/M ratio (with adapted MLVSS in the reactor) then a sequencing system will receive momentously higher F/M ratio during the feed period. The aim of this work is to study and compare the biological and filtration performances of two immersed membranes bioreactors, one is operating in a sequencing way (MSBR), the other one in a continuous way (MCBR). The operations
*Corresponding author. Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.075
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Neither in the continuous operation nor in the sequential operation a stable biomass concen-tration measured as MLVSS was reached. A relatively constant increase in MLVSS was observed despite the presence of cycles of growth and pseudo-stabilisation. During the overall operational period, a 0.044 gMVS/gCODT (0.058 gCODP/gCODT) and a 0.091 gMVS/ gCODT (0.092 gCODP/gCODT) observed sludge yields were obtained for the continuous and sequencing reactor respectively. These values are approximately 5–10 times lower than those measured in conventional activated sludge process [6]. Results point out clearly the higher biomass production in the sequencing reactor presumably for the optimums conditions for biomass replication during the feed period. Though the daily organic loading was the same, the principal metabolic process is different. Fig. 1b shows the evolution of the COD total removal efficiency and the COD of the effluent for both operations. The COD removal efficiency was higher for the continuos operation to reach a value — after a period of biomass adaptation — higher than 97% with a COD in the effluent lower than 50 mg/L. The filtration performance was better for the continuos operation too, where in spite of the continuos increase in MLVSS a nearly constant transmembrane pressure evolution was observed. In this case
were carried on without any sludge extraction to check the Pirt’s theory. 2. Materials and methods The experiments were performed in pilot membrane reactors with a working volume of 50 L. The MBR systems were operated for a long period (5 months) without any sludge purge except for sampling. A complex substrate with respect to soluble and suspended fraction was selected as feed, it was constituted of acetate and meat extracted (Viandox®). A volumetric organic loading of 1 gCOD L–1 d–1 with a COD feed concentration of 2.0 gCOD L–1 was used. An aerobic mixed culture from a municipal wastewater treatment plant was used to inoculate the bioreactor. Polysulphone membranes were used with a pore size of 0.1 mm. The effective surface of the membranes was 0.3 m2. The membrane module and hydrodynamics were designed to reduce fouling. The filtration operations were conducted in sub-critical conditions (J < 5 L m–2 h–1). Operation mode of MSBR was 12 h cycle with 0.32 h of feeding time. 3. Results and discussion Fig. 1a shows the evolution of MLVSS concentration in the reactor for both operations. (b)
(a)
3000
16
12 MSBR 10 8 6 4 MCBR
2
y = 0.0442x + 3.3011 R2 = 0.9278
2500 CODT effluent (mg/L)
y = 0.091x + 3.1474 R2 = 0.9738
90 85
2000
80 1500
75 70
1000
65 60
500
CODT removal efficiency
95
14
MLVSS (g/L)
100
55 0
0 0
10
20
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40
50
60
70
80
90 100 110 120 130 140
Organic matter removed (gCODT/L)
0
50 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (d)
Fig. 1. (a) MLVSS vs. organic matter removed for biomass yield determination. Operation in MCBR (s) and MSBR (u ); (b) Effluent COD: MCBR (s) and MSBR ( ), COD removal efficiency: MCBR (l) and MSBR (O).
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the filtration conditions allowed a long time operation period (3600 h) without necessity of membrane regeneration.
[2]
4. Conclusions The results show better performance for the continuous membrane reactor respect to the biomass production, organic matter removal and filtration. The process of storage–consumption process was not favourable with the operations conditions selected in the MSBR. The results show that a MSBR cannot be consider as only a series of batch reactors jointed end to end.
[3]
References
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R.D. Davis and J.E. Hall, Production, treatment and disposal of wastewater sludge in Europe from
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a UK perspective, Eur. Water Pollution Control, 7(2), (1997) 9–17. E.W. Low and C. Howard, The effect of maintenance energy requirements on biomass production during wastewater treatment, Water Research, 33 (3), (1999). S.J. Pirt, The maintenance energy of bacteria in growing cultures, Proc. Roy. Soc. London, Ser. B., 163, (1965), pp. 224–231. J. Lobos, C. Wisniewski, M. Heran and A. Grasmick, Effects of starvation conditions on biomass behaviour for minimisation of sludge production in membrane bioreactors, Water Science & Technology, 51 (6–7), (2005). M.C.M. Van Loosdrecht, M.A. Pot and J.J. Heijnen, Importance of bacterial storage polymers in bioprocess, Water Science & Technology, 35 (1), (1997). P. Pitter and J. Chudoba, Biodegradability of Organic Substances in the Aquatic Environment, CRC Press, Boca Raton, USA, 1990.
