Transcript Bild 1 - Sustainable Sanitation

Chapter 4 module 3 Treatment
of faeces by composting
How should urine and faeces be treated for safe
handling and reuse in crop cultivation? How can
organic material from households be co-treated?
Goal: After the lecture the student should know the
principles, prerequisites and main characteristics,
properties, advantages and disadvantages of
composting of faeces of kitchen waste.
What is composting?
Gas emissions
Carbon dioxide
ammonia
Odours
Compost:
nitrogen (N)
phosphorus (P)
potassium (K)
sulphur (S)
etc.
Sun shine
Oxygen
Organics:
energy
nitrogen (N)
phosphorus (P)
potassium (K)
sulphur (S)
etc.
Heat energy
The compost process?
Heat energy
pH
Microbes:
bacteriar,
fungi
Organics:
energy
nitrogen (N)
phosphorus (P)
potassium (K)
sulphur (S)
etc.
Oxygen
Water/
moisture
Gas emissions
Vapour
Carbon dioxide
Ammonia
Odours
Compost:
nitrogen (N)
phosphorus (P)
potassium (K)
sulphur (S)
etc.
Degradation – compost – digestion
Composting
• C6H12O6 + 6 O2 6 CO2 + 6 H2O + bacteria
• E= -3880 KJ/mol
• Energy – appr. half to heat, half to bacteria
Digestion
• C6H12O6  3 CO2 + 3 CH4 + few bacteria
• E= - 405 KJ/mol
• Energy – almost all remains in the biogas (-3475 KJ/mol
sugar)
Optimal conditions – moisture/oxygen
Water/oxygen – optimal moisture
Water – bacteria thrive in 100%
Oxygen – compost bacteria are aerobic
(>5% oxygen in pores)
Optimal moisture – as squeezed sponge
Oxygen
Water/
moisture
Too much moisture 
Pores waterlogged – no aeration,
anaerobic, acidic, smelly
Action – add dry structure material & mix
Too little moisture 
Slow degradation – low temperature
Action – add water & mix
Optimal conditions - energy
• Energy
• Too much fast energy 
• Sources – fast release:
Carbohydrates, proteins,
fats.
• Lignin & cellulose  slow
release
• Food waste - rich in fast
energy,
• Faeces: less energy, slower
release
• Woody waste  slow
release
• Oxygen – goes easily
anaerobic
acidic and/or smelly
• Action – add structure
material & mix
• Too little fast energy 
• Too low temperature
• Action – add food waste
and/or insulate
Optimal conditions – C/N-ratio
• C/N-ratio
• C/N > 30 
• Sources of C: the organics
of the substrate.
• Sources of N: both organic
nitrogen (protein) and
inorganic nitrogen, e.g.
NH4+, in the substrate
• Process – might be slowed
down due to lack of N for
bacteria
• Action – add N, e.g. faeces
or urine & mix
• C/N < 15 
• Process – might smell
ammonia – excessive losses
of ammonia
• Action – add C, e.g. park
waste & mix
Optimal conditions - pH
• thermophilic bacteria prefer
pH > 6.5
• Too high pH (lots of lime)

• Slow degradation – low
activity of bacteria
• Action – add food waste
and/or wait
• Too low pH (lots of food
waste) 
Slow degradation
• Action – wait (especially if the
compost is small) or add
ash/lime
Optimal conditions – temperature
• Temperature – for
sanitation >50ºC,
•
for fastest
degradation approx.
55ºC
• Too low
temperature
Sanitation not ensured
• Action – insulate and/or
add fast energy
Compost heat & scales
Airflow
100% rel. humidity
Surface cooling:
Heat production:
20.7 kJ/g
degraded
-temperature
difference
-insulation
Good sanitation 
Good insulation needed
Temperature increase Heat capacity
Insulation crucial
• Theoretical heat loss
•
•
Heat transfer coefficient:
– non-insulated: 13 W C-1 m-2
– Insulated 10cm: 0.053 W C-1 m-2
so the heat resistance was 25 times
larger for the insulated box .
Composts – small scale
These small composts reach high temperatures, thanks to good insulation
Composts - medium scale
These medium size windrow composts reach high temperatures.
For good sanitation they ought to be covered by some insulating material, e.g.
straw.
Composts - large scale
Insulated large scale compost with mechanical
aeration
Compost and plant nutrients
• N: Protein (org. N) + NH4+ 
10-50% loss & of remainder 90-95% is humus (organic
N) + a few % NH4+ + a few % NO3– Usually 10-50% of N is lost to air
– Low availability of N left in compost
•  10% first year and 20-30% in total over the years.
• K: K+, high availability, water soluble and thus large risk
for loss with leachate
• Composting decreases the amount of N, due to losses,
and the remaining N has low availability
Composting - disadvantages
• Difficult to get the whole compost hot – usually cold at
air intake  mixing several times needed for sanitation
• Additional energy rich substrates, e.g. kitchen waste, can
be needed to reach above 50°C
• A large proportion of the nitrogen is lost (often around
50%, more if urine has not been well diverted)
• Mixing - handling - is needed also before sanitation 
hygiene risk
Summary - Why compost?
•
•
•
•
•
•
To sanitise  50ºC required  insulation
To eliminate visual contaminants (toilet paper etc)
To decrease C/N-ratio - to eliminate risk for N fixing
To homogenize – simplifes handling
To produce good humus – high doses can be used
To decrease the amounts to handle
– Organic matter is degraded by 30-60%
– Finished compost has a high dry matter content (50-70%)