The Water Treatment Plant in your Backyard

What sophisticated bioreactors can teach us about anaerobic pond filtration

Healthy pond, healthy fish!
Healthy pond, healthy fish!

Imagine that two busy newlyweds who are holding down two to three jobs (each) want a koi pond. They have a new home, spacious land and a clean slate to work with. Yes, they have dreams and plans. They would appreciate a pond that doesn’t require a lot of water changes. They are also into the Green Movement, so they don’t believe in waste. They have called you because you are the expert; you are the best in the business! How do you create a pond that offers them all the beauty they are looking for but doesn’t create a lot of hard work and maintenance?

The answer can be found in an unlikely place: wastewater treatment plants.

When designing and building a new pond for a customer, the first thought a pond builder has is typically the design. But there’s an equally important thought that any good builder should consider before tackling a new project: “How is this pond going to work for the customer in the long haul?”

A microscopic view of biofilm activity.
A microscopic view of biofilm activity.

From Plant to Pond

Wastewater filtration plants solved the problem of removing organic pollutants and ammonia cost-effectively
by using biological processes.

This discovery lead the way to bigger and better technologies. Once they had tackled pollutants and ammonia, they targeted nutrients — primarily nitrates, due to their propensity to encourage unwanted algal growth that can impede pumps and produce noxious odors.

To combat nitrates, these water treatment plants employed denitrification — a biological process in which
certain bacteria will, under anoxic conditions, utilize the oxygen tied up in oxygen-bearing inorganic compounds (such as nitrate) and chemically reduce them. In the case of nitrates, the compounds are reduced to nitrogen gas. The nitrogen gas is then released harmlessly into the atmosphere, which is naturally composed of 78 percent nitrogen gas.
Nitrates were once considered harm-less in ponds, but now we realize that they have a big impact on aquatic life. Nitrates impact a fish’s immune response and reproduction and reduce metabolic processes. Eventually, exposure to nitrates can result in death.

In order to balance the nutrient levels in our ponds, we have relied too long on the addition of plants, which sometimes crowd the fish out of the pond, and constant water changes. Indeed, one of the primary reasons for frequent pond water changes is nitrate buildup. But now, taking a cue from the wastewater facilities, many pond builders are already taking advantage of the various types of anoxic filtration to reduce nitrates, saving water and labor for their customers. This method allows pond owners to reduce the frequency of water changes while still maintaining water in their ponds that is healthy and aesthetically pleasing.

With denitrification, water changes are needed less frequently.
With denitrification, water changes are needed less frequently.

Anoxic vs. Anaerobic

In discussing the technology behind this nitrate reduction, it is important to understand the difference between anoxic and anaerobic conditions because there are other factors affecting your customers’ ponds — and the success or failure of their filtration systems.

Anoxic conditions exist when the water does not contain elemental oxygen (usually in the form of dissolved oxygen), but there is oxygen in the form of oxygen-bearing inorganic compounds like nitrates (NO3-) or sulfates (SO4-). This oxygen can be used in the oxidation of organics in the pond. As in most oxidation-reduction reactions, when one species is oxidized another has to be reduced. It’s kind of a zero-sum game.

In order to establish an anoxic or anaerobic environment, there are two factors that have to be addressed: oxygen transfer rate and oxygen utilization rate (OUR). This is true whether you are looking to achieve the anoxic condition in a liquid medium or a biofilm. Oxygen can be transferred passively through the air/water interface, or actively through the use of aerators, waterfalls, fountains and the like.

The oxygen consumption rate will be related to the organic load, which is determined by a few factors:

  • How many fish will this pond hold and how many can the bio-exchange really support?
  • How much work will the new owner have to do to maintain the pond? How easily is this system cleaned?
  • Will the mechanical filter remove sufficient waste to allow the bio-reaction to occur naturally?
  • The organic load in the pond creates oxygen demand, and the bacteria convert it to carbon dioxide, water and (additional) bacterial cells. In most cases, in a pond filter it is most desirable to have anoxic rather than true anaerobic activity. The reason for this is that many anaerobic metabolites are malodorous and, in some extreme cases, can be toxic. A pond can even become “too anoxic,” as measured by something called oxidation-reduction potential (ORP), when sulfates are chemically reduced to hydrogen sulfide (the source of that notorious rotten egg smell). If this occurs in the pond in high enough concentrations, it could be harmful to the fish.

    Stocking levels are vitally important to the outcome of water quality.
    Stocking levels are vitally important to the outcome of water quality.

    The Solution

    Fortunately, it is relatively easy to get into the “sweet spot” where water is anoxic enough to reduce nitrates, but not so anoxic that hydrogen sulfide generation occurs. You also avoid true anaerobic metabolism, where a lot of foul-smelling, possibly toxic byproducts like butyric acid can be produced. The answer: an anaerobic biofilter.

    For most anaerobic biofilters for ponds, the biomass grows on a fixed film on the media in the filter. While this biofilm may seem just like a slimy layer on the media, even a thin biofilm contains millions of bacteria per square inch, many layers deep.

    The biofilm is an often misunderstood and vaguely accepted terminology in pond lore. In a biofilm (see diagram 1) you will generally have a thin film of water that clings to the outside of the biofilm due to the adhesive and cohesive properties of water. It is these same properties of water that provide for capillary action in plants and help water get to the top of the tallest of trees. Soluble organics and some inorganics, like ammonia, diffuse into the biofilm through this layer of water.

    In the first layer of the biofilm, there is an aerobic zone. The depth of the aerobic zone is determined by the strength of the water in terms of oxygen demand and loading per unit area of the media, and the dissolved oxygen content in the water. As the oxygen is consumed as it diffuses through the biofilm, and organic and inorganic compounds are still present, an anoxic section develops.

    In this section, any oxygen-bearing compounds will be utilized as what are called “electron acceptors,” providing a means to oxidize the organic compounds to carbon dioxide, while reducing the oxygen-bearing inorganic compounds to nitrogen gas (from nitrates) and hydrogen sulfide (from sulfates). Fortunately, the utilization of nitrate is preferred over the utilization of sulfate and most pond water will have more nitrate than sulfate.

    The process dynamics of diffusion and biological activity in a biofilm.
    The process dynamics of diffusion and biological activity in a biofilm.

    The key to getting the correct dynamics in the anaerobic or anoxic filter is proper sizing of the filter with respect to loading. If a filter is oversized and the loading is too light per unit area of surface area, there will not be enough oxygen demand to create an anoxic zone. However, if the loading per unit of surface area is too great, the film may grow too thick and create a true anaerobic zone, which can lead to undesirable results. So, an oversized or undersized filter may give less than the desired results. In our experience, more people err on the side of the filter being undersized to try to save money.

    It is fairly easy to track the efficiency of your ponds’ anaerobic filters simply by monitoring the nitrate levels. A well-functioning anaerobic filter should keep your nitrate levels below 20 mg/L nitrate. If you find that the nitrate levels are not going down, or even going up if you have installed an anoxic filter, it may be helpful to add a bacterial product that contains effective denitrifiers to quickly establish the population. We have had customers report that their ponds went from 160 mg/L of nitrate to less than 20 mg/L of nitrate after applying a bacterial denitrifying product.

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    While anoxic/anaerobic filtration is relatively new in the U.S. market, it has been popular in Europe for over 10 years. One of the main reasons for this is that it is generally much more expensive to do water changes in Europe. In fact, water changes are even costly in some parts of the U.S., and large-area ponds rarely, if ever, do water exchanges simply because it is too much water to replace.

    One thing to keep in mind is that in addition to doing water changes to lower nitrates, there are other criteria for doing water changes as well. These include phosphate levels and total dissolved solids (TDS) levels, as both can affect water quality and the health of aquatic life. There are limits to how much phosphate can be removed biologically. The same applies to TDS levels, which at some concentration can affect the osmo-regulatory systems of the aquatic life in your pond. Phosphate can also be lowered by phosphate-binding water treatments, as well as vegetation, and through water changes. Lowering TDS levels requires either water changes to dilute out the dissolved solids, usually salts, or more sophisticated treatment technologies like reverse osmosis (RO) or ultra filtration.

    As the specialist, it is your responsibility to know the right size filter for your customer’s pond size, recirculation rate and fish load. Make the right choice, and your customers will be thrilled with their clear water and healthy ponds. Now you have more information to give you the optimal outcome every time!

    One Response to The Water Treatment Plant in your Backyard

    1. architects in coimbatore June 5, 2019 at 11:04 PM #

      This website was… how do you say it? Relevant!! Finally I have found something that helped me.

      Thank you!

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