Oily water treatment units

CORRUGATED PLATE INTERCEPTION SEPARATION (COALESCING PLATE SEPARATORS)

INTRODUCTION

CPI Packages are commonly used to separate oil and fats with low specific gravity. The water flows through corrugated plates (also defined as "coalescing plates") manufactured in polypropylene. Prefabricated units in steel can be provided for flows up to 500 m3-hour. Oil and fats particles adhere to the plates and, due to low density, move upward. A skimming device collects the oil/fats from water surface. Any eventual sludge is collected in a separate section for extraction and disposal.

When the wastewater contains both low density and high density oils/fats, an Air Flotation Package (DAF) must follow the CPS package. Based on the German standard "DIN 1999 part 5" or the new European standard "EN 858" < 5 mg/l of oil in the effluent water can be easily achieved. Separation of dissolved and stable emulsified oils or hydrocarbons is not possible with CPI.

WORKING MECHANISM OF THE COALESCING PLATES

The high efficiency of the system is based on 6 mm or 12 mm short rise distance of the oil droplets which is the exact distance between the plates. E.g. an oil droplet of 20 micron diameter at a density of 850 kg/m³ and a temperature of 20 °C has a rise velocity of 2 mm/min. Just above 3 min retention time within the plate pack the oil droplet will be captured by the under side of the next plate ensuring separation. In standard sedimentation separators a 20 micron oil droplet will need several hours retention time and because of turbulence an upward movement of the oil droplets will not take place.

Hydrodynamic Coalescence
The configuration of the plates will force hydrodynamic coalescence. The merging of very small oil droplets into larger ones will create extremely enhanced coalescence. Before the oil droplets are separated they are captured by the next higher plate. The oliophilic material of the plates ensures that the oil droplets will never reach the separator outlet. They are concentrated in the top of the corrugated plates and will rise through the weep holes to the developed oil layer above the plate packs. The rise velocity is a function of the square of the oil droplet diameter. A twice as large oil droplet will rise four times faster, a triple size oil droplet will rise nine times faster etc... An oil droplet of 3 mm will rise with enormous velocity of 0,74 m/s through the weep holes. (density 850 kg/m³; t=20 °C).

DESIGN AND CONSTRUCTION OF THE SYSTEM

The oily/water mixture enters the first stage over an inlet distributor pipe (a large system has 2 pipes). This, for the new separators enlarged first stage, achieves gross oil and solid separation. Large oil droplets will rise with a high velocity to the developed oil layer and solids will sediment to the bottom of the first stage. The oily water containing very small oil droplets then enters the plate packs (2nd stage) where separation, as described earlier, takes place. The developed oil layer which is above the plate packs is separated from the water outlet by an oil dam and the oil layer will flow by gravity over two adjustable oil skimmers to an integrated oil chamber or an external oil collecting tank. Mainly for prefabricated concrete separators, oil skimmers are not installed and the oil layer will be drawn-off from time to time. The 3rd stage consists of the outlet chamber with a trough where the clean water flows over its edge to the outlet of the separator.

AIR FLOTATION PACKAGES (DAF)

Flotation Packages are commonly used to separate oil and fats having specific gravity close to water. In such a case, the separation occurs only by air flotation. The air bubbles adhere to the oil/fats particles to reduce the density and thus allows the lifting of the particles. A skimming device collects the supernatant while the clarified water is extracted from a dedicate section. Flotation packages include air saturation skid and may be proposed as circular type or rectangular type depending on size. Rectangular type may be equipped with double scraping chain to collect any eventual suspended solid in the water precipitating b y gravity. The units are supplied with full instrumentation,equipment and control panel.

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