As part of the Clean Water Act, the EPA sets pH discharge limits as part of the National Pollutant Discharge Elimination System (NPDES) permit. (between 5.0 and 11.0). The State’s Department of Environmental Quality must follow this permit.
Often States will make limits a little more strict than the Federal limit. By 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 the pH of your wastewater? At it’s most basic, simply grab a sample of your wastewater and test the pH. Record the results in a log sheet (date, time, sample location, sample method, test result, comments, initials) and before too long you have a record of your wastewater pH- you could even graph it. But the reality is this isn’t a fun task and is often a low priority. This can be automated…
If you grab a sample of wastewater and the pH is within limits, that doesn’t mean it’s always within limits. It would be good to grab your samples at different periods of production. In general, brewery wastewater is acidic, around pH 4.5. However there will be spikes both low and high due to cleaning processes, varying from about pH 2 to 12.
When it comes to pH adjustment, this is where you will literally start pouring money down the drain in the form of chemicals used to adjust the pH of the wastewater. Ideally you could hold your high pH CIP wastewater and add it to your low pH wastewater, but this usually isn’t very practical.
Manual pH adjustment is possible. Grab a sample of wastewater and stick a pH strip in there. If the pH is within limits turn a pump on and send it to sewer, maybe through a flow meter. If the pH is not in range, add some chemical to your wastewater tank and mix. Test the pH again. If it’s in range, send it to sewer. Remember to record all of these results on your log sheet. The reality of this is it’s very labor intensive and not precise, but it can be done safely and it does work and with proper planning you can easily upgrade to automated later. You could also forgo the pump and just overflow from this tank to sewer. Lucky you if this is an option, do it for as long as you can get away with it.
The best way to add chemicals to the wastewater is automatically. This requires a pump, tank, pH meter and electrode, 1 or 2 dosing pumps, maybe a flow meter, and controls keep track of it all. Figure at least $10k – $15k for this automation equipment, plus design, shipping, tax, and installation. I regularly design these systems for breweries around the world. Controls are the big issue, I use an off-the-shelf controller that functions similar to a PLC. In addition to the logic and control from the programmable functions, the controller is a pH meter, an auto-dialer and data logger, allows remote access (I can access from my office), automatically emails reports, and receives 4-20 mA and digital input signals- all for about $3300. A screaming deal given that it can do all of that.
The biggest maintenance issue with a system like this is the pH electrode; it requires calibration, verification, and replacement every 6 months or so. As alluded to earlier, you can start with a manual system and upgrade to automated a few years down the road once production picks up and cash starts flowing.
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 buying 50% NaOH and diluting it with water on site. This creates 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 tried using magnesium hydroxide, there are huge benefits and drawbacks to working with this product. It is very safe, you can even drink it. But it can’t freeze- even during shipping, and it needs to be mixed at all times.
To lower pH, a cheap liquid acid is easiest; 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, creating carbonic acid. This works, but pay attention to economics and ventilation. You could also try using CO2 from fermenter blow off, but this is usually not very feasible. In general you will use more caustic than acid for pH adjustment.
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 the fat into soap. Splash some beer or yeast on (mild acid) 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?). Using a flask duct taped to a stick, I dumped the acid in to the caustic flask. As you can imagine, 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…