Impact of composting-induced decomposition on microbial indicator status of spent broiler
litter under sub-tropical environment
Nafiisa SOBRATEE, University of Mauritius
Extended abstract
In Mauritius, animal waste-related environmental problems, particularly regarding the poultry
sub-sector have been exacerbated by the organisational structure of the industry, notably its high
degree of vertical integration and the rise of contract farming (Shane, 1998; Rajkumar and
Driver, 2003). In line with the industrial ecology concept, clean practices that allow degradation,
transformation and re-use of livestock waste need to be researched and applied to prevent
environmental and sanitation hazards. In this context, the main interest with composting as waste
management option lies in its capacity to both lead to stabilization of the exogenous organic
matter and its potential for sanitization. Mohee et al. (2008) investigated a 110-day systematic
composting study for the presence of Salmonella and the survival of total coliforms (TC), faecal
coliforms (FC), Escherichia coli (EC) and faecal enterococci (FE) in three experimental
windrows consisting of SBL/bagasse mixture in a close-sided roofed facility. The spent broiler
litter utilized originated from a pen of 2000 birds originating from a commercial facility
comprising of 40,000 birds. At establishment (Day 1) windrows were 1.2–1.3 m long,
approximately 0.7 m wide at the base, and approximately 0.6 m high. Each windrow consisted of
240 kg broiler litter, 200 kg bagasse mixed with 250 L water. All bacterial enumerations (threetube MPN method) were performed selectively using the procedures for International Standards
Organisation (ISO) methods: ISO 9308 (1990), ISO 4831 (1991), ISO 7251 (1993), ISO 7899-1
(1984)). The Fischer’s Least Significant Difference method was used to compare pairs of
treatment means which was taken to be time of composting.
At a further level, Sobratee et al. (2008) discussed the design of empirical equations that best
describe the behaviour of the faecal bacterial indicators and two decomposition parameters as a
function of composting time. The Levenburg-Marquardt algorithm was used to fit nonlinear
mathematical models to TC, FC, EC, FE, organic-C and volatile solids reduction, VSRed, by the
least squares procedure. The order of rate equations was also identified. The temperature
dependency of degradation rate applicable for composting temperatures was also investigated.
Three equations derived by Haug (1980), Mohee (1998) and Nielsen and Berthelsen (2002) were
also compared in this respect.
Additionally, Sobratee et al. (in press) developed an exposure assessment based on the Source–
Pathway–Receptor approach to investigate the theoretical arithmetic mean exposure of root crops
to enteric bacterial indicators at point of harvest by simple event tree analysis and thereby to
comprehensively quantify exposure scenarios of E. coli from the β-Poisson model.
Log10 reductions of −6.98, −8.03, −8.18 and −5.96 occurred in TC, FC, E. coli and FE
concentrations respectively. As expected, FE exhibited resistance to high temperature compared
to E. coli especially for the first 21 days. Temperature histories revealed hygienisation
attainment. Differences in mean, representing benchmark stages of composting, were highly
significant (P < 0.05) for all pathogen indicators. VSRed (%) validated effective system progress.
The rate equations showed that TC, FC and EC reductions were expressed by second-order decay
kinetics, while FE reduction followed first-order decay and hence, was inactivated at a slower
rate. Temperature elevation, organic-C and VSRed dynamics provided an accurate understanding
of composting-induced decomposition of the broiler litter.
Temperature dependency of the stabilisation rate was verified by applying empirically derived
rate equations. Decomposition rate according to the equation of Haug (1980) showed a definite
tendency to increase rapidly with temperature elevation whereas those of Mohee (1998) and
Nielsen and Berthelsen (2002) emphasized on the reduction in decomposition rate beyond 55oC.
The structured model proposed by Nielsen and Berthelsen (2002), based on the application of
theoretical assumptions about enzyme catalysis and high activation energy-induced spontaneous
deactivation, has specifically indicated system progress rate and under which time/temperature it
can be optimized hence, revealing the relevance of such results for practical purposes in the
technical management of composting facilities. The main implication therefore resides in the
ability to plan the process runs for spent broiler litter composting in Mauritius in such a way to
maximize on both the attainment of stability, through efficient control of degradation
thermodynamics, and hygienisation compliance.
TC, FC, E. coli and FE levels on root crops were reduced to very remote fractions of 0.01826,
0.00046, 0.000132 and 0.000013 kg-1 respectively. The predicted E. coli counts on root crops at
point of harvest provided a basis for estimating the exposure potential by the β-Poisson model.
Probability of exposure was 0.782 for raw SBL mix compared to 1.40 × 10-11 with composting.
It can be concluded that there is a definite advantage in optimally composting SBL mix before
land application since the composting process effectively confers an escalating dilution effect of
the enterobacteria.
References
1. Haug, R.T. (1980) Compost Engineering: Principles and Practice. Ann Arbor, Ann Arbor
Science. MI, USA.
2. ISO 4831 (1991) Microbiology – General guidance for enumeration of coliforms – most
probable number technique. International Standards Organization. Switzerland.
3. ISO 7251(1993) Microbiology–General guidance for enumeration of presumptive
Escherichia coli – most probable number technique. International Standards
Organization. Switzerland.
4. ISO 7899-1(1984) Water quality–detection and enumerating of faecal streptococci Part1:
Method by enrichment in a liquid medium (most probable number technique).
International Standards Organization. Switzerland.
5. ISO9308(1990) Water quality–detection and enumeration of coliform organisms,
thermotolerant coliform organisms and Presumptive Escherichia coli – Part 2: Multiple
tube (most probable number) method. International Standards Organization. Switzerland.
6. Mohee, R. (1998) Composting potential of bagasse and broiler litter and process
simulation using a dynamic model. PhD Thesis, University of Mauritius. Réduit,
Mauritius.
7. Mohee, R., Driver, M.F.B., Sobratee, N., (2008) Transformation of spent broiler litter
from exogenous matter to compost in a sub-tropical context. Bioresource Technology, 99:
128-136.
8. Nielsen, H. and Berthelsen, L. (2002) A model for temperature dependency of
thermophilic composting process rate. Compost Science & Utilization. 10: 249-257.
9. Rajkumar, B. and Driver, M.F.B. (2003) The poultry sub-sector in Mauritius. FANRPAN
Policy Brief No. 2. Available on the Internet at: http://www.fanrpan.org. Accessed on
08.08.08.
10. Shane, S. (1998) Mauritius: industry profile. Poultry International. 37(14), 56-62.
11. Sobratee, N., Mohee, R., Driver, M.F.B., Mudhoo, A. (2008) Survival kinetics of fecal
bacterial indicators in spent broiler-litter composting. Journal of Applied Microbiology,
104(1): 204-214.
12. Sobratee, N., Mohee, R., Driver, M.F.B., (in press) Quantitative exposure of root crops to
indicator enterobacteria from composted spent broiler litter under sub-tropical
environment. Bioresource Technology, doi:10.1016/j.biortech.2008.06.035.