Backyard Aquaponics
A place to discuss aquaponics

One of the first questions that many new APers ask is “what is a good pump to run my system”. The thing is that this should really be one of the last things you consider because in order to know what pump you need you first have to know how high it needs to lift the water. It is very hard to know this until you have designed the drains.

Before I continue a couple of things I want to make clear. The first is that the way I approach AP system design is way more complicated than it needs to be for backyard AP (BYAP) systems. If you are new then your best bet is to read some of the great sources of information like the “IBC of Aquaponics” and copy one of the systems there or copy one of the forum members systems that are detailed in numerous members threads. Though if you do the latter choose a thread that has a long history and read the entire thing so that you know the problems they have had and what they have done to fix them(ie don't copy mine).

The easiest way to work out your drain design is to guess (what most people use to do and I think what most new people still do) but that is rather risky. The second easiest is to copy someone elese’s (what most people should do) as long as it worked for them it should work for you. The third approach is to just add more vertical height to your drain but even using this quick fix can cause new people to come unstuck. Having said all that how do you design a drain?

First you need to know what is your flow (Q, L/hr, m3/s) through your drain. The challenge is that you will not know what it is. If you have read the other threads I’ve done recently (search “xtutex”) you will know that we want to have a flow greater than the FT volume once per hour (If you haven’t you need to because I’ll be referring back to them). With drain design you really need a more precise value but the problem is you can’t give one because you don’t yet know the vertical height that you need to include to allow the drain to work which will effect how much water the pump that you haven’t specified yet will deliver. Normal people call this annoying, engineers call it iteration.

Based on your experience (which may be none) you choose a vertical height that you think will do, which is why I did the explanation on pump pipe and pump sizing before I did this explanation on drain design.

Before I get into the fun stuff (math) we need to go through a number of concepts.

The first is effective fall of the drain versus the height through which the water falls.

The effective fall of a drain is the vertical distance from the top of the top water surface to the top of the bottom water surface with no break in the structure that is holding the water column between the two water bodies. I’ve just written that out three times and it’s not clear to me even though it’s accurate.

Another way to describe it is the depth of the water column in the drain.

Really need pictures to get this across.

This first comparison shows that by increasing the length of vertical drain pipe the depth of fully filled pipe and hence the effective fall of the drain are increased. The effective fall of the drain in example 2 is the same as that in example 3. The comparison of examples 2 and 3 shows that increasing the depth of water column in the drain (example 2) is the same as taking the drain from deeper/lower in the tank.

Examples 4 and 5 shows that it is the difference in the levels of the top water surfaces that is important. In particular example 5 shows why drains that are full of water have a larger effective fall than those that are partially full of water.

Effective fall is important because it is half of the equation to work out how well a drain works.

In a drain there is no pump moving the water it is just moving through the drain due to gravity. The effect of gravity moving the water through the drain is countered by the friction between the water moving through the pipe and the pipe walls. Since friction increases with velocity when water starts moving through the drain it speeds up until the force due to gravity is balanced by the opposite force due to friction.

This is expressed by the formula:

Zf-Zdp=0

Where:

Zf=effective fall of the drain;
Zdp=losses due to friction or essentially the effective negative “fall” due to friction.

This is where it starts to get tricky.

We have to start with an assumed flow of water. So we will begin with the values that we used in the example used in this thread:

http://www.backyardaquaponics.com/forum/viewtopic.php?f=14&t=21099&hilit=xtutex

So we have a pump that will deliver slightly more than 6000L @ 1.42m per hour of total dynamic head but the thing is we have only calculated the dynamic head based on a flow of 5000L/hr. So we need to run the calculations based on a larger flow.

So we pick a flow between 5000L and 6000L and see how close we get. Lets give 5500L/hr a go.

Area of pump pipe is:

App=(πD^2)/4
=3.142 * 0.0516 * 0.0516 / 4
=0.002m2

Q=5500L/hr=5500/1000/60/60=0.00153m3/s

V=Q/A
=0.00153/0.002
=0.765m/s

Zpp=(KV^2)/2g
K=17.8
V=0.765m/s
g=9.81m/s2


Zpp=17.8*0.765*0.765/2/9.81
=0.53m

Dymanic head of Zpp added to the static head of 1m gives a Total Dynamic Head (TDH) of 1.53m.

So looking up the pump curve for the Laguna 7500 we see that actually at 1.53 it will produce 6000L

Running the calcs again (good idea to do yourself a spread sheet for all this)

We get a TDH of 1.58 which gives us a flow just below 6000L/hr. So now we have the flow bracketed we can go to the next step.



The drain shown here is what you would use from a centre drain aquaculture tank with a standpipe.

Same process as before:

1. Start with a flow, 6000L/hr in this case.
2. Add up the fittings: 1 entrance (K=0.5), 90 degree bends * 4 (K=0.5ea in DWV range), 1 butterfly valve (K=0.2, fully open) and one exit (K=1).
3. Total pipe length of 3.85m.

We need to choose a drain pipe size.

50mm was fine to get the water to the fish tank is it the correct size for the drain?

Before you run the friction loss calculation there is another calculation that you need to run because drains carry solids from the fish tank. In pipes from pumps it is a simple case of the bigger the better. The only thing that limits how big you go is really the cost of the larger pipe and fittings. With drains the bigger you go the more likely is the risk that the solids will settle in the pipe which is something you really want to avoid. So that this is not a problem and so that you don’t need to regularly clean out a drain you need to maintain the speed of the water in the drain higher than the minimum flushing speed so that the solids won’t settle in the pipe.

The formula for this is:

V>SQRT(gD)0.58

Where:
V=velocity (m/s)
g=9.81m/s2
D=internal diameter of the pipe (m)

So for a DWV 50mm with an internal diameter of 51.6mm the velocity in the pipe needs to exceed:

V>sqrt(9.81 * 51.6 / 1000) * 0.58

V>0.413m/s

Since our velocity is greater than this we now know we won't have settling in the pipe.

So we now calculate the friction loss.

K for the pipe is based on the Reynolds number (R=VDρ/μ) of ~41,000 which gives a friction factor of ~0.033 (See Fanning Friction Factor Moody Diagram).

Kp=4fL/D
=4 * 0.033 * 3.85 / 0.0516
=9.85

Total K value is:
Ktdp=Kp+Kf
=9.85+0.5+0.5*4+0.2+1
=13.5

Zdp=(KV^2)/2g
=17.02 * 0.797 * 0.797 / 2 / 9.81
=0.406m

Remember that:

Zf-Zdp=0

Or an easier way understanding what result we are trying to find is:

Zf(measured)>Zdp(calculated)

As long as the design fall (Zf) is greater than the calculated friction losses (Zdp) the drain will work better (higher flow rate) than as designed/calculated.

In our sketch we had a fall of 500mm so we can use 50mm drain.

Using a 50mm pipe meets this criterion but what about the standpipe? Not many BYAPers use this sort of arrangement but it was used extensively in RAS (and still is in older systems) and is a good simple design (but there are better newer commercial ones). The velocity can not be too low or the solids won’t be carried up inside the standpipe to the drain.

The way to work this out is to choose an outer pipe, say 65mm, and then work out the internal cross sectional area of the outer pipe and the external cross sectional area of the inner pipe.

Di(65)=63.6mm
Do(50)=56mm
Ai(65)=0.00318
Ao(50)=0.00246

Then we subtract the outer area from the inner to work out the area free to transport water (effective area):
Ae=A65-A50
=0.00318-0.00246
=0.00072

The next step would be to work out the velocity of the water through this area but since the effective area is so much smaller than the cross sectional area of the 50mm pipe we can be assured that a 65mm pipe is too small.

So we run the calculation with 80mm pipe:

Di(80)=76.2mm
Do(50)=56mm
Ai(80)=0.00456
Ao(50)=0.00246

Ae=A80-A50
=0.0021

We can see that Ae and Ao(50) are similar to each other so 80mm will be fine to use but just to be sure:

V=Q/A
=0.00167/.0021
=0.79m/s

The problem with this is that the velocity is a bit higher and that will mean more friction loss. Calculating this friction loss is not a simple matter and I haven’t found a methodology that I am happy with. However, standard practice is to not worry about it and if the drain doesn’t work to cut the inner pipe shorter until it does work

So we are done.

Not quite

This is the combined diagrams of the pump pipe and the drain added together.



This is a pretty standard CHIFT PIST (Constant Height In Fish Tank with the Pump In the Sump Tank) setup where running the GB on a flood and drain cycyle.

One of the problems with these setups is having to raise the FT and GB or lower the sump or do all three. The water level in the sump has to be lower than the bottom of the GB in order to get it to drain fully so we cant change that but if we used a larger pipe could we lesson the fall between the FT and the GB.

Let us see...

First check that the water won't be moving too slow.

V>SQRT(gD)0.58

Where:
V=velocity (m/s)
g=9.81m/s2
D=internal diameter of the pipe (m)

So for a DWV 65mm with an internal diameter of 63.6mm the velocity in the pipe needs to exceed:

V>sqrt(9.81 * 63.6 / 1000) * 0.58

Vmin65>0.458m/s
Vmin80>0.501m/s
Vmin90>0.533m/s

Then we check the velocities with 6000L/hr flowing through the drain.

V@6000L/hr65=0.525m/s
V@6000L/hr80=0.365m/s
V@6000L/hr90=0.286m/s

From these checks we see that 65mm is the largest we can use.

Running the the calculation for friction loss:

R=VDρ/μ
=0.525 * 63.6 / 1000 * 1000 / 0.001002
~33,000

Gives us an f value of f=0.034

Kdp=4fL/D
=4 * 0.034 * 3.85 / 0.0636
=8.23

Ktdp=Kp+Kf
=8.23+0.5+0.5*4+0.2+1
=11.9

Zdp65=(KV^2)/2g
=11.9 * 0.525 * 0.525 / 2 / 9.81
=0.168m

What this means is that we can shrink the difference in height between the FT and GB by about 240mm (Zdp50-Zdp65=406mm - 168mm=238mm)

If you are digging your sump into the ground this means that the depth of the hole can be shrunk by 240mm. Plus there is still a fair bit of lee way because the FT is not completely full.

So by understanding how to design your drain you can make the decision to either spend some more money on some larger drain fittings or dig your sump deeper.

So now are we done? No.

If you decide to not dig your sump in deeper you have reduced the static head by 240mm you now need to go back to your pump curve and check what the flow will be at a the reduced total dynamic head. Since reducing the TDH will increase the flow rate you need to check that the increased flow won't be so much to overload your drain.

You keep going through this process until you get close enough or sick of repeating the calculations and you always add in some fat to allow for things not working as planned. Just like in the case of the standpipe where it can be adjusted after it is built.

Designing your system in this way will not ensure hassle free operation but it will get you in the ball park and can save you money, effort or both.

Thanks for this body of work Stuart, truly valuable
Can I ask you for an eye over my drainage set up.

I am sinking the FT this weekend and cant seem to get my head around which diagram you supplied in this thread represents mine (its the u turn mine has that has me confused)

I dont have much room to play with in relation to head height and obviously cant afford an error here, there is no way I will be able to move the tank once in place
See attached for what I hope is all the information you need.

Ps I just noted an error in one of the measurements there is only about 10 meters of drainage pipe not 14 as stated

Without starting up the cray super computer and with only the use of the "looks right principle" I reckon the 20mm down leg should be 32 or 40mm. the FT water level only needs to be 200 or 300 below the base of the GB's likewise for the FT to ST.

Where to measure the Z distance depends on a number of factors.

First does the return line to the FT stay full of water. Obviously the bit under ground does but what about the inverted U over the edge of the FT? If it does then the bottom point of the Z or fall is measured from the outlet of the pipe. If it doesn't remain full then that means that there will be a free passage of air in the pipe outlet, up the pipe and in the top of the inverted U over the side of the FT. In this case the place to measure the bottom of the fall from would be the top of the horizontal section of pipe of the top of the FT wall (conservative) or more accurately the water level in the horizontal section of pipe.

Hopefully to be clearer if the pipe is full then either 1, 2 or 3 would apply:

If the drain is not completely full then 5 would apply:

4 would only apply if the outlet of the pipe returning water to the FT was below the water level in the FT and as you have drawn it it is not.

The top of the Z value or fall or depth of continuous water column is the height of the water level in the GBs at any particular point of time (accurate), at its maximum when the GB is full or for the sake of conservatism the bottom of the GB or rather where the GB empties to.


That the fall should be 300mm sounds like someones rule of thumb that has worked for them. I guarantee you it will be more than enough in some situations and no way near enough in others. On the other hand I'm sure it would be fine in some situations to

In response to Slowboat the mathematics involved in these calculations is relatively easy. What is hard is understanding the theory and what the numbers mean and why they mean what they mean.

I like doing the calculations because it removes so much of the guess work. For example we don't know what flows we are dealing with so there is no way of knowing if the pipe sizes are appropriate or not. Slow boat has suggested that the down legs need to be 32 or 40mm but I could never recommend a pipe size without knowing what the flow through that pipe needs to be.

i appreciate your love of math Stu but what are you trying to achieve here, its not like we are trying to land an aquaponics system on a comet is it.

Stu I'd like to check your math but after being taught thermodynamics by a calculus fanatic, math by me is only done at a rate of $100 / hour or more.

You are right I have no idea what flow rate the Judester is planning and I have not used bell siphons but even though he tricked me with his diagram (showing 5 GB's and saying 10 GB's) having a 25mm supply pipe and a 20mm drain pipe was an obvious fail point to me.

So judester how big are your GB's going to be, it looks like a good design to me albeit with the drain size typo.

So I basically need to set it up and then see if it is 'too much' 'not enough' or 'just right' in my situation.
Sounds like a disaster on its way


I dont know how I am going to know if the return pipe (horizontal and u section is going to remain full).
The GBs will drain independently so with a 90mm pipe I believe no

What I do know is that in my testing on a single bed (with NO medium-clay)in which I was trying to work out the min and max flow rates at which this siphon worked



These ranges enabled the siphon to work consistently:
Supply filled the 230mm high IBC GB (without any medium-stones) between 18 and 25 min
Once the siphon kicked in the beds were drained between 4 min6sec and 3min25sec.

This told me the 20mm drain leg down from each GB would suffice.
This also tells me that I need a pump size that will deliver this amount of water to all 10 beds which effectively gives me a starting L/hr pump size to work on (But im told pump is one of the last things to consider)

Slow boat: the GBs are set in a frame in which there will be 2 IBC containers together, each will have their own siphon. I dont do math so hence my probing before I destroy something. Original drawing has green line in top view that represents the edge of the 2 GBs sitting next to each other.

Stuart: To play 'safe if I should assume the horizontal section will have air in it. Then I can use the emptied GB to the horizontal pipe as the drop calculation. Aim for 300 ish for this measurement on Friday

If this fails I can then reduce the size of pipe in the U turn and horizontal to hopefully keep it full.
(That is really another question)

Thanks again for your time... you know I will be returning for pump calcs next dont you

The idea of the calculations is to save you time and money. I wasted hundreds of dollars on fittings and have been left with a few unfixable problems in some of my systems because I didn't know how to determine what fall I needed and what pipe size I should use. The suck it and see method works just fine if you can afford it. I much rather recommend people copy someone elses design or learn how to do the calculations. Although I would mind if they paid me to do it for them

Doing the engineering will not land you on the comet to coin Scotty's phrase but it will get your dart stuck in the board. Every system should probably have the means to adjust flows through standpipes, valves or some other means so that the plumbing system can be tweaked. Once running perfectly it may still need adjusting at a later date as bio-films, plant roots and other things effect the flow.


An engineers response to that is to assume it will only be partially full. If only one GB is likely to empty at a time then it most likely will not fully fill. However, you can ensure it stays full by putting a hook at the end of the pipe.


Testing without media is a good idea but once you put the media in everything changes.

The hook idea seems so simple why didnt I think of that .

Another 2 elbows on the FT side it is.

Awesome... Thanks again Stuart.



True, I wanted a siphon and this seemed so much simpler than the bell siphon. It works with these flow rates therefore medium or no medium I will aim to replicate the rates.





I aimed at placing a stand pipe at the far end or start of the drain ie furthest GB from the FT. This in my mind was to assist with flushing if req and if open would prevent siphoning, esp now the return will be full.

Skip arrives today Yay....

why did you make the siphons like that instead of a bell type?

its going to need a very large media guard.

I thought the siphons normally run straight to the tank or into a vented opening to the drain line?

Pretty sure the Cheidys std standpipe for IBC is 25mm

if you drop siphons straight into a sealed 90mm pipe will you get an airlock in the dropper?

this is how Collum did his, 90mm riser with siphon pipe open.

viewtopic.php?f=8&t=23506

or then again save yourself a lot of digging and use the classic system.

The airlock can be avoided/overcome in a number of ways.

One is to have the entrance to the main drain vented.
Two is to increase the fall to overcome the airlock
Three is to drill a medium sized hole in the bottom of the hook. The last one allows the hook to drain so when it has finished flowing the water drains out letting air into the bridge over the wall of the FT. When the drain starts flowing it soon gets to a flow rate where the hook fills up and then the air trapped in the bridge starts to get drawn out via bubbles formed in the inlet to the hook and taken out by the flow and expelled out the end of the drain. As the air gets drawn out the effective fall of the drain increases and all the air get flushed out giving full flow. You have to make sure the hole is not too small so it clogs nor too large so that the hook doesn't fill.

Hi SB


I did look at the bell siphon but felt this one seemed more reliable, less moving parts
I didnt want to go down the path of either without testing so I tested this one first and found it to be so simple and effective never tested the bell.
I have purchased uni-seals for 20mm pipe and have cut the IBCs sides to use as beds, this way I dont have the lids or drain to deal with and will place siphons nearer to a corner
Yes the stone guard will have to be 150mm.
Easier to clean out roots though as my hand will fit in one side of the siphon.

Your point about the siphons draining straight to the FT got me thinking.
If I have 10 siphons working independently but all draining into the 90mm pipe there will be so much variation in flow rates in the 90 mm pipe due to different combinations of beds draining at different times. I wonder if there may be a potential for a 'backup' of water up the 20mm pipe from the 90 which may wreck the siphon action esp if all beds drain at the same time.
I assume- hope - pray because they drain in 3 to 4 min (probably less as media will reduce the volume) the backpressure would be relieved soon enough to not be a big issue.


Yeah I know its lame but aesthetics and space did it for me. I dont really mind digging. I have koi in other below ground ponds and to me its the thing to do


Stuart


I will go for option 3 as it again is a simple fix. Besides I dont understand what one means and 2 is what I am trying to do but dont have that much room to play with
Does 1 mean I have a vent Above head height at the farthest end of the drain on the 90 mm pipe? Grow bed end?


Again appreciate the discussion I have learnt something from every post.
I did 10 barrows today to get in the mood. Start in earnest tomorrow.

Hi Stuart

Can you please explain to me the placement of the vent in the main drain. (your second option for preventing an airlock)
I think adding this and the holes in the bottom of the hook wont hurt.

I am ordering the pipe work this week.
I should post some pics as the sump and FT are in.
Frames have been made for all the beds, just need to plumb up then work out the right pump size.

So EXCITING

Thanks again for posters efforts keeping me edumacated

Sorry I missed your post on the 14th.

The vent that you have drawn near the FT is one way of adding a vent but you may have problems with your siphons not stopping if the fall is not great enough to suck enough air into the siphon to break it. A common fix for small systems (relatively) is to use the 90mm storm water drain for the main drain and instead of plumbing the GB drains directly into the drain from the main drain you have a 90mm riser come up to meet the GB drain without actually being connected. The GB drain just fits inside the 90mm riser. This creates a vent at every GB drain entry.

By adjusting the GB drains so that their ends are higher than the water level in the drain allows air to get into the siphon from both ends when the flow diminishes when the GB is nearly empty.

However, having said that small siphons can be really tricky to get to cut out. Even a small amount of water flow can seal the pipe preventing air getting in to break the flow.

I think someone posted this earlier (Slowboat?) but you probably want to check your pipe sizes. A 50mm main feed pipe with 25mm outlets could be asking for soilds to settle in the 50mm pipe particularly towards the end where the flow will be reduced.