Treatment of Effluents of Petroleum Refining Industry | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of petroleum refining industry.

Sources and Characteristics of Effluents:

Sources:

The major water pollutant in refinery operations is oil, besides chemicals that may be formed during treatment of petroleum products by chemical processes for removal of undesirable constituents.

The effluents may be classified under the following five categories:

(a) Effluents free from oil and dissolved organic material;

(b) Effluents accidentally contaminated with oil;

(c) Effluents continuously contaminated with oil but not contaminated with other soluble organic material;

(d) Process effluent; and

(e) Sanitary and domestic effluent.

1. Effluents free from oil and dissolved organic material:

These originate from:

(a) Boiler blow-down,

(b) Water treatment plant effluent, and

(c) Storm water from oil-free areas.

2. Effluent accidentally contaminated with oil:

These originate from:

(a) Storm water from tank farms; and

(b) Blow-down from circulating cooling water system.

3. Effluents continuously contaminated with oil but free from soluble organic material:

These originate from:

(a) Storm water from oil processing areas, and

(b) Ballast water.

4. Process Effluents:

These originate from:

(a) Dealter,

(b) Condensate from steam stripping operations,

(c) Pump gland cooling water,

(d) Barometric condenser water containing emulsion, and

(e) Wash water from treating plants.

5. Sanitary and Domestic Effluents:

These originate from:

(a) Toilets, and

(b) Canteen.

Characteristics:

General characteristics of the composite oil refinery waste water (before treatment) are likely to be as given in Table 17.7 (see also 17.7).

Note:

Cyanides and ammonia may be present in the waste water from refineries where cracking process is practised.

Effect on Pollution:

The principal public health hazard caused by untreated refinery effluents is pollution of water courses and land with toxic chemical compounds such as oil, hydrogen sulphide, ammonia, mercaptans, phenol, cyanides, fluorides, acids, alkalis, chromates, and traces of heavy metals.

Acids and alkalis are toxic to aquatic life and inhibit self-purification of water courses. Fluorides cause dental fluorises and dental caries which affect teeth, joints and bones, Presence of hydrogen sulphide, mercaptans and phenol produce disagreeable odour in water and, at the same time, increases biochemical oxygen demand of water, Oils, which are difficult to biodegrade, form a thin film over water surface, causing unsightly appearance and interfere with the natural process of aeration and photosynthesis, and also cause fore hazards.

Treatment:

Oil-free waste water should never contain oil and hence, they may bypass waste treatment facilities and discharge directly into the refinery outfall line after decontamination through gravity separators or surge ponds to take care of for accidental contamination, (facilities should be provided for oil removal).

The general treatment methods to handle waste waters from refinery operations are as follows:

1. Primary Treatment:

It consists of free oil removal. Oil removal is also affected by in- plant processes like stripping and extraction. Further, removal of oil from the waste water is carried out principally in two stages the first stage fracturing gravity separation and the second stage floatation with or without addition of chemicals/coagulants.

Gravity separation is meant for removal of separable free oil:

a. Oily effluents originating from various sources in the refinery are collected in a sump routed to an oil separator for removal of free oil. Oil separators are designed considering hydraulic load, density and viscosity of the oil and diameter of oil globules.

The API separator mainly consists of an inlet bay, a middle or a separator bay and an outlet bay. The inlet bay contains sluice gates for commissioning or decommissioning the individual bays of the separator. It contains a vertical hanging baffle to retain the oil and also arrangement to collect the separated oil.

The middle or separator bay is the place where the remaining portion of the oil is separated by reducing the velocity, so that the water becomes almost quiescent. The separated oil is retained in the middle bay by another vertical hanging baffle situated at the end of this bay and the collected oil is skimmed by a skimmer provided near this baffle.

A flight scrapper provided in the middle bay brings oil to skimmer for its removal and also at the same time pushes the sludge collected at the bottom of the sludge pit or hopper for periodic removal.

The outlet bay contains a weir over which the oil-free water flows out for further treatment. Oil content in the effluent after the API separator is normally less than 100 mg/l in water.

b. It is very important that the oily effluent be sent to a primary oil removal basin and to the API or gravity separators by means of gravity flow. Pumps, especially centrifugal type, should be avoided to minimize formation of oil emulsion with water which would call for additional chemical methods for its removal. Where pumping of oily effluent is essential after the primary oil removal basin, screw type or diaphragm type pumps should be used.

c. Stripping:

Stripping is another physical method. It is done with the aid of steam to remove gases like hydrogen sulphide, mercaptans and ammonia and to some extent phenol and free cyanides.

d. Extraction:

Extraction is yet another physical method which is used to remove phenolic compounds in refinery operations. In this process, solvents like tricresyl phosphates and mixed organic esters are used for extraction of phenol from wastes. Use of petroleum fractions of crude oil is also made for extraction in some of the refineries.

2. Secondary Treatment Methods:

The secondary treatment methods may be classified as follows:

(i) Chemical methods, and

(ii) Biological methods.

i. Chemical Method:

The main purpose of a chemical method is to remove emulsified oil with addition of flocculating agents and also to remove suspended solids and toxic substances, thereby conditioning the effluents for further treatment by biological methods. Sedimentation is normally employed to remove suspended solids after chemical treatment.

Various chemical methods available may be further classified under the following four categories:

(a) Neutralization method,

(b) Precipitation and clarification method,

(c) Chemical oxidation method, and

(d) Regeneration method.

(a) Neutralization:

Neutralization methods are applied to many types of refinery wastes as primary treatment. The wastes as primary treatment.

The wastes considered here are:

(1) Dilute acid or alkaline wastes from various refining processes and from water treatment plant-Neutralized to produce a neutral effluent.

(2) Spent caustic solution – Neutralized with either spend acid or acidic stack gases.

(3) Sulphuric acid sludges-Neutralized with alkaline wastes.

(4) Spent Catalyst – Neutralized with spent caustic wastes.

(5) Hydrofluoric acid wastes – Neutralized with caustic soda solution.

(b) Precipitation and Clarification Method:

Precipitation method is used in the treatment of several types of refinery wastes such as:

(1) Spent caustic solutions – Acidic oils in caustic are precipitated by neutralization.

(2) Sulphide water – Precipitated with iron salts.

(3) Solutions of sulphonates – Precipitated by the addition of lime.

(4) Chromates from cooling water – Reduced to trivalent with ferrous sulphate, sulphur dioxide or hydrogen sulphide and precipitated with lime as chromium hydroxide.

(c) Chemical Oxidation Method:

This method may be used for:

(1) Treatment of phenols and cyanides in wastes and

(2) Combustion of certain wastes.

(d) Regeneration Method:

Production of spent caustic solution is reduced by the regeneration of a chemical like monoethanolamine which is used to absorb hydrogen sulphide and mercaptans. It is regenerated by steam stripping for further use.

ii. Biological Method:

This is intended to remove the biodegradable organic substances and toxic substances like phenol. Biological treatment also reduces the residual oil after primary treatment (but the oil is not recoverable). The biological method of treating the waste is done by application of conventional methods like trickling filter activated sludge process and low cost waste treatment methods like aerated lagoon and oxidation pond.

Trickling filters can tolerate oil up to 100ppm, whereas activated sludge process can tolerate oil upto 25 ppm. Presence of oil in oxidation ponds retards reaeration. Besides oil biological treatment methods are influenced by BOD, toxic substance like chromium, lead nickel, hydrogen sulphide, etc., temperature, pH and nutrients.

(a) Activated Sludge Process:

The activated sludge process is an aerobic biological treatment process in which waste waters are treated with microorganisms that are suspended uniformly within a reaction tank into which oxygen is introduced by one chemical means. The microorganism is maintained in the reaction tank by recycling activated sludge which is formed by the microorganisms in the process.

Essentially, the organic matter is removed from the waste waters by synthesis and oxidation. An aerobic environment is necessary to maintain the proper microbial balance in the system. The basic process consists of a aeration tank followed by a sedimentation tank. The activated sludge removed from the sedimentation tank is recycled to the aeration tank to maintain a high concentration of activated sludge.

The activated sludge will continue to build up until it is necessary to ‘waste’, or discard, some of the excess. In most activated sludge systems, sludge wasting is carried out in a continuous basis at a relatively low rate. Activated sludge forms naturally and does not need any special seeding for proper development.

(b) Trickling Filters:

The trickling filter is a bed broken stone or coarse aggregate. A bed is from 1 m to 3 m deep and may be either rectangular or circular. It may be deeper when plastic packing is used. Fixed or moving sprinklers distribute the waste water over the surface of the bed.

A gelatinous film of slime composed of aerobic organisms develops on the surface of the aggregate. As the waste water trickles over the film, 6th dissolved and suspended organic matter are removed by the adsorption.

The adsorbed matter is oxidised by the organism in the slime during their metabolic processes. Air flows through the filter by convection, thereby providing the oxygen needed to maintain aerobic conditions.

The oxidised waste water from the filter is clarified in a sedimentation tank before discharge into a watercourse. The microbial film that develops on the surface of the rocks is responsible for removing the organic materials from solution.

(c) Oxidation and Stabilisation Ponds and Aerated Lagoons:

The oxidation pond is a popular type of biological waste treatment system where land is plentiful. Fundamentally, the oxidation pond depends on the bacteria to aerobically stabilise the organic wastes added to the pond. In heavily loaded ponds, a certain amount of the stabilization occurs anaerobically rather than aerobically.

The oxygen for the bacteria is supplied by surface transfer from the air and from the metabolism of algae that grow in the pond. The production of oxygen by algae occurs only as a result of synthesis of cellular protoplasm. Thus as the algae produce in synthesizing algae protoplasm, the bacterial utilize the oxygen in synthesizing bacterial protoplasm.

The net effect of this process is the production of organic matter in the oxidation pond. In hard- water areas, more organic matter is produced than stabilized. The important relationship, which permits the oxidation pond to produce a more stable effluent than influent, is that the rate of degradation of organic matter for synthesis of cellular protoplasm.

Also, many of the microorganisms settle out in the pond and are not removed in the effluent. The organic loading on the oxidation pond is limited by the oxygen transfer capacity. A broad, shallow pond with considerable surface agitation, caused by wind action, will have a high rate of oxygen transfer.

A small pond with little exposed surface area will be able to handle a relatively low organic load per unit of surface area. Although the tendency is to construct a single pond, there are some areas that require at least two ponds.

The dual ponds tend to reduce short-circuiting and permit retention of floating organics in the first pond. The second pond is usually very tightly loaded in comparison to the first pond and may build up a sufficiently large animal population during the warmer months to remove most of the algae and produce a clearer effluent.

Oxidation ponds have been used to treat the entire plant wastes as well as to polish the effluent from other biological waste treatment processes. Although oxidation ponds have been constructed for refinery wastes, very few data are available on their operation. Retention periods in oxidation ponds range from a little more than 1 day up to 90 days. For the most part, the reduction in organic matter is a function of the retention period and effluent concentration.

Generally, the entire plant wastes are treated in the oxidation ponds, so that the organic concentration applied is quite low. The net result is a low of microbial metabolism, and a long period of retention is required for organic reduction. As a rule, simple oxidation ponds are ineffective if holding time is less than 7 days.

3. Tertiary Treatment Method:

Tertiary treatment has been limited to activated carbon filtration process and ozonisation which are effective in removal of the taste and odour and organics from biologically treated waste waters. However, these methods are extremely costly. Many of the refineries prefer to use oxidation ponds after biological treatment where enough land is available.

4. The treated liquid wastes which satisfy the relevant tolerance limits are finally disposed by controlled solution into the neighbouring stream, river or sea.

Pollution Control Practices:

In waste control, preventive methods other than treatment should be used for treating the amount of polluting wastes.

New technique should be studied under the following headings:

1. Pollution Control Measures:

These may include:

(a) Provision of separate drainage and sewer systems to prevent pollution of large volumes of clean water by small volumes dirty water;

(b) Segregation of water streams may be done, so that non-oily chemical wastes and oily process wastes are collected and treated separately;

(c) Following water conservation measures at refinery will reduce both water supply and cost of waste treatment.

Installation of air cooled exchangers and cooling towers; pumps gland cooling water to serve as cooling tower blow-down and use of oily water emulsion from barometric condenser, and stripes sour water and phenolic waste water as make-up water for desalter.

2. Process Design Modification:

These may include:

(a) Acid sludge or spent clay disposal problems may be eliminated by replacing acid treatment or clay treatment of lubricating oil with propane deaphalting or with hydro finishing process.

(b) Substitution of continues process for batch processes,

(c) Use of processed which include regeneration of chemicals used for hydrogen sulphide and mercaptan removal may avoid problems associated with disposal of caustic wastes,

(d) Use of downgraded chemicals is economical for both process and waste control. Fresh caustic used for washing light hydrocarbon streams is used for gasoline treating,

(e) Use of methods for conversion of hydrogen sulphide to elemental sulphur.

3. Local Pretreatment or Disposal:

In general, it is advantageous to treat or dispose of concentrated and unusual wastes locally. Treatment of highly polluted waste streams at the sources can prevent gross pollution of large volumes of relatively clean waste water. Such treatment is a more economical solution to the problem then discharging directly into the refinery sewer.

For example:

(a) Stripping ammonia and hydrogen sulphide condensate before discharging into sewer.

(b) Neutralising the spent caustic waste before discharging into sewer.