Water Softening By Lime At Twyford Waterworks.
As so often happens, what is historically the most important part of the waterworks is also one of the least spectacular, the importance of the lime softening plant at Twyford is that it is unique, being the only substantial remains of lime softening equipment on a public waterworks in the whole of Britain.
The lime softening process for removal
of hardness in water, as used at Twyford waterworks between 1903 and 1969,
was developed by a Dr. Clark and, as such, is sometimes referred to as Clark's
process. It was first used commercially at a London waterworks in 1854. It
is worth noting that the lime process only removes the hardness caused by
carbonates in the water.
Another process, known as the Lime-soda process, was
patented later, in 1878, which removes all the hardness including hardness
caused by other compounds. The lime-soda process was seldom used in waterworks
(it was not used at Twyford) because it involves more difficulties and costs
than lime softening and because carbonates cause most of the hardness.
Rain falling on the ground is soft and many water supplies extracted from
rivers and surface reservoirs are also soft, however where the water soaks
into the ground through chalk or limestone, carbonates dissolve into the
water making it hard. This is the situation at Twyford where the water is
pumped from many metres underground after it has passed through the chalk
subsoil.
There is nothing harmful health-wise from the hardness of the water;
it is in its uses that problems occur. Soap does not readily lather in hard
water, making thorough washing of clothes difficult. Hardness causes scum
and curd in wash basins and baths, 'Fur' builds up in kettles, boilers and
heating pipes. These problems largely disappear when softened water is used.
The Clark lime softening process consists of three
chemical reactions or stages,
(i) Burning limestone or chalk to produce quick
lime
(ii) Slaking the quick lime with water and diluting to produce lime water.
(iii) Adding the lime water to hard water and removing precipitates.
In stage iii, the highly soluble compound calcium bicarbonate in the water,
which causes the hardness, is converted by the slaked lime into an insoluble
calcium carbonate, which precipitates on formation as 'sludge'. This is either
allowed to settle in a water tank or is filtered out of the now softened water.
The Lime Kilns
The first stage in the softening process took
place in the lime kilns, where chalk was heated to a high temperature.
The 5
kilns at Twyford have identical internal dimensions although the exteriors
appear different, the kilns are of a design known as flare kilns and are
of the "intermittent type". That means the kilns are not burning continuously
but are loaded, heated up, cooled down and emptied.
This results in an inefficient
process both in the use of the kilns because they are not producing lime
continuously and in fuel, because of the heating and cooling of the kiln structure.
The alternate heating and cooling of the kilns caused much stressing of
the brickwork and to strengthen the kilns, ironwork was introduced. In
several places there are wall-plates which in conjunction with tie rods
passing completely through the structure, hold it together.
Iron hoops
surround the domes and chimneys to the kilns for the same purpose. The
kilns themselves are built of ordinary bricks on the outside but lined
on the inside with firebricks.
There is a fire-grate at the bottom with
an ash-pit beneath, then above it the part of the kiln where the lime burning
took place and at the top, a dome with a chimney. The kilns were charged [loaded]
through the doorway at charging floor level, the doorway being bricked up during
the charging.
Flare kilns are usually fired from the stoke hole for the whole
bum of the kiln, however it seems that at Twyford, layers of fuel were intermixed
with the chalk to assist in the burning, an unusual method of working with
this type of kiln. The chalk for the kilns had to be in lumps of at least 3inches(80mm)
across because it is very important to have a free flow of air through the
kiln while it is burning, to carry gases away.
When charging a kiln, first
of all a tunnel of chalk was built over the fire bars at the bottom of the
kiln. Once the tunnel had been constructed, layers of chalk and coke were placed
alternately, which continued until the kiln was full. When charging was complete,
a coal fire was lit on the fire bars which caught the the layers of coke alight
and the fire traveled up through the kiln. The kiln had to reach a temperature
of at least 900°C [1650°F] and be kept at that temperature
for one or two days to allow chalk in the centres of the lumps to change to
quick lime.
The temperature was controlled by the air-flow through the kiln
and the quantity and position of the fuel. For efficient burning it was also
important to maintain an even temperature across the kiln.
Once the lime burner judged that the burning was complete, the kiln was allowed
to cool down and the lime removed. Kilns were emptied both through the charging
door and the stoke holes, the lime being removed to the lime store either in
tipping wagons or manually in baskets. Loading the kiln, firing, cooling and
emptying the kiln took about one week.
