2012 Annual Report - Jesus College - University of Cambridge

20 CHEMICAL ENGINEERING I Jesus College Annual Report 2012
Water, water, everywhere
Ian Wilson
Water, water, every where, / Nor any drop to drink wrote one of the best known students of
Jesus College in 1798. The Ancient Mariner was then referring to sea water but the line
is equally appropriate in many parts of the planet today as the stock of potable water comes
under threat from a growing population, changing living standards and pollution. The
future of mankind depends on water. We drink, wash and play with it while industry uses
large quantities of it for transport, power, as a feedstock and a solvent.
One of the everyday chores performed worldwide that consumes large quantities of
water and generates appreciable volumes of waste is cleaning of dishes and utensils used
in preparing food. Dishwashing also consumes considerable amounts of energy, the other
commodity in limited supply. A bowl of water at 50ºC for dishwashing requires around 10
litres of water and about 0.5 kWh of thermal energy, as well as the chemicals (surfactants,
pH adjusters, etc.) we use to help the ‘soil’ detach from our plates. Many of us choose to
use an automatic dishwasher (over 37% of UK households owned one in 2011) and these
use up to 20 litres of fresh, potable water per wash. The latest devices have reduced this to
7 litres per wash by recycling spent water, and there is a clear need to make these machines
as energy and water efficient as possible.
The same concerns apply to industry. The heaters that pasteurise milk have to be cleaned
daily, and the diaries that process roughly 12 billion litres of milk each year in the UK use
around 1 billion litres of water for cleaning. Our group has been working on understanding
cleaning for some time in order to reduce energy, water and chemical consumption. Jesus
College is actually famous across the world of cleaning as it is also a research area of my
predecessor as Fellow in chemical engineering, Peter Fryer (now at Birmingham), and the
College hosts a major international conference in the area every four years. It has been said
that everyone at the College benefits from this activity as the College gets a detailed
inspection from a hundred or so food hygiene experts at regular intervals.
The science of cleaning can be grouped into three mechanisms. We can exploit
thermodynamics to make a ‘soil’ dissolve – basically it prefers to be in the solvent rather than
on your plate. The problem here is when your soil does not like water, as dry-cleaning
dishes is a non-starter. We can trick fats and grease into forming an emulsion and
dispersing into warm water using surfactants (such as soap), but many others would
require rather aggressive chemicals or more exotic, and thus expensive, surfactants.
The second mechanism is to change the adhesion between the soil and the surface of the
cutlery or dish. This is the science of non-stick, where the differences in chemistry mean
that the soiling material forms weak bonds to the surface which can be readily disrupted
by water or surfactants. The soil layer then comes off when contacted with water and some
force applied. One problem here is that a surface can be designed to minimise adhesion
of certain foods but not all the foods in a healthy diet. The other is that the non-stick
functionality is usually imparted by a coating. The coatings are usually mechanically weak
and can be disrupted by the forces and temperatures we use in cooking, eating or cleaning.
This leaves us with cohesive breakdown, aka elbow grease, where we apply forces large
enough to break the material down and cause it to move off the surface and into solution.
There is a limit here, as the forces have to be smaller than those which will damage the
dishes! Detergents help us to do this by promoting changes in the microstructure of many
foods: dishwashing liquids are mildly alkaline as this promotes the swelling (and