Journal of Environmental Science and Health Part A, 41:1819–1829, 2006 C Taylor & Francis Group, LLC Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934520600778994
Optimization of Nitrogen Removal for Alternating Intermittent Aeration-Type Activated Sludge System: A New Process Modification Guclu Insel, Seval Sozen, Serden Bas¸ak, and Derin Orhon ¨ Environmental Engineering Department, Istanbul Technical University, Maslak, Istanbul, Turkey A new activated sludge process modification was proposed for intermittent aeration process to achieve more stable nitrogen removal performance. A single completely mixed reactor was divided into two compartments in series and operated in intermittent aeration mode by using activated sludge simulation model. The new configuration provided competetive advantage on nitrification as well as denitrification capacity, compared to the intermittently aerated system with a single reactor. In addition, the dissolved oxygen set-point control during air-on periods was found to be an important parameter in terms of nitrogen removal. Key Words: Activated sludge modeling; Intermittent aeration; Nitrification; Denitrification; Aerated fraction; Cycle time ratio; Aeration control.
INTRODUCTION Strict effluent limitations have been imposed to promote efficient nitrogen removal from wastewaters. Activated sludge systems, as favorable technologies in meeting those stringent effluent discharge criteria, are widely used for retrofitting the existing conventional systems to biological nitrogen removal systems cost effectively, especially in sensitive zones.[1] Moreover,
˙ Address correspondence to G. Insel, Environmental Engineering Department, Istanbul ˙ Technical University, 34469 Maslak, Istanbul, Turkey; E-mail:
[email protected]
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the flexibility in process modification/controls provides a great advantage in sustainability of the efficient nitrogen removal, as well. The aeration control is regarded as one of the principal parameters in the optimization of nitrogen removal systems.[2,3] In intermittent aeration type activated sludge process (IAS), time dependent aerobic and anoxic phases are created for effective nitrogen removal within the same reactor by a simple aeration control where aerobic and anoxic phases alternate in a sequence turning the aeration on and off in a cycle.[4,5] Without requiring additional anoxic volume and pumping equipments for internal recycle, the intermittent aeration becomes a particularly convenient tool for retrofitting the existing treatment plants to nitrogen removal systems.[6,7] The complexity of biological reactions as well as dynamic behaviour of the process (i.e., aeration control, intermittent aeration, dynamic influent conditions) requires the use of multicomponent models being increasingly used in process design and optimization of activated sludge systems. In this study, a new process modification is proposed for an intermittently aerated activated sludge system to overcome the drawbacks such as nitrogen fluctuations and unstable total nitrogen concentration in the effluent. The modification relies on splitting-up the single reactor into 2-reactors in series and operating them in an alternating air on-off mode. The advantages of process modification were illustrated with the aid of an intensive simulation study using the activated sludge model adapted from ASM1.[8]
CONCEPTUAL APPROACH The intermittently aerated activated sludge system involves a cyclic operation where aerobic and anoxic phases alternate in each cycle by means of intermittent aeration. The design of the intermittently aerated activated sludge process relies upon two specific parameters in addition to the sludge age.[9] First one is the aerated fraction (AF), which defines the fraction of aerated time (TA ), where nitrification is allowed to proceed with the consumption of NH4 -N and built up of NO3 -N, within a total cyclic period (TC ) as given in Eq. 1. The total cycle time covers the aerated (TA) and anoxic time (TD) periods. The aerated time allows the oxidation of NH4 -N to NO3 -N, whereas the anoxic time (TD) stands for the time devoted to denitrification. The AF fraction is comparable to the anoxic to total volume ratio (VD /V) in predenitrification systems. The default value of this design parameter is proposed as 0.50, based on the empirical approaches in different studies.[5,9] However, experimental justification was also provided for an intermittent aeration system operated with AF around 0.17.[10] AF =
TA TC
(1)
A New Activated Sludge Treatment System
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The second parameter is the cycle time ratio (CTR), which corresponds to the total cycle time (TC ) to the hydraulic retention time (θH ) ratio as given in Eq. 2. Lower CTR values around 0.01–0.07 are reported in the literature for efficient nitrogen removal.[4,10] This parameter can be accepted as analogous to the internal recycle ratio in predenitrification systems. In this respect, the smaller the CTR value, the higher the amount of nitrate recirculated to the anoxic period within TC . CTR =
TC θH
(2)
MATERIALS AND METHODS The simulation studies were performed in order to compare the nitrogen removal efficiencies (ammonia and nitrate) for an intermittently aerated activated sludge system (IAS) and alternatingly intermittently aerated system (AIAS) for domestic wastewater. For intermittently aerated system (IAS), aeration was switched on and off in a continuously fed single reactor. The reactor was splitted up into two equal size reactors (V/2) for AIAS. However, the wastewater was introduced only to the first reactor (Fig. 1). The aeration was alternatingly switched on and off in the reactors where the first reactor was aerobic whilst the second reactor was anoxic, or vice versa. The total hydraulic retention time (θH ) and the total sludge age (θX ) were set to around 15.2 hours and 15 days, respectively. During air on periods, the oxygen was controlled around 2 ± 0.2 mgO2 /L both for IAS and AIAS. First, the results of nitrogen simulation were compared for two systems under steady state conditions for a cycle time (TC ) range of 1 to 16 hours. These cycle times corresponded to the cycle-time ratios (CTR) of 1/15 and 16/15. The aerated fraction (AF) was always set to 0.5 indicating that the air on-off periods were
Figure 1: Implementation of alternating intermittent aerated activated sludge system (AIAS) in WEST software.
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equally distributed within one cycle. In addition, the effect of dissolved oxygen dynamics on total effluent nitrogen levels was investigated under steady state conditions. Secondly, the effect of dynamic influent wastewater conditions on the quantities of effluent nitrogen species were compared for both systems at the selected cycle time as an example. The simulation studies were performed using the endogenous decay model modified from ASM1 model.[8] The model-based influent wastewater characterization and kinetic/stoichiometric model parameters of Istanbul wastewater were adopted from Orhon et al.,[11] Okutman[12] and S¨ozen et al.[13] The average biodegradable COD and Total Kjeldahl Nitrogen (TKN) of the influent wastewater characterization were assumed as 470 mgCOD/L and 42 mgN/L, respectively. Accordingly, the readily and slowly biodegradable COD values were adopted as 50 and 305 mg/L. COST-624 wastewater simulation benchmark was used for representing the dynamic conditions. All simulations were carried out using the WEST modeling and simulation platform (Hemmis NV, Belgium).[14]
RESULTS AND DISCUSSION Steady-State Process Performance Based on the simulation studies conducted at 2 mg/L dissolved oxygen setpoint, it was concluded that operating two reactors in AIAS mode, produced much lower effluent ammonia nitrogen concentration (SNH
