Abstract

McKay Creek Technologies is a member of the Washington Marine Group, the largest shipbuilding, ship maintenance and repair, and marine transportation company in Canada. Inherent to this scale of operation, the Washington Marine Group generates considerable quantities of emulsified oily wastewater from cleaning vessels' bilges, fuel/slop tanks, as well as power washing of engines, machinery, cranes, trucks, and other mechanical parts.

We needed to find a way to deal with our wastewater and through investigation determined that none of the existing commercially available ways to treat this wastewater were acceptable. We then developed our own process based on electrocoagulation. Now, after three years of operating EC at Vancouver Shipyards to treat the Washington Marine Group's wastewater, we have learned how to simply and effectively treat wastewater that is contaminated by free and emulsified oils, suspended solids, and heavy metals.

Electrocoagulation uses an electrical current in an electrochemical cell to coagulate contaminants in wastewater. EC is well suited to treating wastewater with the following contaminants: 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.

Introduction

Wastewater generated by the Washington Marine Group fleet and by Vancouver Shipyards operations is widely variable, both in terms of the contaminants of concern and their concentrations, and in the volumetric rates. Wastewater sources include: bilge water, tank wash water from gas freeing operations, ballast water, and wastewater from pressure washing equipment. Both salt and fresh water is contaminated with oils and greases, diesel, hydraulic oil, and a full range of marine fuel oils as well as suspended solids.

While the large number of Shipyards wastewater generating activities leads to a corresponding large variability in wastewater volumes and quality, in general the contaminants of concern are as follows:

  • Monocyclic aromatic hydrocarbons arising from fuels in the wastewater;

  • Low molecular weight and high molecular weight polycyclic aromatic hydrocarbons, also from fuels;

  • Both low molecular weight and high molecular extractable petroleum hydrocarbons from fuels, lubricants, and de-greasing of mechanical parts; and

  • Metals (primarily iron and zinc) from corrosion of the steel barges and bilges and mechanical parts.

Oil and Water Don't Mix - or Do They?

Petroleum hydrocarbons are present in the wash water both as " free oil" (petroleum hydrocarbons that separate from the 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).

Separation of Free Oil from 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. Vancouver Shipyards WWTP uses a"Vertical Gravity Separator" coalescing plate separator manufactured by Muddy River Environmental Ltd. in Delta, BC.

For typical batches of Shipyards wastewater, the contaminant levels still remaining in the wastewater following free oil removal are generally far too high to allow discharge. Therefore further treatment is necessary.

This content is only available via PDF.
You can access this article if you purchase or spend a download.