Water treatment for aquaria, aquatic mammals and zoological parks

Understanding and use of Media Bed Filtration

Mike Causer Howard

Dryden

Aquarium Filtration

• Biological filtration

• Mechanical filtration

• Water chemistry• Redox potential• Zeta Potential• Etc,.

Biofouling

Every surface in contact with the water will be colonised by bacteria.

•Every m³ of sand in sand filters has a developed surface area of 3000 m²

•Bacterial coagulation of the sand results in anaerobic areas.

•The anaerobic zones in the filters can produce unhealthy bacteria which will become a direct source of contamination in any aquarium.

Bio-mechanical design considerations• The darkened areas in all 3 of the standard designs shown below

illustrate poorly irrigated zones that tend to become anaerobic.• Homogeneity of filter beds are also compromised by pipework

traversing the filter beds.

Poor filter design will compromise performance resulting in:• Uneven flows across the bed• Media bed coagulation and anaerobic zones • Channelling of the media bed• Inefficient backwash performance

WORST

COMPROM

ISED

BETTER

Horizontal FilterVertical Filter with LateralsVertical Filter with Nozzle Plate Bed

Filtration performance

• Filter performance is inversely proportional to water flow in any media bed filter.

• If the environment in the filter is too aggressive flocculated particles will be broken up and will be pushed through the bed.

• At 15 m/hr filtration performance is more than 400% more efficient than at 30 m/hr

Backwash design considerations

• What goes into a filter must come out again.

• A 20% bed expansion is needed to allow trapped particles to escape from the bed.

• 55m/hr is needed in order to achieve this with sand (DIN standards).

Mechanical design considerationsPoor filter design will compromise backwash performance and structural integrity of the filter:• Full DIN standard filters respond to all design concerns and

are our preferred specification.

• During backwash in standard fibreglass nozzle plate bed filters.• the floor of the nozzle plate bed will rise.

• friction with sand will cause failure of the slip joint where the pipe passes through the nozzle plate

• Stress may be transmitted to the filter shell and may cause cracking and failure, especially in fragile bobbin wound filters.

• The bed is homogenous and uninterrupted by pipework.

• There is a large chamber above the bed for media expansion in backwash and for even flow distribution in filtration

BEST

Biofouling

Even with efficient backwash :

•it is impossible to remove all bacteria from sand when used as a filter medium.

•all sand filters suffer from biomechanical instability and wormhole channelling.

AFM an active filter media

• AFM was developed to replace sand used in sand filters for commercial aquaculture.

• AFM is an active catalytic media• AFM also carries a high surface –ve

electrical charge, high zeta potential• AFM will not support bacterial

growth.• By eliminating biofilm there is no

media bed coagulation or wormhole channelling

Electro-mechanical filtration for marine & freshwater aquaria

Clean sand

Sand after 3 days in a filterShowing the surface biofilm

New AFM

AFM after 5 years treating sewage effluent.

No biofilm problems

AFM

• At least 30% more solids are removed by AFM filters in comparison to sand.

• Performance of AFM is maintained because back-flush efficiency is 100%

• Sand filters will discharge retained solids and bacteria in pulses back into the aquarium resulting in increased turbidity and bacteria levels.

• AFM back-wash frequency and amount of water used per back-wash is reduced. Water saving 50% to 75%

AFM reproducible normal distribution on back-flush efficiency. Sand was a variable straight line on back-flush performance as measured by turbidity against time

Back-wash, time in seconds

Turb

idit

y N

TU

, in

filt

er

back

-wash

w

ate

r

AFM back-wash profile

Sand back-wash profile

Back-wash performance profile

Operational criteria

Run phase• Sand & AFM 15 to 22 m/hr

Air purge• Sand & AFM 50 to 95m/hr

Back wash• Sand 50 to 65 m/hr (DIN standards) • AFM 40 to 45 m/hr

AFM

• AFM will mechanically remove particles down to 5 to 10 microns• The surface charge on AFM will also adsorb micron and sub-micron

particles and even dissolved organics.• All particles in the water carry an electrical charge known as their

zeta potential.• The turbidity and clarity of the water is a direct function of the

electrical charge density or zeta potential of the particles in suspension.

• We can now control the zeta potential of colloidal solids. • to reduce the turbidity of the water • to further improve AFM filter performance• to fine tune any filter system

The mechanism behind it

AFM

• 30% to 80% more solids removed• Turbidity levels reduced by around 50% and visibility doubled• Reduced ozone demand in any fish system• Up to 90% chlorine demand reduction in marine mammal systems • Reduced loading on the biofilters reduces nitrate production,

reduced alkalinity consumption and improved pH stability• Back-wash water reduced by up to 75%• Energy savings approximately 15%• AFM should last for the life of the filter• Can be used in any good quality sand filter, but DIN specification

vertical filters are recommended for best results.

Summary