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Use of Revolving Algal Biofilm in Recirculating Aquaculture Systems for Production of Salmonids

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Recirculating aquaculture systems (RAS) have developed significantly in the last 20 years. With stagnant production volumes from ocean net pens and increasing salmon consumption growth of RAS is needed.

One of the largest hurdles that remains in build-out of RAS is the high load of nitrate and phosphorus heavy wastewater. In the United States most locations are moving toward stricter discharge limits. Common standards are 10 mg/L concentration of nitrates, and 1 mg/L of phosphorus. 

Costs to be able to meet these permits will continue to be challenging for farmers. The added operating and capital costs can be the major factor limiting profitability in some locations. 

Kimle Aquaculture is developing algae-based recirculating aquaculture systems. Algae can directly treat and capture nitrogen, phosphorus, and carbon dioxide in one step. Eliminating the need to discharge wastewater to surface waters.

For algal cultivation Kimle Aquaculture uses a revolutionary technology called “revolving algal biofilm” (RAB). The RAB grows algae on vertically oriented conveyor belts, that rotates through a nutrient solution. This provides the ability to grow significant volumes of algae in a small footprint. In addition, algae can be simply scrapped off the belts for easy harvest.

Figure 1: Commercial size revolving algal biofilm system

Trial 1:

In January 2019 Kimle Aquaculture designed and setup a production system with a pilot scale RAB reactor for primary nutrient filtration. The production system was stocked with 60 rainbow trout and were fed at 1% of bodyweight. These fish were harvested and smoked at 1.5lbs average. 

This system was run with essentially no wastewater disposal. The only loss of water was due to evaporation and draining of sludge through the clarifier.

Equipment Setup:

1.     125 gallon circular tank with a hydraulic retention time of 34 minutes. Single drain through bottom.

2.     Clarifier for settler of large solid waste

3.     10 m2 of revolving algal biofilm

4.     Pump Sump

5.     50 micron bag filter, changed twice a day

6.     Supplemental pure oxygen added directly to tank

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Results and Observations

Algae was the primary treatment of all dissolved wastes in this system. While operating these systems there was a number of key lessons learned with operating these systems. Although through the study it was shown to be a stable simple to be run. 

·      The algae belts were harvested every 5-7 days. The goal was always to keep total biomass heavy on the belts, to provide the maximum amount of treatment. If the algae got too thick, it would slough off. Harvest frequency would be increased.

·      Alkalinity is important to algae growth and treatment. While algae has an advantage of consuming less than half the alkalinity of nitrifying bacteria, growth is much better when in range of 150-200mg/L alkalinity.

 

·      Low Hydraulic Retention time can significantly hinder treatment. The pilot scale algae reactors did not have correct ratio of raceway volume to belt area. This sometimes had skewed treatment values.

 

This study proved the efficacy of using revolving algal biofilm as primary treatment in RAS. The system was able to treat: ammonia, nitrates, phosphorus, CO2 and add oxygen. Therefore providing a completely symbiotic system for the fish produced.

Key takeaways:

·      Ammonia was kept well below 1mg/L. Results below show ammonia concentration as loading per m2 of belt area was increased.

·      Nitrite was generally between .5 and 1mg/L. The system was relatively small causing high levels of passive nitrification that had effected the nitrite levels. 

·      CO2 was kept between 8 and 10mg/L with no degassing. Algae provided most of this treatment value.

·      As water passed through the algal reactor, 1- 2mg/L of dissolved oxygen was added.

·      Nitrates ranged from 8 to 12mg/L. Even as we increased feeding rates increased, nitrates did not build up in the system.

·      Phosphorus was tested a few times and it was below 1mg/L

Figure 1: Commercial size revolving algal biofilm system

Next Steps

Kimle Aquaculture is currently designing a facility that will produce 10-15 metric tons of fish per year. This facility will demonstrate capability of algae-based RAS at commercial scale. Algae based systems can simplify design, lower economically competitive size, eliminate need to discharge, and provide farmers valuable algae by-product revenue.