Novel pretreatment enables performance of MBR installed to treat oily wastewater

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Courtesy of Clean Water Technology, Inc.

Two new integrated systems processes, gas-energy-mixing (GEM) flotation – electrochemical catalytic oxidation (ECO) – membrane bioreactor (MBR) and ultra-filtration (UF) – ECO – MBR, were tested in industrial-scale pilots of 200L/h to 3000L/h for the treatment and reuse of steel mill cold-rolling emulsion wastewater. The operation parameters and mechanisms of each module action in the integrated process were analyzed. The influence of operation parameters of two processes on chemical oxygen demand (CODCr ) and fats oil and grease (FOG) removal efficiency were comparatively investigated. The test results showed that the treated water quality could meet the requirement for reuse after treatment by either process. However, the quality of effluent treated by GEM – ECO – MBR was more stable than that of UF–ECO–MBR. Such system was more stable and economic to operate, and can therefore be used in various wastewater treatment plants used to remove emulsified oils and combination of non- and bio – degradable dissolved COD’s.

Keywords: Cold rolling emulsion wastewater; gas-energy-mixing (GEM); electrochemical catalytic oxidation (ECO); ultrafiltration (UF); membrane bioreactor (MBR)

A large amount of oily wastewater is discharged during cold-rolling in the steel industry. Among such effluents, cold-rolling emulsion wastewater is the most difficult one to treat. (Chkrabarty et al., 2008). At present, the conventional treatment methods of oily wastewater are flotation, flocculation, adsorption, bio-chemical, membrane separation process, etc (Yi et al., 2004). However, these technologies still have some disadvantages, such as high operation costs, high chemical consumption, and unstable effluent quality, which make it difficult to achieve the expected treatment efficiency and limit their industry application.

Wastewater treatment plant of Wuhan Iron & Steel Corporation (WISCO) has been using the conventional combination of flocculation – sedimentation – ultra-filtration (UF) – contact oxidation – precipitation to treat the cold-rolling emulsion wastewater. Even under best conditions, CODcr of effluent still remained between 300 mg/L to 500mg/L, which was much higher than that in the Chinese Iron and Steel Industrial Water Pollutant Discharge Standards. As far as we know, there is no efficient process for treatment of cold-rolling emulsion wastewater in iron and steel industry. Researchers are trying to find some new integrated methods to treat such wastewater. UF– ECO – MBR process has been proposed, but not tested, in iron and steel plants in China (Chakrabarty et al., 2008). Therefore, it is necessary to develop more reliable treatment methods to achieve the required effluent quality.

Among new wastewater treatment technologies such as ECO, MBR, GEM and UF, ECO is especially promising new technology. ECO systems provide energy high enough to break the chemical bonds of non-biodegradable organic matters so that those pollutants can be oxidized, decomposed, or even completely mineralized. This gives it unique advantages in treating non biodegradable contaminants (Liu et al., 2005, Panizza et al., 2000). After conversion into biodegradable materials, advanced bioreactor technologies such as MBR can then be used for residual dissolved BOD and COD removal. However, solids and emulsions (oils) have to be removed to prevent electrode fouling in the ECO systems.

GEM, an efficient novel solid liquid separation technology, can provide controlled mixing of contaminants, flocculants and air bubbles by sequentially translating liquid particles (down to molecular size) throughout a centrifugally rotating liquid layer inside liquid hydrocyclone columns. (Colic et al., 2008). Consequently, the floating ability of flocs is improved and the majority of oil and suspended solids (SS) can be efficiently removed (Colic et al., 2008)]. The resulting effluents are then comparable in TSS and oil concentrations to UF effluents.

Therefore, GEM-ECO-MBR and UF-ECO-MBR systems performance in treating cold-rolling emulsion wastewater was compared. In industrial-scale tests, these two processes had been introduced with the support of WISCO, PACT/USA, CWT and Shanghai DEPE. The test results showed that the treated water quality could satisfy the standards for recycling after using the two processes. But the GEM-ECO-MBR process was operationally more stable than UF-ECO-MBR. Therefore it is a promising technology that could be widely applied in a variety of industrial fields that produce emulsified partly non-biodegradable wastewater.

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