Livscyklus-Screening af renseteknologier indenfor fiskeindustrien

Summary

Introduction of cleaner technologies in fishing industries does not normally give rise to questioning the obtained environmental advances. In general, efforts with various, in fact rather simple, technologies have brought about considerable environmental advances in the form of reduced water consumption and reduced pollutant concentrations in process water.

On several occasions, however, accomplishment of overall assessments in the form of Life Cycle Assessments (LCAs) has proved highly relevant in connection with implementation of more advanced (purification) measures and, accordingly, more complicated technologies in order to obtain reduced content of pollutants in process water. More often than not, nevertheless, such measures will derive certain unwanted resource and environmental impacts, e.g. increased consumption of energy, auxiliary materials etc.

Accomplishment of pronounced LCAs is very often a rather complicated matter, as to time as well as to resource consumption. Therefore, this project has its focus on the simplified type of LCAs, below referred to as LCA screenings.

The aims of the project are,

• outlining a method/procedure for LCA screenings concerning purification technologies for process water from fish processing industries.

• accomplishing LCA screenings of thermal flotation and membrane filtration as a combined process water solution at the Skagerak Fiskeeksport in preparation of comparing actual resource and environmental impacts to the reduced wastewater impacts.

• Comparison of solutions and combinations hereof.

The project is based on the ISO 14040 principles and builds on experience gained from a NIRAS research project on life cycle based environmental assessments of various applications of organic waste products. The UMIP environmental assessment method with PC tools to match, designed by the Danish Instituttet for Produktudvikling (Institute for Product Development), has been applied.

Among others, the project has shown that, in certain circumstances, the environmental advances of more extensive measures in fact become neutralised by increased resource consumption and environmental impacts due to the implemented measures.

The selected object for the study is process water conditions at the Skagerak Fiskeeksport, where various types of cleaner technology are already an integrated part of production processes. On implementation of new, advanced measures, the Skagerak Fiskeeksport has made a point of exploiting substances separated from process water for feed purpose. This point has formed the background of our assessment of thermal flotation and membrane filtration methods. The chosen methods and the relevant alternatives have been tested by means of LCA screenings.

The project is based on exploitation of the following technologies,

• thermal flotation
• chemical flotation
• membrane filtration in the form of nano filtration
• treatment at municipal biological wastewater treatment plant with advanced treatment and nutrient removal

Specific technologies

On assessment of the specific technologies exploited, we have focused our attention on extent of environmental impacts and resource consumption in connection with each individual treatment technology, compared to the actually obtained environmental advances.

• Our conclusion is that the environmental effect of thermal flotation is highly dependent on the circumstances:

If heating of process water is based on existing oil heating and the sludge is either used in bio gas plants or as feed (and replaces "fish cut off"), the increased environmental impact and resource consumption will in fact exceed the environmental advances obtained by the thermal flotation, which indeed is a very significant result.

If on the other hand heating of process water is based on natural gas with a higher efficiency than oil heating and the sludge is used for feed purposes (and replaces "whole fish"), the environmental advances obtained by thermal flotation will exceed the disadvantages of increased environmental impact and resource consumption.

It deserves notice that as far as the Skagerak Fiskeeksport is concerned, thermal flotation is not considered suitable as an isolated solution. Under the circumstances, the thermal flotation must be regarded as a pre-treatment process preceding the membrane filtration.

• Chemical flotation of process water with subsequent exploitation of the sludge in a bio gas plant means - apart from reduced discharge into receiving waters - considerable reductions regarding emission and waste, and, moreover, rather reduced resource consumption, which is due to the fact that by bio gasification sludge substitutes combustion of fossil fuels. Another important factor regarding the actual reductions obtained by chemical flotation is to what extent production of chlorine (which is used in ferric chloride production) is connected to emission of mercury.

• Provided that thermal flotation of process water is previously accomplished, we assess that the environmental advances of nano filtration during the treatment process exceed the environmental load and the increased resource consumption attributable to establishment and operation of the plant. However, on estimate at least 50%, perhaps even up to 80%, of the environmental advances will be lost due to increased loads attributable to operation of the nano filtration plant.

• Provided that chemical flotation of process water is previously accomplished, we assess regarding biological treatment procedures that the environmental advances originating from discharge into receiving waters will significantly exceed the environmental load and the increased resource consumption attributable to establishment and operation of the biological treatment plant.

Combined solutions

As regards LCA-screening of the combined solutions (i.e. thermal flotation combined with nano filtration and chemical flotation combined with biological treatment respectively), the project results are as follows:

Two scenarios were put forward as regards thermal flotation in combination with nano filtration,

1) heating by means of existing oil burner, and
2) heating by means of highly efficient natural gas.

In either case, the prior assumption was that the sludge was to be used as feed, substituting fish cut off or whole fish.

Assessment of Scenario 1: By oil heating as implied, environmental advances of thermal flotation combined with nano filtration cannot counterbalance the disadvantages of the increased environmental impacts and the increased resource consumption. This result has proved valid no matter whether the residuals as feed are replacing cut off from herrings or whole fish.

The negative effect will be even more significant, if the nodium hydroxide used for cleaning of the plant is produced by means of mercury cells.

The test results of Scenario 2 came out significantly better than those of Scenario 1, because the environmental advances significantly exceeded the increased environmental impacts and resource consumption. The disadvantages of these impacts, however, will still make heavy inroads into the obtained environmental advances, in particular if the sodium hydroxide production is based on mercury cells. The absolute best results are obtained where whole fish is considered substituted.

On discharge into receiving waters more vulnerable than the actual one, the environmental advances of reduced nutritive salt load can be weighted higher, which in due course will improve the "environmental accounts".

As to chemical flotation combined with biological treatment, the environmental advances and - under certain circumstances the resource profits - will be significantly more weighty than the increase in resource consumption and environmental impact.

Comparing the process of thermal flotation combined with nano filtration with the process of chemical flotation combined with biological treatment we assess that from a comprehensive point of view - resource consumption and the environment - chemical flotation in combination with biological treatment makes up the best solution.

Choosing thermal flotation combined with nano filtration, however, opens up the prospect of future development, refinement and more advanced exploitation of valuable substances from residuals.