Two phase (thermophilic acidification and mesophilic methanogenesis) anaerobic digestion of waste activated sludge

Transcription

1 Two phase (thermophilic acidification and mesophilic methanogenesis) anaerobic digestion of waste activated sludge Prof. Dr. Göksel N. Demirer Department of Environmental Engineering Middle East Technical University Ankara, Turkey Civil, Environmental and Chemical Engineering RMIT University, Melbourne, Australia 26 September 2007

2 OUTLINE OF THE PRESENTATION Personal Background Brief Introduction to Anaerobic Treatment Theoretical Background of the Study Objective of the Study Materials and Methods Results and Discussion Conclusions

3 PERSONAL BACKGROUND EDUCATION Ph.D., Environmental Engineering December 1996 Department of Civil and Environmental Engineering Vanderbilt University, Nashville, Tennessee, USA. M.Sc., Environmental Engineering June 1991 B.Sc., Environmental Engineering June 1989 Department of Environmental Engineering. EMPLOYMENT AND PROFESSIONAL EXPERIENCE Middle East Technical University, Department of Environmental Engineering, Ankara, Turkey. Professor (August 2005-present) Associate Professor (November 1999-August 2005) Assistant Professor (February 1998-November 1999) Instructor/Lecturer (April 1997-February 1998) Research/Teaching Assistant (February August 1992) Washington State University, Department of Biological Systems Engineering, Pullman, WA, USA. Visiting Professor (January 2003-July 2004) Anadolu University, Department of Environmental Engineering, Eskişehir, Turkey. Adjunct Faculty Member (February 2001-January 2003) Tuzla University, Department of Technology, Tuzla, Bosnia-Herzegovina. Visiting Professor (July-August 1998) Vanderbilt University, Department of Civil and Environmental Engineering, Nashville, TN, USA. Research/Teaching Assistant (August 1992-December 1996)

4 Personal Background 2/3 TEACHING Anaerobic Treatment of Wastes (Graduate) Wastewater Engineering Design (Undergraduate) with Dr. M. Kerestecioglu Fundamentals of Environmental Engineering Processes (Undergraduate) Introduction to Environmental Engineering (Undergraduate) Fundamentals of Environmental Engineering (Undergraduate) Treatment and Disposal of Water and Wastewater Treatment Sludge (Undergraduate and Graduate) Cleaner Production (Undergraduate and Graduate) Pollution Prevention (Undergraduate and Graduate) Environmental Chemistry Laboratory (Undergraduate) Natural Systems for Wastewater Treatment (Undergraduate and Graduate) with Dr. S.Chen AREAS OF INTEREST Anaerobic Environmental Biotechnology Wastewater Engineering Cleaner Production/Pollution Prevention Natural Treatment Systems Bioenergy and Biobased Products.

5 Personal Background 3/3 SOME OF THE CURRENT/RECENT PROJECTS High Rate Anaerobic Degradation of Organic Fraction of Municipal Solid Wastes in an Innovative Sequential Reactor (Leaching and Upflow Sludge Bed) Configuration Production of Renewable Energy and Biobased Industrial Chemical Products from Organic Wastes Organic Acid Production from Municipal and Agro-Industrial Wastes Biogas production potential from cotton wastes Enhanced Anaerobic Digestion of Farm Animal Manure Removal of TCE in Sequential (Biological/Chemical) Reactors Cleaner production opportunity assessment for a milk processing facility Integrated/Preventative Environmental Management for Municipalities Domestic Wastewater Treatment in Pilot-Scale Constructed Wetlands Implemented in the METU Campus Application of Anaerobic Technologies for the Management of Industrial Wastewaters

6 BRIEF INTRODUCTION TO ANAEROBIC TREATMENT Anaerobic treatment can be defined as the use of microbial organisms, in the absence of molecular oxygen, for the stabilization of organic materials by conversion to methane and some inorganic end products. Organic matter + H 2 O anaerobes CH 4 + CO 2 + NH 3 + H 2 S + New cells Anaerobic treatment of wastes results in conversion of readily biodegradable organic matter into biogas (20-30% CO 2, 60-79% CH 4, 1-2% H 2 S and other gasses) and water. Using anaerobic treatment, it is possible to convert municipal, agricultural and industrial wastes into useful by-products mainly methane which may be used to provide heat or electrical power.

7 Brief Introduction to Anaerobic Treatment 2/6 Acetate as substrate (Methanosaeta) Sucrose as substrate (mixed culture)

8 Metabolic steps in anaerobic digestion Brief Introduction to Anaerobic Treatment 3/6 The first group of microorganisms secretes enzymes which hydrolyze polymeric materials to monomers such as glucose and amino acids, which are subsequently converted to higher volatile fatty acids, H2 and acetic acid. Microbial groups involved Fermentative bacteria H 2 -producing acetogenic bacteria, H 2 -consuming acetogenic or homoacetogenic bacteria, CO 2 -reducing methanogenic bacteria, and Acetoclastic methanogenic bacteria. In the second stage, hydrogenproducing acetogenic bacteria convert the higher volatile fatty acids e.g., propionic and butyric acids, produced, to H2, CO2, and acetic acid. Finally, the third group, methanogenic bacteria convert H2, CO2, and acetate, to CH4 and CO2.

9 Brief Introduction to Anaerobic Treatment 4/6 WHY ANAEROBIC TREATMENT? Heat Loss 100 kg COD Aeration (100 kwh) AEROBIC Sludge kg Effluent 2-10 kg COD Biogas 35 m 3 or 285 kwh 100 kg COD ANAEROBIC Effluent kg COD Sludge 5 kg

10 Brief Introduction to Anaerobic Treatment 5/6 BENEFITS OF ANAEROBIC TREATMENT production of usable energy in the form of methane no need for aeration and associated energy costs Low production of stabilized sludge Very low nutrient requirements Little if any energy requirement Reduction of green house gas emissions, up to 4 levels! Very high loading rates (up to 35 kg COD/m 3.day) Plain technology (relatively simple in operation and maintenance) biodegradation of aerobic non-biodegradables such chlorinated organics Anaerobic sludge can be stored unfed (provision of seasonal treatment important especially for campaign industries )

11 Brief Introduction to Anaerobic Treatment 6/6

12 THEORETICAL BACKGROUND OF THE STUDY Waste Activated Sludge (WAS) is the excess bacteria in the form of suspended solids that are produced by the biological conversion of biochemical oxygen demand (BOD) in the activated sludge process. Typical chemical composition and properties of waste activated sludge COD (g/l) Total dry solids-ts (%) Volatile solids (% of TS) Protein (% of TS) Nitrogen (N, % of TS) Phosphorus (P 2 O 5, % of TS) Potash (K 2 0, % of TS) ph Alkalinity (mg/l as CaCO3) Organic acids (mg/l as Hac) Energy content (kj/kg) NOT ONLY A WASTE! High polluting potential COD, N, P, TS, etc. High potential for reuse Protein, N, P, Org. acids, etc. High energy content

13 Theoretical Background of the Study 2/10 Typical WAS production is 0.7 lb/10 3 gal (or x kg/10 3 m 3 ) or 0.5 kg/kg of BOD destroyed or g of dry matter per person per day in western countries (Or on average 90 tons/day for a 1 million city) WAS treatment and disposal is receiving increased attention as sludge volumes are becoming higher and higher as a consequence of more stringent criteria for wastewater treatment plant effluent and due to the building of new treatment facilities (Bolzonella et al., 2007). The disposal of WAS poses a significant challenge to wastewater treatment because sludge handling represents 30 40% of the capital cost and about 50% of the operating cost of many wastewater treatment facilities (Vlyssides and Karlis, 2004; Choi et al., 2006).

14 Theoretical Background of the Study 3/10 Anaerobic digestion is the most widely used method of WAS disposal due to its high performance volume reduction and stabilization and the production of biogas that makes the process profitable. However, biological hydrolysis which is the rate limiting step for the anaerobic degradation of WAS has to be improved to enhance the overall process performance as well as the associated cost. Thermal, alkaline, ultrasonic, mechanical, thermal-alkaline, thermochemical, microwave and ozone pre-treatment methods have been investigated to improve hydrolysis and anaerobic digestion performance. Provided that these methods are energy-intensive and costly, biological pre-treatment methods of WAS has recently received attention because of their efficiency and relatively low investment. Two-phase anaerobic digestion process or the application of hydrolysis/acidification step before methanization constitutes one of these methods.

15 Metabolic steps in anaerobic digestion Theoretical Background of the Study 4/10 Two Phase Configuration Acidogenic Phase Methanogenic Phase One Phase Configuration Microbial groups involved Fermentative bacteria H 2 -producing acetogenic bacteria, H 2 -consuming acetogenic or homoacetogenic bacteria, CO 2 -reducing methanogenic bacteria, and Acetoclastic methanogenic bacteria.

16 Theoretical Background of the Study 5/10 The different growth rate and ph optima for acidogenic and methanogenic anaerobic organisms and, thus, different requirements regarding reactor conditions has led to the development of the two-phase AD process. The two-phase configuration has several advantages over conventional one-phase processes. Such as: selection and enrichment of different bacteria in each phase, increased stability of the process, preventing ph shock to the methanogenic population, etc. higher total biogas production Thus, the process can be smaller and more cost-efficient.

17 Theoretical Background of the Study 6/10 Bolzonella et al. (2007) reported the main findings in recent literature with particular attention to two phase anaerobic digestion of WAS (Table 1). Data reported in Table 1 indicates that level of COD solubilization and increase in biogas production is significantly increased with pretreatment (acidification) thermophilic temperatures over 60 C and HRTs of 2-3 days. Table 1. Experimental results from previous studies on two-phase anaerobic digestion of sludge Pretreatment (acidification) step temperature ( C) Pretreatment (acidification) step HRT (day) Dissolved COD (%) Increase in gas production (%) Reference Bhattacharya et al., Watts et al., Watts et al., Roberts et al., Lu and Ahring, Oles et al., Watts et al., Cheunbarn and Pagilla, Gavala et al., Lu and Ahring, Lu and Ahring, Lu and Ahring, 2005

18 Theoretical Background of the Study 7/10 It has to be noted here that the concentrations and thus the organic loading rates (OLR) used in the pretreatment (acidification) studies were significantly high, namely g VS/l.day (Shana et al., 2005; Bolzonella et al. 2007). Considering that high concentrations or OLRs along with low retention times lead to higher acidification in two-phase systems (Ghosh, 1987; Shin et al., 2001), they improved the acidification and led to high performance as presented in Table 1. However, it is well known fact that WAS from different wastewater treatment plants is likely to be less concentrated due to different process configurations and operational practices which lead to lower organic loading rates. IN OTHER WORDS, WE DO NOT KNOW WHETHER THE ADVANTAGES OF ANAEROBIC PHASE SEPARATION APPLY TO WWTPs WITH LOW OLRs OR NOT.

19 Theoretical Background of the Study 8/10 Examples of Pilot- and Full-Scale Applications

20 Theoretical Background of the Study 9/10

21 Theoretical Background of the Study 10/10

22 OBJECTIVE OF THE STUDY Based on the previous studies and the above discussion, this study investigated the performance of thermophilic anaerobic preacidification prior to conventional mesophilic (methanogenic) anaerobic digestion or twophase anaerobic digestion of waste activated sludge (WAS) at low OLRs, namely g VS/l.day. The objective was simply investigating whether the advantages of anaerobic phase separation can still be exploited at low OLRs.

23 MATERIALS AND METHODS Waste activated sludge and anaerobic seed cultures Table 2. Characterisation of the WAS used in the study Table 3. Characterisation of the anaerobic seed used in the study Parameter Concentration Total COD 14550±250 mg/l Soluble COD 280±20 mg/l Total Solids 8180±35 mg/l Volatile Solids 7860±27 mg/l Total P ±8.75 mg PO 3-4 /l Soluble TP 65.5±1.0 mg PO 3-4 /l [PO 3-4 ] Total 64.2±3.0 mg PO 3-4 /l Total N 500±25 mg N/l Soluble Total N 37.5±2.5 mg N/l NH3-N 7.5±0.5 mg NH 3 /l ph 6.82 Parameter Concentration Total COD 30900±0 mg/l Soluble COD 540±0 mg/l Total Solids 19370±230 mg/l Volatile Solids 12860±160 mg/l Total P 2104±124 mg PO 3-4 /l Soluble TP 1013±75 mg PO 3-4 /l [PO 3-4 ] Total 371.5±38.5 mg PO 3-4 /l Total N ±12.5 mg N/l Soluble Total N 625±125 mg N/l NH3-N 468.5±56.5 mg NH 3 /l ph 7.50

24 Materials and Methods 2/7 Experimental set-up Thermophilic Anaerobic Acidification of WAS Twenty daily-fed continuously-mixed acidogenic anaerobic reactors with no recycle were operated at two different HRT (or SRT) values of 2 and 4 days, and three different OLR values of 0.98, 1.97 and 3.93 g VS/l.day. Duplicate reactors were operated for each HRT (or SRT) and OLR pair. Reactor operation involved daily feeding of raw WAS and wasting of corresponding reactor contents (Table 4). Solids and hydraulic retention times (SRT and HRT) applied to each reactor (Table 4) was the same since no recycle of the effluent was practiced. Initially 25 ml of concentrated anaerobic seed was added to all the reactors except Blank (B) and Control (C) reactors (Table 4). The reactors with HRT (or SRT) of 2 and 4 days were fed with 25 and 12.5 ml of original (undiluted, see Table 2) or ½ diluted WAS every day, respectively.

25 Materials and Methods 3/7 Biochemical methane potential (BMP) assay In order to determine the anaerobic biodegradability and biogas production from raw and anaerobically preacidified WAS, BMP experiments were performed. Thirty two batch methanogenic anaerobic reactors (Supelco, Bellefonte, PA, USA) were operated. Duplicate reactors were fed with the daily effluent of each reactor from the thermophilic anaerobic acidification phase as well as their corresponding feeds (or WAS and WAS(½)).The effective and total volumes of the reactors were 50 and 120 ml, respectively. Initially 25 ml of concentrated anaerobic seed was added to all the reactors except Blank (B) reactors (Table 5). The Control (C) reactors contained seed but were fed with water (Table 5). Then, the reactors were flushed with 100% N2 gas for 5 min, sealed with natural rubber sleeve stoppers and maintained in a shaking water bath (Stuart Scientific, Staffordshire, UK) at 35 ± 1 C and 100 rpm and daily gas production was monitored by a water displacement device in each reactor. At the end of the BMP assay (Day No: 30), the reactor contents were analysed for CODt, CODs, and VA.

26 Materials and Methods 4/7 Reactor Table 5. Experimental setup for BMP assay Seed Addition Fed with Feed Volume (ml) C1 Acid + Eff B1 Acid + Eff T1 Acid + Eff C2 Acid + Eff B2 Acid + Eff T2 Acid + Eff B3 Acid + Eff T3 Acid + Eff B4 Acid + Eff T4 Acid + Eff B1-Meth - WAS 25 0 B2-Meth - WAS C1-Meth + WAS C2-Meth + WAS C3-Meth + WAS(½) C4-Meth + WAS(½) Seed* (ml) Eff.: Effluent of the corresponding acidifying reactor WAS: Undiluted raw (not subjected to anaerobic acidification) waste activated sludge WAS(½): Raw (not subjected to anaerobic acidification) waste activated sludge diluted by ½

27 Anaerobic counterpart of aerobic BOD test. Cheap Not labor-intensive Can provide very valuable design if properly designed and conducted properly. Performed in batch reactors Procedure: Add Seed+Waste+Basal Medium Purge the reactors with a gas containing no O 2 for 3-4 min. Incubate the bottles in the hot room at 35±2 0 C. Measure the daily produced biogas by a simple water displacement device Materials and Methods 5/7 Biochemical Methane Potential (BMP) Assay BMP is a standard method to determine anaerobic treatability and biogas production of a waste/wastewater based either total gas or CH4 production. It is typically used as the first step of a series of anaerobic treatability activities. NH 4 Cl MgSO 4.7H 2 O KCl Na 2 S.9H 2 O CaCl 2.2H 2 O (NH 4 ) 2 HPO 4 FeCl 2.4H 2 0 CoCl 2.6H 2 0 KI MnCl 2.4H 2 0 CuCl 2.2H 2 0 ZnCl 2 AlCl 3.6H 2 0 NaMoO 4.2H 2 O H 3 BO 3 NiCl 2.6H 2 0 NaWO 4.2H 2 O Na 2 SeO 3 Cysteine NaHCO 3 Concentration (mg/l)

28 Materials and Methods 6/7 The daily effluents from all the acidogenic reactors were used for the analysis during the operation of the reactors. Furthermore, the effluents from the reactors on the last day (Day 15) of operation were collected and subjected to biochemical methane potential (BMP) assay along with corresponding raw (not subjected to anaerobic acidification) WAS and WAS(½) samples of 25 (HRT or SRT of 2 days) and 12.5 ml (HRT or SRT of 2 days) in separate reactors. Table 4. Experimental setup for acidogenic reactors Reactor Seed Addition Daily feeding with HRT or SRT (days) Daily feeding/wasting volume (ml) OLR (g VS/l.day) C1-Acid + Water B1 Acid - WAS T1 Acid + WAS C2 Acid + Water B2 Acid - WAS T2 Acid + WAS B3 Acid - WAS(½) T3 Acid + WAS(½) B4 Acid - WAS(½) T4 Acid + WAS(½)

29 Materials and Methods 7/7 SUMMARY OF THE EXPERIMENTAL DESIGN Raw WAS Acidogenic Phase Raw WAS Acidified WAS BMP Assay (Methanogenic Phase) Compare The gas production COD reduction VA concentration Continuous Reactors Operated for 15 days Batch Reactors Operated for 30 days

30 ph RESULTS AND DISCUSSION Thermophilic Anaerobic Acidification of WAS Figure 1 depicts the ph profile in all the acidogenic reactors during course of operation. After the onset of the operation, the ph of all test reactors decreased with varying rate and extent which was determined by the combination of HRT (or SRT) and OLR applied. For the same HRT, the rate ph drop during the operation was increased with the increase in the OLR as expected. While for the same OLR, the ph drop was inversely proportional with the increase in HRT of the reactor. This is an expected observation since it is a well known fact that low retention times and high OLRs lead to higher acidification in two-phase systems (Ghosh, 1987; Shin et al., 2001). The extent of ph drop observed for all the reactors did not vary too much and the minimum ph value was obtained in reactor T1 with The relatively high ph values observed can be explained by the alkalinity generated by the anaerobic biodegradation of nitrogenous organic compounds (Speece, 1996) contained in the WAS used in this study C1 C2 B1 T1 B2 T2 B3 T3 B4 T Time (Days)

31 CODs (mg/l) CODs/CODt (%) Results and Discussion 2/6 When a high solids-containing waste is introduced to an anaerobicly acidifying reactor, the particulate organic matter is liquefied through hydrolysis along with acidification (Demirer and Chen, 2004). This mechanism can be quantified with monitoring the soluble COD concentration as well as the ratio of soluble to total COD in the acidogenic reactors. When the soluble COD or CODs/CODt values (below figure) are considered, it is seen that there has been a dramatic increase in the CODs at varying levels. For example CODs for T1 increased from the initial value of 412 mg/l to 3040 and 3380 mg/l on Days 6 and 14, respectively Day 0 (A) Day 14 (A) Day 30 (M) Day 6 (A) Day 0 (M) b Day 0 (A) Day 6 (A) Day 14 (A) c B1-Acid T1-Acid C2-Acid B2-Acid T2-Acid B3-Acid T3-Acid B4-Acid T4-Acid B1-Meth B2-Meth C1-Meth C2-Meth C3-Meth C4-Meth C1-Acid C1- Acid B1- Acid T1- Acid C2- Acid B2- Acid T2- Acid B3- Acid T3- Acid B4- Acid T4- Acid

32 Results and Discussion 3/6 The headspace gas in acidogenic reactors contained 7-22% methane. Ideally, the methane content in the headspace gas produced in acidifying reactors should be negligible. In practice, however, varied amounts of methane of up to 30% have been detected in acid-phase digesters (Eastman et al., 1981; Ghosh, 1987; Shana et al., 2005; Yilmaz and Demirer, 2007). This may be due to either incomplete separation of the two phases, which results in the coexistence of acidogens and methane producers. The HRT (or SRT) values applied in this study (2 and 4 days) were not favorable for the most sensitive anaerobic bacteria type known as methanogens. However, the methane production at such low SRTs could be explained by unintentional extended retention times of microorganisms in the reactors due to very high solids concentration and thus lack of homogeneity during daily wasting of sludge (Ghosh, 1985/1987).

33 Volatile Acids (mg/l as HAc) Results and Discussion 4/6 Volatile Acid (VA) production is another important parameter indicating the performance of acidifying cultures. Therefore, VA analyses were conducted in the effluents of the acidifying reactors as well their feeds namely WAS and WAS(½) on Day 13 of the operation. As it is clear from the below Figure, the VA formation is due to the acidifying activity since the VA concentration in the control reactors were insignificant (less than 24 mg/l). In all the other reactors, the VA formation ( mg/l) was much higher than that of the feed WAS solutions (52-86 mg/l). This indicated that anaerobic acidification increased the VA concentration in the WAS and WAS(½) by 6.3 and 9.8 times, respectively C1-Acid B1-Acid Day 13 (A) Day 30 (M) T1-Acid C2-Acid B2-Acid T2-Acid B3-Acid T3-Acid B4-Acid T4-Acid B1-Meth B2-Meth C1-Meth C2-Meth C3-Meth C4-Meth

34 Results and Discussion 5/6 Biochemical methane potential (BMP) assay After the acidification reactors reached to steady state conditions, the preacidified WAS samples (or the effluents of thermophilic acidogenic anaerobic reactors) and raw WAS samples (or the feeds to the thermophilic acidogenic anaerobic reactors) were subjected to Biochemical Methane Potential (BMP) assay. The BMP assay was run for 30 days at the end of which all the reactors were analysed for CODt, CODs, and VA. When the CODt results are considered, it is observed that all the preacidified WAS samples has led to increased CODt removals than their corresponding feeds. The difference between the CODt removals of preacidified and raw WAS samples were more pronounced for reactors T1 and T2 (13.8 and 16.1%, respectively) than T3 and T4 (8.0 and 11.9%, respectively). CODt reduction in acidogenic and BMP reactors CODt removal (%) Reactor Preacidification BMP Assay B1-Acid T1-Acid B2-Acid T2-Acid B3-Acid T3-Acid B4-Acid T4-Acid C1-Meth 38.4 C2-Meth 36.3 C3-Meth 37.0 C4-Meth 34.8

35 Results and Discussion 6/6 The increase in the CODt removal by up to 16.1% observed during the BMP assay is an important enhancement of the anaerobic biodegradability of WAS. However, when the CODt removal observed for the same samples during acidogenesis along with the methane production is considered, the net effect of preacidification on the process can better be evaluated. This is in agreement with Ghosh (1987) who suggested that the methane from acidification phase could be transmitted to methanogenic phase of the system to increase the overall system efficiency. So, when the total CODt removals are considered (Table 6), it will be seen that preacidification has led to 38.3, 49.4, 26.2, and 30.5% extra CODt removals in reactors T1, T2, T3, and T4, respectively, relative to reactors fed with non preacidified WAS.

36 CONCLUSIONS Thermophilic pre-acidification of WAS at g VS/l.day of OLR and 2-4 days of HRT resulted in % CODt reduction and % increase in dissolved (soluble) COD concentration in acidogenic reactors. This corresponded to times higher acidification levels relative to unacidified WAS feed samples. When the preacidified WAS samples were subjected to BMP assay along with unacidified WAS samples, it was observed that % additional CODt removal or gas production was observed for preacidified samples. When the CODt removals observed for preacidification and BMP were both considered, preacidification (or phase separation) has led to % extra CODt removal. It is apparent from these results that beneficial effect of preacidification on CODt removal is sustained for low OLRs ( g VS/l.day).

37 ACKNOWLEDGEMENTS The Department of Education, Science and Training of the Australian Government is appreciated to support Dr. Goksel N. Demirer s visit to the RMIT University through the Endeavour Research Fellowship. The speaker would also like to thank to Dr. Maazuza Othman and all the other staff of School of Civil, Environmental and Chemical Engineering of RMIT University who contributed and supported this research.

38 THANK YOU FOR YOUR PATIENCE