Environmental Science & Engineering - www.esemag.com - January 2003
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New electrocoagulation process treats emulsified oily wastewater at Vancouver Shipyards

Figure 1: Process flow diagram of the electrocoagulation wastewater treatment plant at Vancouver Shipyards.

The Washington Marine Group is the largest shipbuilding, ship maintenance and repair, and marine transportation company in Canada. It generates large quantities of emulsified oily wastewater from cleaning vessels’ bilges, fuel/slop tanks, and power washing of engines, machinery, cranes, trucks, and other mechanical parts. Both salt and fresh water are contaminated with oil and grease, diesel and a full range of marine fuel oils, as well as suspended solids and metals (primarily iron and zinc) from corrosion of the steel barges and bilges and mechanical parts.

Petroleum hydrocarbons are present in wastewater both as “free oil” (petroleum hydrocarbons that separate from wastewater and float to the liquid surface) and as “emulsified oil” (petroleum hydrocarbons that remain in stable suspension and do not separate from the wastewater).

Many systems are commercially available to remove free oil. Typically configured as coalescing plates, rope skimmers and the like, these systems exploit the characteristic of oils to stick to solid surfaces, concentrate there, and later slough off, and thus separate from the bulk liquid. For typical batches of Vancouver Shipyards wastewater, the contaminant levels still remaining in the wastewater following free oil removal are generally far too high to allow discharge, so further treatment is necessary.

How emulsified oils are in any particular batch of wastewater depends upon the type of oil, the amount and type of cleaning solution used, as well as the extent of mechanical agitation, primarily from power washing and pumping.

Alternative technologies to treat emulsified oily wastewaters Commercially available technologies to treat emulsified oily bilge water that were considered for use at Vancouver Shipyards included: absorbents, chemical coagulants and flocculants, filtration, air flotation, hydrocyclones, membranes, microbes, and oxidation. Each was evaluated according to its complexity of operations, and capital and operating costs. No off-the-shelf system met Vancouver Shipyards’ requirements for simple, effective, robust, reliable, and low cost treatment.

In response to this need, McKay Creek Technologies Ltd. was formed to develop an electrocoagulation process to treat the oily wastewater generated by the Washington Marine Group fleet and shipyards. Now US patented (international patents pending), this technology has been serving Vancouver Shipyards for over three years, and has treated many millions of litres of wastewater.

Electrocoagulation (EC) directly addresses the three main factors that lead to a stable suspension of suspended solids and emulsified oils: ionic charge, droplet or particle size, and droplet or particle density. Electrocoagulation uses an electrical current to dissolve a sacrificial anode and thereby introduces aluminum ions with a positive charge into the wastewater stream. These ions are attracted to the very fine negatively charged droplets and particles of the contaminants. The resulting agglomerations increase in size until they are no longer stable in suspension. Simultaneously, gases formed by hydrolysis form very fine bubbles that associate with the coagulated contaminants and buoy them up for removal by flotation. Fortunately, EC’s pollutant removal mechanisms are not exclusive but rather work together in an operationally simple system.

Table 1: Removal of petroleum hydrocarbons and related contaminants from oily wastewater.

History of EC Electrocoagulation has been an “emerging technology” since 1906 when the first US patent on EC was awarded. At that time, there were insufficient financial or regulatory incentives for industry to adopt the process.

This, of course, is changing. Now a greater awareness of pollution and regulatory requirements has industry seeking low-cost and robust technical solutions to solve their wastewater discharge problems. Sporadically trumpeted as a wastewater treatment panacea, EC’s fundamentals remain poorly understood. Developing EC into a robust industrial process has involved resolving the key issues of electrochemical cell design, electrode fouling, power supply, operating conditions, and providing the most suitable unit operations to support the process.

EC is a good fit to treat wastewaters contaminated with emulsified oils, PAHs, poorly settling solids, poorly soluble organics, contaminants in general that add turbidity to water, as well as negatively charged metal species such as arsenic, molybdenum and phosphate that form co-precipitates with iron or aluminum. Heavy metals and soluble organic compounds are removed from wastewater by EC in association with the removal of emulsified and particulate solids.

At Vancouver Shipyards, wastewater from a wide array of activities is treated on a continuous basis using electrocoagulation. Figure 1 illustrates the process. The McKay Creek process at Vancouver Shipyards treatment plant can treat 6,000 litres of wastewater per hour. This treatment capability was sized to meet the anticipated requirements of Vancouver Shipyards operations. It is not a limitation that is inherent to EC.

Table 2: Chemical stability of solid residuals.

Treatment plant performance Over the past three years, Vancouver Shipyards has treated all of the industrial wastewater generated by the Washington Marine Group fleet and discharged treated water to easily meet environmental requirements for petroleum hydrocarbons, suspended solids, PAHs, BETX, and heavy metals. Table 1 summarizes this performance.

Since EC separates rather than destroys wastewater contaminants, the nature of the resulting solid residuals is an important consideration. Table 2 summarizes the characteristics of the wastewater treatment solid residuals from routine operations at Vancouver Shipyards, de-watered by means of a filter press. Table 2 shows that the contaminant concentration of the solids is very high, confirming that little is added to the wastewater in order to treat it. For example, on a dry weight basis, the concentration of oil and grease was 716,000 µg/g or 71.6%. It looks and feels like axle grease. Of course, for a given wastewater contaminant load, the higher the concentration of the solid residuals, the smaller the volume of residuals that require disposal. Table 2 confirms that the solid residuals are very stable chemically. This non-leachability minimizes the risk of re-contamination of the environment by these wastes.

What EC won’t do The EC process, while extraordinarily effective to remove a wide range of contaminants, is no wastewater treatment panacea. For example, soluble BOD resulting from contaminants such as antifreeze or solvents or sewage is not directly removed by EC. These contaminants may be removed to some degree in association with the removal of TSS or petroleum hydrocarbons.

The costs to build and operate the Vancouver Shipyards wastewater treatment plant were lower than the other technologies considered. In declining order, the factors that drive EC treatment costs are as follows: labour, electrode replacement, electrical power, amortization of capital, disposal of solids residuals, and chemicals.

The requirements for anode replacement depend upon the nature and the concentration of contaminants to be removed. At this point in our understanding of electrocoagulation, the requirements for aluminum are determined through treatability testing, preferably under continuous operations.

Electrical power requirements depend on the concentration of aluminum needed to treat a particular wastewater, on the configuration of the electrochemical cell, and on the conductivity of the wastewater. When needed, road salt can be added to boost conductivity.

Summary Resolving the technical details to make EC industrially robust, mechanically simple, and cost competitive to treat a wide range of industrial wastewaters has been a formidable challenge. Chiefly, these technical details include: electrode design, electrode fouling, operating conditions, and separation of coagulated contaminants following electrocoagulation.

EC can be particularly useful where:

Wastewater contaminants, alone or in combination, of elevated concentrations are difficult to remove by commonly available treatment methods;
There is significant benefit to providing high water quality for reuse or discharge;
The high cost of treatment residuals disposal strongly encourages minimizing their volume; or
Limited available space requires a high capacity and low footprint solution.

The devil in electrocoagulation is truly in the details.

Rob Stephenson and Bruce Tennant are with McKay Creek Technologies Ltd. Don Hartle is with Vancouver Shipyards Co. Ltd.

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