Water treatment technology

Flow chart of the primary water circulation (blue) and the secondary water circulation (grey)
Flow chart of the oceanloop

Solids separation

The water quality, particularly the concentration of suspended solids, is a decisive factor to produce flavorful fish. In conventional systems fish feces, food remains and bacteria pollute the water, so that it has to be changed continually. In contrast the oceanloop technology enables separation of all suspended solids. This is achieved by two different solid separation processes.

 

Studies have shown, that there is no technology, which removes the entire size spectrum of suspended solids efficiently and completely as possible. Therefore the oceanloop combines mechanical separation (drum filter) and boundary surface filtration (skimmer).

 

The mechanical separation primarily removes coarse solids from the water. The size of separated solids depends on the mesh size of the filter panels. The filter panel is rinsed from time to time, whereby the solids are collected in the rinsing water. Generally drum filters with a mesh size of 50-100µm are used in the oceanloop.

 

Fine suspended solids (<50µm) are removed by a skimming process (foam fractionation). In a skimmer extremely fine air bubbles are mixed into the water. The boundary surface between air bubble and water layer acts as filter material for surface active substances, such as protein compounds. The fine suspended solids dock at these compounds and are transported towards the water surface together with the air bubbles. At the water surface they form so-called waste foam which is rinsed by a patented self-cleaning system.

In addition, the process of skimming increases the oxygen concentration in the water and carbon dioxide is stripped effectively.

Nitrifying biofilter

Fish excretes a number of metabolic end products, especially ammonia nitrogen (which is balanced with ammonium nitrogen). It is the end product of protein metabolism. Ammonia can be toxic in very low concentration and must therefore be removed continuously from the water. The most widespread method for removing ammonia is biological nitrification, where bacteria oxidize ammonia via nitrite to non-toxic nitrate.

 

In oceanloop floating plastic media are used, which provide a maximum colonization surface area for the bacteria. This filter type achieves very high filter efficiency per unit of volume, while at the same time offering process stability. In addition the biofilter oxidizes dissolved and particulate organic substances, which are not removed by the solid separation. This process is called mineralization. This process impairs the nitrification capacity of the biofilter. Therefore, an effective solid separation is essential.

Denitrifying biofilter

Nitrate is formed during the nitrification of ammonia. This leads to an increase of nitrate in the process water. Although nitrate is not acute toxic to fish, even in high concentrations, it has to be removed, especially in recirculating aquaculture systems. At higher concentrations nitrate impairs the osmoregulation of fish.

 

Nitrate is removed in an anaerobic denitrification process in which heterotrophic bacteria convert nitrate into gaseous nitrogen. In contrast to the nitrifying biofilter the denitrification operates in a bypass of the total water flow and without aeration. Furthermore, the denitrifying bacteria require a carbon source which is dosed in liquid form (e.g. methanol or acetic acid). 

 

The process of denitrification in the oceanloop is an in-house development of neomar.

Carbon dioxide removal

The respiration of fish, as well as heterotrophic bacteria, increases the carbon dioxide concentration in the water. Excessive concentrations lead to an acidification of fish blood, due to the fact that carbon dioxide cannot be released efficiently from the gills. This impairs the oxygen uptake of the fish. Thus, the fish becomes inactive and stops feed intake. Ineffective feeding negatively influences the fish health, as well as the productivity of the system.

 

The constant removal of carbon dioxide is therefore an important unit of the water treatment. In the oceanloop carbon dioxide is essentially removed by a trickling filter. The water is pumped flat over a solid substrate in such a way, that the boundary surface between the water and the air is as large as possible. The carbon dioxide diffuses following the concentration gradient out of the water into the air. This physical process is supported by strong ventilation. The exhaust air is transported directly outside.

 

In addition to the trickling also other units of the water treatment remove carbon dioxide. In general, this pertains to all units which are aerated, as the skimmer or the insertion of liquid oxygen. For this reason, the trickling filter works with a relatively low head. This reduces the energy demand of the pumps used. 

Disinfection

The control of pathogens is of great importance for fish health in recirculating aquaculture systems. However, in order to maintain a biological vital system, the goal is not a complete sterilization. In fact, pathogens have to be controlled at a tolerable level for the fish.

 

In the oceanloop ozone is used, due to its broad spectrum of antiseptic activity. By an accurate dosage of low amounts of ozone into the skimmer many pathogens can be inactivated.  At the same time, the activity of the bacterial community in the biofilter is preserved. Our experiences during operation have even shown, that the biofilter profits from the use of ozone. Over time biologically non-degradable compounds cumulate in recirculating aquaculture systems. These compounds form the so-called chemical oxygen demand (COD). Ozone pre-oxidizes these compounds in a manner, that they can be further processed by the bacteria in the biofilter. Therefore the use of ozone results in a slight increase of the biological oxygen demand (BOD), which is finally reduced in the biofilter.

 

The biologically non-degradable compounds include humic and fulvic acids, which cause a yellow turbidity. Ozone oxidizes these compounds to ensure high water clarity.

 

In addition, ozone supports a clustering of tiny colloidal solids. This process is called flocculation. This improves the efficiency of the solid separation process, as mechanical filtration and foam fractionation (skimming). Furthermore, ozone denaturates proteins. This supports the process of skimming.

 

In addition, ozone oxidizes compounds as the fish toxic nitrite. Therefore, it also contributes to the alleviation of sudden nitrite peaks, which can occur during an incomplete nitrification or denitrification.

Water conditioning

The physical/chemical conditioning of process water is operated by different systems. These are automatically managed by a programmable logic controller (PLC). This includes the control of oxygen concentration, pH value, salinity and temperature.

 

Oxygen supply

The oceanloop is operated with liquid oxygen. It is directly and demand-oriented dosed into the fish basins. Optical sensors continually measure the oxygen concentration. If the concentration falls below the desired value, proportional valves are opened, corresponding to the difference between the set-point and measured value. There are several methods to insert oxygen into the water. The primary decision criteria for the choice of method are the efficiency of insertion, the energy demand, the maintenance effort and a possible power-off insertion of oxygen. The methods available on the market fulfill these criteria only in parts. For this reason, we have developed an innovative method to fulfill all of these criteria.

 

The demand-oriented insertion of liquid oxygen allows an accurate setting of oxygen concentration. In addition, oxygen is also inserted to the system by the aeration of biofilter as well as skimming. The inserted atmospheric oxygen reduces the amount of liquid oxygen, which has been used to maintain the desired oxygen concentration.

 

pH regulation

It has already been mentioned, that the respiration of fish, as well as heterotrophic bacteria, leads to an increase of carbon dioxide concentration in the water. Thus, the pH value sinks. The removal of carbon dioxide is therefore a crucial component of pH regulation. In addition, buffering systems are required to maintain the pH value in a tolerant level for the fish, as well as the bacteria in the biofilters. The reason is primarily the release of protons during the process of biological nitrification, which also reduces the pH value. Even if this reaction is partially neutralized by the process of biological denitrification, a permanent sinking of the pH value arises in recirculating aquaculture systems. In the oceanloop lime milk is used to prevent this pH drop. Beside the neutralization of protons, the lime also reacts with the phosphate. Therefore, the use of lime milk is also important to set a tolerable phosphate concentration in the system.

 

The pH value is constantly monitored at various measuring points. The dosage of lime milk operates automatically, depending on the pH values measured.

 

Adjusting salinity

The technology of the oceanloop has been optimized in order to reduce water losses as much as possible. As a result no water is exchanged in the systems. Only water losses during the process of solid separation have to be replaced with artificial seawater. The water renewal rate is currently below 1% of the system volume per day.

 

Generally, the artificial seawater is produced from tap water and a mixture of various salts. The composition and the concentration of seawater are adjusted individually, depending on the water quality on-site and the species produced.

 

The mixing of sea salt and water is done in a special tank, supported by an intensive aeration. After that the brine is diluted with additional water and pumped to a storage tank. From there, the seawater can be used automatically or manually.

 

Temperature control

A significant advantage of recirculating aquaculture systems is the fish production at year-round constant and optimal temperatures. This allows considerably shortened production cycles and therefore increases the productivity of the system. To adjust the optimal temperature the supply of heat or cold is required, depending on the location, the farmed fish species, the type of building and the season. As a rule, the supply of cold is related to higher operating costs. In contrast, low-price waste heat is available in many locations. In the scope of system planning the climate management is of crucial importance.