Silt Pollution 1

[Pearse29]

I am an environmental engineer working on a major road project. We are having problems with surface runoff and silt pollution of nearby watercourses. We have tried silt fencing, siltbuster tanks, settlement ponds and silt traps, without much success. 74% of the parent material is classified as silt, and is too fine to settle. Is there any other methods i should be using to remove the silt from surface runoff, any other suggestions are appreciated

 

[Unotec]

If you can collect it before it gets to the water sources you can centrifuge it out

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Hoagie]

allow me to add to unotec's post.

you could collect into a frac tank (or pond) and pump thru a bag filter prior to release

 

[cvg]

managing silt is difficult at best. If you can stabilize the ground to prevent the erosion in the first place, that may help. Otherwise you will have to retain and filter.

 

[Pearse29]

We have settlement ponds in place and are trying to stabilise as much ground as possible, but due to the amount of rainfall, it is almost impossible to treat all runoff. It is a very fine particle size, in some cases smaller than 75 microns. We have had a report from Siltbuster in Wales that talked about, flocculation, chemical pre treatment and running through a siltbuster unit. This did not claim to remove particles less than 2mm. I don’t think that the silt fencing removes this sized particle. We also have found that if you get any volume of water hitting the net it just channels it. It appears to work with a very slow flow were the water can percolate through it. Is there another product we should be thinking about? I have heard of the filter bags, but are they effective for this size of particle. Any input is appreciated.

 

[gerhardl]

Is this in frost-free conditions/landscape?

I believe that you have two possible ways of solving the problem:

a) Artifical marshes.
Will work as 'natural' filters, but requires areas large enough to cope with sometime large quantities.

b) Mechanical filters or centrifugal treatment.
Will require large investments and running costs.

All others will be variatons of this.

Depending on surrounding and total conditions, and amount of water, I would go for artifical marshes or lakes, perhaps in combination and perhaps also combined with smaller coarse pre-treating devices taking the bulk of the largest particles.

 

[cvg]

silt fence should not be used for larger areas, only very small. It must be installed with the contours, otherwise it acts as a diversion levee and channels water to the lowest point. This concentrates the flow and generally creates more erosion than it stops. You might look at some sort of erosion control blanket or soil tackifiers.

 

[bltseattle]

Get your base course down asap. Asphalt treated base is not erosion prone. Perhaps the work should be scheduled so as to not have the grading occur too far ahead of the installation of base course materials.

Also Consider:
polyacrylamide (PAM) or tackifier applied to disturbed soils to hold silt in place. Can be applied as a solid or spray (be careful, treated surface may get very slick)

package electrocoagulation treatment device to remove suspended silt

I've would think that holding ponds dosing runoff (chemical treatment) with "cat-floc" would be able to remove silt, I've seen it done before but the effectiveness would be subject to holding capacity and flow rates.

The challenge for a road however is typically how to collect & convey to the treatment site, so I think the surface treatment is a better option. Why not cover all exposed soils with plastic sheeting when not being worked? It will yield runoff, but won't be silty...

 

[brucedaengineer]

I agree with others - it is important to stabilise the stripped surfaces first - these are probably dispersive soils - these can be covered with non-dispersive material if possible, or ripped and have lime or gypsum placed in , mixed an compacted - but if it keeps raining you will make a mud pie?? Get your geotechnical engineer to solve the source of the problem - the soils.

By the way make sure it is coming off your site and not the landowners place next door say!!!

And in areas of fill the constructor should be using non-dispersive soils; or be stabilizing the fill with lime or gypsum mixed in soil as it is placed and compacted.

For all these issues - Talk to your geotechnical engineer - have them test the areas where runoff is worst quality.

Watching the soccer (oops football) over there - does it ever stop raining? - I live in a ten year drought but we do use silt fences, sedimentation ponds/dams and all the rest on roads projects.

Now this is MAJOR road construction - the environmental management plan should have all this covered and clear sediment control processes should be in place - which I assume they are - you must ensure the documented procedures are in place or stop the job and get them in place.

Let's assume they are and they have worked but it keeps raining and now the dams are full and are discharging the same quality water that is coming in - sound familiar? The problem is the silt is actually silt or clay and it is colloidal - so it won't settle for months - you have to destablise the colloids - in a water treatment plant this is done with a coagulant - say alum (aluminium sulphate) or a ferric/ferrous salt. Alum is easiest to handle (ferric chloride for instance is very acidic (OH&S issues) and stains everything dark brown)-but alum coagulates only in a limited pH range - aim for say pH6.4 - but will be OK pH 5.8 to say pH 7.0 - coagulated clays/silts will settle and stay settled - a "supernatant " turbidity of less than say 25 NTU should be possible - may need two dams in series.

Now dosing the alum into a dam AND MIXING IT is a very agricultural exercise - talk to people who have done it - THIS IS VERY IMPORTANT TO BE SAFE AND SENSIBLE- can broadcast, or put in from a boat, OR MIX IN DRUMS ON BANK AND TIP IN etc etc - definitely something to organise someone else to do

You will need a lot of alum - you need a lot to remove the clay and may need more to drag the pH down into coagulation range - say 100 mg/L - get a chem eng/water treatment person to do your calcs for the form of alum you have.

Also they use hay bales here a lot across drain or swale entering (and leaving) the sedimentation dam - replace when clogged - will reduce load but water still very turbid.

Now a quick bit of chemistry to show the chief you are onto something - soils are dispersive because they lack calcium (too much sodium - look up sodicity or sodium adsorption ratio - SAR) - so add & mix lime Ca(OH)2 or gypsum CaSO4 to stabilise them. Once clay is dispersed in water as colloids - lime or gypsum will do nothing at all (unless you use lime and push pH to pH11/12 (don't do that) - you now need to coagulate the colloids - hence use alum - coagulates (precipitates to form Al(OH)3 as precipitate with high surface charge that attracts clay colloids to form clumps - flocculation - but DO NOT put alum on the soil to stabilise it - it will do no good there at all!
Good luck and get help - need geotech and some chem eng mates to help - surprising how scarce they can be when you have a real problem to deal with!! - I think you have - don't try to solve it - get help these procedures have been used a lot but you need experience.

Final hint - read the contract and get the design basis for the sediment control - usually designed for say 1 in 2 year return rain event - so if you have a 1 in 10 year or 1 in 100 year rain event your system is not designed for it - and presumably all the local streams are running a banker of muddy (silty) water like yours - maybe you are not obliged to do more??