4.6 How to determine the area requirement for the compost - Humanitarian Sanitation Hub

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,

ParameterDescriptionUnit
APad area for the aerated static pile *m2
VS28Total sludge volume in 28 days **m3
VB28Total bulking agent volume 28 daysm3
HHeight of the compost pile ***m

*   The pile area should be increased by 10% for the aerated static pile system for the odour-control filter.
** The equation assumes a 21-day composting period and provides 7 additional days for low temperature, excess precipitation, and malfunctions. Achieved by active aeration.
*** The typical height of the active aerated static pile is between 2 and 2.5m.

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.


ParameterDescriptionUnitValue
APad area for the aerated static pilem2
VS28Total sludge volume in 28 daysm3140
VB28Total bulking agent volume 28 daysm3735
HHeight of the compost pilem2.5

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,

ParameterDescriptionUnit
ACompost area for box composting *m2
VS/dTotal sludge volume per daym3/d
VB/dTotal bulking agent volume per daym3/d
VABVolume aeration box **m3
CCComposting cycle ***d
AABArea of aeration boxm2

*    The calculation does not include the areas necessary for air circulation between the box composts (0.75m) and the access paths.

**  The box dimensions are typically 1.2-1.5m wide, 3-6m long, and 1.2-1.3m high.

*** Nr of days required for composting with passive aeration could range from 30 to 60 days

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.


ParameterDescriptionUnitValue
ACompost area for box compostingm2
VS/dTotal sludge volume per daym3/d5
VB/dTotal bulking agent volume per daym3/d25.25
VABVolume aeration boxm311.7
CCComposting cycled40
AABArea of aeration boxm29

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,

ParameterDescriptionUnit
lLength of windrowm
VS/dTotal sludge volume
per day
m3/d
VB/dTotal bulking material volume
per day
m3/d
wWidth of windrowm
hHeight of windrowm

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,

ParameterDescriptionUnit
AwArea of windrowm2
lALateral lengths around
windrow
m
wWWidth of windrowm
lWLength of windrowm

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


Where,

ParameterDescriptionUnit
ATotal aream2
AwArea of windrowm2
CCCompost Cycled

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:


ParameterDescriptionUnitValue
lLength of windrowm
VS/dTotal sludge volume
per day
m3/d5
VB/dTotal bulking material volume
per day
m3/d26.5
wWidth of windrowm3
hHeight of windrowm1.5


Area for each windrow:



Total area:



The required area for composting is 4,800 m2.

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