Enhanced Electroflotation & Its Application in Offshore & Marine Sewage Treatment System

Authors: Sehul Patel, P.E. and Dana Casbeer

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In wastewater treatment, there are various types of flotation techniques/processes available for the treatment of variety of wastewaters. The flotation technology has been widely used in wastewater clarification and potable water treatment. Basically, flotation is the process of separating solids from a body of liquid by using air bubbles.

The following short paper is focused on describing electroflotation, dissolved air flotation, and a new enhanced electroflotation process developed by the De Nora Water Technologies (DNWT).

What is Electroflotation (EF)?

Electroflotation (EF), which involves flotation by electrically generated bubbles, is the process for removal of floating ions or solid solids, suspended or dissolved in a liquid phase (ref. 1).

Traditional Electroflotation (EF) vs. Dissolved Air Flotation (DAF)

A traditional EF requires a cathode (that may be made of non-oxidizing material) and an anode (that may be made of iron or aluminum). Electrode plates or packs may be connected to an electric power source and positioned within a flotation tank under the water surface. During electrolysis, the electrode plates/packs generate small gas bubbles (e.g., O2, H2) which are then attached to the pollutants (e.g., suspended solids) in the water before they begin to float upward in a flotation tank where they can be more easily collected and removed.

Figure 1: Schematic Presentation of Electro-flotation, Electro-coagulation, and Electro-Flocculation (Photo credit: Ref. 3) 

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by the removal of suspended matter such as oils and greases or solids. The removal is achieved by dissolving air in wastewater under pressure using either an external compressed air supply or an air supply from a blower unit and then releasing the air at atmospheric pressure in a flotation tank basin. The released air forms micro-bubbles which adhere to the suspended matter in the wastewater causing the suspended matter to float to the surface of the water where it may then be removed by a mechanical skimming device.

Figure 2: Flotation Tank (Photo credit: Ref. 4)

Challenges associated with the Traditional EF and DAF Processes:

Traditional EF:

Producing the correct bubble size requires certain electrical power control over the submerged electrode plates/packs. Traditional EF process suffers the drawback of requiring continuous maintenance due to the layer of iron or aluminum hydrates and oxides that forms on the surface of the electrodes, without separating from them, so that over time the electric current decreases and therefore so does the formation of hydrates. Another drawback is that the electrodes are subjected to uneven wear, with the need to replace them often before they are completely worn. Also, maintenance on the electrode plates/packs requires either draining of the flotation tank or removal of the electrodes from the water volume to service. Since the electrode plates/packs mounted in the bottom of the tank often do not produce a 100% coverage of the tank's water surface area, allowing portions of the wastewater to ultimately bypass the flotation process and exit the treatment unit untreated. Furthermore, since the floating solids mass remains on the surface of the water column, similar to the DAF process, various mechanical skimming devices or pumps are required to effectively remove the solids from the tank.

DAF:

It is difficult to produce the most optimum fine bubble size (bubble flux) to effectively float the suspended matter without specialized, costly diffusers and high-pressure air feeds. Also, since bubble diffusers mounted in the bottom of a DAF tank cannot provide 100% coverage of the tank's surface area, allowing portions of the wastewater to ultimately bypass the DAF flotation process without being treated. Furthermore, since the floating solids mass remains on the surface of the water, mechanical skimming devices or pumps are required to effectively remove the solids from the tank. While DAF is more mechanically intensive, it is not suitable for locations such as offshore drilling platforms where equipment maintenance is likely to be neglected.

What is Enhanced Electroflotation (EEF) Process?

This patented DNWT’s treatment process involves electrolysis and an enhanced/mechanically-induced electroflotation stage for efficiently separating suspended solids and for clarifying the water. In a first step, an electrolytic cell(s) can be used to generate disinfectant (e.g., sodium hypochlorite) which helps to oxidize organic matter and create micro-bubbles (hydrogen/oxygen). These bubbles are able to lift any suspended solid matter in a batch tank water volume. In a second step, a small amount of polymer can be mixed in-situ within a mixing spool of piping in the system. The process flow through the mixing spool ensures good dispersion of the blended polymer in the wastewater stream. Polymer added coupled with micro bubbles from the first step create an electroflotation condition to facilitate solid agglomeration and flotation at the liquid surface. Below this floating layer is the substantially clear treated effluent which can be discharged to a receiving stream.

Figure 3: A “cookie”/floating layer on the top of the Treatment Tank

(Photo credit: DNWT)

Application of Enhanced Electroflotation (EEF) Process in the Offshore & Marine Sewage Treatment:

This EEF process is the heart of the OMNIPURE Series 64 G2 treatment system. The following is the summary of the test results of the OMNIPURE Series 64 G2 treatment system which successfully met IMO MEPC.227(64) discharge standards (excluding section 4.2). Note: Total Nitrogen and Total Phosphorus testing were not the scope of the testing program. The IMO and USCG type test unit was continuously operated for more than 240 hours in a time period of total 10 days to capture all operational conditions. The type tests also included performing the testing of the unit at an angle of inclination to 22.5 degrees in four (2) separate planes. 

5-day Biochemical Oxygen Demand (BOD5): >90% removal

Chemical Oxygen Demand (COD): >94% removal

Total Suspended Solids (TSS): >96% removal

Turbidity: >93% removal

Fecal coliform (FC): >99.9% removal

Effluent pH: 6.7-7.4

Effluent residual chlorine (post-dechlorination): <0.5 mg/L

References:

1.    G.B. Raju and P.R. Khangaonkar, “Electroflotation – A Critical Review”, Transactions of the Indian Institute of Metals (Feb 1984). Vol. 37, No. 1, pp 59-66.

2.    D. Casbeer, R. Bariya, S. Patel, R. Matousek, and T. Reeves, "System and Method for Treatment of Wastewater via Enhanced Electroflotation” (May 2018). De Nora Water Technologies. WO/2018/083594.

3. I. Mickova, "Advanced Electrochemical Technologies in Wastewater Treatment. Part II: Electro-Flocculation and Electro-Flotation". American Scientific Research Journal for Engineering, Technology, and Sciences (2015). Vol. 14, No. 2, pp 273-294.

4.   M.N. Adlan, "A Study of Dissolved Air Flotation Tank Design Variables and Separation Zone Performance" (1998). Ph.D. Thesis, University of Newcastle Upon Tyne.