has been a lot of misinformation thrown about recently on the topic of
water recycling. Leaving aside emotional and even religious arguments,
concerns such as David Thackrah’s about pharmaceuticals in treated
water (yesterday, comments) are irrelevant to any of the plans that I
have seen discussed for Australia.
The fact sheets
from the Toowoomba city council clearly state that reverse osmosis will
be used to purify the water. Reverse osmosis (RO) is a system of
filtration through a membrane with sufficiently small holes to block
salt ions (eg chloride or sodium). These molecules are an order of
magnitude smaller than complex molecules such as statins. Reverse
osmosis is a commonly used and well trusted technology.
companies prepare water for injection for all medicines in part using
RO (an exceptional level of purity is needed so other steps are taken
as well), power stations use RO water to prevent scale build up in
their boilers and RO systems are available for domestic users to remove
chlorine from their tap water. The water produced by a reverse osmosis
plant would be more pure than rain water.
Where the error
lies, is in comparing the discharge from a sewage treatment plant and
the discharge from an RO plant. Sewage treatment plants do not have to
meet potable standards. Their effluent has to meet standards of BOD (a
measure of how much material is present in the system that can rot and
deprive rivers and oceans of oxygen) and nutrients (in order to prevent
algal blooms). Sewage treatment plants are not designed to remove
pharmaceutical molecules, although some will be destroyed by the
treatment process. This is not to say that the discharge of treated
sewage into any waterway is not a problem, it is only to say that RO
water discharged into a catchment is subject to higher standards and is
completely different to sewage discharge.
Fear that large
molecules will leak through a filter that holds back small molecules is
like worrying that a bird will get through a fly screen that a mosquito
won’t. Failure of the membrane system is possible, however it is a
simple process to automatically monitor the RO discharge stream for
salt (by measuring conductivity to detect the small salt molecules) and
automatically divert water in the event of membrane failure. This
system can easily be designed so that diversion is the default state
and that the conductivity must be below a certain value before the
system discharges to the catchment.