Composting.ppt 49604a

COMPOSTING

What is composting ? • Composting is a biological transformation of biodegradable waste into water, carbon dioxide, energy and a composted matter. • can help to reduce the volume of waste destined for landfill or incineration •yields a valuable product that can be used for soil amendment and mulch and when properly processed and cured, the compost improves soil nutrients, reduces erosion and help suppress plant diseases.

Compostable materials

• • • • •

yard waste (grass, leaves, small tree branches, etc) organic fraction of MSW ( food waste, thin plastics, papers, etc) partially processed Commingled MSW (waste which were not separated before arriving at the processing site, co-composting of the organic fraction of MSW with wastewater sludge. manure

Process Requirement • • • • •

Composting takes place when these conditions exist i. Organic matter ii. Suitable Microorganisms iii. Sufficient nutrient iv. with Oxygen (aerobic) / without oxygen (anaerobic) depending on the types of microorganisms’ requirement for transformation process.

Process Chemistry • Aerobic decomposition : bacteria Organic matter + O2 + nutrients → new cells + resistant organic matter + CO2 + H2O + NH3 + SO4+ heat • Anaerobic decomposition : bacteria Organic matter + H2O + nutrients → new cells + resistant organic matter + CO2 + CH4 + NH3 + H2S + heat

Process Parameters Parameters

Particle size

Moisture content

Condition smaller particle is encouraged to increase surface area for microbial activity and optimum temperature to be maintained. However too small particle size will reduce void size and hence oxygen viability for aerobic condition. Hence ideal size is in the range of 1.25 – 7.5 cm. this is required by microorganisms to assimilate nutrients and increase colony size. Too little water may affect microbial activity whereas too much water will deplete oxygen concentration and inhibit temperature rise in the pile. Hence ideal range is between 40 – 65 %.

Parameters

Condition

Temperature

The rate of decomposition depends on temperature. However different types of microorganism require different temperature range as shown in table 14-4, pg 676 of your textbook. Since composting process also involves heat generation, thus it is important to control the temperature during the process. This is why ‘cooling’ is performed on the compost pile by turning the compost frequently as well as to aerate the compost while controlling the temperature and microbial activity.

Parameters

Condition

Nutrient

The important nutrients required for the microbial activities are ; carbon, nitrogen, phosphorus and potassium. C : N ratio dominates the microbial activity. A high C : N ratio slows the rate of aerobic decomposition. On the other hand a high rate of decomposition will deplete oxygen concentration and thereby turn to anaerobic conditions. This is undesirable as foul smell is produced (due to the release of H2S). Also high N concentration will lead to higher NH3 is produced which can be toxic to the microorganisms. Hence it is usually acceptable if the range of C : N ratio is between 20 – 25 and higher C concentrations can be considered if high quality of compost is required. Table 14-7 page 689, provides the nominal range of C : N ratio for many types of compostable materials.

Parameters

Condition

pH

the pH level indicates the Hydrogen ion concentration. Microbial activity can be affected by pH less than 5.5 or rise above 9. Hence ideally the pH level is best kept between 6.5 – 7.5.

Time

composting time can take within 6 months or up to 4 years. The time depends on the waste composition. Usually protein, fats, lignin and cellulose will take longer time to break-down or decompose. Besides the decomposition process, another important factor that affects timing for completion is the curing process. The longer the curing period will result in high quality compost.

Composting process Flow

Preprocessing of MSW

Decomposition of the organic fraction

See website Wasteology Systems Ltd

Preparation and marketing of the final compost product.

Pre-processing 1. separate compostable materials from those that cannot decompose through the process of sorting 2. size reduction of the waste 3. The sorting process can either be done manually or with the use of mechanical devices (see page 552 – 561) 4. size of the waste should not be too small to disallow aeration

Composting Methods 1 Windrow composting 2 Aerated Static Pile Composting 3 In-vessel composting systems The windrow composting involve piling the waste into windrows of about in the range of 6’ to 7’ or 8’ to 10’ high and 14’ to 16’ or 20’ to 25’ wide respectively. After initial size reduction process, the moisture content of the waste is adjusted to 50 – 60 % and temperature maintained at around 55 C. During the degradation process, the waste is regularly turned (and odour released) and composting is expected to complete in 3 – 4 weeks followed by the curing process (without turning) for another 3 - 4 weeks.

Figure 14-2 shows the different types of composting methods.

• For static process (needs no frequent turning), the compost piles is laid on perforated pipes equipped with blower and covered by a layer of screened compost for insulation and odour control. For in-vessel composting systems, as the name implies, the waste is held inside a vessel cum reactor where the composting reactions takes place under well controlled conditions. The system can be divided under two major categories; plug-flow and dynamic (agitated bed). In plug flow system, assuming ideal condition prevails, the concentration remains the same at the same position along the flow path at any point in time. On the other hand, for the dynamic agitated system, the vessel can be said to resemble a continuous stirred tank reactor.

4 ISSUES OF COMPOSTING Pathogens • The waste may contain pathogens which may exist if the conditions in the system allows its survival. Hence control on the temperature typically around 55 C and exposure time of between 15 to 20 days are critical to ensure pathogen free compost.

Bad odour . The release of odour during the composting process can be extremely severe during the turning and mixing process and if not managed well can be tormenting to the operators and upset the nearby residents. Hence odour masking agents and enzymes must be used for the temporary control of the odours.

• Cost economics • . The economics of composting will have to consider the capital cost for establishing the facility including siting, cost of operation and maintenance and the marketing and promotions of the product.

Heavy metal toxicity Heavy metal toxicity may arise due to the emission of the dust and particles during shredding of waste. Hence to avoid the production of the dust, it is wise to separate the waste before arriving at the composting facilities or alternatively, the method of co-composting of waste water treatment plant sludges and organic fraction of MSW is highly recommended to reduce the concentration and consequently the potential toxicity.

Benefits of composting Benefits that can be considered such as : 1. savings on the purchase of soil amendments 2. use for streets and parks, 3. revenues from selling the compost, 4. creation of new jobs within a community and 5. most importantly a knowledged nation is generated.