Farm for Fish
Fish farming - touted as a solution to overfishing the oceans - does not enjoy a good ecological reputation. After all, the fish farms pollute the environment considerably with their waste water. Closed recirculation systems could offer a way out - such as the turbot farm in Büsum
Fish is an increasing part of the menu around the world. Fishing companies and breeders are working at full speed: while in 1990 around 100 million tons of fish were caught or farmed, in 2002 it was already more than 130 million tons.
And there is still no end to the lust for fish. The Food and Agriculture Organization of the United Nations (FAO) estimates that demand will increase by almost forty percent to 180 million tons by 2030. At the same time, overfishing has already occurred in almost all of the world's oceans. In the past fifty years, some fishing grounds have been considered almost empty. For example, the stock of cod in the North Sea has fallen to almost zero.
What can be done about the problem of overfishing? The magic word advertised by engineers is "aquaculture". Its oldest variant has been known for several centuries: pond breeding. Although this original type of fish farming has little impact on the environment, pond farming is hardly worthwhile from an economic point of view, since only a relatively small number of fish can grow in a pond.
Flow or Circuit
A flow-through system, in which water from an open body of water is fed through the fish farm, seems much more interesting. Lots of fish can grow up here in a very small space – but with the corresponding ecological consequences: excretions and food waste flow untreated into nature and can pollute the environment extremely heavily. After all, it is illegal, at least in the European Union, to add growth-promoting hormones to feed, and drugs to combat disease can only be administered under veterinary supervision. It is also important to prevent the caught fish from escaping into the surrounding waters, because the escaped fish often displace the native species.
"One solution could be the recirculation system," says Peter Däbritz from Linde Gas. Here water circulates again and again in a closed system and is cleaned by biofilters and enriched with oxygen. Recirculation systems save resources, are independent of the weather and offer protection against bad environmental conditions
(Peter Däbritz) The environmental impact should be minimal: The small amounts of waste water are not chemically contaminated, so that the sludge that forms from leftover feed and metabolic products can be disposed of in agriculture."Recirculation systems also offer numerous other advantages," says Däbritz. "They conserve resources, are independent of the weather and offer protection from poor environmental conditions. The fish are fed species-appropriate food and do not need any medication."
However, recirculation systems, which contain many failure-prone mechanical and electrical components and require large halls, are still quite expensive. In order to optimize the operation economically and last but not least also economically, the Kiel company Ecomares operates a pilot plant in Büsum on the North Sea coast as the first seawater fish farm in Germany with a closed water cycle.
Büsum intensive breeding
How does the system work? First, when the water is reprocessed, a sieve removes uneaten food, faeces and other solid matter from the circulating water draining from the fish tank. The screen is flushed with high pressure, the flushing water is removed from the circuit and the sludge is settled and thickened.
Apart from solids, there is another harmful substance that needs to be disposed of: ammonium. Fish excrete it as a metabolic end product, which becomes too toxic for the animals above a certain concentration. In order to free the water from this, it is fed into bio-filter basins in which small plastic bodies are floating. Bacterial strains have settled on their surfaces, which convert the toxic ammonium into nitrate. Blowers remove carbon dioxide from the bubbling water and excess bacterial growth from the floats.
Next, the water flows through a disinfection path, where UV light or ozone destroys disease-causing bacteria. Finally, the water is enriched with oxygen and returned to the fish tanks. Numerous sensors constantly check the water quality. In addition to the oxygen content, the temperature must also be constantly monitored, since warm water can only dissolve a small amount of oxygen. If the oxygen concentration drops, the fish digest poorly and need more food. The risk of illness also increases.
The breeders don't just use air here, but the gas in its purest form, which because of the higher partial pressure dissolves more easily in water than atmospheric oxygen. "The enrichment of the water with pure oxygen increases the stocking density and enables significant increases in production," explains Däbritz. "For example, an increase in oxygen saturation from ninety percent to one hundred percent results in a thirty percent increase in production."
There is a container in each recirculation system that stores liquid oxygen at a temperature of minus 183 degrees Celsius. After an evaporator has converted the liquid oxygen into the gaseous state, the gas can flow into the water in the breeding facility. "In order to be able to use the advantages of enrichment with pure oxygen in as many different ways as possible, we have developed a series of processes," explains the engineer."They can be individually tailored to the fish species, the respective breeding method and the climatic conditions."
Especially feared is a power failure, after all, this could put the entire fish stock at risk. That is why the fish technicians rely on emergency gas systems that do not require any additional energy: If the power fails, electrically operated solenoid valves open and thus ensure the oxygen supply in the tank.
The closed-loop technology seems to be paying off: the fish farmers in Büsum have been able to reduce their water requirements to such an extent that they only have to replace ten percent with fresh lake water every day. Over the course of a year, the system uses around 60 to 70 cubic meters of water per day, or an average of 3.5 cubic meters per hour – compared to conventional flow systems that require 600 times that.
The farmed fish is obviously doing well: Around 100 tons of turbot grow in Büsum every year and have been on the market since autumn 2002 - and are therefore fresh on the table every Friday.