drastic reduction in chlorine needed when CH lowered

[Guest]

We notice a drastic reduction in the amount of chlorine (bleach) needed to maintain proper levels in the pools that we treat as opposed to the amount needed prior. I will admit that I am not a chemist, but I can't figure out what else could be contributing to the decrease in chlorine dosage.

Hard water is detrimental to many things (heat exchangers, pump shafts, SWCG cells, plaster, waterline tile, water features, etc.), and I see no advantage of maintaining a high CH level (anything up to or over 1,000 ppm). Arid conditions accelerate the accumulation of high ppm numbers, which just means that treatment or replacement must occur more often than less arid areas.

The only reason that I can possibly see for allowing such high concentrations of calcium is to temporarily save money. Continued disregard for these levels, if they indeed do or do not allow for easier propagation of algae, is playing Russian Roulette with your plaster and equipment. I'm not willing to take that chance for a few dollars.

 

[chem geek]

Re: Spa plaster flaking

We notice a drastic reduction in the amount of chlorine (bleach) needed to maintain proper levels in the pools that we treat as opposed to the amount needed prior. I will admit that I am not a chemist, but I can't figure out what else could be contributing to the decrease in chlorine dosage.

That sounds like a topic for The Deep End. High CH shouldn't affect chlorine usage directly, but maybe there is some strange indirect effect. Certainly we've never seen this effect on this forum going from low (vinyl pool level) CH of 50-100 ppm up to higher hard water pools with up to 500 ppm CH.

 

[JasonLion]

If I remember correctly, your water treatment process lowers the levels of all of the dissolved solids, not just CH. If I am correct about that, then it is almost certainly one of the other levels, not the CH level, that is causing the effect you see.

When speaking of "hard water", it is important to distinguish between high CH levels and conditions which promote calcium scaling. Conditions that promote scaling are indeed detrimental to many things. But you don't automatically have conditions that promote calcium scaling when the CH level goes up. Quite a number of factors affect the likelihood of calcium scaling, notably PH and TA, not just the CH level.

With proper management of your PH and TA levels, CH levels around 1,000 are manageable without any risk to the pool. This isn't an ideal situation, lack of attention could result in problems, but it is often preferable to more frequent water replacement/treatment. Arid areas with high CH levels in fill water often have water usage restrictions, limiting the times of year when water replacement is allowed, and water treatment to remove CH can get fairly expensive.

In extremely arid areas with very high CH levels and water usage restrictions, it is preferable, both economically and ecologically, to manage the likelihood of calcium scaling by lowering PH and TA, until CH levels get up to around 1,200 to 1,400. When done with the proper amount of attention to maintaining appropriate levels, this approach poses no significant risk of problems from calcium scaling.

In areas with less extreme conditions, that maximum CH level that it is worth tolerating is lower, often far lower. In most of the mid-atlantic region, where high humidity lowers water evaporation, CH is very rare in fill water, and water is usually plentiful, there is no point in allowing CH levels to go over 400 to 450.