Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1999-07-30
2001-03-20
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S603000, C210S605000, C210S630000, C210S631000, C210S719000, C435S262500
Reexamination Certificate
active
06203700
ABSTRACT:
THIS INVENTION relates to the treatment of water. It relates more particularly to the treatment of acid water. Still more particularly, it relates to a process for treating metal-containing acid water, such as acid mine waste water or effluent.
According to a first aspect of the invention, there is provided a process for treating metal-containing acid water, which process comprises
adding an alkaline aqueous component to metal-containing acid water, thereby to raise the pH of the acid water and to cause metals therein to precipitate;
separating, in a separation stage, the precipitated metals from the water;
passing the water to a biological alkalinity generating stage wherein the alkalinity of the water is increased biologically;
withdrawing the alkaline aqueous component, which is added to the metal-containing acid water, from the alkalinity generating stage; and
withdrawing treated water from the alkalinity generating stage.
The biological alkalinity generation may, more particularly, be effected by means of algae, while the alkaline aqueous component may comprise hydroxide and carbonate anions. The process is thus based on the ability of algae to increase the alkalinity, ie raise the pH, of their surrounding medium.
While, at least in principle, the process can be used for treating any metal-containing acid water, the Applicant believes that the process will have particular application in the treatment of heavy metal-containing acidic effluents or waste waters, such as acid mine drainage. By adding sufficient alkaline aqueous component to the acidic effluent or waste water, the majority of the metals are precipitated as metal hydroxides and metal carbonates as a result of increased alkalinity achieved by adding the alkaline aqueous component thereto. Sufficient of the alkaline aqueous component may be used so as substantially to neutralize the acid water and substantially to neutralize the acidic effluent or waste water.
The alkalinity generating stage may comprise an algal growth vessel or pond. The vessel or pond may be fitted with a mixing device in order to facilitate an even suspension of algal cells and to ensure or promote even distribution of light throughout algae- containing culture present therein.
The process may then include, from time to time, or continuously, as the case may be, withdrawing some of the algae-containing treated water from the vessel or pond, separating algae therefrom, and returning the algae to the vessel or pond. This withdrawal may be effected either actively, eg by pumping the algae-containing aqueous component from the vessel, or passively, eg by means of gravity flow thereof from the vessel. Typically, the pond may comprise a raceway or high rate algal pond.
The separation of the algae from the treated water may be effected by passing the algae-containing treated water through at least one separation device, such as a gravity settler. Instead, in another example of a suitable separation device, the algae-containing treated water may initially be passed through a primary screen located at an outlet from the algal growth vessel or pond, to prevent large amounts of algae from flushing out of the growth vessel or pond with the treated water; thereafter the treated water may be passed through at least one primary filter to trap the majority of the algae which pass through the primary screen, with the primary filter also constituting an algae harvesting stage from which algae which are removed from the treated water are harvested for further processing or return to the algal growth vessel or pond; and thereafter the treated water may be passed through at least one secondary filter which acts as a final screen to remove algae which have passed through the primary screen and the primary filter.
The treated water emerging from the separation device will thus consist of higher pH, eg non-acidic or neutral, water containing a minimum of algal biomass.
The addition of the alkaline aqueous component to the acid water or effluent may be effected in a mixing stage, which may typically comprise a mixer or reaction vessel into which both the alkaline aqueous component and the acid water are introduced. As discussed hereinbefore, the effluent is typically acid mine effluent or drainage water having a low pH, typically around 2-4, and a high heavy metal load, typically around 10-3000 ppm, comprising heavy metals such as iron, copper, lead and zinc. As the pH of the acid water increases due to the alkaline aqueous component mixed therewith, the metals precipitate out. The mixed water components or streams, together with the precipitated heavy metals, pass to the separation stage. Sludge, containing the precipitated heavy metals, is removed from the bottom of the separating stage, which typically comprises a settler, and may be further processed, eg to recover valuable metals therefrom.
The treated water from the vessel or pond may be subjected to further processing. For example, it may be subjected to final polishing to remove trace amounts of metal. Such polishing may comprise treating it with a biosorbent, such as
Azolla filiculoids
. The further processing may also comprise passing the water through an anaerobic digester.
However, heavy metal-containing acid waste waters such as acid mine effluent or acid mine drainage, or zinc refinery waste water, usually contain significant quantities of dissolved sulphate anions. An additional source of sulphate ions may be added, if desired. This may conveniently be a sulphate waste source. The process may then include, if desired, feeding the sulphate-containing waste water from the alkalinity generating stage into a pond or trench; subjecting the sulphate-containing waste water to biological sulphate reduction in the pond or trench, thereby to convert dissolved sulphate anions in the waste water to dissolved sulphide anions; and withdrawing treated waste water from the pond.
When a trench is used, it may be in accordance with ZA 98/3970 or AU 65949/98, which both claim priority from ZA 97/4165, and which are thus incorporated herein by reference. When a pond is used, it may be in accordance with our copending South African and USA patent applications, both claiming priority from ZA 98/3203, which are thus also incorporated herein by reference, and which are entitled “The Treatment of Water”.
The process may, in addition to adding the alkaline aqueous component to the metal-containing acid water, also include adding sulphide anions to the acid water, eg in or ahead of the mixing stage, to aid in precipitation of metals therefrom. Thus, the metals will then additionally precipitate as metal sulphides, which will also be removed in the separation stage.
At least part of the sulphide anions which are added to the acid water may be those obtained by withdrawing treated sulphide-containing waste water from the pond or trench; stripping gaseous components, particularly hydrogen sulphide, therefrom in a stripping stage; returning the stripped waste water to the pond or trench; and adding the gaseous hydrogen sulphide to the metal-containing acid water. However, in another embodiment of the invention, at least part of the sulphide anions which are added to the acid water may instead or additionally be those obtained by withdrawing treated sulphide-containing waste water from the pond or trench, and adding it directly to the metal-containing acid water.
The exact nature of the metal precipitates will thus depend on the pH of the acid water after the alkaline aqueous component has been added thereto, and the proportion of sulphide ions added thereto.
The addition of the sulphide ions can thus be controlled to achieve, in respect of the acid water to which it is added together with the alkaline aqueous component, precipitation of different metal species as hydroxides, carbonates and sulphides. These precipitated metal species can then readily be separated from one another using known separation techniques.
The addition of the alkaline aqueous component to the acid water, ie the addition of carbonate and hydroxide anions, ca
Boshoff Genevieve Ann
Duncan John Richard
Rose Peter Dale
Van Hille Robert Paul
James Ray & Associates
Prince Fred
Simmons David A.
Water Research Commission
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