Use of Two Phase Anaerobic Bioprocess Configuration

Transcription

1 Use of Two Phase Anaerobic Bioprocess Configuration Prof. Dr. Göksel N. Demirer Department of Environmental Engineering Middle East Technical University Ankara, Turkey 1st German-Turkish Biogas Workshop İzmir, Turkey of October 2011

2 BRIEF INTRODUCTION TO ANAEROBIC DIGESTION Acetate as substrate (Methanosaeta) Sucrose as substrate (mixed culture)

3 Metabolic steps in anaerobic digestion Brief Introduction to Anaerobic Digestion 2/4 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.

4 Brief Introduction to Anaerobic Digestion 3/4 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

5 Brief Introduction to Anaerobic Digestion 4/4 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 )

6 TWO PHASE ANAEROBIC DIGESTION Acidogenic and methanogenic microorganisms present in a mixed anaerobic culture diverge, not only in terms of their nutritional and ph requirements, but also with respect to their physiology, growth and nutrient uptake kinetics and in their ability to tolerate environmental stresses. Therefore, conditions that are favorable to the growth of acid-forming bacteria (short HRT, low ph) may be inhibitory to methane forming bacteria. Furthermore, in a conventional (one-phase) anaerobic digester, the ph and organic loading rate are adjusted to suit the slowgrowing methanogenic organisms at the expense of the relatively fastgrowing acidogens and the process efficiency as a whole. The different growth rates and ph optima for acidogenic and methanogenic organisms, and thus different requirements regarding reactor conditions, have led to the development of two-phase anaerobic digestion processes.

7 Metabolic steps in anaerobic digestion Two Phase Anaerobic Digestion 2/3 Two Phase Configuration Acidogenic Methanogenic Phase 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.

8 Two Phase Anaerobic Digestion 2/3 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 efficient. Due to these advantages, the two-phase anaerobic digestion has been applied in the biogasification of wastewater treatment sludge, organic fraction of municipal solid wastes, industrial wastes and sludge, olive mill solid waste and olive pomace, grass, coffee pulp juice, food waste, cane molasses alcohol stillage, spent tea leaves, brewery wastewater, dairy wastewater, abattoir wastes as well as some studies focusing on improving reactor design, control and operational parameters.

9 OBJECTIVE OF THE PRESENTATION The most established application of two-phase anaerobic digestion is for anaerobic digestion of organic fraction of municipal solid wastes. Our research group has conducted several studies on the subject. However they are not included in this presentation. This presentation will summarize our findings on the application of this concept for waste activated sludge as well as unscreened dairy manure.

10 WASTE ACTIVATED SLUDGE 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) High polluting potential COD, N, P, TS, etc. HOWEVER NOT N ONLY A WASTE! High potential for reuse Protein, N, P, Org. acids, etc. High energy content

11 Waste Activated Sludge 2/11 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).

12 Waste Activated Sludge 3/11 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.

13 Waste Activated Sludge 4/11 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 (Thermophilic operation) Batch Reactors Operated for 30 days (Mesophilic Operation)

14 Thermophilic Anaerobic Acidification of WAS Waste Activated Sludge 5/11 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. 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 C1 C2 B1 T1 B2 T2 B3 T3 B4 T4 The relatively high ph values 6.9 observed can be explained by the alkalinity generated by the anaerobic biodegradation 6.6 of nitrogenous organic compounds contained in the WAS used in this study. 6.3 p H Time (Days)

15 Waste Activated Sludge 6/11 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. CODs (mg/l) C1-Acid Day 0 (A) Day 14 (A) Day 30 (M) Day 6 (A) Day 0 (M) 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 b CODs/CODt (%) C1- Acid B1- Acid T1- Acid C2- Acid B2- Acid Day 0 (A) Day 6 (A) Day 14 (A) T2- Acid B3- Acid T3- Acid B4- Acid c T4- Acid

16 Waste Activated Sludge 7/11 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).

17 Waste Activated Sludge 8/11 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 1000 indicated that anaerobic 900 Day 13 (A) 800 Day 30 (M) acidification increased the 700 VA concentration 600 in the WAS and WAS(½) 500 by 6.3 and times, respectively. Volatile Acids (mg/l as HAc) C1-Acid 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

18 Waste Activated Sludge 9/11 Biochemical methane potential (BMP) assay CODt reduction in acidogenic and BMP reactors 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 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

19 Waste Activated Sludge 10/11 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.

20 Waste Activated Sludge 11/11 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.

21 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. I 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.

22 UNSCREENED DAIRY MANURE Concentrated animal feeding operations along with a corresponding absence of suitable manure disposal methods have been shown to cause significant environmental and public health problems, including odors and nutrient enrichment and pathogen contamination of surface and ground waters. Anaerobic digestion (AD) of manure can offer substantial benefits to animal feeding operators and surrounding communities, such as on-site energy generation, production of stable, liquid fertilizer and high quality solid soil amendment, reduction in odors, and reduction in ground and surface water contaminations. Conventional one-phase slurry digestion is not an effective system for wastes containing high solids (>10%) such as animal manure, since they require the manure to be capable of being pumped which in itself necessitates a concentration below 10% solids. This, in turn, results in a significant increase in fluid and digester volume which results in increased capital and operating costs.

23 Unscreened Dairy Manure 2/7 One relevant feature of the two phase approach is that when a high solids containing waste is introduced to the first phase it is liquefied along with acidification. This translates into less liquid addition and, thus, less energy requirements for heating, storing, and spreading for two phase AD systems. The results of several studies have clearly demonstrated the applicability and efficiency of two phase AD for high solids containing wastes. Even though several aspects of two phase configuration including liquifaction might be very significant for efficient AD of dairy manure, its application has been limited to screened dairy manure only. Phased digestion has not been applied to dairy waste. In recognition of this fact and in support of its needed application to high solids waste, the objective of this study was to exploit the advantages of two phase AD for unscreened dairy manure. To this purpose, one phase conventional (R1) and two phase (R2) reactor configurations were run at a range of OLRs and influent VS concentrations. R1 served as a conventional one phase anaerobic digester with a SRT/HRT of 20 days. The total SRT/HRT of the two phase configuration, meanwhile, was 10 days which consisted of an acidogenic first phase with a SRT/HRTof 2 days and a methanogenic second phase with a SRT/HRT of 8 days.

24 Unscreened Dairy Manure 3/7 The Experimental Set-up Dairy Manure Used

25 One Phase Reactor Unscreened Dairy Manure 4/7

26 Two Phase Reactor Unscreened Dairy Manure 5/7

27 Unscreened Dairy Manure 6/7 Comparison of One- and Two-Phase Reactors As seen in Fig. 6a, biogas production in R2 increased with the increase in OLR. Because the SRT/HRT ratio of R2 to R1 is 0.5 (20 versus 10 days, respectively), the biogas production in R2 must be more than 50% that of R1 to realize any beneficial effect of phase separation on AD of dairy manure. Therefore, the biogas production in R2 was normalized over this value and plotted in Fig. 6b. Fig. 6b clearly shows that there is no advantage of the two-phase over conventional one-phase configuration at low OLRs. However, at high OLRs, the normalized biogas production was significantly high, namely and for OLRs of 5 and 6 g VS/L day, respectively (Fig. 6b). These values correspond to 50 and 67% higher biogas production or volume reduction for two-phase AD of dairy manure at OLRs of 5 and 6 g VS/L day, respectively. The advantage of a two-phase configuration is also clear when the biogas production with respect to varying influent VS concentrations are considered (Fig. 6c).

28 Unscreened Dairy Manure 7/7 CONCLUSIONS This study investigated the possible exploitation of the advantages of two-phase AD for unscreened dairy manure which could result in significant environmental and public health problems. The results indicated that the use of a twophase reactor at a SRT/HRTof 10 days (2 days acidogenic and 8 days methanogenic) for AD of dairy manure: Resulted in 50 and 67% higher biogas production or volume reduction at OLRs of 5 and 6 g VS/L day, respectively, relative to a conventional one-phase configuration with SRT/HRT of 20 days. Made an elevated OLR of 12.6 g VS/L day possible which was not achievable for conventional one-phase configuration. Translates into significant cost savings due to both superior performance and reduced volume requirements.

29 ACKNOWLEDGEMENTS Dr. Craig Frear of the Department of Biological Systems Engineering, Washington State University, Pullman, WA, USA is appreciated for his valuable comments and careful proofreading.

30 THANK YOU FOR YOUR PATIENCE The contact details for further info and/or copies of the relevant manuscripts: Dr. Göksel N. Demirer Middle East Technical University Department of Environmental Engineering Inönü Bulvari, Ankara-TURKEY Phone: Fax: E.mail: