As shown, there are several methods for removing particles from a water flow. To evaluate how effectively a filter is functioning and to compare filter systems, the term purification efficiency is commonly used (Fig. 5.9). This can be defined as follows:
Cin=concentration of the actual substance enter-ing the filter
Cout=concentration of the actual substance exitingthe filter.
The concentration of suspended solids entering the filter units is measured and found to be 20 mg/l; on exiting the filter the concentration is measured as 5 mg/l. Find the purification efficiency of the filter.
=((20 − 5)/20) × 100 = 75%
In aquaculture the efficiency is normally ex-pressed as the percentage of the incoming TSS removed by the filter. However, it may also be used for other substances, such as the amount of removed nutrients (total phosphorus, TP or total nitrogen, TN) or as reduction in chemical oxygen demand (COD) or biological oxygen demand (BOD) in the water passing through the filter. The last two measurements tell us how the outlet water will affect the oxygen concentration in the recipient water body.
When considering removal of nutrients, it is important to know the proportion attached to par-ticles and not dissolved, since it is not possible to remove dissolved nutrients with a particle filter. These values depend on the species and the feed composition and utilization. Experiments on salmonids have shown that more than 80% of nitro-gen compounds are dissolved in the water, whereas the situation is reversed for phosphorus with up to 80% being attached to particles. However, phosphorus leach easily from particles lying in the water; this is a major reason to remove the particles from the water flow as quickly as possible.
The efficiency of the particle filter of course depends on the particle concentration and characteristics of the water to be purified. For a good comparison of filter systems, they must be tested on exactly the same water. Typical values for removal of TSS from wastewater from fish farming range from 30 to 80% while some lower values have been reported for water re-use systems.