Hydrogen sulfide (H2S) results from septic conditions during the collection and treatment of wastewater. H2S has long been recognized as a major problem for municipal wastewater systems. This colorless gas, known for its rotten egg smell, is produced by the biological reduction of sulfates and the decomposition of organic material. It forms at virtually every point in a system from interceptors, force mains and lift stations, to holding tanks, mechanical dewatering equipment, and drying beds. H2S also poses a serious corrosion problem to the structural integrity of the collection system caused by the sulfuric acid formed from the interaction of H2S with moisture. Of greatest concern are the safety hazards associated with H2S gas which is acutely toxic. Even at low concentrations in air, exposure to H2S has been linked to fatigue, headaches, eye irritation, sore throats and other health problems.
Case study - CAIROX CR potassium permanganate tablets
CAIROX CR Potassium Permanganate Tablets are a unique controlled-release (CR) formulation that dispenses permanganate slowly into flowing water. CAIROX CR Tablets are designed to treat odors in municipal and industrial wastewater at remote sites, lift stations, or where feed equipment is not practical. The CR technology is very fast reacting with H2S (on the order of seconds) and other organic odor-producing compounds, making it an excellent choice when retention times are very short. Potassium permanganate (KMnO4) oxidizes H2S according to the following equation:
3H2S + 2KMnO4 3S + 2MnO2
Easy and safe to use, the CAIROX CR Tablets provide H2S odor control for many days before new Tablets are needed (Figure 1). CAIROX CR Tablets are a passive treatment option that is deployed in specially-designed polyester mesh bags for rapid oxidation of H2S in wastewater lift stations or gravity interceptors (Figure 2).
In the fall of 2015, a Midwestern Wastewater Treatment Plant (WWTP) with typical daily flows of three million gallons per day was receiving neighborhood complaints about strong H2S odors coming from gravity interceptors. Carus personnel conducted a field test of the CAIROX CR Tablets for H2S treatment and odor mitigation. To obtain baseline H2S concentrations a data logger was suspended in a gravity interceptor located upstream of where the tablets were going to be deployed (Figure 3).
Ten pounds of CAIROX CR Tablets were placed in the mesh bag and placed approximately six inches below the water level in the interceptor. Note the characteristic purple color of permanganate in contact with water in Figure 4.
The chart in Figure 5 provides the untreated and treated H2S concentrations that were continuously measured throughout the CAIROX CR Tablet trial. The blue lines designate the baseline H2S data prior to CAIROX CR Tablet deployment. The orange lines are the measured H2S concentrations during the trial. The highest recorded atmospheric H2S peak was 5.2 ppm during the five-day trial. H2S has a low odor threshold, and its smell may be detected below one ppm. The average treated atmospheric H2S concentration was 0.2 ppm throughout the monitoring period with the majority of odor release occurring between 2 pm and 9 pm daily. In addition to H2S concentrations decreasing and resulting in odor elimination, visual inspection of oil/scum floating on top of the water also indicated evidence of the fats, oils and grease (FOG) layer being attacked.
CAIROX CR Tablets are very effective in controlling H2S and the resulting odors that occur in municipal wastewater applications. Based on performance, practicality and cost effectiveness, the Midwest WWTP continues to use CAIROX CR Tablets. H2S levels continue to be extremely low and customer complaints have been eliminated.