The waste fluid of water-based metalworking fluids is an oily wastewater that is difficult to treat among industrial wastewaters. It is hard to achieve the legally required compliant water quality using simple treatment methods and equipment.
1. Characteristics and Treatment Challenges of MWF Wastewater
Regarding the characteristics of the waste fluid, the mineral oil content, COD (Chemical Oxygen Demand), and BOD values are relatively high. Therefore, the load placed on the treatment equipment is extremely large. In particular, emulsified products contain a massive amount of organic amines and surfactants that are easily dissolved or dispersed in water. To completely remove these substances, several treatment methods must be used simultaneously.
2. Generation and Collection Strategies
Metalworking fluid waste is generated when the degraded working fluid is replaced with new fluid. Because its occurrence time and generation volume are uncertain, it is difficult for users to formulate a precise treatment plan. Thus, it is generally treated by mixing it with other industrial wastewater.
In factories of a certain scale, they have their own waste fluid treatment facilities. They collect the metalworking fluid waste together with other waste fluids and drainage. This keeps the treatment volume and pollutant load relatively stable, which is beneficial for subsequent treatment operations. In addition, when the volume of metalworking fluid waste is significantly greater than other drainage, it is more economical to use separate preliminary treatment before mixing it with other drainage.
3. Core Pre-treatment Methods
Its pre-treatment method mainly consists of two parts:
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Step 1: Rough Separation
This involves simple oil and impurity removal. The separated tramp oil must be disposed of as hazardous waste (HW09 category). The separated sludge must be treated as solid waste, which is also considered hazardous waste. -
Step 2: Concentration and Reuse
This involves recovering reclaimed water for reuse. The concentrate is then discharged into the waste fluid pool for further treatment. The treatment methods for the waste fluid involve various techniques and their combinations.
4. Multi-Stage Treatment Process: Primary, Secondary, and Tertiary
Coagulation treatment methods (such as sedimentation, dissolved air flotation, etc.) are used as the primary treatment method. However, it is very difficult to obtain legally compliant water quality using only primary treatment. Therefore, secondary and tertiary treatments are required.
Microbial treatment methods, such as the activated sludge process, are frequently used as the secondary treatment method. In areas with strict water quality limits, tertiary treatments such as activated carbon adsorption and reverse osmosis may also be required.
5. Cost Control and Fluid Lifespan Management
When outsourcing the disposal of waste fluid to an external company, the converted cost often exceeds the procurement price of the original metalworking fluid concentrate. This is a massive cost investment. Therefore, minimizing the generation and discharge of waste fluid is the universally recognized ultimate goal.
By properly managing the working fluid, inhibiting its degradation, and extending its lifespan, the treatment frequency will decrease. Consequently, the treatment cost will be reduced, and the overall comprehensive cost will also drop.
However, no matter how well the working fluid is managed, it can never be used indefinitely. The problem of waste fluid treatment cannot be entirely avoided. This is because spoilage is only one of the reasons for discarding the working fluid. Other reasons include natural aging, external contamination, and the inability of its functions to meet current demands.
