Wastewater Treatment - Timing for the Use of Heavy Metal Chelating Agents

 

With the rise of environmental awareness among the public and the increasing stringency of regulations and enforcement, surface treatment operators are compelled to address the environmental investments for new factory establishments. Additionally, addressing how existing factories can meet future, stricter environmental standards is a current challenge. Currently, there are numerous wastewater treatment agents available in the market, each designed for specific treatment purposes. When it comes to the issue of heavy metals in wastewater treatment, especially in cases where there is limited space to add wastewater treatment equipment or when the wastewater contains multiple heavy metals, the use of heavy metal chelating agents (hereinafter referred to as chelating agents) is considered an optimal treatment method.

 

How to Determine the Need for Chelating Agents

 

    However, not all heavy metal issues require the use of chelating agents. It is recommended to follow these steps for assessment:

 

1. Analyze the main heavy metal components in the wastewater (e.g., zinc, nickel, copper).
2. Select the heavy metal component with the highest concentration and adjust the wastewater's pH to the lowest pH at which the hydroxide of that metal dissolves (e.g., adjust zinc to pH 9.5).
3.  Analyze the supernatant for the heavy metal components identified in step 1 and compare whether they meet discharge water standards.
4.  If the heavy metal concentrations are below the discharge standards, chelating agents may not be necessary.
5. If metal concentrations remain above standards, choose a suitable chelating agent to complement the wastewater treatment system.

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     Choosing the right chelating agent has several advantages, as shown in Table 1. If the use of chelating agents is deemed necessary, selecting the appropriate one is a crucial process.

 

	Suitable Coagulant	Unsuitable Coagulant
Dosage of Chemicals	Decrease in dosage	Increase in dosage
Operating Costs	Saving costs	Increasing costs
Chemical Oxygen Demand (COD)	COD unaffected	COD significantly increased
Effluent Appearance	Colorless and clear	Yellow and turbid
Sludge	Reduced sludge	Increased sludge

 

Sodium Sulfide Type Chelating Agent

 

The most common chelating agent on the market is sodium sulfide or its derivatives, typically used in concentrations of 10-15%. It is recommended to add it in an alkaline environment to avoid the production of lethal gas - hydrogen sulfide. However, sodium sulfide, besides the potential for lethal gas formation, also generates a strong odor on-site, which is detrimental to workers and site operations. Moreover, sodium sulfide is not effective in treating chemical nickel or metals with chelating properties and may not meet discharge standards.

 

Highly Effective Zinc-Nickel Chelating Agent

 

As environmental requirements become more stringent, traditional agents struggle to meet standards, and their dosage continues to increase. This highly effective zinc-nickel chelating agent does not require additional chelation breaking and reduces the dosage of chemicals and equipment input. It can be easily added without altering existing wastewater treatment processes. The agent demonstrates excellent metal capture capabilities, achieving the most stringent standards in current use, with nickel concentrations below 0.1 ppm.

 

Additionally, for high-concentration wastewater, chelating agents can achieve effluent standards. However, for high-concentration nickel or copper wastewater, it is advisable to evaluate the use of alkaline precipitation or oxidation methods, combined with chelating agents for pre-treatment. This approach reduces the cost of treating high-concentration wastewater and avoids overloading the wastewater treatment system, ensuring compliance with effluent standards.

 

Environmentally Friendly Low-Toxicity Patented Chelating Agent

 

Environmentally friendly low-toxicity patented chelating agents are available in the market as alternatives to sodium sulfide-based chelating agents. These agents effectively remove heavy metals, have no significant odor during operation, and reduce the public safety risks associated with hydrogen sulfide. Excessive addition has lower toxicity to the microorganisms in subsequent biological ponds, minimizing the impact on the toxicity of water discharged into natural bodies. This preparation aims to anticipate the future requirements of regulations becoming stricter concerning biological toxicity.

 

There is a diverse range of chelating agents available, and selecting the suitable one requires an understanding of the wastewater's characteristics and how to integrate it with the treatment process. This allows wastewater treatment agents and treatment equipment to function effectively, achieving the increasingly strict effluent water standards. Gong Long hopes to leverage the experience and technology of treating surface treatment wastewater from Europe and the United States, introducing more environmentally friendly electroplating solutions and pre-treatment agents. This collaboration aims to assist Taiwan's surface treatment industry in reaching advanced environmental standards, enhancing its environmental processes and capabilities, and leaving behind a beautiful and sustainable environment for future generations.

 

[1] Generally recommended not to be lower than pH 6.0.

 

If your wastewater issue is not heavy metals but rather reducing COD in wastewater, feel free to refer to the following articles related to biological treatment.

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