The basic design and construction of the system, as shown in Fig. 1, and the mechanical equipment selected appear to be excellent. Pumps, blowers, electrically-operated valves and other mechanical apparatus function properly. Activated pressure or float switch alarms to signal malfunction were installed at critical points. Electrically-operated valves have a mechanical override permitting manual operation in case of a power outage.
A 22-kilowatt diesel-powered generator is available to supply electrical energy to the recycling plant, should a prolonged commercial power outage occur. This stand-by power source was not included in the original plan, but was strongly recommended by the Consultant. The generator has the capacity to supply energy to operate all electrical equipment necessary to maintain a favourable environment for fish, but biofilter backflushing would not occur.
The primary settling tanks and the biofilters function very efficiently. Sludge is drained from the settling tanks four times daily automatically through clock-controlled, electrically-operated valves. Biofilters are backflushed every five to seven days with compressed air supplied by blowers and water from the storage tank.
The biofilter medium was selected after conducting small-scale tests on several types and sizes of materials as recommended by the Consultant. Clinoptilolite, a natural occurring zeolite found in Hungary, of a particle size of 3–5 mm was chosen. This material provides adequate substrate for nitrifying bacterial growth, is easily backflushed and has the added advantage of adsorbing some ammonia.
Fish rearing space is designed to be flexible enough to accommodate a wide variety of research and/or production requirements. Thirty m3 of space available consists of twelve 2.5 m3 concrete tanks. These containers will be used primarily for brood fish or fingerlings. The remaining 50 m3 in the rearing hall have fibreglass reinforced plastic tanks and troughs of various sizes and shapes. These containers may easily be rearranged in the rearing hall or, if a specific need arises, be removed and other containers substituted.
During the first few months of operation, the schedule and technique of water sampling for chemical analyses were not standardized. There was, therefore, wide variation in the day to day measurements of some parameters. Nevertheless, certain conclusions were apparent:
The pH fluctuated slightly but remained near 8.6.
Dissolved oxygen content of the water entering the rearing tanks exceeded 90 percent of saturation which indicates that the aeration system is functioning properly.
Ammonia nitrogen (NH3-N) decreased dramatically in the biofilter indicating the presence of an active nitrifying bacterial culture.
The nitrite nitrogen (NO2-N) remained at a low level. There was a slight increase in the NO2-N at the lowest level sampled in the biofilter indicating, perhaps, a slight reduction in the numbers of Nitrobacter which exidize nitrite to nitrate.
Nitrate nitrogen (NO3-N) increased as expected, as nitrification was accomplished in the biofilters. The NO3-N level will remain well within acceptable limits as long as there is loss of some water from the system and new water added. The rate of replacement has been from 5–8 percent of the total volume per day.
Water temperature has been maintained at 20–40°C. The water is heated in the storage reservoir by a heat exchanger supplied with 43°C geothermal artesian water. A steam generator, soon to be installed, will be capable of increasing the water temperature and providing more precise temperature control (± 0.5°C).
