Trickling Filters

The "Do it Yourself Biofilter"

The beginning aquaculturist needs a simple, effective and inexpensive biofilter that will allow them to build a recirculating system without breaking the bank.  Trickling filters fill that role perfectly.  Trickling filters are one of the oldest types of biological filters.  They are still the most common type of biological filter for several reasons.  They are reliable, simple, effective, rugged and inexpensive when built properly.  There are much fancier, complicated and sexy biological filters available for aquaculture but fancy, complicated and sexy typically mean more money.  In the late 1800 the first trickling filters were built for sewage treatment.  These filters were filled with rock or coal.  Incredibly, some trickling filters are still built with rock or gravel today.  Unfortunately, the high capital cost and maintenance costs associated with those old types of filters lead some people to think that trickling filters are an outdated and inefficient type of biological filter.

Modern systems are much more efficient and less expensive to build than the old gravel filled types.  A modern trickling filter utilizes light weight plastic packings or media rather than rock or gravel.  The water to be treated is sprayed over the top of the media and collected in a sump underneath the media.  The surface area provided by the media or packing provides the substrate for the growth of a biofilm.  In some systems, air is forced into the filter with a fan.  However, most small aquaculture filters rely on natural convection and diffusion to move air throughout the filter.

Trickling filters are rugged and easy to operate.  They can be very simple to build.  Trickling filters are completely scalable; they can be built to handle water flows from 4 GPM to 4 million GPM.  They have the ability to treat a wide variety of nutrient levels.  Properly designed systems can handle solids very well. One of the big advantages of a trickling filter is that the water can leave with more oxygen than it entered. Because trickling filters have a large - air water interface, they also act as strippers to remove CO2, H2S, N2 or other undesirable volatile gases.  Very few other types of biofilters perform all these functions.  There are only two minor disadvantages to trickling filters.  One is the energy cost required to pump the water to the top of the filter. However, trickling filters are typically more energy efficient than bead filters. The other disadvantage to trickling filters is their size.  They are larger and take more space than some other types of biofilters.

The first step in the design of a trickling filter is to pick the right packing or media. Over the years many different materials have been used for trickling filters but today, the best packing is structured media.  Structured media is composed of sheets of rigid PVC that are corrugated and glued together to form blocks. This media is very lightweight but very strong.  A cubic weighs about 2.5 lbs.  A cubic foot of gravel with an equivalent amount of surface area will weigh over 100 lbs.  For an in depth review and analysis of packing materials, refer to the paper "A Review of Biofiltration Packings".

Here is an example of a typical structured media with a few sheets cut away to show the internal construction.

There are numerous types of structured media so there is a great deal of flexibility in the design and construction of trickling filters.  One of the big advantages for aquaculturists is the ability to build a biofilter without a vessel. Since the vessel is typically the major cost of a biofilter, a biofilter with no vessel can be a real money saver. Structured media can be stacked on a frame or any flat surface.  It can be located over a culture tank or have its own water collecting sump. No sides are required because the packing is self supporting. A minimal frame is recommended for permanent installations. 

 The most important design consideration for any trickling filter is a good water distribution system. In order to achieve full performance, the water must be distributed evenly across the top of the media.  There are two common ways to do this.  A pressure spray system with splash guards at the top is probably the simplest.  The only drawback is the additional pressure drop required to operate the nozzle. 

Here are two examples of solid cone, square pattern nozzles with standard pipe threads.

These nozzles are easy to use and inexpensive to buy.  You can see flow rates and prices on the web site at Nozzle Information and Data

The other system involves the construction of a shallow, water distribution pan with several gravity flow nozzles in the bottom of the pan. Here is a simple schematic.

Here is a picture of the gravity flow nozzle. 

These "vessel less" trickling filters are extremely simple to build and operate.  They are truly do-it-yourself biofilters.  Any aquaculturist worth of the name can build one.  The ability to glue PVC pipe is probably the most difficult skill required.  Very minimal carpentry skills are needed to construct a spray shield from plywood.  For longevity of supports and spray shield one should probably either fiberglass the wood or use plastic lumber. However, the choice of construction materials is left to the discretion of the builder.

Sizing the Trickling Filter

The big question in most peoples mind is: "How big do I make it?"  The sizing of a biofilter starts with three basic pieces of information:

1.         Maximum quantity of feed that will be used per day.

2.         % crude protein contained in the feed.

3.         The desired ammonia concentration in the culture tank.

For a simplified, step by step procedure on how to size a trickling filter, go to Sizing a Biofilter.  The procedure outlined there will give a slightly undersized estimate of the size required.  For a more accurate calculation, drop us a note at info@biofilters.com.

There are a few important things to remember when designing and building a trickling filter.

1.         Be sure that the water is evenly distributed across the top of all of the media.  If you don't wet the surface of the media, the bugs won't grow there.

2.         Be sure that you have a minimum water loading rate of 4 gpm/ft2 of plan area.  Higher water loading rates will make the filter more efficient.  Practically speaking, you can't put too much water over the top of the media.

3.         Put the trickling filter in a place where it will be well ventilated.  Trickling filters exhaust CO2 and a build up of CO2 in a building is dangerous for people and unhealthy for fish.

4.         Crisscross layers of media when you build the filter.  This helps the water distribution and makes the stack of media more structurally secure.

5.         Structured media is relatively cheap.  It is better to slightly oversize your filter than undersize it.  Fish don’t die because the biofilter is too big.

6.         Shorter turnover times for the water in the culture tank generally results in better water quality.  When in doubt, use higher recirculating flow rates.  Never restrict the flow out of the recirculating pump with a valve.  It just wastes energy and reduces the effectiveness of the system. If a particular component can't handle the full flow from the pump, then use a partial bypass around it. 

            Trickling filters are the easiest, cheapest and most effective biofilter that you can build.  They are truly the "do-it-yourself" biofilter.

©2003 by L. S. Enterprises.  All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Published by L. S. Enterprises

PO Box 13925

Gainesville, FL  32604 USA

Author: Matt Smith

Office 1-352-379-5626

Mobile 1-239-851-1175

Fax 1-866-706-1775

Email: mattsmith@biofilters.com

Ver 8/07/2013