RAS inlet water: Good results with nano-filtered seawater.

The data from the MEMBWELL (Membranes for Welfare) project, where we compared our membrane filtered water with ordinary brackish water on the welfare of post-smolt salmon, has now been fully analyzed!

In this article you get a good insight into what was the purpose of the project and how this was carried out.

Briefly summarized, the results shows:

Good fish welfare
Increased growth
Reduced sulphate content
- reduced H₂S risk
Stable good water quality

Nofima and the Department of Chemistry at NTNU participated in the completion of this experiment.

Project participants:

Trine Ytrestøyl, Lene Sveen, Aleksei Krasnov, Jascha Gerwins, May Britt Mørkedal, Gerrit Timmerhaus, Jelena Kolarevic, NOFIMA

Øyv ind Mikkelse n, Vilde Larsen, NTNU

Morten Kojen, Karen Nessler Seglem, FiiZK Akvafresh AS

The effects of nanofiltered inlet water

In MEMBWELL, fish health and fish welfare were the primary focus. FiiZK Aqua (formerly AkvaFresh) investigated the use of nanofiltration for partial desalination of inlet seawater in a RAS facility for post-smolt production. This was done under experimental conditions where post-smolt salmon in RAS received nanofiltered seawater instead of the traditional mixture of seawater and fresh water. We also included a control RAS to compare against fish that received traditionally mixed water.

In the partial desalination of seawater, a different ion composition is obtained than in ordinary brackish water, for example, in relation to sulfate content. The purpose of the experiment was to test the effect of nanofiltered intake water on salmon physiology, welfare and growth as well as water quality in the RAS system.

Hydrogen sulphide (H₂S) is a lethal gas known to result in high fish mortality if undetected. Sulphate was therefore given special focus, as the sulphate-reducing properties of membrane technology can contribute to a reduced risk of H₂S formation in the water.

The nanofiltering system used in MEMBWELL.

But what is nanofiltered water?

Nanofiltration is used for partial desalination of seawater, brackish water and salty groundwater.

A membrane is a material that lets something through and holds something else back – a selective barrier. A specific membrane can control the movement of components of a certain size or with certain chemical properties. In membrane filtration of water, pressure is used to get purified water through the membrane, while unwanted components are withheld.

Membranes can be used to select particles that are too large to pass through pores.

Membranes for filtration of water can be characterized by pore size. The pore size determines which components are held back (as shown in the figure below). The desired water quality for the product water determines which filtration process is used.

Nanofiltration removes particles, pathogens and some of the salts in water.

Membrane system for controlled inlet water

FiiZK designed a membrane system for this experiment.

Membrane filtration was used to reduce the salinity of the inlet water to a RAS – from 32 to 12‰.

As a control, a RAS was used where salinity was adjusted to 12 ‰ by mixing raw water from the sea and fresh water.

At the start of the experiment, fish with an average weight of 140g were transferred to both the control RAS and the nanofiltered RAS. After 12 weeks, both RAS systems were switched to sea water flow through to simulate sea transfer without handling.

Setup of experiments with nanofiltration of inlet water (blue RAS) and control with mixing of seawater and fresh water (orange RAS).

Overview of tests and measurements performed

Sampling was carried out at (a) the start of the experiment, (b) transferring to seawater, and (c) at the end of the experiment (as shown in the diagram below).

Histology and measurements of gene expression:

Gills
intestine
skin

Blood tests:

Cortisol
Blood Plasma ions

Throughout the experiment, fish welfare and growth were assessed.

Diagram with timeline and sampling plan for the MEMBWELL project

So how did nanofiltered water affect the fish themselves?

Post smolt fish exposed to the nanofiltered water had higher average weights (400g) than the control fish that were exposed to mixed water (355g).

Sample weights gathered at the end of the sea water transfer phase showed that average weights remained significantly higher in the fish exposed to nanofiltered water (556g) than the control fish (491g) in the RAS.

Average weight of fish in nanofiltered water (blue) and control water (orange) in the MEMBELL experiment.

There were no differences between the treatments in operational welfare indicators.
No differences in the salmon’s skin were observed in the two production systems.
There was no difference in the ion concentration in the blood of the salmon in the two systems.
In both RAS systems, some light gill irritation was observed, and this was reversed in both cases after transfer to seawater.

Analyses at NTNU

The goal of analysis at NTNU has been to study the effect of nanofiltration to remove sulfur. In addition, it was to document the effects of nanofiltration on water composition with regards to pH, salinity (conductivity), and the changes in selected ions. NTNU has been responsible for various analyses in the project, including (a) pH (b) analysis and review of sulphate and total sulfur concentrations at different locations in the test systems (c) analysis of levels of selected main ions and trace elements (d) levels of organic matter.

Sulphate and sulfur

Analysis of water samples from the two systems indicates that the nanofiltration system is an effective method of removing sulphate and various sulfur compounds. In the tests conducted, up to a 93% reduction in total sulfur was observed. When comparing nanofiltered water to the control water, nanofiltered water had on average 91% less sulfur and 86% less sulphate. Thus, the results indicate a marked reduction in the amount of sulfur – important for reducing the risk of H2S formation.

pH and other selected factors

pH showed no significant difference between the nanofiltered water and the control system. Other selected elements analyzed from the inlet water were magnesium, silicon, potassium, calcium and bromide.

When comparing nanofiltered water with the control system, there was a slight increase in bromide. At the same time, a decrease in levels of magnesium, silicon and calcium was observed. Reductions in concentration levels were also observed for several trace elements.

In relation to organic matter, no significant difference in levels was observed between nanofiltered water and the control. However, in both systems, an increase was observed over time in the facilities. This may indicate a possible accumulation of organic matter in the systems as fish grow larger. However, this observation is not related to the nanofiltration system.

Summary of the MEMBWELL project

In summary, MEMBWELL has demonstrated that nanofiltration can be used to produce a water quality with reduced sulphate content and desired salinity, and that this water quality mainly has a positive effect on growth and fish health.