4.5 How to determine the composting processing parameters

Moisture content and the Carbon-to-Nitrogen (C:N) ratio are two pivotal parameters that influence the microbial activity, decomposition rate, and overall effectiveness of the composting process. They should be determined before initiating the composting process as establishing the right conditions from the start is important for effective composting.

How to determine the C:N ratio of the mix:

Quantifying carbon and nitrogen content in sludge or bulking materials would require specialised equipment and an elemental analyser that is only sometimes feasible. Therefore, in this case, it is advised to use local or regional values from the literature or research the  C:N values from existing databases, like the Phyllis2.

Dewatered faecal sludge typically contains a high level of nitrogen. As an example, a study by Cofie and Koné (2009) in Ghana measured the C:N ratio of dewatered faecal sludge to be 18:1 ± 11. To balance this ratio, it is necessary to add carbon-rich materials as a bulking agent, i.e. wood chips with a C:N ratio ranging between 100 – 500:1.

To estimate the C:N ratio of the mixture of dewatered sludge and bulking materials, we can use the following equation:

Where,

Parameter	Description	Unit
CNmix	Carbon to nitrogen ratio of the compost mix	–
CS	Carbon content in sludge	%
NS	Nitrogen content in sludge	%
CB	Carbon content in bulking material	–
NB	Nitrogen content in Bulking material	–

Exercise:

Assuming we want to process 5m³ of dewatered faecal sludge daily, we conducted a market study and found that compost could be used for horticulture and landscaping. We identified wood chips from local woodwork manufacturers and rice husk from rice mills as locally available bulking materials. Let’s estimate the required mixing ratio of bulking material and dewatered sludge to achieve an optimal C:N ratio between 25:1 and 35:1.

The following table presents the basic assumptions:

Parameter	Description	Value
CS	C dewatered sludge	25 %
NS	N dewatered sludge	2.5%
CW	C wood chips	46.8%
NW	N wood chips	0.27%

Let’s calculate the C:N ratio for a mix of dewatered sludge and wood chips.

Solution:

The C:N ratio is 26, within the optimal range for composting 25:1 – 35:1, indicating an ideal mixture of 1:1, of equal parts of sludge and wood chips.

That means that 5 m³ or 5,250 kg of dewatered sludge (with a density of 1,050 kg/m³) is mixed with 5,250 kg of wood chips.

How to adjust the C:N ratio of a mixture:

The adjustment of the C:N ratio in the mixture involves determining the appropriate quantities of different materials to achieve the desired ratio. After defining a target ratio, the adjustment factor can be determined based on the following formula.

Where,

Parameter	Description	Unit
CNmix	Carbon to nitrogen ratio of the compost mix	–
CS	Carbon content in sludge	%
NS	Nitrogen content in sludge	%
CB	Carbon content in bulking material	–
NB	Nitrogen content in bulking material	–
XB	Bulking material adjustment factor	–

Exercise:

We want to mix 5 m³ of dewatered sludge with rice husk. The target is to achieve a C:N ratio of 30. How much rice husk is required?

The following table presents the basic assumptions:

Parameter	Description	Value
CS	C dewatered sludge	25 %
NS	N dewatered sludge	2.5%
CR	C rice husk	44%
NR	N rice husk	0.5%
XR	Rice husk adjustment factor	–

Solution:

That means that 5 m³ or 5,250 kg of dewatered sludge (with a density of 1,050 kg/m³) needs to be mixed with 1.72 parts rice husk, resulting in 9,030 kg of rice husk that is used as bulking material.

How to calculate the moisture content of the compost mix:

For effective composting, ensuring that the compost mixture’s moisture levels are within the 50 – 60% range is crucial.  If the moisture content is too high, air cannot flow through the gaps, leading to anaerobic zones. If there is too little moisture, microorganisms that facilitate composting can dehydrate and die off.

To calculate the moisture content of the compost mix, we use the following formula:

Where,

Parameter	Description	Unit
MCmix	Moisture content of mixture	%
MCS	Moisture content of sludge	%
MCB	Moisture content of bulking material	%
PS	Portion of sludge	–
PB	Portion of bulking material	–

Exercise:

Let’s calculate the moisture content for the combination of dewatered sludge and rice husk. The following assumptions are made:

Parameter	Description	Unit	Value
MCmix	Moisture content of mixture	%	–
MCS	Moisture content of sludge	%	70
MCR	Moisture content of rice husk	%	10
PS	Portion of sludge	–	1
PR	Portion of rice husk	–	1.72

Solution:

The moisture content of the mixture is only 32%, which is lower than the recommended level. In this case, we can consider the following alternatives.

How to adjust the moisture content of the compost mix:

The result of the previous exercise suggests that we must increase the compost mix’s moisture content. It can be achieved by one of the following options:

1. Increase the moisture content of dewatered sludge by removing it from the drying beds earlier
2. Using wetter bulking materials
3. Adding water to the mixture

Let us explore the three options for our case.

(a) Increase the moisture content of dewatered sludge by removing it from the drying beds earlier

Assuming we want a 50% moisture content for the compost mix, let’s explore the required moisture content of faecal sludge (X).

The result shows that even if using the faecal sludge with a moisture content of 100% will not achieve the required moisture content of the mix.

(b) Using wetter bulking materials

Further, we could explore the provision of wetter rice husks by washing and soaking rice husk in water prior to composting. Let us explore what moisture content is required to be achieved or identified for rice husk for optimal compost mix.

The rice husk with a moisture content of 38.4% is required for the given compost mix.

(c) Adding water to the mixture

However, instead of pre-treatment and soaking the rice husk in water, we could explore the addition of water directly to the compost mixture. The following is known to us from our previous calculations:

• Moisture content of the initial mix:         32%
• Required moisture content:         50%
• Total mass of the initial mix:        5,250 kg/d of dewatered sludge + 9,030 kg/d of rice husk = 14,280 kg/d

We then use the following formula to estimate the required amount of water to be added:

Where,

Parameter	Description	Unit	Value
Mx	Amount of water to be added	kg/d	–
W	Total mass of the initial mix	kg/d	14,280
Md	Desired moisture content	%	50
Mi	Moisture content of the initial mix	%	32

Per day, 5141 kg of water is to be added to reach the required moisture.

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