CHAPTER 3: MATERIALS AND METHODS
3.4 Characterisation of the dried sludge
Characterisation of the samples was performed to determine the effect of solar thermal drying on the physicochemical properties of the sludge. Each analysis was performed on the wet and dried samples to determine the effects of the solar drying on the quantitative and qualitative aspects and included replicates in order to determine the uncertainty of the measurement. Table 3-2 shows the summary of the quantitative and qualitative properties analysed.
Table 3-2: Summary of characterisation tests of the dried FS categorised into quantitative properties and qualitative observations
QUANTITATIVE PROPERTIES
Physical Moisture content Ash content Shrinkage Density Thermal Calorific values Heat capacity Thermal
conductivity
-
Elemental nutrients
Carbon Nitrogen Sulphur -
Molecular nutrients
Ammonium Nitrates Orthophosphates -
QUALITATIVE OBSERVATIONS Crusts Cracks Visual aspects Odour
3.4.1 Quantitative properties Moisture content
Moisture content was determined by placing the samples in ceramic crucibles and drying the samples in an oven at 105 °C for a minimum period of 24 h. The moisture content (M) can then be defined in either a dry (db in index) or wet basis (wb in index). The moisture content is defined by equations (3-1) in dry basis, and (3-2) in wet basis.
% 100
% 0
f f
db w
w
M w (3-1)
% 100
%
0
0
w w
Mwb w f (3-2)
Where: wo is the initial mass of the sample before being oven dried.
wf is the dried mass of the sample.
Page | 50 The moisture content in wet and dry basis are related by equation (3-3):
% 100 100
%
wb wb
db M
M M (3-3)
The average moisture content of the wet FS used in this study was found to be 78% in wet basis (355% dry basis).
Ash content
Ash content is the amount of material that remains after combustion of the FS in an oven at 550
°C for 2 hours (for VIP FS samples). It represents then the inorganic material that remains after the removal of all water and organic matter components by combustion. It is calculated by equation (3-4).
%
2 100
f f f
w w content w
Ash (3-4)
Where: wf2 is the mass of material residue after combustion.
The average ash content of the wet FS used in this study was found to be 0.474 g/g dry sample.
Thermal analysis of faecal sludge (FS)
The characterisation of the thermal properties of the sample is relevant for the thermal treatment processes of FS and the determination of the potential of resource recovery from the dried product.
The thermal properties analysed were calorific values, heat capacity and thermal conductivity. The calorific values was determined by combusting a pre-weighed known mass of FS in an oxygen bomb calorimeter (Parr 6200). Both the thermal conductivity and heat capacity were determined using a thermal conductivity analyser (C-Therm TCi) which uses the modified transient plane source (MTPS) technique in describing the thermal conductivity and heat capacity of materials.
Page | 51 Nutrient analysis
Nutrient analyses were performed on the dried product in order to ascertain the effect of solar drying on the agricultural value of the sludge as a nutrient supplement or/and soil conditioner. The nutrients that were investigated can be split into two categories, which are elemental and molecular nutrients. The elemental nutrients include carbon (C), nitrogen (N) and sulphur (S). The concentration of these nutrients in sludge were determined using a microwave plasma-atomic emission spectrometer (4200 MP-AES) equipped with a concentric nebuliser.
The molecular nutrients investigated were ammonium (NH4+), nitrates (NO3-) and orthophosphates (PO4-3). A Spectroquant photometer (Pharo 300) and the relevant commercial test kits specific to the analysis were utilised to determine the concentrations of the molecular nutrients. Prior to each test, a known mass of the sample was added to a known volume of water and centrifuged for liquid- solid separation. The resultant liquid was taken and analysed.
Density
The effect of drying on the density of the sludge was investigated. The average height of sample values were recorded using a ruler calibrated in millimetres before and after drying for each experiment so that the volume properties of the sample could be attained. The mass of the sample was measured using a mass balance, with an accuracy or 0.0001 g.
𝜌 =
𝑚𝑉
=
𝑚𝜋𝑟2ℎ (3-5)
Where: ρ is the density of the material m is the mass of the sample
V is the volume of the sample
r is the radius of the petri-dish
h is the height of the sample
Page | 52 Shrinkage
Shrinkage was measured in proportion to the initial volume. This was physically calculated using the measurements for the initial and final volume after experimentation on the samples.
𝑆ℎ𝑟𝑖𝑛𝑘𝑎𝑔𝑒 = 𝑉𝑓
𝑉𝑖 = 𝜋𝑟2ℎ𝑓
𝜋𝑟2ℎ𝑖 (3-6)
Where: 𝑉𝑓 is the final volume of the sample, ℎ𝑓 is the final height of the sample 𝑉𝑖 is the initial volume of the sample, ℎ𝑖 is the initial height of the sample 3.4.2 Qualitative observations
Crust
Crusting of the surface is a common occurrence in drying. Formation of crusts hinder mass transfer off the surface of the water and hence affect drying rates. A qualitative look through visual observation and crust feel on crust formation was carried out on both samples, for open air sun and solar drying.
Cracks
Cracks can form on hard surfaces, thus during drying, formation of crusts could be coupled with cracks. Cracks are positive for the mass transfer and so they are desirable when there is crust formation. A visual observation at crack formation was carried out on both samples, open air sun and solar drying.
Colour
Quality of dried produce is concerned with the colour and visual aspects of the samples. These were made as a relative comparison of the before and after aspects.
Odour
The odour of FS directly influences the acceptance of FS by communities. A qualitative analysis of the odour strength was carried out before and after experimentation on all samples. This was done by smell in order to compare the odour emission before and after drying.
Page | 53