Elsevier

Bioresource Technology

Volume 98, Issue 11, August 2007, Pages 2065-2075
Bioresource Technology

Operation and model description of a sequencing batch reactor treating reject water for biological nitrogen removal via nitrite

https://doi.org/10.1016/j.biortech.2006.04.033Get rights and content

Abstract

The aim of this study was the operation and model description of a sequencing batch reactor (SBR) for biological nitrogen removal (BNR) from a reject water (800900mgNH4+NL-1) from a municipal wastewater treatment plant (WWTP).

The SBR was operated with three cycles per day, temperature 30 °C, SRT 11 days and HRT 1 day. During the operational cycle, three alternating oxic/anoxic periods were performed to avoid alkalinity restrictions. Oxygen supply and working pH range were controlled to achieve the BNR via nitrite, which makes the process more economical. Under steady state conditions, a total nitrogen removal of 0.87 kg N (m3 day)−1 was reached.

A four-step nitrogen removal model was developed to describe the process. This model enlarges the IWA activated sludge models for a more detailed description of the nitrogen elimination processes and their inhibitions. A closed intermittent-flow respirometer was set up for the estimation of the most relevant model parameters. Once calibrated, model predictions reproduced experimental data accurately.

Introduction

In Wastewater Treatment Plants (WWTPs), the supernatant from centrifugation of anaerobically digested sludge (reject water) contains up to 25% of the total nitrogen load in a flow, and it is usually returned to the head of the sewage treatment works (Macé and Mata-Álvarez, 2002). Biological nitrogen removal (BNR) from this wastewater (800–900 mg NH4+NL-1) can be achieved in the existing WWTP. An efficient alternative is the use of a sequencing batch reactor (SBR) for the treatment of this highly loaded water, since nitrogen removal efficiencies of more than 90% have been reported (Arnold et al., 2000, Rostron et al., 2001).

The BNR process is divided into two steps: the oxidation of ammonium to nitrate (nitrification) and the nitrate reduction to nitrogen gas (denitrification). Nitrification is defined as a two-step process, where ammonium is firstly oxidized to nitrite (nitritation, Eq. (1)) and subsequently nitrite is oxidized to nitrate (nitratation, Eq. (2)).NH4++3/2O2NO2-+2H++H2O(Ammonium oxidizing biomass)NO2-+1/2O2NO3-(Nitrite oxidizing biomass)Denitrification is then the reduction of NO3-NO2- (Eq. (3)) and further on to N2 (Eq. (4)) by the catabolism of heterotrophic bacteria. This process is carried out under anoxic conditions and with a biodegradable carbon source, such as acetate, as electron donor.4NO3-+C2H4O24NO2-+2CO2+H2O8NO2-+3C2H4O2+H2O4N2+8OH-+6CO2Some authors (Abeling and Seyfried, 1992, Hellinga et al., 1999, Wett and Rauch, 2003) have discussed the beneficial effects of performing the BNR process via nitrite, since it suggests a saving of 25% of the aeration costs and 40% of the external carbon source needed during denitrification, as well as a reduction in the amount of sludge produced. Many ways have been described in literature to achieve the BNR process via nitrite. Anthonisen et al. (1976) determined the effect of ammonia (NH3) and nitrous acid (HNO2) concentration upon the ammonium oxidation and the nitrite oxidation kinetics. These authors demonstrated that nitrite oxidizing biomass (NOB) is inhibited at concentrations higher than 0.2–2.8 mg HNO2 L−1 and/or 0.1–1.0 mg NH3 L−1, while ammonium oxidizing biomass (AOB) is inhibited by unionised ammonia concentrations higher than 10–150 mg NH3 L−1. Wett and Rauch (2003) corroborated these experimental results and experienced a partial inhibition of nitrite oxidizers in a SBR treating extremely ammonium loaded landfill leachate and reject water. Grunditz and Dalhammar (2001) also reported that NOB kinetics are more affected by basic values of pH than AOB. Furthermore, many authors (Guisasola et al., 2005, Pollice et al., 2002, Ruiz et al., 2003) have reported that, at reduced dissolved oxygen concentrations, ammonium oxidation is favoured over NOB activity due to a greater oxygen affinity for the first step of nitrification. Another technique to achieve the inhibition of NOB consists of the SHARON process (Hellinga et al., 1999). This process is based on the careful selection of a low sludge retention time (SRT) and a high operating temperature (35 °C), which enables the proliferation of AOB and the total wash-out of NOB.

Activated sludge models (Henze et al., 2000) represent the most widespread and successful approach to characterise the nutrient removal process for design and control (Copp et al., 2002, Seco et al., 2004). The biological nature of wastewater treatment processes implies that their model parameters must be determined (model calibration) according to the local situation (Vanrolleghem et al., 1999). Respirometry is the most popular tool used for model calibration and it consists of the measurement and analysis of the biological oxygen consumption under well defined experimental conditions (Rozich and Gaudy, 1992, Spanjers et al., 1998).

On the other hand, the IWA models (Henze et al., 2000) describe nitrification as a single step process, since nitrite does not usually appear as an intermediate product under the typical temperature range and ammonium concentrations of secondary biological reactors from municipal WWTP. However, these models must be modified to describe properly high ammonium loaded wastewater treatments and/or to study the BNR via nitrite.

In this study, a BNR via nitrite strategy in a SBR was implemented for the treatment of a specific reject water (Barcelona Metropolitan Area). Moreover, the SBR working sequence under steady state conditions was characterised by means of a modified version of the IWA ASM models (Henze et al., 2000) previously calibrated using respirometry.

Section snippets

Lab-scale SBR reactor

The BNR process was carried out in a jacketed lab-scale SBR (3 L). Operating temperature was maintained by means of a heating system (RM6 Lauda). Fill and draw stages were performed by two peristaltic pumps (Cole-Parmer Instrument 7553-85). Air flow inside the reactor was controlled by an electromagnetic valve. External carbon source was added through a peristaltic pump (EYELA Micro Tube Pump MP-3). The SBR was also equipped with a thermocouple, a pH electrode (Crison pH 28) and a DO probe (Oxi

Reject water characterisation

Table 1 shows the average composition of the reject water used in the experiments. This wastewater was mainly characterised by a high ammonium concentration (800–900 mg NH4+NL-1) and a high temperature, which made its treatment at 30 °C feasible. The bicarbonate to ammonium ratio on a molar basis was approximately one, which means that the alkalinity of the wastewater was not sufficient to buffer the complete nitrification process but only about half the process. Consequently, the reactor

Conclusions

In this study, a SBR was used to carry out BNR via nitrite for the treatment of a real reject water from a municipal WWTP. This highly ammonium concentrated wastewater had a reduced buffer capacity and a very low BODST concentration. Consequently, an external carbon source for denitrification was required for its treatment.

Nitrogen was completely removed when operating with three cycles per day, temperature 30 °C, SRT 11 days and HRT 1 day. During the operational cycle, three alternating

Acknowledgements

The authors are grateful for the grants received from the Spanish government (Joan Dosta) and from the University of Barcelona (Alexandre Galí). This research has been supported by the CICYT (CTM 2005-02877/TECNO).

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