SLOW SAND pH PROBLEMS

[swindlar]

Hi, I am currently running a slow sand filter pilot plant and am gaining some interesting results. The raw water is from a highland stream that is subject to intense freshes. We have placed a pretreatment stage in front of the slow sand that keeps the NTU generally below 5. The filter is not covered, the water entering is saturated with oxygen and pH of approximately 7.3. Exiting the filter the pH is raised to above 9 and the DO is approximately 1 mg/l below saturation. The treated particle count exibits an intense diurnal change, increasing to a peak at midday and dropping away at night. pH and DO exibit very small diurnal changes. NTU is generally above 2 coming out. Flowrate is 0.1m/h. Water level 1.5 m above the sand. Sand level 1m. Temperatures around 18°C.

I have attributed the ph and diurnal particle changes to algae growth in the filter. I see the pH as one of the major problems as I dont see too many organisms being able to survive at such a pH let alone provide the treatment we require.

How do we counter this and get NTU and particles down consistently.

Cover the filter?
Lower Water Level?
Higher Flowrate?

Any comments appreciated.
Does anyone else have the high pH problem, (is it a problem?)

 

[bimr]

A good starting point would be to obtain complete water analyses of the water entering and exiting your filter. That way, you will not have to guess what is happening.

 

[swindlar]

Inlet

pH 7.7
Alkalinity 18
DO 9
Temp 16
TOC 1.6
Turbidty 1.5
Diatoms & Crytophyta Present - Cryptomonas sp. Abundant

Outlet

pH 9.1
Alkalinity 33
DO 7
Temperature 18
TOC 1.5
Turbidity 1.4
Cryptophyta + Diatoms + Chlorophyta + Small Flagellates present Cryptomonas sp. most abundant

 

[bimr]

Do you have the nitrate concentrations? Do you know the source of the TOC?

 

[swindlar]

Dont have nitrate conc at this stage.
Assuming TOC is of algae and vegetable origin from stream.
It is seemingly a low value anyway?
Arent highland streams usually 5 to 15 mgC/Litre.

 

[bimr]

That's correct, the TOC value is below average. Considering the problems that you are having, there must be a problem with the sand that you are using as indicated by the efflent NTU.

Maybe you can describe your pilot test equipment and your sand. Is it possible that the sand has been contaminated with an alkali?

 

[swindlar]

Thanks for your comments

The filter consists of a concrete manhole base and riser (shell), pvc underdrains covered with 0.2m pea metal followed by 44/20 graded sand,(described as sub angular quartz crystals) uniformity coeff 1.53, Calcium oxide 6.3% Magnesium oxide 0.42% calcium carbonate equivalent 2.1

PS Distribution % Passing
425um 96%
300um 84%
212um 53%
150um 8%

The filter has been in operation for some time hence I have discarded the possibility of leaching from the concrete.

I figured the poor turbidity was due to poor schmutzdecke development due to the pH. Algae in filter removing CO2 raise pH during the day, the problem I thought is the lack of the pH dropping back during respiration at night.

 

[bimr]

The filter media which you are using does not appear to be correct. Does the filter sand meet the AWWA Standard B100-01? The calcium oxide seems to be too high. Where did you get the filter media? My experience is that there are very few suppliers of sand filter media.

Sand needs to be of a fine grade ( 0.15-.35 mm is recommended), uniform (the UC should always be less than 3 and preferably less than 2), and be washed free of loam, clay, and organic matter. Fine particles will quickly clog the filters and frequent cleaning will be required. A sand that is not uniform will also settle in volume, reducing the porosity and slowing the passage of water. Sand manufacturers should be able to supply or blend sand to these specifications.

http://www.thomasregister.com/olc/72842941/sand.htm

http://www.northerngravel.com/products.html

http://www.filtermedia.co.uk/Sand.htm