One of the questions that crops up from time to time is…..”Do sand beds need cleaning?”
No form of so-called ‘aquaponics’ is a perpetual motion machine. Every ‘system’ type requires periodic maintenance – and iAVs is no exception to the rule.
Everything that one inputs into a ”system’ remains in that ‘system’ until it is removed in the form of fish and plant tissues – or mechanically by the system operator.
The longevity of a iAVs biofilter will be influenced by many factors. Such factors include:
- The filter media type, extent (relative scale), effectiveness/capacity.
- The stocking density and age (size) of the fish.
- The feed input rate and its elemental composition.
- The feed conversion ratio realized (to a lesser degree the particular fish species cultured).
- The amount and type of plants and the predominant growth phase of your cropping schedule.
- The microbial populations (range/scope and density/activity).
- The availability of molecular Oxygen for aerobic metabolism.
- Other parameters such as temperatures, pH, alkalinity, EC, CEC.
One of the many benefits of sand as biofilters is that sand can be repeatedly cleaned and reused.
Suggested methods to prolong useful cycle ‘life’ in a sand biofilter:
- Stock the system with fish sufficient to meet the nutritional needs of the plants. Remember, iAVs is a horticulture system where fish production is the means to an end rather than the end itself.
- Do not over feed the fish. Ideally, ALL the feed should pass through the digestion tracts of the fish.
- Highly efficient mechanical filtration (related to particle size) of the solid fraction with ‘wastes’ exposed to atmospheric O2 concentrations and hydration such as at the surface of (and in the upper strata) of a reciprocating sand biofilter.
- Maintain a full compliment of beneficial soil organisms to effectively bio-process all ‘wastes’ into plant available nutrient forms.
- Maintain plants at relatively high density across the entire biofilter surface and with most plants in the log growth phase. Try to not have all plants either very young or very mature.
- Employ a feed composition that is balanced for the fish species but also for plants’ assimilation requirements (e.g., not excessive in Sulphur or any metals – which at least some commercial rations are).
- Grow plant species with ‘high’ nutrient demand requirements in proportion to feed input composition (e.g., not mostly lettuce and other leaf crops).
Some aspects to monitor for clues when you will need to change/clean your sand of unwanted accumulations:
- Monitor percolation/drainage times through the vertical column of the biofilter (other than at the immediate surface/in the furrows) – suggest establishing a baseline interval between initial irrigation on cycle to the initiation of return drainage in the ‘fresh’ biofilter – compare this duration at intervals as time progresses.
- Investigate bottom layer of biofilter every 6 months or so for signs of refractory solids.
- Sight, periodically evaluate coloration and turbidity of the drainage water.
- Smell, when investigating lower volume of biofilter, attempt to detect any slight odor , especially Hydrogen Sulfide (rotten egg smell).
Some thoughts on cleaning sand of accumulated/ excess material
- Sand can be reused indefinitely.
- Have a back-up/contingency plan e.g., replacement sand and or additional/alternative biofilter(s) in advance of need.
- In septic and waste water treatment “slow sand filter” systems, eventual sand rejuvenation is typically limited to the first few millimetres of the surface. The intervals between ‘cleanings’ are measured in years (1-5), but such ‘systems’ do not benefit from ongoing mineral assimilation provided by plant production.
- Sand can be mechanically agitated and screened (even successively if required) to remove accumulations of all particles smaller than the recommended grain size.
- The more one can spread out a sand volume into a greater surface area (shallower depth), the easier/faster cleaning via any method will become.
- option; remove sand from biofilter, spread out onto well drained ground and allow rain and sun to wash it for you (over time), stir/mix occasionally – the necessary time duration will depend on your climate/precipitation patterns.
- If climate/weather is not conducive, construct a sand washing unit – visit sand quarries to observe/learn their methods.
- To loosen ‘problem’ (resistant) coating/films, place on moving conveyor passing thru dilute Hydrogen peroxide solution bath/shower (recycle solution). Rinse with freshwater before returning to use as a media.
- Decouple biofilter drainage return from the grow out tank(s) and wash with fresh water and/or H-peroxide in place. Work as small batches in a screened tray. Capture drainage and loosened minerals and apply outside the ‘system’ such as a soil amendment.
- When moving qualities of sand any distance, employ mechanical aids such as belt conveyors, containers on roller conveyor, wheeled carts, or sloped chutes or slides (w/ or w/o hydraulic assist) when/where possible. For smaller volumes, a shovel and wheel-barrow may suffice.
Viable/economic options for (potential) ‘disposal’ of used sand include:
- amendment to counteract compaction in clay dense soils.
- golf-course dressings, turf production soil regeneration.
- potting soil ingredient.
- amendment for improved drainage/aeration to ‘organic’ gardeners.
Once we come to grips with the idea that all growing systems (including iAVs) require periodic maintenance, the next question to arise is…”How often will iAVs sand biofilters need to be cleaned?”
The simple answer is that we don’t actually know.
The investigation of iAVs happened over a period of a couple of seasons. Similarly, the USDA-sponsored commercial trial was limited to a couple of years. While the sand biofilters were still operating effectively at the conclusion of the research – and the commercial trials – it is not possible to say with any certainty what their lifespan would have been.
What can be said in support of an extended lifespan is that the commercial trial was conducted with far heavier fish loads than were necessary to support the plants that they grew. Indeed, the fish biomass in their system was sufficient to support at least double the number of grow beds they used. That those sand beds were still operating effectively under such circumstances speaks well of their durability.