Ask the WWM Crew
|Please visit our Sponsors|
There seems no end to pro/con arguments for/against different filter media and their applications for biological ponds; bizarre plastic, ceramic, crushed rock of different types, grades, and configurations among others; even hair curlers!
Ultimately, all these varieties have their pro/antagonists and place. Which type(s) you might/should choose will depend on what affect you're trying to achieve coupled with costs of procurement, installation and operation (emphasis mine).
This piece describes various commonly available, utilized media, rationale and function of their employment as relates to individual conditions.
Filter media are intended for three rough purposes; to optimize and stabilize 1) biological, 2) mechanical and 3) chemical/physical conditions. Let's understand these in order of importance:
# 1. Biological:
Of premiere importance is the expedient removal or conversion of noxious chemicals and physical materials of organic or not derivation. Most everyone is aware of some aspects of bio-geo-chemical nutrient cycling> ammonia to nitrites to nitrates, carbon, phosphate...
These changes must occur readily through biological, chemical and/or dilution mechanisms to support life/optimize capacity, growth, color, fecundity... basic biological functions.
Filter media serves to promote these activities in several important ways:
(1) They provide surface area for attachment/trapping of habitat particles.
(2) They promote gas exchange for aerobic (& possibly intentional anaerobic) conversion.
(3) Flow for sweeping away activity wastes.
(4) Mineral substrate for continuing operation, including buffering capacity for countering the reductive (acidifying) nature of aerobic biological conversion.
(5) A serviceable method for periodic partial cleaning away (e.g. vacuuming, back-washing) of excess particulates.
Unless you're on an 'open system' with new "good" water continuously flushing through your ponds you must/will address the issue of biological filtration. Use nutrient balance to your advantage.
# 2. Chemical/Physical Filtration:
Including carbons, Zeolites (clyptinolites), ion-exchange resins, aluminum sulfate, et al. can and do function as physical and at times very efficient biological filter adjuncts.
Their primary role is in contacting (ab/adsorbing) of undesirable or exchange of less-noxious material (e.g. hardness, chemical therapeutics) for more noxious.
For most systems/users continuous use of chemical filtrants is not practical due to purchase costs. They do have their facility as initial treatments of water, as in removing chlorine, chloramine and/or ammonia, in temporarily crowded conditions like a fish show, or emergencies, & for periodic or special removal of color or treatment chemicals.
* One note of caution concerning the casual use and chemical filtrants. Under changing conditions of pH, dissolved oxygen, temperature... they may be dangerous. As an example, some carbons when close to "being used up" are known to un-contact/release undesirable materials into the water. Be aware of the degree of saturation of these media and remove/replace them as they are exhausted.
A mention of ozone and ultraviolet filter modes. I'm leaving these out as strictly non-media, but do want to tangentially acknowledge their possibility/usefulness as adjuncts; UV will be covered in the a later piece.
# 3. Mechanical Filtration:
As a matter of size, micro- & macro-configuration, charge, other surface characteristics, filter media serve to "trap" particulates. This may be a simple affair of larger 'stuff' getting stuck in smaller openings, 'glomming' itself betwixt or aggregating amongst spaces in and between filter particles/openings.
Too often, mechanical filtration is considered as foremost in importance (for looks); for your system's organisms sake it is not. As presented here, mechanical/physical filtration is dead third behind biological and chemical considerations.
Removal/control of suspended matter and color is chiefly an aesthetic (human) desire. This concern is largely and best addressed through biological and chemical controls; when your system is "balanced" in your favor biologically/chemically it most likely will be clear. Simply transparent liquid may be unable to support quality life. White vinegar is beautifully clear as well.
The Media Themselves in No Particular Order of Preference:
Swimming Pool/Spa Technology.:
Some media, like cartridge and diatomaceous earth filtration, are only good mechanical filters. Their two dimensional nature renders them of limited service/application in biological systems as they require high pump pressure and frequent cleaning (aka short cycle times).
Are plastic sleeves with determinate pore sizes. As two- versus three-dimensional surfaces, their pleats all too quickly clog, creating excess back pressure, reduced flow-rate and anaerobic conditions. They may be used in conjunction with other adequate biological and chemical methods. Cartridge filters are best positioned after biological and before chemical modules/components; most sensibly soon after a (the) source of pressurized water (that is, adjacent to the pump).
Diatomaceous Earth Filters:
Are more pool/spa tech. that has little place in most applications due to short cycle times, cost of operation (mainly pump electricity). Siliceous (silicate, SiO2) skeletons of single-cell diatom algae are used to coat a semi-porous plastic sleeve in which water is pushed through under considerable pressure. Used for small particulate (less than one micron) removal as matter becomes trapped in the coating of the diatom skeleton's small porosities.
Sand: Used in pressurized (i.e. rapid sand filters) and non-pressurized (fluidized bed filters); these have a slightly improved use as strictly mechanical/physical filters (over cartridge and D.E. filter modes).
Most sands used are Silicas; pretty much two-dimensional (flat) and chemically inert (un-reactive). Silica is the chief ingredient in glass.
Pressurized filters are biologically worthless in general owing that rapid and large changes in pressure are deadly to micro-organisms. More to their disfavor is the effect of back-washing; the vigorous rubbing of the sand particles in the process of periodic cleaning all but denudes the filter bed.
Fluidized-bed filters do have their place in some situations. Combined with other filter modes and provided sufficient aeration they can support very active and expansive conversion abilities. These are reverse-flow (bottom up) devices used in conjunction with pre-filters to remove most gross particulates.
A note here regarding sand size: the grade specified, like # 10, 12, 20, 30 is an average of the number of grains per linear inch. As an example, #30 denotes that about 30 pieces laid end to end make up about an inch in length.
A further note re media size in general: barring any other considerations, grade (media size) is inversely relational to the depth of the filter bed. What I'm getting at in plain English:
(1) the deeper the bed the lower the flow rate (@ 2 gallons/cubic foot/minute) &
(2) with smaller grade sizes you need a faster (up to about 4 gallon/cubic foot/minute) of flow.
Appropriate Technology/Pond Media:
Crushed 'Rock' Materials (aggregates): are the most commonly used media, They are good for biological and some degree of mechanical and possibly chemical filtration. A classification schema of characteristics of types encountered might read as follows:
(1) Chemically inert (e.g. siliceous material like volcanic rock) and carbonaceous (e.g. crushed Chondritic boulders used for concretes, asphalts),
(2) "Naturally" occurring, as in above, or not; as in crushed brick.
(3) Dimensionality: two for silicates, three for more cubical media like crushed aggregates and volcanic rock.
(4) Surface area per unit volume, degree of nook and cranniness.
(5) Subjective evaluation by the user (as to which/why one is better than another).
Let it be known (my opinion) that all "crushed rock" media are useful for different applications. Some observations, facts, commentary and advice:
Volcanic (igneous) rock
Is available in different grades and qualities. Some are too soft & fall apart too easily; avoid most red cinders. Benefits of using volcanics include their at times low cost, availability, relative low density, uniformity in size, and for folks with hard, alkaline water, chemical inertness.
Their downside is their two-dimensional flatness, sharp, pock-marked configuration, leading to interlocking, channeling and resistance to cleaning/backwash, & lastly their real lack (!) of surface area. On a microscopic level volcanic (igneous) rock surfaces are typically micro-smooth. Not really as supportive of microbial life as they are oft-touted.
Lack of adequate flow combined with low-buffering capacity (and possibly other mineral limitation) can also mal-affect the filter bed. Many of the processes that occur in our filters are reductive; they drive the pH of the system down. In fact, one way to measure a filter's activity is to monitor pH, RedOx, alkalinity/acidity, dissolved oxygen of the water before and after the filter.
A distinct advantage of using carbonaceous instead of volcanic filter material is that is has some capacity for liberating carbonate (CO3 and the balance it affords in modifying/stabilizing the micro-environment in the filter.
Some folks still swear by their volcanic rock though these beds require more frequent disruption, cleaning and removal, more readily channelize and require more force to adequately backwash.
I'd rather swear at igneous material or use them as decorative toppers over other more practical media in non-pressurized, reverse-flow filters.
Carbonaceous rocks (e.g. crushed aggregates) are better in being more 3D, alkaline reserve offering, easier to thoroughly backwash... but admittedly worse in being more dense than volcanic.
Most celebrated as wet/dry, trickle filter substrate for marine 'reef' aquaria, these various and sundry materials range from hair curlers, shotgun wadding, turnings, to high-tech sewage treatment Pall rings and other novel mixed-usefulness shapes, sizes, &... (see ads/claims in "fish" magazines everywhere!).
Basically plastic media are good for increasing surface area for biological and physical conversion and "outgassing". Towards these ends, the most surface area per unit volume (Thomas Muffin'ness?), tied with optimized flow pattern, the better.
I strongly endorse the use of this artificial media; at least in some part of each system, for various reasons: changes in the filter/system due to predation/competition, nutrient limitation, pump failure, therapeutic poisoning, et al.. Other media at times suffer 'biological collapse' (something like aquarium 'wipe-out' syndrome) with most all the (desirable) microbes dying off. This is where the plastic media's crew can save your (and your fishes') keister. By picking up the conversion slack, resisting chemical bottle-necking & aiding in quickly re-populating the other filter beds with beneficial micro-life.
Ideally, I'd place the "plastic" or auxiliary system towards the end of my filter module string/array in a non- to low-pressure container, and adjust it's water flow rate for the amount of media used.
A personal note: Some years back (about the time of Three Island notoriety) I became enamored of evaporative cooling material utilized in cooling tower operations. We tried a few of these slanted plastic units in custom filters and the 'drum' variety we manufacture without being able to colonize any appreciable bacterial activity. Perhaps this compound and/or configuration is designed/engineered to resist biological recruitment?
Other Media: Ceramics, "Beads", Other Chemically Inert Materials:
For those who have had organic chemistry you can recall the tremendous increase in efficiency/purity from using glass rings in a Hempel column in fractional distillation operations.
Benefits of using these miscellaneous non-reactive materials include uniform size, shape, and porosity that facilitates back washing/cleaning and reduces compacting, channeling and consequent anaerobiosis.
The good news is they work and last; the bad is they are expensive per unit volume.
In layered or separated filter module arrangements it may be very appropriate to utilize these media as your pocket book allows. They should be offered system water only after it has been cleared of slime and particulates.
Sometimes we are faced with a situation where the structural design, construction, plumbing, mechanicals (pump, filters, etc.) have already been 'done'... Don't fool yourself into believing and behaving as if you're stuck with any given option. Consider the long term operational costs/savings/benefits to be had by the selection and use of the most appropriate pump (pressure and flow rate, service factor, noise, heat...), filter(s), and media (type, grade, placement) for the desired result. Initial cost for media, filters and pumps, indeed everything else is secondary to long term operational costs for labor and electrical.
As can be seen, there is no absolute right or wrong media; more of a balance of form and function to optimize balance.
CHOSEN MEDIA MUST BE CAREFULLY MATCHED/ENGINEERED/APPLIED PER SYSTEM VOLUME & CONFIGURATION, BIOMASS (LIVESTOCK), FOODS AND FEEDING, PUMP PRESSURE AND VOLUME, CIRCULATION AND AERATION, LIGHT AND HEAT PARAMETERS.
Application of this understanding will maximize operation and appearance; that is, get you the cleanest, best-looking on-going results with the least cost and maintenance.