Solid Waste Engineering

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This section covers the definitions of waste, solid waste and solid waste management, the concept of integrated sustainable solid waste management, the sources, types and composition of solid waste, and the sources and characteristics of hazardous and clinical wastes.

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Definitions and the Waste Hierarchy • Waste is any substance or object which the holder discards, intends to discard or is required to discard.

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The definition turns on the intention of the holder rather than on any property of the material, which is why the same substance can be a waste in one hand and a raw material in another — a point examined directly. • Solid waste is the non-liquid, non-gaseous residue of human activity, comprising discarded solid material and semi-solids such as sludge.

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Municipal solid waste (MSW) is the solid waste generated by households, commercial premises, institutions and street sweeping within a municipal area; it excludes industrial process waste, construction and demolition waste, and hazardous waste, although these are often mixed with it in practice. • Solid waste management is the systematic control of the generation, storage, collection, transfer and transport, processing and disposal of solid waste in a manner consistent with public health, economics, engineering, conservation, aesthetics and other environmental considerations. • The waste management hierarchy, in order of preference, is the organising principle of the whole chapter: • 1.

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Prevention or avoidance — do not generate the waste at all; always the cheapest and most effective option. • 2.

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Minimisation or reduction — reduce the quantity and the hazard of what is generated. • 3.

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Reuse — use the article again for the same or another purpose without reprocessing. • 4.

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Recycling — reprocess the material into a new product. • 5.

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Recovery — extract value, typically energy, from what cannot be recycled. • 6.

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Treatment and disposal — the last resort, with landfill at the very bottom. • Note that reuse ranks above recycling, because reuse avoids the energy and material cost of reprocessing, and this ordering is frequently asked.

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Integrated Sustainable Solid Waste Management • Integrated sustainable solid waste management (ISWM) is an approach that considers the whole waste system rather than individual elements, and it rests on three dimensions which must be known as a set: • 1.

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The stakeholders — households, local government, private operators, informal waste pickers, NGOs, donors and national government — all of whom must be engaged, because a system designed without the people who operate and use it does not work. • 2.

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The waste system elements — generation, separation, collection, transfer, treatment, recovery and disposal. • 3.

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The sustainability aspects — technical, environmental, financial and economic, socio-cultural, institutional and political, and legal.

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A scheme that is technically sound but financially or institutionally unsustainable will fail, which is the central lesson of ISWM and the usual explanation of why donor-funded facilities in developing countries stop working within a few years. • The informal sector deserves particular mention: waste pickers in Nepali and South Asian cities recover a very large share of the recyclable material at no cost to the municipality, and ISWM holds that they should be integrated into the formal system rather than displaced by it.

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Sources, Types and Composition Source Typical generators Typical waste Residential Single and multi-family dwellings Food waste, paper, plastic, textiles, glass, metal, ash, bulky items, some household hazardous waste Commercial Shops, hotels, restaurants, markets, offices Paper, cardboard, plastic, food waste, glass, metal Institutional Schools, offices, hospitals, prisons As commercial, plus specific hazardous and clinical waste from hospitals Construction and demolition Building sites, road works, demolition Concrete, brick, timber, steel, soil, rubble — very heavy and bulky Municipal services Street sweeping, parks, drains, treatment plants Street dirt, leaves and garden waste, drain silt, treatment sludge Industrial Factories and processing plants Process residues, packaging, ash, scrap; sometimes hazardous Agricultural Farms, feedlots, dairies Crop residue, animal manure, pesticide containers • Composition by material is usually reported as organic or putrescible, paper and cardboard, plastic, glass, metal, textile, rubber and leather, inert and other. • The defining characteristic of municipal solid waste in Nepal and in developing countries generally is its very high organic content, commonly 60-70 per cent by weight, with a correspondingly high moisture content and a low calorific value.

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Four consequences follow directly and should be stated together: • Composting and anaerobic digestion are the appropriate treatment routes, since the material is largely biodegradable. • Incineration without supplementary fuel is impracticable, because the calorific value is too low and the moisture too high; mass-burn incinerators proposed for South Asian cities have repeatedly failed for exactly this reason. • Leachate production is high, because the waste itself contains so much water. • Segregation at source gives a very large benefit, because the organic fraction can be diverted entirely. • By contrast, waste in high-income countries contains far more paper and plastic, less organic matter and less moisture, with a calorific value two or three times higher — which is why incineration is viable there and not here.

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This comparison is examined directly. • Generation rate is expressed in kg per capita per day and increases with income and urbanisation.

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Nepali urban generation has commonly been reported at around 0.2-0.5 kg per capita per day, with Kathmandu Valley at the upper end; figures vary between surveys and should be checked against current data.

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Hazardous and Clinical Waste • A waste is hazardous if it exhibits ignitability, corrosivity, reactivity or toxicity, or if it is specifically listed as hazardous.

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These four characteristics must be known as a set and are asked for directly. • Sources: industry — chemicals, tanneries, metal finishing, batteries, paint; hospitals and laboratories; agriculture — pesticides and their containers; and households — the small but troublesome stream of batteries, fluorescent tubes, paint, solvents, pesticides and medicines, which is dangerous precisely because it is dispersed and enters the municipal stream. • Clinical or healthcare waste is the waste generated by healthcare establishments.

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The critical fact is that only about 10-25 per cent of healthcare waste is hazardous; the remaining 75-90 per cent is general waste no different from household waste.

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It follows that segregation at the point of generation is the whole of the problem: if the hazardous fraction is separated at the bedside, only a small quantity needs expensive special treatment, whereas if it is mixed, the entire stream must be treated as hazardous.

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This is the most examined point in the section. • Categories of healthcare waste: infectious; pathological or anatomical; sharps, which are hazardous both as infectious material and as a physical injury risk; pharmaceutical, including cytotoxic waste from chemotherapy, which is the most hazardous category of all; chemical; radioactive; pressurised containers; and general non-hazardous waste. • Colour coding is the operational basis of segregation, and although the exact scheme differs between national guidelines, the universal principle is that sharps go into a rigid, puncture-proof container which is never filled beyond about three-quarters and is never recapped by hand, since needlestick injury is the principal occupational hazard. • Treatment of clinical waste: autoclaving for infectious waste, which is the preferred method since it produces no dioxins; microwave disinfection; chemical disinfection; incineration at high temperature for anatomical, pharmaceutical and cytotoxic waste; and encapsulation or inertisation of sharps and pharmaceuticals before landfilling.

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Open burning of clinical waste, which remains common, is the worst possible practice, since low-temperature combustion of chlorinated plastics produces dioxins and furans.

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This section covers the physical, chemical and biological properties of solid waste, the proximate and ultimate analysis of solid waste, waste to energy calculations, and the production of green hydrogen from solid waste.

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Physical Properties • Density (specific weight) is essential to the design of every container, vehicle and landfill.

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It varies enormously with the degree of compaction, so the condition must always be stated: loose waste as generated is commonly 100-300 kg/m³, waste in a compactor vehicle 400-700 kg/m³, and waste compacted in a landfill 600-1000 kg/m³ or more.

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Quoting a density without saying in what condition it was measured is meaningless, and the point is examined. • Moisture content is expressed on a wet basis as (wet weight − dry weight)/wet weight × 100, or on a dry basis with the dry weight in the denominator.

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The wet basis is the convention in solid waste work, and the basis must always be stated because the two give very different numbers.

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Nepali municipal waste commonly has a moisture content of 40-60 per cent because of its high organic fraction. • Field capacity is the quantity of water the waste can retain against gravity.

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It is the single most important physical property for landfill design, because leachate is produced only once the field capacity has been exceeded — up to that point infiltrating water is simply absorbed.

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Typical field capacity is about 20-30 per cent by volume for uncompacted mixed waste and less for compacted waste. • Other physical properties: particle size and distribution, which govern the design of shredders, screens and separators; permeability, which controls the movement of liquid and gas through the mass; porosity; and compressibility, which determines the settlement of a completed landfill and hence the restrictions on its after-use.

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Chemical Properties and Analysis • Proximate analysis reports four fractions determined by standardised heating: moisture, volatile combustible matter, fixed carbon and ash.

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Ultimate analysis reports the elemental composition — carbon, hydrogen, oxygen, nitrogen, sulphur and ash, and sometimes chlorine.

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The distinction is examined directly: proximate analysis is the routine commercial test, while ultimate analysis gives the elemental data needed for combustion air calculations and for the carbon-to-nitrogen ratio. • Calorific value is the heat released on complete combustion.

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The gross or higher calorific value assumes the water formed is condensed and its latent heat recovered; the net or lower calorific value assumes it leaves as vapour.

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The gross value is therefore always greater, and the gap is widest for wet, hydrogen-rich material — which is precisely the case for municipal waste. • Typical calorific values: mixed municipal waste in a high-income country about 9-13 MJ/kg; mixed waste in Nepal about 4-8 MJ/kg because of its moisture; paper 16-18; plastics 30-40, comparable with fuel oil; food waste only 4-6 as received; and refuse-derived fuel 15-20 after separation and drying. • The rule of thumb for incineration is that waste requires a net calorific value of at least about 7 MJ/kg to burn without supplementary fuel, and much Nepali waste falls below this, which is the quantitative statement of the point made in 7.1. • Dulong's formula estimates the calorific value from the ultimate analysis:

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HCV (kJ/kg) ≈ 337C + 1419(H − O/8) + 93S + 23N, with the percentages by mass.

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The term (H − O/8) allows for the hydrogen already combined with oxygen in the fuel, which contributes nothing on combustion, and this correction is the feature of the formula most often asked about. • The carbon-to-nitrogen (C/N) ratio governs biological treatment: the optimum for composting is about 25-30.

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A higher ratio means nitrogen is limiting and decomposition is slow; a lower ratio means excess nitrogen, which is lost as ammonia, causing odour and wasting the nutrient.

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Food waste and green material are nitrogen-rich with a low C/N, while paper, straw and dry leaves are carbon-rich with a high C/N, so the two are blended to reach the optimum.

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Biological Properties • Biodegradability is the fraction of the waste that micro-organisms can decompose.

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The volatile solids content overstates it, because lignin resists degradation almost completely; the biodegradable fraction is therefore estimated from the lignin content, falling as lignin rises.

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Newsprint has a high volatile solids content but a low biodegradability because it is lignin-rich, whereas food waste is almost entirely biodegradable — a comparison examined directly. • Odour arises from the anaerobic decomposition of the organic fraction, producing hydrogen sulphide, mercaptans, ammonia and organic acids.

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It is controlled by prompt collection, covering, and aerobic rather than anaerobic conditions; in a hot climate waste becomes anaerobic and odorous within a day or two, which is why collection frequency matters more in Nepal than in a cold country. • Vector breeding: flies complete their life cycle in uncollected waste in about a week in warm weather, and rats and other vermin are attracted by food waste.

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Collection at intervals shorter than the fly breeding cycle is therefore a public health requirement, not merely an amenity.

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Waste to Energy • The routes are thermal and biological, and they suit different wastes: • Incineration with energy recovery — suits dry, high calorific value waste; reduces volume by about 90 per cent and mass by about 70 per cent. • Refuse-derived fuel (RDF) — the combustible fraction is separated, shredded and dried to give a fuel of 15-20 MJ/kg, which can be burned in a cement kiln or a dedicated boiler.

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Cement kilns are particularly suitable because their high temperature and long residence time destroy organics completely and the ash is incorporated into the clinker. • Gasification and pyrolysis — convert the waste to a combustible gas or oil; technically attractive but sensitive to feedstock variability, which is the reason so many municipal waste gasification projects have failed. • Anaerobic digestion — the appropriate route for the wet organic fraction, producing biogas of 50-70 per cent methane and a digestate usable as a soil conditioner.

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For Nepali waste, with its high organic and moisture content, digestion is far more suitable than combustion, and this judgement should be stated in any answer comparing the routes. • Landfill gas recovery — collects the gas generated in a landfill, roughly 50 per cent methane and 50 per cent carbon dioxide, for use as fuel or for flaring. • The energy calculation follows a simple sequence: energy available = mass of waste × net calorific value; electrical output = that energy × overall conversion efficiency, commonly 20-25 per cent for a waste-to-energy plant; and the power rating follows by dividing by the operating hours.

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For digestion, the biogas yield in m³ per tonne of volatile solids is multiplied by the calorific value of the gas, about 20-25 MJ/m³. • Green hydrogen from solid waste is an emerging route and should be described accurately.

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Hydrogen can be obtained from waste by gasification of the dry fraction followed by the water-gas shift reaction and gas separation; by steam reforming of biogas from anaerobic digestion; by dark fermentation, in which bacteria produce hydrogen from organic waste without light; and by electrolysis using electricity generated from the waste.

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The hydrogen is described as green only when the process is net renewable and the carbon is of biogenic origin.

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All these routes remain at pilot or early commercial scale, are costly, and depend critically on a consistent segregated feedstock — which is exactly what mixed municipal waste does not provide.

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The honest conclusion, which an examination answer should reach, is that waste-derived hydrogen is promising but not yet a practical option for a Nepali municipality, whereas composting and biogas are.

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This section covers waste handling methods, on-site and off-site waste management, the methods and services of solid waste collection, the hauled and stationary container systems, and the location and design of transfer stations.

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On-site Handling and Storage • The functional elements of a solid waste system, in order: generation → on-site handling, separation and storage → collection → transfer and transport → processing and recovery → disposal. • On-site handling and storage is the responsibility of the generator and is where segregation at source must occur.

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The container should be of adequate size for the collection interval, covered to exclude flies and rain, durable, easy to lift and clean, and placed where the collection vehicle can reach it.

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Storage capacity is matched to collection frequency: a longer interval requires a larger container, and in a warm climate the interval is limited by odour and fly breeding regardless of the capacity provided. • Separation at source into organic, recyclable and residual fractions is the single most important operational decision in the whole system, because once wet organic waste is mixed with dry recyclables, both are contaminated: the recyclables lose most of their value and the organics become unsuitable for composting.

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Separation after mixing is always far more expensive and less effective than separation at source, and this statement is examined directly. • Collection accounts for 60-80 per cent of the total cost of a municipal solid waste system, which is why most of the engineering effort and most of the savings lie here rather than in treatment or disposal.

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This cost proportion is a standard examination figure.

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Collection Methods and Services • Types of collection service: kerbside collection, in which the householder places the container at the kerb on a set day; alley collection; set-out set-back service, in which a crew member carries the container from the premises and returns it, which gives the best service at the highest cost; block or community bin collection, in which households bring waste to a shared container; and door-to-door collection by a small vehicle or handcart, which is the commonest arrangement in Nepali towns. • Collection systems are classified in two groups, and this classification is the most examined content of the section: • Hauled container system (HCS): the container itself is hauled to the disposal or transfer point, emptied there and returned, either to its original location or to the next.

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It suits large, concentrated generators such as markets, construction sites, hotels and industrial premises, where the quantity justifies a dedicated container.

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Its advantage is that only one container is handled per trip and the vehicle is simple; its disadvantage is that the vehicle carries only one container's worth of waste per journey, so it is inefficient where the haul is long or the generators are small and scattered. • Stationary container system (SCS): the container remains at the point of generation and its contents are transferred into the collection vehicle, which then moves on to the next container, collecting from many before travelling to the disposal point.

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It suits scattered residential generators with small quantities.

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Mechanically loaded SCS uses a compactor vehicle; manually loaded SCS uses a crew, which is the normal arrangement in Nepali residential collection. • The distinction in one line, which should be memorised: in HCS the container travels with the waste, and in SCS only the waste travels. • Time elements in collection analysis: pick-up time, haul time, at-site time at the disposal point, and off-route time, which covers everything unproductive — travel to and from the depot, breakdowns, breaks and waiting.

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Off-route time is commonly taken as about 10-15 per cent of the working day and must be allowed for explicitly, since ignoring it is the commonest error in collection calculations. • Route planning rules, which are asked for as a list: start and finish near the depot or main road; avoid duplicating travel over the same street; collect on one-way streets starting from the upper end; on steep hills collect downhill, so the vehicle gains load as it descends and the crew works with gravity; make right turns in preference to left in left-hand drive (or the reverse), to avoid crossing traffic; collect the heaviest loads early in the shift; and schedule collection in congested commercial areas outside the busiest hours.

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The downhill collection rule is a favourite question and matters particularly in Nepal's hill towns.

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Transfer Stations • A transfer station is a facility at which waste from small collection vehicles is consolidated into larger vehicles for long-distance haul to the disposal site. • The justification is economic, and it should be stated as an economic argument: a transfer station is worthwhile when the saving in haul cost from using large vehicles over a long distance exceeds the capital and operating cost of the station plus the cost of handling the waste twice.

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It follows that the longer the haul to the disposal site, the stronger the case; for a short haul a transfer station is simply an additional cost.

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This conditional answer is what the examination looks for. • Other circumstances favouring a transfer station: small collection vehicles such as handcarts, tractors or tempos that are unsuitable for long haul; a disposal site remote from the city; the opportunity to recover recyclables at the station; and the need to reduce traffic and congestion in the city. • Types: direct discharge, in which collection vehicles discharge straight into the transfer vehicle, which is simple and cheap but requires the two to be synchronised; storage discharge, in which waste is discharged into a pit or platform and loaded later, which decouples the two at the cost of a larger facility; and combined.

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By capacity they are small (under about 100 tonnes a day), medium and large. • Siting criteria: as near as practicable to the centroid of the waste generation area, since every kilometre from it is travelled by the small vehicles; with good access to a major road and without routing heavy vehicles through residential streets; on a site large enough for queuing, manoeuvring and future expansion; where the environmental and social impact is acceptable and public opposition manageable; with utilities available; and where the land can be acquired at reasonable cost.

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The requirement to be near the centroid of generation while also being acceptable to neighbours is the central tension in siting, and it is why transfer stations are so often placed badly. • Design requirements: adequate capacity for the peak arrival rate, not the average; a weighbridge; dust, odour, noise and litter control; leachate collection and drainage; fire protection; washing facilities; and provision for recovery of recyclables if that is intended.

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This section covers the types and methods of landfilling, the design of landfill sites, criteria for site selection, the mechanism of waste decomposition in a landfill, leachate calculation and treatment technologies, and the disposal of hazardous and special wastes.

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Types and Methods of Landfilling • The distinction between an open dump and a sanitary landfill is fundamental and is asked in almost every paper: an open dump is uncontrolled tipping with no liner, no leachate collection, no gas management, no daily cover and no monitoring, producing odour, flies, rodents, fires, leachate contamination of groundwater and uncontrolled methane release; a sanitary landfill is an engineered facility with a liner, leachate collection and treatment, gas management, daily and final cover, compaction, surface water control and long-term monitoring.

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The difference lies in the engineering and the operation, not in the material deposited. • Methods of landfilling: • Trench method — a trench is excavated, the waste placed in it, compacted and covered with the excavated material.

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Suits flat ground with a deep water table and available cover material. • Area method — waste is spread and compacted in layers on the existing ground surface, with cover material imported.

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Suits ground where excavation is impracticable — a high water table, rock, or a site being used to fill a depression. • Ramp or slope method — a combination, in which cover material is excavated from immediately ahead of the working face.

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Suits gently sloping ground and is economical because the cover is won on site. • Depression or canyon method — a natural or excavated depression, quarry or ravine is filled. • Operational concepts: a cell is the waste deposited and compacted in one day together with its cover; a lift is a complete layer of cells across the site; and the final cover or cap is placed when the lift reaches design height.

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Daily cover of about 150-300 mm of soil controls flies, rodents, blowing litter, odour and fire, and is the defining operational practice of a sanitary landfill.

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Site Selection and Design • Site selection criteria, which should be given as a structured list: • Hydrogeology — a deep water table, low permeability natural strata, and no connection to an aquifer used for supply; this is the single most important group of criteria, because the principal risk from a landfill is groundwater contamination. • Distance — adequate separation from habitation, water bodies, wells and springs, and from an airport, because birds attracted to the waste are a hazard to aircraft. • Topography and geology — stable ground, not on a fault, a landslide-prone slope or a flood plain. • Capacity — sufficient volume for a design life of at least about 10-20 years, since finding and acquiring a site is so difficult. • Cover material — available on site or nearby, since importing it is a major running cost. • Access — all-weather road access adequate for heavy vehicles, and a haul distance from the city that is economically acceptable. • Land use and social acceptability — compatible with surrounding use, with the lowest practicable population affected.

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Public opposition is in practice the binding constraint on landfill siting everywhere, and in Nepal it has repeatedly closed sites and halted projects, so community consultation and benefit sharing must begin at the outset rather than after selection. • The engineered components of a sanitary landfill, from the bottom upward: a prepared subgrade; a liner system, typically a composite of compacted clay of low permeability with a geomembrane above it; a leachate collection and removal system of perforated pipes in a granular drainage layer laid to fall; a protective layer; the waste in compacted cells with daily cover; a gas collection system of vertical wells or horizontal trenches; a final cap of low permeability to exclude infiltration, with a drainage layer and topsoil above it for vegetation; surface water diversion around and over the site; and groundwater monitoring wells upgradient and downgradient. • The composite liner is used because the two materials fail in different ways: a geomembrane is almost impermeable but can be punctured, while compacted clay is permeable but self-healing and limits the leakage through any puncture.

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This complementary failure argument is the point examined. • Monitoring wells must be sited both upgradient and downgradient of the site, because the upgradient well establishes the background quality against which any downgradient change is judged — without it, contamination cannot be attributed to the landfill.

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Decomposition and Landfill Gas Phase Condition Products Duration I — Initial adjustment (aerobic) Oxygen present in the voids CO₂, water, heat; temperature rises Days to a few weeks, until oxygen is consumed II — Transition Oxygen depleted, nitrate and sulphate reduced Onset of anaerobic conditions Weeks III — Acid phase Acidogenesis dominant Volatile fatty acids, CO₂, hydrogen; pH falls to 5-6; leachate strongest, with very high BOD and high metal concentration Months to a few years IV — Methane fermentation Methanogens established CH₄ and CO₂ in roughly equal proportion; pH returns to 7-8;

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BOD falls sharply Years to decades; the principal gas-producing phase V — Maturation Readily degradable material exhausted Gas production declines; leachate weak but rich in refractory humic compounds Decades • The practical consequence of the phase sequence, and the point most often examined: the age of the landfill determines the character of its leachate.

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A young landfill in the acid phase produces a strong leachate with BOD of several thousand mg/L and a BOD/COD ratio above about 0.5, which is readily biodegradable and suits biological treatment; an old landfill in the maturation phase produces a weak leachate with a BOD/COD ratio below about 0.1, dominated by refractory humic substances and ammonia, which resists biological treatment and requires physico-chemical methods.

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Designing a single fixed treatment plant for a landfill that will operate for decades therefore does not work, and the treatment must be able to change. • Landfill gas is typically 45-60 per cent methane and 40-55 per cent carbon dioxide, with traces of hydrogen sulphide, ammonia and volatile organic compounds that give it its odour. • Why it must be managed: methane is explosive between about 5 and 15 per cent in air and migrates laterally through the ground, so it can accumulate in nearby buildings and services; it is a greenhouse gas with a warming potential far greater than carbon dioxide; and it kills vegetation on the cap by displacing oxygen from the root zone. • Capturing and flaring landfill gas is beneficial even when the energy is not used, because burning it converts methane to the much less potent carbon dioxide — a point asked directly.

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Leachate Calculation and Treatment • Leachate is the liquid that has percolated through the waste, carrying dissolved and suspended material from it.

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Its sources are infiltrating rainfall, the moisture originally in the waste, groundwater intrusion if the site is below the water table, and water produced by decomposition. • The water balance method is the standard calculation: leachate = precipitation − surface runoff − evapotranspiration − change in moisture storage in the waste.

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No leachate is produced until the field capacity of the waste has been exceeded, as set out in 7.2.

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Computer models such as HELP perform the calculation layer by layer. • The design conclusion that follows is the most useful one in landfill engineering: because precipitation is the dominant source, the most effective leachate control measure is to keep water out of the waste — by a low-permeability cap, by diverting surface water around the site, and by filling and capping progressively rather than leaving a large area open to the rain.

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Preventing leachate is always cheaper than treating it. • Treatment technologies: recirculation back into the waste, which is simple, promotes decomposition and reduces volume by evaporation, and converts the landfill into a bioreactor; biological treatment by aerated lagoon, activated sludge or SBR for young, biodegradable leachate; anaerobic treatment for high-strength leachate; physico-chemical treatment — coagulation, air stripping of ammonia, activated carbon adsorption and chemical oxidation — for old, recalcitrant leachate; membrane processes, chiefly reverse osmosis, which produce a high-quality effluent but a concentrate that must itself be disposed of; constructed wetlands for polishing; and discharge to a municipal sewer for treatment with sewage, which is often the cheapest option where a works with spare capacity exists nearby, subject to agreement on strength and load. • Ammonia is the parameter that most often governs leachate treatment design, because it persists at high concentration long after the organic load has fallen, and biological nitrification-denitrification or air stripping is required to remove it.

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Hazardous and Special Wastes • Hazardous waste management follows the cradle-to-grave principle: the generator remains responsible for the waste from its production through transport, treatment and final disposal, tracked by a manifest system. • Disposal routes: physical and chemical treatment to neutralise, precipitate or oxidise; stabilisation and solidification, which immobilise the contaminants in a cement or polymer matrix; high-temperature incineration, the preferred route for organic hazardous waste, requiring adequate temperature, residence time and turbulence with rigorous flue gas cleaning; secure engineered landfill with double liners, leachate collection and long-term monitoring; and deep well injection where the geology permits. • Special wastes and their handling: • Electronic waste (e-waste) — contains lead, mercury, cadmium, hexavalent chromium and brominated flame retardants, together with recoverable gold, silver, copper and rare metals.

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The hazard arises overwhelmingly from informal recycling — open burning of cable to recover copper and acid leaching of circuit boards — which exposes workers and contaminates land, and the control is formal collection with extended producer responsibility. • Used batteries — lead-acid batteries are highly recyclable but informal smelting is a serious lead exposure problem; lithium-ion batteries present a fire hazard in collection vehicles and at facilities, which is now a significant operational problem worldwide. • Waste tyres — not readily degradable, occupy large volume, collect water and breed mosquitoes, and burn with dense toxic smoke that is extremely difficult to extinguish; they are used as fuel in cement kilns, retreaded, or crumbed for road surfacing and playground material. • Construction and demolition waste — very large in volume and weight but largely inert, and highly recyclable as crushed aggregate, which is the cheapest and most effective diversion available to a municipality. • Asbestos — must be wetted, double-bagged, labelled and buried in a designated cell without compaction, and never broken, cut or dry-swept. • Agricultural pesticide containers — triple rinsed with the rinsate returned to the spray tank, then punctured and disposed of; never reused for water or food.

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This section covers the 3R principles, the recycling of paper, plastics, metals and glass, composting and its types, incineration and its types, and good recycled products.

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The 3R Principles and Material Recovery Facilities • Reduce, Reuse, Recycle — in that order, because the order expresses the priority.

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Reduce prevents the waste existing; reuse keeps the article in service without reprocessing; recycle returns the material to manufacture, which costs energy and usually degrades quality.

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The sequence is sometimes extended to 5R with Refuse and Recover, or 6R with Repair.

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Stating the order and the reason for it is what the examination expects. • A material recovery facility (MRF) is a plant that receives, separates and prepares recyclable materials for sale to end markets. • Clean MRF versus dirty MRF — the comparison most often asked: a clean MRF receives source-separated recyclables and produces high-quality, high-value material with little residue; a dirty MRF receives mixed waste and must separate the recyclables from it, which gives a much lower recovery rate, contaminated material of low value, a large residue for disposal, and poor working conditions.

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Clean MRFs are therefore preferred wherever source separation can be achieved, and the existence of a dirty MRF is a symptom of failed source separation rather than a solution to it. • Separation technologies: manual picking from a conveyor, which remains the most flexible and is dominant in South Asia; trommel screens for size separation; magnetic separators for ferrous metal; eddy current separators for non-ferrous metal, chiefly aluminium; air classifiers for light fractions; optical and near-infrared sorters for plastics by polymer type; and float-sink separation by density.

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The eddy current separator deserves note because it is the standard means of recovering aluminium, which is not magnetic: a rotating magnetic field induces currents in the conducting non-ferrous particles, which are then repelled off the end of the belt.

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Recycling of Individual Materials Material Process Benefits and constraints Paper and cardboard Pulping, de-inking, screening, re-forming Saves trees, water and energy; the fibre shortens each time, so paper can be recycled only about five to seven times before the fibre is too short; cannot be recycled if contaminated with food or grease Plastics Sorting by polymer, washing, shredding, melting and pelletising Must be sorted by polymer type, since mixed polymers are incompatible; quality degrades with each cycle, so it is usually downcycled;

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PET and HDPE have established markets, others much less so Aluminium Melting and recasting Saves about 95 per cent of the energy of primary production from bauxite; can be recycled indefinitely with no loss of quality; the most valuable common recyclable by weight Steel and iron Magnetic separation, melting in electric arc furnace Easily separated magnetically; saves about 60-75 per cent of primary energy; indefinitely recyclable Glass Colour sorting, crushing to cullet, melting Indefinitely recyclable with no quality loss; must be sorted by colour; cullet lowers the furnace temperature and saves energy; heavy, so transport cost often exceeds its value over long distances Organic waste Composting or anaerobic digestion The largest fraction in Nepali waste and the greatest diversion opportunity; requires source separation to produce clean compost • Aluminium is the material to quote when asked which recycling saves the most energy: recycling saves about 95 per cent of the energy required to produce it from bauxite, because the electrolytic reduction of alumina is extraordinarily energy intensive. • Glass and metals can be recycled indefinitely without loss of quality; paper and plastics cannot, because the fibre shortens and the polymer degrades.

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The term downcycling describes recycling into a product of lower value, which is the normal fate of mixed plastics. • The constraint that actually governs recycling in practice is the market: material is only recycled if someone will buy it, so a collection scheme established without a secure end market simply accumulates sorted waste.

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This is the point most often missed, and it is why recycled-content requirements and green public procurement matter as much as collection systems.

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Composting • Composting is the controlled aerobic biological decomposition of organic waste into a stable, humus-like product.

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That it is aerobic is the defining condition: if oxygen is excluded the process becomes anaerobic and produces odour, acids and methane instead of compost. • The controlling parameters, which must be known with their values: carbon-to-nitrogen ratio of about 25-30; moisture content of 50-60 per cent, since below about 40 per cent biological activity slows and above about 65 per cent the pores fill and the mass goes anaerobic; particle size of 25-75 mm, small enough for surface area but large enough to maintain porosity; aeration by turning or forced air; pH of 6-8; and temperature. • The temperature profile is the feature examined: the process passes through a mesophilic phase, then a thermophilic phase reaching 55-65 °C, then a cooling phase and finally a curing or maturation phase.

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The thermophilic phase is essential because it destroys pathogens and weed seeds, and a compost that has not reached and held that temperature throughout the mass is not safe for use on food crops.

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Turning serves both to aerate and to bring the outer material into the hot centre. • Types of composting: • Windrow — long piles turned periodically by machine or by hand; simple, low cost, suits large volumes, needs land and is affected by rain. • Aerated static pile — air blown or drawn through a pile on a perforated base, with no turning; faster and needs less land than windrow but requires power and control. • In-vessel — enclosed reactor with full control of aeration, moisture and temperature; fastest, best odour control, smallest footprint, highest cost. • Vermicomposting — earthworms, typically Eisenia fetida, process the material at ambient temperature.

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It gives an excellent product rich in nutrients and microbial activity, but it is slower, requires careful control of moisture and temperature, and the worms are killed by the thermophilic phase — so vermicomposting does not reach pathogen-destroying temperatures and material should be pre-composted if pathogen kill is required.

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This limitation is examined directly. • Home and community composting — small-scale pit, bin or basket composting, which is the most cost-effective diversion available to a Nepali municipality because it removes the heaviest and most troublesome fraction of the waste before it ever enters the collection system. • Indicators of maturity: dark brown to black colour, earthy smell, temperature fallen to ambient, no recognisable feedstock, C/N ratio reduced to about 10-15, and no re-heating when turned.

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Immature compost damages plants, because it continues to decompose and robs the soil of nitrogen and oxygen.

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Incineration • Incineration is controlled combustion of waste at high temperature.

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It reduces volume by about 90 per cent and mass by about 70 per cent, destroys pathogens and organic toxins, and can recover energy.

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What it does not do is eliminate the waste: it leaves bottom ash, which is largely inert and may be usable as aggregate, and fly ash, which concentrates heavy metals and dioxins and is itself a hazardous waste requiring secure disposal.

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That fly ash is hazardous while bottom ash is generally not is a standard examination point. • The three Ts of good combustion: temperature, commonly 850 °C for municipal waste and 1100 °C where chlorinated material is present; time, a residence time of at least two seconds at that temperature; and turbulence, to ensure complete mixing of the combustion gases with air.

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Inadequate control of any of the three allows products of incomplete combustion, notably dioxins, to survive. • Dioxins and furans form in the low-temperature combustion of chlorinated material and re-form on cooling in the window around 250-400 °C, which is why rapid quenching of the flue gas through that range is a design requirement. • Types of incinerator: mass-burn moving grate, the standard for unsorted municipal waste; fluidised bed, which gives excellent mixing and lower temperature operation but requires prepared, size-reduced feed; rotary kiln, used mainly for hazardous and clinical waste because of its long residence time and tolerance of varied feed; and modular or starved-air units for small installations such as hospitals. • Air pollution control is the greater part of the cost of a modern incinerator, and the train is selective non-catalytic or catalytic reduction for NOx; a spray dryer or scrubber with lime for acid gases; activated carbon injection for dioxins and mercury; and a fabric filter for particulates. • The judgement for Nepal, which should be stated plainly: incineration of unsegregated municipal waste is not appropriate, because the calorific value is too low and the moisture too high to sustain combustion, the capital and operating cost is far beyond what municipal budgets can bear, the emission control required is demanding to operate, and the waste composition is better suited to composting and digestion.

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Incineration remains appropriate for clinical and certain hazardous wastes, where destruction rather than energy recovery is the object. • Good recycled products: the demand side of recycling.

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Compost and vermicompost; recycled paper and handmade paper, which is an established Nepali product; plastic lumber, road-surfacing material and pavement blocks incorporating waste plastic; crushed concrete aggregate; glass used as aggregate or in decorative work; and upcycled goods made from waste textiles, tyres and metal, which support livelihoods in Nepali cities.

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The decisive point remains that a recycling system without a market for its products does not function, so developing and protecting those markets through standards, labelling and public procurement is as much a part of waste management as collection.

1

This section covers the status of solid waste management in Nepal, the challenges and gaps, and the Solid Waste Management Act and Rules.

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Status of Solid Waste Management in Nepal • The general position should be described accurately, and the figures treated as indicative because surveys differ and conditions change: • Generation — urban generation has commonly been reported at around 0.2-0.5 kg per capita per day, with the Kathmandu Valley at the upper end; generation rises with income and urbanisation, and Nepal is urbanising rapidly. • Composition — 60-70 per cent organic, with plastics the next largest fraction and growing, which means the greatest diversion opportunity lies in the organic fraction. • Collection coverage — incomplete, with coverage substantially better in the core urban areas than in the newly declared municipalities, many of which have little or no service. • Disposal — the great majority of collected waste goes to open dumping, riverbank dumping or roadside tipping, with very few engineered sanitary landfills in operation; the Sisdol and subsequently Banchare Danda sites serving the Kathmandu Valley have been the subject of repeated disruption, and river dumping remains widespread, with the Bagmati and other urban rivers receiving a great deal of waste. • Recovery — recycling is carried out overwhelmingly by the informal sector of waste pickers, itinerant buyers and scrap dealers, who recover a substantial share of the plastics, metal, paper and glass at no cost to the municipality; formal material recovery facilities are few. • Finance — user fees are collected in some municipalities but rarely cover the cost of the service, so solid waste management is subsidised from general municipal revenue and competes with other priorities.

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Challenges and Gaps • These should be given as a structured list, since the question is usually asked in that form: • Institutional — overlapping and unclear responsibilities following federal restructuring; limited technical capacity in municipalities, many of which are newly created and have no engineer with waste experience; weak enforcement of existing rules; and frequent changes of personnel. • Financial — cost recovery is very low; willingness to pay is limited where the service is poor, which creates a vicious circle of poor service and poor revenue; and capital for facilities depends on donors or central transfers. • Technical — almost no engineered disposal capacity; very limited treatment infrastructure; inadequate and poorly maintained vehicles; and no reliable data on generation and composition, which makes planning guesswork. • Social — low public awareness and poor participation in segregation; the NIMBY problem, with communities near every proposed site opposing it, which has repeatedly closed landfill sites and halted projects; and the social marginalisation and poor occupational health of waste pickers. • Geographical — difficult terrain in hill towns, limited flat land suitable for disposal, high rainfall generating large volumes of leachate, and seismic risk affecting facility design. • The gap that matters most is the absence of segregation at source: without it, neither composting nor recycling can produce a clean product, so every downstream option is compromised.

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An answer identifying this as the single greatest gap is making the right point. • Opportunities: the very high organic fraction makes composting and biogas directly applicable; the informal recovery network already exists and can be built on; community-based and private-sector operation has worked well in several municipalities; and household composting removes the heaviest fraction at source at almost no public cost.

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The Legal Framework • The Solid Waste Management Act, 2068 (2011) is the principal statute, and it replaced the Solid Waste (Management and Resource Mobilization) Act, 2044 (1987).

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Its key provisions should be known: • Responsibility is placed on the local body for the construction, operation and management of infrastructure for the collection, treatment and disposal of solid waste within its area, and for the management of waste generated in its jurisdiction.

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That primary responsibility rests with local government is the most examined provision. • Duty of the generator — any person or institution generating waste is required to segregate it at source into organic and inorganic fractions and dispose of it as prescribed by the local body, and hazardous, chemical, industrial, medical and harmful waste must be managed by the generator itself at its own cost under the prescribed standards.

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The principle that the generator of hazardous waste is responsible for managing it is the second most examined provision. • Private sector participation — the local body may involve the private sector or a community organisation in solid waste management through a competitive process, with a licence and an agreement. • Service fee — the local body may levy and collect a service fee for solid waste management, set by considering the type and quantity of waste. • Prohibitions and penalties — the Act prohibits the indiscriminate throwing or dumping of waste in public places, the open burning of waste, and the disposal of waste in rivers and streams, and provides for penalties. • Solid Waste Management Technical Support Centre — established under the Act to provide technical assistance to local bodies. • The Solid Waste Management Rules, 2070 (2013) prescribe the procedural detail — standards and procedures for segregation, collection, transport, treatment and disposal; licensing of private operators; the format of agreements; the fixing of service fees; and the requirements for landfill sites. • Related instruments: the Environment Protection Act, 2076 and Rules, 2077, under which environmental assessment of a landfill is required; the Local Government Operation Act, 2074, which allocates functions to local levels under the federal structure; and the Public Health Service Act, together with healthcare waste management guidelines issued by the Ministry of Health and Population.

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The Constitution of Nepal, 2072 assigns solid waste management to the local level in its schedule of exclusive powers. • A caution that should accompany any answer on this topic: responsibilities have been redistributed by federal restructuring, and the legislation is amended from time to time, so the current text of the Act and Rules and the current institutional arrangements must be verified against official sources before any professional reliance.