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%)