Mature landfill leachate treatment by denitrification and ozonation

Abstract

The removal of nitrate from a mature landfill leachate with high nitrate load in a lab-scale anoxic rotating biological contactor (RBC) was studied. Under a phosphorus-phosphate concentration of 10 mg P-PO₄³⁻ L⁻¹ and nitrogen-nitrate concentrations above 530 mg N-NO₃⁻ L⁻¹ the reactor achieved nitrogen-nitrate removal efficiencies close to 100%, without nitrite or nitrous oxide accumulation. Although the reactor presented a very good denitrification performance, the effluent carbon concentration was still above the legal discharge value. In order to increase the biodegradability of the leachate recalcitrant carbon load, a pre-ozonation was further investigated. The pre-ozonation led to a total organic carbon (TOC) removal of 28%. The sequence of treatments, leachate ozonation followed by RBC denitrification did not affect the denitrification efficiency. 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. The anoxic RBC showed to be a promising technology for removing nitrate from landfill leachate.

Introduction

Water and wastewater contamination by nitrate (NO₃⁻) constitutes a major environmental concern worldwide. Biological nitrate reduction (denitrification) is the most widely used method to remove nitrate due to the high specificity of denitrifying bacteria, low cost and high denitrification efficiency [1]. In this process, under low oxygen levels, microorganisms first reduce nitrate to nitrite (NO₂⁻) and then produce nitric oxide (NO), nitrous oxide (N₂O), and, finally, nitrogen gas (N₂). The production and accumulation of nitrite and other intermediary products are undesirable, since they are toxic, and are often referred to as incomplete denitrification. To ensure complete denitrification, since most denitrifiers are heterotrophs, sufficient carbon must be available. Denitrification efficiency is strongly susceptible to type of carbon source, concentration of carbon source, the carbon to nitrogen molar ratio (C/N) and the biomass activity. Phosphorus has also an important effect on denitrification efficiency [2], [3].

Anoxic rotating biological contactors (RBCs) started to be used for denitrification of groundwater and synthetic wastewaters in the last decade [2], [4], [5]. An anoxic RBC unit typically consists of a series of closely spaced discs that are mounted on a common horizontal shaft and are partially or completely submerged in the wastewater to be treated and inserted in a tightly closed case to avoid air entrance. Similarly to an open RBC, the pollutants contained in the wastewater are removed by the biofilm that is established on the entire surface area of the discs, which continually rotate. These reactors offer advantages such as compact design, simplicity of operation, low operating and maintenance costs, short hydraulic retention time (HRT), high biomass concentration, high specific surface area, resistance to toxic loads and relatively small accumulation of sloughed biofilm. RBCs have been applied for removal of ammonium and organic substances in the treatment of landfill leachate with high performance [6], [7], [8]. However, to the authors’ knowledge, reports of nitrate removal from landfill leachate in anoxic RBCs are not found in the literature.

Landfill leachate has been generally known as a high-strength wastewater that is most difficult to deal with. Leachate generated from mature landfills is typically characterised by high ammonium (NH₄⁺) content, low 5-day biochemical oxygen demand (BOD₅) to chemical oxygen demand (COD) ratio (BOD₅/COD), or, in other words, low biodegradability and high fraction of refractory and large organic molecules [9]. In many cases, after treatment by a series of oxidation processes, mature landfill leachate still presents high concentrations of recalcitrant compounds and nitrate. When treating this type of leachate, biological methods are ineffective, while physico-chemical and advanced oxidation processes are expensive. By combining several treatment technologies, economical savings and process optimisations could be achieved due to the degradation of the refractory compounds into biodegradable organic matter and the use of these products as a carbon source for denitrification.

Ozone (O₃) has proved to be an effective oxidant for landfill leachate, due to its oxidation potential [10]. During ozonation, organic compounds with long chains can be fragmented in lower chains, with an increase of their biodegradability, or degraded to carbon dioxide. Geenens et al. [11] used a combined treatment comprising ozonation before the biological process and verified that landfill leachate biodegradability increased after ozonation, resulting in a higher carbon removal by the biological process.

The purpose of this investigation was to evaluate the removal of nitrate from a mature landfill leachate with high nitrate load by denitrification in a lab-scale anoxic RBC, with previous ozonation to favour the biodegradability of the refractory organic load. Previously, the anoxic reactor was operated to optimise some parameters that affect denitrification efficiency such as phosphorus concentration and C/N ratio. In order to clarify the results obtained with the combined treatments in continuous mode, batch experiments were also performed to evaluate the substrate removal rate.