PART 6: Mats, Pads and Biofall Filters

In the last issue, the topic covered was biological filtration basics. This article will begin to cover the actual filters on the market.

To begin, there is no perfect filter. We will begin with the most popular biological filter, the biological waterfall. Many companies manufacture these units. They all have a similar design. This is an upflow design, meaning that the water comes in the bottom and then flows up and out of the unit. The output of this filter is designed to be the beginning of a waterfall. The unit is comprised of either a plastic or fiberglass container. Two or three polyester pads are placed horizontally in the container. These are supported off the bottom by pipes or a rack system. Water flows in under the pads and is supposed to flow up through the pads.

Bags of rocks or something similar are set on top of the pads. The manufacturer says this material allows for more filtering to occur. In fact, they are there to hold the pads down to prevent the water from pushing the pads up. Some manufacturers provide a container for plants at the top of the unit.

Let’s first consider the down side of these units and then take a look at the up side. The first problem is using an upflow design with polyester pads. As the pads start to grow bacteria they begin to clog and the water finds it is easier to go around the pads than to go through the media. This is because the water pushing up helps to push the pads away from the walls of the container. With no water going through the pads, the bacteria are deprived of oxygen and die. Then there is the matter of the bags of material on top. There are usually two to three bags for media. Again, water is usually going to go around the bags of media instead of inside. In addition there is the size of the container versus the flow of water through it. In most cases the water flows through the container in a minute or less. At that rate the bacteria that converts nitrite to nitrate can’t survive.

Although I stated I would include the positive features of this type of filter, there really aren’t that many. While there is some filtering occurring, it is not much. The bacteria can grow on the walls of the piping going to the unit, the walls of the container, the top and bottom of the pad assembly, and the sides of the bags. If there are plants in the top, you will get some benefit from them. The problem is that this isn’t much surface area so not a lot of bacteria is growing.

Next let’s talk about downflow filters with pads. One manufacturer of some of the best is Patio Ponds, Ltd. They produce two very good ones with polyester pads and one with Matala pads. The Big Sister and Little Sister filters have been around for years. The Vista came on the market last year.

First let’s cover the Big and Little Sister filters’ design. These two filters are very similar with the differences being the size of the container and the material they are made of. They have two chambers. The first contains a spray bar that the water from the pond is pumped through. The water travels horizontally through this chamber, passing through a barrier of brushes. It enters the next chamber by going over a weir. The water then goes down through pads to the exit pipe and flows back to the pond.

First let’s review the good points of these filters. The water enters these filters by going through a spray bar. This adds oxygen to the water. As discussed earlier, bacteria needs lots of oxygen. The water passes through brushes removing most of the debris in the water plus providing a large surface area for bacteria to grow. Next the water flows into the second chamber, going through the pads to exit the filter. How, you might ask, is this different from the example of the waterfall filter?

In the downflow design the manufacturer is using the movement of the water to force it through the pads and not around them. As the pads get clogged up, the water pressure pushes the pads down against the bottom of the container. This seals the pads against the container and prevents water from going around them. This means that the bacteria are now using all the inner surface area of the pads. What happens if these pads get totally clogged up and water can’t get through? The design of the filter is such that the water goes through the pads to the open end of the exit pipe. The water then goes up this pipe and out of the filter. If the pads get clogged up, the water in the chamber rises to a point where the top of the exit pipe is located. Here the water is allowed to flow over the top and out of the filter. This also indicates when the filter should be cleaned. The bottom of each chamber is sloped to drains. Valves are mounted on these drains, allowing debris in each chamber to easily be drained from the filter by opening the valves. The spray bars regulate the flow rate of water going through the filter. So the minimum time the water takes to get through the filter is at least 5 minutes.

Now let’s talk about the down sides to these filters. There are two that I can think of. First, the larger of the two filters is good for a maximum pond size of 4500 gal. Second is that the water coming out of the filters has to flow back via gravity. Because of this the filter has to be above the pond level, meaning the rectangular box needs to be disguised or hidden.

We’ve talked about polyester pads being used in upflow and downflow filters. Next is a filter that uses polyester pads in a horizontal flow filter. Last year, Emperor Aquatics came out with the Hydro Max filter, using polyester pads in a horizontal design. Water from the pond is pumped through a spray nozzle. The filter is one chamber. Water passes horizontally through a row of brushes then through a series of pads, proceeding from coarser to finer. The pads are held vertically in the filter chamber by slots molded in the sides and bottom of the container. Similar to the downflow filter, as the pads get clogged up, they push tighter against the slots and seal to prevent water from flowing around them. If the pads get too clogged to let water through them, the chamber fills and the water goes over the top of the filter, flowing out a drain pipe back to the pond. The last part of the chamber is designed so one or two UV lights can be fitted into the filter. This filter can handle a 3000 gal pond.

The down sides to this filters are the same as those stated for the Patio Ponds filters. As these filters are new to the market, there may be problems once the pads become older (1 year or more). My thoughts are that once the pads are older and lose their stiffness, they may start bowing in the center, pulling the edges of the pads away for the slots, letting water go around them. If this is the case, the pads would need to be replaced before they are in actuality worn out.

It is apparent that depending upon the media used, different types of flow design can work well or may not work at all, as I have shown with the upflow with polyester pads versus downflow or horizontal flow with the same type of pads. This is true with most medias used in filters. Depending upon the type of media will determine which flow design works best. This doesn’t mean that another flow design won’t work but it may not work as well or may present other problems.

In the next issue I will continue to talk about filter types. We will cover filters such as bead filters, trickle town, vortex, fluidbed sand, bio reactors, and other types. This will probably take more than one article as there is so much material to cover.

©2004 all rights reserved to Mike White