GATE Environmental Engineering Solid and Hazardous Waste Management Syllabus

Write the simplified overall reaction for the aerobic decomposition of organic matter.

$$\ce{Organic matter + O2 -> CO2 + H2O + NH3 + SO4^{2-} + heat + humus}$$

What are the four phases of anaerobic digestion?

(1) Hydrolysis (complex polymers $\to$ monomers), (2) Acidogenesis (monomers $\to$ volatile fatty acids), (3) Acetogenesis (VFAs $\to$ acetate, $\ce{H2}$, $\ce{CO2}$), and (4) Methanogenesis (acetate/$\ce{H2}$ $\to$ $\ce{CH4 + CO2}$).

What is the approximate composition and calorific value of biogas from anaerobic digestion?

Biogas is about $55$–$65\%$ $\ce{CH4}$ and $35$–$45\%$ $\ce{CO2}$ (with traces of $\ce{H2S}$, $\ce{N2}$). Its calorific value is roughly $20$–$25$ MJ/m$^{3}$ (about $5000$–$6000$ kcal/m$^{3}$).

What is vermicomposting?

A biological process in which earthworms (e.g., Eisenia fetida) consume and digest organic waste, producing nutrient-rich vermicast (worm castings); it operates at lower (mesophilic) temperatures than thermophilic composting and yields a high-quality soil conditioner.

Compare windrow composting and aerated static pile / in-vessel composting.

Windrow: long piles turned periodically for aeration; simple, land-intensive, slower. Aerated static pile: air forced/drawn through stationary pile, no turning. In-vessel: enclosed reactor with controlled aeration/temperature; fastest, smallest footprint, best odour control, highest cost.

What are the three main thermal energy-recovery (waste-to-energy) processes and their oxygen conditions?

Incineration — full combustion with excess air. Gasification — partial oxidation with limited air/oxygen producing syngas ($\ce{CO + H2}$). Pyrolysis — thermal decomposition in the absence of oxygen producing char, oil and gas.

Define incineration and state its main advantage and concern for MSW.

Incineration is controlled high-temperature combustion (~$850$–$1000^{\circ}\text{C}$) of waste with excess air. Advantage: up to ~$80$–$90\%$ volume reduction plus energy recovery. Concern: air emissions (dioxins/furans, particulates) and low efficiency for high-moisture, low-calorific Indian MSW.

What is refuse-derived fuel (RDF)?

A fuel produced by shredding, drying, screening and removing inerts/non-combustibles from MSW, yielding a higher and more uniform calorific value combustible product (pellets/fluff) suitable for co-firing in boilers and cement kilns.

What minimum calorific value is generally considered necessary for self-sustaining (autogenous) incineration of MSW?

About $1000$ kcal/kg (~$4200$ kJ/kg). Below this, auxiliary fuel is needed — a common problem for high-moisture Indian MSW.

Define a sanitary landfill and how it differs from an open dump.

A sanitary landfill is an engineered facility where waste is spread in thin layers, compacted, and covered with soil daily, with liners, leachate collection and gas management. An open dump has no engineering controls, causing uncontrolled leachate, odour, fires and disease vectors.

What are the typical engineered components (from bottom to top) of a modern sanitary landfill?

Bottom: compacted clay/geomembrane liner $\to$ leachate collection and removal system $\to$ compacted waste cells with daily cover $\to$ gas collection system $\to$ final cover/cap (clay + geomembrane + soil + vegetation).

What is landfill leachate and what factors control its generation?

Leachate is the contaminated liquid that percolates through waste, carrying dissolved/suspended pollutants. Its quantity depends on precipitation, surface runoff, infiltration, waste moisture above field capacity, and cover/evapotranspiration.

Write the water-balance basis for estimating leachate generation in a landfill.

$$L = P - R - E - \Delta S$$ where $L$ = leachate, $P$ = precipitation infiltrating, $R$ = surface runoff, $E$ = evapotranspiration, and $\Delta S$ = change in moisture storage of waste up to field capacity.

What are the typical phases of landfill gas generation over time?

(1) Aerobic phase (short, $\ce{O2}$ consumed, $\ce{CO2}$ produced), (2) Anaerobic acid phase ($\ce{CO2}$, $\ce{H2}$, organic acids), (3) Initial methanogenic phase, and (4) Stable methanogenic phase (steady $\ce{CH4}$ ~$50$–$55\%$, $\ce{CO2}$ ~$45$–$50\%$).

Why is methane from landfills both a hazard and a resource?

Hazard: $\ce{CH4}$ is explosive at $5$–$15\%$ in air and is a potent greenhouse gas (~$25\times$ $\ce{CO2}$ over 100 yr). Resource: it can be collected and combusted for heat/electricity (landfill gas to energy).

What is the purpose of daily, intermediate and final cover in a landfill?

Daily cover (~$15$ cm soil) controls vectors, odour, litter and fire. Intermediate cover is for inactive areas for longer periods. Final cover/cap minimizes infiltration (reducing leachate), controls gas escape, and supports vegetation after closure.

How is the required landfill area/volume estimated for a given population?

From the volume of compacted waste plus cover soil: $$V = \frac{P \times q \times t}{\rho_{c}} \times (1 + f_{cover})$$ where $P$ = population, $q$ = per-capita generation, $t$ = design period, $\rho_{c}$ = compacted density, and $f_{cover}$ accounts for cover soil (often ~$0.2$–$0.25$).

Define hazardous waste as per Indian regulations.

Waste that, because of its physical, chemical, reactive, toxic, flammable, explosive or corrosive characteristics, causes danger or is likely to cause danger to health or environment — whether alone or in contact with other wastes/substances (Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016).

What are the four key characteristics that classify a waste as hazardous?

Ignitability (flammable), Corrosivity (e.g., $\text{pH} \leq 2$ or $\geq 12.5$), Reactivity (unstable/explosive/reactive with water), and Toxicity (e.g., by TCLP leaching test). These are sometimes remembered as ICRT.

Name the major sources/generators of hazardous waste.

Chemical and petrochemical industries, pharmaceuticals, pesticides, paints and dyes, metal finishing and electroplating, tanneries, petroleum refining, pulp and paper, and pesticide/fertilizer manufacture; plus electronic and battery waste.

What does TCLP stand for and what is its purpose?

Toxicity Characteristic Leaching Procedure — a laboratory test that simulates landfill leaching to determine whether toxic constituents leach above regulatory limits, thereby classifying a waste as hazardous (toxic) or not.

What is meant by the 'fate of a hazardous material in the environment'?

The physical, chemical and biological transport and transformation processes a contaminant undergoes after release — including advection, dispersion, sorption/desorption, volatilization, hydrolysis, oxidation/reduction, photolysis and biodegradation — which determine its persistence, mobility and ultimate distribution.

Define bioaccumulation and biomagnification.

Bioaccumulation is the build-up of a persistent contaminant in an organism over time (uptake exceeds elimination). Biomagnification is the increasing concentration of the contaminant at successively higher trophic levels of a food chain.

What is the retardation factor in contaminant transport through soil/groundwater?

$$R = 1 + \frac{\rho_{b}\,K_{d}}{n}$$ where $\rho_{b}$ = bulk density, $K_{d}$ = distribution (sorption) coefficient, and $n$ = porosity. $R$ indicates how much slower a sorbing contaminant moves relative to groundwater.

Give the one-dimensional advection–dispersion equation governing contaminant transport.

$$\frac{\partial C}{\partial t} = D\frac{\partial^{2} C}{\partial x^{2}} - v\frac{\partial C}{\partial x}$$ where $C$ = concentration, $D$ = dispersion coefficient, and $v$ = seepage (pore) velocity. It combines dispersive spreading and advective transport.

List common physico-chemical treatment methods for hazardous wastes.

Neutralization, chemical precipitation, oxidation/reduction, ion exchange, adsorption (activated carbon), solvent extraction, air/steam stripping, and membrane processes (RO/UF) — used to reduce toxicity, mobility or volume before disposal.

What is solidification/stabilization of hazardous waste?

A treatment that immobilizes contaminants by mixing waste with binders (e.g., cement, lime, fly ash, pozzolans) to form a solid, low-permeability mass — chemically fixing and/or physically encapsulating pollutants to reduce leachability before secure landfilling.

What is a secure landfill for hazardous waste and how does it differ from a sanitary (MSW) landfill?

A secure landfill has stricter engineering — double liners with leak-detection between them, leachate collection above each liner, restricted/compatible waste segregation, and long-term monitoring — to contain toxic wastes, whereas a sanitary landfill (single liner) is for non-hazardous MSW.

Why is incineration often preferred for organic hazardous wastes, and what conditions ensure destruction?

High-temperature incineration achieves high destruction and removal efficiency (DRE, often $\geq 99.99\%$) of toxic organics. Effective destruction follows the '3 T's': high Temperature (~$1100^{\circ}\text{C}$ for hazardous), sufficient residence Time, and adequate Turbulence (mixing), plus excess oxygen.

What are the management practices mandated for hazardous waste under the cradle-to-grave principle?

Generators must identify/characterize waste, minimize generation, ensure safe on-site storage and labelling, transport via authorized carriers with a manifest, treat/recycle where possible, and dispose only at authorized TSDFs — with tracking and authorization at every stage from generation to final disposal.