Strong Chelates Cause Problems in Waste Treatment

In the metal finishing industries strongly bonded chelates have a long tradition. The superior electrochemical properties of chemicals such as EDTA, NTA, cyanide, polyamines and others, have made these compounds invaluable during the production of plastics and metals. However, some of these compounds can seriously inhibit the ability of a wastewater treatment plant to meet the desired discharge limits.

If the wastewater contains chelating compounds such as polyamines, traditional precipitation treatment is often inadequate, meaning newly developed methods such as the ultraviolet advanced 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 following chemistries may be used:

  1. Hydroxide Precipitation
  2. Iron Reagent
  3. DTC ( Dimethyl Dithiocarbamate )

1. Hydroxide Precipitation

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.

2. The Iron Reagent

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 chelate 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.

Process Chelators Classical Treatments Modern Method
Electroless nickel Organic acids, ammonia Rinse waters only Rinses and concentrates e.g. Enviolet®
Electroless copper A Organic acids (tartrate, citrate) Rinse waters only Rinses and concentrates e.g. Enviolet®
Electroless copper B Polyaminocarboxylate (EDTA and other complexes) Already difficult for rinse waters Rinses and concentrates e.g. Enviolet®
Zinc-nickel Polyamines (EDTA, cyanide [generated during plating] and other complexes) Already difficult for rinse waters Rinses and semiconcentrates e.g. Cyanomat® P
Cyanide Cyanide Good to satisfactory Rinses and concentrates e.g. Cyanomat®

Table 1: Chemical composition of strongly chelated electrolytes and treatment processes.

3.  DTC (Dimethyl Dithiocarbamate)

DTC has been very successful in applications. The DTC is used in the presence of all chelating agents, including those with tartrate, quadrol, EDTA etc. DTC will react with a number of metals, forming an insoluble metal / DTC compound which will precipitate as sludge. The concentration of the chelated material has a significant impact on the dosage rate. The disadvantage of DTC treatment is that it cannot be safely adjusted to treat fluctuations in feed metal concentrations. An advantage is that DTC produces less sludge than the Iron process. This treatment route can be used where the metal concentrations are relatively low and where there is adequate equalization and full process segregation. For treatment of concentrated solutions such as electroless nickel and copper bail out the cost of treatment may be prohibitive.

Conventional Treatment of Electroless Nickel

With a classic lime precipitation, a large amount of nickel as well as phosphite can be separated. Depending on the concentration of the chelates though, a significant amount of nickel may still remain in the solution. The concentration of the chelates can rarely be reduced through precipitation therefore the resulting wastewater filtrate still contains large amounts of nickel and chelates.

Hypophosphite does not build a low solubility calcium salt like phosphate and it also cannot be separated with lime. There have been many attempts to change hypophosphite and phosphite to phosphate. Some applications use calcium hypochlorite in excess to achieve oxidation and precipitation. Permanganate has also been suggested and tried. The use of calcium hypochlorite, not only creates unwanted AOX, but also causes a strong increase in salt load. This causes subsequent problems with flocculation and filtration, sludge generation can also be significant. In practice, the disposal of rinse waters and used baths is often undertaken by special waste disposal companies.

Advanced Oxidation - Decomplex Treatment

EDTA and other organic chelates are widely employed in electroless plating processes used in the PCB and metal finishing industries. The main concern about these compounds is their poor biodegradability. Chelates can still be present as much as 15 years after their disposal, with EDTA being among the most persistent. European legislation on the discharge of chelating agents and general organics is becoming ever more stringent. The legislation is especially restrictive in Germany, where Annex 40 of the wastewater legislation only allows EDTA based processes to function where the EDTA is either recycled or completely destroyed.

Belmar has addressed the issues of treating chelated waste solutions with the application of Advanced Oxidation (AOP). The process uses hydrogen peroxide in combination with UV to produce hydroxyl radicals that subsequently oxidise species such as EDTA to a level where after treatment the wastewater can be simply treated via hydroxide precipitation. In addition, the process also eliminates other sources of COD such as formaldehyde etc.

The wastewater is treated in a batch tank from where it is recirculated through a UV reactor system. The key to success is the use of a performance regulated UV-reactor and application of subsequent process steps. After completing approximately 16 treatment steps, the wastewater is free of EDTA, the COD is significantly decreased and the wastewater can now be treated with conventional treatment techniques. AOP yields a waste suitable for discharge that meets future legislation and greatly reduces operating costs compared with off site haulage or DTC treatment.

  Wastewater will be treated by sulfide or organo-sulfide   The complexing agent gets oxidized, precipitation is simply done with lime
- Separate filtration is necessary + State of the art technique
-- High chemical costs
Organosulfide: £40-65/m3
Flocculant aids: ca. £7/m3
+ Moderate chemical costs:
H202/acid/catalyst: ca. £6/m3
UV-light/power: £2.50/m3
pH correction: ca. £1.50/m3
- Precipitation difficult: Danger from forming of colloids which need flocculant aids + Compact and easy disposable sludge
- Difficult to automate + Full automation possible
-- Only with dilution can the limiting value for copper be achieved ++ No trouble to achieve limited values
- High labour costs + Low labour costs
+ Moderate investment costs - By comparison higher investment costs
- Hazardous chemical storage + No chemical issues
- High toxicity 0 Normal
- Decomplex chemical is a fish toxin, with the risk of consequential costs + Low risk of consequential costs.
- EDTA and high COD discharge + Best available technology.

Table 2: Treatment of CuEDTA. Comparison of sulfide – DTC Precipitation and UV-Oxidation/Precipitation

Element Concentration in the bath Concentration after alkaline precipitation
Copper 5,000-6,000 mg dm-3 0.2-0.5 mg dm-3
EDTA 25,000-35,000 mg dm-3 < 10 µg dm-3
Formaldehyde 6,000 mg dm-3  n.n.
COD 43,000-60,000 mg dm-3  Approx. 1,000 mg dm-3

Table 3: Electroless CuEDTA bath composition and contamination levels after treatment

Treatment of cyanide contaminated process waters

The traditional cyanide destruction is achieved by hypochlorite treatment. This method generally produces acceptable results as far as the metal levels are concerned.

If, however, the wastewater contains metals such as nickel or silver, these can cause major problems. If total cyanide limits are imposed and Ferro cyanides are present, then traditional Hypochlorite treatment will rarely achieve the required discharge consent criteria. In addition this treatment has another disadvantage in that the transformation of cyanide with hypochlorite creates large amounts of toxic chloramines. The chloramines result from the reaction of the excess of hypochlorite with ammonia created in the process.

UV Cyanide Treatment Process

  • CN- + H2O2 --> CNO- + H2O
  • CNO- + 2 H2O --> NH4+ + CO32-

...further acidification

  • + 2H+   ->  NH4+ + CO2 + H2O

...and, if further oxidation occurs

  • à N2 + CO2 + H20

The UV light enhances the formation of OH - radicals:

  • H2O2  + UV à 2 OH

Advanced Oxidation System for Chelate and Cyanide Destruction