Use of Floating Bead Filters to Purify Water in Koi Ponds

7/28/2019 Use of Floating Bead Filters to Purify Water in Koi Ponds

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USE of FLOATING BEAD FILTERS to PURIFY WATER in KOI PONDS

Introduction

Clear, clean waters form an essential linkage between koi and their keepers. Floating

bead filters and UV lights provide the modern koi hobbyist with the opportunity to

enjoy their fish without large burdensome filtration systems used by early koi

enthusiasts. An excellent clarifier, the floating bead filter also provides biofiltration,

while the UV light assures control of phytoplanktonic algae blooms in the sunniest

ponds. Simplified backwashing approaches prevent filter clogging problems while

constantly diverting captured solids away from the pond where they would contribute to

depression of the essential oxygen supply . The result is a well aerated pond where fish

can be clearly seen as they swim in waters that are free from toxic metabolites such as

ammonia and nitrite.

Bead Filter History

Bead filters, utilizing floating plastic media, have been used since the mid 1970's as

biofilters to support high density rearing of food and game fish in Idaho. Although

successful, the operation of these early air washed filters was poorly understood and

their use was limited. In the late 1980's, work at Louisiana State University

demonstrated that a hydraulic washed bead filter was capable of supplying solids

control and biofiltration for a high density catfish rearing system. These results

stimulated additional academic interest in bead filters. Development of mechanically

washed units (U.S. Patent Nos. 5,126,042 and 5,445,740) that were compact and simple

to operate overcame many of the operational difficulties experienced by earlier designs.Shortly thereafter, the favored "bubble-washed" or "hourglass" configuration (U. S.

Patent No. 5,126,042) was developed and tested for use on outdoor ponds. Since 1989,

bead filters have been tested on systems holding food fish (such as tilapia, catfish,

striped bass, trout), along with a wide variety of specialized applications (including

ornamental fish, alligators, crayfish, crabs, oysters).

Koi, with their natural habit of "mouthing" bottom sediments, assure that no sludge

deposits accumulate on the pond's bottom. The bead filters easily capture the sediments

once they are disturbed by the koi, resulting in their rapid removal from the tank. In

effect, the koi and bead filter work together to keep the pond clean. Both mechanically-

washed and bubble-washed bead filters can be used to purify the water in outdoor pools.The filters are similar in operation, providing both clarification and biofiltration for the

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pond's waters. Large pools supporting several hundred pounds of koi are most

commonly serviced using the mechanically-washed filters; whereas, smaller pools

containing a moderate number of fish are well served by the bubble-washed units and

their simple mode of operation. This document focuses on the use of the bubble-

washed units since the use of the mechanically washed units is well documented

elsewhere.

Description

Figure 1 illustrates the cross-section of the typical "bubble-washed" bead filter. When

viewed from the side, the unit displays an hourglass shape and is referred to as an

"hourglass" bead filter. The filter itself contains no moving parts. The filter casing has

three major zones: I) the filtration chamber, 2) the washing throat, and 3) the expansion

chamber. Water enters the filter through the slotted inlet pipe at the bottom of the filter,

passes upward through the bead bed that is contained in the filtration chamber and exits

through the slotted discharge pipe at the top of the filter. The inlet pipe also serves as

the sludge discharge line. The filter is also equipped with an air inlet line that terminates below the center of the washing throat. The filter is normally operated with four valves

(inlet, sludge, air, and discharge). However, check valves are sometimes used on the air

inlet and water outlet lines to simplify the backwash operation.

Figure 1.

The "hourglass" or "bubble-washed" bead filters use a bed of floating plastic beads,

providing for simultaneous clarification and biofiltration.

The beads used in the filters are spherical in shape, approximately 1/8 inch in diameter,

and are made from a food-grade, low density polyethylene plastic with a specific gravity

of 0.91. They float and fill the filtration chamber when the filter is operational. The

beads contain about 400 square feet of surface area for every cubic foot of beads that is

used for bacterial growth. The beads are very durable and never have to be replaced.

Theory of Operation

Bead filters function both as a solids capture device and a biofilter. They operate very

much like a submerged rock bed or undergravel filter except that they are designed to

facilitate cleaning. Bead filters dramatically simplify operations by providing two of the

most important water reconditioning processes, clarification and biofiltration in a singleunit. Clarification is the process of removing suspended solids from water. Suspended



solids in a pond consist of small particles of partially digested food, debris, algae,

bacteria, clays, and silts, which are small enough to stay suspended in the water column

for an extended period of time. Fine suspended solids tend to make the water opaque,

interfering with the clarity of the water. The larger organic particles, on the other hand,

represent a serious wasteload problem and can consume tremendous amounts of oxygen

while adversely impacting the hygienic pond ecology. Bead filters are consideredexcellent clarification units, capable of maintaining display quality water at high waste

loading rates. Bead filters remove suspended solids by four different mechanisms as the

recirculated pond water is passed through the plastic bead bed. Physical straining is

probably the most dominant mechanism for the larger particles (>80 microns). The bulk

of the suspended particles (20-80 microns) are probably removed by interception, a

subtle process caused by collisions between the particle and the bead media surface. The

finest particles (<20 microns) are removed by bead filtration, but at a slower rate. It is

believed that bioabsorption, the capture of particles by the bacterial biofilm, is the

dominant process attacking these fine particles.

Biofiltration depends on the development of a filter bed that encourages the growth of beneficial bacteria, which extract dissolved chemicals from the water and convert them

to particulate biomass or harmless dissolved compounds. Given the proper

encouragement, the bacteria grow in a thin film, coating the surface of each of the

600,000 beads contained in every cubic foot of media. There are literally hundreds of

species of bacteria at work in a biofilter. Most of the bacteria fall into the category of

"heterotrophic" bacteria, which actively break down organic materials into carbon

dioxide and water. The most critical however, are broadly described as nitrifying

bacteria, consisting primarily of the genera Nitrosomonas and Nitrobacter. Nitrifying

bacteria are responsible for the conversion of the toxic nitrogen forms of ammonia and

nitrite to the relatively harmless nitrogen form of nitrate . Success and a wide variety of

parameters that influence bead biofiltration have been identified. The pond hobbyist

should be aware of these factors. However, the pond itself generally contains a very

active population of nitrifying bacteria, so optimization of the filter's operation can be

relaxed.



Sizing a Bead Filter

Table 1 presents the relationship between the volume of a bead filter, feedrate, pounds

of fish, and pond size for typical applications. The bead filter sizes indicated are the

minimum recommended. Larger bead filters will not necessarily perform better, but will

require less frequent backwashing and have a larger safety factor.

Table 1

The minimum size of a bead filter for a Koi pond is primarily controlled by the daily

feedrate.

A properly sized UV light will assure control of algae blooms.

Bead Filter

Size (ft3)

UV Light

(Watts)+

Flow Range

(gpm)

Maximum Daily

Feedrate (lbs)

Amount of

Koi (lbs)*

Typical Pond

Volume (gal)

1 25 5 - 15 0.5 25 - 50 500 - 1000

2 40 10 - 30 1.0 50 - 100 1000-3000

3 80 15 - 45 1.5 75 - 150 1500-5000

5 120 25 - 75 2.5 125 - 250 2500-10000

10 240 50 - 150 5.0 250 - 500 5000-20000

+ Wattage requirements are approximate and will vary with quality of bulb and

flowrates.

* Aeration backup systems are recommended to protect heavily loaded ponds from

power interruptions.

Backwashing

Bead filters spend most of their time in the filtration mode (Figure 1). As the

recirculating pond waters pass through the bed, solids consisting of fish feces, algae,

zooplankton, debris, and biofloc accumulate, gradually clogging the filter. For most

pond applications, the clogging process generally occurs over a one to two week period.

Figure 2

Bubble-washed bead filters are designed to be self-washing when drained. The

discharge of the filter is equipped with a valve (or check valve) that prevents the backflow of air into the filter when the sludge (or drain) valve on the bottom is opened.



This causes a vacuum to form within the filter housing. An air valve on the side of the

filter is opened so that air can be sucked into the filter as it drains (Figure 2). This air

injection line is located below the washing throat, in the middle of the filter. This

constriction is critical to the washing operation. Water dropping out of the filtration

head causes the beads to fluidize downward and pass through the narrow throat where

they are scrubbed further by the rising bubbles. The washing process is complete oncethe filter is drained and all the beads have dropped into the expansion chamber (Figure

3). The next filtration cycle is started by readjusting the valves and refilling the filter

with the recirculation pump (Figure 4).

Figure 3 Figure 4

The energy for backwashing is generated from the hydrostatic pressure stored in the

filtration chamber. Draining alone is not sufficient to wash the beads; so, the bubble-

washed filter is configured to release waste while sucking in air below the washing

throat. The rate at which the water stored in the filter is drained, and thus the vigor of

the washing, is controlled by the siphon head formed between the elevation of the

internal end of the air injection line and the elevation of the first air break in the sludgedischarge line (Figure 6). Typically, hourglass filters are designed with a minimum

siphon head of about six inches, which is sufficient for most applications. If the filter is

mounted 12 to 24 inches above the ground, then a drop pipe can be attached to the

sludge discharge line and the siphon head can be increased to 12 to 18 inches, doubling

or tripling the rate of draining. This increased siphon head will translate into a quicker

and more vigorous wash. Drop pipes dramatically increase the filter's resistance to

clogging. Their use increases the amount of fish the filter can support without

shortening the backwash interval.

Backwash Frequency

The bubble-washed bead filters are generally washed once or twice a week during the

summer months in a typical koi pond application. They may be washed more often

without adversely impacting nitrification performance. If the filters are not washed, they

slowly clog, gradually shutting off the return flow to the pond. This decline in the return

flow is usually visually evident, providing a convenient reminder for the need to

backwash.

Bubble-washed bead filters perform better with frequent backwashing. Backwash

frequencies of two to four times daily are used to carry heavy commercial loadings.

Backwashing the filter once or twice a week is all that is required for most koi ponds. In



the cooler winter months, backwashing frequency can be reduced to as little as once

every two weeks.

Disposal of Backwash Waters

Bubble-washed filters are designed to lose 10-15 gallons of water per cubic foot of beads. The discharged waters will display the same high quality as the pond water;

however, it will be burdened with a suspended solids load of 500-1500 mg/l, reflecting

the scouring of the solids accumulated in the filter. These solids are generally already

well decomposed by the bacteria in the bead filter and are generally discharged directly

onto the lawn or a nearby garden. The backwash waters are usually enriched with

phosphorus and nitrogen (principally nitrates), thereby, acting as a low grade fertilizer

for the area of application. The solids show little potential for odor generation and are

quickly assimilated into the underlying soils.

Alternatively, the backwash waters can be discharged directly to the household sewer

line. The wash waters are entirely compatible with centralized or household treatmentsystems, which are based in a large part on the same bacterial communities that

effectuate treatment in the bead filters.

Algae Control

Bead filters can not eliminate algae blooms (green water) in ponds exposed to strong

sunlight. They are effective at removing suspended particles as small as 10 microns, but

cannot harvest the fast growing small 5- 10 micron algae well enough to assure really

clear water since they continually grow. To overcome this problem, the bead filters are

often operated with an ultraviolet light (W). UV units produce a high intensity of light

in the ultraviolet wavelengths which are very damaging to most living organisms. The

bulbs are capable of killing most bacteria and algae on their first pass through the unit.

The sizing of a UV light is critical to its success at combating planktonic algae. The

pond's volume must be turned over more rapidly than the algae can grow (i.e., pond

turnover rate must be greater than algae growth rate). During the hot summer months,

many algae that may inhabit a koi pond can divide every six to twelve hours.

Subsequently, the UV unit must be sized to process the entire pond's volume two to four

times a day with an intensity of at least 15,000 µwsec/cm 2.

Oxygen

Bead filters with their powerful biofiltration and clarification capacities tend to keep the

water in a koi pond very clean allowing pond keepers to substantially increase the koi

population. As their numbers rise, the fish and bacteria will increase their demand for

oxygen. At some point, additional aeration capacity will be dictated. Fish in heavily

loaded ponds should be watched for respiratory stress, particularly in the early morning

when dissolved oxygen levels are typically ebbing. Respiratory stress is indicated by a

lack of appetite or when a large number of fish are seen at the surface of the water

gasping. This problem will be most acute during the warmest days of summer went the

feeding rate is high and oxygen saturation levels in the water are lowest. The problem

can be remedied by addition of a supplemental waterfall, spray head or aerator to the

system. Two aeration systems are definitely better than one, preferably plugged intolines served by separate circuit breakers.

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A more subtle danger arises whenever the recirculation flow is interrupted. Suddenly,

the entire fish population must depend on the natural ability of the pond to supply

oxygen. Waterfalls, spray heads and air stones deliver oxygen 10-100 times faster than

the pond surface alone. Expensive fish or favorite pets should be protected by

maintaining a low fish population. If the fish population is too high to be supported by

the pond's natural aeration, a 12 volt aerator or a compressed oxygen tank should beinstalled for backup. A simple way to determine the need for a backup aerator is to turn

all your pumps off and observe the fish. If the fish come to the surface within a couple

of hours, immediately turn your pumps back on and consider a reduction in population

or a backup system. No damage to your fish will occur if the pumps are restarted as

soon as the fish are seen gasping.

Intake Screens

Intake screen design and placement is an important but often overlooked aspect of pond

design. Intake screens provided with submerged pool pumps are generally inadequate to

deal with the quantity and variety of debris that is associated with an outdoor koi pond.They can clog frequently, incapacitating the filter endangering the fish and reducing

aeration, while creating a maintenance headache.

Bead filters operate best when protected by a pre-screens with a mesh size of about 1/8

of an inch. This mesh size is commonly used in swimming pool screens and in-line

catch baskets. Use of an easily accessed inline catch basket on the intake side of the

pond's main recirculation pump will eliminate most problems with the pump and the

bead filter. But, for ponds in the vicinity of trees and shrubs, an in pond screen box or

slotted intake pipe is required for care-free pond operation. The intake structure should

be positioned 8-12 inches above the deepest part of the pond where leaves and debris

can settle below the intake structure without clogging it. The settled leaves can then be

removed at your convenience. Placement of the intake structure above the bottom will

not adversely impact water quality of the pond since the fine solids which control water

quality will be suspended by the koi where they can be removed by the filtration

system. A properly designed intake structure should require servicing no more

frequently than once a week. Off bottom placement not only reduces cleaning

frequency, but also, provides a safety reservoir for the fish should the wrong valve be

left on and the pond accidentally pumped dry (almost).

Acknowledgments

This work was funded, in part, by the Louisiana Sea Grant College Program, an element

of the National Sea Grant College Program, under the direction of NOAA, U.S.

Department of Commerce. Pond sizing information was reviewed by Mr. Burt Nichols

of Water Gems, Inc in Marion Texas. Dr. Ron James, President of Aqua Ultraviolet in

Temecula, California provided technical support for UV light sizing criterion.

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Use of Floating Bead Filters to Purify Water in Koi Ponds

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