pH of run-off water

[kbs]

I may be in the wrong forum, thought I'd start here. We have a few spray pads in our parks. All but a couple are concrete structures. These others have asphalt for the pads. The pads are like a cooling fountain for kids to run around in. The water runs into the drains, into a tank, pumped through filters, treated then pumped out again. (Conservation). The pH is easy to maintain on the concrete pads, however the asphalt pads are very difficult to maintain neutrality, it climbs dramatically, well above 8.5. Should asphalt have this affect?

 

[bimr]

Asphalt generally should not affect the pH. Concrete is more likely to effect the pH because the cement in the concrete is alkaline.

Not sure what time frame that you are talking about in regards to your samples. But what happens in a cooling tower for instance, is that the water evaporates and causes the alkalinity to increase with a corresponding rise in water pH. What is commonly done in cooling towers is to feed acid to lower the pH.

If you are feeding well water containing carbon dioxide, you may also be getting an effect where the pH increases as the carbon dioxide is being stripped into the air.

You may want to increase your blowdown and increase your water makeup to counter the effects of evaporation.

If you have a complete description of your system and water analyses, it would be helpful for further diagnosis.

 

[kbs]

Hi bimr
Thanks for your reply. These pads are essentially large, group showers. On a hot sunny day the kids in the park run up to it, push a button and the water flows out a "shower head" in a 10 or 15' diameter for a timed period. The water is contained on the pad, drains through a filter into an underground water tank (5000 gal) and is treated before it gets pumped back up to the shower head. The concrete pads require very little treatment to maintain pH 7.2 or so. The asphalt pads rise in pH to 8.5 in one day. Because of evaporation, our city water keeps the levels up in the tank. Our city water is soft and about 6.4 pH. With the makeup being higher in the asphalt pad (more evaporation) we would expect lower pH. Not the case. It has been suggested the oil in the asphalt is causing this.
Any thoughts?

 

[bimr]

I doubt that the oil has anything to do with it.

You probably get more evaporation from the asphalt units because the dark material absorbs more of the sun's heat than the lighter color of the concrete.

With a higher evaporation rate, the alkalinity will increase more in the asphalt units than the concrete units, driving the pH up. You also have some carbon dioxide in your water as evidenced by the low pH. The carbon dioxide will also air strip, driving the pH up.

Why don't you check the temperature in the ponds and see if each pond is the same temperature. Also, see if you can measure the water input to each pond.

 

[kbs]

I think your leading me down the right track. I don't think the air scrubbing of the CO2 is a large part of the equation as we don't have the problem on the identical equipment with concrete pads. I believe the answer must be the evaporation. As we won't be starting the spray pads up for a couple of months, I think our best bet is to install meters on the make-up lines and measure the difference between the two types of pads. Unfortunately, the curious minds will have no relief for a few months. Thanks for your input.