Composting, aerobic and anaerobic composting

Municipal solid waste management. Aerobic and anaerobic composting

1. Composting MODULE 4

2. Composting It is a popular method used for the disposal of biodegradable wastes. During composting microorganisms like bacteria and fungi decompose the plant and animal waste into organic manure. • Waste needs to be separated into biodegradable and non-biodegradable wastes before composting. Biodegradable wastes is dumped in underground trenches and covered with earth/old manure and left for decomposition. Organic matter is decomposed by bacteria and the refuse is finally converted into powdery brown coloured mass called compost which can be used in agriculture

3. Contd … organized method of producing compost by adopting and accelerating the natural decomposition phenomenon Compost : an organic manure containing N,P,K and other micro-nutrients Compost should be used with other chemical fertilizers for optimum results

4. Contd … •Methane gas is also released during the process of composting, which can be utilized by composting the waste in a biogas plant. •In several parts of India compost plants are in operations,e.g . in Ahmedabad, Calcutta, Mumbai, Chennai, Delhi, Pune, and Hyderabad. Composting needs air,water , Food (Organic materials) Two category food is available Browns categories Are dry and dead plant material such as straw, dry brown weeds, autumn leaves and wood chips or sawdust. Need to be moistened before they are put into a compost system

5. Contd … Greens categories : Are fresh (and often green) plant materials such as green weeds from the garden, kitchen fruit and vegetable scraps, green leave,coffee grounds and tea bags, fresh horse manure, etc. Have more nitrogen than browns.

6. Advantages and Disadvantages Advantages Waste is converted into useful manure which enhances the productivity of soil. • Industrial solid wastes which are biodegradable, can be composted. Manure can be sold easily, thereby reducing cost of waste disposal Disadvantages Non-biodegradable waste must be separated collected. Non-biodegradable waste must be disposed off separately. The technology still not widely used due to problems in implementation.

7. Composting Site

8. Composting Techniques • Anaerobic Composting • Aerobic Composting Conventional windrow Composting Rapid Composting Vermi -composting Mechanical Composting

9. Aerobic composting: Using aerobic microorganisms End products are CO 2 , NH 3 , nitrates, etc Carbon is the source of energy Exothermic reaction and temp. rises all pathogens are killed It takes 2-4 months No foul smell

10. Contd … Compost bacteria combine carbon with oxygen to produce carbon dioxide and energy

11. Anaerobic composting Using anaerobic microorganisms reduction reaction End products are CH4, CO2, H2S, etc Temperature does not increases pathogens are not killed It takes 4 to 6 months Odour problem Large space requirements

12. Factors affecting Aerobic Composting Microorganisms Shredding of refuse C/N ratio Temperature Moisture content Windrow details Aeration/turning

13. Micro-organisms In the aerobic systems, there are many organisms actively participating bacteria, actnomycetes , fungi, others • In the initial stages: mostly mesophilic form (temp 30 to 40 o C) • then: mostly thermophilic bacteria and fungi (60 to 70 o C) In final stage: mesophilic bacteria and fungi Bacteria: breaks down mostly protein and other organic matter • Fungi and Actinomycetes decomposes lignin and cellulose • Most of the organisms for composting are readily available in the MSW itself.

14. Shredding of the MSW • For better bacterial invasion • Shredding destroy the natural resistance of the vegetation to microbial invasion • Provides larger surface area • Better availability of oxygen • Uniform and rapid decomposition • Materials become more homogeneous • Better fly control/moisture control Desirable size for composting: less than 2 inch • Material size governs (to certain extent) the size of the finished product

15. C/N ratio • C/N ratio available is important as all C present may not be available for degradation • For rapid composting, initial C/N ratio of the waste = 30 is ideal. (range 26 to 32) • If C/N ratio is >>30, add N sources like blood, sewage sludge, slaughter house wastes, etc If C/N ratio is <<30 add C sources like straw, saw dust, paper, etc For the finished compost product: C/N ratio = 16- 20 is the best

16. Moisture Content Optimum : 45 to 55 % by weight • If moisture is considerably less than the optimum, the physiological needs of the organisms are not met. • If moisture is considerably greater than the optimum, pores may be blocked affecting the oxygen supply. System may turn anaerobic • If moisture is < 40%, just add water • No composting if moisture is < 12 %

17. Windrow Dimensions For aerobic composting, materials should be loosely packed. • If windrow height is too much, self weight causes some compression and the pore space reduces • If windrow height is too low, heat loss will be high and temperature goes below optimum • Height can be more in cold season/countries • For MSW, for simple windrow, height shall not be more than 1.5 to 1.8 m and not less than 1 to 1.2 m , width: 2.4 m to 3.6 Height is in between 1.8 to 2.7 m for the windrows with air circulation arrangement at the center. In this case, width is 3.6 m to 6.1 m • Turning / mixing equipment also decides the height of the windrow

18. Finished Product – Ripe Compost Colour : black brown or black • Odour : earthy • Crumbly in nature • Weight: about 50 % of the original waste • It is neither very dry nor watery • N,P,K content > 1% (each) • C/N ratio < 20

19. MSW Compost Plant Major expenditure (capital and operational) is for the Separation of the biodegradable from the inert • An array of separation techniques may be required • Separation may be carried after or before Composting Air Classification - To separate light materials Magnetic Separator - For ferrous metals Screens - Size Separation Manual Separation - Materials that can be hand picked

20. Vermi -Composting (Rapid Composting) Bio-degradable are degraded by the aerobic bacteria • Earthworms feed on the organic matter. During the passage through the worm’s alimentary canal, it is converted to a simpler humus rich material due to the action of the enzymatic secretions and bacteria Earthworms are effectively used for maximizing the growth of aerobic bacteria • Time requirement: about 1 to 1.50 months

21. Earthworms They are invertebrates, Phylum: Annelida • More than 3000 species • In India, there are about 500 species • Only surface dwelling earthworms are useful a composters • Deep burrowing type is not preferred • Most suitable species: Eudrilus Eugeinae and Eisenia Foetida

22. Positive and Negative of vermi -composting Simple to construct and operate • No mechanical/electrical parts • Minimum cost High process stability • Elimination of pathogens • Large land requirements • Uncertainty and lack of understanding • Slow nature ( about 1 to 1.5 months ) Vermi -Composting is a viable and ecofriendly technology for MSW management • But mostly feasible for small community or campus (not suitable on a large scale) • There should be a good market for vermi compost for self-sustainability of a vermi compost project

23. COMPOST QUALITY

24. Rotary Drum Composting The rotary drum composter uses large amounts of ventilation to reduce composting time to 2–3 weeks To decrease the composting time to about 7- 10 days to get green compost • Another 10 to 15 days maturation to get ripe compost Consistent rotation of the compost and the side air vents allow the organic waste to remain in contact with oxygen, while the drum retains heat. • To get very high quality compost (all N, P. K about 3%) • Rotary drum composting is a sort of High rate Compost

25. Contd … Rotary drum provides agitation, aeration and mixing of the compost, to produce a consistent and uniform end product without any odor or leachate related problems. In warm, moist environments with ample amount of oxygen and organic material available, aerobic microbes flourish and decompose the waste at a quicker pace. The four phases include: 1) the mesophilic phase; 2) the thermophilic phase; 3) the cooling phase; and 4) the curing phase. Compost bacteria combine carbon with oxygen to produce carbon dioxide and energy.

26. Advantages and Disadvantages of Aerobic Digestion Disadvantages of composting It requires an initial investment Its effectiveness is contingent on the quantity of organic waste. Composting creates a smell that is unpleasant. Snakes, rats, and bugs could be attracted by it. Many hours of work are involved. It requires time. A lot of space is needed. Advantages of composting: It enhances the soil nutrients such as nitrogen and phosphorus. It enhances the water retention capacity of the soil. It increases the fertility of soil by adding humus. It works as a natural waste disposal system

27. Design consideration of Composting (Aerobic conversion) If the organic materials, excluding plastics , rubber and leather are separated from solid waste and are subjected to bacterial decomposition ,the end product is called compost or humus. The entire process involving both the separation and bacterial conversion of organic solid wastes is known as composting. The composting operation involves three steps (1) preparation of solid waste – include receiving , sorting, separation, size reduction, Moisture and Nutrient addition.

28. Contd … (2) Decomposition of solid waste (3) Product preparation and marketing – includes fine grinding, blending with various additives, granulation, bagging, storage, shipping and in some cases , direct marketing.

29. Important Design Considerations for Aerobic Composting process

30. Contd …

31. Anaerobic composting Anaerobic composting takes place naturally in nature as the decompostion of organic materials by microorganisms takes place without the involvement of oxygen producing quality anaerobic compost. Micro-organisms in a back yard compost heap, called  Aerobes  need oxygen to help them grow, multiply and decompose the waster materials when there is insufficient oxygen available inside the compost pile, these aerobic micro-organisms cannot survive and eventually die. As theses aerobes die off, another type of bacterial micro-organism called  Anaerobes  take over the job of decomposition producing anaerobic compost. Anaerobic organisms  both live and thrive in an environment with little or no oxygen (oxygen deprived). These anaerobic organisms work at a much slower rate than their air breathing aerobic counterparts, making the process of decomposition and therefore composting a much slower process A naerobic composting process is the slow bio conversion of organic matter in the absence of oxygen using fermentation, and is similar to the fermentation processes which occur naturally in bogs, swamps, marshes and other such wetlands that produce peat compost and moss.

32. Contd … Anaerobic Composting  works best with nitrogen-rich materials as they are mainly wet, but almost any organic material can be processed in an anaerobic composter such as waste paper and cardboard, grass cuttings, left over food, animal slurries and manure, etc , or any other type of feedstock that has a particularly high water content. it is important to avoid adding too much carbon rich materials such as dry leaves, sawdust and woody garden waste. A high concentration of these types of dry materials will retard or even halt the anaerobic composting process either killing of the anaerobic microbes completely producing a smelly mess or turn it into an aerobic composting pile anaerobic composting is a low temperature process so the compost pile will not heat up due to the high percentage of liquid. So any pathogens, diseases or weed seeds present within the mixture may not be killed in the fermentation process. However, the pathogens will gradually disappear over time due to the hostile environment The wet biomass feedstock is sealed in containers allowing the anaerobic bacterial microbes to thrive in an oxygen free environment as liquid composting anaerobic breakdown of organic waste which occurs naturally in landfill sites. The anaerobic digestion of the feedstock happens naturally over time, producing a useful biogas, usually methane, that can be collected and used to generate electricity or heat

33. Types of Anerobic Digestors There are basically two types of anaerobic digesters: Dry Digesters and Wet Digesters. Dry Digesters Dry Anaerobic Digesters  not uses additional water or liquids to promote the biological process. The compostable materials that contain sufficient water content such as cut grass, fleshy plants and stems, or food waste are finely chopped and shredded, sealed in the their respective air tight containers and left for several weeks or months to naturally decompose The decomposition time of the solid waste feedstock as well as the quantity of biogas produced is considerably affected by the type of waste materials used and the external temperature during fermentation. The carbon to nitrogen ratio or C/N ratio as well as the biodegradability of the feedstock materials to be used

34. Contd … Wet Anaerobic Digesters It  use animal and poultry manures, slurries, effluent’s and green organic materials mixed with water as a form of liquid composting. The water content of the liquid feedstock is very high at more than 80% of the volume.  Wet Anaerobic Composting  converts liquid organic waste into a rich source of renewable energy in the form of “Biogas”. This biogas contains methane and carbon dioxide which can be used for energy generation directly, as well as a stable semi-solid digested material called the “digestate”. This digestate can be separated into a liquid compost, affectionately called “compost tea” which can be used as a liquid fertiliser high in water soluble nutrients and a nutrient rich solid fraction as a soil conditioner. Liquid composting and liquid anaerobic fertilisers can be cheaply made at home by using a standard watering can as a wet anaerobic digester. One of the main disadvantages of wet anaerobic composting by the direct use of animal manures and sludge’s, is the risk of plant and human contamination by pathogens such as  ecoli  and  salmonella . Heating of the liquids and/or solids produced above 55 o C would destroy these pathogens but an additional energy resource is required to do this.

35. Anaerobic composting-Summary Using anaerobic microorganisms reduction reaction End products are CH4, CO2, H2S, etc Temperature does not increases pathogens are not killed It takes 4 to 6 months Odour problem Large space requirements

36. Design consideration of Anaerobic Digestion It is the process used for the production of methane from solid waste. Three steps are involved in this process. Preparation of the solid waste- includes separation and size reduction This step involves addition of moisture and nutrients, blending, pH adjustment to about 6.7, heating of slurry to between 328 and 333K and anaerobic digestion in reactor with continuous flow, in which the contents are well mixed for a period of 5-10 days. This step involves capture, storage and separation of gas components involved during the digestion process.

37. Important design considerations for Anerobic Digestion

38. Contd … The stochiometric equation of anaerobic digestion is used for calculation of Methane gas production.