Gravitational filters, bottom drains and Airlift placement

Here you will learn the basics of this system.
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Gravitational filters, bottom drains and Airlift placement

Post by Pssymon »

Good day,

In this topic I will explain the basics of a gravitational filter. This is important as the gravitational setup is the main requirement for using an Airlift system.
Many pond professionals and veteran Koi keepers know about the concepts I'm about to explain, but for many newcomers they can seem complicated and are often overlooked.
I speak from personal experience as I've been keeping sturgeons in my pond for over five years now, and in those first years I never really learned about gravity systems and bottom drains.
And believe me, I read a lot of pond related material online. It's easily overlooked if you're not actively trying to learn more.

As most people will know, any pond that is stocked with a larger number of fish will need a filter. Especially when keeping Koi (which is what most serious pond keepers use their pond for), as they are fairly large fish that require a lot of food, produce a lot of waste, and at the same time are really sensitive to bad water quality.
How a pond filter works in detail, is something I'll get into in another topic.

Pump fed filtration

When using a pump fed filtration system, the filter is usually placed hidden away near the edge of the pond. A water pump is used to pump the water through the filter which then overflows back into the pond.
This is the easiest way to filter a pond. Especially when the pond is already dug, lined, and full of water and fish. Since a water pump can simply be placed in the pond with the hose going over the edge connecting to the filter.

I made a couple of simple drawings to show the differences between pump fed, gravity fed, and gravity fed with Airlift setups.

This is basically what a pump fed filter setup looks like:



While this setup is the easiest way to filter a pond, there are a number of drawbacks when comparing to a gravitational filter.
The pump being in the pond, usually has small slits or holes that prevent large pieces of debris entering the pump and blocking up the impeller.
While this does work, it means the pump will easily lose efficiency as the pump cover becomes covered with debris that sticks on there.

The debris that does fit through,can get cut up by the pump's impeller before reaching the filter. Making the mechanical filtration less effective.
Over time, these smaller particles will still clog up the inside of the pump. This requires having to get the pump out of the pond and cleaning it.
A pond pump usually does not suck in debris on the pond floor in a 360 degree radius. This means it's possible that areas with little flow occur around the pump on the pond floor where debris accumulates, where a drain would effectively get rid of the debris.

When using a UV unit on a pond with a pump fed filter, the UV unit has to be connected before the filter. This means that unfiltered pond water first passes through the UV unit, where particles or algae can build up on the quartz sleeve, making the unit less effective.

Another downside to this setup is the fact that usually your pump connection and power cord are visible over your pond edge.

Gravity fed filtration (pump return)

To use the Airlift system we must have a gravity fed filter. Basically this means we need a filter that is directly connected to the pond, and will retain the same water level.
In most applications this means that the entire filter is dug into the ground, but when connected to a raised pond, the filter has to be raised to the same level.

Contrary to the pump fed system, as the name suggests, the gravity fed system automatically fills the filter chambers by using the principle of communicating vessels.
This principle states that when gravity and pressure are constant in two or more connected vessels, the water level will always find an equal level.
In pond filter applications gravity and pressure in the filter chambers will naturally always be the same.

When filtering a pond we can use this concept to our advantage. The filter line can be connected to the pond by using one or more bottom drains.
A bottom drain is a large drain that is built into the pond floor when building the pond. The drain connects to a pipe which directly connects to the first filter chamber.
Since the drain is built into the floor, debris and dissolved ammonia can effectively be removed from the lowest part of the pond.
From the drain(s) water can flow into the first chamber, and from there into the next chambers. At the end of the filter line a dry mounted water pump is placed, which then pumps the clean water through the UV unit.
The flow is achieved by pumping out the last filter chamber, which causes a chain reaction where the previous filter chamber will start to fill it back up to even the water level, which causes the chamber before it to flow into it, and so on.
After the pump and the UV unit the water simply flows back to the pond where it returns above water level.

The advantages compared to pump fed filters are numerous, as I said before, particles are removed by the filter before they ever reach the pump, which reduces maintenance and is beneficial for the longevity of the pump. It is also easier to get to the pump. The UV unit does not become dirty as quickly, and will work more effectively as the UV rays can target the single cell algae particles that remain after the filter. These are the particles that turn the water green.
Using bottom drains provides more advantages such as the bigger inlet that will hardly ever clog, the ability to keep a large area of the pond floor clean without obstructions in a 360 degree radius, and the fact that it can be neatly built into the pond without any visible unsightly piping in and around the pond.

* Alternatively, side drains can also be used to connect to a gravity filter. This simply means going through the pond wall or liner, sealing it with a transit and connecting to the filter this way. A side drain won't be able to filter solid debris from the pond floor, and will usually take water higher up in the pond. This makes it less effective than a bottom drain, but it's a good solution to retro fit a gravitational filter with a dry mounted pump or Airlift.

Gravity fed system:



Gravifty fed filtration using Airlift

Now I can explain how an Airlift factors in to this.
The Airlift we use is designed for low power consumption and high flow rate. While we can achieve this the limiting factor when using this type of Airlift is the discharge head, and the water supply diameters.

Here you can see two concept drawings I made to show how an Airlift connects to a gravity fed filter:



When using a gravity fed filter, you can eliminate discharge head completely.
Secondly, the Airlift requires a large diameter water supply with as little obstructions as possible. Bottom drains are a great way to achieve this, as they often come in 110mm. Under normal circumstances a 110mm bottom drain should be able to let through 8000 liters per hour when running an Airlift. This may seem a low estimate but you have to consider that the Airlift is pumping with little force, unlike a similar performing water pump, you can easily halt the flow with your hand. This also explains why the discharge head needs to be low.
The good news is that the system does not need to pump with a lot of force. We only need the large water displacement through the filter, which is exactly what the Airlift will do.

After the last filter chamber, the Collector is placed in the ground. Usually the Collector is about two meters long, to provide enough space for the Airlift. This depth is needed for the Airlift to perform at optimal efficiency. Changing the length of the Airlift will change the flow rate and energy consumption. For the standard model with a 20W Secoh air pump attached, the best length is either 170cm or 145cm depending on the required flow rate (15.000 litres at 9W, 20.000 litres at 12W).

Compared to the gravity fed filter with a water pump return, mentioned above, there are a number of advantages.
The saving on energy consumption is the most important factor for most people. As many Koi ponds require to have the complete volume of the pond to be filtered every hour or two hours, you need a big pump.
Comparing a water pump that is rated to pump 20.000 litres, to the Airlift we use, shows that the Airlift is over 9 times as efficient, an energy saving of 89,1%.
I compared this to the most efficient pump I could find, which pumps 20.000 litres at 110W. Every other water pump on the market uses more energy (to achieve 20.000 litres).

Another thing is to consider is that you are using your air column to pump water. Your oxygen pump which is normally working alongside the water pump is now doing two jobs. So actually the energy saving is even greater.
Also,there now is one less pump to go wrong and break down. And a proper air pump will run for many years without failing.

Then there is the possibility to skim proteïn with the Airlift. Although this can be a bit tricky to achieve and requires a couple of extra modifications.

I have left out the UV unit in the Airlift drawings, because the conventional UV sterilizers have an enclosed case, and are normally dry mounted outside of the pond.
If we would use such a unit with an Airlift, the flow would suffer considerably. For this purpose it is better to use a submersible UV unit, which can be placed in a transit tube or in the collector next to the Airlift. The placement should not considerably restrict the flow, and can vary depending on how the filter is constructed.

The downside of using Airlifts, when the conditions above are met, is probably just the placement of the Collector which can be a bit tricky.

I hope this topic has cleared some things up for members who are new to these concepts.

Greetings :)
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Re: Gravitational filters, bottom drains and Airlift placeme

Post by Pssymon »

I just noticed that the images in this topic do not appear properly on my phone, so mobile devices will probably display only about half of the image. I'll look into the problem, but for now, I advise readers to use a pc / laptop to view this topic.
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Re: Gravitational filters, bottom drains and Airlift placement

Post by gilespye »

Hi, I am a complete newbie to airlifts, and actually to bottom drains also.. Thanks for a great website and explanations.
I am digging a pond, for my aquaponics fish, but which will also have quite a few plants, lilies etc.
I am wondering whether to install a bottom drain, but for sure I would like to use air lift technology.
From your tutorial, I dont understand why the return has to be below water level.
Does this mean that I cannot return the water from the air lift system through a waterfall, about 2 feet above water level?
Thank you,
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Re: Gravitational filters, bottom drains and Airlift placement

Post by Pssymon »

Hi Giles,

Using bottom drains gives a huge advantage. I am currently running a pond and two pools without them and it's a big hassle. Using bottom drains eliminates the risk of blockage and a drain is much more efficient in pulling in debris in a 360 degree radius. When using airlift, remember that each drain can only give about 8000l an hour reliably. So I would use a minimum of two drains. Remember to use the least amount of bends in your pipes as possible. Try to never use 90 degree bends, use 45 degree bends instead. Alternatively you can use side-drains which also give all the benefits of a bottom drain, except for the fact that you will not be able to pull water from the bottom of the pond, causing debris to settle.

The return absolutely has to be exactly up to, or preferably 3-5cm below water level. Discharge head requires power. It's very simple. Conventional pumps lose less output when you increase discharge head. With an airlift, output plummets dramatically. It is nearly impossible to match a water pump in efficiency when trying to run a waterfall or raised filter for example. That is the only downside of using airlifts and I would strongly advise to abide by this rule. 2 feet is ridiculously high for an airlift, it would not work.

If you have to have a waterfall, you can do what I plan to do. Run your filter gravity fed with an airlift. Then run the waterfall with a small energy efficient water pump with a large diameter hose. This way you can filter extremely effectively at a really low energy consumption and still have an ornamental waterfall that has a relatively low running cost.
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Re: Gravitational filters, bottom drains and Airlift placement

Post by damolaonline »

I am not fortunate to dig a metre whole for an airlift? what options do I have?
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