Novel autotrophic nitrogen removal system using gel entrapment technology
Highlights
► A pilot-scale nitritation–anammox plant using gel entrapment technology was operated. ► Nitrifying bacteria were immobilized in gel cubes, resulting stable nitritation rate. ► Anammox bacteria were entrapped in gel cubes, resulting high denitrification rate. ► The overall nitrogen removal efficiency of combined both process was 84.4%. ► Stable nitrogen removal was attained even at high influent SS concentration.
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.
Section snippets
Experimental setup
Fig 1 shows a schematic of the pilot plant for nitrogen removal from digester supernatant. The wastewater was taken three times a week from a centrifugal dewatering machine in a municipal WWTP and stored in a 4 m3 tank before being fed into the nitritation reactor, whose effluent was then fed into the anammox reactor. The composition of the supernatant obtained from the activated sludge digestion facilities of the Mooka WWTP (Japan) is given in Table 1. The volumes of the nitritation and anammox
Nitritation
Fig 2a shows time courses for the nitrogen load and nitritation rate in the nitritation reactor. After the startup of the plant, no significant nitritation activity was observed for 15 days. Then, the nitritation rate started to increase rapidly and reached 1.4 kg N m−3 d−1 on day 29. This result indicates that the nitritation process using gel carriers, in which nitrifying bacteria were pre-entrapped, can be started within one month. As a result of the stepwise increase in the influent nitrogen
Conclusions
In the present study, a novel nitrogen removal system from anaerobic digester supernatant using a gel entrapment technique was demonstrated at the pilot-plant scale. Stable nitritation rate of 1.7 kg N m−3 d−1 and nitrogen conversion rate of 5.0 kg N m−3 d−1 were obtained on average. Since the suspended solids (SS) and gel carriers were separated by a screen, stable nitrogen removal performance was observed even with a high influent SS concentration.
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