Page 88 - Membranes for Industrial Wastewater Recovery and Re-Use

68 Membranesfor Industrial Wastewater Recovery and Re-use

every 12 minutes, as might be appropriate for a dead-end filtration of a
municipal wastewater, incurs a loss of 8% of the product water if the backflush
flow is three times that of the forward flow. The cleaning cycle employs less
product water and generally a shorter percentage downtime but demands the
use of acidic, caustic and/or oxidative chemicals and so produces extra transport
and handling problems and generates a waste stream which may not be so
readily disposed of as the backwash water. CIeaning, of course, also incurs the
additional cost of both the cleaning reagent(s) and system.
Cleaning chemicals generally fall into one of seven categories, with some
overlap between them:

0 Acids
0 Alkalis
0 Chelants
0 Enzymes
0 Surfactants
0 Biocides
0 Oxidants

A wide variety of proprietary formulated membrane cleaning products have
been developed and marketed mainly by the membrane suppliers but also by
other companies, with the number of products proliferating over the last decade
or so. Notwithstanding these commercial developments, many membrane plant
operators still tend to employ conventional generic cleaning chemicals and
formulations (Table 2.16). It is also the case that cleaning solutions tend to be
employed on a reactive rather than routine preventative basis. It is generally the
case that employing cleaning once fouling of the membrane has become severe is
less effective than routine maintenance cleaning which removes incipient
fouling. Also, in many cases no single cleaning reagent will recover the flux
entirely, and it is often advantageous to use a combination of cleaning reagents
in sequence.

Table 2.16 Common foulants and appropriate cleaning solutions, reverse osmosis
Foulant Cleaning solution
Calcium carbonate Hydrochloric acid pH 4
2% citric acid ammoniated to pH 4
Calcium, barium 2% citric acid ammoniated to pH 8
sulphate EDTA sodium salt, pH 7-8
Metal oxides 2% citric acid ammoniated to pH 4
1% sodium hyposulphite
Silica Caustic soda pH 11
Organics Caustic soda pH 11
Alkaline detergent
Biological fouling 1% formaldehyde
Hypochlorite