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Dr Simon Judd, Cranfield University, UK, compares two types of submerged membrane bioreactors currently operating at sewage

treatment plants; one in the UK and the other in France.

Submerged Membrane Bioreactors: Flat Plate or Hollow Fibre?

T m e membrane bioreactor (MBR) represents an important embrane process development. The association of

iological treatment with membrane filtration produces a compact process able to comply with stringent discharge criteria. The process can be adapted to urban and industrial wastewater and has been used, extensively in Japan, for water recycling. The presence of membranes replacing a conventional darifier allows the complete retention of biomass within the biological reactor, and thus a high mixed liquor or sludge concentration, enabling high hydraulic loading and tow waste volumes.

The first generation of MBRs were sidestream systems with the membranes placed in a recirculation toop external to the bioreactor. In such systems, the mixed liquor is pumped, so as to enable a high crossflow velocity in the membranes, and the concentrated mixed liquor returned to the activated sludge tank. More recently-developed MBRs are based on a submerged

configuration, with the membrane placed directly in the aeration tank containing the mixed liquor. These rely on low-flux operation to reduce fouling as much as possible and permit operation at low transmembrane pressures (TMPs). This makes them well suited to relatively large-scale applications, such as sewage treatment.

The plants at Portock, UK, and Perthes en Gatinais, France, are both examples of submerged MBR technology. They are of comparable size (Table 1), have been operational for about the same length of time and are both employed for sewage treatment. Both use coarse-bubble aeration of vertically- mounted membranes as the principal means of promoting mass transfer, and therefore permeate product flow, and the technologies give similarly high-quality effluent demanded by the discharge consents. They are, however, markedly different in design (Table 2) and operation (Table 3).

Parameter Perlock Perthee en Getinais

Owner Wessex Water Vivendi Environnement Contractor MBR Technology Vivendi Environnement Year of Commissioning 1998 1999 Capacity [m3/day) 1900 1500 Total number of tanks 4 1 Total membrane area [m 2) 2880 2580 Pretreatment 3 mm perforated screen 1 mm screening

Parameter

Membrane type Flat plate MF Hollow fibre MF Membrane supplier Kubota Zenon Mode of operation Out-to-in Out-to-in Membrane element/module 0.4 m x 0,8 m panel ~2000 1.9 mm O.D. fibres

(2 m (h) x 0.7 m (w) x 0.2 m (t)) Membrane cassette 150-element cassette 8-module cassette Cassette area (m 2) 128 368

Flate Plate vs, Hollow Fibre

The principal differences arise from the membrane module configuration. The Zenon system is based on hollow fibre membrane elements, which are less expensive to produce than plate-and-frame modules and are also backflushable. On the other hand, because the hydrodynamics is less readily controlled in such systems, they

are more prone to fouling than either flat plate or tubular modules and so require more frequent washing and cleaning. The flat plate Kubota membranes cannot, in fact, be backflushed making it more critical that they are operated at a flux below that at which fouling becomes significant, the so-

Parameter Porlock Perthes en Gatinais

Coarse bubble aeration rate (approach velocity) TMP (bar) Flux (I/m2/h) Backflush cycle time [s) Backflush duration (s) Backfiush volume Clean-in-place frequency External chemical cleaning

0.6 Nm3/h per element, (~0.05 m/s) 0.1-0.15 20-27 Continuous

0.5% hypochlorite for 5 hours after 9 months

38-45 m3/h per module (~0.1 m/s) 0.2-0.5 23-33 600 45 ~ 25% of permeate product 1-2 times a week (hypochlorite) 1-4 times a year {hypochlorite/acid soak)

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Figure 1: Process flow sheet of the Perthes en Gatineis MBR plant.

called 'critical flux'. In fact, it is impossible to completely prevent 'irreversible' fouling (that is fouling by adsorbing solutes and colloidal materials which can only be removed chemically), but the fouling rate can be significantly reduced by adopting the appropriate operating conditions.

The backflushing and cleaning cycles at Perthes en Gatinais demand 25% of the product water. Backflushing recovers most of the flux by removing 'reversible' fouling material, but the residual irreversible foulant leads to an increase in the TMP over the course of a few days. The fouling rate of the Kubota membranes at Porlock appears to be much lower, but the membrane modules themselves are also more expensive (perhaps 20-25% more than the Zenon membrane).

The overall sewage treatment scheme also differs at each site. The plant at Perthes en Gatinais (Figure 1) includes a somewhat finer screen than that at Porlock, since dogging of the hollow fibre membranes is extremely onerous. The plant also includes a storage tank to regulate the flow, and therefore suppress potentially damaging hydraulic shocks. It achieves phosphate removal by dosing with ferric chloride, the phosphate being removed with the excess sludge. The plant also has a small sludge dewatering facility which produces a 25 wt% sludge product employed for local agriculture. At the Porlock plant (Figure 2), the four tanks sit in a single stone-faced building which was purpose-built so as to compliment the landscape. The constraints on available space demand that all processing of the 3-6.5 m3/day of 2 wt% sludge generated is carried out off site.

In many respects, however, the MBR plant performance at each site is similar - particularly with reference to water quality

improvement, since both~employ membranes of similar pore size. Both achieve greater than 5-1og removal of faecal coliforms and significant nitrification. Both operate at high mixed liquor concentrations, typically 15-18 g/l, associated with high sludge retention times (>20 days) and low hydraulic retention times (<4hr), and around 0.3-0.5 kg sludge per kg BOD loading is produced at both sites. There are no significant odours from either process and, aside from possible foaming incidents (to which all submerged MBR processes are susceptible due to the high aeration rates required), operation since commissioning appears to have been reliable.

Any assessment on the relative merits of the two systems is therefore likely to be based solely on cost. The Zenon membranes are certainly less expensive than the Kubota membranes, but the actual process is made slightly more complex by the backflushing requirement. Also, the Kubota process tends to lead to lower hydraulic resistances due to the greater hydrodynamic control. However, the most significant component of the operating costs is the aeration, and figures presented for this cost seem to vary widely from as low as 0.3 KWh/m 3 to as high as 2 KWh/m 3 permeate product. Clearl); the effectiveness of the aeration is a key aspect of MBR technologies. It remains to be seen whether technical improvements in aeration systems can yield lower MBR operating costs, in addition to continuing efficiencies in fabrication techniques lowering the cost of the membranes. O

Contact: Simon Judd (Dr), Reader in Water Sciences,

Cranfield University, Cranfield, Bedfordshire MK43 OAL, UK.

Tel: +44 1234 754842; Website: www. cranfield, ac. uk /s ims/water

More information on the use of membranes in the French water industry Is available at a special meeting being held at Cranfield Universitg UK, on 18 July 2002 (contact the Short Course Office on +44 1234 754176; E-mail: shortcourse@cranfield.ac.uk). Cranfield University.is also running a two-day short course on MBRs between t5-t6 July 2002, details of which are also available from the Short Course Office.

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Washing tank Membrane units

Figure 2: Schematic of the Porlock sewage MBR plant.

Filtration +Separation June 2002 3 1