Wastewater from the metalworking industry contains high concentrations of heavy metals, which harms the environment and threatens public health and biology. Therefore, wastewater needs to be pre-treated before being discharged.
Traditional processing methods include chemical methods, heavy metal precipitation, flocculation, sedimentation and discharge, all of which are very time consuming. The precipitation of heavy metals reduces the concentration of contained metals. The key is the solubility of the precipitated metal compounds.
That is, if the metal can form an insoluble compound, the compound can be removed by purification and filtration.
The two heavy metal precipitation methods currently used are hydroxide and sulfuration. In both of these methods, metal cations react with OH- or S2- to form the corresponding insoluble metal hydroxide or sulfide. The precipitation of heavy metals such as hydroxides is pH sensitive. Therefore, the optimal pH must be determined before wastewater treatment. Most heavy metal hydroxides precipitate at pH 8.5-9.5. Outside this range, the metal hydroxide will re-solve or decompose.
Heavy metal hydroxides precipitated as colloidal charged particles require the addition of a coagulant to reduce the charge and increase the size of the precipitated particles. To promote settling, flocculating agents are used to flocculate the solidified particles. Metal floes / sludge settles in a purifier or sedimentation tank. If the supernatant reaches the regulatory discharge standard, it can be discharged, and the sludge is filtered by a filter press to form semi-solid waste.
Each step of this treatment method needs to be performed in a separate tank. The entire treatment process needs to adjust the pH value and add acid, coagulant, lime or alkali and polymer flocculant multiple times. In addition, this process produces large amounts of hazardous sludge / waste containing high concentrations of heavy metals that require further processing.
1 sludge disposal
If the pH drops below the optimal range, the precipitated heavy metals will resolubilize. The decomposition of municipal solid waste and humic substances will produce organic acids (lower pH), which will increase the solid solubility of heavy metals, and sulfides and cyanides can react with these organic acids to generate toxic hydrogen sulfide and hydrogen cyanide gas. In general, disposal of waste in landfills or landfills has the least impact on groundwater. However, where these conditions are not available, a collection system needs to be installed to collect the leachate and remove heavy metals by precipitation or other means.
The chemical treatment method was adopted to reduce the exudation rate of sludge containing heavy metals. Sludges containing heavy metals are treated with chemicals to make them stable or poorly soluble. This method is used in most landfills. However, these treatments increase the high cost of wastewater treatment, which usually accounts for 60% -80% of waste disposal costs.
2 Stabilization of non-leaching substrate
The best technology to remove heavy metals and other harmful pollutants in wastewater is to keep the non-leachable substrate stable so that once the garbage is placed in the landfill, these heavy metals and other harmful pollutants will not pose a danger to the environment. This is usually handled by reacting the discharged waste with chemicals.
The latest treatment method is to use chemicals to remove pollutants from wastewater. This agent has a strong affinity for heavy metals, and this method is less limited by pH compared to the precipitation of metal hydroxides. Metals can react with pharmaceuticals at levels up to 10,000 mg / L or higher. High levels of metals are not limited by pH or oil, surfactants, chelating agents, complexing agents, suspensions, and hardness. This process requires adding chemicals to the stirred wastewater of the treatment tank. After dense floating blocks are formed in the treatment tank, it quickly precipitates and dehydrates, which can be performed on existing equipment. The resulting waste does not require post-treatment. The chemical components in the treatment agent stabilize pollutants, especially heavy metals, in the waste and reduce the solid solubility of the metal. Therefore, the waste has passed the toxic leaching test to meet the regulatory requirements of non-hazardous materials.
3Removing heavy metals through metal removal technology
Metal removal technology (MRT) is a cost-effective technology for removing heavy metals from industrial wastewater. It is called MRT-100, and it is a filter tank containing two parts of granular activated carbon (GAC) and methylbenzotriazole (MeBT). When metal-containing wastewater flows through this tank, MeBT combines with metal ions and adsorbs on the GAC to remove water pollutants. It is not affected by pH and does not require adjustment of chemical additives, reducing chemical and labor costs. However, MeBT is highly selective for heavy metals and transition metals, so it does not bind non-target metals. This further extends its service life, reduces frequent replacement of filter media, and ultimately reduces user costs. Once carbon is saturated, it can be reused after the metal is removed by some solvent. In addition, MRT-100 is simple and efficient.
Summary As the country attaches great importance to environmental protection, the metal processing industry must strictly abide by the existing regulations and national standards on wastewater discharge, actively transform to green production and environmentally friendly direction, and prepare for the long-term development of enterprises.