Water Management Technologies
Efficiency of a Hydrotech Drumfilter as Particle Separator in a recirculated system:
Introduction: The efficiency of Hydrotech´s filters in aquaculture applications are proven in several tests. However, different mesh sizes have not been emphasised enough. One of the most extensive investigations has now been carried out at DIFTA in Denmark in a recirculated eel farm as part of a "blue" label project supported by the Commission of the European Union(Fair CT-9158).
Method: The test system used for the experiment is a research unit consisting of 18 circular shaped tanks with a volume of 450 L each. A hydraulic load on the drumfilter of 27 m3/h(model HDF1201-1H) directly from the fish tanks and two submerged biofilters combined with one trickling filter. Return flow through oxygen cones. Three different mesh sizes, 30, 60 and 100 microns, were tested each for one month. Performance of the filter was satisfactory during the entire test period. The drumfilters operated on liquid level control. Filtered water was used as rinse water for the backwash system, spraying the particles down into the sludge trough.
Results: Relative water consumption for the drumfilter at a feeding level of 10 kg/day with 30, 60 and 100 micron mesh:
| Mesh size | 30 micron | 60 micron | 100 micron |
|---|---|---|---|
| Litre/kg feed | 200 | 100 | 50 |
| % of recycled water | 0,23 | 0,11 | 0,06 |
As could be expected the relative water consumption as well as the fraction of the recycled water used for spraying is decreasing with increasing mesh size from 200 to 50 litre pr. Kg feed and 0,23 to 0,06 % of the internal flow.
Water exchange was very acceptable for a recirculated system.
Mass balances for suspended solids, COD, nitrogen and phosphorous.
Based on measures of internal flow, water consumption of the drumfilter and the results of the chemical analysis on samples taken at the termination of each test period the mass balances for suspended solids, COD, nitrogen and phosphorous has been established for each of the three mesh sizes.
Efficiencies are calculated as follows:
The energy equation:
Feed Intake = Growth + Respiration + Excretion + Feces
Efficiency: in sludge x 100 / with feed¸ (in fish + respiration+ excretion)
This efficiency is a measure of how big a fraction of the material in the feed not used for growth or respiration by the fish, that is recovered in the sludge from the drumfilter. Removal efficiencies of the drumfilter with 3 different mesh sizes:
| Mesh Size | Suspended Solids | COD | Nitrogen | Phosphorous |
|---|---|---|---|---|
| 30 micron | 99 % | 92 % | 103 % | 101 % |
| 60 micron | 48 % | 54 % | 49 % | 91 % |
| 100 micron | 21 % | 20 % | 24 % | 47 % |
The sludge generated was examined closely and it has to be emphasised that removal efficiency figures include suspended solids in the form of bacteria flocs from the biological filters, hence the values are higher than 100 %.
It was concluded that at 30 microns, 25 % were accounted for by this fraction. The true efficiency on removal of suspended solids can therefore be estimated to approximately 75%.
Henrik Mortensen, Hydrotech AB
MSc. Biology
Ph: +4575694822, fax: +4575694824
E: henrik [dot] mortensen [at] hydrotech [dot] se
PS. For a comphrehensive book on recycle principles in fish farming from this author, please email us at info [at] w-m-t [dot] com. Please specify CD or hard copy.