An Indian refiner experienced some challenges in maintaining quality control specifications on nitrogen removal in the Effluent Treatment Plant (ETP). Like most refineries, their effluent needed to comply with their operating permit and local wastewater discharge regulations.
Ammonia is one of the quality specifications that is often a challenge to consistently achieve in a refinery’s ETP outlet.
Biological treatment is one of the most economical methods to remove nitrogen. The typical biological nitrogen removal process is an Anoxic/Oxic activated sludge process. Ammonia is oxidized to nitrite and then to nitrate in an aerobic (oxic) tank through nitrification reactions. The oxidized water is recycled back to an anoxic tank, in which nitrite and nitrate are reduced to nitrogen gas through denitrification reactions.
Nitrification is achieved through specific strains of nitrifying bacteria (e.g. Nitrosomonas and Nitrobactors). These specific bacterial cultures need to be cultivated and promoted as a thriving culture as part of the general biological population. However, the nitrifying bacteria are very slow growing and vulnerable to unfavorable water conditions (e.g. toxins, inconsistent ammonia inlet concentration, poor bicarbonate concentration, excessive free oil, etc.). If the culture is eliminated through toxic conditions, it often requires more than a month to reestablish an effective population of nitrifying bacteria to recover the ammonia oxidation. If this occurs, there is a risk of ETP effluent not meeting the ammonia discharge limit.
The refiner identified that the dominant issue causing the variation in ammonia concentration in the ETP outlet was the variability in both magnitude and frequency of the ammonia concentration at the ETP inlet. This variability was identified as primarily due to crude oil feed variability.
While the refiner wanted to maintain flexibility in processing whatever crude oils were the most economically attractive, they wanted to be able to manage this variability in crude diet and achieve more consistency in achieving the ammonia quality specifications in the ETP outlet.
The refiner consulted SUEZ on a potential solution and the outcome of these discussions was to conduct a trial for bio-augmentation, which is the practice of enhancing the performance of an indigenous bacterial population through the addition of bacterial strains with specific derivative abilities.
Specifically, for this situation, the addition of bacterial cultures capable of ammonia degradation under stressed conditions was explored. BioPlus* BA2912, a concentrated liquid mixture of nitrifying bacteria, designed to provide removal of ammonia- nitrogen from wastewater, was used. BioPlu
BA2912 contains Nitrosomonas and Nitrobacter strains, as well as other bacteria capable of oxidizing ammonia.
The dosing rates were calculated based on the specific plant operating conditions, including ammonia demand and hydraulic load. Ongoing optimization of the dosing rates is required as impact of the toxins in the effluent water is undefined. This presents an ongoing requirement to provide a unique response to the toxins, and dose of the BioPlus BA2912 accordingly.
After implementing the BioPlus BA2912 dosing program, overall performance of the activated sludge significantly improved in terms of ammonia -nitrogen (NH3-N) reduction. The time required to achieve this reduction was significantly faster than previous recoveries, and was in fact achieved within a few days.
Nitrification rate increased in the aerobic compartment with higher nitrate concentration (NO3) observed across the aeration tank.
The ammonia concentration of the biologically treated water was more consistent in control even with significant variation (higher) of the ammonia concentration in the feedwater. The operating target for the ETP effluent was to maintain the ammonia concentration at less than 1.5 ppm. This target was lower than the discharge permit, however the cost of non-compliance would have been very significant because of the limited resources to impound out-of- specification water.
The refiner appreciated the positive impact of the bio-augmentations and it remains part of the tools available to the refiner to manage their ammonia reduction through the ETP.
The primary benefit to the refiner is that there is more consistent performance of the bio-plant in terms of ammonia reduction. The incidents of quarantining off specification water has reduced, further supporting the refiner’s priority of maintaining effluent discharge quality specifica-tions.
It also allowed the refinery to relax the limits of ammonia in the inlet feed whilst still achieving more consistent ammonia outlet of the biological process. This helped the refinery to maintain the crude margin as the ETP plant did not require the planners to restrict any crude selection based on nitrogen content.