EFFECTS OF THERMAL HYDROLYSIS PRE-TREATMENT ON ANAEROBIC DIGESTION OF SLUDGE Pallavi Bishnoi Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Master of Science In Civil Engineering John T. Novak (Chair) Gregory D. Boardman Amy J. Pruden July 31, 2012 Blacksburg, VA Keywords: anaerobic digestion, thermal hydrolysis, solids removal, biosolids, odor Copyright © 2012, Pallavi Bishnoi EFFECTS OF THERMAL HYDROLYSIS PRE-TREATMENT ON ANAEROBIC DIGESTION OF SLUDGE Pallavi Bishnoi ABSTRACT The increased demand for advanced techniques in anaerobic digestion over the last few years has led to the employment of various pre-treatment methods prior to anaerobic digestion to increase gas production. These pre-treatment methods alter the physical and chemical properties of sludge in order to make it more readily degradable by anaerobic digestion. The thermal hydrolysis process has been used in several treatment plants around the world, but none currently operate in the US. Thermal hydrolysis causes cell walls to rupture under the effect of high temperature and high pressure and results in highly solubilized product which is readily biodegradable. The performance of the process was evaluated for a treatment plant located in Dallas, TX. The performance assessment was based on various characteristics including pH, solids removal, COD removal and gas production. The study was conducted in two phases to investigate the effect of change in mesophilic temperature (37oC and 42oC) and the effect of solids retention time (SRT) (15 days and 20 days). Thermally hydrolyzed combined (1:1) primary and waste activated sludge was fed to a Thermal Hydrolysis (TH) anaerobic digester and its performance was compared to a conventional mesophilic anaerobic digester receiving non pre-treated sludge. The thermal hydrolysis pre-treatment was found to be more effective as compared to the conventional anaerobic digester. The efficiency of the process varied slightly with increase in temperature but the change in SRT was seen to have a greater impact on the digester’s performance. The pre- treatment technique was observed to deliver the best results at a 20 day SRT. ACKNOWLEDGEMENTS First and foremost I would like to offer my sincerest gratitude to my advisor, Dr. John T. Novak, who supported me throughout this research with his patience and valuable guidance. I attribute the success of this work to his incessant enthusiasm and effort. His encouragement helped me to strive for betterment; personally, academically and professionally. Without him this thesis, too, would not have been completed or written. One simply could not wish for a better advisor. It will always be an honor to be counted as one of his disciples. I would also like to extend my heartfelt gratefulness to my committee members, Dr. Gregory D. Boardman and Dr. Amy J. Pruden for their guidance and timely advice. I acknowledge the monetary support provided by CDMSmith for this research work. I thank Samir Mathur for coordinating this project. I appreciate the efforts put in by Juanita Miller and Edward Keenan for shipping the sludge samples every week to Virginia Tech. The laboratory set-up and analyses would not have been possible without the precious help and support from Julie Petruska and Jody Smiley. I thank Julie for bearing with the smelly experiments and for training me for various methods during the course of this study. I would like to express a special note of gratitude to Jody, not just for training me for using the GC but for being my life-line. I thank her for going out of the way to help me out. I am also grateful to Beth Lucas for printing all those shipping labels for me every week with a smiling face. I would like to thank Betty Wingate for her assistance in all my administrative issues. I was blessed to have the most awesome group of lab-mates ever: Evan C. Bowles, Kartik Radhakrishnan, Ritika Kacker, Vidula Bhadkamkar, Renzun Zhao, Jen Miller, Ross Varin, iii Laurel Ackinson, Anna Kasassi, Abhinav Gupta and Sudhir Pathak. I thank Evan for having the answers to all my questions, no matter how stupid they were. I am grateful to everyone who has turned off the COD oven for me sometime. I would like to extend a special note of thanks to Nashita Naureen for taking care of my digesters in my absence and for being the best classmate ever. I thank all my friends for being my support system during good and bad times: Farheen Noorie, Shilpa Elizabeth Jacob, Natasha K Das, Karl S. Pereira, Adrian Pinto, Shalini Ramesh, Ravikiran Dinakar, Sudarshan Gopinath, Nikhil Jain, Navish Wadhwa, Neha Rajora, Sumedh Katkar, Ramandeep Matiana, Jaspreet Bhullar, Poulomi Dasgupta and Yash Chheda. Above all I would like to thank my family for their undying love and support. I thank my brother, Siddharth Bishnoi for being my confederate at all times. I would never have come so far if it was not for my Maa and Papa, thank you for having faith in me. All photos by the author,2012. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... ii ACKNOWLEDGEMENTS ........................................................................................................... iii TABLE OF CONTENTS ............................................................................................................. v LIST OF FIGURES ..................................................................................................................... vi LIST OF TABLES ..................................................................................................................... viii Chapter 1. Introduction ............................................................................................................... 1 Chapter 2. Literature Review ...................................................................................................... 4 2.1 Anaerobic Digestion ....................................................................................................................... 4 2.2 Pre-treatment prior to Anaerobic Digestion .................................................................................. 10 2.3 Thermal Hydrolysis Pretreatment ................................................................................................. 11 2.4 The CAMBI Process ..................................................................................................................... 12 2.5 Anaerobic Digestion of Thermally Hydrolyzed Sludge ................................................................ 14 2.6 Ammonia Inhibition ...................................................................................................................... 15 Chapter 3. Research Objectives................................................................................................. 17 Chapter 4. Methodology ............................................................................................................. 19 4.1 Digester Setup and Operation ....................................................................................................... 19 4.2 Analytical Methods ....................................................................................................................... 23 Chapter 5. Results and Discussion ............................................................................................ 27 5.1 Research approach ........................................................................................................................ 27 5.2 Experimental results ...................................................................................................................... 28 5.3 Summary ....................................................................................................................................... 52 Chapter 6. Conclusions ............................................................................................................... 54 REFERENCES ............................................................................................................................ 56 APPENDIX- Sludge Characterization ......................................................................................... 62 v LIST OF FIGURES 2-1 Multi-step nature of anaerobic operations …………………………………… 5 2-2 Schematic representation of Cambi Process…………………………………. 13 4-1 A representation of the experimental setup of anaerobic digester design …… 20 4-2 Laboratory setup of anaerobic digesters at Virginia Tech, The “bomb” used for thermal hydrolysis of sludge……………………………………………… 21 5-1 pH of anaerobic digesters during phase I (20 day SRT) ……………............... 29 5-2 pH of anaerobic digesters during phase II (15 day SRT) ……………………. 29 5-3 Overall pH for the digesters during phase I and II…………………………… 30 5-4 Average total solids reduction in anaerobic digesters during phase I and II…. 32 5-5 Average volatile solids reduction in anaerobic digesters during phase I and II 32 5-6 Average COD reduction in the digesters during phase I and II………………. 34 5-7 Correlation between VSR and CODR for all digesters……………………… 35 5-8 Specific gas production values for anaerobic digesters for phase I and II at steady state……………………………………………………………………. 36 5-9 Total VFA concentration in anaerobic digesters for phase I and II……….. 39 5-10 VFA concentration in feed and effluents from anaerobic digesters during phase I………………………………………………………………………… 40 5-11 VFA concentration in feed and effluents from anaerobic digesters during phase II……………………………………………………………………….. 40 5-12 Total ammonia concentration in feed and effluents from the anaerobic digesters during phase I and II………………………………………………... 42 5-13 Total ammonia concentration in anaerobic digesters varying with time……... 43 5-14 Phosphate concentration in control and TH-20D-42 digesters……………….. 45 vi 5-15 Variation of phosphate concentration in control and TH-20D-42 digesters over time……………………………………………………………………… 45 5-16 Experimental data for determining the optimum shearing time……………… 47 5-17 TVOSC generation pattern of sludge cakes sheared at 45s and 55s…………. 48 5-18 Total volatile organic sulfur compounds concentration in the samples from anaerobic digesters during phase I and II……………………………………. 50 5-19 TVOSC generation pattern for samples from different anaerobic digesters… 52 Appendix 1 A comparison of total solids concentration in different samples……………... 63 2 A comparison of volatile solids concentration in different samples………….. 64 3 Soluble COD concentration in the samples…………………………………... 65 vii LIST OF TABLES 4-1 Anaerobic digester acronyms used during analysis and operational characteristics………………………………………………………………. 22 5-1 %Methane data for the biogas collected from anaerobic digesters………... 37 5-2 Total VFA Concentration for feed and effluent in different phases……….. 39 5-3 Total ammonia concentration in feed and effluents for different phases….. 42 5-4 Average values for TKN in feed and effluents from anaerobic digesters…. 44 5-5 Optimum polymer dose and cake solids concentration for effluents from different digesters………………………………………………………….. 49 viii Chapter 1 Introduction Over the last century, anaerobic digestion (AD) has emerged as a reliable treatment solution for the stabilization and disintegration of sludge. The process was initially used for the treatment of household waste and sewage sludge in the municipal treatment plants. But over the past 20 years, the true potential of anaerobic digestion has been explored and major advances in reactor design, configuration and operation and in our understanding of the nature of the microbial biochemistry; physiology and ecology have been reported [Craik et al., 1995]. The growing interest of the researchers in this process is a testimony to the viability and applicability of the process. The growing population and industrial expansion have both led to the production of increasing volumes of wastewater and thus, the demand for a sustainable method for the treatment and disposal of this wastewater has also been amplified. Due to the multiple advantages of anaerobic digestion over aerobic processes [McCarty, 1986], researchers are exploring ways to enhance the process and make it more pertinent for a variety of applications. Also, the market requirements such as energy recovery, reduction of greenhouse emissions, high quality biosolids, and a low odor product and high-rate reactor systems have driven the research on various advanced anaerobic digestion techniques. 1 Pre-treatment of sludge prior to anaerobic digestion has attracted the attention of many researchers over the past decade. Various pre-treatment techniques such as mechanical grinding, chemical pre-treatment etc. have been utilized to alter the quality and characteristics of the sludge entering the anaerobic digesters. Thermal hydrolysis pre-treatment has been one of the most popular pre-treatment methods for conditioning the sludge prior to anaerobic digestion. It is known for its highly efficient sludge disintegration capability enhancing the biogas production and for destroying pathogens. Anaerobic digestion with thermal hydrolysis as pre-treatment has shown effective results in VS removal, COD removal and biogas generation [Camacho et al., 2008]. The thermal hydrolysis pre-treatment solubilizes organic particulate matter and causes lysis of cell walls. The main advantage of the process is that the hydrolysis, which is considered to be the rate limiting step in anaerobic digestion, occurs via physical rather than biological means [Wilson et al., 2008]. The problem of biosolids handling and disposal has also been improved by the thermal hydrolysis pre-treatment technology. Certain commercial thermal hydrolysis processes like Cambi claim to produce Class A biosolids with minimal odor issues [Higgins et al., 2006]. The thermal hydrolysis process is often associated with ammonia build-up issues in the anaerobic digesters causing inhibition of methanogenic activity in the system. This depends primarily on the sludge characteristics and the ammonia concentration in the feed. An aerobic digester downstream to anaerobic digester has proved to be effective to avoid the ammonia inhibition issues in the system [Tanneru et al., 2009] The main objective of this research is to evaluate the performance of a thermal hydrolysis process as a pre-treatment step prior to anaerobic digestion for the Trinity River treatment plant 2