Reducing Total Suspended Solids in Zinc-Nickel Lines

Treating the Hard to Treat

A question from a customer: “We are a zinc-nickel plater with a discharge of 80,000 gpd. While we are able to meet our zinc and nickel discharge numbers, we struggle with our total suspended solids (TSS). We have a small floc formation with slow settling. We have solids coming over our weir. What can we do to help solve this problem?”

A: When we look at hydroxide precipitation of metals, we see that we need anionic flocculants too for the larger floc that settles rapidly. Often, the answer is just any anionic flocculant/polymer will work. However, that is not always the case.

Flocculants come in three different charges, cationic or positive charge, anionic or negative charge, and nonionic or no charge. Nonionic flocculants are rare with very specialized usages that do not cross over into plating wastewater treatment. Each charge has different molecular weights, different strand lengths, and different percentages of charges. Depending on the hardness of the water used to make the flocculant into usable forms, the pH of the wastewater stream, and the other contaminants in the wastewater such as organics from wetting agents or surfactants from cleaners, we may find a flocculant with a different charge percentage as well as a different chain length and molecular weight may work better. Bench testing with different flocculants will help determine if we are using the correct flocculant.

Over-dosing of the flocculant to try to force a larger floc is often seen in cases where the wrong flocculant is used. When this happens, floc can switch charges and float rather than settle. It can also leave a “hazy” appearance to the effluent. This can account for the slow settling. Bench testing will show what percentage of make-down of the flocculant works best and what dosing should be used to prevent overdosing. We recommend a make-down of 0.1-0.2% when making flocculants in day tanks. This equates to 4-8grams of dry or 4-8mL of emulsion per gallon of water.

If we see a buildup of solids in the clarifier between the veins, we may need to increase the time the sludge transfer pump runs. It is recommended to keep the sludge blanket at the bottom of the clarifier between 2-3feet. This will assist the sludge settling times and help prevent solids from going over the weir.

The best practice is to have the sludge transfer pump on a basic on/off solenoid valve with a timer. When the pump is on, it will transfer sludge from the bottom of the clarifier to the sludge thickening tank. We want to ensure that we are only pulling sludge and not excessive amounts of water from the clarifier to the sludge thickening tank. It may take some adjustments to get the timing right.

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