Organic and nitrogen removal in a two-stage rotating biological contactor treating municipal wastewater

doi:10.1016/j.biortech.2003.06.005

Bioresource Technology

Volume 93, Issue 1, May 2004, Pages 91-98

https://doi.org/10.1016/j.biortech.2003.06.005 Get rights and content

Abstract

A laboratory scale rotating biological contactor (RBC) predenitrification system incorporating anoxic and aerobic units was evaluated for the treatment of settled high-strength municipal wastewater. The system was operated under four recycle ratios (1, 2, 3 and 4) and loading rates of 38–182 g COD/m² d and 0.22–14 g Oxid-N/m² d on the anoxic unit and 3.4–18 g COD/m² d and 0.24–1.8 g NH₄–N/m² d on the aerobic. The average removal efficiency in terms of chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), total suspended solids (TSS) and total nitrogen (Total-N) was 82%, 86%, 63% and 54%; settling of the RBC effluent increased COD and TSS removal to 94% and 97%. An increase in hydraulic loading resulting from higher recirculation, had limited negative effect on organic removal but improved nitrogen removal, and in terms of Total-N removal efficiency increased up to a ratio of 3 and then decreased.

Introduction

Fixed-film systems have been successfully used for organic matter stabilization and nutrient control. Rotating biological contactors (RBC) have been employed in recent years for the treatment of various types of substrates, including municipal wastewater (Grady, 1983; Akunna and Jefferies, 2000; Griffin and Findlay, 2000; Nowak, 2000), and studies have been conducted to ascertain the effect on RBC performance of factors such as disc rotation speed (Friedman et al., 1979), recirculation (Klees and Silverstein, 1992), temperature (Pano and Middlebrooks, 1983), presence of organic particulate matter (Figueroa and Silverstein, 1992), hydraulic conditions (Kugaprasatham et al., 1991), use of supplemental air (Surampalli and Baumann, 1992) and scale-up (Wilson et al., 1980).

Biological nitrogen removal involves aerobic and anoxic conditions. Various schemes have been developed for nutrient control involving suspended or attached growth processes, and the RBC system has been used for this purpose. Carbon removal and ammonia nitrification have been studied in multi-stage RBC systems using synthetic (Weng and Molof, 1974; Stover and Kincannon, 1975) or municipal wastewater (Torpey et al., 1971; Murphy et al., 1977; Pano and Middlebrooks, 1983), and the effect of the organic strength/NH₄–N ratio on the process has been examined (Okabe et al., 1996; Radwan and Ramanujam, 1997). Denitrification is accomplished through the addition of an anoxic step following or preceding the aerobic step (post or predenitrification). Postdenitrification in the RBC process has been developed since the late 1970s (Murphy et al., 1977; Soyupak and Murphy, 1979), and involves the addition of an external carbon source (such as methanol). Predenitrificaton does not require the addition of external carbon, but depends on the recycle of effluent to the anoxic unit; this method has been mainly employed in the activated sludge process and limited work has been carried out on biofilm reactors, including RBC (Tzimas and Grigoropoulos, 1996) and moving-bed systems (Rusten et al., 1995). Finally, simultaneous nitrification and denitrification accompanied by carbon removal has been reported in single or multi-stage RBC configurations (Watanabe et al., 1994; Gupta and Gupta, 1999).

The aim of this research work was to study a simple, low cost two-stage RBC system, consisting of single-stage anoxic and aerobic reactors, for the treatment of settled high-strength municipal wastewater, and to evaluate its performance under various hydraulic and organic loading conditions and recycle ratios.

Section snippets

Experimental system

The two-stage RBC system consisted of an anoxic and an aerobic reactor made of plexiglas (Fig. 1). The covered anoxic unit had a 1.0-l working volume; four fully immersed biodisks, 7 cm in diameter and with a 0.0343-m² total surface area, were connected via a stainless steel shaft to a motor and rotated at 2 rpm parallel to the direction of wastewater flow. Influent waste and recycled effluent were fed to the unit through inlet ports in the cover and its effluent was directed by gravity to the

General performance

The daily variations in COD, SCOD and TSS values and in NH₄–N, Oxid-N and Total-N values for the RBC system influent, intermediate (anoxic effluent) and effluent are shown in Fig. 2, Fig. 3; daily average samples of the influent and effluent and grab samples of the anoxic effluent were tested. From the first days of operation effluent COD was at a low level, below 100 mg/l; however, during two periods (days of operation 65–78 and 102–126) effluent quality deteriorated as a result of the washout

Effect of the organic strength to nitrogen ratio

The COD/NH₄–N ratio in the wastewater fed to the aerobic reactor (based on the anoxic unit effluent) had an average value of 9.9 (ranging from 4.5 to 20.4). It should be noted that according to Radwan and Ramanujam (1997), who treated medium-strength synthetic wastewater, at a COD/NH₃–N ratio below 23 nitrification decreased when the rotational speed was 6 rpm and increased when it was 12 rpm (it was 8 rpm in the present study). Watanabe et al. (1994), who used a single reactor with partially

Conclusions

The RBC anoxic–aerobic system was operated at a constant settled municipal wastewater feed rate of 4 l/d (a total HRT of 22.5 h) and varying effluent recycle ratios (1–4 times), with the fully and partially (35%) immersed biodisks rotating at 2 and 8 rpm. The organic and nitrogen loading rates applied averaged 104 g COD/m² d (59 g BOD₅/m² d) and 4.9 g Oxid-N/m² d in the anoxic unit and 9.2 g COD/m² d (5.9 g BOD₅/m² d) and 0.99 g NH₄–N/m² d in the aerobic. The COD removal efficiency was in the range of

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Organic and nitrogen removal in a two-stage rotating biological contactor treating municipal wastewater

Abstract

Introduction

Section snippets

Experimental system

General performance

Effect of the organic strength to nitrogen ratio

Conclusions

Water Res.

Water Res.

Performance of family-size sequencing batch reactor and rotating biological contactor units treating sewage at various operating conditions

Water Sci. Technol.

Standard Methods for the Examination of Water and Wastewater

Standard Methods for the Examination of Water and Wastewater

The effect of particulate organic matter on biofilm nitrification

Water Environ. Res.

Effect of disc rotational speed on biological contactor efficiency

J. Water Pollut. Control Fed.

Modelling of fixed films––A state of the art review

Process and engineering improvements to rotating biological contactor design

Water Sci. Technol.

Wastewater Treatment: Biological and Chemical Processes

Improved biological nitrification using recirculation in rotating biological contactors

Water Sci. Technol.

Effect of short-term and long-term changes in hydraulic conditions on nitrifying biofilm

Water Sci. Technol.

Wastewater Engineering, Treatment, Disposal and Reuse