Composting is the controlled decay of organic matter in a warm, moist environment by the action of bacteria, fungi, and other organisms. The organic matter

Mixed MSW

Mixed MSW

Flow Diagram Composting Plant

Mainly Air to organic fraction atomosphere

FIGURE 3.19 Process flow diagram for MRF used to process commingled waste for recovery of recyclable materials. (Source: H. Leverenz, G. Tchobanoglous, and D. B. Spencer, "Recycling," in G. Tchobanoglous and F. Kreith (Eds.), Solid Waste Handbook , 2nd ed., McGraw-Hill, New York, 2002, Chapter 8.)

Mainly Air to organic fraction atomosphere

FIGURE 3.19 Process flow diagram for MRF used to process commingled waste for recovery of recyclable materials. (Source: H. Leverenz, G. Tchobanoglous, and D. B. Spencer, "Recycling," in G. Tchobanoglous and F. Kreith (Eds.), Solid Waste Handbook , 2nd ed., McGraw-Hill, New York, 2002, Chapter 8.)

may be in municipal solid waste, wastewater sludge, septage, agricultural waste, manure, leaves and other yard waste, or combinations of these materials and other organic wastes. Composting is becoming an increasingly popular waste management option as communities look for ways to divert portions of the local waste stream from landfills. The principal applications of composting are for (1) yard wastes, (2) the organic fraction of MSW, (3) partially processed commingled MSW, and (4) co-composting the organic fraction of MSW with wastewater sludge. Because of the importance of composting in meeting mandated waste diversion goals, the number of composting facilities has increased significantly over the past 10 years. The uses of compost and constraints to its uses; a

TABLE 3.15 Important Technical Considerations in the Planning and Design of


Step 1: Feasibility Analysis

Function of MRF The coordination of the MRF with the integrated waste management plan for the community. A clear explanation of the role and function of the MRF in achieving landfill waste diversion, and recycling goals is a key element.

Conceptual design, including What type of MRF should be built, which materials types of wastes to be sorted will be processed now and in the future, and what should be the design capacity of the MRF? Plan views and renderings of what the final MRF might look like are often prepared.

Siting While it has been possible to build and operate

MRFs in close proximity to both residential and industrial developments, extreme care must be taken in their operation if they are to be environmentally and aesthetically acceptable. Ideally, to minimize the impact of the operation of MRFs, they should be sited in more remote locations where adequate buffer zones surrounding the facility can be maintained. In many communities, MRFs are located at the landfill site.

Economic analysis Preliminary capital and operating costs are delineated. Estimates of revenues available to finance the MRF (sales of recyclables, avoided tipping fees, subsidies) are evaluated. A sensitivity analysis must be performed to assess the effects of fluctuating prices for recyclables and the impacts of changes in the composition of the waste.

Ownership and operation Typical ownership and operation options include public ownership, private ownership, or public ownership with contract operation.

Procurement What approach is to be used in the design and construction of the MRF? Several options exist, including (1) the traditional architect-engineer and contractor process, (2) the turn-key contracting process in which design and construction are performed by a single firm, and (3) a full-service contract in which a single contractor designs, builds, and operates the MRF


Step 2: Preliminary Design

Process flow diagrams One or more process flow diagrams are developed to define how recyclable materials are to be recovered from MSW (e.g., source separation or separation from commingled MSW). Important factors that must be considered in the development of process flow diagrams include (1) characteristics of the waste materials to be processed, (2) specifications for recovered materials now and in the future, and (3) the available types of equipment and facilities.

Materials recovery rates Prediction of the materials flow to the MRF is necessary to estimate the effectiveness or performance of the recycling program. The performance of a recycling program, the overall component recovery rate, is generally reported as a materials recovery rate or recycling rate, which is the product of three factors: (1) participation factor, (2) composition factor, and (3) source recovery factor. Component capture rates for the recyclable materials most commonly collected in source separation recycling programs must be estimated. Composition factors are measured in waste composition studies.0

Materials balances and loading One of the most critical elements in the design and rates selection of equipment for MRFs is the preparation of a materials balance analysis to determine the quantities of materials that can be recovered and the appropriate loading rates for the unit operations and processes used in the MRF.

Selection of processing Factors that should be considered in evaluating equipment processing equipment include: capabilities, reliability, service requirements, efficiency, safety of operation, health hazard, environment impact, and economics.

Facility layout and design The overall MRF layout includes (1) sizing of the unloading areas for commingled MSW and source-separated materials; (2) sizing of presorting areas where oversize or undesirable materials are removed; (3) placement of conveyor lines, screens, magnets, shredders, and other unit operations; (4) sizing of storage and out-loading areas for recovered materials; and (5) sizing and design of parking areas and traffic flow patterns in and out of the MRF. Many of these layout steps are also common to the layout and design of transfer stations.

Depends on type of MRF (i.e., degree of mechanization). Refine preliminary cost estimate prepared in feasibility study. Important environmental issues include groundwater contamination, dust emissions, noise, vector impacts, odor emissions, vehicular emissions, and other environmental emissions. Important health and safety issues are related to worker and public access issues.

Plans and specifications will be used for bid estimates and construction. The necessary environmental documents (e.g., Environmental Impact Report) are prepared. A detailed engineer's cost estimate is made based on materials take-offs and vendor quotes. The cost estimate will be used for the evaluation of contractor bids if the traditional procurement process is used. A bidding process is used to obtain supplies, equipment, and services related to the construction, operation, and maintenance of the facility.

aTypical component recovery rates may be found in Leverenz et al. (2002).

Source: H. Leverenz, G. Tchobanoglous, and D. B. Spencer, "Recycling," in G. Tchobanoglous and F. Kreith (Eds.), Solid Waste Handbook, 2nd ed., McGraw-Hill, New York, 2002, Chapter 8.

description of the compost process; descriptions of some of the more common composting processes, important design and operational considerations, and the implementation of the compost process are considered in the following discussion. Additional details on the compost process can be found in Refs. 12-15.

Uses of Compost and Constraints to Its Use

Compost improves soil moisture retention; it is a good soil conditioner but a poor fertilizer. Compost, depending on the waste source and its composition, may be used as a soil amendment for agricultural soil and landscaping in municipal parks, golf courses, gardens, and green belts; sod growing; home gardens; and nursery and greenhouse use. Compost may also be used as landfill cover, land reclamation, animal litter, and possibly animal feed. It may also be used as an additive to fertilizer, as a fuel, or in building materials.

The presence of toxic levels of pesticides, heavy metals, and pathogens should be determined and evaluated to ensure the levels are compatible with the intended


Economic analysis Environmental issues

Health and safety issues

Step 3: Final Design

Preparation of final plans and specifications Preparation of environmental documents Preparation of detailed cost estimate

Preparation of procurement documents

TABLE 3.16 Maximum Allowable Metal Concentrations for Class I and II Compost and Allowable Usage

Parameter Concentration

(ppm dry weight) Class Ia Class II6

Parameter Concentration

(ppm dry weight) Class Ia Class II6






Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

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  • aida tesfay
    What is composting in environmental chemistry?
    1 year ago

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