Sanitary landfilling is the most used and accepted method to eliminate municipal solid waste
worldwide due to its economic advantages. The generation of leachate is an inevitable
consequence of this practice. Landfill leachate is a high-strength wastewater with great
chemical complexity and diversity. In order to avoid discharges to the environment causing
negative impacts to the biota or public health, it must be properly collected and treated before
In Portugal, in many leachate treatment plants, the leachate after withstanding a series of
biological and physico-chemical processes, still presents very high concentrations of nitrate
The main objective of this work was to evaluate the removal of nitrate from a landfill leachate
with high NO3- load by denitrification in an anoxic rotating biological contactor (RBC).
Accordingly, the study began by assessing the denitrification process in an anoxic RBC, for the
treatment of synthetic wastewater, under two carbon to nitrogen ratios (C/N) (1.5 and 3). For
the tested conditions, the ratio C/N=1.5 was the most advantageous. The anoxic RBC showed
a very high performance in reducing the nitrate concentration working with a relatively short
hydraulic retention time. Moreover, the increase of carbon-acetate and nitrogen-nitrate influent
concentrations had only a slight negative effect in terms of substrate removal. As the biofilm
structure and activity are determinant to the reactor performance, at the end of the continuous
experiment, biofilm characteristics, composition and activity were evaluated. It was verified
that, in spite of a lower thickness, the biofilm grown under a carbon/nitrogen ratio of 1.5 was
more active than the biofilm grown at C/N=3.
After that, the denitrification performance of the biofilm grown on the reactor disks using pretreated
landfill leachate with high nitrate load was evaluated and the effect of initial nitrate load,
phosphorus concentration and C/N ratio assessed. Under a C/N=2, the reactor achieved NNO3-
removal efficiencies above 95% for nitrate loads up to 100 mg N-NO3-!L-1. The highest
observed denitrification rate was 55 mg N-NO3-!L-1!h-1 at a nitrate load of 560 mg N-NO3-!L-1.
Although the reactor has revealed a very good performance in terms of denitrification, effluent
chemical oxygen demand (COD) concentrations were still high for direct discharge. The results
obtained in a subsequent experiment at constant nitrate load (220 mg N-NO3-!L-1) and lower
C/N ratios (1.2 and 1.5) evidenced that the organic matter present in the leachate was nonbiodegradable.
A phosphorus concentration of 10 mg P-PO43-!L-1 promoted autotrophic
denitrification, revealing the importance of phosphorus concentration on biological nitrate
In order to improve the biodegradability of the pre-treated landfill leachate, Fenton’s oxidation
(Fe2+/H2O2) and different ozone-based Advanced Oxidation Processes (AOPs) (O3, O3/OH- and
O3/H2O2) were also tested. The effect of initial pH, oxidant agents concentration and reaction
time on the performance of each AOP tested was evaluated in terms of COD, total organic
carbon (TOC), BOD5, nitrogenous compounds and aromaticity. The results indicated that
Fe2+/H2O2, O3/OH- and O3/H2O2 processes, comparatively to ozone at natural and neutral pH
values, resulted in higher COD, TOC and UV254 removal efficiencies and improvement of
landfill leachate biodegradability. These results confirm the enhanced production of hydroxyl
radical under such conditions. Although Fe2+/H2O2 is the most economical system to treat the
landfill leachate, for practical purposes O3/OH- was chosen for further work.
Finally, the performance of the sequence of treatments, leachate ozonation followed by RBC
denitrification was analyzed. The pre-ozonation led to a TOC removal of 28%. The global
system did not affect the denitrification efficiency, which remained close to 100%. In fact, it was
possible to attain a denitrification rate of 123 mg N-NO3-!L-1!h-1. The moderate decrease in the
carbon load of the final effluent indicated that some recalcitrant compounds were still present
after ozonation. These results were confirmed by the denitrifying activity tests carried out at the
end of the continuous experiment.
From the experiments performed with landfill leachate, considering the nitrate load applied,
nitrate removal efficiencies and the negligible accumulation of intermediates, the anoxic
rotating biological contactor showed to be extremely efficient and constitutes a promising technology for removing nitrate from landfill leachate.
Publication Type: PhD Theses