Significant industrial users (SIUs) are regulated industries within a municipality or sewer district that discharge either:
1) a volume of wastewater that is above a minimum threshold as being representative of a certain percentage of flow entering the treatment facility, or
2) a wastewater stream containing a contaminant(s) that could adversely impact the operations of the treatment facility.
SIUs are commonly regulated on wastewater contaminants such as TSS (total suspended solids), BOD (biological oxygen demand), pH, and flow. For instance, a common pH range for acceptable discharge is from 6.0 to 9.0. Therefore, if an industry has a wastewater stream that has a pH less than 6.0, it will be required to feed an alkaline additive to bring the final effluent pH to above 6.0 prior to discharge to the municipality.
More recently, sewer districts are requiring SIUs to increase the pH of their effluent wastewater, perhaps from a minimum of 6.0 up to 6.5 or 7.0. This is being done to minimize the impacts of further pH reduction and corrosion that can occur while the sewage passes through the collection system (which will be discussed in the next section).
The industry standard alkaline additive used to increase the pH of acidic wastewater is sodium hydroxide (NaOH), also called caustic soda, and it is typically sold as a 50% concentrate. In colder climates, the typical product concentration is 25% NaOH, which freezes at the same temperature as water, while 50% NaOH freezes at 60oF.
All about caustic soda: the benefits and the drawbacks
One of the benefits of caustic soda is that it is relatively inexpensive, being a byproduct of the chlor-alkali process that produces chlorine gas. Another benefit is that it is a pure liquid and a strong base that reacts instantly within the wastewater stream to give the desired pH within seconds. As a liquid, it is relatively easy to feed using a standard chemical metering pump.
The negatives of caustic soda are also related to its high rate of reactivity. First, if the dose in the wastewater is too high, the pH can readily spike up to a value above 12, resulting in a need to feed an acid-based product to bring the wastewater pH back down into regulatory compliance. That means the wastewater operator has to handle two hazardous products – a strong base and a strong acid. This brings us to the next concern with sodium hydroxide: it is a highly hazardous chemical to handle. The primary industrial use for NaOH is as a cleaner, because of its rapid hydrolysis reaction to break down proteins and carbohydrates that can line the walls of storage tanks and transfer lines. That is why “caustic cleaners” are extremely common products in the sanitation market. Unfortunately, our skin and eyes consist of proteins and carbohydrates that are equally susceptible to the extreme reactivity of caustic soda, and serious caustic burns are far too common in the industry. For this reason, most wastewater operators do not enjoy working with caustic soda.
In addition, an industrial wastewater operator typically does not have the same amount of resources, support, or training available to them as compared to a municipal operator. At a municipal wastewater treatment facility, the entire purpose is to achieve a high-quality effluent stream that meets or exceeds their state permit. The effluent being discharged to the environment is their “product” – something to be extremely proud of. However, for industries like food processing or metal plating, a clean wastewater stream is not a sellable product. Their products are pork chops and chrome fenders, and the need to pretreat the wastewater for discharge is purely an overhead expense. Therefore, the Environmental Manager responsible for the industry’s pretreatment process often runs a leaner operation with less personnel and training. For these reasons, it’s in their best interest to help their wastewater operator to work with less hazardous chemicals. From the very beginning of IER, this has been our primary mission: to help industrial wastewater operations convert from the use of caustic soda to magnesium hydroxide, to make the life of the wastewater operator a little safer each day.