More On Sand Beds

[jhnrb]

(PART-1)

Sand Bed Systems

by: Sam Gamble



In last month's article the danger of tracking in nuisance algae problems was discussed. The important factor was the organic content of the rocks that we sometimes use. Next, a similar problem occurs with sand used for the sand bed itself. Because we have built the sand bed filter to function a prescribed way, the mechanisms for problems become a little more complex and interrelated. When problems occur it's more than a load of nutrients and a conditioned surface.

In general, the important components of the sand bed are biological mediators, sand type, and grain size. Correctly orchestrating these contributing elements will achieve the balance and equilibrium the system needs. Like rocks, the amount of organic compounds in the sand is a contributing stress factor and will be treated seperate.

The focus begins at the sand to water interface and diffuses into the bed beneath. With the construction of the plenum space we have effectively produced an environment that is not reliant on the anaerobic condition. In most cases, once sand bed filters are cured, it is hard to find areas in the sand bed with less than 1 mg/l dissolved oxygen.

In general the surface of the sand bed is very active with oxygen demanding heterotrophs. They begin the sequence of reduced compounds, reduced oxygen, and lowered redox. The list of microbes species is long, as is the list of chemical transitions. The important thing is the balance of the consortium. Where you get the inoculating live sand will determine the ratio of organisms. From there, the natural mechanisms proceeed with microbial colonization of the constructed sand bed. The rate of development and the end result will be influenced by what you start with.

The area where the live sand was collected will determine the number and types of microbes it contains. In very general terms, live sand from near shore will be the same as live sand that has its origin from a fore reef area. NOTE THE WORD GENERAL. This will not be good enough to get you off to an advantageous start. Chosing a more specific sampling of microorganisms will start a profoundly better successional colonization of the virgin bulk sand,that makes up your sand bed. Sand sampled from a successful aquarium will do it. A quality live sand sampled from the top layer of the sandy bottom in a fore reef will also. This is where some cultured sands get their starting stock.

Inadequate sand is deficient in other important components, like sand type. Sand type involves the sand's geological history before you got it. It s understandable that a preferred sand comes from an area resulting from the deposited remains of previous reefs. For example, this doesn't include Virginia Beach.

Hence we have the GEOlogical importance of aragonite sand. When dug up and processed the aragonite sand is basically fossilized reefs, that contain the basic elements to build new reefs, without any recent organic deposits. Most importantly this is usable calcium and all essential trace elements. In our particular case, the new reef being built is in the aquarium. Historically the mechanisms regulating elemental depositing and release, have been governed by natural processes. We have recently learned how to harness these mechanisms, technically and/or naturally.

Processing the sand includes grading the sand for size and uniformity. Size is important because large grain size sand will contain less bacteria than small grain size. It's a function of surface area. The cut off is the size regarded as silt. Whereas silt will have more surface area, it inhibits flow around the sand by diffusion. Silt will have a tendency to collect organic compounds and have little or no oxygen associated with it. Producing an anaerobic environment is not what we want. It's the reduced oxygen or anoxic environment we're looking for.

A sand that has an average grain size of 2 mm has shown to produce the reduced oxygen environment needed for the designed anoxic - passive filtration. Uniform grain size will aid the overall efficiency for this goal. Uniform pore water around the sand grain will help the diffusion of elements to and from the sand bed. It is impossible to produce and maintain a perfectly homogeneous sand bed, but having sand that is relatively uniform to start with is a big plus. The chemical transitions of compounds and elements back and forth from the living cells (microbes) will be greatly enhanced by the ability to move, diffuse.

Diffusion is a very good process over short distances. With our sand bed filtration we are counting on it. To limit it in some way, we are effectively reducing the capability to metabolize and cycle organic material and the elemental by-products. In simplest terms, this is CHEMICAL transformation and interaction.

Putting together some of these general concepts, we have BIOlogy, GEOlogy. and CHEMICAL pathways for the mechanisms of the sand bed filtration. Does "biogeochemical pathways" ring a bell from the first article? When we neglect the components of good sand, then some of the functionality of the filtering capacity is sacrificed. When that happens compounds like "nutrients" end up in the undesirable places and accumulate. What happens with strong light and an abundance of nutrients? Killer algae! This is an example of organic content being produced by imbalance. It can also originate from sand laden with organic compounds.

(CONT. TO PART-2)

 

[jhnrb]

Part-2

(CONT. FROM PART-1)

At the beginning of the article I mentioned organic load can be present in starter sand. Sand from near shore or grassy areas can have high organic content and undesirable microbial situations. Reefs are reputed as being nutrient poor environments where the likelihood of having a sand with reduced organic content is favored. Near shore and estuaries, this is not the case. If you add live sand with a poor selection of microbes, that is packed with nutrients, what would you expect to get? Killer algae!

Going to all the time and trouble to build and stock a new aquarium, but use the wrong sand, you've shot yourself in the foot. Or perhaps, you've tried to follow all the guidelines, and still have algae or incomplete filtration, e.g. nitrate above 20 mg/l, what then? It's time for remedy and treatment, Rx and Tx. You'll need some water quality information to base Rx & Tx. It's basically an evaluation of where to place the emphasis; bio, geo, or chemical problems.

If algae problems start to occur but the pH, alkalinity, and nitrogen compounds look acceptable, then the situation may stem from nutrients like phosphate being present on the substrate surface. Rx, most often is the laborious task of removing the algae by hand. Sometimes the addition of snails and crabs will help control it, but they are redistributing the nutrients essentially. When the nuisance algae is in response to a high nutrient content of the sand, then off loading the problem is quicker. When combined with natural predators it becomes a little more effective. However, it will not be a quick process. You'll have weeks and perhaps months to work with the problem. Tx, chemical additives to absorb nutrients, only work on the compounds dissolved in the water that pass through the media. Phosphate has a tendency to coat surfaces, so it won't be effected too much. One of the reasons removing the algae by hand is effective. All that has gone into the algae to build it, is removed with the algae. Nature packages it, and you do the shipping on a oneway ride.

If your water quality starts off within an acceptable range, but begins to deteriorate, e.g. pH drops below 8.2, and nitrate gradually increases, then the problem may stem from microbial problems. If the microbes responsible for filtration and cycling are not growing and functioning, the water quality starts to accumulate nutrients. Rx and Tx, basically have two routes, to remove the bad water (water exchanges) or RX, give the microbial populations help, reinforcements.

Water changes are merely diluting the problem until the filtering and cycling work force can catch up to the load presented to it. The microbes are more essential to maintain. I have used a couple of good ways to succeed in this task. One is to accelerate the reaction rate of the microbial cells. This is done simply by adding the correct spectrum of biological enzymatic catalysts. The other is to add more microbes.

Adding more microbes can be done by adding more live sand. But, if the live sand is the same that resulted in the problem in the first place, not much is gained. Would recommend a new source of live sand. Try to get sand that comes from a reef environment, another well functioning aquarium, or a well reputed cultured sand.

It stands to reason, if you add a better quality live sand and give it a boost with catalysts, you are promoting a lasting change. The biological and chemical pathways just got a shot in the arm to increase natural capabilities. An ecosystem to remain healthy must continually change and try to achieve balance., and not just remove nutrients.

In this method you are not subtracting anything. You are providing more natural tools and mechanisms to cycle components to usable form, or dispose of them through the pathways inherent to equilibria. Metabolism occurs in an appropriate proportion to the conditions given. That's the crux of many problems; " to the conditions given".

Much of the emphasis on sand bed filtration is the treatment of nitrate as described by natural nitrate reduction. All aquariums have natural nitrate reduction to some extent. It can occur in any sand bottom, rocks, hidden detritus, biofilms, algal mats, micro algae, macro algae, and etc. In some cases it is reducing the NO3 molecule to its elements. In others, it is assimilated to new compounds like NH3.

Without the energy of a balanced sand bed, you are not getting some of the natural sources and sinks for biogeochemical processes that are mediated by microbial organisms. Most of which are single cells living in colonies. They are naturally selected for the metabolisms that the nitrogen cycle is only a concomitant part of. The ecology of the sand bed is also responsible for self regulating carbon sources, a natural and self regulating buffering capacity, resident sulfur reduction and regeneration, steady source of trace elements, energy flux, and a natural tendency to promote equilibria during stress.

To get this industrious natural package, you have to use good ingredients. Just a bucket of "live sand" may not do it. Read the label and check the ingredients, so to speak. Your sand needs to be environmentally friendly for your new aquarium. It needs the correct BIOlogical representatives, in the correct GEOlogical conditions, carrying out essential CHEMICAL transitions to obtain anything that resembles equilibria of a substrate oriented natural filter.

This illustrates several of the common situations with substrates. Next time some of the negative situations that animals cause will be illustrated. How many reef keepers have gone to their favorite aquarium store looking for a large, healthy mantis shrimp?


More follows.
posted jhnrb

 

[jhnrb]

(PART-1)

Sand Bed Systems

By Sam Gamble


From the last article, we have begun to picture some of the microscopic events that control our aquarium. Our methods of investigation are to measure important indices and interpret the findings. Even with the aid of good data like pH, dissolved oxygen, and ORP, solving new problems that the ecology of the aquarium may have, can be frustrating and perplexing.

Historically the problem with the greatest track record is nuisance algae. So let's start there. The cause is simple actually; water (medium), strong light (source of radiant energy), and nutrients (food, a.k.a. more energy and building blocks). Many people have tried the frontal attack, using hand to hand combat (removing the junk by hand), or chemical warfare (removing the junk by absorbents and starving it out). At any rate, it often takes months to be successful. When we see it start to disappear, we figure our battle plan has worked. I would like to suggest there was more to it than that.

Usually our maintenance (holy wars) for algal mats begins when we start to see the preliminary little tufts starting to grow. Then it can quickly become an emergency situation. Essentially you are starting out a couple of steps behind mother nature. The contention is that mats effectively retain nutrients for potential use in biomass production as needed. Mats illustrate a nearly steady state condition. Enough nutrients are generated in the mat by oxidative processes over 24 hours, to fuel primary production during the day (photosynthesis, etc.). Then it takes only a SMALL additional external source recruited, to balance the small amount of carbon buried in the mat. Bottom line they are structured and suited to thrive in water depleted of basic nutrient elements. Worse yet, they can easily seed new areas. Once you see them, they are ready to maintain and spread themselves even in low nutrient conditions.

This is the scope of the primary problem with the nuisance algae when we start a new system. The microbes that will be the heart and soul of our filtration (cycling) reaches the steady state or balanced condition a bit more slowly than the more adapted mats (the problem). The metabolically diverse, autotrophs and heterotrophs play vital roles in the cycling and balance of marine organic and inorganic carbon and nitrogen. Daily turn over of carbon and nitrogen due to growth, reproduction, and death by these groups, represents by far the bulk of mediated carbon and nitrogen cycling. Also, the rest of the consortium of microbes are also vital cogs in cycling. Getting this scenario in place quickly will give biofilms and mats the necessary competition.

When nutrients are supplied at higher levels, mats can jump ahead in line until microbes can eventually dominate the use of nutrients. The process seems eternal sometimes. But generally, we are at fault from the beginning. One source is rocks.

How many can identify with starting their new sand bed system (SBS) with the rocks from a previous aquarium? Or an analogous situation; to buy rocks from your favorite store that has had them in the holding system for several months. And despite good water quality, perhaps the rocks have absorbed a sizable amount of nitrogen and phosphorous compounds. A large biomass that partially dies off is essentially the same thing. The situation sounds unavoidable. So what do you do?

Light: There is the commonly used start up procedure to keep the lights off or raised to evade one of the necessary elements for algae; strong light. It works pretty well, but there are some side effects. The rocks will loose most of the other things that also need strong light. For example, you may loose some of the highly prized coralline algae. Which we pay a high price for, when purchasing the rocks in the first place. Much of the life that diminishes when the light is removed, does return, but not as "natural" looking as before.

Nutrients: The other method is a natural competition of sorts. When starting a new SBS, you cannot start with a desert. There are, and will have to be, nutrients to feed the growing microbial populations. Obviously, trying to keep it as low as possible works best. The nutrients that are present will effectively be handled quicker if the balanced consortium of microbes is quickly settling into the empty niches, that you have so purposefully provided with the constructed sand bed.

There are a few major keys to this protocol, but perhaps the most important in this case is the ratio of carbon to nitrogen. In coastal marine benthic environments the ratio of amounts of C:N is usually around 6:1. Okay, where can you find a carbon and nitrogen ratio test kit? No can do, but the concept is important.

When nutrients get out of hand, it is often the result of the N part of the ratio getting bigger. This leaves either reducing nitrogen or increasing carbon to again reach balance (equilibrium). In the past , the most common methods of reducing nitrogen compounds has meant removal with protein skimmers, chemical absorbents, water changes, and that sort of thing. Increasing carbon sources has in "specialized cases", meant adding things like alcohols and sugars. But, given the chance nature will do all of the above.

Nitrogen can be cycled by nitrogen fixation (constructed into NH3) and/or destructive routes like denitrification. Also, carbon is very importantly coupled to the process. Close by in the same neighborhood are the sulfur reducing bacteria, which are noted for carbon transitions.

(CONT. TO PART-2)

 

[jhnrb]

Part-2

(CONT. FROM PART-1)

During sulfate reduction, energy flow is decoupled from carbon cycling. About 75% of the energy content of the organic matter which is decomposed is transferred and stored in hydrogen sulfide. When the sulfide and other reduced sulfur compounds are later oxidized, this energy is released and used by other microbes close by.

The microbes associated with nitrogen, and carbon cycling plus concomitant carbon involvement with sulfur bacteria, dramatizes the importance of getting the starting "live sand" growing and colonizing. This includes the ecology typical of the water to sediment surficial interface. Just a bucket of sand won't help you here.

When the sand bed has been inoculated with a sufficient spectrum, the microbes will generally take care of nitrogen and carbon. Your goal is to see that the microbial growth rates are ahead of algal mat development. Sometimes you win and sometimes you loose.

The balance or equilibrium of the C:N ratio is important on many fronts, but another factor outside of these energy cycles is the more kinetic phosphorous. With regard to rocks, sand, and nuisance algae, it is more a physical action. Phosphorous compounds like phosphate in our aquariums, have a tendency to go in and out of solution and onto surfaces.

Over generalized perhaps, but when we see the red, brown, and green hairy or slimy stuff, it has something to do with phosphate being on or near the surface of the substrate. Phosphate in high concentrations in the water has the ability to precipitate. It chooses calcium carbonate surfaces as one of the favorites. Microbial mineralization is also a major source of phosphate at the sediment to water interface. The additional sources produce enough fertilizer for mat proliferation. It tips the scales too far in the nuisance algae's favor.

There are other factors, but attention to carbon, nitrogen, and phosphorous will be affective and enough to keep you busy.

Ultimately the problem stems from organic compounds. You've most likely read things about dissolved organic compounds (DOC) and particulate organic compounds (POC). There are a few others, but they are derivatives. These compounds can be added or hitch hike into your system. If your rocks have come from an aquarium that had a history of high nitrate or other nutrient problems, then you are going to have preliminary problems.

If you build the sand bed with sand of small grain size (less than 1mm.) and from a source that has a sizable organic content (beaches, shallow water,or estuaries), you will also have problems. This theoretically may also include the wrong spectrum of starter bacteria.

All of these factors essentially do what the algal mats dearly love. You're putting the competition on the bench and the algae on the field with the ball. Touchdown! Slowly the mats use up their stockpile of energy sources, while the microbial populations grow and function well enough to play a competitive game. In the meantime there is some tough maintenance. Lots of time outs.

In a stable and well constructed tank, most algae problems will straighten out. This automatically assumes nutrients are not outrageous and microbial populations are colonizing. However, because of their adaptive characteristics to achieve a steady state, primitive algae can easily dominate conditioned surfaces once they have a foothold.

Constructing the ecological environment for the SBS to be more competitive by tank friendly microbial populations, is a good line of defense. Poor quality live sand, and nutrient rich rock substrates are a big drawback to this aim and usually manifest algae problems. All too many of us have learned the hard way. Unfortunately, we are the cause and have merely shot ourselves in the foot, by overlooking basic concepts of energy cycling and what causes it to happen.

In the past obtaining a healthy and functional Nitrogen Cycle was the primary goal and maintenance centered around sustaining the microbes involved. Keeping the detritus to a minimum and enough turnover through the filter to feed the bacteria and scrub the water. Now, we are talking balance, steady state, and equilibria. We're adding new words and concepts to our aquariums to make them simpler and more efficient. Seems paradoxical doesn't it.

Next time let's return to the subject of sand and take a closer look at one of the other common mistakes when building the SBS; cheap - o sand. In doing so we will have to return to the concept of biogeochemical pathways as illustrated by information from benthic ecology.

(MORE FOLLOWS)

POSTED jhnrb

 

[jhnrb]

Sand Clumping

(PART-1)

Sand Clumping
By Sam Gamble

In spite of our best efforts, we haven't achieved the "in situ", all natural aquarium. We're doing better all the time. Communication is a very helpful part of it. Thanks to people like D. R. Martin (Aquarium.Net), communication on the INTERNET is helping many. Problems and observations, as well as, theory and solutions are quickly exchanged on many tropics.

One such topic is "clumping' of sand substrate. The clumps or coatings are actually calcite deposits caused by precipitation. The observation is new. New in the sense it has become noticed with the use of sand substrates for passive filtration. In other words since going beyond under gravel filters and the Berlin Method. This is important to consider because each time new methods of aquarium filtration are implemented, there is the tendency to carry over some technical aspects. Some bring new puzzling observations.

This is the case of sand bed filtration and clumping. Several factors from previous methods contribute significantly. Kalkwasser and buffering are two. The contributing ingredients are calcium compounds, and particularly the Ca++ ion. In natural seawater calcium carbonate (CaCO3) is supersaturated and the important element being calcium (Ca++). Since our closed system aquariums are finite and don't have an infinite supply of important constituents like calcium, we add them.

The second factor that influences calcite precipitation ("clumping") is less obvious, but important to consider in closed systems. Magnesium inhibits the chemical precipitation of calcite under natural conditions. So then we essentially have two sides of the coin; adding too much and removing too much. A reoccurring situation in closed system aquariums. A familiar battle between maintenance of a finite system (the aquarium) versus the observations in a relatively infinite system (the sea).

These are the predominant pathways that Richard Greenfield, Marine Geologist of CaribSea gave in a personal communication. However, as he says, it is a bit too complicated to issue a general statement. You can say it is a bit unsightly, but doesn't harm anything. Many find it easily controlled by sand sifting organisms.

For the sake of understanding, lets take a look at the pathways that cause calcite to precipitate from solution, forming crystals. Thereby, perhaps preventing it in the first place. The first path is simple supersaturation in respect to CaCO3 through the audition of Kalkwasser solution (Ca(OH)2), calcium chloride (CaCl2), and buffer (in the form of carbonate (CO3) or various combinations of these chemicals.

Sea water is supersaturated with respect to Ca to begin with. One can increase supersaturation easily by increasing the ionic strength of these ions. It can be done by direct addition or even evaporation. Another way could be done is by raising the pH with repeated water exchanges.

Kalkwasser, however, seems to be the common denominator. This would mean that precipitation is often caused by increasing the degree of supersaturation and increasing the pH (keep your eye on the OH- ion). It is important since the distribution of Ca++ ions by Kalkwasser is probably not as active as other inputs such as aragonite or CaCl2. Importantly, a person can get precipitation of calcite even at low Ca++ and carbonate concentrations. It occurs in a system that has naturally high pH levels (8.3 and above).

When a carbonate precipitates, it prefers to do so on other carbonate materials such as aragonite reef sand. Without sand in the system you would expect calcium carbonate to come out of solution in the more energetic areas of the system such as the pump and plumbing. This was an often observed event before the use of a sand substrate. Pumps and plumbing needed periodic cleaning to remove scale.

The easy solution to most "clumping' is to reduce Kalkwasser input. A word of caution for changing regular protocol; if you decide to reduce the input of Kalkwasser, do so gradually. The presence of the hydroxyl (OH-) ion may play a significant part in the respiration of hard corals particularly in aquariums that tend to accumulate dissolved carbonate or have rather sluggish circulation or both.

(CONT. TO PART-2)

 

[jhnrb]

Part-2

(CONT. FROM PART-1)

The second pathway is more insidious in that it involves the subtraction from solution of a rarely monitored ion, namely Mg++. Magnesium is roughly three times more abundant than calcium in natural sea water (1.295g/kg versus 0.412g/kg). Yet it plays a vital part in the whole carbonate scheme of things. This is what keeps the oceans supersaturated in respect to calcite.

Magnesium poisoning (a chemical term that has no implications to living organisms) inhibits the chemical precipitation of calcite at normal ocean values of pH and ionic strength. The process is by coating the surfaces of growing calcite crystals with very unstable combination of Ca and Mg carbonates and hydroxides. If magnesium were to be subtracted from a closed system in significant amounts you could expect to see accelerated calcite crystal growth. It could be observed even at normal pH, Ca, and carbonate ionic strength. So how could you subtract this seemingly spectator ion out of a closed system?

There are two ways of doing this. First, every reef keeper's favorite algae is the pink coralline variety which is depositing high magnesium calcite (geologists call it high mag calcite). Sometimes it grows at a spectacular rate. This calcite can be 30 mole % magnesium. Also, the calcite crystals growing on aragonite substrate can contain a similar percentage. Since calcium, not magnesium is the only +2 ion regularly replaced outside of water changes, you may deduce that many closed systems can experience a steady decline in dissolved magnesium. Hence a commensurate tendency to chemically deposit calcite results. This is a good time to remind ourselves, that aragonite does not incorporate magnesium. Hard coral and Halimeda growth should not affect dissolved magnesium levels.

The best way to insure enough magnesium in a system is probably through regular water changes. However, as marine system technology and practice improves, water changes tend to become less frequent. Magnesium additions can be made from magnesium chloride or hydrated magnesium sulfate (Epson salts). I don't know of any commercially available test kits for magnesium. So if readers do want to make magnesium additions, do so carefully and at your own risk.

Again, "clumping" does not present any threat to aquatic life (including denitrifying bacteria) nor affect the performance of aragonite substrate outside of some reduction in porewater space. The calcite crystals ("clumping") themselves are metastable at whatever pH they were formed. This makes it likely you may even expect some buffering capability at elevated pH levels.

Also, assuming that these two major mechanisms postulated here are essentially correct, the conditions that cause calcite crystals to grow in the substrate have nothing to do with the substrate itself. In the past (before substrates) "clumping" would appear in the form of "chalky deposits" on and in aquarium fittings. Basically, when water conditions favor calcite deposition, it is going to come out of solution somewhere. The substrate is just the first choice.

When you see the calcite crystal formation on the substrate, you are probably seeing an overdose of Ca++. Since closed systems or particularly reef tanks, are decreasing the need for frequent water change maintenance, the increased Ca++ could be coupled with a Mg++ deficiency. It does contradict the notion that supersaturation of calcium is the best thing for coralline algae growth. How many times have we felt that when the coralline growth is below par, that the increase in alkalinity is will help?

The understanding that clumping is a result of several concomitant events and pathways is useful. It deters from the reasoning that perhaps another grain size of calcium based sand will change the mechanism. Secondly, it helps the understanding of mineral deposition by coralline algae. Most importantly, it's good to know that there isn't any particular danger for the closed system in which it happens. It is a little unnatural looking though. Who has ever seen in the reef environment, what looks to be melted ice cream? Then again, what reef develops input of calcium beyond supersaturation to the point of precipitation?

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POSTED jhnrb

 

[jhnrb]

More Sand Clumping

Sand Clumping

By Sam Gamble

SAND CLUMPING - ANOTHER VIEW

In the previous article sand clumping was discussed from the chemical and physical properties. The Marine Geologist at CaribSea, Rick Greenfield, helped us understand that the observation generally has two pathways, 1. to supersaturate the water of the system with calcium ions, 2. the lowering of magnesium concentration allows calcite formation to become easier.

This understanding is based on physical and chemical aspects of our sand bed aquariums. It's not hard to see that when the abundance of calcium ions becomes great enough they congregate to an easily used surface (precipitation). The calcium ions and the medium in which they are suspended are positively charged. The sand bed and the grains of sand are mostly negatively charged. The precipitation onto the sand grains is a coating that satisfies charge and abundance of ions. A path of least resistance to a desirable surface.

Since it is usually observed on the surface of the sand bed, let's focus our attention there. Also, let's take a shift from physics and chemistry to biology. This is where a very dynamic part of our aquarium is cycling organic material, to gain energy for existence and reproduction. Some people are still referring to the living process as filtration. But, to truly understand it, we must visualize the cell and its environment.

Our living sand system consists of a sand bed, which provides suitable surface area for microbial cells to attach themselves. If we do it correctly there are populations of varied species in colonies large enough to use up the compounds added and produced in the closed system. We depend heavily on the natural checks and balances of the natural systems from which we have borrowed the ingredients. I equate success, with balance of living cells to use and produce energy for maintenance of their immediate environment at equilibrium. To satisfy this qualification the macro environment thrives.

The micro environment of cells on the surface of the sand can be termed, "biofilm". A biofilm consists of cells immobilized at a substratum and frequently embedded in an organic polymer matrix of microbial origin; a surface accumulation, which is not necessarily uniform in time or space; may be composed of a significant fraction of inorganic or abiotic substances held together by the biotic matrix (protein goo).

A biofilm system consists of the biofilm, the overlying liquid layer, and the substratum on which the biofilm is immobilized. A biofilm is composed of microorganisms immobilized at a support surface. This is generally in association with the organic polymer matrix. However, the biofilm may also contain degradable and/or inert particles and sometimes may include microorganisms. The biofilm system has been classified in terms of phase and compartments.

Many compartments can be defined in a biofilm system:

the substratum
the base film
the surface film
the bulk liquid

Each compartment is characterized by at least one phase (gas, liquid, or solid), using the term "phase" in the strict thermodynamic sense. Thus, each compartment can be described in terms of its thermodynamic and transport properties. This includes the transport and transformation processes that dominate within the compartment. The system encompasses all of the "compartments," the phases, the process compartments (properties and processes), and the geometry of the system (e.g., a rotating biological contactor or a sand bed).

The substratum plays a major role in biofilm processes during the early stages of biofilm accumulation and may influence the rate of cell accumulation as well as the initial cell population distribution. The substratum can sometimes also serve as the substrate, the rate limiting nutrient for growth.

Biofilms serve beneficial purposes in the natural environment or engineered biological systems. For example, biofilms are responsible for removal of dissolved and particulate contaminants from natural streams and in waste water treatment plants (fixed film biological systems such as trickling filters, rotating biological contactors, and fluidized beds). Biofilms in natural water, called mats, frequently determine water quality by influencing dissolved oxygen content and by serving as a sink for many materials. Biofilms provide opportunity for syntrophic and other community interactions between microorganisms, and a means for survival of microorganisms in natural habitats.

When microbial cells in the biofilm are reproducing and expanding their colony, perhaps the sudden appearance of calcite deposits could be inhibited by the cells when attaching their fibrils to form the matrix. An early phase in the biofilm process is the conditioning of the substrateto which they attach. With regard to precipitation of calcium, it would seem to be a controlled factor when there is biofilm formation of fibrils and matrix during the beginning stages of the colonization process.

This condition would fit the observations; sand-bed systems tend to have unstable chemistry in the early stages, and reported clumping seems to be early in the sand bed setup. The interesting thing to note is that seeding with live sand has a positive effect. Sand beds seeded with live sand do not tend to have as many reports of clumped sand. If applied to the process of conditioning the substrate surface and the recruitment of microbial populations for ecological succession, it makes sense.

But, we must caution ourselves from making too many hardened statements. The high cell densities and the (electro)chemical properties of the matrix influence transport of soluble and particulate components. Conceptually, the diffusion process can be described with present available models. However, the factors influencing detachment (erosion and sloughing) processes are not so obvious. Since detachment influences several processes, including the population distribution, more attention to causes and effects of detachment is necessary. In addition, the integration of the biofilm with the inorganic chemistry of the bulk water has not been addressed sufficiently. The exact role of calcium precipitation has not been determined.

Various environmental variables (e.g., pH, temperature, salinity) influence microbial stoichiometry and kinetics and, thus, physiological ecology, in a biofilm. This is an important consideration if application to clumping is valid. With the success of live sand seeding, it is possible to accept the importance of diversity to balance the environment. Microbial ecology is concerned with the interactions between different microorganisms and between organisms and their environment, e.g. high concentrations of calcium ions and/or decreased magnesium concentrations.

If any correlation to inhibition of clumping and the inoculation of a well functioning biofilm is valid, then the matrix and microbial conditioning of the substrate is a plausible influence. It stands to reason microbial biofilms are biotic and abiotic barriers between the ionic attraction of calcium carbonate to the substrate surface. The natural tendency for the biofilm to grow and mature would also be an inhibitive force against clumping, since it is the matrix that conditions the substrate.

To me a more basic question arises when considering the initial phases of biofilm formation. If, it is part of the biofilm's succession in growth and establishment, to condition the substrate and maintain its environment, then is a steady supply of supersaturated calcium ion necessary for health and reproduction? Does it actively achieve equilibria in the system that the biofilm is part of?

I tend to side with the idea that in sand bed systems where a diverse and healthy population of microbial cells has been supplied, buffering is adequate. pH fluctuations are for reasons other than total alkalinity. Thereby it is much less crucial to add a supersaturation of calcium and magnesium ions. The micro environment will, with its own time line, maintain balance for the macro environment. Of course the catch in the statement is "diverse and healthy". Those essential elements can be easily abused.

END

POSTED jhnrb