If the wastewater contains chelating compounds such as polyamines, traditional precipitation treatment is often inadequate, meaning newly developed methods such as the ultraviolet oxidation technique described below are required. Typical comparisons of traditional and modern techniques in treating various chelated processes are shown in Table I. Due to increasingly higher specification requirements in surface finishing, chelates based on polyamines are being introduced more extensively. Chelates such as EDTA that were largely replaced in the past, have found their way back into surface finishing technology.
Traditional Treatment Options
Depending upon the nature of the wastewater produced and the degree of metal removal desired, one or more of the chemistries will meet these requirements. The three main chemistries for use are:
- Hydroxide Precipitation
- Iron Reagent
- DTC ( Dimethyl Dithiocarbamate )
In the field of wastewater treatment, hydroxide precipitation of metals has been the most widely used treatment method due to its simplicity of use and relatively inexpensive costs. However, there are limitations to its use. The hydroxide precipitation is ineffective in breaking the complexing bonds produced by various chelating agents.
The sodium hydroxide reaction for metals reduction is as follows:
M2+ + 2NaOH => M (OH2) + 2Na+
Sodium hydroxide will react with any non-chelated heavy metal present producing an insoluble metal hydroxide. Each metal hydroxide has its characteristic hydroxide solubility curve which determines the limits of complete removal of the element as the hydroxide. However, Co-precipitation with other metals as cations can often yield lower solubility than would be expected from the solubility of a single metal hydroxide. Chelating agents are specifically designed to prevent hydroxide precipitation from occurring under normal conditions. Therefore, simple hydroxide precipitation is rarely acceptable for the PCB or similar applications.
Hydroxide chemistry is a very effective chemistry for use in treating non-chelated wastewater. The most important consideration in specifying its use is the presence of any chelates in the wastewater which may complex with copper or other metals, keeping them in solution, rendering the hydroxide solution useless.
The Iron Process
Iron can be used in the presence of chelating agents, effectively breaking a number of complexing bonds. The solution is introduced into the wastewater in sufficient amounts to reduce most metals and break the chelates bonds. The iron reagent is added to the solution in a reaction tank at a pH of 2.0 - 3.0. The tank is sized with a minimum of 20 minutes reaction time. This step is very important to safeguard consent conditions as well as low iron dosage rates. The solution is then raised in the second reaction tank to a pH of between 8.5 - 11, where the metals, including iron, precipitate as metal hydroxides.
The primary disadvantage of the iron reagent chemistry is that the reagent is added on a volumetric basis; therefore, the dosage must be predetermined and is not related to the actual concentration of metals in the feed. The iron reagent process is very difficult to optimize and control when there are wide fluctuations in feed metal concentrations. This can be overcome by providing adequate feed water equalisation. One further consideration is the increase in sludge volume associated with the addition of the iron reagent. Please note this process will result in a higher iron discharge value, the EDTA will complex the iron instead of the copper or Nickel for example.