|
Manganese
Manganese
(Mn) The Water Supply Regulations limit
is 0.050 mg/l or 50 µg/l.
Manganese is too reactive
a metal to be found in elemental form in nature
but its ore, the black dioxide pyrolusite,
is widely distributed. The purified metal
or its salts are used extensively in industry
for the manufacture of glass, ceramics, batteries,
paints, varnishes, inks, dyes and fireworks.
It occurs in two forms (divalent
and trivalent). Although the chlorides, nitrates,
and sulphates are soluble in water, the oxides,
carbonates and hydroxides are only sparingly
soluble and manganese is rarely found in natural
surface waters in concentrations above 1.0
mg/l or 1000 µg/l. However, in ground
waters subject to reducing conditions manganese
can be leached from the soil and occur in
high concentrations. Manganese often accompanies
iron in ground waters.
At high concentrations in
water it will cause; an unpleasant taste,
deposits on food during cooking, stains on
sanitary ware, discolouration of laundry,
deposits on plumbing fittings and cooking
utensils, and will foster the growth of micro-organisms
in water supply systems.
Manganese is commonly found
dissolved in borehole waters, and at first
glance, fresh from the pump, the water may
appear crystal clear. However, after it has
come in contact with oxygen in the air it
will oxidise into an oxide of manganese and
deposit visible solid particles or coat exposed
surfaces.
There are treatment methods
to remove dissolved manganese from water supplies,
powerful oxidants such as chlorine will cause
oxidation and the production of particles
that may be filtered out.
For private water supplies
chemical addition is not always practical
or recommended and a well established method
of removal is by apparent adsorption onto
manganese dioxide. A manganese dioxide mixture
or similar is retained within a filtration
vessel, the water to be treated is passed
through a column of the media and manganese
levels are reduced. After a period of operation
the column is automatically backwashed to
flush off the deposited manganese particles
and the process can continue.
pH level is very important
in the process of manganese oxidation and
removal. It is usual for borehole waters to
contain high levels of carbon dioxide and
consequently carbonic acid; this may depress
the pH below the optimum range for manganese
removal. CO2 removal
must be addressed before manganese removal
can take place. |
