I mentioned pH limits earlier. As part of the Clean Water Act, the EPA sets pH discharge limits as part of the National Pollutant Discharge Elimination System (NPDES) permit. The State’s Department of Environmental Quality must follow this permit. Often States will make limits a little more strict than the Federal limit. With the same logic, local limits may be stricter than State limits. Getting a change to your local pH discharge limits is possible but hopefully the limits you receive from your municipality are generous to begin with.
How to go about measuring and adjusting the pH of your wastewater? The simplest method to measure is getting a good, well mixed grab sample and sticking a pH strip in the water. Beyond this, things get a little more complicated. An electronic tester of any sort will require calibration and buffers. That being said, pH measurement is a about as good an application for automation as it gets. For most purposes, a meter and a separate pH electrode are better than an all in one unit.
For pH adjustment, this is where you will literally start pouring money down the drain. Ideally you could hold your high pH CIP wastewater and add it to your low pH wastewater, but this usually isn’t very practical since it requires a lot of tank capacity. Therefor the cheapest method of chemical addition is to put some in a cup and pour the cup in to your wastewater tank. Not automatic or precise, but it can be done safely and it does work.
The best way to add chemicals to the wastewater is automatically. The requires a pH meter and electrode plus 1 or 2 dosing pumps. Figure at least $3000 for this equipment. This will require calibration, verification, and replacement electrodes every 6 months or so. But it solves a big problem and is mostly trouble free.
Types of chemicals for pH adjustment. To raise the pH, 50% caustic (NaOH) is the cheapest source. However this freezes at about 50F and likes to find a way to leak. Going with 30% caustic with some potassium hydroxide (KOH) cut in will lower the freezing point and make it easier to work with, but more expensive. I have had success with 50% NaOH and diluting it with water. This causes an exothermic reaction, but not severe. Depending on how much water you add, this can lower the freezing point to 32 F- making it much easier to work with. I have not had good luck with magnesium hydroxide. There are huge benefits and drawbacks to working with this product. Freeze protection and mixing requirements top the list of drawbacks, plus sludge buildup. Safety is the biggest benefit.
To lower the pH, a cheap liquid acid is easy. 96% sulfuric acid is usually the cheapest source. Another way to do it is to submerge an old carbonation stone and bubble CO2 through the wastewater. This will create carbonic acid, lowering the pH. This works, but pay attention to economics and ventilation. I’d love to use CO2 from fermenter blow off, but this is usually not very feasible.
Safety is a huge concern with these pH adjustment chemicals. You will go through a lot of them and they are very dangerous, especially to sensitive tissues like eyes and lungs. Read the MSDS and follow the safety precautions. One funny trick to get caustic off your skin is to use beer or spent yeast. Caustic is hard to wash off with water because it reacts to the fat in your skin, turning it to soap. Splash some beer or yeast on and the caustic is instantly neutralized.
Once I did a small experiment to appease my curiosity. I had a small beaker of 50% NaOH and added a roughly equal amount of 96% sulfuric acid. This was done outdoors, with ‘proper’ safety gear (is there such a thing in this case?), and using a flask duct taped to a stick to dump the acid in. Anyway, the reaction was violent. No flames, but it boiled and rocked the beaker and released a lot of fumes. Wouldn’t want to be around if that happened on a bigger scale. But I digress…