Unit 2.6: Introduction to composting
Self-assessment questions
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Composting achieves:
A: When all bacteria and viruses are killed
B: When waste stabilization occurs and weight
reduction is done up to 70%
C: When most of the pathogens are killed and volume
is reduced up to 70%
During composting:
A: Ammonia may accumulate and cause odor
problems
B: Fluctuation of pH has a severe effect on the
degradation rate
C: Worms/insects play a primary role
Typical C:N ratios for starting and final products of
composting are –
A: 10 : 1 and 25 : 1
B: 25 : 1 and 10 : 1
C: 20 : 1 and 15 : 1
D: 15 : 1 and 20 : 1
What is the purpose of composting?
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Name the two main phases of the composting
process.
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Why maturation of compost is so important before
applying in the field as soil conditioner?
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Which parameters are used to judge stable compost?
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Which of the following statements is true for
composting?
A: A high moisture content of the composting material
is desirable because heat produced will evaporate
water and thereby increase the amount of O2.
B: At low oxygen content, anaerobic conditions occurleading to the production of methane which is
desirable in composting
C: In order to supplement the growth of bacteria,
certain amounts of N and C must be provided
C
A
B
The main purpose of the composting process is the
disposal of the biodegradable part of waste. Composting
can be seen as a recycling and revalorization of the
biodegradable part of the waste. This process reduces the
volume, mass and humidity of the biodegradable waste
while transforming it into a valuable soil conditioner.
 Precomposting (high-rate composting): Initial
mesophilic state (400C), Thermophilic range (40 –
700C)
 Maturation (curing) or slow rate composting.
Immature compost continues to consume oxygen after
application to the field and thereby reduces the
availability of oxygen in the soil to the plants. Immature
compost can also contain high levels of organic acids or
have a high C/N ratio. The high C/N ratio results in a
competition with the plant roots for the available nitrogen
in the soil. So, immature compost can damage crops and
plants.
 C/N ratio
 Oxygen demand
 Temperature
 Odour
C
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Why are salt and heavy metals in the composting
material not desirable?
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Name two different types of application of
composting in the ecosan concept.
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What are the roles of amendments and bulking agents
in composting?
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Which is true?
A: Vermicomposting is a special type of composting
done by certain earthworms
B: Vermicomposting cannot be done to treat faeces
C: Insects, mold and bacteria play major role in
vermicomposting
Which factors determine the degradation of organic
matter in vermicomposting?
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When composting toilet is deployed to treat
household organic waste, the C/N ratio –
A: Will increase
B: Remains same
C: Will decrease
D: First increases, after that remains steady and then
decreases again
Why is the C/N ratio so important in composting?
Composting is a concentrating process. So salts and heavy
metals will also be concentrated after composting. That’s
why leaching tests will have to be done before applying
the compost in the field as soil conditioner
 Thermophilic composting (55-650C) as secondary
treatment of faeces;
 Mesophilic composting (350C) in composting toilets
for primary treatment of faeces
 To reduce the bulk weight and moisture content of
the composting material
 (To increase the air voids for proper aeration (e.g.
sawdust, straw, recycled compost, rice hulls)
 Provide structural support and increase the porosity
of the mixture for effective aeration (e.g. wood chips)
A



A
Amount of degradable waste
Maintenance of the aerobic condition
Absence of toxic compounds
A C/N ratio of 25 is optimum for composting. When C/N
ratio is below 20, the available carbon is fully utilized
without stabilizing all of the nitrogen. The excess nitrogen
may then be lost to the atmosphere as ammonia and
odour can become a problem. When C/N ratio is much
higher, for example higher than 40, a longer composting
time is required for the microorganisms to use the excess
carbon. As the substrate loses faster its carbon than its
nitrogen during composting, the C/N ratio decreases
slowly to about 10 in the end product.
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Unit 2.5: Introduction to constructed wetlands
Self-assessment questions
1. Name two issues that distinguish the design of constructed wetlands for grey water from that for
domestic wastewater.
2. Name four factors/prerequisites that should be kept in mind before designing constructed wetlands.
3. Mention three areas of ecological sanitation where constructed wetlands can be used.
4. Give three advantages and three disadvantages of constructed wetlands systems.
5. Classify constructed wetlands.
6. Plant species selected for constructed wetlands(A) Should be exotic and imported
(B) Should have low biomass production and an extensive rooting system
(C) Should be able to withstand shock loads and short dry periods
(D) Should always be of emergent plant type
7. The area requirement for a horizontal sub surface flow (HSSF) constructed wetland is
(A) 3 – 5 m2/PE
(B) 2.5 – 4 m2/PE
(C) 5 – 10 m2/PE
(D) 2 – 3 m2/PE
8. What do we mean by hybrid constructed wetland systems?
9. Name the three processes by which nitrogen and phosphorus can be removed in a constructed
wetland.
10. For a constructed wetland system(A) Pathogens can be removed by protozoa and by dieing-off from unfavorable environmental
conditions
(B) Heavy metals and trace organics cannot be removed
(C) Particulate BOD is removed only by settling and is converted to insoluble BOD by hydrolysis
(D) Redox conditions are maintained constant at different parts
11. Mention three roles that aquatic plants play in constructed wetland systems?
12. What are the maximum BOD loading rates in g/m2/day for FWS, HSSF and VSSF constructed wetland
systems respectively?
(A) 4 – 6, 7.5, and 8.0
(B) 8.0, 7.5, and 4 – 6
(C) 6.5, 9.0, and 7.5
(D) 7.0, 5.5, and 8.0
13. Write water balance for constructed wetland systems?
14. The filling material for constructed wetland systems(A) Should have a low hydraulic conductivity to prevent excessive water movement
(B) Should be locally available
(C) Have lower clogging risk when sorption capacity is high with a lower conductivity
15. Mention some key features of “French System” constructed wetlands.
Answers
1. (i) Nitrogen and phosphorus removal are not important (much lower values in grey water than in
domestic wastewater)
(ii) Pathogen removal is also not a big issue (low levels of pathogens in grey water)
2. (i) Wastewater to be treated should not be too toxic for bacteria and plants
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(ii) Organic loading should not be too high
(iii) Detention time should be long enough
(iv) Large space requirement because it is a low rate system
(i) Grey water treatment
(ii) Faecal sludge treatment (less common)
(iii) Post treatment after anaerobic treatment of black water
Advantages:
(i) Siting flexibility compared to natural wetlands
(ii) Easy operation and maintenance, no need of highly technical know-how
(iii) Can be integrated into landscaping
Disadvantages:
(i) Mosquitoes can be spread in the free water surface systems
(ii) Large space is required
(iii) Performance is not consistent
Classification based on water flow characteristics:
(i) (free water) surface flow (SF)
(ii) Subsurface flow (SSF)
Classification based on plant species characteristics:
(i) Floating plant
(ii) Submerged plants
(iii) Emergent plants
C
A
It is a two step constructed wetland consisting of a horizontal sub surface flow (HSSF) and a vertical sub
surface flow (VSSF) flow bed. The horizontal bed is maintained anoxic where denitrification occurs
whereas the vertical bed is a aerobic one where nitrification occurs. The provision of recycle flow
improves denitrification in the horizontal first compartment. Hybrid systems are more efficient
especially for nitrogen removal.
Nitrogen removal:
(i) Plant uptake
(ii) Nitrification/denitrification in biofilms
(iii) Volatilization as ammonia at pH > 8.5
Phosphorus removal:
(i) Plant uptake
(ii) Retention in the soil (adsorption)
(iii) Precipitation with Ca, Al, Fe etc.
A
(i) Nutrient uptake and heavy metal accumulation in the tissue
(ii) Stems and root systems of plants act as a mechanical filter and help in biofilm attachment
(iii) Reduce water velocity, thereby increasing settling.
B
Change of storage = Inflow – Outflow
OR, Change of water volume over time = (Influent wastewater/grey water flow + Precipitation) –
(Effluent + Evapotranspiration)
B
Key features of “French system” constructed wetlands:
(i) It is a two stage vertical flow constructed wetland
(ii) It accepts raw sewage directly onto the first stage and treats the primary sludge on the surface of
the first stage beds
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(iii) Each stage of the integrated sludge and wastewater treatment wetland has parallel operated filter
beds: under normal conditions 3 beds in the first stage and 2 beds in the second stage
(iv) The treated sludge from the first stage has to be removed every 10 – 15 years and is usually
directly valorized by land spreading
Unit 2.2: Conventional on site sanitation
Self-assessment questions
1. Why do people prefer onsite sanitation instead of a sewer system?
2. What is the major disadvantage of onsite sanitation?
3. As a rule of thumb, what population densities can be considered in low density, peri-urban areas and
slums respectively?
(A) < 100 people/ha, > 800 people/ha, > 240 people/ha
(B) < 100 people/ha, 100-240 people/ha, > 800 people/ha
(C) > 100 people/ha, < 240 people/ha, 800-1000 people/ha
4. Pit latrines are not suitable if
(A) The ground is underlain by impervious rock and the groundwater table is deep
(B) The ground allows rapid groundwater movement and the groundwater table is deep
(C) The ground is underlain by pervious rock and the groundwater table is shallow
5. What is the amount of faecal sludge that is produced from a septic tank?
(A) 1.0 L/cap/day
(B) 1.5 L/cap/day
(C) 2.0 L/cap/day
6. The solids content of typical faecal sludge is:
(A) 25 kg/m3
(B) 20 kg/m3
(C) 25 mg/L
(D) 25 g/m3
7. For selecting the best onsite sanitation technique, what factors should be considered?
8. What is brown water?
(A) Wastewater from shower and basin
(B) Blackwater with urine
(C) Domestic wastewater from the kitchen
9. What is the COD percentage of urine in a wastewater composed of grey water, urine and faeces?
(A) 41%
(B) 12%
(C) 21%
(D) 15%
10. What are the advantages of decentralized sanitation systems over the centralized systems?
11. What should be the minimum distance between an unlined pit and a well extracting water for
drinking?
(A) 15 m
(B) 20 m
(C) 10 m
(D) 25 m
12. Why round pits are more suitable than rectangular or square pits?
13. What is the role of ash in UDD toilets?
14. What is the typical height of the scum layer being considered for septic tank design?
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(A) 20-30 cm
(B) 25-40 cm
(C) 15-30 cm
15. Which nutrient fraction present in urine is considered as “Oil of the future”?
(A) Nitrogen
(B) Calcium
(C) Phosphorus
(D) Potassium
Answers
1. People prefer onsite sanitation(i) To save construction and maintenance cost
(ii) To save water
(iii) To serve remote locations which are distant from the central sewer systems
(iv) To serve slums, refugee camps
(v) To get better flexibility with respect to population growth and decline
2. It is not suitable for an area where population density is high and the people have low income. Periurban and slums are the best example of this.
3. B
4. C
5. A
6. A
7. (i) Sustainability of available options (Social, technical, economic, environmental, public health,
institutional aspects)
(ii) Local conditions (Whether groundwater used for drinking, whether water is scarce and expensive,
income of people)
8. B
9. B
10. (i) Existing systems are decentralized
(ii) Specific treatment and reuse can be deployed for the specific waste
(iii) Easier to plan and implement
(iv) Less expensive
11. B
12. Round pits are more suitable because they have natural arching effect. So the earth pressure is evenly
distributed through the circumference of the round pits. Whereas rectangular pits have a tendency to
collapse due to the uneven distribution of earth pressure.
13. Ash is added to UDD toilets mainly to increase the pH and to reduce the moisture.
14. A
15. C
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