Watermark > Spring 2003 > Feature: Vancouver Shipyards treats emulsified oily wastewater using electrocoagulation

Features

Vancouver Shipyards treats emulsified oily wastewater using electrocoagulation

By Rob Stephenson, Bruce Tennant, McKay Creek Technologies Ltd.; Don Hartle and George Geatros, Vancouver Shipyards Co. Ltd.

Vancouver Shipyards

At Vancouver Shipyards, managing the oily bilge water, fuel/slop tanks, tank wash water from gas freeing operations, ballast water, and wastewater from pressure washing machinery was becoming a big and expensive problem. Both salt and fresh wastewaters are contaminated with emulsified oil and grease, diesel, hydraulic oil, and a full range of marine fuel oils, plus heavy metals and suspended solids.

The commercially available technologies to treat these wastewaters were evaluated. It was concluded that no system had even a remote likelihood of reliably meeting effluent discharge requirements for the wide range of Shipyards’ wastewaters at reasonable cost.

Subsequently, through good fortune and even better timing, we stumbled across an old technology called “electrocoagulation” or EC. We could see that, hidden behind innumerable false starts and misinformation about the process, it actually worked! Of course, there are many steps between eureka and an operating wastewater treatment plant. In brief, two years after our rediscovery of EC, the plant was built at Vancouver Shipyards. It has now been operating for four years. This article describes how.

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How electrocoagulation works

Electrocoagulation uses an electrical current to dissolve a sacrificial anode and thereby introduce chemically reactive aluminum ions into the wastewater stream. These positively charged aluminum ions are attracted to the very fine negatively charged droplets and particles of the contaminants in suspension. The resulting agglomerations of droplets or particulate solids 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 to the surface of the treated wastewater for removal by flotation.

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.

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EC treatment plant conditions

Each batch of wastewater is treated by the WWTP according to the following eight processing steps:

From vacuum trucks, tanker trucks, or from storage tanks, wastewater is pumped to the receiving tank of the plant.
Free oil (that which readily separates by gravity) is removed from the feed tanks by a coalescing plate filter (Muddy River Environmental, Delta, BC). This oil is then sent out for rerefining as BC Special Waste.
The emulsified oily wastewater, now with most of the free oil removed, is pumped to feed tanks.
The emulsified oily wastewater is then pumped through the electrocoagulation cell at a flow rate of 70 to 100 litres per minute, depending on the contaminant load.
An anionic polymer is pumped into the EC treated wastewater just prior to flowing through an in-line mixer and then to a flotation cell.
A flotation cell splits the treated wastewater stream into two portions: (1) a treated water underflow that is largely free of suspended solids, and (2) a separated solids overflow that contains a concentrated slurry of coagulated contaminants.
The coagulated contaminants in the separated solids stream are de-watered by means of a plate and frame pressure filter and are disposed of in 45-gallon drums as BC special waste. The liquid filtrate from the pressure filter flows to the sump for return to the feed tank to be treated once again.
The treated water is discharged through activated carbon filters to the GVS&DD sanitary sewer. Spent activated carbon is managed as BC special waste.

Ec Treatment plant performance

For almost four years, Vancouver Shipyards has discharged treated water to meet GVS&DD requirements for petroleum hydrocarbons, suspended solids, PAHs, BETX, and heavy metals. The treated discharge is consistently clear and colorless.

Characteristics of treated wastewater treatment solid residuals

Electrocoagulation does not destroy most contaminants, but rather separates them from the bulk volume of the wastewater. The solid residuals look and feel like axle grease and are managed as BC Special Waste.

WWTP limitations and implications for acceptance of wastewater

The WWTP at Vancouver Shipyards, while extraordinarily effective at removing a wide range of contaminants, does not remove soluble BOD. Therefore, waste streams that contain contaminants such as antifreeze or solvents or sewage are not accepted for treatment at Vancouver Shipyards WWTP.

Summary

EC is a good fit to treat wastewaters contaminated with emulsified oils, PAHs, poorly settling solids, poorly soluble organics, contaminants 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 extraordinarily effective removal of emulsified or particulate solids. The multiple mechanisms of EC fortuitously work together to simply and effectively remove a wide range of contaminants from a broad spectrum of industrial wastewaters, not just oily wastewater.

Electrocoagulation can be particularly useful where:

Wastewater contaminants, alone or in combination, of any concentration are difficult to remove by commonly available physical, chemical, or biological 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.