4.6 How to determine the area requirement for the compost

A composting facility consists of various infrastructures designed to facilitate the composting process efficiently and in accordance with environmental regulations. Key infrastructures in a composting facility include:

• Reception area (for incoming sludge and bulking material)
• Pre-processing area (for pretreatment of sludge or bulking material)
• Active composting area (for the actual composting process)
• Storage areas for finished compost (before distribution or sale)
• Pathways (for easy access)
• Leachate and stormwater management systems (to prevent runoff and potential environmental impacts)
• Buffer zones (to help control odours and as a barrier between the facility and neighbouring properties)
• Washing facilities (for workers)

The active composting area is recommended to be capable of accommodating 30 days of compost production. The specific area requirements for the actual composting depends on the composting method used as each requires a different approach.

In this topic, we shall look into determining the area required specifically for the compost, relevant to the following composting methods:

• Aerated static pile composting
• Passive aerated box composting
• Windrow composting

Aerated static pile composting

The formula to calculate the required composting area is as follows:

Where,

Parameter	Description	Unit
A	Pad area for the aerated static pile *	m2
VS28	Total sludge volume in 28 days **	m3
VB28	Total bulking agent volume 28 days	m3
H	Height of the compost pile ***	m

Exercise:

We want to compost 5 m³ of dewatered faecal sludge per day, with a mixing ratio of dewatered sludge to bulking agent as 1:1. The bulk density of wood chips is assumed to be 200 kg/m³. The height of the compost is assumed to be 2.5 m as an excavator would load. As a reminder, the volume of the respective feedstock materials is determined by multiplying the mass with density.

Parameter	Description	Unit	Value
A	Pad area for the aerated static pile	m2	–
VS28	Total sludge volume in 28 days	m3	140
VB28	Total bulking agent volume 28 days	m3	735
H	Height of the compost pile	m	2.5

Solution:

The required area for composting of the aerated static pile is 385 m².

Passive aerated box composting

The formula to calculate the required composting area is as follows:

Where,

Parameter	Description	Unit
A	Compost area for box composting *	m2
VS/d	Total sludge volume per day	m3/d
VB/d	Total bulking agent volume per day	m3/d
VAB	Volume aeration box **	m3
CC	Composting cycle ***	d
AAB	Area of aeration box	m2

Exercise:

We want to explore the area requirement for passive aerated box composting as an alternative to active aeration due to the lack of power and availability of low-cost labour. Specifically, we want to determine the required composting space to compost 5 m³ of dewatered sludge, assuming a mixing ratio of 1:1 for dewatered sludge to bulking agent (as previously established for optimal C:N). The bulk density of wood chips is assumed to be 200 kg/m³. The composting box dimensions are 1.5 m in width, 6 m in length, and 1.3 m in height. The composting cycle will take 40 days with passive aeration.

Parameter	Description	Unit	Value
A	Compost area for box composting	m2	–
VS/d	Total sludge volume per day	m3/d	5
VB/d	Total bulking agent volume per day	m3/d	25.25
VAB	Volume aeration box	m3	11.7
CC	Composting cycle	d	40
AAB	Area of aeration box	m2	9

Solution:

The required area for composting is 961.5 m².

Windrow composting

Firstly, we use the following formula to determine the length of each windrow:

Where,

Parameter	Description	Unit
l	Length of windrow	m
VS/d	Total sludge volume per day	m3/d
VB/d	Total bulking material volume per day	m3/d
w	Width of windrow	m
h	Height of windrow	m

Secondly, we are required to add 1.5 – 2 m for each side of the pile for the lateral circulation area and calculate each site and the area of each pile.

Where,

Parameter	Description	Unit
Aw	Area of windrow	m2
lA	Lateral lengths around windrow	m
wW	Width of windrow	m
lW	Length of windrow	m

Finally, we calculate the total area when a number of piles is equal to the number of days in the compost cycle.

Where,

Parameter	Description	Unit
A	Total area	m2
Aw	Area of windrow	m2
CC	Compost Cycle	d

Exercise:

We want to explore the area requirement for windrow composting as an alternative. Specifically, we want to determine the required composting space to compost 5 m³ of dewatered sludge, assuming a mixing ratio 1:1 for dewatered sludge to bulking agent (as previously established for optimal C:N). The bulk density of wood chips is assumed to be 200 kg/m³. The windrow is triangular, and because it is turned manually, the height is 1.5m, the width is 3 m, additional lateral area is assumed to be 2 m per side. The composting cycle is assumed to be 60 days.

Solution:

Determination of the length of windrow:

Parameter	Description	Unit	Value
l	Length of windrow	m	–
VS/d	Total sludge volume per day	m3/d	5
VB/d	Total bulking material volume per day	m3/d	26.5
w	Width of windrow	m	3
h	Height of windrow	m	1.5

Area for each windrow:

Total area:

The required area for composting is 4,800 m².