10. PREPROCESSING OF BIOMASS AND ITS MANAGEMENT.pptx

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PREPROCESSING OF
BIOMASS AND ITS
MANAGEMENT

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WASTES
 Waste (or wastes) are unwanted or unusable
materials. Waste is any substance which is discarded
after primary use, or is worthless, defective and of no
use.
SOLID WASTES
 Solid Waste (also referred to as rubbish, trash, refuse,
garbage, or junk) is unwanted or unusable materials.

z TYPES OF SOLID WASTE
 Municipal waste
 Hazardous waste
 Biomedical waste
 Electronic waste

SOLID WASTE DISPOSAL
Types of materials or refuse commonly transported to a disposal site.

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CLIMATE CHANGE/GLOBAL WARMING
 Green House Effect
Some gases naturally exist in the atmosphere, the so called
Greenhouse Gases (GHGs) that form a blanket surrounding the
earth and keeps the earth warmer. This is called Greenhouse Effect
 Enhanced Greenhouse Effect
Human activities (fossil fuel burning, depletion of sinks like forests
etc.) has been increasing the concentration of GHGs in the
atmosphere and is leading to rise in temperatures. This is called
Enhanced Greenhouse Effect.
 Global Warming/Climate Change
Rise in temperatures of earth and other associated climatic changes
as caused by the Enhanced Green House Effect is called “Global
Warming” and in broader term “Climate Change”

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POTENTIAL OF GREEN HOUSE GASES
Name Formula GWP (CO2 eq.)
1. Carbon- dioxide (CO2) 1
2. Methane (CH4) 21
3. Nitrous oxide (N2O) 310
5. Per- fluorocarbons (PFCs) 92,00
4. Hydro- fluorocarbons (HFCs) 11,700
6. Sulphur hexafluoride (SF6) 23,900
Sinks (carbon sequestration)

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CLASSIFICATION OF SOLID WASTES
Solid wastes
Based on
Source
1. Residential
2. Commercial
3. Industrial
4. Institutional
5. Municipal
6. Agricultural
Based on Type
1. Biodegradable
2. Non- biodegradable

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CLASSIFICATION BASED ON TYPE
Biodegradable Non-biodegradable
consist of organic matter and
can be degraded
consist of inorganic
and recyclable
materials which
cannot be degraded
Paper, food waste, vegetables, fruit peels,
wood, etc.
Plastics, glass and metals

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TYPE DESCRIPTION SOURCE
Garbage Wastes from the preparation, cooking and serving of food, market
refuse, waste from the handling, storage, and sale of produce and
meat.
Households, institutions and
commercial concerns such as
hotels, stores, restaurants, market,
etc
Combustible and non-
combustible
Combustible (primarily organic) paper, cardboard, cartons, wood,
boxes, plastic, rags, cloth, bedding, leather, rubber, grass, leaves, yard
trimmings etc.
Ashes Residue from fires used for cooking and for heating building cinders
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Bulky wastes Large auto parts, tyres, stoves, refrigerators, other large
appliances, furniture, large crates, trees branches, stumps etc
Streets, sidewalks, alleys,
vacant plots etc.
Street wastes Street sweepings, dirt, leaves etc.
Dead animals Dogs, cats, rats, donkeys etc.
Abandoned vehicles Automobiles and spare parts
Construction and
demolition wastes
Roofing and sheathing scraps, rubble, broken concrete,
plaster, conduit pipe, wire, insulation etc
Construction and demolition
sites

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Type Description Source
Industrial
wastes
Solid wastes resulting from industrial processes and manufacturing
operations, such as food processing wastes, boiler house cinders, wood,
plastic and metal scraps, shaving etc
Factories, power plants etc
Hazardous
wastes
Pathological wastes, explosives, radioactive materials etc. Households, hospitals,
institutions, stores, industry
etc
Animals &
agricultural
wastes
Manure, crop residues etc Livestock, farms, feedlots
and agriculture
Sewage
treatment
residue
Coarse screening grit, septic tank sludge, dewatered sludge. Sewage treatment plants
and septic tanks.
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CATEGORY TYPE OF WASTE APPROXIMATE TIME TAKEN TO
DEGENERATE
Biodegradable
Organic waste such as vegetable
and fruit peels, food waste etc
A week or two
Paper Upto 30 days
Cotton cloth 2-5 months
Woollen items 1 year
Wood upto15 years
Non-biodegradable
Tin, aluminum, and other metal
items such as cans
100-500 years
Plastic bags One million years?
Glass bottles Undetermined
DEGENERATION TIME-SOLID WASTES

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a. Domestic/Residential Waste
 This category of waste comprises the solid wastes that
originate from single and multi-family household units.
These wastes are generated as a consequence of household
activities such as cooking, cleaning, repairs, hobbies,
redecoration, empty containers, packaging, clothing, old
books, writing/new paper, and old furnishings. Households
also discard bulky wastes such as furniture and large
appliances which cannot be repaired and used.
b. Municipal Waste
 Municipal waste include wastes resulting from municipal
activities and services such as street waste, dead animals,
market waste and abandoned vehicles.
 However, the term is commonly applied in a wider sense to
incorporate domestic wastes, institutional wastes and
commercial wastes.

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c. Commercial Waste
 Included in this category are solid wastes that originate
in offices, wholesale and retail stores, restaurants,
hotels, markets, warehouses and other commercial
establishments. Some of these wastes are further
classified as garbage and others as rubbish.
d. Institutional Waste
 Institutional wastes are those arising from institutions
such as schools, universities, hospitals and research
institutes. It includes wastes which are classified as
garbage and rubbish as well as wastes which are
considered to be hazardous to public health and to the
environment.

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e. Industrial Wastes
 In the category are the discarded solid material of
manufacturing processes and industrial operations. They
cover a vast range of substances which are unique to each
industry. For this reason they are considered separately
from municipal wastes. It should be noted, however, that
solid wastes from small industrial plants and ash from
power plants are frequently disposed of at municipal
landfills.
f. Agricultural wastes
 Agricultural waste is waste produced as a result of various
agricultural operations. It includes manure and other
wastes from farms, poultry houses and slaughterhouses;
harvest waste; fertilizer runoff from fields; pesticides that
enter into water, air or soils; and salt and silt drained from
fields

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Biodegradable
 Biodegradable wastes are those
waste materials that can be
degraded by natural factors
like microbes (e.g. bacteria,
fungi and a few more), abiotic
elements like temperature, UV,
oxygen, etc.
 Microorganisms and other
abiotic factors together
contribute towards breaking
down complex substances into
simpler organic matters.
 These substances eventually
suspend and fade into the soil.
 The whole process is natural
which is sometimes slow and
sometimes rapid.
Non-Biodegradable
 Non - Biodegradable objects or
materials are those which do not easily
decompose by natural factors.
 Non - Biodegradable Waste is the kind
of waste that cannot be decomposed
by biological processes.
 Most of the inorganic, plastic and
artificial waste are non-biodegradable.
 Non-biodegradable wastes are of two
types.
 The kind of non-biodegradable that
can be recycled are known as
“Recyclable waste” and those which
cannot be recycled are known as “Non-
recyclable waste”.
 Most of the non-biodegradable waste is
non-recyclable waste making them
extremely harmful and dangerous for
the environment as well as human
health.

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SOLID WASTE MANAGEMENT
 SWM is the control of generation, storage, collection,
transfer and transportation, processing and disposal of
solid wastes
 This includes all technological, financial, institutional and
legal aspects involved to solve the whole spectrum of
issues related solid waste

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SOLID WASTE MANAGEMENT
 Solid waste management may be defined as the discipline
associated with the control of generation, storage, collection,
transfer and transport, processing and disposal of solid wastes in
a manner that is in accord with the best principles of public
health, economics, engineering, conservation, aesthetics and other
environmental considerations and that is also responsive to public
attitudes.
 In its scope, solid waste management includes all administrative,
financial, legal, planning and engineering functions involved in
solutions to all problems of solid wastes. The solutions may
involve complex interdisciplinary relationships among such fields
as political science, city and regional planning, geography,
economics, public health, sociology, demography,
communications and conservation, as well as engineering and
materials science

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GENERATION
 Generation of solid waste is a result of natural, human
and animal activities
 Knowledge of generation of solid waste is important in
the planning, designing and operation of solid waste
management system.
 Generation has two aspects: One is the quality of solid
waste and the other is the quantity of solid waste.
 Quality includes the sources, types and typical
composition of solid waste along with its properties
whereas the quantity represents the generation rates
and total quantities and volumes of waste generated.
 The handling, storage and separation of solid waste at
the source before they are collected is a critical step in
the management of residential solid waste

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WASTE HANDLING
• Handling refers to activities associated with managing
solid wastes until they are placed in the containers
used for their storage before collection or return to
drop-off and recycling centers.
• The specific activities associated with handling waste
materials at the source of generation will vary
depending on the types of waste materials that are
separated for reuse and recycling and the extent to
which these materials are separated from the waste
stream.
• Depending on the type of collection service, handling
may also be required to move the loaded containers to
the collection point and to return the empty container
to the point where they are stored between collections

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WASTE STORAGE
 The first phase to manage solid waste is at home level. It
requires temporary storage of refuse on the premises.
 The individual household or businessman has
responsibility for onsite storage of solid waste.
 For individual homes, industries, and other commercial
centers, proper on-site storage of solid waste is the
beginning of proper disposal, because unkept solid waste or
simple dumps are sources of nuisance, flies, smells and
other hazards.
TRANSFER AND TRANSPORT
 Transfer and Transport refers to the means, facilities,
appurtenances used to affect the transfer of wastes from
one location to another, usually more distant location.
 Typically, the wastes from relatively small collection
vehicles are transferred to larger vehicles and then
transported to distant locations.

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RESOURCE RECOVERY AND PROCESSING
 Resource recovery is a partial solid waste disposal and
reclamation process.
 It can be expected to achieve waste reductions in
future landfill volume requirements.
 Resource recovery must recognize what is worth
recovering and the environmental benefits.
DISPOSAL
 Most of the organic content after segregation may be
subjected to bacterial decomposition with an end
product called humus or compost.
 The entire process involving both separation and
bacterial conversion is known as “Composting”
 Decomposition of solid waste may be accomplished
aerobically or anaerobically.

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WASTE TREATMENT METHODS
 Incineration
 Landfill
 Recycling
 Anaerobic digestion
 Composting

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INCINERATION
 Burning is a very effective method of reducing the volume and weight of
solid waste. In incinerators the waste is burned inside a properly
designed furnace under very carefully controlled conditions.
 The combustible portion of the waste combines with oxygen, releasing
mostly carbon dioxide, water vapour and heat.
 It can reduce the volume of uncompacted waste by more than 90 %,
leaving an inert residue of ash, glass, metal and other solid materials
called bottom ash.
 The gaseous by-products of incomplete combustion, along with finely
divided particulate material called fly ash, are carried along in the
incinerator air-stream.
 Fly ash includes cinders, dust and soot. In order to remove fly ash and
gaseous by-products before they are exhausted into the atmosphere,
modern incinerators must be equipped with extensive emission control
devices. Such devices include fabric baghouse filters, acid gas scrubbers,
and electrostatic precipitators.
 Bottom ash and fly ash are usually combined and disposed of in a
landfill.

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LANDFILL
 Land disposal is the most common management strategy for MSW.
 Refuse can be safely deposited in a sanitary landfill, a disposal site
that is carefully selected, designed, constructed and operated to
protect the environment and public health.
 One of the most important factors relating to landfilling is that the
buried waste never comes in contact with surface water or
groundwater.
 Engineering design requirements include a minimum distance
between the bottom of the landfill and the seasonally high
groundwater table.
 Most new landfills are required to have an impermeable liner or
barrier at the bottom, as well as a system of groundwater-monitoring
wells.
 Completed landfill sections must be capped with an impermeable
cover to keep precipitation or surface runoff away from the buried
waste.
 Bottom and cap liners may be made of flexible plastic membranes,
layers of clay soil, or a combination of both.22

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LANDFILL GAS CAPTURE SYSTEM INCLUDING LANDFILL LAYERS
AND COLLECTION WELLS

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RECYCLING
 Before any material can be recycled, it must be separated from
the raw waste and sorted. Separation can be accomplished at
the source of the waste or at a central processing facility.
 Source separation, also called curbside separation, is done by
individual citizens who collect newspapers, bottles, cans, and
garbage separately and place them at the curb for collection.
 The best practice is to have citizens separate refuse into a
limited number of categories, including newspaper; magazines
and other wastepaper; commingled metals, glass, and plastics;
and garbage and other non-recyclables.
 The newspaper, other paper wastes, and commingled recyclables
are collected separately from the other refuse and are processed
at a centralized material recycling facility, or MRF. A modern
MRF can process about 300 tons of recyclable wastes per day.

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 At a typical MRF, commingled recyclables are loaded onto a
conveyor. Steel cans are removed by an electromagnetic
separator, and the remaining material passes over a vibrating
screen in order to remove broken glass. Next, the conveyor
passes through an air classifier, which separates aluminum and
plastic containers from heavier glass containers. Glass is
manually sorted by colour, and aluminum cans are separated
from plastics by an eddy-current separator, which repels the
aluminum from the conveyor belt.
Reuse
 Recovered broken glass can be crushed and used in asphalt
pavement. Colour-sorted glass is crushed and sold to glass
manufacturers as cullet, an essential ingredient in glassmaking.
 Steel cans are baled and shipped to steel mills as scrap.
 Aluminum is baled or compacted for reuse by smelters.
 Recycling of plastic is a challenge. Mixed thermoplastics can be
used only to make lower-quality products, such as “plastic
lumber.”

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 Old newspapers are sorted to remove corrugated materials and then
baled to paper mills, where they are reused in the making of more
newspaper. Although the processes of pulping, de-inking, and
screening wastepaper are generally more expensive than making paper
from virgin wood fibres, the market for recycled paper has grown with
the establishment of more processing plants.
 Rubber is sometimes reclaimed from solid waste and shredded,
reformed, and remolded in a process called revulcanization, but it is
usually not as strong as the original material.
 Shredded rubber can be used as an additive in asphalt pavements and
artificial turf and is also sold directly as an outdoor mulch. Discarded
tires may be employed as swings and other recreational structures for
use by children in “tire playgrounds.”
 The most difficult problem associated with the recycling is finding
applications and suitable markets.
 Recycling by itself will not solve the growing problem of SWM, because
completely valueless solid residue needs final disposal.

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ANAEROBIC DIGESTION
 Anaerobic digestion is a process through which bacteria break down
organic matter—such as animal manure, wastewater biosolids, and
food wastes—in the absence of oxygen.
 Anaerobic digestion for biogas production takes place in a sealed
vessel called a reactor, which is designed and constructed in various
shapes and sizes specific to the site and feedstock conditions.
 These reactors contain complex microbial communities that break
down (or digest) the waste and produce resultant biogas and digestate
which is discharged from the digester.
 Multiple organic materials can be combined in one digester, a practice
called co-digestion. Co-digested materials include manure; food waste
(i.e., processing, distribution and consumer generated materials);
energy crops; crop residues; and fats, oils, and greases (FOG) from
restaurant grease traps, and many other sources.
 Co-digestion can increase biogas production from low-yielding or
difficult-to-digest organic waste

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COMPOSTING
 It is a biological process in which the organic portion of refuse is
allowed to decompose under carefully controlled conditions.
 Microbes metabolize the organic waste material and reduce its
volume by as much as 50 percent.
 The stabilized product is called compost or humus. It resembles
potting soil in texture and odour and may be used as a soil
conditioner or mulch.
 The steps involved in the process include sorting and
separating, size reduction, and digestion of the refuse.
Sorting and shredding
 The decomposable materials in refuse are isolated from glass,
metal, and other inorganic items through sorting and separating
operations.
 Shredding or pulverizing reduces the size of the waste articles,
resulting in a uniform mass of material. It is accomplished with
hammer mills and rotary shredders.

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Digesting and processing
 Pulverized waste is ready for composting either by the open
windrow method or in an enclosed mechanical facility.
 Windrows are long, low mounds of refuse. They are turned or
mixed every few days to provide air for the
microbes digesting the organics.
 Depending on moisture conditions, it may take 5-8 weeks for
complete digestion of the waste.
 Because of the metabolic action of aerobic bacteria,
temperatures in an active compost pile reach about 65 °C (150
°F), killing pathogenic organisms that may be in the waste
material.
 Digested compost must be processed before it can be used as a
mulch or soil conditioner. Processing includes drying, screening,
and granulating or pelletizing.

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ALTERNATE TREATMENT METHODS
 Gasification
 Hydrothermal carbonization
 Hydrothermal liquefaction
 Mechanical biological treatment (sorting into selected fractions)
 Refuse-derived fuel
 Mechanical heat treatment
 Molten salt oxidation
 Pyrolysis
 UASB (applied to solid wastes)
 Bioconversion of biomass to mixed alcohol fuels

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GASIFICATION
 Gasification is a process that converts organic or fossil-based
carbonaceous materials at high temperatures (>700°C), without
combustion, with a controlled amount of oxygen and/or steam
into carbon monoxide, hydrogen, and carbon dioxide. The
carbon monoxide then reacts with water to form carbon dioxide
and more hydrogen via a water-gas shift reaction. Adsorbers or
special membranes can separate the hydrogen from this gas
stream.
 Simplified example reaction
C6H12O6 + O2 + H2O → CO + CO2 + H2 + other species
In the above reaction uses glucose as a surrogate for
cellulose. Actual biomass has highly variable composition and
complexity with cellulose as one major component.
 Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)

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PLASMA GASIFICAION
 Plasma gasification is an extreme thermal process using plasma which
converts organic matter into a syn-gas which is primarily made up
of hydrogen and carbon monoxide.
 A plasma torch powered by an electric arc is used to ionize gas
and catalyze organic matter into syn-gas, with slag remaining as a by-product.
 It is used commercially as a form of waste treatment and has been tested for
the gasification of refuse-derived fuel, biomass, industrial waste, hazardous
waste, and solid hydrocarbons, such as coal, oil sands, petcoke and oil shale.
 Small plasma torches typically use an inert gas such as argon where larger
torches require nitrogen. The electrodes vary from
copper or tungsten to hafnium or zirconium, along with various other alloys.
 A strong electric current under high voltage passes between the two electrodes
as an electric arc. Pressurized inert gas is ionized passing through the plasma
created by the arc. The torch's temperature ranges from 2,000 to 14,000 °C.
 The waste is heated, melted and finally vaporized. Only at these extreme
conditions can molecular dissociation occur by breaking apart molecular
bonds. Complex molecules are separated into individual atoms. The resulting
elemental components are in a gaseous phase (syn-gas). Molecular
dissociation using plasma is referred to as "plasma pyrolysis."

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HYDROTHERMAL CARBONISATION - HTC
 HTC is a chemical process for the conversion of organic compounds to
structured carbons.
 It can be used to make a wide variety of nanostructured carbons, simple
production of brown coal substitute, synthesis gas, liquid petroleum
precursors and humus from biomass with release of energy.
 Biomass is heated together with water to 180 °C in a pressure vessel.
The pressure rises to about 1 MPa (10 bar). During the reaction, oxonium
ions are also formed which reduce the pH to pH 5 and lower. In this case,
at low pH values, more carbon passes into the aqueous phase. The effluent
reaction is exothermic, that is, energy is released.
 After 12 hours, the carbon of the reactants is completely reacted, 90 to
99% of the carbon is present as an aqueous sludge of porous brown coal
spheres with pore sizes between 8 and 20 nm as a solid phase, the
remaining 1 to 10% of carbon is either dissolved in the aqueous phase or
converted to carbon dioxide. The reaction equation for the formation of
brown coal is:
C6H12O6  C6H2O +5H2O

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HYDROTHERMAL LIQUEFACTION - HTL
 HTL is a thermal depolymerization process used to convert wet biomass, and
other macromolecules, into crude-like oil under moderate temperature and
high pressure.
 The crude-like oil has high energy density with a lower heating value of 33.8-
36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals.
 Carbon and hydrogen of an organic material, such as biomass, peat or low-
ranked coals (lignite) are thermo-chemically converted into hydrophobic
compounds with low viscosity and high solubility.
 Depending on the processing conditions, the fuel can be used as produced for
heavy engines, including marine and rail or upgraded to transportation fuels,
such as diesel, gasoline or jet-fuels.
 Operating temperature is between 250-550 °C and high pressures of 5-25 MPa
as well as catalysts for 20–60 minute.
 At these temperatures and pressures, the water present in the biomass
becomes either subcritical or supercritical, depending on the conditions, and
acts as a solvent, reactant, and catalyst to facilitate the reaction of biomass to
bio-oil.
 The exact conversion of biomass to bio-oil is dependent on several variables
like Feedstock composition, Temperature and heating rate, Pressure, Solvent,
Residence time & Catalysts

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MOLTEN SALT OXIDATION
 Molten salt oxidation is a non-flame, thermal process that destroys all
organic materials while simultaneously retaining inorganic
and hazardous components in the melt.
 It is used as either hazardous waste treatment (with air) or energy
harvesting similar to coal and wood gasification (with steam).
 The molten salt of choice has been sodium carbonate (m.p 851°C), but
other salts can be used.
 Sulfur, halogens, phosphorus and similar volatile pollutants are
oxidized and retained in the melt. Most organic carbon content leaves
as relatively pure CO/CO2/H2/H2O and the effluent only requires a
cold trap and a mild aqueous wash.
 It has been used for safe biological and chemical weapons destruction,
and processing waste such as scrap tires where direct
incineration/effluent treatment is difficult.
 The major downside of the process compared to direct incineration is
the eventual saturation of the melt by contaminants, and needing
reprocessing/replacement.

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PYROLYSIS
 Pyrolysis is the gasification of biomass in the absence of oxygen.
 In general, biomass does not gasify as easily as coal, and it
produces other hydrocarbon compounds in the gas mixture
exiting the gasifier; this is especially true when no oxygen is
used.
 As a result, typically an extra step must be taken to reform
these hydrocarbons with a catalyst to yield a clean syngas
mixture of hydrogen, carbon monoxide, and carbon dioxide.
 Then, just as in the gasification process for hydrogen
production, a shift reaction step (with steam) converts the
carbon monoxide to carbon dioxide. The hydrogen produced is
then separated and purified.

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UASB
 Upflow anaerobic sludge blanket (UASB) technology, normally
referred to as UASB reactor, is a form of anaerobic digester that
is used for wastewater treatment.
 UASB uses an anaerobic process whilst forming a blanket of
granular sludge which suspends in the tank.
 Wastewater flows upwards through the blanket and is processed
by the anaerobic microorganisms.
 The upward flow combined with the settling action
of gravity suspends the blanket with the aid of flocculants. The
blanket begins to reach maturity at around three months.
 Small sludge granules begin to form whose surface area is
covered in aggregations of bacteria. In the absence of any support
matrix, the flow conditions create a selective environment in
which only those microorganisms capable of attaching to each
other survive and proliferate. Eventually the aggregates form into
dense compact bio-films referred to as "granules".

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 Biogas with a high concentration
of methane is produced as a by-product,
and this may be captured and used as an
energy source, to generate electricity.
 The technology needs constant monitoring
when put into use to ensure that the
sludge blanket is maintained, and not
washed out.
 The blanketing of the sludge enables a
dual solid and hydraulic (liquid) retention
time in the digesters. Solids requiring a
high degree of digestion can remain in the
reactors for periods up to 90 days.

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 Liquid waste can be defined as such Liquids like wastewater,
fats, oils or grease (FOG), used oil, liquids, solids, gases, or
sludges and hazardous household liquids.
 These liquids that are hazardous or potentially harmful to
human health or the environment.
Types of Industrial Wastewater
 Inorganic wastewater
 Organic wastewater

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FAECAL SLUDGE MANAGEMENT
 When human excreta collects in a pit latrine, the solids settle at
the bottom and form a slurry called faecal sludge.
 Over time the sludge accumulates and periodically needs to be
removed and disposed of. This process presents several
challenges because the sludge is offensive, a potential danger to
human health and highly polluting if dumped indiscriminately
into the environment.
 Faecal sludge management (FSM) is a set of processes
designed to ensure that people and the environment are
protected from these hazards. It includes the storage, collection,
transport, treatment and safe end use or disposal of faecal
sludge.
 FSM is a significant problem in towns and cities in many
developing countries. Key issues are who is responsible for
collecting sludge and where and how it is disposed of.

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PIT EMPTYING
 The process of pit emptying is sometimes called desludging.
There are manual and mechanical methods for desludging, but
the manual removal of faecal sludge from pit latrines poses
severe risks to those undertaking the task.
 Whether using manual or mechanical methods, the personal
safety of anyone employed in pit emptying should be of primary
importance. Operatives should wear gloves, masks and
protective clothing.

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VACUUM TRUCKS
 Vacuum trucks are vehicles equipped with a storage tank and
pump with a hose that is lowered into the pit to suck the sludge
up and out into the storage tank.
 The sludge can then be easily transported to a suitable disposal
site.
 Vacuum trucks are quick, powerful and efficient, but they are
large vehicles and so access to the pits can be a problem.
 The size of the truck can limit their use in areas where roads are
narrow and twisting.
 Truck operators will charge a fee for their services, which is
another factor that needs to be considered

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VACUTUG
 The Vacutug is basically a smaller version of the vacuum truck.
 It was devised by UN-Habitat as a system that could replace
manual emptying.
 It is a mechanical system that can be manufactured locally
using readily available components.
 It is affordable, easily serviceable, able to operate in narrow
passageways where vacuum trucks cannot go and is capable of
sucking out waste sludge for transportation to a larger tanker
vehicle.
 It can empty pits down to 2 m deep.

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HAND OPERATED PUMPS
 Even the Vacutug cannot get everywhere. The Sludge Gulper is
an example of a smaller hand-operated pump.
 These can be taken to pit latrines that are inaccessible to larger
pumps. It is a simple design consisting of a PVC pipe containing
two valves and can be built using locally available materials. The
sludge is pumped up by hand, collected in a container and
taken away for disposal. Care is needed to ensure that the
operator and other helpers do not come into contact with the
sludge and that it is not spilled.

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DISPOSAL OF SLUDGE
 For disposal of the collected sludge, it can be put directly onto
land and used as a soil conditioner, but this is only possible if it
has been left untouched for at least two years.
 Fresh, untreated wet sludge poses high risks for human health
and so should not be put on land used to grow crops.
 Drying the sludge will kill most pathogens. This can be achieved
using drying beds, where sludge is put into shallow tanks to a
depth of about 300 mm. The base of the tank is sloped and
covered with a layer of sand to allow liquid to drain out of the
sludge.
 The sludge can also be composted by mixing it with vegetable
matter, or biogas can be obtained by anaerobic digestion.
 Whichever method is used, faecal sludge disposal must be
carefully managed and operated in order to ensure that the
associated risks to health and the environment are avoided.

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SEPTIC TANKS
 A septic tank is an underground and watertight tank made of
concrete, fibreglass or PVC in which sewage is collected and partially
treated. These are used for water-flushed systems that are not
connected to a sewer.
 Wastewater enters the tank, remains there for a time (retention time
– 24hrs), and is displaced out of the tank by new wastewater coming
in. There are no pumps or mechanical parts. In this period, the solid
matter in the sewage settles to the bottom of the tank, where it is
partially degraded by anaerobic micro-organisms.
 The liquid above the sludge is relatively free of solids, but it does
contain dissolved organic and inorganic chemicals that are not
treated. Light substances such as oil and grease form a scum and
float to the top. The position of the outlet ensures that only water from
the middle of the tank is displaced outwards. This effluent is disposed
of to a soakaway or to a drainfield.
 The soakaway is a covered, unsealed pit lined with bricks or stone.
The wastewater from the septic tank seeps into the soil through the
base of the pit and through the spaces in the lining material.

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 A drainfield is a field where the effluent from a septic tank is disposed of. It can be
constructed by laying a network of perforated pipes in excavated soil which has a
bed of gravel. Soil is placed over the top of the perforated pipe, and plants are
grown on top of the drainfield. The wastewater is sent into the perforated pipe and
percolates through the gravel and goes into the ground, replenishing the
groundwater.
 Septic tanks need to be desludged when the sludge depth is approximately two-
thirds the depth of the tank. A vacuum truck is usually used to suck the sludge
out for disposal
 If a sewer network is installed in an area where there are septic tanks, the sewage
flow can be connected directly to the sewer, thus bypassing the septic tank. This
allows for better treatment of the sewage, and of course eliminates the need for a
septic tank.

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WASTEWATER TREATMENT
 Septic tanks remove solids from sewage, but treatment is
minimal and the effluent contains high levels of dissolved
organic matter and ammonia. Other options for better treatment
are available.
The aim in wastewater treatment is to:
 reduce the amount of biodegradable material and solids
 remove toxic materials
 eliminate pathogenic micro-organisms.

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WASTE STABILISATION PONDS
 Waste stabilisation ponds are natural or
constructed ponds used for treating sewage or other
wastewaters biologically by harnessing the power of
sunlight and wind.
 In a typical waste stabilisation pond system, effluent
that has passed through a screen is sent through a
series of ponds with a total retention time of between
10 and 50 days.
 Bacteria in the ponds degrade the organic waste and
work symbiotically with algae, which provide oxygen
through photosynthesis.
 Oxygenation also occurs through the action of wind
and by diffusion from the air.
 No mechanical equipment is used in the ponds, so
operation and maintenance costs are very low. Land
requirement is, however, high.

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FACULTATIVE PONDS
 Responsible for biodegradation of the sewage. These are 1–1.5 m
deep, with retention time of between 5 and 30 days.
 Here, the upper layers are aerobic and the lower layers of water
are anaerobic. Solids settle to the bottom and are anaerobically
digested, so sludge removal is rarely needed.
MATURATION PONDS
 They are placed after facultative ponds for the purpose of
pathogen reduction. These are usually 0.5–1.5 m deep with a
retention time of between 15 and 20 days.
 These ponds serve to inactivate pathogenic bacteria and viruses
through the action of UV radiation from sunlight and the greater
algal activity in these shallow ponds, which raises the pH above
8.5. The long retention time in the maturation ponds also
enhances the sedimentation of the eggs of intestinal parasitic
worms.

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ANAEROBIC PONDS
 If the wastewater has a very high level of pollutants, anaerobic ponds can be used
ahead of the facultative ponds. These are 2–5 m deep and are nearly devoid of oxygen.
Their retention time is one to seven days.
 Solids settle to the bottom, forming a sludge, and anaerobic digestion takes place,
producing methane. Up to 60% of the organic material can be removed in this process.
 To prevent sewage from leaching away, and to preserve the effluent for reuse later, the
ponds should have a liner. This can be made of clay, asphalt, compacted earth or any
other impermeable material.
 To prevent run-off from entering the ponds and to prevent erosion, a protective raised
earth barrier can be constructed around the ponds using the excavated material from
their construction. Finally, a fence is needed to keep people and animals out.
 Any scum that builds up on the surface of the facultative and maturation ponds should
be removed to allow sunlight to reach all the algae and also to increase surface aeration.
Large plants that are present in the water should be removed.
 Treated sewage can be reused in crop irrigation if safe limits of faecal bacteria and
intestinal parasite eggs are achieved in the treatment process. At the same time as
treating wastewater, pond systems have been used to increase protein production
through the rearing of fish (such as Tilapia) and ducks in maturation ponds

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REED BEDS
 These are artificially constructed wetlands with emergent plants, have been used to
treat sewage in many parts of the world. It is ideal for warm countries where plants
grow rapidly. They have low operational costs but they do require a lot of land.
 The plants (usually reed species Phragmites australis or Phragmites communis) are
grown in rows in beds of soil or gravel lined with an impermeable clay or synthetic
liner.
 The effluent requiring treatment is fed into the bed, which typically has a depth of 600
mm. The base of the reed bed has a slope to enable collection of the effluent after
treatment.
 The effluent is distributed through pipes and nozzles onto the reed bed and then
percolates down to the roots and rhizomes of the reeds. The root and rhizome system
provides a mix of aerobic and anaerobic conditions that encourage a diversity of
microbial species in the soil. As a result, the reed bed system has potential for treating
a wide range of pollutants.
 For example, although micro-organisms that are capable of biodegrading many
synthetic chemicals are found in soil, they not normally present in effluent treatment
plants, so reed beds can be effective for treating effluents that contain these types of
chemicals.

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MECHANICAL - BIOLOGICAL WASTEWATER
TREATMENT
 Wastewater treatment can be undertaken using a sequence of processes in
a mechanical-biological system.
 This treatment is faster than using natural systems such as waste
stabilisation ponds or reed beds and requires less space. These factors make
it desirable for sewage treatment in towns with large populations where there
is not enough land for natural systems.
 However, mechanical-biological systems are more expensive because of the
equipment required and the need for skilled personnel to operate them.
 These systems typically have three main stages: preliminary, primary and
secondary treatment

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Preliminary treatment
 In this first stage screens remove large items such as pieces of
wood, metal, rags, paper or plastic that have got into the
sewerage system.
 Removing them protects the structures and equipment in the
wastewater treatment plant.
 Paper and rags in the wastewater flow are sometimes shredded
by comminutors, which are rotating, slotted drums equipped
with cutting blades. The shredded material can then be returned
to the flow further downstream without causing harm.
 Small stones and grit have to be removed from the flow.
 Grit is comprised of very small pieces of sand, stone and
possibly also glass and metal.
 All these materials can increase the rate of wear of mechanical
equipment and can also settle easily in pipes, causing
blockages. The grit settles out in grit channels and can be
removed daily by manual or mechanical means and dumped at a
landfill site.

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SCREENING OF WASTEWATER GRIT CHANNEL
RECTANGULAR SEDIMENTATION TANK

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Primary treatment
 In the primary treatment stage, fine solids in the wastewater
are removed by settlement in a sedimentation tank.
 A properly designed and well-operated primary sedimentation
tank will reduce the suspended solids content of the wastewater
by between 50 and 70%, and the biochemical oxygen demand
(BOD) by between 25 and 40%.
Secondary treatment
 This is the biological stage of treatment. Here the organic matter
in the sewage is biodegraded by micro-organisms using oxygen.
 Oxygen levels are increased artificially by various means to
ensure removal of organic matter. Also in this stage, ammonia in
the sewage is converted to nitrate.
 This is followed by a second sedimentation stage to remove
solids produced by the microbial activity.
 The treated effluent should be clear, free of pathogens and safe
to discharge into a river or possibly be reused for irrigation.

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SULLAGE MANAGEMENT
 Many people simply throw sullage onto the ground or into the
street. In small quantities this may be acceptable, but in densely
populated urban areas proper handling and disposal is required.
 Some of the disadvantages of improper disposal of sullage
include the potential to contaminate the soil, pollute water
sources and create favourable breeding conditions for disease
vectors.
 Sullage can be discharged to sewers or septic tanks in areas
where these facilities exist.
 If not, it is necessary to construct a pit filled with gravel or sand
near the household to dispose of sullage properly.
 A sullage pit keeps the wastewater in one place and encourages
it to soak quickly into the ground.

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STORM WATER MANAGEMENT
 Stormwater has a polluting potential,
as well as being a possible cause of
flooding. In towns and cities,
stormwater should be directed into
stormwater drains.
 These should be kept clear of rubbish.
Climate change means that many areas
are experiencing heavy and prolonged
rainfall, leading to flooding when the
stormwater drains are unable to cope.


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