ЗВІТ З НДР 29-81 ЗА 2007 – 2009 Р 101 Journal homepage: www.fia.usv.ro/fiajournal Journal of Faculty of Food Engineering, Ştefan cel Mare University of Suceava, Romania Volume XV, Issue 2-2016, pag. 101 - 107 PERFORMANCE OF COMPLETE-MIX AND PLUG-FLOW SYSTEMS DURING TREATMENT OF LOW LOADED NITROGEN DEFICIENT WASTE WATER – SIMULATION WITH ASAL1 MODEL *Aleksandar SLAVOV1 Department Environmental Engineering University of Food Technologies, Plovdiv, Bulgaria aleksander_slavov@yahoo.com *Corresponding author Received April 13th 2016, accepted 24th May 2016 Abstract: Based on Modified Ludzack-Ettinger process, the performance of biological complete-mix and plug-flow units are compared during simulative treatment of low loaded nitrogen deficient wastewater. BOD-based model ASAL1 is applied. Different variants of nitrogen deficiency, Anoxic- Aerobic volume distributions, as well as diverse SSVI3.5 are manipulated. It is shown that plug-flow reactors give better effluent quality over complete-mix systems in relation to indicators: total nitrogen, total BOD and total suspended solids. Nitrogen removal is primarily diminished with the increase of Anoxic against Aerobic zone, while SSVI3.5 up to 200 ml/g has a less pronounced effect on this parameter. However,the optimal nutrient composition gives the lowest ni- trogen in the outcome stream – 95.53 % for plug-flow and 94.75 % for complete-mix series. Keywords: biological nitrogen removal, complete-mix, plug-flow, STOAT, ASAL1 1.Introduction Elevated amounts of nitrogen compounds in wastewaters have harmful influence over the life in aquatic ecosystems. To meet water quality standards different methods for nitrogen removal are applied [1,2]. Common technologies include acti- vated sludge processes with various nitrifi- cation and denitrification steps realized as complete-mix or plug-flow reactors [3,4,5]. One of the most widespread sys- tems is modified Ludzack-Ettinger process where nitrogen removal is carried out in pre-anoxic and aerobic zones without the necessity of easily biodegradable carbon supplementation [6]. Nevertheless beneficial effects of biologi- cal nitrogen elimination, microorganisms involved in activated sludge have special requirements on the basis of wastewater composition. Proper biomass development and effective contaminant elimination re- quires the BOD:N:P ratio in the secondary influent to be 100:5:1 [7]. However, many sewage and industrial waste streams are below such ratio, including food production ones with high nitrogen defi- ciency – brewery, fruit processing, beverage [8,9,10], which can lead to operational problems and low quality effluents [11]. Laboratory investigations of many differ- ent conditions on the behavior of wastewater treatment plant during purifica- tion of high BOD:N containing wastewater is an expensive and time-consuming work. It can be realized considerably faster with the application of mathematical modeling of biological nitrogen removal [12], where diverse initial conditions can be applied http://www.fia.usv.ro/fiajournal mailto:aleksander_slavov@yahoo.com Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 102 within models for various design options and analyses. The aim of this work is to simulate and compare the performance of complete-mix and plug-flow systems in the treatment of low loaded wastewater with diverse amounts of nitrogen deficiency. Nomenclature BOD – biological oxygen demand MLSS – mixed liquor suspended solids SSVI3.5 – stirred sludge volume index at MLSS 3500 mg/dm3 WRc – Independent Centre of Excellence for Innovation and Growth 2.Methods 2.1. Biological unit description The biological wastewater treatment unit is designed graphically in the modeling and simulation environment STOAT® 5.0 [13] to purify wastewater with a constant flow- rate of 2400 m3/d (Fig. 1). Figure 1. Schematic representation of biological wastewater treatment unit in STOAT®. It consists of a biological activated sludge reactor with a total working volume of 1000 m3 and a secondary sedimentation tank with a surface area of 500 m2. Initial conditions for activated sludge reactor and secondary sedimentation tank are repre- sented in Table 1. Activated sludge reactor is divided into Anoxic and Aerobic zones (A/O) on the basis of modified Ludzack-Ettinger pro- cess. Three A/O percent volume ratios are investigated: 25/75, 50/50 and 75/25. Sludge wastage and internal cycle flow- rates are 5 m3/h and 500 m3/h, respective- ly. These parameters are maintained con- stant in all simulation variants. Table 1. Initial conditions in: a) activated sludge reactor; b) secondary sedimentation tank. a) Parameter Anoxic stage Aerobic stage Soluble BOD, mg/dm3 O2 5 5 NH3, mg/dm3 1 1 Dissolved oxygen, mg/dm3 0 2 MLSS, mg/dm3 3000 3000 Viable autotrophs, mg/dm3 100 100 Viable heterotrophs, mg/dm3 1000 1000 b) Parameter Stages 1-3 4-7 8 Soluble BOD, mg/dm3 O2 5 5 5 NH3, mg/dm3 1 1 1 Dissolved O2, mg/dm3 2 2 2 MLSS, mg/dm3 0 300 6000 Viable autotrophs, mg/dm3 0 100 2000 Viable heterotrophs, mg/dm3 0 10 200 2.2. Wastewater treatment plant modeling WRc’s activated sludge model: ASAL1 is chosen as a model for the description of bacterial growth and decay processes, both of autotrophs and heterotrophs. It is BOD- based and is recommended for normal ac- tivated sludge processes with hydraulic retention time higher than 4 h and NH3 concentration lower than 40 mg/dm3. The model is applied for complete-mix and plug-flow reactors according to author’s instructions [13]. ASAL1 works with the respective settling tank model: SSED1. Model ASAL1 doesn’t include the growth and decay of filamentous microorganisms, solely, although these heterotrophs directly affect development of activated sludge with good settling properties in high BOD/N ratios. Activated sludge bulking as a result of nitrogen deficite in wastewater is achieved with the variation in SSVI3.5, being higher than 150 ml/g. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 103 2.3. Influent characterization The wastewater characteristics include BOD-based profile for constant influent pattern, included in STOAT®. Influent in- dicator values are pointed at Table 2. Table 2. Influent characteristics Parameter Value Flow, m3/h 100 Temperature, °C 15 Soluble BOD, mg/dm3 O2 150 Particulate BOD, mg/ dm3 O2 90 Volatile solids, mg/dm3 180 Non-volatile solids, mg/dm3 60 NH3, mg/dm3 vary, according to BOD/N ratio Assuming that free NH3 in the influent at pH 7 and temperature 15° C is negligible small (less than 0.0027 mole/dm3), nitro- gen concentration is calculated on total BOD basis for BOD/N ratio: 100/5, 100/3 and 100/1, respectively, in the form of NH4+-N. All other parameters, included in the model, are kept at default values. 3. Results and discussion. In a set of 48 h simulations the ability of complete-mix and plug-flow activated sludge units to remove nitrogen from low nutrient wastewater is investigated. In addition, 25/75, 50/50 and 75/25 A/O volume distributions and different SSVI3.5 values are analysed. Summarized results are presented in Fig. 2a, Fig. 2b and Fig. 2c. Model data show high purification levels over 70 %. Close results are obtained for complete-mix and plug-flow reactors in all BOD/N ratios. However, plug-flow units show slightly better performance over complete-mix series which is probably due to the reactor differences in equation for steady state effluent concentration, described by [14]. Exceptions are at BOD/N ratio: 100/3 and at BOD/N ratio: 100/5, both with Anoxic/Aerobic volume relation: 75/25 in the range of 2.5 and 5.9 %, respectively higher than other variants (Fig. 2b, Fig. 2c). Low effluent nitrogen is a result of consecutive NH3 oxidation and NO3¯ reduction from wastewater, which can be achieved with proper Anoxic/Aerobic volume calculations. Results obtained show that lower or equal dimensions of Anoxic basin towards Aerobic give better performance of biological nitrogen removal unit, notwithstanding the reactor type. This counteracts with the conclusions, received by [15] and [16], where optimal A/O ratios are 0.75 and 0.64, respectively. However, the mentioned authors analyse processes with sufficient amount of influent nitrogen. At analysed BOD/N ratios exceptions are also observed, but the differences are less of the percent (Fig. 2a, Fig. 2b, Fig. 2c). Switching to bigger Anoxic vs. Aerobic zone reflects over NH3 transformation into NO3¯, which rises total N in the effluent. This process is apparent at the highest BOD/N ratio with differences over 16 % for complete-mix and more than 22 % for plug-flow reactors against other two A/O zones (Fig. 2c). To simulate the effect of sludge bulking SSVI3.5 values of 150, 200 and 220 ml/g at which effluents fit standards for total nitrogen, total BOD and total suspended solids are explored (Fig. 2a, Fig. 2b). Obtained results show that development of filamentous organisms in large numbers doesn’t reflect nitrogen removal capacity of the system for the investigated SSVI3.5 interval and period of simulations. The highest nitrogen removal is achieved at optimal BOD/N ratio 100/5 and equal A/O volumes – 95.53 % for plug-flow reactor and 94.75 % for complete-mix unit (Fig. 2c). Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 104 a) b) c) Figure 2. Total nitrogen removal in biological complete-mix and plug-flow unit from wastewater with BOD/N ratio: a) 100/1; b) 100/3; c) 100/5. 8 9 .7 5 8 9 .8 1 8 7 .8 2 9 0 .5 5 9 0 .7 3 8 8 .9 8 8 9 .5 6 8 9 .9 9 8 7 .8 2 9 0 .5 5 9 0 .7 3 8 8 .9 8 8 9 .1 5 8 9 .3 9 8 7 .4 1 9 0 .4 5 9 0 .7 3 8 8 .9 8 0.00 20.00 40.00 60.00 80.00 100.00 A/O = 25/75 Complete- mix A/O = 50/50 Complete- mix A/O = 75/25 Complete- mix A/O = 25/75 Plug-flow A/O = 50/50 Plug-flow A/O = 75/25 Plug-flow T o ta l N r e m o v a l, % Anoxic/Aerobic zone volume distribution, % Nitrogen removal at BOD/N ratio: 100/1 SSVI3.5 = 150 ml/g SSVI3.5 = 200 ml/g SSVI3.5 = 220 ml/g 9 3 .8 3 9 3 .9 3 8 7 .9 4 9 4 .5 6 9 4 .7 5 8 4 .6 1 9 3 .7 6 9 3 .8 9 8 7 .9 1 9 4 .5 0 9 4 .7 5 8 5 .1 2 9 3 .5 3 9 3 .6 5 8 7 .9 9 9 4 .4 0 9 4 .7 2 8 5 .7 0 0.00 20.00 40.00 60.00 80.00 100.00 A/O = 25/75 Complete- mix A/O = 50/50 Complete- mix A/O = 75/25 Complete- mix A/O = 25/75 Plug-flow A/O = 50/50 Plug-flow A/O = 75/25 Plug-flow T o ta l N r e m o v a l, % Anoxic/Aerobic zone volume distribution, % Nitrogen removal at BOD/N ratio: 100/3 SSVI3.5 = 150 ml/g SSVI3.5 = 200 ml/g SSVI3.5 = 220 ml/g 9 4 .6 4 9 5 .3 4 9 4 .7 5 9 5 .5 3 7 8 .0 2 7 3 .1 7 0.00 20.00 40.00 60.00 80.00 100.00 Complete-mix Plug-flow T o ta l N r e m o v a l, % Anoxic/Aerobic zone volume distribution, % Nitrogen removal at BOD/N ratio: 100/5 A/O = 25/75 A/O = 50/50 A/O = 75/25 Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 105 At BOD/N ratio 100/3 the best nitrogen reduction is the same for plug- flow reactor with A/O ratio 50/50 at SSVI3.5 from 150 to 200 ml/g – 94.75 %. Complete-mix system gives better performance at equivalent A/O volume and SSVI3.5 200 ml/g – 93.89 % (Fig. 2b). At BOD/N ratio 100/1 the lowest nitrogen values are obtained with identical A/O ratios at all three SSVI3.5 – 90.73 %. Complete-mix type achieves 89.99 % at same A/O distribution and SSVI3.5 200 ml/g (Fig. 2a). Figure 3. Total nitrogen in the effluent at different reactor system and BOD/N ratio. Figure 4. Total BOD in the effluent at different reactor system and BOD/N ratio. 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 1 6 11 16 21 26 31 36 41 46 T o ta l n it ro g e n , m g /d m 3 Time, h Total nitrogen at different BOD/N ratio Complete-mix and BOD/N: 100/5 Plug-flow and BOD/N: 100/5 Complete-mix and BOD/N: 100/3 0.00 5.00 10.00 15.00 20.00 25.00 1 6 11 16 21 26 31 36 41 46 T o ta l B O D , m g /d m 3 O 2 Time, h Total BOD at different BOD/N ratio Complete-mix and BOD/N: 100/5 Plug-flow and BOD/N: 100/5 Complete-mix and BOD/N: 100/3 Plug-flow and BOD/N: 100/3 Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 106 Figure 5. Total suspended solids in the effluent at different reactor system and BOD/N ratio. A comparison between 48 h effluent profiles is done for the best obtained values from complete-mix and plug-flow reactors for all BOD/N ratios. The obtained results are represented in Fig. 3, Fig. 4 and Fig. 5. Analysed indicators: total nitrogen, total BOD and total suspended solids from all variants cover European standards for effluent quality from wastewater treatment plants – 2 mg/l, 25 mg/l O2 and 35 mg/l, respectively [17]. Furthemore, total nitrogen concentration in the purified wastewater is less than 0.7 mg/l (Fig. 3). Nevertheless, it should be taken into account that very low effluent nitrogen under 0.5 mg/l can provoke additional sludge bulking even in well performed wastewater treatment plant [18]. Better performance of plug-flow reactors over complete-mix ones have been achieved in all simulation series, which confirms the comparative statements of the two systems, summarized by [11]. Station- ary conditions in total nitrogen removal are attained after 21st h. Only complete-mix system for BOD/N: 100/5 reaches earlier equilibrium – after 11th h (Fig. 3). Ob- tained result may be explained with opti- mal nutrient composition of the wastewater. Plug-flow systems don’t differ too much in their effluent BODs. Faster substrate bio- degradation by heterotrophs corresponds with earlier BOD balance in the outgoing stream, gained between 16th and 21st h (Fig. 4). Complete-mix units don’t reach dynamic equilibrium for 48 h. Only the effluent at BOD/N: 100/1 has two times higher BOD loading at 7th h in comparison with the other ideal-mixed reactors. The result is connected with the bulking sludge which increases particular BOD, giving higher total BOD in the effluent (Fig. 2a, Fig. 4). Complete-mix and plug-flow series for BOD/N ratios: 100/5 and 100/3 are equal in their effluent profiles for total suspended solids. Average balance is reached around 21th h. At BOD/N: 100/1 the plug-flow unit follows the same behavior. In contrast, bigger SSVI3.5 overloads the complete-mix system – about 4 h total suspended solids are over permissible for the effluent quality – 35 mg/dm3 (Fig. 5). 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 1 6 11 16 21 26 31 36 41 46 T o ta l s u sp e n d e d s o li d s, m g /d m 3 Time, h Total suspended solids at different BOD/N ratio Complete-mix and BOD/N: 100/5 Plug-flow and BOD/N: 100/5 Complete-mix and BOD/N: 100/3 Plug-flow and BOD/N: 100/3 Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XV, Issue 2 - 2016 Aleksandar SLAVOV, Performance of complete-mix and plug-flow systems during treatment of low loaded nitrogen defi- cient wastewater – simulation with ASAL1 model, Food and Environment Safety, Volume XV, Issue 2 – 2016, pag. 101 – 107 107 4. Conclusion As a result of investigated analysеs on the biological nitrogen removal in complete- mix and plug-flow reactors the following more important conclusions can be made: 1. Plug-flow reactors give better perfor- mance over complete-mix units with lower effluent concentrations of total nitrogen, total BOD and total suspended solids; 2. Better nitrogen removal can be achieved if Anoxic basin is smaller or equal to Aer- obic; 3. Rising SSVI3.5 up to 200 ml/g doesn’t affect nitrogen elimination from wastewater; 4. The best nitrogen removal is reached in a plug-flow reactor with equal Anoxic and Aerobic zones, at optimal nutrient compo- sition BOD:N = 100:5 – 95.53 %. 5. References [1]. LUO X., YAN Q., WANG C., LUO C., ZHOU N., JIAN C., Treatment of Ammonia Nitro- gen Wastewater in Low Concentration by Two- Stage Ozonation, International Journal of Envi- ronmental Research and Public Health, 12: 11975- 11987, (2015) [2]. BREISHA G. 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