Page 89 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 69

Acid cleaners are generally used for inorganic scale removal, principally
calcium carbonate and ferric hydroxide. Reagents employed include
conventional low-cost inorganic acids (sulphuric, hydrochloric, nitric and, less
commonly, phosphoric) and organic acids (citric, oxalic, acetic and sulphamic).
The stronger, in terms of acid strength per equivalent concentration, inorganic
acids are correspondingly more effective that the weaker organic salts but are
also more corrosive and potentially more damaging to the membrane element,
module or plant pipework. Careful pH control is therefore necessary, with pH
values above 2 being maintained for most applications. Citric acid solutions,
which have historically been employed for scale removal, must be amminated
and adjusted to a pH of 4 to avoid complexation with the ferrous ion, since this
can precipitate and blind the membrane. Oxalic acid is particularly effective
against iron, but forms a calcium salt of limited solubility. Some organic acids act
through chelation, i.e. through the formation of stable complexes with
multivalent metal cations. The most commonly used chelants are ethylene
diamine tetracetic acid (EDTA), phosphonocarboxylic acid, gluconic acid, citric
acid and various polymers that have a chelating ability. Of these EDTA, as a
hexadentate ligand, is significantly the stronger but also has adverse health
effects.
Alkaline cleaners, primarily caustic soda, are employed for the removal of
organic acid foulants, including proteins, and silica. The solution may need
buffering with bicarbonate or phosphate to maintain the pH, however, and the
addition of surfactants can improve the detergency action of the alkaline
solution. It is common to combine caustic with acid cleaning and other cleaning
chemicals, applied sequentially, to maximise the flux recovery. Surfactants
themselves can be non-ionic (e.g. ethylene oxide), anionic (soaps and
alkylsulphonates) or cationic (quaternary ammonium compounds or QACs).
QACs, which are also bacteriostatic (i.e. prevent the growth of bacteria), cannot
be used with membranes with a high surface concentration of carboxylate ions,
such as polyamide thin-film composites, due to irreversible complexation. Non-
ionic surfactants may also exhibit this behaviour, though to a lesser extent.
Enzymic cleaners are effective against many biodegradable organic foulants,
such as proteins, and are chemically non-aggressive. They are, however, also
slow acting and expensive, and so are generally only employed when other
cleaning chemicals have failed. Since they are also proteins in themselves, an
additional alkaline chemical clean is required to remove any residue.
The most commonly used biocide is chlorine, although aromatic polyamide
membranes are degraded by chlorine and other oxidants. Generally chlorine
levels of around 10 pprn are sufficient for routine sanitisation following a
chemical clean, but concentrations of 100 pprn may be used for specific
bactericidal duties such as pyrogen removal. Chlorine is generally employed as a
sodium hypochlorite solution, adjusted to a pH of 10-1 1 to reduce its chemical
aggressiveness, though this also reduces its bactericidal potency. A 2-3%
formaldehyde solution can also be used for sanitisation, and this reagent is also
used at lower concentrations (0.5-1%) for storing membranes as an alternative
to the more potent hypochlorite. However, it must be removed by thorough