slow sand filters to treat ground water 2

[handywater]

We have a few groundwater sites that suffer from occassional turbidity spikes, have researched the solutions and the best from a total economic stand point appears to slow sand filters BUT we have no experience of groundwater treated by SSF, we have experience with surface waters on SSF. Has anybody operated SSF on groundwater, looking for experience, problems etc.
Thanks

 

[RWF7437]

Can't see any reason why SS filters can't be used to treat groundwater. You will not get any mineral removal in a SS filter, however, so if that is a concern you should consider other or additional treatment. But bacterial removal and SS removal efficiencies should be very good.

good luck

 

[hydrae]

Handywater

If I were in your position I would conduct a pilot study to verify the filter performs, SS filters use the bacteria in the top layer to perform a portion of the filter function, with ground water is there enough oxygen for this bacteria to live? is the water warm enough?

Hydrae

 

[ch81pc]

Hi Hnadywater,

I have worked at a plant using SSF's for both river and ground water. The problems we encountered in using them with groundwater were:

1. Temperature (above 10 degrees C ideally)
2. Not enough Dissolved Oxygen (aerobic conditions must be maintained in the filters)
3. Not enough "nutrients" in the water to quickly attain an adequate schmutzdecke (biological filter skin).

It would be worth installing some form of aeration step before the SSF's in order to increase the DO and oxidise any iron you may have present in the feed water. Also depending on where this plant will be operating it may serve to warm the feed water by a crucial degree or so.

Due to the lack of nutrients in the feed water our SSF's fed with groundwater always ran to waste for longer periods before an adequate schmutzdecke would form and the filter could be brought back on line. But in another respect we acheived significanlt longer run times before the filters needed skimming.

one last thing, how big are the turbidity spikes you are encountering and how regular are they?

Hope this is usefull
Any other questions please let me know

Paul

 

[jmj5152]

ch81pc: Good tips.

 

[handywater]

Hi ch81pc / Paul,
Thanks for your very interesting comments. I have been away a lot and have only today managed to see your valuable comments.
Please let me know if you have tried seeding SSF to create a schmutzdecke either with SSF skimmings or other?
Any info on filtration rates and periods between sand skimming.
Once a schmutzdecke is formed what is the problem with temperature below 10 degrees C as long as its above 4 degrees C?
Regarding our spikes these happen after heavy rainfall due to the vertical fractures in the aquafer when the turbidity spike can exceed 100 NTU and can last up to 18 hours. During these spikes we plan to divert and store the turbid water and turn down/isolate the SSF and when turbidity reduces to below 10 then bleed in the turbid water.
I had not mentioned but we cannot waste water (no nearby watercourse or sewer).
Await your answer.
thanks
Handywater

 

[bimr]

Slow sand filters work the same on groundwater as on surface water.

 

[ch81pc]

Hi Handywater,

Typical filtration rates I have used with feed water of <5 NTU turbidity are in the region of 0.1 m3/m2/hr (cubic meters of water per square meter of sand) to 0.2. I get typical run times of 3-4 months, depending on how bad the algae is.

I have tried seeding filters in the past with sand from the schmutzdecke of an estabilished filter, but to be honest didn't find any significant decrease in the maturation time.

Ideally the filters will need to be in service at all times (even if at low flow) in order to keep a supply of dissolved oxygen to the bugs. If you shut the bed in, the bugs within the sand will very quickly use up all the available oxygen and could give rise to problems with nitrites/ammonia and bacterial break through.

Running the beds at temperatures below 4 degrees just means that all the biological activity of the bed will slow right down. So there is a danger that the beds will be nothing more than oversized strainers.

Hope this helps

Paul

 

[bimr]

Well water is anerobic and if the well was properly constructed, there should be no bacteria in the well. If water is aerobic and not sterile, you will need to periodically chlorinate, so that the sand filter does not get fouled up with organic growths.

I would suspect that the suspended solids that you are concerned with are inorganic. If so, you are only going to be filtering them. You need to determine what the turbidity spikes consist of. Is the turbidity organic or inorganic?

There are biological filters that are used in wastewater treatment. But these filters are used to remove dissolved organics and that application is completely different than filtering ground water.

 

[bimr]

A 1984 report titled Effective Filtration Methods for Small Water Supplies, USEPA Cooperative Agrrement CR808837-01-0, NTIS No. PB84-187-905 EPA 1984 suggests that an acceptable source water should have turbidity less than 5 NTU.

If you are in a northern climate subject to freezing, the installation will also require indoor installation.

Because the sand used for a slow sand filter is smaller than for a rapid sand filter, the removal of solids is almost entirely in a thin layer at the top. For this reason, high turbidity water is not a good application because the filter will foul quickly. The technology is quite attractive for treating high-quality surface water.