Category: Technical Articles

Performance of a two-stage partial nitritation-anammox system treating the supernatant of a sludge anaerobic digester pretreated by a thermal hydrolysis process


A two-stage system (partial nitritation (PN) and anammox processes) was used to remove nitrogen from the dewatering liquor originating from the thermal hydrolysis/anaerobic digestion (THP/AD) of municipal WWTP sludge. Two strategies were tested to start up the PN reactor: 1) maintaining a fixed hydraulic retention time (HRT) and increasing the ammonium loading rate (ALR) by decreasing the feeding dilution ratio and 2) feeding undiluted dewatering liquor and gradually decreasing the HRT. With diluted feeding, the reactor performance had destabilization episodes that were statistically correlated with the application of high specific ammonium (> 0.6 g NH4+-N/(g TSS·d)) and organic (> 0.7 g COD/(g TSS·d)) loading rates. The second strategy allowed stable PN reactor operation while treating ALR up to 4.8 g NH4+-N/(L·d) and demonstrating that dilution of THP/AD effluents is not required. The operating conditions promoted the presence of free nitrous acid levels (> 0.14 mg HNO2-N/L) inside the PN reactor that inhibited the proliferation of nitrite oxidizing bacteria.

Batch activity tests showed that the inhibitory effects of organic compounds present in the THP/AD dewatering liquor on the ammonia oxidizing bacteria activity can be removed in the PN reactor. Thus, aerobic pretreatment would not be necessary when two-stage systems are used. The PN reactor effluent was successfully treated by an anammox reactor.

An economic analysis showed that using two-stage systems is advantageous for treating THP/AD dewatering liquor. The implementation of an aerobic pre-treatment unit is recommended for WWTPs capacities higher than 5·105 inhabitants equivalent when one-stage systems are used.

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Application of Annamox-Based Processes in Urban WWTPs: Are We on the Right Track?


The application of partial nitritation and anammox processes (PN/A) to remove nitrogen can improve the energy efficiency of wastewater treatment plants (WWTPs) as well as diminish their operational costs. However, there are still several limitations that are preventing the widespread application of PN/A processes in urban WWTPs such as: (a) the loss of performance stability of the PN/A units operated at the sludge line, when the sludge is thermally pretreated to increase biogas production; (b) the proliferation of nitrite-oxidizing bacteria (NOB) in the mainstream; and (c) the maintenance of a suitable effluent quality in the mainstream. In this work, different operational strategies to overcome these limitations were modelled and analyzed. In WWTPs whose sludge is thermically hydrolyzed, the implementation of an anerobic treatment before the PN/A unit is the best alternative, from an economic point of view, to maintain the stable performance of this unit. In order to apply the PN/A process in the mainstream, the growth of ammonia-oxidizing bacteria (AOB) should be promoted in the sludge line by supplying extra sludge to the anaerobic digesters. The AOB generated would be applied to the water line to partially oxidize ammonia, and the anammox process would then be carried out. Excess nitrate generated by anammox bacteria and/or NOB can be removed by recycling a fraction of the WWTP effluent to the biological reactor to promote its denitrification.

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