GB2129786A

GB2129786A - Sludge treatment or dewatering process

Info

Publication number: GB2129786A
Application number: GB08228562A
Authority: GB
Inventors: James A Prejean
Original assignee: Cosden Technology Inc
Current assignee: Cosden Technology Inc
Priority date: 1982-10-06
Filing date: 1982-10-06
Publication date: 1984-05-23
Also published as: GB2129786B

Abstract

Solids and liquids contained in an aqueous dispersion, which may also be oil-bearing, are separated by adding a flocculating agent to the dispersion while flowing through a pipe, and then subjecting the flowing aqueous dispersion to a flow restriction to mix the flocculating agent and to flocculate the solids. The thus treated dispersion is discharged onto a surface permitting free liquid run-off and deposition of the floc. The process is particularly applicable to oil-bearing waste water sludges and waste oil sludges and facilitates recovery of the oil and land forming of the solids.

Description

SPECIFICATION Sludge treatment or dewatering process This invention relates to the treatment of aqueous dispersions of solids to facilitate separation and disposal of the solids. More particularly, this invention pertains to the flocculation and separation of solids contained in the slurries or sludges produced in the treatment of waste waters. Still more particularly, this invention pertains to the flocculation and separation of solids contained in oil-bearing slurries or sludges produced in the treatment of waste waters and waste oils to facilitate the recovery of the oil and the separation and disposal of the solids contained therein.

Many industrial or manufacturing operations generate considerable quantities of contaminated process waste water requiring treatment prior to disposal. Contaminants include undissolved inorganic and organic materials and dissolved inorganic and organic materials. Primary and secondary treatment are usually employed and are well known in the art.

Process waste water is first subjected to a skimming or screening operation to remove gross solid materials. Suspended solids are then removed from the waste water in the primary treatment. This process, commonly called Dissolved Air Floatation, consists of the injection of air into the waste water together with a suitable flocculating agent. The rising air floats the suspended solids as a floc to the surface where it is scraped or raked off as a froth or sludge and collected. This material is referred to as DAF froth or DAF sludge.

The effluent from the primary treatment passes to a secondary treatment aerator basin. This basin contains microorganisms which, in the presence of dissolved oxygen, destroy or metabolize dissolved organics. This process also produces insoluble material which is separated as a sludge commonly referred to as biomass, biological sludge or aerator basin sludge.

Both the DAF sludge and the aerator basin sludge require further treatment for effective separation of the liquid and solids. The two sludges can be separately but similarly treated or combined and treated as one. While these two materials are commonly referred to as sludges, they are, in reality, a very fluid liquid slurry having a rather low solids content permitting it to be handled as a liquid. The combined sludges commonly contain as low as 4% solids by weight.

Petroleum refining and certain petrochemical manufacturing operations generate considerable quantities of liquid wastes requiring treatment prior to disposal. These liquid wastes are comprised of process waste water and waste oils, commonly referred to in the trade as slop oils. These waste or slop oils have their origin in leaks, spills and other waste causing episodes commonly encountered in petrochemical or petroleum refining operations.

Normal waste collection procedures result in the co-mingling of process waste water and waste oil streams. The resultant total waste stream contains oil, water, suspended inorganic and organic solids, and dissolved inorganic and organic materials. Economic and environmental considerations dictate the need for the recovery of oil contained in such waste streams and the separation of the water and solids for their ultimate disposal.

Over the years, a rather standard method of processing such waste streams has evolved in the industry. The process waste stream is first subjected to a skimming operation followed by treatment in a conventional API separator to remove most of the contained oil and oil-water emulsions. The oil and oil-water emulsions are collected and subjected to a thermal treatment at about 1800F (82 C) with the addition of polymeric flocculant. This treatment promotes the breaking of oil-water emulsions. The free oil is recovered and the water and any remaining oil-water emulsions with suspended solids collected for further treatment and separation. This water with suspended solids is commonly referred to as slop oil solids, slop oil emulsion solids, slop oil sludge, or waste oil sludge.In spite of the prior treatment, considerable oil remains in the slop oil sludge either as an oil-in-water emulsion or occluded in the solids contained therein. Solids separated therefrom can contain as much as 10% or more by weight ofoil.

The water from the API separator is then subjected to a conventional primary treatment for the removal of suspended solids. This process, commonly called Dissolved Air Floatation, consists of the injection of air into the waste water together with a suitable flocculating agent. The rising air floats the suspended solids as a floc to the surface where is is scraped or raked off as a froth or sludge and collected. This material is referred to as DAF froth or DAF sludge. It contains some oil that escaped prior removal.

The effluent from the primary treatment passes to a secondary treatment aerator basin. This basin contains microorganisms which, in the presence of dissolved oxygen, destroy or metabolize organics.

This process also produces insoluble material which is separated as a sludge commonly referred to as biomass, biological sludge, or aerator basin sludge.

The DAF sludge, aerator basin sludge and slop oil sludge require further treatment for effective separation of the liquids and solids. While these materials are commonly referred to as sludges, they are, in reality, a very fluid liquid slurry having a rather low solids content permitting them to be handled as a liquid. It is not uncommon for these sludges to contain as low as 4% solids by weight.

The solids in such sludges are usually anionic. To separate the solids, a cationic flocculant is added to the sludge in a suitable flocculating tank. The tank contains a mechanical agitator to facilitate mixing and floc formation. The agitation must be gentle to avoid excess shear which could redisperse the floc.

The addition of the sludge and flocculant to the tank is continuous.

According to one of the most commonly used techniques, the material from the flocculating tank overflows onto a continuously moving endless belt where free water and oil flow off and the floc is deposited. As the belt with the deposited floc advances, the floc passes under pressure rollers removing some of the entrained water and oil in the floc. The resultant cake contains about 20% solids but has the handling characteristics of a solid material facilitating its being trucked to a land fill or land farm for disposal. In some cases, the cake is incinerated for energy recovery.

The DAF sludge and aerator basin sludge can be separately but similarly treated or combined and treated as one to separate the solids and liquids.

Slop oil sludges are normally treated similarly but separately.

The techniques of the prior art are obviously exceptionally costly both as to capital costs and continuing operating expense. Furthermore, oil separation and recovery using the above described technique is far from complete. In any sizable operation, the resultant cake obtained as outlined above would contain some quantities of oil which would be totally lost in subsequent land filling or land farming of the cake.

It is an object of this invention to provide an improved simplified process for the treatment of sludges to flocculate the solid material contained therein and separate the floc.

According to the present invention, there is provided a process for treating waste water and waste oil sludges, which process comprises flowing the oil-bearing sludge to be treated through a pipe, injecting a flocculant into the flowing sludge, flowing the sludge containing the flocculant through a segment of pipe containing a flow restriction which facilitates mixing of the flocculant and formation of the floc, and discharging the treated material on a surface permitting the run-off of free oil and water and deposition of the floc. In a specific embodiment of this invention, after injection of the flocculant and formation of the floc as above described, the treated material is discharged onto land where the floc is deposited, free oil and water run-off diverted to a collection basin or sump, and the oil layer is recovered while the deposited floc may be tilled into the land.

The novel process of this invention has a number of advantageous results. The absence of any significant mechanical shearing action on the floc results in much less entrained water and oil in the floc as discharged. The cake as discharged, after run-off but without any compression of the cake, contains about 20 per cent solids, the same as the cake after the belt press treatment in currently-used techniques. The process of this invention eliminates the need for the belt press and separate flocculating tank. Waste water sludge and waste oil sludges can be combined and treated as one.

The cake, after discharge and free liquid run-off, is resistant to redispersion. It has been found that the cake deposited by discharging from the treatment pipe directly onto land is unaffected and not washed away by subsequent heavy rainfalls.

The locale of treatment is not limited by any permanent installation of expensive capital equip ment, such as a belt press or a flocculating tank.

Since the injection of the flocculant and flocculation occurs near the discharge end of the pipe, the sludge, prior to any chemical treatment, can be moved inexpensively by pipe to any desired remote location, treated in the pipe, and promptly discharged. Treatment and discharge can be constructed and installed as to eliminate trucking the sludge to disposal. Furthermore, since the distance of travel of the sludge after flocculation is very limited, redispersion of the floc as the result of excessive pipe travel does not occur.

Land farming of oil-bearing sludges is a commonly used waste disposal technique. In land farming, waste sludges are deposited onto the land and then tilled into the land. Microorganisms in the land, in the presence of adequate oxygen, metabolize or destroy the organic materials contained in such waste sludges. Land farming of oil-bearing sludges, without any prior chemical treatment, flocculation of the solids, and separation of the solids and liquids, has not proved to be too satisfactory. Spreading such dilute untreated oil-bearing sludges onto the ground does not result in an effective separation and run-off of the oil and water from the solids.It has been found that when depositing such untreated oil-bearing sludges on the ground, as much as twelve months' standing time is required for evaporation of water and sufficient absorption or partial metabolization of oil to produce a surface and subsurface sufficiently hard and caked to permit tilling with conventional farm implements. It is for this reason that, according to current practice, such oil-bearing waste sludges are separately chemically treated to produce flocculation of the solids and a gross separation of the floc solids and the oil and water accomplished on a belt press. The resulting cake, having the handling characteristics of a solid material, is then trucked to a land farm area where it is spread on the ground.

The present invention eliminates the need for a separate flocculating tank and a belt press and, in a sense, permits the direct land farming of the oilbearing sludge itself. It has been found thatfiocculat- ing the solids and discharging the material in accordance with this invention results in a much more thorough and clean separation of the oil and water from the solids. When such treated oil-bearing sludge is deposited onto the land, the run-off of free oil and free water is practically instantaneous and clearly visible. Diversion and collection of the run-off liquid is simple and recovery of the oil layer by vacuum truck easily accomplished. The separation of liquid and solid is so complete that the deposited solids reach a state of consistency permitting conventional tilling within 14 days after being deposited.

The present invention will now be illustrated further by the following Examples.

Example 1 DAF sludge and aerator basin sludge obtained from the treatment of the process waste water in a petroleum refining operation and contained in a common storage tank was pumped through a three inch (7.6 cm) underground pipe approximately 3,000 feet (900 meters) to an area of vacant land. The sludge contained approximately 4% solids by weight. At the edge of the vacant land, the pipe extended vertically four feet (1.2 meters) where it was joined by a three inch (7.6 cm) pipe extending horizontally over the land approximately 20 feet (6 metres).

The horizontal pipe was downwardly inclined so that the point of discharge from the pipe was approximately two feet (0.6 metres) above the land.

Approximately 12 inches (30 cm) from the juncture of the vertical and horizontal pipes, a one and one half inch (3.8 cm) orifice was inserted into the horizontal pipe.

The flocculant as a 1% solution of the active ingredient in water was separately prepared nearby in a 200 gallon (757 litre) tank. The flocculant solution was pumped by means of a metering pump from a tank through a one-half inch (1.3 cm) line. A one-half inch (1.3 cm) coupling was welded into the three inch (7.6 cm) vertical pipe near ground level for chemical injection. The flocculant was introduced into the pipe at the inside surface of the pipe. The flocculant employed was a moderately cationic high molecular weight polyacrylamide commercially available from Nalco Chemical Company under the registered trademark Nalclear 7123.

In operation, the sludge, consisting of approximately two parts by weight of aerator basin sludge and one part by weight of DAF sludge, was pumped through the pipe at approximately 10 gallons per minute (gpm) (38 litres/min.). The 1% solution of the flocculant was metered into the pipe and flowing sludge to provide a concentration of the active flocculating agent in the sludge of approximately 500 - 700 ppm.

Upon discharge of the treated sludge from the end of the horizontal pipe the separation of free water and floc was clearly visible and, upon deposition of the treated material onto the ground, free water run-off was practically instantaneous. The deposited cake, after run-off but with no further treatment, contained approximately 20% solids.

While the above described preferred embodiment involved the treatment of a mixture of DAF sludge and aerator basin sludge, the process is equally applicable to either waste sludge alone of any other sludge wherein the solids are capable of flocculation.

Example 2 DAF sludge, aerator basin sludge and waste oil sludge obtained in a petroleum refining operation and contained in a common storage tank were pumped through a three-inch (7.6 cm) underground pipe approximately 3,000 feet (900 meters) to an area of vacant land approximately four acres (1.6 hectares) in size. The sludge contained approximate ly 4% solids by weight. At the edge of the vacant land, the pipe emerged perpendicular to the surface of the land. The pipe extended vertically four feet (1.2 meters) where it was joined by a three-inch (7.6 cm) pipe extending horizontally over the land appro ximately 20 feet (6 meters). The horizontal pipe was downwardly inclined so that the point of discharge from the pipe was approximately two feet (0.6 meters) above the land.

Approximately 12 inches (30 cm) from the juncture of the vertical and horizontal pipes, a one and one-half inch (3.8 cm) orifice was inserted into the horizontal pipe.

The flocculant as a 1% solution of the active ingredient in water was separately prepared nearby in a 200-gallon (757 litre) tank. The flocculant solution was pumped by means of a metering pump from the tank through a one-half inch (1.2 cm) line. A one-half inch (1.3 cm) coupling was welded into the three-inch (7.6 cm) vertical pipe near ground level for chemical injection. The flocculant was introduced into the pipe at the inside surface of the pipe. The flocculant employed was a moderately cationic high molecular weight polyacrylamide commercially available from Nalco Chemical Company under the registraton trademark of Nalclear 7123.

In operation, the sludge was pumped through the pipe at approximately 10 gallons per minute (gpm) (38 litres per minute). The 1% solution of the flocculant was metered into the pipe and flowing sludge to provide a concentration of the active flocculating agent in the sludge of approximately 500 to 700 ppm. To some degree, the composition of this sludge varied during continuous operation. The primary and secondary treatment of process waste water is a continuous operation and the amount of DAF sludge and aerator basin sludge produced per increment of time will be rather consistent. In this operation, approximately 2 parts by weight of aerator basin sludge for every one part by weight of DAF sludge was contained in the common storage tank.

Since the generation of waste oil is more episodic in nature, the oil content and waste oil sludge content of the total sludge is more variable. The waste oil sludge content was at times as high as 20% by weight of the total sludge composition. The oil content of the total sludge composition in this operation was found to conventionally vary from one to ten percent by weight of total sludge composition. Furthermore, no mechanical means of agitation was provided in the common storage tank for the three sludges.

Upon discharge of the treated sludge from the end of the horizontal pipe the separation of free water, free oil, and floc was clearly visible and, upon deposition of the treated material onto the ground, free water and free oil run-off was practically instantaneous. The deposited cake, after run-off but with no further treatment, contained approximately 20% solids.

Near the point of discharge of the treated sludge onto the land, the land was channelled to divert the liquid run-off to a trench bordering the edge of the land which led to a sump at the corner of the land where the liquid was collected. The oil layer was recovered by means of a vacuum truck. In one operation extending over a period of 48 hours, approximately 100 barrels (13,350 litres) of oil were recovered.

In this operation it was found that within 14 days after deposition of the floc onto the land, the land was sufficiently dry and caked to permit the use of bulldozers to spread the deposited floc and then to till the land with conventional farm implements.

Four acres (1.6 hectares) of land were sufficient to handle the land farming requirements to dispose of the waste load of this operation. By contrast, over 20 acres (8.1 hectares) of land were required when an attempt was made to land farm the waste sludge load without any prior chemical treatment and gross separation of the solids from the liquids.

The size or diameter of pipe used for mixing and clocculation, as well as the degree of flow restriction and distance between flocculant injection and flow restriction, are all subject to substantial variation.

These three factors coact to provide a flocculant mixing and residence time to cause flocculation followed by subsequent release of the floc and separation of fluid in the absence of any substantial shearing action.

In the preferred embodiment set forth above, the residence time of the sludge in the vertical pipe was approximately 9.2 seconds, with a linear flow of approximately 0.43 ft/sec (0.13 meters/sec). In the system described, sludge flow can be ncreased to 20 gpm (7.6 liters/min) resulting in a linear flow of 0.87 ft/sec (0.26 meters/sec) with a residence time of 4.6 sec in the vertical pipe. Equally good flocculation without shear and excellent water separation is realized under these latter conditions. Generally, a linear flow much in excess of 1 ft/sec (0.3 meters/sec) begins to result in excessive shear, decreasing the sharpness of the water/floc separation. When desired capacity would require too high a linear flow, the use of pipe having a larger diameter and, hence, a lower linear flow would be the proper approach.

Claims

1. A process for the separation of solids contained in an aqueous dispersion thereof, wherein the solids are flocculated by adding to said dispersion a flocculating agent and free water then separated from the flocculated solids, which process includes the step of flowing the aqueous dispersion through a pipe, injecting a flocculating agent into the aqueous dispersion while flowing through the pipe and subjecting the flowing aqueous dispersion containing the flocculating agent to a restriction in flow through the pipe to mix the flocculating agent and flocculate the solids.

2. A process according to Claim 1, wherein the aqueous dispersion of solids is a sludge obtained from the treatment of waste water.

3. A process according to Claim 2, wherein the aqueous dispersion of solids is a sludge obtained from the primary treatment of waste water.

4. A process according the Claim 2, wherein the aqueous dispersion of solids is a sludge obtained from the secondary treatment of waste water.

5. A process according to any one of the preceding claims, wherein the flocculating agent is injected as an aqueous solution of the flocculating agent.

6. A process according to Claim 5, wherein the flocculating agent is injected as an aqueous solution containing less than about 2% by weight of the active flocculating agent.

7. A process according to any one of claims 2 to 6, wherein the flocculating agent is injected at a rate to provide from 500 parts to 1,000 parts by weight of the active flocculating agent per million parts by weight of the flowing waste water sludge.

8. A process according to any one of the preceding claims, wherein the flocculating agent is cationic.

9. A process according to Claim 1, for the separation of solids contained in an oil-bearing aqueous dispersion thereof, which process includes the steps of flowing the oil-bearing aqueous dispersion through a pipe, injecting a flocculating agent into the dispersion while flowing through the pipe and subjecting the flowing aqueous dispersion containing the flocculating agent to a restriction in flow through the pipe to mix the flocculating agent and flocculate the solids, separating free water and oil from the flocculated solids and recovering free oil.

10. A process according to Claim 9, wherein the oil-bearing aqueous dispersion of solids contains oil-bearing sludges obtained from the treatment of waste oils.

11. A process according to Claim 9, wherein the oil-bearing aqueous dispersion of solids is an oilbearing sludge obtained from the treatment of waste oils and waste water.

12. A process according to Claim 10 or 11, which comprises flowing the sludge through a pipe, injecting a flocculating agent into the sludge while flowing through the pipe, subjecting the flowing sludge containing the flocculating agent to a restriction in flow through the pipe to mix the flocculating agent and flocculate the solids, discharging the thus treated sludge onto land, diverting free oil and water to a collection basin and recovering the free oil and tilling the deposited floc into the land.

13. A process according to Claim 12, wherein the oil-bearing waste sludge is waste water sludge and waste oil sludge.

14. A process according to Claim 12 or 13, wherein the flocculating agent is injected as an aqueous solution of the flocculating agent.

15. A process according to Claim 14, wherein the flocculating agent is injected as an aqueous solution containing less than about 2% by weight of the active flocculating agent.

16. A process according to any one of Claims 12 to 15, wherein the flocculating agent is injected at a rate to provide from 500 parts to 1,000 parts by weight of the active flocculating agent per million parts by weight of the flowing sludge.

17. A process according to any one of Claims 12 to 16, wherein the flocculating agent is cationic.

18. A process according to Claim 1, substantially as described in any one of the foregoing Examples.