Page 287 - Membranes for Industrial Wastewater Recovery and Re-Use

256 Membranes for lndustrial Wastewater Recovery and Re-use

sludge retention time of 20 days. This range of mixed liquor concentration is well
known to provide optimal performance of the MBR system and is necessary to
reduce operational problems that occur at either lower of higher values.
The membrane bioreactor plant (Fig. 5.18) consists of a suspended growth
bioreactor into which six double-tier membrane modules are submerged
providing a total membrane area of 960 m2. The membranes are hydrophilic
polyethylene hollow fibres rated at a pore size of 0.1 pm. The system is designed
to treat 140 m3 d-l under a constant flux of 5 LMH. Flow through the
membranes is generated by a combination of hydraulic head and permeate
suction providing a total transmembrane pressure of between 25 and 40 kPa
(0.2 5-0.4 bar) depending on the level of fouling. Fouling control is achieved by a
combination of air scour over the membrane fibres and periods of intermittent
operation which relaxes the fouled layer and enables the air scour to remove
more material. Chemical cleaning is required every six months by an overnight
soak in 0.1% sodium hypochlorite and 4% sodium hydroxide solution. The
membranes have as yet not required replacement during the 4 years of
operation.

Reverse osmosis
The final stage of the treatment train is a reverse osmosis plant arranged in a
2:2: 1 array. The membranes are polyamide spiral-wound modules supplied by
Toray. The plant contains a total membrane area of 70 m2 and operates at a flux
of 24 LMH equating to an overall recovery of 67%. Chemical cleaning occurs
once a month by means of a chemical feed washing with a solution of ethanol
(15%)atpH 12.

5.8.3 Performance
The treatment plant went online in May 1998 and is still being used today to
supply both wash water and boiler feed. The MBR plant effectively removes the
majority of the organic and suspended pollution found in the wastewater. The
BOD is reduced from 2240 mg I-' in the feed to less than 1 mg 1-1 in the MBR

F:

(a) 0-4
Figure5.18 (a) OutsideviewoJwatertankand(b) MRRatN-foodtreatmentplant