Two-Stage Start-Up to Achieve the Stable via-Nitrite Pathway in a Demonstration SBR for Anaerobic Codigestate Treatment Nicola Frison, † Silvia Lampis, ‡,§ David Bolzonella, ‡,§ Paolo Pavan, † and Francesco Fatone ‡,§, * † Department of Environmental Sciences, University of Venice, Calle Larga Santa Marta, Venice, Italy ‡ Department of Biotechnology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italy § Interuniversity Consortium “Chemistry for the Environment”, Via delle Industrie 21/8, Marghera, Italy ABSTRACT: The supernatant effluent from full scale anaerobic codigestion of waste-activated sludge and the organic fraction of municipal solid waste was treated by a demonstration sequencing batch reactor for short-cut nitrogen removal. After inoculation with conventional municipal activated sludge, the strategy of two-stage start-up allowed us the fast speciation of ammonia oxidizing bacteria (achieved within 20 days), then the achievement of the maximal treatment potential (0.8 kgN m -3 d -1 ) for nitritation-denitritation. By automatic indirect control of the free ammonia and the free nitrous acid concentrations, the via- nitrite pathway was fully and stably achieved under aerobic conditions (DO of 1.5 mg L -1 ) and T of 15 °C. Under ordinary operation, the specific nitritation rates were 15-20 mgN gMLVSS -1 h -1 , while the denitritation rate was 45-50 mgN gMLVSS -1 h -1 . In-situ and ex-situ respirometry and gene-based molecular techniques demonstrated the stable presence of a dominant, restricted ammonia oxidizing bacterial population after the first-stage aerobic start-up. We therefore demonstrated that codigestion and advanced nitrogen removal from anaerobic supernatant may optimize the performances of an integrated municipal treatment plant. ■ INTRODUCTION Nitrogen contained in organic matter, specifically in proteins, gets released when organic compounds are degraded. As a result, liquors effluent from anaerobic digesters represent a high- strength ammonia stream, which should be adequately treated before disposal or recirculation to the headworks of the wastewater treatment plants (WWTPs). 1,2 In case of mesophilic digestion, the reject water from sludge dewatering may contain 12-25% of the influent nitrogen loading which is suitable for separate treatment and cost optimization. Ammonia concen- tration in reject water is typically in the range 0.7-2 gN L -1 and the molar ammonia/alkalinity ratio is about 1, while the average ratio of COD/N is 0.5-2, but less than 25% of the COD is biodegradable. 1 In addition, the separate treatment of anaerobic liquor would be even more desirable wherever the waste activated sludge (WAS) is codigested with other organic waste containing protein nitrogen, such as organic fraction of the municipal solid waste (OFMSW). 3,4 In fact, the anaerobic codigestion enhances the biogas production, while increasing the content of ammonia nitrogen in the supernatant recirculated to the WWTP headworks. 5 Even though physical/chemical methods are of interest because nutrients are recovered as fertilizer, 2,6,7 biological methods are more cost-effective, 7 especially when short-cut nitrogen removal is operated. 8-10 To date, several sludge liquor treatment plants for short-cut nitrogen removal are in operation around the world because of the small investment and operation cost. Many of them operate the nitritation-denitritation in sequencing batch reactors (SBRs) as this requires less reaction volume than the chemostats. 6,7 In addition, the stable partial nitritation is the first crucial step even to achieve the complete autotrophic nitrogen removal. 11-14 To date, an optimal process conditions suppressing nitrite oxidation has not been investigated for effluent from codigestion of WAS and OFMSW. However, a number of experimentations and “key-factors” for achieving partial nitrification (Table 1) were widely reported for sludge liquors. Fux et al. 15 reported that high temperature (about 30 °C) and low solids retention time (SRT), in SHARON and SBR pilot scale process, favored NOB wash-out because AOB grow faster than NOB at temperature above 20 °C. On the basis of full scale results, 16 van Kempen et al. 17 suggested to maintain the SRT between 1 day and 2.5 days. In fact, it is well-known that AOB growth can be favored by an appropriate regulation of SRT in suspended-growth system, due to the different minimum required times. Recently, Mayer et al. 14 observed stable nitritation-denitritation in pilot scale SBR treating real anaerobic supernatant of OFMSW and WAS: the authors concluded that the cause of this phenomenon was uncertain, while attributing the role of major drivers to low dissolved oxygen (DO) (<1 mgDOL -1 ) in the reactor. This work deals with a reliable and reproducible operational strategy to achieve stable partial nitrification by controlling environmental conditions favorable for AOB growth, while treating real supernatant from the anaerobic digestion of WAS and OFMSW. Further to the process parameters, the desired speciation of the bacterial community is actually demonstrated and discussed on the basis of both respirometry and gene-based analytical techniques. Received: April 13, 2012 Revised: October 13, 2012 Accepted: November 1, 2012 Published: November 1, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 15423 dx.doi.org/10.1021/ie3009742 | Ind. Eng. Chem. Res. 2012, 51, 15423-15430