In manufacturing, metalworking fluids are widely used for cooling, cleaning, and lubrication of workpieces, resulting in a great amounts of wastewater.
It is estimated that the worldwide annual usage of cutting fluids exceeds 2x10^9 L and the waste could be more than ten times the usage, as the metalworking fluids have to be diluted prior to use.
The treatment of oily wastewater is problematic and costly for many enterprises since its treatment requires sophisticated project designs and high-tech equipment. In fact, cutting fluid wastewater contains emulsified oil, additives, and impurities such as waste oil, dust, and metal chips which are difficult to treat.
Nowadays the processing of metals by cutting becomes impossible without the use of emulsions and lubricants. In this scenario, which are the most effective methods for the treatment of cutting fluid wastewaters?
Continue reading this article to find out.
What Are Water-mix Metalworking Fluids, Cutting Liquids, and Emulsions Wastewaters?
Metalworking fluids are complex mixtures of oils, detergents, surfactants, biocides and anti-corrosion agents. They are widely used as coolants, lubricants, and swarf or metal chip removers during machining operations. They also help to prevent surface corrosion of the machined parts and prolong the life of machine cutting tools.
This kind of wastewater has many toxic components that are very difficult to remove and their disposal costs are often very high.
Water-mix cutting fluids are supplied as concentrates and are diluted with water before use to produce an emulsion of 2-10% by volume oil in water. A typical composition of cutting fluid and oily wastewater is shown in the table below.
Water is the main ingredient of metalworking fluids, but separating it from the emulsion is not an easy task. For this reason, an appropriate wastewater treatment system can treat cutting fluids waste and cut down discharge volumes and disposal costs for enterprises.
Discharge Parameters to Pay Attention to for Cutting Fluid Wastewaters Disposal
In the recent years regulations governing the discharge of industrial wastewaters, including spent metal working fluids, have become more stringent all around the world.
Now, companies must take any and all measures necessary to ensure the safe collection and disposal of waste oils and must, as far as possible, give priority to the processing of waste streams. As a result, businesses need to treat spent fluids before the waste water component can be discharged to sewers.
The most important parameters to consider for cutting fluids and oily wastewater treatment:
Chemical Oxygen Demand (COD): Measure of the amount of oxygen that is needed to oxidize soluble and particulate organic matter in water;
Biological Oxygen Demand (BOD): Biological method used for the measurement of the total amount of dissolved oxygen (DO) used by microbes in the biological process of metabolizing organic molecules present in water;
Total Suspended Solids (TSS): Suspended particles in water that are not dissolved;
Depending on the country, the discharge limits for metal cutting oils can vary, but the most cost-efficient wastewater treatment methods remain the same.
What Is The Best Wastewater Treatment Technology For Spent Cutting Fluids?
Treatment and disposal options applicable to water-mix metalworking fluids depend on the type and volume of wastewater and wastewater composition, thus the treatment solutions for a specific fluid are often difficult to determine without accurate testing.
The treatment methods for cutting fluid wastewater are divided into primary, secondary and tertiary:
The best solution often relies in the combination of these three treatment methods, so to efficiently remove pollutants from oil emulsion wastewaters and reuse the water in the production lines.
Cutting Fluid Wastewater Treatment Technology
Cutting fluid wastewater treatment can be divided into three main approaches: physical, chemical and biological methods.
For example, membrane separation is a typical physical treatment approach, while the traditional coagulation method is a chemical method. Both of these methods are effective in the treatment of spent metalworking fluids.
Instead, because the composition of cutting fluid wastewater is complex and toxic, it cannot be subjected to separate biological treatment. The biological treatment unit can be combined with the oxidation process to improve the treatment results.
Based on the state-of-the-arts of cutting fluid wastewater treatment technology, new innovations are optimizing the treatment of oil and emulsion wastewaters.
Primary Treatment for Metalworking Fluids Wastewater
The purpose of primary treatment is to separate the waste cutting liquid into two waste streams, categorized by hazard level. Removal of tramp oil and suspended solids will result in a waste stream which is less hazardous and therefore cheaper to dispose of or easier to treat downstream.
Some of these methods include skimmers, settling tanks, floatation and centrifuges.
Another method is by using a chemical coalescing agent, which is suitable for cutting fluids disposal applications where tramp oil and metal particulates are the main contaminants.
Secondary Treatment for Metalworking Fluids Wastewater
Secondary treatment methods involve separation of the emulsified oil from the spent cutting oils.
Some of the most used treatment technologies are evaporators, electrocoagulation, and ultrafiltration systems.
Cutting Fluid Wastewater Treatment by Vacuum Evaporators
Wastewater evaporation has become an effective method for removing contaminants and concentrating liquid waste from cutting fluids. This technology can remove salts, heavy metals, and hazardous materials, reducing COD, BOD and TSS very effectively.
In cutting fluid wastewater treatment, evaporators can reduce COD levels to discharge limits, and the recovery rate can reach up to 85% as reported in some projects.
In addition, a vacuum evaporation equipment uses no expensive chemical for cleaning wastewater and has much lower energy consumption when compared to other types of products on the market. The last improvement brought by MVR evaporators make this technology the preferred choice for large volumes of cutting liquids and oily wastewater treatment applications.
Electrocoagulation for Cutting Fluid Wastewater Treatment
Electrocoagulation can effectively separate the oils in cutting fluid from the water body. This method can separate emulsions by using electrochemical reactions, so it’s becoming the most suitable choice for oily wastewater treatment.
In cutting liquid wastewater treatment, electrocoagulation is used for the removal of oils and contaminants from the water contained in metalworking fluids. This technology can be used as pre-treatment for wastewater evaporators. In fact, an electrocoagulation equipment followed by a filter press can reduce COD in cutting fluid wastewaters by 80%, and greatly increase the efficiency of vacuum evaporators.
The contained costs of this technology, combined with the minimal use of chemicals, make it a good choice for oily wastewater treatment applications.
More information about electrocoagulation be found here: Electrocoagulation System for Water and Wastewater Treatment.
Tertiary Treatment for Metalworking Fluids Wastewater
This treatment option involves techniques for further improve the treatment of the end stream from the secondary treatment process to reduce the COD to a very low level, so that the recovered water could be re-used on-site.
These techniques are: reverse osmosis, nanofiltration, carbon absorption and ion exchange resins.
In oily wastewater treatment, the water recycling can be furtherly improved by installing a reverse osmosis or nanofiltration equipment as the last step of the wastewater treatment system. These filtration technologies can reduce pollutants in wastewater by 99.9%.
Instead, the other methods are based on the filtration of the water body through active carbons and ion exchange resins to reduce pollutants and soften the treated water.
The Most Cost Effective Treatment Method For Cutting Oil-Water Emulsion Wastewater
In wastewater treatment the costs can be divided into capital and operating. In the table below these costs are compared to the treatment efficiency of each technology.
Overall, evaporation and chemical methods are the most suitable treatment technology for oily wastewaters.
Instead, electrocoagulation is the most cost-effective solution to break down emulsions and reduce pollutants, but the effluent requires further treatment either with evaporation or membrane filtration methods. For this reason, electrocoagulation is a great choice as pre-treatment or for improving the treatment effects of evaporators and membrane equipment on cutting oil wastewaters.
Finally, membrane systems are effective in separating the water from the cutting oil, but the operating costs are very high because the membranes need to be replaced frequently.
What Is The Best Treatment Technology For Spent Metalworking Fluid Wastewaters?
The best technologies for the treatment of emulsions and spent metalworking fluids wastewater are evaporation and electrocoagulation. Such treatment system can recover more than 80% of the water from cutting liquid and oily wastewaters.
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