Review of composting and anaerobic digestion of MSW & a methodological proposal for a mid-size city

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Merry Reynolds
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1 Review of composting and anaerobic digestion of MSW & a methodological proposal for a mid-size city By M. R. Q. Silva & T. R. Naik UWM Center for By-Products Utilization University of Wisconsin-Milwaukee Milwaukee, WI, USA

2 OUTLINE Introduction Overview of composting and anaerobic digestion: fundamentals & processes Methodological proposal: a MSW Treatment for Milwaukee, WI, USA Conclusions 2

3 INTRODUCTION Garbage challenge in the world In 2003, in USA: MSW generation: 2kg/person/day 2.4x10 8 kg of MSW generated 55% landfilled 14% combusted 31% recycled or composted Composting industry USA Recycling of MSW increased 9% Composting of MSW increased 5%

4 INTRODUCTION Therefore: Key-word: SUSTAINABILITY Garbage should not be thrown away because it is possible to generate composting from MSW, while producing energy from one of the by-products of the process, the methane gas ( cleaner energy ) and harvesting materials from MSW to either recycle or reuse in cement industry. 4

5 OVERVIEW OF COMPOSTING AND ANAEROBIC DIGESTION: FUNDAMENTALS & PROCESSES 5

6 Biological fundamentals ANAEROBIC DIGESTION Microorganisms involved: hydrolytic, fermentative, acetogenic, and methanogenic bacteria Two phases: Acid-former phase Methanogenic phase 6

7 Biological fundamentals COMPOSTING Microorganisms: cryophiles, mesophiles, and thermophiles Bacteria - 80 to 90% of the microbial activity Actinomycetes - degrade cellulosic components Fungi break down debris and organic residues 7

8 Chemical fundamentals Carbon (C) and nitrogen (N) C/N ratio Optimum range: Phosphorus (P) and potassium (K): Important to the growth of the plants. Heavy metals: Some countries limit the concentration O 2 and CO 2 : Indication of composting activity. 8

9 Physical fundamentals Moisture content: Assimilation of nutrients. Desirable range: 50% and 60% Particle size: Affects moisture retention, free air space, and porosity Temperature: Optimization of decomposition rate and inactivation of pathogenic organisms 9

10 Thermodynamics fundamentals Designer of a composting plant: Microbes Reactions obtain energy Environmental conditions Conditions to kill pathogens First & Second Law of Thermodynamics: Conservation of energy and direction of spontaneous process 10

11 Composting Composting may be performed in various ways. Currently, the leading concepts are: non-reactor systems (windrows and static pile) and reactor systems (vertical flow, horizontal and inclined flow processes, and nonflow processes).

Windrows Height, width, and shape of the windrows vary Naturally aerated or forced aerated Turning: Aeration & loss of water Frequency: nature of the material,

12 Windrows Height, width, and shape of the windrows vary Naturally aerated or forced aerated Turning: Aeration & loss of water Frequency: nature of the material, pathogen kill, uniformity of decomposition, and the rapidity Arrangement for mechanical turning of the composition desired by the [Diaz et al. 1993] operator 12

13 Static Pile Passively aerated Assisted passive aerated Aerated static piles: fans aerate the composting materials. simple, require less space, not required to be turned regularly Compaction, short circuiting of air, and inconsistent decomposition within a batch of compost Passively aerated windrow system (PAWS) with perforated PVC pipes [Stofella & Kahn 2001] 13

14 Horizontal and Inclined solids flow Tumbling solids bed Agitated solids bed Static solids bed. Tumbling solids bed reactor [Haug 1993] Static solids bedreactor [Haug 1993] Agitated solid bed reactor [Haug 1993] 14

15 Vertical flow Agitated solids bed Solids are agitated during movement down the reactor, with forced aeration. Feeding can be continuous or intermittent Some mixing in reactor. Packed bed Periodic transfers of solids from the bottom to the top of the reactor occur Vertical flow, packed bed reactor [Diaz et al. 1993] 15

16 Nonflow Very simple systems Materials are loaded in the box reactor and remain there for 7 to 14 days Curing: windrows for several months afterwards Double T composting system [ ] 16

17 Anaerobic digester Currently, the leading concepts of anaerobic digestion are dry continuous system, dry batch systems, wet continuous systems, and co-digestion

18 Dry systems Continuous system Fresh substrate added continuously and an equivalent amount is withdrawn once each day or two. Dry batch system Material is not added or withdrawn Each batch: 2-3 weeks and percolate is recirculated to stimulate mixing and digestion 18

19 Wet systems Conventional slurry systems Co-digestion of animal manure, MSW and other wastes Anaerobic digesters Have two- or multi-phases systems First reactor: hydrolysis and acidification Second reactor: methane fermentation 19

20 ANAEROBIC DIGESTION Plug flow reactor Contact aerobic digester ANAEROBIC DIGESTERS Phased digesters Contact stabilization anaerobic digester Phased digester 20

21 Vermicomposting Best T: 13 C - 29 C Indoor or outdoor. Supplies mineral balance Improves nutrient availability Could act as complex-fertilizer granules Great reduction of pathogenic microorganisms. Decrease of bioavailable heavy metals 21

22 METHODOLOGICAL PROPOSAL: A MSW TREATMENT FOR MILWAUKEE, WI, USA

23 2,000,000 kg/day MSW Proposal 600 kg BSW 400 kg compostable material 190,000kg liquid fertilizer 9,000 kg compost 135,000 m 3 biogas/day 192,188 kwh/day 23

24 CONCLUDING REMARKS Composting and anaerobic digestion Multivariable processes Open or closed reactors Intelligent: Convert organic matter into soil amendment Manage dwindling landfill space Potential source of renewable energy 24

25 CONCLUDING REMARKS Proposal: plant with anaerobic digester followed by vermicomposting. Good quality of compost Generates liquid fertilizer and biogas May not need to blend compost with other minerals Combination of unit operations reduce total process time Co-compost MSW with sewage sludge, adding wood ash, fly ash, lime-kiln dust, and /or limestone quarry dust 25

26 QUESTIONS?

The Biology of Composting

The Biology of Composting Mark King, Sustainability Division mark.a.king@maine.gov MAINE DEPARTMENT OF ENVIRONMENTAL PROTECTION Protecting Maine s Air, Land and Water Systems Covered First We ll Discuss