Human mankind is
encountering with increasing problems for municipal and industrial waste waters
which shall be purified and reused. Physical, chemical, and biological
purification processes have been developed.

Electroflotation is an
established purification method especially for industrial waste waters. Low voltage
(5-24V) direct current is applied to achieve decomposition of water to hydrogen,
oxygen and other gases depending on the inorganic ion in water. Electricity
causes various secondary reactions in the solutions which cause solubility of
colloidal particles to decrease. The particles form flocks which bind to generated
gas bubbles and are lifted by their buoyance forces to the water surface and
form relatively stable foam or froth which can be then removed. Purification
and microbial disinfection of waste waters are interconnected tasks. Removal of
organic load from waste water removes also microbes. When biologically purified
water is released to the nature, it almost always still contains microbes which
may interfere with the natural habitats. Pathogens may cause aberrations in the
ecosystems although they will not directly infect humans. The possible harmful
effects largely depend on the climatic and soil/drainage basin conditions of
the waste discharge environment. However, even occasional infections may be
harmful and therefore there is a need to secure that the discharge liquids are
microbiologically safe.

The disinfection of
contaminated waters can be divided into chemical and physical techniques. The
chemical ones include treatment with active chlorine compounds. Ozone is popular
in disinfection of drinking water. Various methods to produce ozone and
chlorine gas or compounds which release them are in use. Chemical methods have
the drawback of yielding residues of unnatural chemicals and their reaction
products.     

Low-voltage direct
current has been deployed for disinfection of waste waters in various formats.
Solar cells are especially attractive source for electricity. In situ production of oxidative chlorine
compounds from concentrated NaCl solutions and chlorine gases from anode are introduced
to water to be disinfected. The gases contain also other disinfecting compounds
like reactive oxygen, ozone, together with generated alkali and acids. The electrodes
may include semipermeable membranes or porous filters so that gases at anode
and cathode do not mix. In situ
production of disinfection gases has the advantage of production of poisonous
chemicals without need of their storage and transport. The main disadvantage of
the in situ processes is the use of
semipermeable membranes which tend to clog and must be regularly cleaned and/or
changed.

The electrochemical
cathode reactions produce basic hydroxyl ions and molecular hydrogen. The
hydrogen is generated about 0.4 liter (NTP) per Ah. The hydrogen formation is less
useful for the disinfection while it is important for obtaining the flotation
effect. In proper conditions the reduction power at cathode can be transformed
to hydrogen peroxide formation which may be used as a disinfectant.          

Anodic corrosion is a
serious problem in electroflotation, as well as, in the disinfection systems.
It can be diminished by specific coatings of the electrodes. High current
density increases the generation of electrochemical oxidants. Because the
stability of the grainy nano-coating is dependent on the polarity of the
electrode, the polarity cannot be changed to prevent the clogging of electrodes.
Electrode corrosion (dissolving) is also exploited in waste water purification
to produce metal ions, like Fe- and Al- salts, which precipitate biomolecules
and phosphate. This process avoids the need to add them separately as salts.

Especial hindrance for
the adoption of electric technologies in waste water purification is the realization
of convenient and long-life maintenance-free equipment. The primary problems
arise from electrode passivation by deposition of materials on the electrodes.
When considering the overall economy, usual low-corrosion anode materials, like
titanium and stainless steel may be optimal allowing change of the polarity of the
electrodes. Here is presented a new device construction, to work out electroflotation
and microbial disinfection, which can be used to produce drinking water.