Although the need for water remains, the quality of the water must
be controlled.
Water normally contains many impurities and contaminants taken
from its surroundings.
The contents of a particular water depend on its local environment
and history. For example, in the granite regions of Scotland, calcium
levels below 10mg/l are not uncommon. Similarly the calcite rocks
of Derbyshire and East Anglia (England) are associated with waters
containing 350mg/l of calcium carbonate.
The type and quantity of impurity often determines the quality
of the water and the problems that can arise from its use in industry.
Bio-Fouling
Natural waters can contain a variety of living organisms that,
if provided with the correct environment, can increase in numbers
to such an extent that the water becomes completely unacceptable
for either potable or industrial use. Cooling systems and hot
water circuits frequently provide ideal habitats for micro-organisms
such as bacteria, fungi, algae and protozoans, whilst systems
using sea water may suffer from fouling by larger creatures such
as mussels, barnacles etc.
The use of carefully chosen biocides coupled with regular monitoring
and general "good housekeeping" can significantly reduce
biofouling and its associated problems.
Metallic Corrosion
Corrosion may be defined in many ways but it is convenient to
say that it involves reactions that result in metals returning
to their oxides.
In their natural state metals are generally combined as ores,
which are essentially in a stable, low energy form. The extraction
process elevates the metal to a less stable, higher energy condition
and in consequence, there is a natural tendency for it to revert
to its former state.
Metallic corrosion reactions are so extensive it is unlikely
that a single set of mechanisms can explain all cases. It is however
accepted that generally the corrosion of metals in aqueous environments
is caused by electrochemical processes.
Corrosion is a heterogeneous reaction, which is often diffusion
controlled. In order for the reaction to proceed electrochemically,
there are three necessary conditions which must be met simultaneously:
(i) There must be a potential difference.
(ii) Mechanisms for charge transfer between electronic and electrolytic
conductors must exist.
(iii) A continuous conduction path must be available.
In aqueous systems ions in solution complete the conduction path
when water is in contact with the metal.
The discrete oxidation and reduction reactions at anode and cathode
are the charge transfer mechanisms.
Once condition (i) can be met, corrosion will proceed.
The potential difference may arise in many ways, often associated
with a difference in the surface states of zones, e.g. areas covered
with scale or other deposits compared to clean metal. Differences
in oxygen concentration, especially in crevices where corrosion
can be severe, may provide the potential required as can the use
of dissimilar metals for construction, which produce galvanic
cells, depending on the electromotive series.
The greater the potential difference, the more rapid should be
the rate of corrosion. Other factors effecting the rate of corrosion
include temperature, flow rates, prescence of corrosive compounds
such as dissolved carbon dioxide, pH levels and the operating
conditions of the plant.
Certain physical properties can also influence
corrosion.
Flaws in the crystal structure of the metal can increase susceptibility
to corrosion and corrosion cells may be set up across grain boundaries.
Stress can also set up potential differences across the metal
surface in addition to weakening the structure.
Dissolved solids can be a major problem within high flow systems
where erosion and damage to protective films occur. Similarly,
cavitation caused by the impingement of gas bubbles may result
in serious consequences typically on feed pump impellers when
there is insufficient pressure on the suction side of the pump.
When determining the most suitable water treatment programme
these factors should be considered but in general terms corrosion
inhibition will be afforded by preventing the cathodic and/or
anodic reactions. This is achieved in a number of ways including
removal of aggressive species, control of pH, and the uses of
inhibitors to produce protective films or to stabilise the natural
metal oxide film on the metal surface.
Scale Formation
Scale and sludge formation are the result of the precipitation
of compounds that are no longer soluble under the conditions prevailing
at the point of deposition. This typically occurs in the regions
of highest temperature, normally heat exchange surfaces, and low
flow areas, such as cooling tower ponds.
Every solid has a definite solubility in a particular solvent
under specific conditions of temperature and pressure.
In general, as temperature increases so does the solubility.
However, in some cases increasing temperature reduces solubility,
such is the case with calcium carbonate.
This reduction in solubility and subsequent precipitation of
the solid is a consequence of maintaining the most stable energy
levels within the chemical system in much the same way, as corrosion
is a consequence of the metal tending towards its lowest energy
condition. Scale and sludge may also occur due to chemical reactions
producing less soluble compounds as in the decomposition of bicarbonate:
Ca (HCO3)2 --------Heat---------> CaCO3
precipitate + H20 +CO2
Calcium carbonate is one of the most common components of scale
and sludge in untreated boilers and cooling systems. Other potentially
scale-forming compounds are:
- Calcium sulphate
- Calcium phosphate
- Other calcium salts
- Magnesium salts
- Barium salts
- Silica and silicates
Scale formation can be reduced in a number of ways ranging from
the use of pre-treatment plant to remove the scale forming salts
throught, to the addition of threshold agents and dispersants
to chemically extend the solubility range and to prevent compacting
of precipitating crystals.
For further information about any issues raised or details of Accepta's
specialist water treatment products and services please call Accepta
on +44 (0) 161 877 2334 or e-mail info@accepta.com.
