These are the guidelines that underpin the design and operation of an iAVs sand bio-filter.
- Use a washed sand with good hydraulic conductivity (see also) – the rate at which water will infiltrate and percolate through the sand. Refer to the Sand Selection Guide and Sand vs Gravel articles if you haven’t done so previously.
- Use a container formed of, or lined with, an impermeable food-grade material to contain both the water and the sand.
- A base-line fish tank volume to sand bio-filter volume ratio of 1:2 with a sand depth of 300 to 330 mm (12 to 13 inches)) is recommended. This will provide for adequate nitrification (etc.) at rational stocking densities and sensible feed rates. At the v:v 1:2 ratio and a depth of 33 cm, this provides 6 square meters of growing surface area for each cubic meter of grow-out tank volume (or 6.6 m2 at 30 cm deep). The volumetric ratio can also be managed dynamically – increasing the filter size with the feed input rate over time – something we intend to address in a later post.
- Orient the sand bed so that there is a slight gradient down toward the drain end. For well-drained sand a slope of 2 cm per meter length works well. Beds longer than 3± meters and/or sand that is less hydraulically conductive will likely benefit from additional drainage provisions (see part 2).
- Install a drain outlet at the lowest end so that the sand bed drains back into the fish tank – to allow water to exit the bed freely. There are various ways in which this can be facilitated. A gap or holes could be inserted in the end of the sand bed – or a bulkhead fitting or UniSeal could be inserted into the base of the bed. Either way, there needs to be some means of preventing the sand from exiting the grow bed with the water. Drainage pipe covered with filtration socking (see illustration in Part 2) is another option, especially suited to larger beds.
- Level the sand in the bed by flooding it (temporarily restrict drainage) and allowing the water to indicate the level.
- Insert multiple furrows across top surface to distribute the watery solids as uniformly as possible. The bottoms of all furrows also need to be level and connected to one another.
- The operating regime for iAVs requires that the sand in the beds be saturated at two-hour intervals throughout the day time. While the duration of the flood cycle will depend on the flow rate of the pump and the size of the sand bed, the iAVs systems built by Mark typically featured a flood duration of 15 – 20 minutes and a drain cycle (OFF time) of around 100 minutes. The pump does not operate at night – by design. (We’ll explain why in a later post)
More details in Part 2 (here).