Solid and Hazardous Waste Management

Composting of biodegradable waste

Composting is the aerobic biological decomposition of biodegradable organic waste, such as kitchen peelings, leaves and garden trimmings, into a stable humus-like product. Microorganisms oxidise carbon to CO_2 and use nitrogen for growth, so the process depends on aeration, moisture, temperature and the balance of carbon and nitrogen. Windrow composting places the waste in long piles that are turned periodically to supply oxygen. Vermicomposting uses earthworms, which ingest organic matter and secrete enzymes that break complex molecules into simpler compounds that microbes can use. Vermicomposting is generally faster than windrow composting, but it is seldom used on a large commercial scale because the worm population and bedding must be managed carefully.

Windrow height must be controlled. A pile that is too tall compacts under its own weight, loses air spaces and becomes anaerobic, so excessive height should be avoided, although a pile must still be large enough to retain heat for the thermophilic phase. Moisture of about 50 to 60 percent is the usual optimum. Drier piles slow biological activity, while excessively wet piles block air movement and make aerobic conditions difficult to maintain.

The C/N ratio is the key nutrient criterion. At a low C/N ratio, nitrogen is in excess and ammonia is released, which causes odour and nitrogen loss. At a high C/N ratio, nitrogen is limiting and the biological activity is impeded. Odour in a well-managed aerobic pile is mainly caused by anaerobic pockets rather than by the aerobic process itself. Carbon is not fully converted to stable compost within a few days; stabilisation usually takes several weeks.

To design a mix, the carbon and nitrogen contents of each waste stream are converted to dry-basis masses. The total carbon is divided by the total nitrogen for the blend, and the proportion of the second stream is solved from the target ratio, for example 25 or 30. Moisture in wet wastes must be removed from the calculation when the target is stated on a dry basis.

The air requirement for aerobic composting follows from a stoichiometric balance of the waste formula. Oxygen is calculated from the carbon, hydrogen and oxygen balance, with nitrogen leaving as NH_3 if the ammonia escapes. The mass of oxygen is converted to air using its mass fraction in air, and an excess-air allowance is then added. For a closed-vessel facility with 100 percent excess air, the actual air mass is twice the theoretical value.

Proximate and ultimate analysis of solid fuels

Proximate analysis determines the moisture, volatile matter, fixed carbon and ash content of a solid fuel or waste. Moisture is lost on drying at about 105 °C. Volatile matter is the mass lost when the dried sample is heated at high temperature, such as 950 °C, in the absence of air. Fixed carbon is the residue left after volatile matter is removed, and ash is the inorganic residue after complete burning. When a sample is heated in the presence of air, the fixed carbon also burns, so the extra mass lost compared with heating without air gives the fixed carbon content.

Ultimate analysis gives the elemental composition, namely carbon, hydrogen, oxygen, nitrogen, sulphur and ash, on a mass basis. The carbon to nitrogen ratio is derived from ultimate analysis, not proximate analysis, which is why it is not part of the proximate parameters. Proximate analysis describes how a sample behaves on heating, while ultimate analysis describes its elemental makeup.

For mixed samples, the carbon content of the blend is found by weighting each sample's carbon percentage by its mass. Simple averaging of percentages is wrong when the sample masses differ. Total carbon mass is the sum of the carbon masses of the individual samples, divided by the total mass.

The maximum CO_2 emission from a waste is calculated by assuming all carbon is burned completely and none is retained in the ash. Each kilogram of carbon gives 44/12 kg of CO_2 on combustion, so the carbon mass per kilogram of waste is multiplied by 44/12. Hydrogen and oxygen do not contribute CO_2.

Integrated waste management hierarchy

Integrated solid waste management ranks options by their environmental preference. The order from highest to lowest preference is reduction at source, reuse and recycling, energy recovery, and finally disposal in a sanitary landfill. Reduction is ranked first because it prevents waste from being generated. Reuse and recycling recover materials and save raw materials. Energy recovery through incineration or refuse derived fuel recovers heat or electricity from residual waste. Landfilling recovers neither material nor energy and is therefore the least preferred option.

A mass balance of an integrated system distributes the daily waste among these routes. Each fraction is calculated as a percentage of total generation, and any efficiency of the processing step reduces the mass available to the next step. For energy recovery, the mass of refuse derived fuel is multiplied by its calorific value to obtain the thermal energy, which is then multiplied by the conversion efficiency to obtain electrical energy. The electricity is expressed in MWh by dividing the value in MJ by 3600.

MSW processing equipment

Municipal solid waste processing uses a sequence of mechanical equipment, and each machine has a distinct function that must be matched correctly. A trommel is a rotating cylindrical screen that separates the waste by size, so it performs screening. A hammer mill reduces the size of waste by impact, so it performs size reduction. A magnetic separator uses magnetic attraction to recover ferrous metals such as iron and steel. An eddy current separator uses induced eddy currents to repel non-ferrous metals, mainly aluminium, from the stream.

For a matching question, each equipment must be paired with its one correct function, and pairs that describe a different machine are wrong. The most common error is to assign size reduction to screening equipment or aluminium recovery to a magnetic separator.

Refuse derived fuel is produced from dry, segregated waste by a standard flow-sheet. Ferrous and non-ferrous metals are removed first, then dense inert materials such as glass and stones are separated by density or ballistic separation. The remaining combustible fraction is shredded to a suitable size and then pelletized. Shredding must precede pelletizing, and the inert separation must precede shredding so that the pellets consist mainly of combustible material such as paper, plastics and textiles.

Hazardous waste transboundary movement rules

The Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 of India govern the import and export of hazardous wastes. Import is permitted only for purposes that recover value from the waste. These purposes are recovery, reuse, recycling and utilization, including co-processing in industrial processes. Import of hazardous wastes for disposal is not permitted under the Rules. Disposal options such as disposal in abandoned mines or safe disposal in engineered landfills are therefore excluded from permitted imports.

The Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal is the international framework for such movements. It sets out strategies and guidelines for controlling the export, import and disposal of hazardous wastes across national borders. It is distinct from agreements on greenhouse gases such as the Kyoto Protocol, which address CO_2 and CH_4 emissions, and from the UNECE convention on long-range transboundary air pollution, which addresses acid-rain precursors. Only the Basel framework deals with the movement of hazardous wastes.

Bio-medical waste categories (India)

The Bio-Medical Waste Management Rules, 2016 classify bio-medical waste into colour-coded categories, and each category has defined containers, treatment methods and disposal routes. The yellow category covers anatomical waste such as body parts, soiled waste, expired medicines and chemical waste. Red covers contaminated recyclable plastics such as tubing and catheters. White (translucent) covers sharps, and blue covers glassware and metallic body implants. A colour or category not listed in the schedule of the Rules is not a designated bio-medical waste category, and the colour green is among these.

Human anatomical waste is placed in yellow bags and treated by incineration or deep burial, or by autoclaving or hydroclaving where the Rules permit. Bio-medical waste must be sent to a common bio-medical waste treatment facility (CBWTF) where one is available. On-site treatment is allowed only in rural areas where no common facility exists. A standard autoclave cycle at 121 °C and 15 psi requires a defined residence time, and higher temperature cycles shorten that time, so any claim about the reduction in time must be checked against the standard cycle.

Hazardous waste categories (India)

The Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 cover listed hazardous wastes, which are identified by their source and composition in the schedules of the Rules. Sludge from wet scrubbers used in hazardous waste treatment, and chromium-bearing residues and sludge from leather tanneries, are listed hazardous wastes and are therefore covered by the Rules. Domestic hazardous waste from households is regulated under the Municipal Solid Wastes Rules, not under these hazardous waste rules.

Radioactive waste, which is called nuclear waste in some contexts, is regulated under the Atomic Energy Act and related atomic energy regulations, not under the hazardous waste rules. A waste that is outside the Rules' scope must be identified before deciding which regulatory regime applies.

A waste is classified as a characteristic hazardous waste if it exhibits one of four properties. These are ignitability, corrosivity, reactivity and toxicity. Radioactivity is not a characteristic under the Hazardous Waste (Management and Handling) Rules, 2016, because radioactive materials fall under separate atomic regulations.

Plastic types and applications

Each common plastic has a characteristic density, crystallinity and mechanical behaviour, which determine its applications. High-density polyethylene (HDPE) is rigid, has low permeability and is used for geomembranes that line landfills, as well as for bottles and drums. Low-density polyethylene (LDPE) is flexible and is used for garbage bags, films and bubble packaging. Polyethylene terephthalate (PET) is a polyester used for water and beverage bottles. Polystyrene (PS) is a foamed plastic used for insulation and disposable items such as foam cups.

Waste plastics can be recovered in several ways. Pyrolysis, which is thermal cracking in the absence of oxygen, converts plastics into fuel oil and fuel gas, whereas direct burning recovers the energy as heat. Shredded waste plastic can be blended into hot bitumen for road and pavement construction. PET (resin code 1) and HDPE (resin code 2) are the most widely collected and recycled plastics.