Partial nitrification with a continuous biofilm airlift reactor of a high-strength ammonium wastewater

Resumo

Partial nitrification of a high-strength ammonium wastewater was studied in this thesis. Partial nitrification is the oxidation of ammonium to nitrite, but not to nitrate. To achieve partial nitrification, the subsequent oxidation of nitrite must be prevented. This shortcut in the biological ammonium removal process has several advantages with respect to the complete nitrification (ammonium oxidation to nitrate): (i) a 40% reduction of organic matter demand during denitrification; (ii) 63% higher rate of denitrification; and (iii) 300% lower biomass production during anoxic growth. The main factors affecting ammonium-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB) activities in a different degree and useful to achieve partial nitrification are temperature, pH and nutrient concentrations. First the pilot plant used in this thesis was designed and built. The pilot plant consisted on two airlift reactors of 127 L of volume each one, equipped with dissolved oxygen (DO), pH, temperature and TAN (NH3 + NH4+) probes. These probes were connected to a data acquisition platform which also was connected to a process computer for control and monitoring purposes. In the first period of operation, a specific control strategy to achieve and maintain full partial nitrification to nitrite (100% conversion of TAN to nitrite) was designed. The control strategy is an advanced control technique, a ratio control, in which a ratio of DO to TAN concentrations in the reactor are set to provide the necessary oxygen limiting conditions to obtain full partial nitrification. This strategy was firstly tested manually using off-line measurements of TAN concentration in the reactor. After a successful trial, the control loop was closed automatically and linked to the oxygen control loop. This strategy has been named Automatic coNtrol for partial nitriFIcation to nitrite in BIOfilm reactors (ANFIBIO®). The oxygen limiting conditions required for full partial nitrification were explored under automatically controlled conditions, through ANFIBIO®. The minimal oxygen limiting conditions were estimated to be in the range 0.25+RSP=[DO]/[TAN]<0.35 for this reactor configuration. Thereby operating at a RSP=[DO]/[TAN]¿0.25 full partial nitrification is assured. Partial nitrification to nitrite was obtained with a volumetric nitrogen loading (NLRv) rate of 1 g N-TAN L-1 d-1 at 30 ºC. Experiments proved that (i) partial nitrification in biofilm reactors is possible at high DO concentration values; (ii) fast reversible switching from complete to partial nitrification depending on the ratio imposed of [DO]/[TAN] concentrations in the bulk liquid is possible. A continuous settler to retain the granular biomass of the effluent reactor was designed and installed in order to achieve higher treatment capabilities. With the settler, an extremely high NLRv of 6.1 g N TAN d-1 L-1 and high nitrite accumulation of 99% were achieved. The start-up of the partial nitrification system was optimized by bioaugmentation. An AOB enriched inoculum was used in a start-up to assess the advantages of bioaugmentation in the start-up of a full partial nitrification system. The period required for the start-up when using a bioaugmented inoculum was as short as 30 days, achieving a NLRv of 0.65 g N-TAN L-1 d-1 at 30 ºC, with a 99% of nitrite accumulation. The stability of ANFIBIO® at room temperature was tested. A NLRv of 0.6 g N TAN L 1 d-1 and a percentage of nitre accumulation of 99% at 20 ºC were achieved. Finally, the cost of applying five different technologies to retrofit WWTPs EU to meet with the regulations regarding nitrogen discharge was estimated. The key factor in obtaining effluents under nitrogen limit is to remove the nitrogen in the liquor that results from the dewatering of digested sludge (reject water). The evaluated technologies were defined as: (i) the classic technology which consists of the recirculation of reject water back to the main influent of the WWTP and upgrading the secondary treatment with nutrient removal; and advanced alternatives based on a specific and separated treatment of the reject water: (ii) ANFIBIO® plus heterotrophic denitrification, (iii) SHARON® plus heterotrophic denitrification, (iv) ANFIBIO® plus ANAMMOX® and (v) SHARON® plus ANAMMOX®. The results shows that the alternatives with ANAMMOX® process are clearly cheaper than the alternatives with heterotrophic denitrification and that the alternatives based on the ANFIBIO® process are slightly cheaper than those alternatives including SHARON® process. The estimated savings if ANFIBIO® plus ANAMMOX® alternative was implemented in Spain and in EU would be k€57600 and k€581500, respectively.