Treatment of oily wastewater from the machinery industry using the oil separation + air flotation + filtration + adsorption process

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China Encyclopedia Network Mechanical factories generate large volumes of oil-containing wastewater. The oil in this wastewater exists in three forms: free oil, dispersed oil, and emulsified oil. The particle size of the floating oil is ≥100 μm; it rises to the surface relatively quickly after being left to settle, forming a floating layer as an oil film in the continuous phase on the water’s surface. In machinery factories, lubricating oils and fuel oils that end up in wastewater due to leaks are mostly floating oils. The size of dispersed oil droplets is 10–100 μm; they remain suspended and dispersed in the aqueous phase. Given sufficient time or under the influence of external forces, they can coalesce into larger oil droplets that rise to the water surface. Alternatively, they may become smaller and transform into emulsified oil. Dispersed oil exists in a suspended state in wastewater; the oil in the oily wastewater discharged during the cleaning process before machining mechanical parts is dispersed oil. The particle size of the emulsified oil ranges from 0.1 to 10 μm, and it appears in an emulsive state in wastewater. A stable film formed by molecules of emulsifiers (surfactants) covers the surface of the oil droplets, preventing them from merging and thus maintaining stability over time; even after being left undisturbed for a long period, these droplets do not rise to the surface. The oil in the emulsion wastewater generated by ordinary machinery factories is emulsified oil. 　　The main treatment methods for oily wastewater include physical methods, chemical methods, and physicochemical methods. Physical methods are further divided into gravity separation, centrifugal separation, coarsening, filtration (membrane separation), etc. The chemical methods mainly include coagulation and salting-out. The main physico-chemical methods include flotation (air flotation). 　　1 Sources of wastewater Oil-containing wastewater mainly includes: emulsions discarded during machining processes (this wastewater is treated separately before being sent to the wastewater treatment plant); cleaning fluids discharged during the cleaning of components ； Water used for cleaning various workshops and supporting equipment, as well as wastewater generated from flushing floors. 　　2 Design scale and influent/effluent water quality
2.1 Wastewater treatment scale
The drainage system of this plant is a combined sewer system; initial rainwater is directed to the wastewater treatment plant for treatment. Design scale: Flow rate for dry flow, Q_dry = 40 m ； Combined flow: 9 confluences = 80m (interceptive multiplier no=1). 　　2.2 Design of inlet and outlet water quality Data on the wastewater discharge quality in the machinery industry require that the treated wastewater meet the reclaimed water quality standards for landscape use as specified in the \"Design Code for Sewage Reuse Projects\" GB50335-2002. The designed influent and effluent water qualities of this wastewater treatment plant are shown in Table 1. 　　3 Wastewater treatment process
3.1 Process description
In the East Plant of Puyuan Branch Company, oily wastewater flows by gravity into a collection tank after floating materials and large-particle impurities are removed in a grit chamber; it is then pumped by sewage pumps into a regulating and oil-separating tank. The initial rainwater enters the wastewater treatment plant, and after air flotation treatment, it is discharged via overflow from the intermediate tank. The regulating oil separator not only regulates water quality and quantity, but also removes floating oil. The oil floating in the pool is collected by an oil collector and then recycled. The oily wastewater enters the flotation equipment after passing through a regulating oil separator, with coagulants being added. It enters the intermediate tank after demulsification and oil-water separation. The wastewater is pumped up and fed into mechanical filters and activated carbon adsorption towers, where it is subjected to thorough treatment before being reused. 　　Mechanical filters and activated carbon adsorption towers are backwashed regularly, and the backwash water is discharged into the regulating oil separation tank. The oil sludge from the oil separation tank and the scum generated by the air flotation equipment are regularly discharged into the sludge tank. The oil sludge is pumped to a sludge drying bed using a sludge pump for drying, and the dried sludge is then transported to the boiler room for incineration. The process flow is shown in Figure 1. 　3.2 Main Structures and Equipment (1) Grid tank. 1 unit, LxB=2.5mx1.0m, H=2.5In. One mechanical grille cleaner is to be installed, with a grille width of 1.0 m and a gap between the grille bars of 10 mm. The motor power is 1.1 kW. 　　(2) Collection tank. 1 unit, LxB=6.0m x 3.0m, effective water depth = 3.0 ITI. Three submersible sewage pumps are provided, with 2 in operation and 1 as a spare; Q=40–60 m3/h, H=17–11 in, N=4.0 kW. The water pump has both automatic and manual control modes; the automatic mode is controlled by a level controller. Start the pump at high liquid level and stop it at low liquid level. 　　(3) Adjust the oil separator. 1 unit, divided into 2 compartments; L~B=15.0m X 4.5m, H=4.0m. It regulates water quality and quantity while removing floating oil with a particle size > 160 μm. The designed residence time is 4.0 h (the residence time adjusted for the combined flow rate is 2.0 h). One set of oil collector is provided for each compartment (including an oil floating collector and an oil-water separator). N = 1.1 kW. 　　(4) Complete set of air flotation treatment equipment. 2 sets, with a processing capacity of 40 mR; the total power of the complete equipment is N=I1 kW. 2 sets of dissolving and dosing equipment, with N=1.1 kW per set. (5) Intermediate water tank. 1 unit, LxB=4.0m x 4.0m, H=4.0m, effective volume 5613. . Three submersible sewage pumps are installed, with 2 in use and 1 as a spare; Q = 40–60 m3/h, H = 25–190 m, N = 7.5 kW. The water pump operates in both automatic and manual modes. The automatic mode is controlled by a level controller. Start the pump at high liquid level and stop it at low liquid level. 　　(6) Mechanical filter. 1 unit, with a processing capacity of 40 m3/h per unit. It shares a backwash water tank with the activated carbon adsorption tower, and the backwash wastewater flows into the grid tank. 　　(7) Activated carbon adsorption tower. 1 unit, with a processing capacity of 40 mR per unit, and it comes equipped with a backwash water tank. Activated carbon is used as the adsorbent, and the backwash wastewater flows into a collection tank. 　　(8) Reuse water tank. 1 unit, LxB=9.0m x 4.0in, H=4.0in, effective volume 126113. The recycled water tank is equipped with 1 submersible sewage pump, with Q=20–45 m³/day, H=32–22 m, and N=5.5 kW, featuring a variable frequency speed control device. 　　(9) Sludge tank. 1 unit, reinforced concrete structure. It is used to store the oily scum separated by the air flotation equipment, as well as to regulate the oily sludge discharged from the oil separation tank. LxB=3.0m~3.0in, H=3.0nl, effective volume 20in. Two sludge pumps are installed beside the tank, one in use and one as a backup; Q=5.0 m3/h, H=15 m, N=1.5 kW. 　　Sludge drying tank. 1 unit, built on top of the regulating oil separator tank. ×B=9.0m x 2.0m. H=1.2m. Effective volume is 18m³. Perforated plastic pipes and 40-mesh stainless steel wire mesh are installed in the tank; the tank is filled with pebbles and river sand. The filtrate from the sludge drying tank is returned to the regulating oil separation tank for further treatment. The dried sludge is sent to the boiler room for incineration. 　　4 Operating performance
Since its successful commissioning in December 2005, the project has been operating stably, with good quality of the treated water. The monitoring results regarding the quality of both the influent and effluent waters, conducted by the Changsha County Environmental Protection Monitoring Station in February 2006, are shown in Table 2. 　　The results in Table 2 show that the process of oil separation + air flotation + filtration + adsorption is effective in treating oily wastewater from the machinery industry. The concentrations of COD and petroleum are low, and the removal efficiency for various pollution factors is high; thus, the quality of the treated water meets or exceeds the standards for reclaimed water intended for landscape use as specified in the \"Design Code for Sewage Reuse Projects\" (GB50335-2002). For more relevant technical documents, please refer to http://www.dowater.com. 　　5 Technical and economic indicators (1) Project investment: 900,000 yuan for civil works, 1.4 million yuan for equipment, materials, and installation, 200,000 yuan for commissioning and other expenses; the total investment is approximately 2.5 million yuan. 　　(2) Operating costs: chemical cost is 0.04 yuan/m, electricity cost is 0.44 yuan/m, labor cost is 0.03 yuan/m, and wastewater treatment cost is 0.51 yuan/m. Operating cost: 127,500 yuan per year. 　　(3) Economic benefits: After the implementation of this oily wastewater treatment project, 100% of the treated water will be reused for purposes such as floor washing, car cleaning, landscaping, and landscape irrigation, achieving zero discharge of oily wastewater. The annual reduction in pollution discharge fees amounts to 276,000 yuan; 576,000 yuan is saved in terms of fresh water usage, and 8,000 yuan is saved through oil recovery. The net benefit is 733,000 yuan per year. 　　(4) Environmental and social benefits: After the implementation of the project, 9.2 tons of petroleum-based pollutants, 3.6 tons of SS, and 3.2 tons of COD will be reduced in emissions each year. This will **reduce the harm caused by wastewater to the environment, contributing to the protection of the water quality and ecological environment in the lower reaches of the Liuyang River. At the same time, it improved the relationship between enterprises and local residents, made rational use of resources, promoted the sustainable development of enterprises, and yielded significant social benefits. 　　6 Conclusions 　　(1) The process of oil separation + air flotation + filtration + adsorption for treating oily wastewater from the machinery industry offers high treatment efficiency and low operating costs. The actual operating results show that after treatment using this process, the removal rates of COD and petroleum substances in the wastewater are 81% and 99%, respectively. The treated effluent meets the requirements for reuse. 　　(2) Over the more than 3 years since the project began operation, its treatment performance has remained stable. The treated water is reused for purposes such as floor washing, car washing, landscaping, and landscape irrigation, thereby generating significant economic and environmental benefits.