THE INS AND OUTS OF KOI POND BUILDING
by Mike White, White Water Filters
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
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
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
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.
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.
1 - Planning a New Pond || Part
2 - Design || Part 3 - Circulation
|| Part 4 - Mechanical and Chemical
Filtration || Part 5 - Biological
Filtration || Part 6 - Biological
Filters - Mats, Pads and Biofalls || Part
7 - Biological Filters - Bead, Tower and Vortex Filters || Part
8 - Fluid Bed, Bio-Reactors and Nexus Filters || Part
9 - Planning for Pond Expansion
©2004 all rights reserved to Mike White