and fluidized bed (b).
Source: Image (a) courtesy of Chris Limcaco, Algaewheel Technologies, LLC, and (b) courtesy of Catherine Hadfield, National Aquarium.
Ecological Scrubbers
In natural systems, bacteria break down organics rapidly to supply algae and plants with nutrients to grow. As a result, nutrients typically do not accrue in natural systems (without outside inputs like nitrogenous fertilizer). In artificial systems, when algae and plants are discouraged to grow, organic and inorganic end products build up. Biological filtration is the process in which unnaturally large populations of relatively few bacteria species convert organic waste products. Ecological filtration is the natural process centered on using balanced, diverse populations of vascular plants, algae, microorganisms (bacteria, archaea, protozoa), and/or macroinvertebrates to form an environment that cycles and/or removes waste products in the aquarium while balancing water chemistry. These organisms can also provide a rich source of nutrition for herbivores and omnivores. This type of living refugium filtration is simple to design and operate, and is usually inexpensive compared with traditional methods. This holistic approach may improve animal health, habitat operations, and aesthetics.
Figure A3.12 A sulfur denitrification system.
Refugium filtration based on the previously described deep sand bed filters and the use of live rock have become common in tropical reef aquariums where living corals are the predominant animals. Live rock consists of rock and aragonite originally formed by corals or other calcareous organisms and is heavily colonized by a host of micro‐ and macro‐organisms that live in and on it. These organisms perform anaerobic and aerobic types of biological filtration in an ecologically balanced way. There are many artificially established varieties of live rock available on the market today; these reduce the need to harvest from the ocean environment.
Algae‐based refugium filters are often called algal turf scrubbers or algal scrubbers. They consist of a shallow trough or raceway or rotating wheels connected in‐line with the main system (Figure A3.13). They often have screens to increase the surface area. They need to be well‐lit by light emitting diodes (LEDs), high‐intensity metal halide, or very high‐output (VHO) fluorescent lamps with ample levels of photosynthetically active radiation (PAR) in a cycle opposite to that of the main habitat (Adey and Loveland 2011). Natural sunlight to augment treatment during the day can also be used, if access to unfiltered sunlight is possible. The balanced nature of these filters should provide a robust and diverse microbiome to naturally compete with potential pathogens, rather than serving as a “pathogen reservoir”.
Figure A3.13 Algal scrubber wheels.
Source: Image courtesy of Chris Limcaco, Algaewheel Technologies, LLC.
In tropical seawater, red and blue‐green algae and diatoms usually dominate. With high‐intensity lighting, diatoms will colonize the turf first, followed by the blue‐green algae. Green and brown algae are always present, but in smaller numbers. Once the algal turf matures, it must be continually grazed (e.g. by amphipods or chironomid insects living in these systems) or it goes through succession and produces a less productive macroalgal forest (Adey and Loveland 2011). Periodic human intervention to manually harvest the algae with a tool like a plastic putty knife is occasionally needed.
In the past, aquarists have tried to force the ecological scrubbers to grow edible algae such as Caulerpa spp. While this appears to be a good idea, it has resulted in collapse and the sudden release of nutrients and metals as the plants decay. Ecological scrubbers should by managed with what naturally grows and evolves as this will lead to a much more stable system.
Water Disinfection
After mechanical and biological filtration, other materials such as color‐producing organics and infectious agents (primarily bacteria and viruses) need to be addressed through disinfection. Point contact disinfection is a process whereby water is diverted in a side‐stream fashion to be in contact with a sterilizing agent (e.g. UV or ozone) and then recycled back to the system. To improve water quality, the rate of side‐stream treatment must exceed the rate of system contamination (i.e. bacterial growth in the region holding the animals). UV can have an additive or synergistic effect when used with ozone in series, reducing the required applied dose of each. Bulk‐fluid disinfection is a process whereby a residual oxidant (e.g. liquid chlorine, chlorine dioxide, hydrogen peroxide, or sodium hypochlorite) is added to the main system. This should never be used with fish and aquatic invertebrates.
Ultraviolet Light Disinfection
UV light in the 254 nm range irradiates pathogens and proteinaceous material, physically destroying them. It requires adequate bulb output (measured in mJ/cm2 or mWs/cm2) and an appropriate contact time and flow rate (Figure A3.14). This technique is most effective when used in flow‐through systems, with irradiation of the incoming water. It is less effective in closed and semi‐closed systems, since the effect is localized and may not control the microbial density nearest the animal. The efficiency of UV varies and it should be continuously monitored. Target organism kill rates can be established based on contact time and flow rate. UV quality deteriorates over time and tubes need to be changed regularly (e.g. at least every six months). The quartz sleeves around the bulbs may need to be wiped down as frequently as daily or weekly to reduce biofouling.
Ozone Disinfection
Ozone (O3) is a very strong oxidizer, which is useful for disinfection and water clarification but carries high risks and high equipment and operational costs. Ozone oxidizes therapeutics, color‐producing organics, ammonia, nitrite, and micro‐organisms, and it reduces turbidity through a process of microflocculation (Johnson 2000; Overby 2002). To work efficiently and safely, ozone disinfection needs:
1 Suitable ozone gas generation.
2 Appropriate gas‐to‐liquid mass transfer (mixing).
3 Adequate contact time for reaction (often ~10 minutes).
4 Destruction of residual ozone and oxidation by‐products.
Ozone is created by exposing dry O2 to high AC voltage across a discharge gap (Figure A3.15a). Ozone must be generated on site since it is labile. Ozone is injected into a side‐stream contact chamber with system water flowing through it; this is where the oxidation occurs. The contact chamber may be a dedicated ozone chamber, trickle filter, or foam fractionator; use of a foam fractionator improves the efficiency (Figure A3.15b).
Figure A3.14 Diagram showing ultraviolet light disinfection.
Source: Image courtesy of Sarah Chen, copyright reserved.
Ozone exposed to salt water can create residual oxidative by‐products (particularly hypobromous acid and hypochlorous