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I have a project where I will be diverting water from an aquaculture system when the solids content reaches a certain level.

I am struggling with the decision to use TDS or TSS for the decision point in triggering the diversion.  Testing of the influent water shows a normal concentration of 150-300 ppm TDS and 250-500 ppm TSS.

During periods of heavy runoff, the TDS and TSS values rapidly increase to an unacceptable level for the operation of the fishery.  The contaminants that enter the water are mostly clay silt with a small amount of sand.

All of that being said, what would be a recommended type of sensor and what parameter (TDS or TSS) should I use to trigger the diversion of influent?  Any insight to the issue is greatly appreciated.  


Its been a long time since I did water quality, but ..
As I recall it is a turbidity sensor, basically a photocell.  

Use the TSS value, because that, being Total suspended solids, includes both the suspended and the dissolved solids.  The dissolved particles are the ones that pass through the filter after the sensor looks at it.  The suspended solids get caught.

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus


I was visiting a fish hatchery in the NC mountains earlier in the year. They filter the raw water coming into the hatchery as the fish may be adversely affected by suspended solids in the water.

You would want to use TSS.

TDS includes total dissolved solids and total suspended solids. Fish are not as susceptible to a change in TDS. The TDS should not change as much unless you are close to the ocean or source of runoff containing water soluble salts.



Hi Eryp, My question is this, what is the reason for the diversion?  
Also, what methods are you using to measure TSS and TDS?  

BTW, TSS does NOT include TDS.  The TSS test measures the suspended solids whereas TDS is dissolved solids.  Dissolved solids are not suspended and vice versa.

Conductivity is often used as an indirect measure of TDS.  So, you could use a Conductivity sensor in the stream(?) and then divert if it gets to a set specific setpoint value.  TSS requires obtaining a sample, filtering, drying etc. - i.e. its a little more involved.  You could also measure and divert on turbidity - which is not the same as TSS but would give you some indication that solids were on the rise.

Regards, Kevin   


TDS and TSS are mutually exclusive. Solids are dissolved or not; if they are not, they will remain suspended or they will settle out.

Total Solids include TDS, TSS and settleable solids.

5.8 Total Solids



Use a Turbidimeter for detection of high Suspended solids concentration (sand and Silica). Normally, the first run off will be highly charged with SS. After a while you could expect a drop in SS and the TDS will be also low as dilution come into effect.


Thank you all for your ideas on this project.

The basic reason for diversion is that the fish health is adversely affected when the TSS rises.  Also, it has become a great expense for the fishery to remove the settled solids from their ponds after large amount of solids settle out.

I have decided to use a basic turbidity sensor with a 0-1000 NTU range.  I expect the normal in-fluent to be around 10-50 NTU.  I will program the sensor to divert water away from the fishery at 500 NTU initially to prevent any false diversions.  After a few weeks of data collection I will likely decrease the trip point for diversion after consulting with the fishery operator.


you might be better off using a flow meter or stage gage on the river since TDS is more or less directly related to flow rate



The problem with monitoring river flow is that the solids are a result of irrigation upstream of the water source.  The irrigation is a small portion of the stream flow but can cause a wide swing in the solids content.

Also the water source is a combination of well and surface waters.


Others have made the correct distinction between TDS and TSS.  TDS is only measuring ionic dissolved solids and doesn't reflect dissolved organic content as well as the suspended solids.

What is key here is TSS is really easy and cheap to clean up when compared to TDS.

A centrifugal strainer for large TSS, 70 microns and larger, bag filters, cartridge filters, etc. for the smaller stuff.

TDS is a different issue, here you are looking at reverse osmosis to reduce TDS much more costly.

Big Brand Water Filter


The trouble with asking engineers anything is that we are by nature problem solvers, and in the absence of data we'll usually make it up.  

It sounds like you've reached a workable solution to your issue and that the event duration that you are considering makes diversion a workable answer where filtration would be difficult, expensive, and manpower intensive (all things that a aquaculture has problems with).



Update/Closeout of the project:  Thanks to all for your help.  The project now sits in the owners hands.

After inital programming and commissioning, the owner has the documentation to adjust the turbidimeter's trip setpoint as he wishes.  The incoming water ranged from 10-20 NTU so the decision was made to adjust the trip point to 200 NTU initially.  Last I talked to the owner he had lowered the trip point to 150 NTU.  The valve cycle time was around 20 seconds.  Fast enough to divert water before a large amount of sediment entered the system.  I programmed the system to wait until 30 minutes of turbidity readings below the setpoint would close the diversion and allow water into the system again.

The spring runoff will truly test the system but I do not expect any problems.

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