Precipitation & ultrafiltration address heavy metal ion removal from wastewater
Today’s manufacturers should be aware of the problems and penalties associated with discharging industrial wastewaters into sewage treatment systems. Corrosion and other interference with those systems, exposing workers to toxic substances and hazardous fumes, expensive sludge disposal costs and the pass-through of toxic pollutants into surface waters are just some of the results of improper or insufficient wastewater treatment.
The removal of soluble heavy metal ions from wastewater is a common industrial treatment requirement in modern manufacturing. The processes that generate waste streams containing heavy metals include:
- Electroless nickel plating;
- Printed circuit board manufacturing;
- Metal forming operations;
- Battery recycling; and
- Mining operations.
Precipitation and ultrafiltration provide two ways to remove heavy metals from wastewater that are superior to conventional solutions. Both can help manufacturers avoid costly consequences, and a reputable provider can advise which process will deliver the best results based on a variety of application-specific criteria. First, it is helpful for the manufacturer to understand the importance of wastewater treatment beyond a business standpoint.
Why Companies Should Care
On a global level, wastewater treatment is a critical topic of discussion that has been addressed at the highest levels of government and major corporations. To find a sustainable approach, companies can employ many different strategies to go beyond compliance and begin the process of positively impacting global water quality.
According to a United Nations World Water Development report, more than 80% of all the wastewater from industry, homes, cities and agriculture is released to the environment without adequate treatment and flows back into the ecosystem via lakes, rivers and other bodies of surface water. This process repeats every day across the planet, polluting the environment while losing valuable nutrients and other recoverable materials in the process.
Water is a finite resource with rising international demand. Each year in March, World Water Day serves as a reminder from the United Nations that a daily commitment is necessary for successful wastewater treatment and reuse. The theme for World Water Day 2019 was “Leaving No One Behind,” the central promise of the UN’s 2030 Agenda for Sustainable Development.
A Commitment to Wastewater Treatment
The goal of this agenda is to cut the proportion of untreated wastewater in half and increase safe water reuse by 2030. Guy Ryder, the director-general of the UN International Labor Organization (ILO) and the Chairperson of UN-Water, believes that to achieve this goal, there must be a commitment to improve the management of wastewater from both the business community and the general public.
As wastewater treatment has become such an important aspect of plant operations over the last decade, manufacturing companies across all industrial sectors are prioritizing wastewater policies. Compliance with tightening federal regulations for wastewater treatment, handling and disposal—such as the Clean Water Act (CWA), the Resource Conservation and Recovery Act (RCRA) and the Safe Drinking Water Act (SDWA)—requires manufacturers to be focused on the wastewater issue. There are additional regulations on the state and local level that companies must adhere to as well.
Additional motivating factors that are potential drivers for treating wastewater include strengthening a company’s public image, improving the overall working environment, boosting employee morale, and adhering to ISO 14001 initiatives. Promoting a green operation by putting a priority on environmental awareness and sustainable operations will translate to happy workers, loyal customers and satisfied investors.
The ultimate motivator almost always is related to cost. From a monetary standpoint, it makes sense for all manufacturers to adopt a formal wastewater treatment policy that addresses heavy metals, as doing so allows them to cut rising operational costs while increasing profitability.
The concentration and chemical form of the soluble heavy metals in a particular wastewater stream varies, depending on the industry and the mix of operations at a processing site. For example, in the plating industry, specialty chemical suppliers have developed alloy technologies such as zinc, nickel and other alloy coatings to maximize the high corrosion resistance of the metal.
The zinc and nickel of choice for today’s surface finishing industry is an alkaline electrolyte chemistry instead of an acid electrolyte chemistry. This is due to the alloy distribution required by the plate thickness throughout the entire part.
Unfortunately, alkaline chemistry contains chelators and complexors and organic additives to allow for the co-deposition of the nickel in the deposit with the zinc. These chelators and complexors make the wastewater difficult to treat through conventional chemical or physical treatment methods.
Zinc and chrome wastewater can be treated through conventional methods, such as precipitation and the use of polymers in a lamella clarifier, followed by a filter press to dewater the sludge. The highly complexed alkaline zinc and nickel, however, does not want to precipitate and the chelated metals tend to stay in solution.
Additionally, the metal concentrations are too high to discharge to the local publicly owned treatment works (POTW). Nickel is a hazardous constituent and is not exempted. The addition of wastewaters from the zinc and nickel process will cause the filter press cake to become a listed RCRA hazardous waste with the F006 designation, resulting in significant increases to disposal and regulatory costs.
Typical removal strategies involve precipitating the metals in an insoluble form—such as hydroxides, sulfides, carbonates or some combination—and removing the precipitate with tubular ultrafiltration (UF) (for high-quality filtrate) or conventional clarification. The resultant sludge then is collected, thickened and dewatered for landfill disposal and listed as F006 (but at a much smaller volume) and the water phase is returned to the UF process.
Unlike conventional clarifier techniques, tubular UF is an effective method to remove virtually all of the precipitated metal hydroxide, sulfides and carbonates from the treated wastewater stream. The resulting high-quality permeate can be fed directly to reverse osmosis equipment for reuse, reclaimed as is or discharged to the POTW. Tubular UF can process solids concentrations of up to 18% (w/w), but the most efficient operation normally occurs between 3 and 5% solids.
The sludge normally is drawn off and passed through conventional thickening and filter press processes or similar processes to produce dewatered solids, typically for landfill disposal. Supernatants from the sludge handling processes are recycled through the tubular membrane filter system so that the only outputs are the filtered water and the compact solids cake.
Hydroxide precipitation is a common method as it is relatively simple to operate. Sulfide precipitation has some advantages, but pH and oxidation-reduction potential (ORP) must be carefully controlled to minimize the risk of producing toxic levels of hydrogen sulfide gas. Using hydroxide and sulfide precipitation in two sequential steps also is an option, particularly where complexes or chelates are present. Carbonate co precipitation using sodium or calcium carbonate also can be helpful.
Finally, there are membrane technologies available that utilize a proprietary additive in conjunction with a chelate breaker for precipitating the chelated metals.