Chapter 4 module 3 Treatment of faeces by composting How should urine and faeces be treated for safe...
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Transcript of Chapter 4 module 3 Treatment of faeces by composting How should urine and faeces be treated for safe...
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?
Oxygen
Organics:energynitrogen (N)phosphorus (P)potassium (K)sulphur (S)etc.
Sun shine
Compost:nitrogen (N)
phosphorus (P)potassium (K)
sulphur (S)etc.
Gas emissionsCarbon dioxideammoniaOdours
Heat energy
The compost process?
Organics:energynitrogen (N)phosphorus (P)potassium (K)sulphur (S)etc.
Compost:nitrogen (N)phosphorus (P)potassium (K)sulphur (S)etc.
Gas emissions VapourCarbon dioxideAmmoniaOdours
Heat energy
Oxygen Water/moisture
pH
Microbes:bacteriar,fungi
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
Oxygen
Water/moisture
Water/oxygen – optimal moistureWater – bacteria thrive in 100%Oxygen – compost bacteria are aerobic (>5% oxygen in pores)Optimal moisture – as squeezed sponge
Too much moisture Pores waterlogged – no aeration, anaerobic, acidic, smellyAction – add dry structure material & mix
Too little moisture Slow degradation – low temperatureAction – add water & mix
Optimal conditions - energy
• 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
• Too much fast energy • 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• Sources of C: the
organics of the substrate.
• Sources of N: both organic nitrogen (protein) and inorganic nitrogen, e.g. NH4
+, in the substrate
• C/N > 30 • 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
Heat production: 20.7 kJ/g degraded
Temperature increase -Heat capacity
Surface cooling: -temperature difference-insulation
Airflow100% rel. humidity
Good sanitation Good insulation needed
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%)