Simultaneous nitrification and denitrification process in a new Tubificidae-reactor for minimizing nutrient release during sludge reduction
Introduction
The broader application of activated sludge process in domestic and industrial treatment plants has amplified sludge problems. With increasingly stringent requirements on the discharge of excess sludge, the management of excess sludge presents many technical challenges (Wei et al., 2003). An ideal way to solve the problem is to reduce sludge production during treatment process rather than the post-treatment of excess sludge produced (Mahmood and Elliott, 2006). Thus intensive attention has been received about the technologies applied in the wastewater purification processes based on the following mechanisms: lysis-cryptic growth (Kamiya and Hirotsuji, 1998, Chen et al., 2001), uncoupling metabolism (Mayhew and Stephenson, 1997, Low et al., 2000), maintenance metabolism (Wagner and Rosenwinkel, 2000, Rosenberger et al., 2002), and micro-fauna’s predation (Ghyoot and Verstraete, 2000, Wei et al., 2003). Compared with other strategies, micro-fauna’s predation as a cost-efficient way (Rensink and Rulkens, 1997) presented a potential solution to reduce sludge production in wastewater treatment process. Recently, many studies have focused on sludge reduction induced by aquatic worms. With the advantages of high trophic level, wide feeding habits, large appetite and big size, aquatic worms may have most potential for waste sludge reduction in aerobic wastewater treatment process (Liang et al., 2006). The existence of worms was reported to be accompanied by lower sludge production rates (Huang et al., 2007, Hendrickx et al., 2009a).
However, full-scale applications of micro-fauna’s predation are still challenge as there are two major disadvantages including unstable worm growth in the worm reactor and nutrients release induced by sludge predation (Elissen et al., 2006, Wei et al., 2009b). The problem of unstable worm growth can be overcome by using a proper worm reactor which is operated under the optimal process condition (Huang et al., 2007, Guo et al., 2007, Hendrickx et al., 2009a, Hendrickx et al., 2009b, Wei et al., 2009a). But an increase of phosphorus, nitrogen, and even soluble chemical oxygen demand (sCOD) in effluent during micro-fauna’s predation seems to be unavoidable. In wastewater treatment process, parts of nutrients in the influent are removed by incorporating into the biomass and then withdrawn with excess sludge. These nutrients may be released again during the sludge mineralization owing to the predation of the worms. Wei et al. (2009b) reported that the rates of sCOD, ammonia and phosphorus produced in the predation of Tubifex tubifex increased in correspondence with dry weights of T. tubifex organisms, and their increasing rates were 0.09 mgCOD mg Tubifex−1 d−1, 0.03 mg NH4+–N mg Tubifex−1 d−1 and 0.0006 mg TP mg Tubifex−1 d−1, respectively. The same phenomenon of the nutrients release for the worms consuming also has been reported by Hendrickx et al. (2009b). The results showed that a release of 0.22 mg total ammonia–N g ww−1 d−1 was produced during the worms consumed the waste sludge. As mentioned above, sludge predation might induce nutrients release which was discharged with the effluent of worm reactor. The nutrient must be further removed to meet the increasing downstream nutrient removal requirements and prevent eutrophication in the receiving waters. Worm reactor thereby must be followed by recycling of the predated portion to the wastewater treatment system, which may increase hydraulic load and nitrogen load of WWTP influent flow. For instance, recycling of the predated portion discharged from a novel aquatic worm reactor may increase hydraulic load by 5–15% of the WWTP influent flow, while the nitrogen load would increase by 5% (Hendrickx et al., 2010b). Considering the impact on the wastewater treatment system, the concentration of nutrients in the predated portion of sludge must be minimized.
Recently, some researches reported that denitrification could take place in the process of sludge reduction induced by Lumbriculus variegatus’ predation. They observed that dissolved nitrate and nitrite completely disappeared from the water phase while the ammonium accumulation occured during the experiment, which was conducted in Petri dishes with no aeration (Buys et al., 2008). Conversely, the ammonium accumulation and the process of denitrification has not been reported by other literatures in which the worms (Tubificidae or Aeolosomatidae) are mixed with the sludge by aeration (Huang et al., 2007, Guo et al., 2007). In these reactors, the released ammonium can be reduced by nitrification process, while denitrification process is inhibited at high DO concentration. Comparing the process condition applied in the Petri dishes and worm reactors, the aeration condition and the DO concentration were the critical conditions to the existence of nitrification process and denitrification process. Buys et al. (2008) indicated that denitrification taking place in the Petri dish without aeration was a consequence of simulating the sediment conditions, which could not be met in a mixed worm reactors (Guo et al., 2007, Huang et al., 2007).
To sum up, simultaneous sludge reduction and nitrogen removal can be achieved by creating suitable conditions, particularly suitable aeration condition and DO condition which are essential for stable worms’ growth and efficient simultaneous nitrification–denitrification. The efficient simultaneous nitrification–denitrification may reduce the pollutants release induced by sludge predation, especially organic matter and nitrogen release. In this study, as distinct from the current worm reactors, we proposed a Static Sequencing Batch Worm Reactor (SSBWR) in which the Tubificidae are immobilized in a special poly carrier (Tian et al., 2010), as shown in Fig. 1. The reactor applied a combined aeration system composed of continuous and intermittent aeration and cycle operation consisted of predation stage and mixing stage, with which a sludge layer may be formed on each carrier during most of the operation time. As a result of dissolved oxygen (DO) concentration gradients arising from diffusional limitations, there exist aerobic microzone and anoxic microzone in the sludge layer that allow simultaneous nitrification–denitrification to take place in the SSBWR. The initial experiments (results not shown) conducted in a 10 L reactor SSBWR proved that the high apparent denitrification efficiency can be obtained in this reactor concept. Further research in this paper was carried out in 100 L SSBWR which had the same structure compared to the initial one. The study was focused on: (1) evaluating the effect of structural characteristic of the SSBWR on the sludge reduction and nutrient removal, (2) analyzing the simultaneous nitrification–denitrification process for reducing the sCOD and NH4+–N release in the SSBWR, (3) providing further insight into the nutrient (sCOD and nitrogen compounds) transformation during the worm predation with the mass balance of the nitrification–denitrification cycle.
Section snippets
Perforated panels
An SSBWR with the working volume of 100 L was designed as schematically shown in Fig. 1. Twenty-five perforated panels with special poly carrier were loaded in the worm reactor to enhance the resident area for Tubificidae. The space between each perforated panel was 2 cm and 10 cm in the plane direction and the upright direction, respectively. The total surface area of perforated panels was 0.66 m2.
Special poly carrier
The sediment-dwelling Tubificidae was the main sludge predator in SSBWR. They lived partially
Potential of sludge reduction and denitrification in SSBWR
As well as the stable sludge reduction, effective denitrification was observed in the SSBWR during the long-term performance of 76 days. As shown in Fig. 2, the sludge reduction rate and apparent denitrification efficiency could be maintained at 31.1% and 97.1%, respectively, indicating that effective sludge reduction and nitrogen removal could be obtained simultaneously in the SSBWR. The total sludge reduction was a sum of worm predation, endogenous decay and sludge accumulation. In this
Conclusions
The effective sludge reduction and nutrient removal were obtained in a SSBWR and then a list of the specific results that were achieved in batch experiments and a long-term observation:
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Effective sludge reduction and nutrient removal were obtained in the SSBWR owing to the novel structure of perforated panels, combined aeration system and cycle operation. The results showed that the sludge reduction rate of SSBWR could reach 33.6%, while the NH4+–N release, NO2−–N + NO3−–N concentration and TN
Acknowledgements
This study was supported by the National High-tech R&D Program (863 Program) of China (No. 2007AA06Z348), the National Natural Science Fund of China (No. 50978071) and the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No.2008TS01).
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