Operating Thresholds for Single Stage Nitrification in Municipal Ifas and MBBR Systems as Measured in Terms of Minimum Hydraulic Retention Times and Mixed Liquor McRt

Abstract

This paper presents suggested guidelines based on the results of research on several IFAS (Integrated Fixed Film Activated Sludge) and MBBR (Moving Bed Bioreactor) full scale and pilot plants which were retrofitted into existing municipal activated sludge systems. The research has identified methods to compare the operating thresholds of each of four systems:(1) activated sludge, (2) IFAS with fixed media, (3) IFAS with moving bed, and (4) MBBR.A three-step method is presented that may be used to determine the type of system (IFAS / MBBR) and the type of media to apply in the upgrade of the wastewater treatment plant. Operating thresholds in terms of the aerobic HRT and MCRT and the MLSS levels that the wastewater treatment plant can support are presented. For comparing various systems, this paper keeps the loading and temperature constant, based primary effluent with 250 mg/L COD and 25 mg/L TKN, and a mixed liquor temperature of 12 C.The first operating threshold for the four systems can be expressed in terms of the aerobic mixed liquor MCRT (Aer MLSS MCRT). If the Aerobic MLSS MCRT is less than the washout MCRT of nitrifiers, the biofilm surface area must be adequate for maintaining nitrification on the biofilm. For the municipal wastewater with no significant inhibition of nitrification, the washout MCRT was computed to be 5 days. Above the washout MCRT, the amount of biofilm surface area decreases until the ratio of MCRT to washout MCRT is adequate to handle the diurnal peak loads.Based on an threshold on MLSS of 2,500 to 3,000 mg/L and a hydraulic retention time of 4 hours, the washout MCRT can be related to the specific surface area of media that needs to be applied in the system. IFAS and MBBR systems with media installed at specific surface areas less than 200 m 2 /m 3 need to operate at Aerobic MLSS MCRT operate at greater than the washout MCRT of nitrifiers. This includes cord media and sponge media. IFAS systems and MBBR systems that have specific surface area greater than 200 m 2 /m 3 can be operated at Aerobic MLSS MCRTs that are less than the washout MCRT of nitrifiers.A second threshold identified was the MLSS for operating each of the four systems. In activated sludge and IFAS fixed bed systems, this threshold is determined by the secondary clarifier capacity and Sludge Volume Index (SVI). For high rate clarifiers operating at 500 to 600 gpd/sf (20 to 25 m 3 /m 2 /d), the operating threshold for MLSS in an activated sludge system is 3,000 mg/L. For IFAS moving bed systems and MBBR systems, the operating threshold is also determined by the screens installed in the activated sludge basin. Three 6217 factors influence the operation of the screens: the amount of stalked ciliates in the mixed liquor, the Nocardia foam, and the prescreening.Observations of IFAS moving bed systems showed that when the MLSS exceeded 2,000 mg/L at 20 C or 2,500 mg/L at 15 C, and the Aerobic MLSS MCRT was above 5 days, long tailed stalked ciliates could form a mat on the screens if wedge-wire screens are used and the shear...

Side-by-Side Comparison of IFAS and CAS Processes at Demonstration Scale at the Lakeview WWTP

“…The calculated specific nitrification rate of the carriers falls in between 0.2 to 0.5 gN/m 2 .d in the summer, and 0.4 to 0.8 gN/m 2 .d in the winter. The range matches well with values found in the literature measured at the same plant in 2003/04 by Maas et al (2006) and Ross et al (2004), and at other plants (Johnson and McQuarrie (2002); Sen et al (2005)). It can be argued that higher rates in the cold season are a direct result of the assumptions used in the calculation.…”

“…At Lakeview the length: width ratio is 3.6. This is greater than the maximum value of 2 to 3 recommended by Sen et al (2005) to prevent bunching.…”

“…The data collected here does not allow nitrification rates to be calculated directly for the fixed and suspended biomasses individually. It was possible to approximate specific rates for the fixed biomass using assumptions found in Sen et al (2005). The assumption is that the carriers contribute to 80% of the total nitrification when the suspended SRT is equal to or below the nitrifier washout limit.…”

“…M. parvicella causes foaming at cold temperature (< 18 o C) and low loading rate (Noutsopoulos et al (2006); Knoop and Kunst, 1998) while Nocardia sp. rather thrives at warmer temperatures (Sen et al (2005) Reactor configuration and operational mode in biological nutrient removal (BNR) systems are known to play a significant role in M. parvicella proliferation (Noutsopoulos et al, 2002). In particular, anoxic zones which are not completely devoid of oxygen have been identified as an operating condition that favours M. parvicella growth (Martins et al, 2004).…”

Effects of Fluctuating Iron Dosage on Nitrification in Integrated Fixed Film and Conventional Activated Sludge Processes

“…These values apply to cell 2a only, nevertheless literature confirms that for a whole IFFAS train operated below the critical SRT, a contribution of 80% is typical for floating polyethylene carriers and it can reach up to 90% (Sen et al, 2005;Maas et al, 2006).…”