pH discharge limits are the most common pretreatment requirement for a brewery. When it comes to designing, building, and selling things, automated pH adjustment systems are our main product.
As part of the Clean Water Act, the EPA sets pH discharge limits as part of the National Pollutant Discharge Elimination System (NPDES) permit to 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. That being said, not everyone enforces pH limits. Clear as mud.
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 with a pH strip. 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. However it 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, and drops if the wastewater sits around in a tank, down to about pH 4.5. However there will be spikes both low and high due to cleaning processes, varying from about pH 2 to 12.
Manual pH adjustment is possible. A somewhat effective option is to collect and neutralize spent CIP and sanitizer water before it goes down the drain. All other wastewater goes direct to drain, ideally with no tank or lift station. Collecting CIP and sani water will take away those spikes to 2 or 12. The remaining wastewater will go to the sewer ‘fresh’, it has not been allowed to sit around and ferment which would lower the pH, and your wastewater should be between 6-8 or so with this method. Drawbacks to this method is it’s time consuming and there is no record of what actually went down the drain- and some cities require a record.
Another manual method is to pump wastewater in to a small tank. To test pH first mix the tank, then grab a sample of wastewater and stick a pH strip (or electrode) in there. If the pH is within limits open a bottom drain valve 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, with proper planning you can easily upgrade to automated later.
If the brewery will start out small with plans to grow, it may be best to start with a manual system and upgrade to automated after a few years. This spreads out the cost in to two phases which can be helpful to the people paying the bills. This is something we have done quite often.
For production breweries, roughly 5000+ bbls/yr, an automated system usually makes more sense. This requires a tank, pumps, pH meter and electrode, 1 or 2 dosing pumps, maybe a flow meter, and controls keep track of it all. We regularly design these systems for breweries around the world. 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 mentioned earlier, with good planning 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.
To raise the pH, straight 50% caustic (NaOH) is the cheapest source. But it’s heavy, freezes at about 50F, and likes to find a way to leak. Some brewers use their cleaning caustic for pH adjustment; this works, but it’s expensive. Cleaning chemicals are ‘built’ with detergents and chelants which aren’t needed for pH adjustment. 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 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. One alternative to sodium hydroxide is 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. I’ve tried it with suboptimal equipment and it was a nightmare.
To lower pH, a cheap liquid acid is easiest; 96% sulfuric acid is usually the cheapest source. Less severe chemicals such as citric acid also work, but it’s more expensive. 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 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 acids) and the caustic is instantly neutralized. I’ve even used beer to clean caustic contaminated concrete with good results.
Years ago 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 or melting, but it boiled and rocked the beaker and released a lot of questionable fumes. Wouldn’t want to be around if that happened on a bigger scale. But I digress…