Oil-containing effluent treatment by gravity separation

Oil-containing effluent is treated by passing it through a gravity separator and obtaining (a) a wet oil fraction, (b) an aqueous fraction of reduced oil content and (c) a wet oily sludge. Fractions (a) and (c) are then combined and passed through a pressure filter so that solids are retained and oil and water passed through. The oil and water are then passed to a separator and a reusable oil fraction is recovered and a water fraction which is returned to the gravity separator.

This invention relates to a method for treating (1) water-and 
oil-contaminated solids and (2) solids-and water-contaminated oil, 
particularly the top oil and bottom solid residues formed as by products 
from the gravity treatment of oil contaminated water, such as refinery or 
other industrial plant effluent. 
As a first step in the treatment of oil contaminated water it is common 
practice to effect a rough separation in a primary gravity separator to 
remove the bulk of the oil and suspended solids. Suitable separators 
include API Separators, parallel plate separators and settlement tanks. 
API Separators are described in the "Manual on Disposal of Refinery 
Wastes", published by the American Petroleum Institute in 1969. 
During this separation, oil usually associated with stabilising 
contaminants rises to the surface, oily sediment settles to the bottom and 
water containing relatively small concentrations of oil and suspended 
solids passes through the separator, over the effluent weir and on to 
further treatment if necessary. 
The separated oil is not pure but also contains suspended solids and water 
and often traces of bacteria. Since these interfere with subsequent 
processing it is necessary to treat the separated oil before recharging it 
to processing equipment. (This is generally done by heating it in special 
tanks known as recovered oil or emulsion break tanks and allowing oil, 
water and sediment to settle out. At the end of the settling period three 
or four layers of material will exist. The first or top layer will be 
clean oil which is suitable for processing. Under this there will usually 
be a residual layer of unbroken sludge emulsion. The third layer will be 
water still containing soluble components, suspended solids and oil; and 
the fourth or bottom layer will be a wet oily sludge. The water and sludge 
may be recycled to the separator. 
Recovered oil can be reprocessed or used as a source of energy and the 
aqueous layer from this tank may be returned to the primary gravity 
separator. The residual sludge emulsion from the second layer, if present, 
may present considerable difficulty in treatment.) 
A suitable treatment for the aqueous effluent from the primary gravity 
separator is disclosed and claimed in our British patent specification No. 
1,449,342 which claims a method for treating effluent water containing 
suspended oil which method comprises passing the effluent water firstly 
through a sand filter to remove the suspended oil and subsequently through 
a biological percolating filter. 
The third product from the primary gravity separator, the oily bottom 
sludge, builds up over a period of time and the removal and disposal of 
this gives rise to problems. Oily sludge can have a water content in the 
range 20-98% by weight, an oil content in the range 1-70% by weight and 
the remainder is solid material such as silt (including sand, clay and 
chalk), rust, carbon, etc. 
The only environmentally acceptable method of dealing with this sludge 
currently available is combustion, but this requires the provision of 
special incinerators which are expensive to purchase, install, operate and 
maintain. 
We have now discovered that this oily sludge can be converted to a 
tractable material by a simple procedure. 
Thus according to the present invention there is provided a method for the 
treatment of oil-containing aqueous effluent which method comprises the 
steps of: 
(1) passing the effluent through a gravity separator and obtaining (a) a 
wet oil fraction which may be associated with a sludge oil emulsion, (b) 
an aqueous fraction of reduced oil content and (c) a wet oily sludge, 
(2) heating Fraction (a), 
(3) combining the heated Fraction (a) with Fraction (c), 
(4) passing the resulting mixture through a filter in such manner that 
solids are retained and oil and water passed through and 
(5) passing the oil and water to a separator and recovering a reusable oil 
fraction and a water fraction. 
The water fraction of Stage (5) is preferably recycled to the primary 
separator. 
The filter of Stage (4) is preferably a pressure filter such as a candle, 
leaf or plate filter. 
Fraction (c) requires to be heated to permit the water, oil and solids to 
separate, but supplying heat in a conventional manner through a heat 
exchanger will, because of the nature of the material, result in rapid 
fouling. 
Fraction (a), however, can be heated without any difficulty. Thus by 
heating Fraction (a) and subsequently combining it with Fraction (c), heat 
is also supplied to the latter without risk of rapid fouling. 
When the filter is exhausted with respect to solids it may be cleaned by 
stopping the feed, forcing through the bulk of the oil still attached to 
the solids by passing hot water, steam or solvent in the same direction of 
flow as the feed and recovering a relatively oil-free solid. If desired, 
the solid may be dried, e.g., by air blowing. Alternatively, the wet oily 
solids may be scraped from the filter and treated in a similar manner 
elsewhere. 
Our copending British patent application No. 51,189/75 discloses and claims 
a method for the treatment of oily sludge which method comprises the steps 
of filtering the sludge, treating the filtration residue with a 
hydrocarbon solvent and steam stripping the extracted residue. 
By the method of the present invention it is possible to prevent the build 
up of oily sludge within the separators since the rate of offtake can be 
adjusted to the rate of formation and to convert this material to a 
concentrated solid containing about 5% by wt oil or less. Such a material 
is suitable for land fill operations. 
Furthermore all the oil collected by the primary separator is ultimately 
recovered in a form suitable for further processing and all the water 
initially associated with the oil is recovered and recycled to the 
separator.

With reference to FIG. 1: 
Feedstock enters the system through line 1 flowing into a primary gravity 
separator 2 which may be open or enclosed. The advantages of a closed 
separator are reductions in both evaporative losses and atmospheric 
pollution. In the separator 2, three layers form: oil, sludge/emulsion; 
water; and wet oily sludge. The water passes by line 3 to secondary 
treatment 4, e.g. the process described in British patent specification 
No. 1,449,342. The oil and sludge/emulsion are withdrawn from the top of 
the separator by line 5 and passed to a heated recovered oil tank 6. In 
the tank 6 oil and water may separate to a further extent. Oily water may 
be recycled to the separator 2 by the line 7 and wet oil passes on through 
the line 8 where it joins wet oily sludge withdrawn from the bottom of the 
separator 2 by the line 9 to enter a pressure filter 10 which retains 
solid material. Liquid emerges through line 11 and passes to a second 
recovered oil tank 12 in which oil and water again separate. The oil, of 
useable quality, is removed by line 13 and the water may be recycled to 
the separator 2 by line 14. 
With reference to FIG. 2: 
A typical existing refinery system may be adapted as follows: 
Oil refinery effluent is fed by line 21 to an API Separator 22. Aqueous 
effluent still containing traces of oil is removed for further processing 
by the line 23. Oil and sludge emulsion 24 are removed by line 25 to a 
recovered oil sump 26 from which they are pumped to a heated recovered oil 
tank 28 through the line 27. 
In the tank 28 oil and water may separate. Water may be recycled to the API 
Separator 22 by the line 29 and heated, wet oil is withdrawn through the 
line 30. 
Oily sludge 31 is withdrawn from the base of the separator 22 by line 32 
and bled into the heated oil line 30. Oil and sludge are then pumped into 
a pressure filter 33 which retains the solids contained in the mixed 
sludge and oil. The oil passes on through line 34 to a second recovered 
oil tank 35 in which oil and water again separate. Heating is not normally 
required in this tank but an existing typical recovered oil tank will 
probably have heating facilities which may be useful in cold climates or 
if the recovered oil is waxy. The oil, of processable quality, is removed 
by line 36 and the water may be recycled to the separator 22 by line 37.