Cationic polymer application in DAF units

Dissolved Air Floatation (DAF) technology is one of the proven purification technologies already used in the world. It is designed to meet all costumer requirements in different version and models. It has been used for a potable water clarification/filtration, industrial water treatment and many other applications. Its efficiency can be increased by using cationic polymer flocculant.

M. Sever, D.C. Kan

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Article Summary

Water treatment
Water always has been an important natural resource of the earth and has been essential moreover imperative for the existence of all kind of living things. Water usually contain various substances such as suspended solids, dissolved solids, and gases.

DAF Removal of emulsified oil and suspended solid is essential to improved operation of the secondary treatment plant. Experience shows that removal efficiency without the use of any chemical treatment can be max 50 %. (in most cases it is even much lower)

DAF is very efficient and is widely used in treating the oily industrial wastewater from oil fields, oil refineries, petrochemical and chemical plants, natural gas processing plants, and similar industrial facilities.

The main target of DAF unit, in oil & gas application field, is to remove total suspended solids (TSS) and oil/grease (O/G) from water to be treated.
Figure1 shows a picture of a typical DAF unit.

In DAF system, part of the effluent water is recycled, pressurized, saturated with air, and then mixed with the incoming feed. When the recycle stream is depressurized it releases the air bubbles that attach themselves to both any free oil and solids contained in the feed and float to the surface of the vessel. As a result of this, some solids also settle to the bottom of the DAF where they are scraped off (by scrappers) and removed periodically.

Polymer flocculants are water-soluble polymers which can form flocs from individual small particles. They are high molecular weight polymers and have hydrogen-bonding groups and/or ionic groups in the molecule.

Flocculants may be inorganic or organic, small molecules or polymers. Polymer flocculants have collecting and precipitating function. Based on their ionic nature, polymeric flocculants can be classified as anionic, nonionic, and cationic. Among the polymer flocculants, cationic ones have better flocculating properties.

Cationic polymer is very effective in sludge dewatering and DAF operations. Polymer is used as cationic flocculant before several laboratory and jar tests over a wide range of conditions especially performed in the field. Since water characteristic are different for a Refinery wastewaters, different dosages rates within a range are proposed.

TSS is a fundamental measure of water quality. Improving the removal efficiencies of the TSS and O/G to increase the effluent quality in the outlet of the DAF units.
In addition, the load for the biological units is aimed to be lowered with the chemical treatment especially cationic polymer application
Benefits of the treatment
   - Improved turbidity/TSS & oil removal
   - Lower COD/BOD load to Biological wastewater plant that lowers oxygen consumption
   - Produce a much more concentrated oily sludge

Recommended dosage rates are based on the jar test results performed. But real field conditions might be different, so dosage rates recommended within a range.

A refinery which wastewater is treated in physically and biologically two different Wastewater Treatment (WWT-1; WWT-2) systems. Both System have same treating facilities equipment with different sizes.

System typically consists of
   ⎫ Storm water discharge canals
   ⎫ Sand and Silt Removal Unit
   ⎫ API Oil separators
   ⎫ Additional settling ponds
   ⎫ Flotators (DAF)
   ⎫ Aeration tanks
   ⎫ Secondary settlers (Clarifiers)
   ⎫ Sand Filter

WWT-1 receives relatively cleaner wastewater compared to WWT-2. Each of them has 2 DAF units

WWT-2 receives oilier wastewater stream from desalters so that oil content and TSS content for the WWT-2 is higher than WWT-1

We have started an industrial test consisting in using a cationic flocculant at the inlet of the flotation units. After several lab tests performed on representative wastewater samples collected in different periods of the year.

The polyelectrolyte, after a suitable dilution with raw water, is injected in the two pipelines that WWT-1 and WWT-2, through a dosing skid.

Dosage recommendations are based on the laboratory test and jar test that trial we performed in the field.

The average dosages suggested for both systems are typically 3-6 ppm. In the beginning, the first phase of the test was characterized by several problems occurred to the metering pumps. Replacing with a more efficient model, thee new pumps have ensured a constant and regular product addition.

The performance of the cationic polymer application was evaluated by checking mainly two parameters:
  - oils removal
  - suspended solids removal

The analyses were performed by taking some samples at the inlet and outlet of the systems several times per week. From the beginning the results were positive showing, for both parameters, an average removal higher than 80 % (Lab results)

As shown Table-1 without chemical application DAF Units performance (% efficiency) regarding with COD, TSS and Oil/Grease are 14-19-20 %

As shown Table 3 as a result of chemical application for the WWT-1, average oil removal efficiency 49 % and for the WWT-2 average oil removal efficiency goes up from 20% up to 68% (increased)

In the same way; total suspended solid value (TSS) average removal efficiency for the WWT-1 goes up from 14% to 68% and for the WWT-2 from 26% to 45%

Muammer Sever is Process Superintendent with Tüpraş¸ Izmir refinery, with 20 years’ experience in process improvements, crude oil distillation, Merox process and Isomerization, furnace and boiler combustion control and safety systems, burners, waste heat boilers, hydrocarbon loss management, waste water treatment, operations maintenance coordinating (OPC) and energy reduction studies He holds a BSc in chemical engineering and a MBA.

Doga Can Kan is Process Chief with Tüpraş¸ Izmir   refinery, with 5 years’ experience in operating and design domestic, municipal and Industrial Wastewater Treatment systems and technologies in Refineries. He holds a BSc in environmental engineering.

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