DESIGN OF EFFLUENT TREATMENT PLANT FOR A REFINERY

PROPOSED EFFLUENT AND EMMISION STANDARDS FOR PETROLEUM OIL REFINERIES

Proposed Minimal National Standards (MINAS) for discharge of effluents

S.No.	Parameter	Limiting value for Conc. (mg/l)	Limiting value for Quantum (kg/1000 tonne of crude processed, except Ph)
1	PH	6.0-8.5	-
2	Oil and Grease	5	2
3	BOD_{3 days, 27oC}	15	6
4	COD	125	50
5	SS	20	8
6	Phenols	0.35	.14
7	Cr(VI)	0.1	.04
8	Total Cr	2.0	.8
9	Benzene	0.1	.04
10.	Benzo(a) Pyrene	0.2	.08

1. Concentration limits shall be met at the outlet,discharging effluent (excluding discharge from sea water cooling systems) to receiving environment (surface water bodies,marine systems or public sewers).In case of reuse of effluent directly for irrigation/horticulture purposes and make-up water for cooling systems, the concentration limits shall also be met at the outlet before taking the effluent for such reuse.

2. Limit for quantity of effluent discharged (excluding blow-down from sea water cooling) shall be 400m³/1000 tonne of crude processed. However, for refineries located in high prone area, limit of quantity of effluent only during rainy season shall be 700 m³/1000 tonne of crude processed.

Influent Characterization

It involves

• Identification of wastewater streams

eg. Process wastewater ,Spent Caustic , Oily water

• Flow rate and its distribution

• Pollutant loading

Methods used for measurement of pollutant concentrations in the effluent

S.No.	Parameter	Method
1.	PH	By electrometric method using pH meter.
2.	Oil and Grease	Soxhlet solvent extraction method
3.	BOD_{3 days, 27oC}	Incubation followed by Winkler’s idiometric titration using Azide modification.
4.	COD	Dichromate oxidation open reflux method followed by titration.
5.	SS	By gravimetric method 103-105⁰C
6.	Phenols	Distillation followed by Direct Photometric method.
7.	Cr(VI)	Spectrophotometric method using Diphenyl Carbazide
8.	Total Cr	Oxidation followed by spectrophotometric method using Diphenyl carbazide.
9.	Benzene	Gas Chromatograph method(Purge and Trap Technique)
10.	Benzo(a) pyrene	Liquid-liquid extraction Chromatographic method

AN OVERVIEW OF AN EFFLUENT TREATMENT PLANT

Sewage (or domestic wastewater) treatment incorporates physical, chemical and biological processes which treat and remove physical, chemical and biological contaminants from water following human use. The objective of the treatment is to produce both a clean wastestream (or treated effluent) suitable for discharge or reuse back into the environment, and a solid waste or sludge also suitable for proper disposal or reuse.

The site where the process is conducted is called a sewage treatment plant. The flow scheme of a sewage treatment plant is generally the same for all countries:

Mechanical treatment:

Influx (Influent)
Removal of large objects
Removal of sand
Pre-precipitation

Biological treatment:

Oxidation bed (oxidizing bed) or Aerated systems
Post precipitation
Effluent

Chemical treatment (this step is usually combined with settling and other processes to remove solids, such as filtration.

Treatment stages

Primary treatment

Primary treatment is to reduce oils, grease, fats, sand, grit, and coarse (settleable) solids. This step is done entirely with machinery, hence the name mechanical treatment.

Influx (influent) and removal of large objects

In the mechanical treatment, the influx (influent) is strained to remove all large objects that are deposited in the sewer system, such as rags, sticks,cans, fruit, etc. This is most commonly done using a manual or automated mechanically raked screen. This type of waste is removed because it can damage the sensitive equipment in the treatment plant.

Sand and grit removal

This stage typically includes a sand or grit channel where the velocity of the incoming wastewater is carefully controlled to allow sand grit and stones to settle but still maintain the majority of the organic material within the flow. This equipment is called a detritor or sand catcher. Sand grit and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages. Sometimes there is a sand washer (grit classifier) followed by a conveyor that transports the sand to a container for disposal. The contents from the sand catcher may be fed into the incinerator in a sludge processing plant but in many cases the sand and grit is sent to a land-fill.

Screening or maceration

The grit free liquid is then passed through fixed or rotating screens to remove floating and larger material such as rags. Screenings are collected and may be returned to the sludge treatment plant or may be disposed of off site by landfilling or incineration. Maceration, in which solids are cut into small particles through the use of rotating knife edges mounted on a revolving cylinder, is used in plants that are able to process this particulate waste. Macerators are, however, more expensive to maintain and are less reliable than physical screens.

Sedimentation

In almost all plants there is a sedimentation stage where the sewage is allowed to pass through large circular or rectangular tanks. The tanks are large enough that faecal solids can settle and floating material such as grease and plastics can rise to the surface and be skimmed off. The main purpose of the primary stage is to produce a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed. Primary settlement tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.

Primary sedimentation tank at a rural treatment plant

Secondary treatment

Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of industrial plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota require both oxygen and a substrate on which to live. There are number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc particles. Secondary treatment systems are classified as fixed film or suspended growth. In fixed film systems - such as roughing filters - the biomass grows on media and the sewage passes over its surface. In suspended growth systems - such as activated sludge - the biomass is well mixed with the sewage. Typically, fixed film systems require smaller footprints than for an equivalent suspended growth system; however, suspended growth systems are more able to cope with shocks in biological loading and provide higher removal rates for BOD and suspended solids than fixed film systems.

Roughing filters

Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial. They are typically tall, circular filters filled with open synthetic filter media to which sewage is applied at a relatively high rate. The design of the filters allows high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant liquor is usually within the normal range for conventional treatment processes.

Activated sludge

Activated sludge plants use a variety of mechanisms and processes to use dissolved oxygen to generate a biological floc that substantially removes organic material.

Filter Beds (Oxidising beds)

Trickling filter bed using plastic media

In older plants and plants receiving more variable loads, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonised coal), limestone chips or specially fabricated plastic media. Such media must have high surface areas to support the biofilms that form. The liquor is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the medias' surfaces and eat or otherwise reduce the organic content.

Rotating plates and spirals

In some smaller plants slowly revolving plates or spirals are used which are partially submerged in the liquor. A biotic floc is created which provides the required substrate.

Secondary sedimentation

The final step in the secondary treatment stage is to settle out the biological floc or filter material and produce sewage water with very low levels of organic material and suspended matter.

Secondary Sedimentation tank at a rural treatment plant

Tertiary treatment

Tertiary treatment provides a final stage to raise the effluent quality to the standard required before it is discharged to the receiving environment (sea, river, lake, ground, etc.) More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process.It basically involves Effluent polishing using processes such as Filtration, Lagooning,Wetland Constructions etc.

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