contents are flushed to municipal sewer outlets; this is known as a back‐flush or back‐wash. Canister filters contain a variety of filter media within cartridges or bags that are easy to remove for servicing. A cartridge may consist of pleated filter media. Bead filters use buoyant polyethylene beads that are electrostatically charged to attract negatively charged particulates as the water flows through them from the bottom up. Back‐washes can be driven by propeller or bubbles to dislodge the filtrate from the media and send it to municipal sewer outlets; this may be automated or done manually.
Figure A3.3 Sock filters (a) and a rotating drum filter (b).
Settling/Sedimentation Tanks
Settleable particles have a relatively high specific gravity compared to the water they are in, so they tend to settle to the bottom. Suspended particles have a specific gravity the same as, or slightly higher than, the water they are in, so they tend to stay in suspension and will only drop out slowly. A settling or sedimentation tank is designed to minimize turbulence and allow suspended particles to settle out of the system water as it flows (Figure A3.5). They can then be removed by a bottom drain or by suctioning them off the bottom at regular intervals.
Foam Fractionators
Foam fractionation (also known as protein skimming or protein fractionation) is a process that mimics the natural process of wind and wave movement on the ocean surface – this mixing of air with seawater creates a thick foam of surfactant materials, known as spindrift, which blows up onto the shore and so is taken out of the aquatic system. In a foam fractionator, this is accomplished by pushing microbubbles through a venturi injector into the base of a tall cylindrical column. System water flows through the column from top to bottom, countercurrent to the bubbles rising in the column. As the bubbles rise, long‐chained organic molecules stick to them. As the bubbles contact the surface, they coalesce into a thick film, releasing air that then burps the foam up and out of an inverted funnel collection device (Figure A3.6). This collection device is usually auto‐rinsed with potable water to municipal sewer on a regular cycle. When applied correctly, this provides continuous removal of proteins, amino acids, and lipids from the decomposition of organic waste in the aquarium. In addition, this sticky surfactant waste tends to also carry secondary waste products that include trace metals and bacteria through adsorption, coagulation, and microflocculation. Foam fractionators also increase aeration of the system water. Foam fractionators have to be closely managed to maintain the correct flow for efficient operation. They are not effective in freshwater; they typically require a salinity of >15 g/L to provide the necessary surfactant pressure.
Figure A3.4 Sand filter (a), canister filter (b), bead filter (c).
Source: Image (b) courtesy of Catherine Hadfield, Seattle Aquarium.
Figure A3.5 Settling tank.
Figure A3.6 Foam fractionator.
Source: Image courtesy of Catherine Hadfield, Seattle Aquarium.
Relatively small doses of ozone enhance the removal of organics and provide disinfection. This is typically on a side‐stream equivalent to about 20% of the main filtration rate but is fed directly from the aquarium for optimum performance. This combination of foam fractionation and ozone provides increased pH, increased dissolved oxygen concentration, increased oxidation reduction potential (ORP), improved water clarity, and reduced dependency on turnover rate, sand filters, and other traditional LSS components.
Activated Carbon
Granular activated carbon (GAC) is a highly porous carbonaceous, crystalline, granulated material with a tremendous surface area‐to‐volume ratio (Figure A3.7). It is made by heating a carbon‐based material to very high temperatures and then opening up its crystalline pore structure with steam or acid. Carbon sources include bones, coal, nut shells, peat, petroleum residues, sawdust, sugarcane pulp, wastewater sludge, and wood. Lignite coal‐based carbons have a pore size ideally suited for water treatment. GAC physically adsorbs heavy metals (e.g. copper) and dissolved substances (e.g. medications, organic carbon). In aquarium systems, GAC filters are commonly used to deal with the gradual yellowing of water due to the buildup of humic and fulvic acids (Spotte 1991). GAC filters adsorb these organic materials and improve water clarity.
GAC also removes oxidants such as ozone and chlorine by oxidation on its outer surfaces (not within its pore structure).
GAC can be part of the main‐stream filtration, used on a side‐stream, or plumbed as an auxiliary device which allows it to be turned on and off to manage water clarity or for short‐term removal of a target compound (e.g. praziquantel). Pulse‐treatment is often preferred, as there are concerns that critical organics and nutrients can be depleted by its overuse. It is most efficient when placed in‐line after mechanical and biological filtration.
GAC filters can easily perform chemical, mechanical, and biological water treatment but are exhausted once the binding sites are filled. Bacteria readily utilize the GAC as a growth surface, which also decreases its efficiency over time. GAC filters must be replaced when exhausted. This may be carried out when there is increased yellowing of the water or when post‐filtration levels of target substances (e.g. chlorine) are outside of acceptable ranges.
Figure A3.7 Granular activated carbon.
Source: Image courtesy of Catherine Hadfield, Seattle Aquarium.
Flocculation
Flocculation uses chemicals to bind particulates in order to make them bigger and easier to remove. It focuses on dissolved organic carbon. Common flocculants used in freshwater include aluminum sulfate (alum) and natural and synthetic polymers (cationic polyelectrolytes). These act by reducing surface charges on dissolved or suspended particles, allowing them to collide and coagulate. They are most effective at pH < 7.5. The effects on fish and invertebrate species are not fully known and they should be used with caution, especially where other techniques are available.
Mechanical and Physicochemical Filtration Troubleshooting
Common problems include:
Inadequate size for the organic load: The filtration should be designed to handle the highest organic load that might be found in the system.
Inappropriate positioning: Most mechanical filters should be positioned between the system and dedicated biological filters,