1. The Field of the Invention
The present invention relates to methods and apparatus for removing arsenic contaminants from water. More particularly, embodiments of the present invention create conditions that remove arsenic contaminants from anoxic or oxic water using continuous in-stream and batch process methods at atmospheric or above-atmospheric solution pressures followed by negative pressurization of the solution. Negative solution pressurization within the treatment sequence rapidly and surprisingly increases particulation and reduces aqueous concentrations of arsenic.
2. Background
Domestic water supplies often come from underground aquifers which contain anoxic water that has leached through and otherwise contacted minerals, sediments and rock layers for extended periods of time. These minerals, sediments and rocks often contain high concentrations of minerals, metals and other elements and compounds that are deleterious to human health. As a consequence of this contact, water in these aquifers becomes contaminated with some of the indigenous contaminants rendering the water unsafe for human consumption. Other water sources that may be used for domestic water supplies may also become contaminated with metallic ions and other contaminants through industrial pollution and other processes. These contaminated waters, prior to human consumption, will require remediation treatments.
Arsenic is one element that is often found in water sources and is pathological, in terms of human health of specific population segments, at all levels of concentration in drinking water concentrations. Several methods are known for removing arsenic species from water, however each has limitations and or disadvantages, which make embodiments of the present invention preferable in many applications.
Methods using electrolytic cells for electrochemical insolubilization of metallic ions using sacrificial anodes are known for arsenic removal, but require an energy source to power the electrolytic cell and do not address the issue of removing dissolved inorganic arsenic III species.
Other known methods require significant changes in pH levels to the bulk solution to effectuate precipitation of arsenic species from solution (i.e. lime addition). These processes additionally require a readjustment of the pH level after treatment to near neutral conditions. These two requirements are chemically intensive and equipment intensive. Another method requires ultra-filtration and the addition of anti-scalants as pre-treatments for reverse osmosis treatment systems that remove arsenic species. This treatment method is ineffectual in terms of removing dissolved inorganic arsenic III species without chemical oxidation, which in turn is deleterious to the systems membranes. Other methods utilize adsorptive materials that are rapidly consumed and become solid waste along with the regenerative solutions required by the system (i.e. active alumina, ion exchange). Reverse osmosis removes arsenic As(III) and AS(v) species with out the addition of oxidants but systems employing reverse osmosis have high capital and operating costs as well as loss through rejection of a significant percentage of the inlet water.
In some instances, a treatment method will efficiently remove dissolved arsenic species from a specific water source but be far less efficient when the same operational parameters are applied at a second autonomous water source. In particular, inconsistency of performance capabilities of the iron coagulation treatment system is problematic.
To achieve efficiency with nearly all anoxic ground water sources this method often requires large additions of either ferric chloride or ferric sulfate to the bulk solution. Some of these water sources require the addition of nearly 40-mg of ferric chloride per liter of treated water to achieve a residual arsenic level less than 5 parts per billion. This chemically intensive practice of adding voluminous ferric chloride solutions results in a significant pH shift in the bulk solution towards the acid region. To counter the acid shift, the treatment facility is engineered to add base solutions to the bulk solution causing a return of the bulk solution pH to near neutral conditions. After physical separation of the newly created particulate matter from the bulk solution the resulting accumulation of solid waste is massive, which in its self is problematic in terms of disposal requirements.
Many known methods of arsenic removal also fail to reduce arsenic concentrations to acceptable levels. Some methods only reduce arsenic concentration to a level of approximately 50 parts per billion. This may meet some current standards, however standards are likely to become more stringent in the future rendering these methods obsolete and unusable for purifying drinking water. Furthermore, the increased protection provided by methods that significantly reduce contaminant levels in drinking waters is a benefit to consumers as well as water suppliers.
Most of these methods are overly complex, labor intensive, produce large waste streams, require large facilities and land, are expensive and require the addition of large quantities of various chemicals for precipitation and pH adjustment.
Another prior art method provides for the removal or reduction of dissolved inorganic arsenic and other metals in anoxic or oxic aqueous solutions treated by the addition of iron salts to the solution.
The prior art method teaches that pressurization prepares dissolved inorganic arsenic III species to become particulate during the subsequent depressurization and ambient reaction. The pressurization step of this prior art treatment method lasts for a short duration of time (less than 5 minutes) followed by a depressurization step to near ambient pressures. The depressurized aqueous solution may reside in an ambient pressure reaction vessel or conduit in a quiescent state (batch process) or free-flowing state (continuous in-stream) for a short period of time (less than 5 minutes) prior to physical separation. Particulate and precipitated arsenic-containing solids are stabilized and are then separated from the solution by pressure filtration, sedimentation or other solid-liquid separation methods.
Arsenic removal efficiencies for this prior art method, based upon efficient filtration capabilities for physically removing particulate matter at 5 micron in size and greater, are such that final effluent concentrations are typically less than 2 parts per billion (ppb) arsenic. During continuous in-flow processes, pressurization may be achieved by pumping the solution into a pressure tank or by utilizing the head pressure of pumps associated with ground water sources to pump directly into pressure tanks. An inverted siphon or discharge into a tank of sufficient depth to achieve the desired pressure are other methods of achieving pressurization during in-flow processes.
The present invention is directed toward a method for the removal of arsenic and other metals from an aqueous solution. The solution may be pressurized by filling a tank followed by pressurization by electric pump or other means. After a short reaction time under pressure has elapsed, the solution is then depressurized to ambient pressure and then further depressurized to negative pressure (below ambient) to allow for the final particulation reactions to occur. In a small-scale operation, pressurization and evacuation may be achieved by the use of a hand pump. Pressurization levels between 10 psi and 120 psi have been found to significantly reduce both dissolved inorganic arsenic V and III species levels in anoxic and oxic aqueous solutions, however pressures between about 30 psi and about 60 psi are preferred for solutions with average arsenic contamination. Negative pressurization levels occur in the range of 0 to 25 in.Hg of vacuum, more preferably in the range of 10-20, in.Hg of vacuum. Accordingly, reaction velocities involving the particulation of the dissolved inorganic arsenic species is dependent upon the pressures applied during the pressurization and depressurization steps. The greater the initial pressure utilized the greater the reaction kinetics and the greater the negative pressure utilized in depressurization, the greater the final particulation, where such pressures are within the ranges disclosed.
The processes of the present invention may be advantageously used for emergency water treatment in small, hand-pumped, pressure tanks. Likewise, these methods may be used for large-scale water treatment operations where hundreds of cubic feet per minute are treated. Surprisingly, it appears that elevated pressurization and subsequent negative depressurization of the treated aqueous solution improves the precipitation-and-stabilization reaction compared to treatment sequence involving pressure as an initial condition followed by depressurization to ambient pressure alone or within a treatment sequence including ambient pressure as an initial condition followed by pressurization then followed by depressurization to ambient pressure.
Accordingly, it is an object of some embodiments of the present invention to provide improved methods and apparatus for reducing the concentration of arsenic species in an aqueous solution.
It is also an object of some embodiments of the present invention to provide methods and apparatus for improving the quality and potability of a water supply.
Another object of some embodiments of the present invention is to provide methods and apparatus to improve continuous in-stream reduction of the concentration of dissolved inorganic arsenic in an anoxic or oxic aqueous solution.
A further object of some embodiments of the present invention is to provide methods and apparatus for batch-process to improve the reduction of the concentration of dissolved inorganic arsenic in an anoxic or oxic aqueous solution.
These and other objects and features of the present invention will become more fully apparent from the following, description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.