The present invention relates generally to the use of chemically modified electrodes. More particularly, the present invention relates to an improved treatment system and method for automatically controlling the chemical feed of dithiocarbamates ("DTC") in fluids, for example wastewater, containing metal ions so as to produce metal ion precipitation.
As a result of the continual contamination of rivers, waterways and the like by wastewater from industrial sources, the Environmental Protection Agency has enacted strict regulations in an attempt to stop such contamination. For instance, a mandatory requirement exists to control metals in wastewater below certain prescribed concentrations. Particularly stringent regulations have been established for heavy metals harmful to humans, such as mercury, cadmium, zinc, copper, lead, nickel and silver.
Due to the harmful effects caused by these metals, the EPA continually lowers the permissible discharge levels of metals in process wastewater streams. While many industries producing metal bearing liquid effluents still illegally discharge untreated waste effluents into streams and sewers, the stepped-up enforcement of such EPA standards coupled with excessive fines have persuaded those in the industry to finally cope with the problem of water contamination. The two major methods for complying with metal discharge regulations are the storage and transport of untreated, unconcentrated waste to hazardous waste disposal sites, or, alternatively, on-site treatment. Storage and transport of waste is very expensive for all but the very small volume waste producers. Thus, the only economical alternative for the majority of the industry is on-site treatment.
On-site treatment is an effective means for ensuring compliance with disposal regulations. The most viable chemical techniques for on-site treatment of metal bearing effluents include electrolytic deposition, metallic replacement, ion exchange, chemical reduction and chemical precipitation. While electrolytic deposition, metallic replacement, ion exchange and chemical reduction are all reasonably effective means, chemical precipitation is believed to be the most effective method for removal of metals from wastewater effluents.
Chemical precipitation involves the addition of a precipitant into the wastewater causing metal ion precipitation. Naturally, a treatment scheme that supplies an appropriate amount of the precipitant to the wastewater will be an effective on-site treatment. However, without the use of some type of control feed system, over-feeding of the precipitant can easily occur. Such over-feeding is one of the major problems the industry must face when utilizing a chemical precipitation system.
Oxidation reduction potential ("ORP") electrodes and ion-selective electrodes are both known for assisting in automating chemical feed control and continuous metal ion precipitation processes by monitoring the electrochemical potential therein. Although these electrodes have provided a means to control precipitant feed, disadvantages exist with both types of electrodes.
Oxidation reduction potential electrodes are disadvantageous for at least two reasons. First, ORP probes are non-specific. ORP electrodes are typically made from gold or platinum and measure a voltage difference (potential) between itself and a reference electrode. They respond to the presence of metal ions, but also respond to a multitude of electrochemically-active components that may be present in the wastewater. Such components may include hydrated or chelated metal ions, oxidants (hypochlorite), or reducing agents (bisulfite). These components contribute to an observed cumulative potential, thereby making the monitoring of a single component, such as the disappearance of a particular metal ion, impractical.
Second, ORP probe measurements are sensitive to pH fluctuations in process streams. This sensitivity occurs because hydromium ions (H.sup.+) are often involved in the oxidation or reduction reactions occurring in aqueous systems. Moreover, the lack of knowledge of the actual reactions occurring at the electrode surface makes it impossible to predict the effect of pH on the measured potentials.
Although ion-selective electrodes provide an alternative to oxidation reduction electrodes, these electrodes possess similar problems as those associated with the ORP probes. An ion-selective electrode often used in the treatment of wastewater is a sulfide electrode. While the sulfide electrode provides a means to control precipitant feed, a system incorporating such a sulfide electrode is only responsive to the presence of excess sulfide that does not always allow for accurate detection in resulting precipitant feed. Such ion-specific electrodes are known to plateau under precipitant over-feed conditions. Thus, similar to the oxidation reduction electrode, precipitant over-feed may occur without viable detection.
Still further, ion-selective electrodes, similar to the oxidation reduction electrodes, are potentiometric detection methods. A potentiometric mode measures a cumulative potential based upon components within a solution. Since these electrodes measure a cumulative potential, a variety of common interferents to these electrodes, such as metal sulfide complexes, will obstruct the detection of a particular precipitant.
Therefore, a need exists for an improved selective controlling device, as well as system incorporating same, for optimizing the control of metal precipitant feed in wastewater streams.