Tech Guru – Biofilters
One of the most important components of a system and in many cases, the least understood is biofiltration. While the calculations for biofilters are published in several textbooks and a multitude of professionally reviewed papers exist on the sizing of them, there is still a lot of fuzzy logic that gets used in the industry. The challenge is understanding the pitfalls of improperly sized biofiltration.
There are several schools of thought on which type of biofilter is most efficient, most reliable, as well as if both mechanical and biological filtration can occur in the same vessel. I have been rather vocal throughout my career about the last comment of having both biological and mechanical filtration in the same vessel.
For the most part, I do not like to have dual filters in systems. There are a couple of reasons why. The first is that mechanical filtration encourages heterotrophic bacteria growth which competes directly with nitrifying bacteria for space. It also means that during the cleaning of this type of filter, the nitrifying bacterial growth is disrupted which can cause a spike in either or both the ammonia and nitrite levels in the system.
Many conversations have been had over the years discussing this very thing with all schools of thought. While that is my personal opinion, several facilities have been able to use these combination filters successfully. It should be noted that I am pretty risk-averse so many times a conservative approach is the best approach.
Water Quality Concerns
One of the details that usually gets lost in the maintenance of biofilters is the water quality changes that nitrifying bacteria causes during the reduction process. The actual process of reducing ammonia to nitrate is pretty well known. The actual species of bacteria that perform that task has been discussed but in general, the genus Nitrosomonas converts ammonia to nitrite and the genus Nitrobacter oxidizes the nitrite to nitrate. During that process, the pH is lowered due to the release of the hydrogen ion. This reduction in pH can consume alkalinity which needs to be mitigated with regular water quality maintenance. The choice of buffers should be determined by the type of system that is being used (ie. marine, freshwater, aquaponics, etc.)
Types of Biofilters
While there are a plethora of styles of filters available, we will only go over a few of the most popular.
Moving Bed Bioreactor
One of my favorite style of filters is the moving bed bioreactor. This style filter utilizes a neutrally buoyant media that is tumbled using aeration. This movement helps scour the media and keep it from retaining waste which would harbor heterotrophic bacteria.
There are several designs of both the media and the vessels that house the media but the concept remains the same. In general, the diameter (or width) should not exceed three times the height of the vessel. The aeration should be sufficient to keep the media in motion and is also sized to handle the oxygen demand of the nitrifying process. When properly sized, these filters keep the oxygen levels stable throughout the process and allow for efficient reduction of nitrogen.
A bonus for this type of filter is that it is one of the most robust styles out there. If the system pumps fail, the aeration will keep the bacteria alive. If the aeration fails, then the water movement will keep the bacteria alive. Lastly, if everything loses power, the media will stay suspended and typically the bacteria stay alive for longer than other types of filters. One of the drawbacks is capital cost. The media for these filters can be expensive depending on which brand is used. The size of the filter vessel can be a bit large in comparison with a fluidized bed sand filter. Or a trickling filter using polystyrene.
These are some of the oldest styles of filters still available. They are any vessel in any configuration that holds pretty much any style of media and has water running through it. It is the most flexible in terms of space and style. Some of the more common media used in these filters are Brentwood style corrugated plastic blocks, bio-balls, bio-barrels and some even use soda bottle rings and shaved PVC. Each media has different levels of available surface area for bacterial growth and is chosen based on availability and preference. Unlike the moving bed bioreactors, the trickle filter can be used in any geometry so long as the media stays moist.
The positives for this filter is its flexibility to fit the space and types of media that can be used. Some of the drawbacks for this style filter is that it can channel and collect waste. Over time parts of the filter can become unused as they dry out and they can be difficult to clean. Another concern to keep in mind is that any disruption in water flow can cause the bacteria to die off. A good back up plan with these filters is key.
Fluidized Bed Filters
These filters generally use an aggregate media with reverse water flow. In many cases, the media is a type of sand but other media types will also work. These are high surface area filters in which are helpful in applications that require a small footprint. We really only recommend using these in cool or cold-water applications with good mechanical filtration prior. The media can also capture solids which can change the dynamics of the filter and cause it to be unable to fluidize evenly which is why mechanical filtration is so critical. In warm water applications, the media can over colonize which also changes the fluid dynamics as well.
Some of the drawbacks to these filters is that when the bed collapses, the media tends to stick together and becomes anoxic which causes the bacteria to die. If the water flow is restored quickly, it can rebound. Another issue to consider is that the flow rate must be stable or the bed can either over expand, sending the media downstream of the filter or collapse. These filters have a tight tolerance for flow which must be maintained.
This is a type of filter that can work as either a biofilter, mechanical filter or both. They come in several configurations and tend to work efficiently as a biofilter. These filters can be either pressurized, like a converted sand filter with the reverse flow) or unpressurized. They are also available in a bubble backwash, prop backwash and what is also referred to as a drop backwash. They typically use a plastic bead media which comes in several sizes and shapes.
These filters work well in any temperature but will also collect solids. If they are being used for biological filters only, then proper mechanical filtration should be used prior. They are advertised as a combo filter so that is also an option.
Here comes the lesser spoken about details of biofilters. The process for sizing a filter includes total volume of feed, percentage of fat and protein for the feed and the water quality parameters that are expected for the system. In some cases, rules of thumbs are used based on the weight of the fish and type of fish which assumes an average feed rate and water quality. While this can work, it is a little dangerous to use this method since there are many variables that are ignored. A true mass balance should be done for proper sizing your biofilter.
In most cases, the result of these calculations is the surface area that is required. That being said, the surface area of media varies greatly by manufacturer. If you add that variability to the fact that some manufacturers advertise usable surface area (which is the surface area that remains open to bacterial growth) while others advertise total surface area, it can get challenging.
Think back to the ceramic media that was popular in the 1990s. The advertised surface area was listed at 82,000 ft2 per ft3. That surface area included all the void spaces within the ceramic. Once the bacteria colonized the media, the usable surface area is closer to the surface area of the outside of the media only, something close to 1400 ft2 per ft3.
If the filters using this media were sized based on total, they would underperform after the voids became clogged. The same is true of other medias as well. That is why it is important to understand the capabilities of the media that is chosen given the application that it will be used. This can make or break a project in a hurry if an unsuitable media is chosen.