Physical strength of aerobic granular sludge

Transcription

1 Physical strength of aerobic granular sludge A. Nor-Anuar *, Z. Ujang*, M.C.M. van Loosdrecht** and G.Olsson *** *Institute of Environmental and Water Resource Management (IPASA), Universiti Teknologi Malaysia, Johor, Malaysia.( **Delft University of Technology, Kluyver Laboratory for Biotechnology, Department of Biochemical Engineering, Julianalaan 67, NL-2628 BC Delft, The Netherlands. ***Lund University, Department of Industrial Electrical Engineering and Automation S Lund, Sweden. Abstract Aerobic granular sludge (AGS) has a number of advantages over conventional activated sludge flocs, such as cohesive and strong matrix to allow organisms to grow in fixed positions, fast settling ability, high biomass retention, and ability to withstand high organic loadings, leading towards a compact reactor system. However, to date studies on the strength of AGS physical structure are scarce. In this study, samples of AGS from two different types and sources have been studied, i.e AGS developed at 20 o C and 30 o C. AGS developed at 20 o C were sampled from a 3-litre laboratory scale sequencing batch airlift reactor (fed with synthetic influent) and also from a 1.4 m 3 pilot plant bubble column at Ede wastewater treatment plant, the Netherlands (fed with pre-treated sewage). Meanwhile, AGS developed at 30 o C were sampled from a 3-litre laboratory scale reactor in which two samples were compared, i.e. AGS fed with synthetic and actual wastewater (pre-treated sewage) as influent. A procedure was developed in this study and the determination of a stability coefficient (S) was introduced to evaluate the strength of the AGS. Based on the procedure developed, the results clearly showed that AGS developed at 20 o C and 30 o C, fed with synthetic are very stable against a wide range of shear stress. For AGS fed with pre-treated sewage, the AGS developed at 30 o C in a 3-litre laboratory scale reactor are more stable than the AGS developed at 20 o C in a pilot plant. Keywords: Aerobic granular sludge; shear stress; strength of AGS; stability coefficient. INTRODUCTION Aerobic Granular Sludge (AGS) is a recent innovation in biological wastewater treatment. Granulation is a self-immobilization process in which microorganisms agglomerate and develop to dense and compact biomass granules. AGS have a number of advantages over conventional activated sludge flocs, such as regular and strong structure, good settling ability (sludge volume index (SVI) <50 mlg -1 ), high biomass retention (up to 20 g SSL -1 ), and ability to withstand high organic loadings (Beun et al.,1997). Furthermore, they are able to convert organic substrates, nitrogen compounds and phosphate simultaneously at high removal efficiencies (Dulekgurgen et al., 2003). In order to be able to design a robust system with the AGS technology, knowledge about the strength and stability of the granules is essential. Sufficient shear stress is needed for the formation of stable and dense granular sludge (Liu and Tay, 2002). However, extensive shear stress in the reactor (e.g. by mechanical mixing, aerated mixing or design of the airlift) effects the AGS settling characteristics (Nor-Anuar et al., 2007) or worse, might damage the granules. In addition, sludge handling (processing surplus sludge, inocculation of new reactors etc.) often requires pumping of a water/granule mixture, which also leads to high stress on the granules and thus to breakage. Damaging AGS during reactor operation or sludge handling can lead to decreased settling ability and subsequent washout of the biomass. Changes in granule diameter might influence the efficiency of simultaneous nitrification and denitrification, because of a changed anoxic volume inside the granules (De Kreuk and van Loosdrecht, 2004). In order to understand the capability of granules to withstand shear, more knowledge about the physical strength of AGS is needed.

2 In addition, with regards to the design of a large scale wastewater treatment plant that is operated with AGS, an adequate method has to be developed that reliably and objectively describes the physical strength of AGS. Ideally, the method can be used to evaluate the stability of AGS against the shear stress that occurs in the reactor due to the reactor operation, i.e. mixing process and sludge handling. This study aimed to develop a procedure for ease of reference to evaluate the AGS strength based on the stability of AGS against shear stress. In addition, determination of a stability coefficient (S) was also introduced as an indicator of AGS strength. The physical strength of AGS developed at 20 o C and 30 o C respectively was compared in this study as well as AGS fed with synthetic influent and actual wastewater (pre-treated sewage). MATERIALS AND METHOD Aerobic Granular Sludge (AGS) samples Samples of AGS from two different sources have been studied, i.e AGS developed at 20 o C and 30 o C. A description of the sources of the AGS samples is found in Table 1. Samples Table 1. Description of AGS samples Sample Sources AGS developed at 20 o C AGS developed at 30 o C AGS-20-synthetic AGS-20-sewage AGS-30-synthetic AGS-30-sewage Reactors 3-L laboratory scale 1.4 m 3 pilot plant 3-L laboratory scale Temperature 20 o C 20 o C 30 o C 30 o C Influent feed Synthetic Pre-treated Synthetic Pre-treated sewage sewage Experimental set-up Figure 1 shows the schematic diagram of the experimental set-up to study the shear sensitivity of the AGS. The strength was measured by shear experiments in a 2000 ml standard geometry vessel (T v = 0.133m) at 200 rpm stirrer (D = 0.075m) speed. The shear rate applied is calculated according to Van 't Riet and Tramper (1991) : Where γ is the shear rates (s -1 ); D is the blade diameter (mm); N is the stirrer speed (s -1 ) and T v is the vessel diameter (mm). (1)

3 Figure 1. Picture of experimental set up for shear experiment Analytical Procedures Before conducting the shear experiments, physical characteristics of the AGS were analysed. Size distributions of AGS were determined by sieving the granules using 0.2, 0.4 and 0.6 mm sieves. The total suspended solid (TSS) concentration of the total sample and the dry weight of each fraction were determined by drying the samples for 24 hours at 105 C (APHA, 2000). Biomass density was determined with a dextran blue method (Beun et al., 2001). Changes in diameter of the AGS (φ AGS) before and after the shear test were observed using an Image Analysing System (PAX- it version 6). Determination of Strength of Aerobic Granular Sludge (AGS) Strength of the AGS was measured based on the stability of AGS against shear stress during reactor operation. Therefore, a procedure was developed as described in Table 2, to evaluate the strength of AGS. This test procedure was based on the premise that if the AGS were subjected to fluid shear stress beyond a certain limit, the quantity of sludge released in the surrounding fluid would be a function of shear strength of the AGS. The shear stress was introduced indirectly and approximately by mixing a well-defined AGS sample in a small impeller-stirred vessel as described in the experimental set-up. RESULTS AND DISCUSSION Physical characteristics of Aerobic Granular Sludge (AGS) At the beginning of the experiment, the density and the size distribution of the AGS sample based on total suspended solids (TSS) concentration was determined, in order to analyse the differences between all types of granules. The results are summarized in Table 3. More than 50% of each sample had diameter more than 0.6 mm. The fraction between 0.4 and 0.6 mm differed slightly; more than 10% of the AGS from the laboratory scale reactor were found in this fraction compared to only 4% for AGS pilot plant. The remaining fraction passed the 0.2 mm sieve was not considered as granule and therefore not taken into account in the shear experiment. The density of AGS (ρ AGS ) synthetic-fed developed at 20 o C was 5 times higher compared to AGS sewage-fed and 2 times higher for 30 o C-AGS. This was mainly due to the different type of feeding materials; sewage contains many types of COD (readily biodegradable COD as well as colloidal and particulate COD), leading to more irregular and less dense structures (De Kreuk and van Loosdrecht, 2004). On the other hand, synthetic influent fed into laboratory scale reactor contains only sodium acetate as organic source for microorganism growth. The AGS of the pilot plant were less dense compared to the laboratory scale reactor AGS, due to the fact that operating variables in laboratory scale were easier to control with minimum uncertainties, even though both samples were fed with pre-treated sewage.

4 Table 2. Procedures for evaluation of AGS strength and determination of stability coefficient (S) and percentage of change of φags, (Σ) No. Procedure Outcome Remark Step 1 For each one of the samples, prepare four fractions of granule samples and label as (a), (b), (c) and (d). Each fraction contains approximately 1000 of granules ( 100 ml) with diameter (φ) greater than 0.2 mm Step 2 Step 3 Sample (a) will be used to determine the total dry weight (TSS concentration) using Standard Method (APHA, 2000). Before conducting shear test, analyze the average diameter (φ) of AGS sample through microscopic examination. Pour sample (b) into a small impellerstirred vessel and fill with water until the volume reaches 300ml. Then, stir the sample at 200 RPM for 10 minutes. X = total dry weight of AGS (gtssl -1 ) A = mean φags before shear test The strength was measured by shear experiments in a 2000 ml standard geometry vessel (T v = 0.133m) at 200 rpm stirrer (D = 0.075m) speed. The shear rate applied is calculated according to Equation 1. Samples used in this study are shown in Table 1. This TSS concentration value also represents the total dry weight of other samples. Microscopic examination conducted using Image Analyser D Step 4 T v Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Collect the sample after stirring and sieve over a 0.2mm sieve. Determine the dry weight of AGS residual. For particles greater than 0.2 mm, reanalyze the size with IA. Sample (c) and (d) will be used in a repeat test for accuracy. The fraction of AGS residual is used for evaluation of granule strength or in other perception the stability of AGS against the shear stress. The results are expressed in term of stability coefficient, S defined herein as the ratio of AGS residual to the total weight of AGS, expressed in percent. The stability of AGS against the shear stress has also been evaluated by analysing the changes of AGS size before and after shear stress was introduced to the AGS. The percentage of change (Σ) was calculated based on the differences of mean φ AGS before and after the shear test. X = dry weight of AGS residual (gtssl -1 ) B = mean φags after shear test S is defined as follows: S =(X / X) 100 (%) S = Stability Coefficient (%) X = Total dry weight of AGS residual (gtssl -1 ) X=Total dry weight of AGS (gtssl -1 ) An equation to calculate the Σ is determined as follows: Σ=(A-B) / B 100 (%) Σ=Percentage of change, φags (%) A=φAGS before shear test (mm) B=φAGS after shear test (mm) Similar ratio as in reactor was used, in which 1/3 and 2/3 from total working volume is granules and water. Figure above shows the schematic diagram of experimental set up to study the shear sensitivity of AGS. A particle with size less than 0.2 mm is considered as AGS residual. AGS residual was considered as a broken/damage granule during shear test. A particle with size greater than 0.2 mm is considered as AGS. Average value is reported in results and discussion S is related to the stability of AGS against shear stress, which can thus be interpreted as an indicator for stability of AGS. The lower the value of S, the greater is AGS strength. In other words, the better is the stability of AGS. S is not an accurate tool to measure the exact shear strength in the bioprocess; however it is reasonable and logical to demonstrate the index indicative of strength of the AGS against shear stress. The lower the value of Σ, the greater is the AGS strength.

5 Table 3. Physical characteristics of AGS developed at 20 o C and 30 o C Physical characteristics AGS developed at 20 o C AGS developed at 30 o C AGS-20- synthetic AGS-20- sewage AGS-30- synthetic AGS-30- sewage Size Distribution (% of TSS) > 0.6 mm mm mm < 0.2 mm ( AGS) ρ AGS (gtssl -1 of granules) Strength of Aerobic Granular Sludge (AGS) The strength of AGS is presented in the form of stability coefficient (S) and percentage of change (Σ) as shown in Table 4. The lower the value of S and Σ, the higher the stability of AGS. S value of the AGS synthetic-fed is similar even though it was developed under different temperature. Meanwhile, for AGS fed with sewage, the S value of AGS developed at 20 o C is greater than the AGS developed at 30 o C. A similar trend was also observed in the Σ value. The result showed that the percentage is 10% for AGS-20-synthetic and AGS-30-synthetic approximately. For AGS-20- sewage, shear stress introduced caused a change of 40% in which greater than AGS-30-sewage (24% of change). Table 4. Strength characteristics of AGS developed at 20 o C and 30 o C AGS developed at 20 o C AGS developed at 30 o C AGS-20-synthetic AGS-20-sewage AGS-30-synthetic AGS-30-sewage TSS total (gl -1 ) TSS fines (gl -1 ) S (%) φ AGS before (mm) φ AGS after (mm) Σ (%) This study also observed that there is a correlation between the strength and the density of AGS as shown in Figure 2. Figure 2. Correlation between stability coefficient (S) and AGS density (ρ AGS ). ( ) AGS-20-synthetic (Δ)AGS-20-sewage (Ο)AGS-30-synthetic (apple)ags-30-sewage

6 It shows that S decreases for increasing ρ AGS. For a higher ρ AGS, the value of S will be lower, i.e. a better strength. Similar relations between shear stress, strength and density are also reported for biofilm studies (Gjatelma et al., 1997; Beun et al.,1999; Villaseñor et al., 2000 and Tay et al., 2005, Ghangrekar et al., 2005) and for anaerobic granular sludge (Verschuren et al.,2002). The local shear stress in the well mixed laboratory scale airlift reactor and the pilot plant bubble column are expected to be different as well (De Bruin et al., 2005), resulting in lower strength for the pilot plant granules (AGS-20-sewage). Correlations between S, ρ AGS and Σ with temperature (T) were also studied. Statistical results of correlation obtained are summarized in Table 5. Table 5. Statistical results of correlation between stability coefficient (S), AGS density (ρ AGS ) and percentage of change of AGS size (Σ) with temperature (T) AGS-synthetic AGS-sewage Correlation correlation r value a p-value b correlation r value a p-value b equation equation ρ AGS vs T 191e -0.01T e -0.02T S vs T 2.5e 0.02T e 0.01T Σ% vs T 7.5e 0.01T e 0.01T a : perfect positive correlation; : perfect negative correlation b p 0.5 : less significant; p 0.05: significant; p or 0.001: highly significant From the correlation curve observed, the results were then re-calculated, whereas the value is fixed with the equation and thus, overall conclusion on correlation between S and ρ AGS at different T is illustrated in Figure 3. In addition, Figure 4 illustrates the relationship between S and Σ. Figure 3. The stability coefficient (S) as a function of the AGS density (ρ AGS ) for different temperatures (T). The functions are derived from the data.

7 Figure 4. The stability coefficient (S) as a function of the percentage of change of AGS diameter (Σ) for different temperatures (T). The functions are derived from the data. The functions in Figure 3 and 4 can be expressed by following simple models : S = S o. exp (-α. ρ AGS ) (2) S = S o. exp (-α. Σ ) (3) The values of S o, S o and α, α respectively for the experiments are shown in Table 6. Table 6. Correlation function parameters of the experiment Temperature ( o C) S vs ρ AGS S vs Σ S o α S o α Throughout these experimental results, a guide was developed for ease of reference to evaluate the physical strength of AGS, as described in Table 7. The illustrations that give an overview on the phenomena of AGS breakage during against the shear stress also given in Table 7. Based on the guide developed, the results clearly showed that AGS developed at 20 o C and 30 o C, fed with synthetic are very stable against shear stress. For AGS fed with pre-treated sewage, the AGS developed at 30 o C in a 3-litre laboratory scale reactor are more stable than the AGS developed at 20 o C in a pilot plant. Tay et al., (2005) also found similar observations, in which the granules developed in the laboratory-scale reactor were stronger then those in the pilot scale reactor (both granules fed with synthetic wastewater). The different hydrodynamic shear stress encountered in the pilot and lab-scale reactors might be the reason for the observed phenomena (De Bruin et al., 2005 and Tay et al., 2005). Factors such as reactor diameter and wall effect, as well as size and placement of air diffusers, would influence the hydrodynamic conditions in the reactors, which in turn determine the properties of AGS.

8 Table 7. Guide for evaluation of strength of AGS Indicator AGS strength Very Strong Strong Not strong Stability of AGS against the shear stress Very stable stable Not stable S (%) S < 5 5 S 20 S > 20 ρ AGS (gtssl -1 ) ρ AGS > ρ AGS 120 ρ AGS < 10 Σ (%) Σ <10 10 Σ 40 Σ > 40 Phenomena of AGS breakage CONCLUSIONS A procedure was developed in this study to evaluate the strength of AGS. Based on the procedure developed, the results clearly showed that AGS developed at 20 o C and 30 o C, fed with synthetic wastewater are very stable against shear stress. For AGS fed with pre-treated sewage, the AGS developed at 30 o C in a 3-litre laboratory scale reactor are more stable than the AGS developed at 20 o C in a pilot plant. This was mainly due to the fact that operating variables in laboratory scale reactor were easier to control, having fewer uncertainties. ACKNOWLEDGEMENT The authors would like to express thanks to the Universiti Teknologi Malaysia (UTM) collaboration with Delft University of Technology (TUDelft), The Netherlands for the support of this study, and Ministry of Science, Technology and Innovation for financial grant (Vot:79004). REFERENCES APHA (2000). Standard methods for the examination of water and wastewater. American Public Health Association: Washington Beun, J.J., Hendriks, A., van Loosdrecht, M.C.M., Morgenroth, M., Wilderer, P.A. and Heijnen, J.J. (1999). Aerobic granulation in a sequencing batch reactor. Water Research, 33 (10), Beun, J.J., Van Loosdrecht, M.C.M. and Heijnen, J.J. (2001). N-removal in a granular sludge sequencing batch airlift reactor. Biotecnol Bioengineering, 75 (1), De Bruin, L.M.M., Van der Roest, H.F., de Kreuk, M.K. and van Loosdrecht, M.C.M (2004). Promising results pilot research aerobic granular sludge technology at WWTP Ede. In: Bathe et al., Aerobic Granular Sludge (p ). IWA: London, UK. De Kreuk, M.K. and Van Loosdrecht, M.C.M. (2004). Selection of slow growing organisms as a means for improving aerobic granular sludge stability. Water Science and Technology, 49 (11-12), De Kreuk M.K., Aerobic Granular Sludge Scaling up a new technology, PhD Thesis. Technische Universiteit Delft, The Netherlands (2006). Dulekgurgen, E., Ovez, S., Artan, N. and Orhon, D. (2003). Enhanced biological phosphate removal by granular sludge in a sequencing batch reactor. Biotechnology Letters, 25 (9),

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