Water treatment - calculating solids loading to rapid gravity filters 1

[Andy2005]

I wonder if anyone can help? I'm currently carrying out a project on factors that affect rapid gravity filter performance in water treatment, and one aspect I'm looking at in particular is the effect of solids loading.

In order to calculate solids loading, I believe I need to convert the influent turbidity value (in NTU) to mg/l suspended solids. Several values have been suggested to me as being 'the conversion factor that you use' however, I'm struggling to find any literature that supports any of the claims. Examples of what I have been told are:

1.5 if converting raw water turbidity to mg/l ss.
2.0 if converting interstage turbidity to mg/l ss.

I have also seen a model that uses 1.1 as the conversion factor. Please can somebody clarify what are the correct values to use, and if possible the title of a reference book or paper that supports the claim?

Many thanks...Andy

 

[orenda1168]

Andy2005:

From the data I have seen, there appears to be no single conversion factor to relate NTU to TSS. At the very best, this relationship would seem to be site-specific and, at that, would depend on a host of uncontrolled variables such as consistency of TSS particualte size, etc., though you might be able to develop an empirical relationship for your system with measurement over time of NTU and TSS.

Why not just measure TSS over time to establish the filter's influent solids loading?

Orenda

 

[semo]

I will tend to agree with Orenda.

The turbidity measurement most likely includes not only suspended solids; but, also dissolved solids. Particles under 0.45 microns are classified as being dissolved; but, they still can reflect and absorb light which is used to measure turbidity. Your particular water will have different variables than others so the correlation would be different. The correlation can also change as the time of year, rainfall, etc. changes the constituents of your water.

Part of the treatment process many times is to get these dissolved particles out of solution so that they might settle and/or be caught in the filter.

I think testing the TSS would be a better representation of the loading on the filter.

 

[GarySCWSVI]

That's the first time I have heard that the turbidity measurement most likely includes not only suspended solids; but, also dissolved solids. Particles under 0.45 microns are classified as being dissolved. Can anyone else confirm that?

Dissolved solids are not filterable. They will have no effect on the loading of a filter. In any event a rapid gravity filter would not be expected to remove particlate in the 0.45 submicron range

Gary Schreiber, CWS VI
The Purolite Co.

 

[quark]

Bottom of this link gives you an approximate correlation and some interesting comments.

http://www.duluthstreams.org/understanding/param_turbidity.html

Some dissolved organic matter reduces clarity of water sample and this may add something to an NTU reading.

Regards,

 

[cub3bead]

GarySCWSVI:

I have followed your convention also; anything that passes 0.45 micron is assumed to be ionic and all else is colloidal or otherwise.

Specifically, I follow this convention when running silica by AA or ICP. Unfiltered results represent total silica with the filtered results being the reactive or ionic silica. The delta is the colloidal silica. This method is much faster than doing the acid digestion to convert "all" of the colloidal silica into reactive silica then analyzing via the molybdate blue method.

Regards,

 

[bimr]

The standard methods tss test uses a filter that will retain 98% of the particles smaller than 1.5 µm (0.00150mm). Therefore, particles under 1.5 µm are not included in the suspended solids measurement.

The .45 micron filter is not used in the determination of suspended solids or dissolved solids. .45 micron equals .000450 mm.

The standard for the turbidity test is a suspension of silica of specified particle size selected so that 1.0 mg/l suspension measures as 1.0 NTU. For what you are doing, this conversion factor is as good as any that you will find. If you want something better, you can empirically determine it, using your own water source.

 

[orenda1168]

Gary, Semo and Quark:

Gary, you are correct as usual.....0.45 micron and smaller particulate is considered to be in a solubilized (dissolved) state, and of course will not and cannot have a direct effect on filter loading, though with possible effect on NTU.

Semo and Quark, dissolved organic matter, if truly in molecular solution and not a low micron - submicron dispersion, may have an effect on color which must be blanked out in a NTU measurement, but by definition will not be a component of TSS. By the same token, particulate of any micron - submicron size can and generally will have an effect on NTU measurements. The disclaimer in the posted Diluth reference makes it clear that their formulated relationship of NTU and TSS may be valid for a single sample, but subject to wide and unpredictable variations sample to sample over time.

 

[orenda1168]

Bimr:

Why, if the TSS filter has a particulate retention of 98% smaller than 1.5 microns, is that material so retained not included in the TSS measurement?

Also, if the NTU standards are formed from silica of specified particla size, how does the mg/l to NTU relationship developed from this, which you indicate is usable for Andy2005's purposes, relate to real-world waters with widely varying particulate size and composition?

Regards,

Orenda

 

[bimr]

OK, a word was incorrect. The standard methods tss test uses a filter that will retain 98% of the particles greater than 1.5 µm (0.00150mm). Therefore, particles under 1.5 µm are not included in the suspended solids measurement.

All I said was that this conversion factor is as good as any that he will "find". I would expect that the factor will vary depending on season, storm event, etc. I suggested that if Andy2005 wants something better, he can empirically determine his own, using his own water source and lab determinations.

 

[bimr]

Turbidity is not an exact determination of the mass of the suspended particles. Instead, it a measure of the opacity of the water as compared with certain arbitrary standards; this differs with varying materials and degrees of fineness. Thus a water issuing from under a glacier and containing a very finely divided white rock flour, a muddy river water, and a water containing suspended ferric hydroxide could hardly be expected to contain equal weights of insoluble materials merely because their opacities happened to be equal. However, this usually is a matter of slight importance as far as raw surface waters are concerned because the turbidity of a given water will usually vary from time to time, over such a wide range, that a rapid method of approximating the suspend matter is generally all that is required. Where the suspended matter is always of the same nature, it is possible to measure the divergence between the observed turbidity and the actual weight of the suspended matter and apply a correction factor to subsequent readings.

 

[Andy2005]

Thank you all for your comments. The common thread linking all replies seems to be that any NTU to TSS conversion factor would need to be determined on a site specific basis, although the degree of accuracy (and consistency) of the factor will be determined by the variability of raw water and interstage quality. Very useful,

Thanks again...Andy

 

[semo]

Sorry to cause such a stir. It looks like what I said was construed in multiple ways. Sometimes I need to read what I say instead of thinking what I say.

Not to get into the technicalities of my wording; my point was that there are smaller particles that will show up in the turbidity readings that won't be caught by the filter or show up on the TSS test.

The amount of these particles will vary dependent upon the source, time of year, weather conditions, etc. and I agreed that I didn't think one conversion constant will hold true for every water.