Benny Carlsson was an engineer onboard a Stena Line ferry in 1990 when he began to think about the problem of dealing with oily water discharge from the vessel's bilge tank. IMO rules state that discharges of water from vessels at sea must contain less than 15 ppm of oil. Achieving this level is very difficult largely because of emulsion formation in the bilge tanks. Responsible operators such as Stena and others are forced to bring the contents of bilge tanks onshore and send it out for treatment, with obvious cost implications. However, there are others who for whatever reasons do not do so and discharge excessively oily water into the oceans.
Carlsson and another Stena engineer, Peter Lanzen, began work in 1991 to develop an onboard system to get oil levels down consistently below the limit and so allow the company to increase discharges at sea. Initially spending only just over US$11,000, they came up with a floculation-based process to overcome the problem. Prototype treatment units were built and fitted on board two Stena vessels, the Stena Danica and Stena Jutlandica, which were successful in producing water from the bilge tanks with oil contents as low as 0-1 ppm. Stena's bilge water treatment bills for these two vessels fell from US$80,000 per year to US$23,000.
The level of interest from other shipping operators led Carlsson and Lanzen in 1994 to negotiate with Stena to set up an independent company to market the new product. Called Marinfloc, the small firm has now supplied around 120 of its process units over the past five years, costing about $50,000 each. The units have been tested and approved by all the leading authorities, including Lloyds, DNV, and the US Department of Transportation.
The units employ established science and technology to reduce the oil levels in the discharge water. Flocculation is widely used for both purifying drinking water and treating sewage. The key to the Marinfloc process, according to Lanzen, is getting the process temperatures and the residence times right and engineering the units to work reliably under rough sea conditions.
Sub-3 ppm discharge
The unit takes contaminated water from the base of the bilge tank into a first vessel where steam coils are used to heat it to 50C. This promotes physical separation and enables a fraction of the oil to be fed straight back into the bilge tank. The stream is then dosed with various flocculants, the most effective of which over a wide pH range is polyaluminum chloride, and is then transferred to settling tanks.
After a set period, the flocs are removed from the base of the tank - they are transferred back to the vessel's sludge tank - and the water is then pumped through three separate filters to remove the last traces of floc. These filters are mainly needed because the movement of the vessel makes it difficult to remove the flocs formed purely by settling. The water emerging from the unit typically contains less than 3 ppm oil. Oil levels in the discharge are constantly monitored and, if necessary, the water can be returned through the process a second time.
As part of the same package, Marinfloc has developed a separate sludge tank dewatering unit that is capable of reducing the water in the sludge tank by up to 85%, which provides savings on sludge handling costs.
The company is now established sufficiently to be examining other applications. It is developing equipment to deal with so-called gray and black wastewater produced as a result of everyday life on board ship. It is also turning its attention to a different industry, as Lanzen explains:
"All our effort has gone into the shipping side so far. But we are aware that the offshore industry has all kinds of similar problems to handle; the removal of oil from drilling fluids is probably the best example. We feel that there could be a number of areas where we could contribute. The challenge is going to be to modify the process to handle larger volumes and throughputs.
"Currently, the units we supply are capable of treating about 30,000 liters of water per day on a batch basis. We recognize we will need to have a continuous process to meet the needs of the offshore market and are working on one or two options at the moment."
For more information contact Peter Lanze, Marinfloc. Tel: +46 304 10498, Fax: +46 304 10051, E-mail: peter@ marinfloc.com