Novel autotrophic nitrogen removal system using gel entrapment technology

Abstract

A pilot plant involving a nitritation–anammox process was operated for treating digester supernatant. In the preceding nitritation process, ammonium-oxidizing bacteria were immobilized in gel carriers, and the growth of nitrite-oxidizing bacteria was suppressed by heat-shock treatment. For the following anammox process, in order to maintain the anammox biomass in the reactor, a novel process using anammox bacteria entrapped in gel carriers was also developed. The nitritation performance was stable, and the average nitrogen loading and nitritation rates were 3.0 and 1.7 kg N m⁻³ d⁻¹, respectively. In the nitritation process, nitrate production was completely suppressed. For the anammox process, the startup time was about two months. Stable nitrogen removal was achieved, and an average nitrogen conversion rate of 5.0 kg N m⁻³ d⁻¹ was obtained. Since the anammox bacteria were entrapped in gel carriers, stable nitrogen removal performance was attained even at an influent suspended solids concentration of 1500 mg L⁻¹.

Introduction

Although anaerobic digestion is an excellent process for energy recovery from waste sludge in municipal wastewater treatment plants (WWTPs), plant operators sometimes hesitate to incorporate this process because of the ammonium-rich digester supernatant, which might be returned to the mainstream wastewater treatment system. This recycled nitrogen load, which has been estimated to be 10–20% of the raw influent, is significant (Fux et al., 2002). Therefore, treatment of sludge supernatant is an effective method to keep nitrogen concentrations in the effluent low. However, digester supernatant has low concentrations of biodegradable organic compounds against high ammonium concentrations (low C/N ratio). Therefore, it is costly to treat the sludge supernatant by using a conventional nitritation–denitrification system because additional organic carbon sources have to be supplied for the denitrification process.

In the 1990s, a novel metabolic pathway, anaerobic ammonium oxidation (anammox), was discovered (Van de Graaf et al., 1990, Strous et al., 1999, Strous et al., 2006). In the anammox reaction, ammonium is oxidized to nitrogen gas using nitrite as an electron acceptor by specific kinds of bacteria (“anammox bacteria”). Autotrophic bacteria still need carbon sources, which are inorganic carbon. Conventional denitrification most likely use organic matter for heterotrophic bacteria, which organic matter is electron donor and carbon sources. In addition, the amount of excess sludge can be reduced as compared with conventional denitrification due to extremely small growth yield of the bacteria (Strous et al., 1998). Moreover, the anammox process can achieve high nitrogen removal rates of up to 26 kg N m⁻³ d⁻¹ (Tsushima et al., 2007). Thus, the anammox process has begun to be used to treat sludge supernatant as well as specific ammonium-rich industrial wastewaters (Van der Star et al., 2007).

Since nitrite and ammonium are needed for the anammox process, part of the ammonium in wastewater has to be oxidized to nitrite with a pre-treatment system like a nitritation process, which involves partial nitritation of ammonium to nitrite. Because the growth rate of ammonium-oxidizing bacteria (AOB) is slow, retention of AOB in a reactor and inhibition of the growth of nitrite-oxidizing bacteria (NOB) are both necessary in order to establish a stable nitritation process. Recently, the authors have developed a novel nitritation process using nitrifying bacteria entrapped in gel carriers (Isaka et al., 2008a). In this process, AOB are immobilized in gel carriers, and the growth of NOB is suppressed by heat-shock treatment. This technique is based on the fact that NOB can be inactivated by heat shock but AOB cannot. Although nitritation performance has been investigated using synthetic wastewater in a laboratory-scale reactor (Isaka et al., 2008a), digester supernatant from municipal WWTPs has not. In addition, the long-term stability of the nitritation performance should be investigated. In the present study, the nitritation performance using real digester supernatant was evaluated on a pilot-plant scale. In the present study, the nitritation performance of the process using real digester supernatant from municipal WWTPs was evaluated at the pilot-plant scale.

As for the anammox reactor, since the growth rate of anammox bacteria is extremely slow, it is important to retain a sufficient amount of anammox biomass in the reactor in order to achieve high nitrogen removal performance. Many types of anammox reactors, including a sequencing batch reactor (SBR) and a fluidized bed reactor (FBR) (Strous et al., 1998, Van de Graaf et al., 1996, Sliekers et al., 2003), have been reported. In addition, gel entrapment techniques have been used to immobilize and grow anammox bacteria in a reactor (Isaka et al., 2007, Isaka et al., 2008b). Separation of suspended solids (SS) and anammox biomass is an important factor in developing a stable anammox process for real digester supernatant because high concentrations of SS might be contained in the water fed into the reactor when there is a problem in the dewatering process. It has been shown that gel-entrapment techniques are robust against high SS loading. Therefore, gel entrapment technique was used for immobilization of anammox bacteria. Treatment of digester liquors of raw garbage, livestock manure, and sewage sludge by using the anammox process with gel carriers has been reported (Furukawa et al., 2009). However, the nitritation and anammox processes were operated separately using lab-scale reactors.

In the present study, a pilot-scale combined nitritation–anammox reactor was operated for the treatment of digester supernatant from a full-scale WWTP. In the nitritation process and subsequent anammox process, gel carriers were used to immobilize AOB and anammox bacteria, respectively. This is the first time that stable nitrogen removal from digester supernatant from municipal WWTP was achieved with a nitritation–anammox process using gel entrapment technology.