Implementation of a selector for sludge settling enhancement in an activated sludge system treating petrochemical wastewater

Resumen

The objective of the thesis is to enhance sludge settling in existing full—scale petrochemical activated sludge system focused on organic matter removal, in order to accomplish future more stringent emission levels. Two issues must be faced to accomplish this goal: i) To overcome frequent low food-to-microorganism (F/M) filamentous bulkings and ii) To limit unintended biologiical nitrification. The approach to solve the filamentous bulking has been to implement a selector in the activated sludge system. The performance of aerobic and anoxic selectors has been tested in a pilot-scale activated sludge system supplied with petrochemical substrate and inoculum. Both selectors allowed to improve sludge settling by reducing the excessive proliferation of filaments, though the aerobic selector configuration showed a more consistent operation. Nevertheless, the inclusion of a selector in the biological system increased unintended nitrification. The operational and design parameters of the aerobic selector were optimized, with the goal to provide guidelines to build a full-scale selector. The optimum hydraulic retention time was characterized at 30 min, whereas F/M should be in the range of 30 to 35 g COD g-1VSS d-1. Biodegradable (mínimum tested 10.4 g BOD g-1VSS d-1) and low particulate matter content substrates (máximum recommended of 65 g tCOD g-1VSS d-1) should be supplied to the selector. The mixed liquor parameters were also optimized to enhance sludge settling for a system including a selector. Even with the inclusion of a selector, enough dissolved oxygen in the main reactor, according to its F/M, must be guaranteed so as to provide a good sludge settling. Unintended biological nitrification was limited through the addition of cost effective doses of folic acid to bench-scale biological reactors, which did not supply toxicity to the effluent. However, the vitamin also had an effect on the operational parameters of the activated sludge system. Therefore, to its full-scale implementation it must be taken into account that doses of 0.4 g g-1VSS d-1 requiere spare aeration capacity and doses of 0.9 g g-1VSS d-1 worsened sludge settling. Future experimental work is recommended, to test additional dosing strategies for folic acid and also to develop an action mechanism of folic acid. This thesis provides guidelines for petrochemical engineers to upgrade their wastewater treatment processes, though the results and methodology could also be extrapolated to other sectors with a similar characterization of the effluents. Additionally, the ability of folic acid to limit nitrification could start a new research field in the agricultural sector, where nitrification is the main cause for the losses of nitrogen and for nitrogen oxides emissions.