Application of improperly treated compost from composting toilet is one of the causes of bacterial contamination in the field, crops, food and water. The study characterized the die-off represented by kinetic inactivation rate coefficient, k (h- 1 ) of Enterococcus in clay and sandy loam soils, determined the effect of temperature, compost-to-soil ratio and soil type on the inactivation rates of Enterococcus and evaluated the health risk associated with the amendment of compost from composting toilet in real conditions using local climatic data . The soils were amended with compost to soil ratios of 1:10, 1:25, 1:50 and 1:100 held at different temperatures (30°C, 40°C and 50°C). Inactivation of Enterococcus (pathogenic bacteria) in the soil with high temperature under different compost application rates was tried in the laboratory test and the Quantitative Microbial Health Risk evaluated. The study results indicated the inactivation rates of Enterococcus in clay soils as 0.015 - 0.027 h- 1 , 0.246 - 0.322 h- 1 , 0.397 - 0.571 h- 1 whilst sandy loam soils recorded 0.056 - 0.130 h- 1 , 0.348 - 0.447 h- 1 and 0.475 - 0.630 h- 1 for 30°C, 40°C and 50°C respectively. Inactivation rates of Enterococcus in soils amended with compost from the composting toilet depended on temperature and soil type but not on the compost-to-soil ratios and compost from the composting toilet amended to the soils is safe for use in six (6) days.
The rural model of composting toilet has been designed and installed in consideration of local material availability and affordability [
Comprehensive information on die-off periods of several pathogens in the soil-plant-waste system has been previously reported [
The study by Kagambéga et al. [
Considering actual practices, solarisation is one of the main processes for disinfection of enteric pathogens, because sunlight could be obtained in any place of farmland. The solarisation relates to the ambient temperature, while the temperature is not constant as shown in
Compost used for the experiment was collected from a pilot site installed in a family in Kamboinse, Burkina Faso. Nine people use the toilet and the compost has been used for 8 months on the site. Soil samples (Clay and sandy loam) were taken from the Kamboinse pilot experimental site on the campus of the International Institute for Water and Environmental Engineering (2iE). The characteristics of the sandy loam and clay soils used are described in
Particle size (%) | Sandy loam soil | Clay soil |
---|---|---|
Silt | 23 | 31 |
Fine sand | 23 | 9 |
Coarse Sand | 35 | 2 |
Total N | 0.04 ± 0.01 | NM |
C | 0.54 ± 0.02 | NM |
SOM | 0.93 ± 0.03 | NM |
**NM, not measured.
M C = W w e t − W d r y W w e t × 100 (1)
where, Wwet and Wdry are wet and dry weights of compost sample (g). The moisture content was set to 25% with ultra-pure water, because the moisture content at field capacity for sandy loam and clay soils are 14.7% and 22.6% respectively [
Enterococcus ATCC 19433 strain was purchased from American Type Culture Collection (ATCC) and was grown in a 10 ml Nutritif Nutrient broth (Difco, France) by incubating at 37˚C over night. Zero-point three millilitre (0.3 ml) of the broth solution was inoculated into 20 g of the compost by a pipette on the surface and then mixed for 5 mins with a sterile spatula to homogeneously distribute the Enterococcus. The concentration of Enterococcus in the inoculated solution was about 106 CFU/ml. After 3 h, each 20 g-dry of the inoculated compost was mixed well with the soil at the specified ratios described in
Enterococcus was cultured following the modification of method 9215A in Standard Methods for the Examination of Water and Wastewater [
Samples | Temperatures (˚C) | ||
---|---|---|---|
30 | 40 | 50 | |
Compost-to-soil ratio (g/g) | Corresponding quantity ratio of compost on soil (g/g) | ||
1:10 | 20:200 | ||
1:25 | 20:500 | ||
1:50 | 20:1000 | ||
1:100 | 20:2000 |
Disodium Phosphate Anhydrous 3.5, and Potassium Dihydrogen Phosphate 1.5. Ten grams of the compost sample were added to a 90 ml volume of peptone water and agitated for 3 mins with vortex mixer. After adequate dilution (101 - 107 times) with sterilized Ringer solution, each diluted extract was isolated in Chromocult coliform ES agar (Merck KGaA) by simple layer method. The media were incubated at 37˚C for 24 h, and then, Enterococcus colonies were counted. The limit of detection of bacteria was 10 CFU/g.
Concentration versus time data obtained from the inactivation experiments were fitted to a first order kinetic model. Nakagawa et al. [
ln ( C / C o ) = − k t (2)
where, C is concentration of microorganism in soil sample in dry basis at time, t (CFU/g-dry solid), Co is initial concentration of microorganisms in soil samples in dry basis (CFU/g-dry solid), t is reaction time (h). After the estimation of inactivation rate coefficients, k, we tried to evaluate the effect of temperature with the Arrhenius equation described as follows;
k = A exp ( − E a R T ) (3)
where, A is pre-exponential factor (h−1), Ea is activation energy (J/mol), R is the universal gas constant (J/mol/K), T is the temperature (K). A statistical study (nonparametric Kruskal-Wallis test) was carried out to determine significant difference (p ≤ 0.05) in temperature, the compost-to-soil ratio and the soil type. The analysis was done with IBM SPSS, version 12.0 (IBM Corporation).
Quantitative Microbial Risk Assessment (QMRA) was used to predict the likelihood of salmonellosis transmission during the amendment of compost. The farmer exposure was predicted via a Monte Carlo technique in three scenarios: bottom, middle and top temperature of the soil. The β-Poisson equation was used to assess the dose response of salmonellosis. The N50 and α used are 17,700 and 0.23475, respectively. The random number is applied for estimation of variables with distributions for simulation with the appropriate equations. The simulation was repeated 10,000 times [
One-week temperature was measured during February, 2015 in the soil with the aid of ThermoManager sensors. They were placed in the soil at the bottom, middle and top at 10 cm, 5 cm and 1 cm respectively. The temperature distribution is show in
Enterococcus concentration in the compost amended clay soils at 30˚C for 8 h did not record any change in concentration for all ratios, as shown in
Increase in temperature showed to increase the inactivation rate of all soil types. The first order kinetics inactivation rate coefficient, k, of Enterococcus
Sample | Temperature (˚C) | ||
---|---|---|---|
30 | 40 | 50 | |
First order kinetic inactivation rate coefficient k (h−1) Enterococcus | |||
Clay soil ratio of compost to soils (g/g) | |||
1:10 | 0.015 | 0.246 | 0.397 |
1:25 | 0.027 | 0.386 | 0.509 |
1:50 | 0.026 | 0.418 | 0.541 |
1:100 | 0.027 | 0.322 | 0.571 |
Sandy loam soil ratio of compost to soils (g/g) | |||
1:10 | 0.056 | 0.348 | 0.475 |
1:25 | 0.075 | 0.365 | 0.578 |
1:50 | 0.102 | 0.392 | 0.591 |
1:100 | 0.130 | 0.447 | 0.630 |
increased with temperature as summarised in (
Enteric bacteria have a faster die-off in soils possessing a high pH with pH of 6 to 7 being optimum for bacteria survival, and dying quickly under acid soil conditions [
Results showed that, the type of soil influenced the inactivation rate of Enterococcus. The inactivation rate coefficient in all compost-amended sandy loam soil formulations was higher than the clay soil formulations. This indicated that the soil type had an influence on the survival of bacteria. Soil type affected the die-off rate because finer soils, especially, clay minerals and humic substances increase the survival of bacteria [
Temperature | Compost to soil ratio | ||||||
---|---|---|---|---|---|---|---|
Temp_30 | Temp_40 | Temp_50 | 1:10 | 1:25 | 1:50 | 1:100 | |
Mean | Mean | Mean | Mean | Mean | Mean | Mean | |
Clay soils | 0.024a | 0.343b | 0.504c | 0.219a | 0.306a | 0.328a | 0.307a |
Sandy loam soils | 0.091a | 0.388b | 0.569c | 0.293a | 0.339a | 0.362a | 0.402a |
retention capacity for bacteria [
Compost-to-soil ratio showed a variation on the inactivation rate coefficient in our study. In the compost-amended clay soil formulations, the inactivation rate constant values were higher when the soil volume was increased (
The statistical results, however, showed that there was no effect among the performance of the different application rate of the compost. Results showed that the differences of compost-to-soil ratio on Enterococcus inactivation was not statistically significant Kruskal-Wallis test, p > 0.05 for both sandy loam and clay soils. Crane et al. [
The average diurnal ambient temperature in Ouagadougou varied from 36˚C to 47.2˚C, these temperatures could reduce the concentration of pathogens in an improperly treated compost amended to the soil as fertilizer to minimise the serious health risk as previously observed [
The study simulated the conditions in real situations with the measured temperature in the soil. The result of the change in concentration and annual risk of Salmonella in the soil was estimated.
Inactivation of Enterococcus (pathogenic bacteria) in the soil with high temperature under different compost application rates was tried in the laboratory test. As a result, 1) the inactivation rates of Enterococcus in clay soils were 0.015 - 0.027 h−1, 0.246 - 0.322 h−1, 0.397 - 0.571 h−1 for 30˚C, 40˚C and 50˚C, respectively. Sandy loam soils were 0.056 - 0.130 h−1, 0.348 - 0.447 h−1 and 0.475 - 0.630 h−1 for 30˚C, 40˚C and 50˚C, respectively. 2) Inactivation rates of Enterococcus in soils amended with compost from the composting toilet depended on temperature and soil type, but not on the compost to soil ratios. This study would be a useful information for researchers and farmers to understand the behaviour of pathogenic bacteria in the sandy loam and clay soils. The experimental conditions in this study are different from the real field situation because temperature and humidity change over time due to strong sunlight under field conditions. Therefore, care must be taken when interpreting the results of this study to estimate the die-off rates in real field conditions. 3) Compost after 24 h of post-treatment period amended to the soil would be safe in 6 days. Further research is required to understand the behaviour of pathogenic bacteria in field conditions of a hot semi-arid climate. This study succeeded to evaluate the risk of pathogens with initial biological parameters and operational conditions.
The authors declare no conflicts of interest regarding the publication of this paper.
Darimani, H.S. and Ito, R. (2020) Health Risk Assessment of Compost-Amended Soils. Journal of Agricultural Chemistry and Environment, 9, 59-72. https://doi.org/10.4236/jacen.2020.92006