Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Reexamination Certificate
2002-04-18
2003-12-16
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
C205S751000, C205S757000, C205S761000, C204S554000, C204S269000, C204S276000, C204S278500
Reexamination Certificate
active
06663766
ABSTRACT:
The present invention relates to an electro-flocculation process and apparatus for water treatment and especially for the production of drinking water.
More particularly, the present invention relates to a flocculation process and apparatus using trivalent ions such as those of aluminum and iron, accompanied by in-situ disinfection, without the need for flotation and sedimentation steps as described in the prior art.
Surface water intended for drinking water supply at present are treated for particle removal until extremely low turbity is reached.
Tertiary or advanced wastewater treatment is a requirement in some countries prior to disposal of water, and is one of the most important demands for wastewater reuse. Common tertiary or advanced wastewater treatment schemes include chemical coagulation (flocculation) of secondary clarifier effluent followed by sedimentation, and/or filtration and disinfection. The physical processes associated with classical coagulation are rapid mixing (when chemical coagulants are quickly and uniformly dispersed in the water and particles are destabilized), and then slow mixing or flocculation (when particles slowly aggregate and form settleable or filterable flocs). Chemical flocculation processes are known to aggregate wastewater constituents having a size ranging from 0.1 &mgr;m to about 10 &mgr;m.
Aluminum or iron salts may be added at various stages in a potable water or wastewater treatment plant to enhance solids removal. The metal salt (coagulant or flocculant) destabilizes colloidal solids that would otherwise remain in suspension and thus can be used to improve effluent quality. In spite of the increased use of chemical coagulants, coagulation theory still fails in providing the use of the process in an optimal manner, particularly under transient conditions. This could lead to diminished effluent quality, increased chemical costs by routine overdosing, or both. An improved coagulation process would alleviate its adverse effects on sludge dewaterability and disposal.
Coagulation has been the subject of much research, some of which have proposed coagulation in the context of water treatment. However, wastewater treatment differs from potable water treatment in several ways: particulate matter is present in substantially greater concentrations in wastewater; the average particle size is also greater. These factors are likely to affect both coagulant demand and flocculation behavior. The particulates to be removed include a much greater proportion of organic material than in the case of water treatment for drinking purposes. The more hydrophilic surfaces of these particles may react differently to a coagulant.
The mechanisms of coagulation by Al
+3
salts may involve some chemical factors including the hydrolyzing and polymerizing tendencies of the Al
+3
ion, the adsorptivity of such aluminum hydroxide species; the solubility of such species; the nature and extent of aluminum hydroxide precipitation, including interaction with other colloidal surfaces and effects of other solutes of surface properties on the metal hydroxide species. In light of this apparent complexity, it is not surprising that a number of different models have been proposed to explain the way aluminum salts can destabilize colloids.
The earliest explanation on particle destabilization considered only double-layer compression by Al
+3
ions as such. Other explanations suggested that the hydrolysis products of aluminum play a more important role in particle destabilization. More current explanations of coagulation with alum or iron flocculants hypothesize the existence of two distinct mechanisms, which are charge neutralization and adsorption of positively charged, polynuclear aluminum species to the particles; or the physical enmeshment of particulates in an aluminum hydroxide precipitate.
Electrochemical processes in water treatment which are gaining ground are electrofloatation and disinfection. The main advantage of the electrochemical process of disinfection is the production of disinfective chemicals in situ in the treatment device.
Floatation is a gravity separation process in which air bubbles are attached to individual solid particles, thereby reducing their density so that they float to the surface of the liquid. In the electrofloatation process gas bubbles are generated by electrolysis of the liquid. A 5-20V direct current is used at a current density of approx. 100 amp/m
2
of electrode The particular attraction of floatation, as compared to sedimentation, is that particles' rising rate is generally much higher than their settling rate, so that the size of the unit for a given duty is approximately one third of that of a clarifier.
In WO 95/15295, EP 668 244, GB 2,045,803 and DE 3,641,365, as well as in WO 97/35808, there are described electroflocculation processes involving the release of aluminum ions through an electrolytic cell. The first four documents refer to the electroflocculation-flotation process with regard to wastewater and teach the formation of trivalent ions in situ by electrolysis of a metallic aluminium or iron electrode, while the latter document, although referring to the use of said process for producing drinking water, does not teach the apparatus or method which would enable said process to be carried out
With this state of the art in mind, it is an object of the present invention to combine known electrochemical processes for water and wastewater treatment with the introduction of trivalent ions directly into the solute by electrolysis and the use of specially designed apparatus, as defined and described hereinafter.
More particularly, the present invention provides an electroflocculation process for water and wastewater treatment comprising providing a stack of electrically-conducting perforated spaced-apart plates serving as metallic electrodes, said electrodes being selected from the group consisting of metallic aluminum electrodes and metallic iron electrodes, and said plates being suspended in a vessel with their peripheral edges in proximity to a wall of said vessel, each plate being electrically insulated from adjacent plates and having apertures misaligned relative to apertures of adjacent plates; and supplying a voltage to said electrodes in the range of between 2 to 60 volts to form trivalent ions in situ by electrolysis of said metallic electrodes, which trivalent ions function as coagulation agents, wherein water to be treated is caused to ascend sequentially through apertures provided in said stack of parallel metallic plates serving as said electrodes, each plate being charged with a polarity opposite to plates adjacent thereto, apertures of adjacent plates being vertically misaligned, whereby turbulent flow and mixing is created by the sequential passage of said water to be treated from the apertures of the lowest of said plates to the apertures of the plates thereabove to exit from the highest of said plates.
In preferred embodiments of the present invention the distance between said plates is between 0.5 and 1.5 cm. The present invention also provides an apparatus for water and wastewater treatment by the electroflocculation process, comprising
a) a processing vessel having a lower inlet for impure water, and an upper outlet for purified water,
b) a stack of electrically-conducting perforated spaced-apart plates serving as metallic electrodes, said electrodes being selected from the group consisting of metallic aluminum electrodes and metallic iron electrodes, and said plates being suspended in said vessel with their peripheral edges in proximity to a wall of said vessel, each plate being electrically insulated from adjacent plates and having apertures misaligned relative to apertures of adjacent plates; and
c) a first and a second electrically conducting member, said first member being electrically connected to the upper plate of said stack and to alternate lower plates and insulated from other plates, said second member being electrically connected to plates electrically unconnected to said first m
Adin Avner
Vescan Nicholai
Phasge Arun S,.
Yissum Research Development Company of the Hebrew University of
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