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Aquaculture-fish breeding in a closed water system with simultaneous biofiltration


by Adam Onken

Only a few years ago, the development of aquaculture seemed to be the best method so far devised of rapidly eliminating the international problem of malnutrition. But meanwhile the euphoria has dissipated, and has been replaced by a more sober assessment of the potential amounts of protein obtainable by breeding water organisms under controlled conditions.

As far as the developing countries are concerned, it can be stated that in those regions which had already an existing aquaculture tradition e.g., Indonesia, The Philippines or Birma, its value has been recognized reinvigorated and in some cases even substantially promoted over the last few years. On the other hand, hardly any progress can be observed in those countries which do not have such an aquaculture tradition.

In industrialized countries, and in particular in the Federal Republic of Germany the new term aquaculture gained some recognition as a new scientific discipline and was able to attract considerable funds. These, however, were channelled primarily to those projects aiming at highly intensive, mostly semi-industrial fish breeding.

However, the results were modest, due mostly to hygienic problems, and failed to come up to expectations by far. Furthermore production was primarily concentrated on species which must be regarded as luxury foods and can only be produced at a very low level of conversion efficiency like trout, eels, shrimps, etc.

During the last few years, very effective biological methods of sewage water treatment have been developed, which with appropriate modifications and a more ecological approach might also be applicable for fish waste water treatment. Aquaculture in a closed water cycle, though difficult and ambitious, is worth further efforts since one of its most attractive feature is that it renders fish breeding independent of an abundant water supply.

It is this aspect of decentralization which is a very attractive and rewarding feature of the closed aquaculture system since fish can be produced more closely to consumer demand and even smaller and much less intensive units may guarantee a sufficient supply.

Aquacultural/Agricultural Methods

Both in the USA and in the Federal Republic there has been no shortage of attempts to design fish breeding units with water recycling on ecological lines. These efforts were based on the many different forms of integrated agricultural/aquacultural systems in Asia, which represent a symbiosis of plant and animal production. The end-products of metabolism secreted by fish and other animals serve as nutrients for plants and are thus removed productively from the water.

Aquaculture systems which are based on this universal principle generally use vegetable green as the plant component of the unit. Usually they are cultivated in gravel or other non-earth materials as "hydroponics". The assimilation capacity of the plants, i.e., the removal of the nutrients contained in the water and their fixation in vegetable biomass, is vital to the function of the system.

However, with this concept of water purification through plant biomass increment some important facts seem to have been neglected.

Since the nitrogen in cultivated plants such as lettuce or cabbage accounts for no more than 0.5 to 1% of the dry mass even with intensive plant growth, only limited amounts of nitrogen can be absorbed. On the other hand, to achieve a satisfactory fish biomass increment, which certainly is the aim of every aquaculture activities, at least 2 9 of nitrogen per kg of fish have to be added as protein feed every day. Of this about 50% is absorbed by fish while the remaining is released into the water. To remove it effectively, a prerequisite for all closed aquaculture operations, at least 100 to 200 times more green biomass has to be produced. This, however, also means a tremendous increase in evapotranspiration and thus water losses, which can amount for 60 mm per day. In addition thermal losses would increase considerably, especially with the more productive warmwater fish-farming.

Water treatment with simultaneous biochemical reactions

In order to avoid the difficulties described above, which result from hydroponics in combination with aquaculture, a different approach to water treatment with plants has been adopted. Continuing investigations into the use of emergent aquatic plants for domestic sewage treatment, the common reed (Phragmites) was studied with regard to its suitability as a plant component in a closed water cycle for fish-breeding. Like other aquatic plants, reed has a wide-meshed air-retaining tissue in the interior of its stalks, and this tissue extends down to the tips of the roots. As a result of active oxygen transport in this tissue, an oxidation reduction potential develops directly at the roots even under anaerobic soil conditions, and this makes high rates of conversion of matter possible

The special functions fulfilled by emergent aquatic plants in a filtering unit may be summarized as follows:
1. The plants grow through the filtering unit with a dense root meshwork to a depth of up to 2 m, keeping the filter open mechanically even when there is considerable contamination by organic matter. The effective cleaning volume is large in relation to the surface area.
2. The oxygen from the air, transported through the roots, leads to nitrification of the ammonium nitrogen (NH4-NO2-No3) from the fish culture in areas near the roots.
3. In anaerobic areas of the filter further away from the roots, the nitrate-nitrogen formed by nitrification is denitrified (NO3-N2) and escapes as gas.
4. The fecal sludge settles on the surface of the filter between the stalks of the reed. Secondary roots emerging from the nodes of the stalks cause the sludge to dehydrate within a short time. Finally it is mineralized.

Thus, when used for this application, the plants contribute primarily to an intensification and acceleration of microbial processes. The proximity of the nitrification and denitrification processes promises a very high degree of purification.

The Kassel Aquaculture
System

A system with a capacity of 4 m³ was set up on these principles in Kassel. It has been in operation for several years now, and thanks to its modular construction can be enlarged to any desired size. The basic components are two cylindrical tanks which can be made at very low cost on a do-it-yourself basis from 1 mm thick translucent fibre-reinforced polyester strips. The tanks are stabilized as cylindrical forms by the pressure of the water in them. The first tank is for fish growth, the second of approximately the same size is used exclusively for water treatment and has a filling of granular porous earthenware-like material with a large sorption-active surface, which plays an important part in the degradation of larger organic molecules.

The filter tank is planted with reed, about 4 to 6 rhizomes per m². After about two vegetation cycles the filter material is mostly penetrated by a dense network of roots. Water from the fish tank containing fecal sludge is driven continuously into the biofilter by air lift pumping. While the sludge settles on the surface of the filter between the stalks and is allowed to dry out, the water flows vertically downward through the filter. After passing through the filter, hydrostatic pressure causes it to flow back into the fish tank via a rising tube.

During the growing season this system can be operated as warm water aquaculture even under temperate middle-European climatic conditions requiring no auxiliary equipment.

Once the installation has reached maximum working capacity it can handle a load of approx. 10 9 of fish per 1 litre of water. Even under these semi-intensive conditions of production, nitrogen concentrations remain for the most part below detectable levels. By analyzing the nitrogen dynamics it has been found that 1 m³ of biofilter volume removes about 6 9 of nitrogen from the water cycle per day. However, only 2 percent of this quantity is assimilated by the plants, the remainder is degased by denitrification. Therefore, simultaneous aerobic and anaerobic degradations within the filter are vital processes. In this combination they are extremely effective in preventing any accumulation of nutrients in the recycled water.

However, the capacity of the biofilter is primarily limited by its conductivity more than by its biochemical reactivity. After two years of growth the vertical flow-velocity attained in the filter tub becomes constant, permitting a hydraulic load of about 1 litre per second per m². Since the surface area is small in relation to the effective filter volume, losses due to evaporation are relatively low. On average the weekly water losses do not exceed two percent of the total water quantity contained in the system. They even can be reduced by using

Not only suitable for fish breeding

Fish breeding in a closed water recycling system is feasible and can be managed with rather simple technology, making use of ecologically sound principales. With reed in a special filter bed and simultaneous aerobic and anaerobic degradation of noxious compounds water qualities are attainable that permit aquaculture with high productivity. Only limited amounts of extra energy for ventilation and water circulation are required which come to not more than 10 kWh per kg of fish biomass gained. Most of these energy requirements could also be met by alternative sources of energy.

A system like this undoubtedly can be employed for a wide range of applications. It could be a great help for breeding fish fry and sensitive water organisms as well as all sorts of food organisms.

In horticulture it could be a very interesting and rational expansion of existing activities even in smaller gardens in town and could contribute to an ecologically more stable food production and to local self-sufficiency.

Average Efficiency of a Simultan Biofilter

Data of Filter

Data of Water quality

Filter volume (H 135 cm, 0cm)

0.70

pH

mg/l

7.5

Surface

0.50

COD

mg/l

2.0

Hydraulic Load

m³/m² d

60

BOD5

mg/l

2

Flow [Kf]

m/sec

0.0004

Sediment

mg/l

0

Suldge Load [DM]

g/m³

4.80

NH4-N

mg/l

0.44

Organic Load(BOD5)

g/m³

0.9

NO3-N

mg/l

0.22

N-Load

g/m³

0.3

NO2-N

mg/l

0.037