Nafiisa SOBRATEEPhD Candidate

Department of Agricultural and Production SystemsFaculty of AgricultureUNIVERSITY OF MAURTIUS

Background Methodology Results Model justification Discussion Conclusion

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Compost sanitisation research dates back to several decades

But issues associated with compost quality and hygiene continue to be relevant as more waste will have to be recycled for sustainability reasons.

The present work has been an attempt to respond to the quest to improve the state of knowledge, regarding the type of waste management to be adopted in the poultry farming industry.

Aim: This study investigates differences in bacterial growth response in broth amended with compost-substrate extracts periodically bypassed during broiler litter composting to mimic a contamination scenario

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1. Compost samples, suspended in diluent were mixed with 2X broth. Ampicillin selective (0.3 g l-1) E. coli and E. faecalis were separately seeded. Growth was measured by viable cell count.

2. Microfit© application generated information of direct microbiological interest: increasing λ and decreasing µmax

for both bacteria with time.3. TableCurve 3D v.4.0.05 software to

obtain a unifying model to identify regrowth possibilities of the seeded enteric bacteria

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As a means to integrate the findings of this research, an attempt was made to unify three parameters of interest to poultry litter composting in a mathematical relationship, namely: ◦ explanatory variables:

time of composting (to which the temperature prevailing in the windrows is associated)

the decomposition rate, k calculated from the mathematical expression of Nielsen and Berthlsen (2002) based on heat driven decomposition

◦ response variable = µmax, of inoculated enteric bacteria (E. coli and E. faecalis) in submerged culture with compost extracts of different maturity

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Post thermophilic high mumax

Post thermophilic high mumax

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The experiment has benchmarked the role of the temporally-different compost extracts, representing increasingly non-host environments, in suppressing the growth of seeded E. coli and E. faecalis.

The most salient outcome pertains to:◦ the increase in lag time (E. coli: 1.78 h, E. faecalis:

1.28 h)◦ decreasing maximum specific growth rate, µmax, (E.

coli: 0.95 h-1 h, E. faecalis: 0.69 h-1) for both bacteria in the matured compost extract = Week 15

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R2 = 0.8190 [1] R2 = 0.831 [2] where,

◦ µmax = maximum specific growth rate (h-1)◦ t = time of composting (days)◦ k = decomposition rate (mg O2 g-1 VS) h-1

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kttt ln046.000000177.00003.0019.0136.1 32max

kttt

424.00000114.0002.0093.0942.2 32

max [Eq 1]

[Eq 2]

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Regrowth potential present

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Regrowth potential present

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By using simplifying idealisations as a compromise between the complexity of the biological system and the available data, a practically usable mathematical relationship becomes available.

The use of fourth or even higher order polynomials to represent the environment-dependence of kinetic parameters may not necessarily upgrade a model (Baranyi and Roberts, 1995).

Hence, in the present context, third order polynomial functions have been fitted even though the R2 statistic values< 0.9.

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The most salient feature demonstrated by both E. coli and E. faecalis is their propensity to have an increased µmax in the broths corresponding to:◦ Day 49 week 7(µmax = 1.66 h-1)

◦ Day 35 week 5((µmax = 0.92 h-1) respectively This occurred when temperatures <55 oC were

recorded in all windrows after the post-thermophilic phase

the post-thermophilic environment permits a boosted growth of both seeded enteric bacteria preceded by decreasing µmax till Day 105.

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In E. coli, the decomposition rates of days 14, 22 & 49 are very similar (1.014, 1.012, 1.064 mg O2 g-1 VS h-1 respectively), yet the µmax value (1.66 hr-1) for Day 49 is highest of the three.

Therefore, decomposition rate (and by extension the temperature) seem not to be responsible for this increased rate of growth.

Plausible incriminating factor that can be attributable to this effect = the ecology of the compost immediately after the thermophilic phase as a vacuum because of the subsiding thermophilic ecology and the nascent stage of the 2nd mesophilic microflora.

Due to this vacuum, the seeded E. coli is able to thrive better, hence the increased µmax value for Day 49.

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By using, the maximum specific growth rate (µmax), a kinetic parameters of bacterial multiplication rate under submerged culture, deduce = the indigenous microbial community developed during composting is responsible for enteric microflora deactivation.

This research has grown out of previous works to the extent that it has identified, by means of mathematical functions, the susceptibly weak points during the composting process where the newly stabilised material may be prone to regrowth especially when the composting ecosystem has not yet established its post-thermophilic microflora.

Thus, a week-7 compost extract had the highest post-thermophilic µmax value (1.66 h-1), indicating a higher risk for upsurge for this time of composting in the event of short-circuiting and/or contamination.

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The combined results of this work suggest that the success of composting lies in its being :◦ optimally and thermodynamically triggered to

initiate the process◦ maintain a thermophilic ceiling◦ and managed (turning) such that all feedstock is

exposed to the hygienisation regime to prevent incomplete stabilisation short-circuiting regrowth of pathogens, especially at the onset of the

2nd mesophilic phase

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