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Anaerobic Digestion Classified

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  • ANAEROBIC DIGESTION OF CLASSIFIED

    NN NICIPAL SOLID WASTES

    By

    Michael K. Stenstrom

    Associate Professor and

    Principal Investigator

    and

    Adam S . Ng

    Prasanta K. Bhunia

    Seth D . Abramson

    Graduate Research Assistants

    A Final Report to Cal Recovery Systems, Inc .

    and Southern California Edison Company

    October 1981

  • ACKNOWLEDGMENTS

    This work was supported by the Southern California

    Edison Company . The first year it was funded directly by

    Southern California Edison . During the second year it was

    funded by Edison through Cal Recovery Systems of Richmond,

    California .

    The authors are appreciative of the help and sugges-

    tions of Mr. Lory Larsen of Edison and Dr . Luis Diaz of

    Cal Recovery Systems .

    i

  • ABSTRACT

    Municipal Solid Waste is presently a serious problem in urban areas

    such as Los Angeles . There is an increasing demand for more landfill

    area, but political pressure and the mushrooming cost of real estate

    makes acquisition of new landfill sites nearly impossible .

    In response to the need for municipal solid waste disposal, a number

    of alternative disposal systems have been proposed . Many are based on

    classification of waste followed by some type of thermal processing . An

    alternate method is to use anaerobic digestion of a portion of the organic

    materials in the waste . Anaerobic digestion has the advantage of produc-

    ing a medium BTU gas which can be used for electric power production or

    can be upgraded to produce home heating fuel .

    This report discusses the results of a two-year investigation to

    determine the suitability of Los Angeles area municipal solid waste for

    producing digester gas . An experimental study was conducted using four

    50 gallon pilot-scale digesters . The digesters were operated at organic

    loading rates ranging from 0 .10 lb VS/ft3 day to 0 .25 lb VS/ft3 day and

    over hydraulic retention times ranging from 15 to 30 days . Feed solids

    concentration ranged from 3 .1 to 10 .1% VS . In all cases the municipal solid

    waste was blended with raw, primary sludge obtained from the Hyperion

    Treatment Plant in a ratio of 80% waste to 20% sludge, on a volatile solids

    mass basis .

    The results of the experimental investigation show that a medium BTU

    gas (55-60% methane) can be produced at a rate of 6 .5 to 7 .5 ft3 gas/lb VS

    ii

  • applied . The highest gas productions were obtained at the lowest load-

    ing rate . At higher loading rates reduced gas productions were observed,

    and this reduction is attributed to the inability to adequately mix the

    digesters .

    iii

  • iv

    TABLE OF CONTENTS

    ACKNOWLEDGEMENT

    ABSTRACT

    TABLE OF

    LIST OF TABLES

    LIST OF FIGURES

    INTRODUCTION

    CONTENTS

    i

    ii

    iv

    vii

    viii

    1

    LITERATURE REVIEW 3

    I . Background 3

    II . The Anaerobic Digestion Process 5

    III . Technical Evaluation 7

    A. Sources, Quantity, and Characteristics of MSW 9

    1 . Moisture Content 19

    2 . Particle Size 19

    3 . Particle Density 20

    4 . Chemical Composition 20

    5 . Mechanical Properties 21

    B. Preprocessing Unit Operations for Resource Recovery 21

    1 . Shredders 22

    2 . Screens 23

    3 . Air Classifiers 24

    4 . Magnetic Separators 24

    C. Preprocessing Systems for Energy Recovery25

  • vD. Pretreatment26

    E . MSW Digestion Performance Parameters 32

    1 . Nutrient Requirements 32

    2 . Organic Loading Rate and Hydraulic Retention

    Time

    33

    3 . NSW Feed Slurry Concentration 35

    4 . Temperature35

    5 . Mixing36

    6. Gas Quantity and Quality 38

    7. Dewatering Characteristics and Residue Disposal 40

    8 . Reactor Design 42

    IV . Economic Studies 43

    V .

    EXPERIMENTAL

    Summary

    PROCEDURES

    48

    51

    I . Feedstock 51

    II . Experimental Apparatus 53

    III . Start-Up Procedure 53

    IV . Feeding Procedures 55

    V . Analytical Methods 55

    A. pH, Alkalinity, Ammonia55

    B . Volatile Fatty Acids, Gas Composition 56

    C . Solids Analysis 57

    RESULTS AND DISCUSSION 58

    I . Santa Monica Survey 58

    II . Digestion Results 58

  • vi

    III . Effects of Organic Loading Rate and Hydraulic

    Retention Time in Digesters

    74

    CONCLUSIONS AND RECOMMENDATIONS 81

    REFERENCES CITED 84

  • 1 .

    2 .

    3 .

    4 .

    5 .

    6 .

    7 .

    8 . Methods for Treatment of Cellulose to Increase Digestibility 30

    9 .

    10 .

    vii

    LIST OF TABLES

    Typical Operational Criteria for the Anaerobic Digestion

    Process- Mesophilic Range

    8

    Projection of Municipal Solid Waste Generation Rates 10

    Refuse Composition Data 11

    Estimated Average Municipal Solid Waste Composition, 1970 15

    Quantities and Percentages of Municipal Refuse 16

    Municipal Refuse Composition, Percent by Weight 17

    Comparative RDF Characteristics28

    Alternate Reactor Designs44

    State of the Art Design Criteria for MSW Digestion 50

    Survey Categories for Santa Monica Municipal Solid Waste 52

    Results of the Santa Monica Municipal Solid Waste Survey 59

    Gas Production per Unit Volatile Solids Summary65

    Digester Parameter Summary 66

    Municipal Solid Waste to Methane Studies

    82

  • viii

    LIST OF FIGURES

    1 . Anaerobic Digestion Preprocessing Systems 27

    2 . Flow Process Diagram of Anaerobic Digestion Used

    for Dynatech Economic Analysis

    45

    3 . Pilot Scale Digester 54

    4 . Gas Production for Digester Number 1 60

    5 . Gas Production for Digester Number 2 61

    6 . Gas Production for Digester Number 3 62

    7 . Gas Production for Digester Number 4 63

    8 . Digested Sludge pH as a Function of Time 67

    9 . Digested Sludge Alkalinity 68

    10 . Digested Sludge Ammonia Concentration 69

    11 . Digested Sludge Total Volatile Acids Concentration 70

    12 . Digested Sludge Total Solids Concentration 71

    13 . Digested Sludge Volatile Solids Concentration 72

  • LIST OF FIGURES (Continued)

    ix

    14 . Gas Production as a Function of Organic Loading Rate 76

    15 . Volatile Solids Destruction as a Function of Organic

    Loading Rate

    77

    16 . Total Volatile Solids as a Function of Organic

    Loading Rate

    78

    17 . Gas Production as a Function of Hydraulic

    Retention Time

    80

  • INTRODUCTION

    The recent upward trend in energy costs has created renewed

    interest in novel, or heretofore uneconomical energy production

    techniques . A number of alternate technologies have been evaluated,

    including well-known methods such as passive solar heating, to poorly

    understood methods such as wave energy generation .

    Energy production from biomass has become an important research

    topic . Many methods are being developed, including fermentation tech-

    niques to produce alcohols, combustion of waste biomass, and novel pyroly-

    sis techniques . Anaerobic digestion of wastewater derived sludges is a

    particularly well-known biomass energy production technique, and has

    been used extensively at wastewater treatment plants to reduce mass

    and volume of waste sludges . Anaerobic digestion of waste sludges

    with energy recovery has been a commercially viable energy production

    technology for over 50 years .

    Application of anaerobic digestion to other wastes has not found

    widespread commercial acceptance . A number of farm wastes, such as cow

    manure, have been treated in anaerobic lagoons for many years, but this

    application is primarily a waste disposal technique, rather than an energy

    production technique . One application of anaerobic digestion which appears

    attractive is the anaerobic digestion of classified municipal waste (MSW,

    garbage and refuse which has been shredded and sorted) .

    Solid waste production is an increasingly more important problem in

    urban areas . The volume of waste production is increasing rapidly, while

  • the availability of landfill sites is declining . Existing landfills

    are being exhausted and the legal and financial problems of opening

    new landfills are causing delays which severely limit the availability

    of disposal sites . Anaerobic digestion of municipal solid waste is

    finding renewed interest due to this shortage .

    Anaerobic digestion of classified MSW is attractive since it pro-

    duces a medium BTU gas (550-650 BTU/ft 3 ) without producing the air pollu-

    tion associated with incineration . In many urban areas, especially areas

    with air pollution problems like Los Angeles, this can be a major advan-

    tage . Unfortunately, much of the pilot scale experience with digestion

    of classified MSW has been poor, in direct contrast with bench scale

    studies . The difficulties with larger scale devices is partly attributed

    to insufficient classification and pretreatment, and partly due to mixing

    problems . It appears that many of the pretreatment techniques do not re-

    move a sufficient portion of the indigestibles .

    The objective of the study herein was to evaluate the Cal Recovery

    preprocessing technique in medium scale pilot (50 gallon) digesters to

    determine if economical gas production rates could be obtained . The

    effect of organic loading rate, influent solids concentration and reten-

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