Inhibition Assessment Case study



A chemical refining plant was investigating adding a new process that would introduce a caustic waste stream to their wastewater treatment system (WWTS). The plant had concerns about the new waste stream potentially causing inhibition to the system biomass, effluent permit excursions, and/or aquatic toxicity to the receiving stream. The facility contracted Environmental Business Specialists, LLC (EBS) to perform a continuous flow bench scale bioreactor study to determine if any of these impacts should be expected. Bioreactors were used to simulate the client’s system. One reactor was used as a control, while the second reactor was fed the proposed caustic waste stream. The study ran under quasi-steady-state conditions for approximately six months. The data from the study was used to assess whether any of these negative outcomes was likely, and to determine any changes required for the system to treat the new waste stream.


The bench scale reactors were a scaled-down replicate of the client’s WWTS. This was done to make the reactors suitable for bench-top laboratory operation. The influent flowed into an eight-liter denitrification (DENIT) basin, which was an anoxic bioreactor where the denitrification process occurred. An in-situ probe continuously monitored the oxygen reduction potential (ORP). The mixing speed in the DENIT basin was adjusted to maintain the ORP in the desired range for denitrification.  

The nitrification (NIT) basin was a 21.6-liter aerobic bioreactor where the nitrification process occurred. Diffused aeration was supplied with humidified air to minimize the impact of surface evaporation from the heated basins. The pH was controlled with an automatic sodium bicarbonate solution pump to prevent the basin’s pH from dropping below the set parameter. The NIT basin had a return activated sludge (RAS) line to the DENIT with a set flow rate. The basin had a baffle that created a quiescent zone which allowed the biomass to settle for clarification, and effluent gravity flowed over a weir in the quiescent zone.  


The influent, DENIT, NIT, and effluent for both reactors were routinely tested for pH, total suspended solids (TSS), volatile suspended solids (VSS), total and soluble biological oxygen Demand (tBOD, sBOD), total and solid chemical oxygen demand (tCOD, sCOD), alkalinity, 30-minute sludge settling volume (SSV30), sludge volume index (SVI), zone settling velocity (ZSV), total and soluble nitrogen (total N, soluble N), nitrite (NO2-N), nitrate (NO3-N), and ammonia (NH3-N).


System Changes 

 After introducing the caustic waste stream, EBS could identify relatively minor system changes that would help the client’s WWTS accommodate the addition. EBS found that the system’s pH increased to potentially inhibitory levels, so improved influent pH control would be beneficial. Floc characteristics also changed over the study, and the test reactor required more mixing than the control to keep the solids suspended in the NIT, so additional mixing might be required. It was also found that some components in the new waste stream were carried over into the effluent with discharged solids. Filtration or another method of tertiary solids removal after the secondary clarifier would reduce this. 

Biomass inhibition 

For the first half of the study, the biomass in the test bioreactor experienced nitrite lock. Nitrite lock is when nitrite is not converted to nitrate efficiently during the nitrification process. Eventually, the biomass was able to acclimate to the waste stream and return to complete nitrification. Waste stream components also initially began bioaccumulating onto the solids in the test system. The component concentration in the reactor was higher than it was in the influent during this period. However, the biomass acclimated and was able to biodegrade more of these components so that the reactor concentrations were lower than the influent concentrations by the end of the study. BOD and NH3-N removal were never affected by the addition of the waste stream. 

Figure 1. Nitrite concentration was measured throughout the trial.  



Effluent Quality and Aquatic Toxicity 

All effluent samples were within current permit limits for the existing WWTS. In addition, samples were collected from the test bioreactor effluent and were analyzed for acute Whole Effluent Toxicity (WET) testing according to the client’s permit requirements. The results indicated 100% test organism survival at the permitted in-stream dilution factor. 



This study showcased EBS’s ability to perform a bench-scale study that simulated a client’s biological nitrogen-removal process in their WWTS. EBS identified system changes that would improve the existing WWTS’s ability to treat the proposed waste stream fully. EBS’s environmental specialists were able to show that the client’s existing WWTS, possibly with some minor adjustments, had the ability to overcome inhibition, acclimate to the caustic waste stream, and produce an effluent that met existing and predicted permit requirements, including no increase in toxicity in the receiving stream.