Growing liquid waste causes many things one of which is omnipresent urbanization and consumptive style of life of contemporary society. Burning or storing are the easiest ways to manage the increasing amount of waste. However, neither of these options provides a solution for waste water, which often contains organic substances and food leftovers. A much more economical and sustainable solution is to make use of waste water in the farming sector (Jakubas 2006).
Macroelements, nitrogen, phosphorus, magnesium, sulfur are all present in dehydrated waste water sludge, all of which can contribute to harvesting processes in agriculture. There are requirements regarding the content of fungi, bacteria, viruses, parasitic eggs and heavy metals that sludge intended for use in agriculture must comply with (Napora, Grobelak 2014). However, dehydrated sludge is prevented from being used in the farming sector if excess heavy metals are present in waste water sludge. That is not to say that it cannot be processed and used as an alternative fuel (Środa et al. 2006).
Research on Dry Mass of Waste Water Sludge
The Research Laboratory of waste water sludge department in Municipal Water and Waste Water Administrative Body in Radom carried out a determination of the dry mass of waste water sludge. Tests were firstly carried out in line with the reference method, EN 12880:2004 and the dry mass content was found to be 22.2%. Following on from this, a small sample of the product was subjected to tests based on microwave radiation. The PMV 50 microwave moisture analyzer, manufactured by Radwag Wagi Elektroniczne, Poland was used to carry this out. The construction and operation of PMV 50 are shown in Figures 1 and 2.
Employing Radwag’s PMV Moisture Analyzer
The latest solution using the microwave radiation for determining water content and dry mass is the PMV moisture analyzer manufactured by Radwag. Absorption of the microwave radiation by polar compounds of the sludge (mainly water) increases sludge temperature.
Reorientation of dipoles of polar compounds occurs because of radiation absorption, which leads to molecular friction (Al-Harahsheh M, Al-Muhtaseb and Magee 2009). This induces a fast increase in temperature within the whole product volume and causes short analysis duration. The power and frequency of the emitted microwaves dictate the efficiency of the drying process, as well as the product structure and its chemical content (Soysal 2004; Kamińska and Ciesielczyk 2011). As a modern and productive piece of weighing equipment, the PMV microwave moisture analyzer offers:
- short drying time (2 – 5 minutes maximum),
- products, drying programs, completed drying procedures databases,
- statistical analysis for water content determination of selected product,
- interactive menu with definable buttons, info fields, permission levels, etc.,
- programmable proximity sensors
- drying process visualization, presented as a drying curve,
- export / import of products and drying programs databases, remaining data,
- communication via RS 232, USB, Ethernet, Wi-Fi.
Figure 1. PMV 50 microwave moisture analyzer. Image Credit: Radwag Balances & Scales
Figure 2 shows a diagram of the main subassemblies of the PMV 50 microwave moisture analyzer, manufactured by Radwag Wagi Elektroniczne, Poland.
Figure 2. Diagram of PMV 50 microwave moisture analyzer. Image Credit: Radwag Balances & Scales
The electromagnetic system (1) measures the sludge mass. This is made up of carefully selected monoblock components. This enables the weighing system to maintain stability even when operating in unstable ambient conditions. There is a weighing pan (4) inside of the drying chamber (2), which is loaded with a sludge sample (6) and placed between two glass fiber filters (5). Microwaves that cause a dipole effect of water molecules in a sludge (6) are emitted by a magnetron (3). The sludge temperature increases as a result. An infrared sensor (7) installed in the top part of the drying chamber (2) is used to monitor the temperature. A system controlling the power of the magnetron (3) uses the information obtained. The analyzed sludge temperature drops, the emitted microwaves' power changes, and its intensity matches dynamics of the process.
Dry mass content is analyzed until the condition of sludge mass stability is met. In this case, it means complete desorption of water from the tested sludge structure. Figure 3 demonstrates the means of operation of this mechanism.
Figure 3. Diagram of PMV 50 microwave moisture analyzer. Image Credit: Radwag Balances & Scales
The following equation enables automatic calculations of dry mass content of waste water sludge (%D):
&D – dry mass content
m1 – wet sludge mass, prior to analysis start
m2 – dry sludge mass, upon analysis completion
The Results and a Debate
Using the reference method EN 12880:2004, the initial step of the test was to determine dry mass in waste water sludge. A value of 22.20% of dry mass was obtained, which was then taken as a reference point in determining accuracy of the method based on microwave radiation (PMV 50 moisture analyser). A small layer of sludge was placed in between two glass fiber filters and the prepared sample was dried until a constant weight value was obtained.
The optimum temperature for analysis performance, for Auto 2 selected as the process finish mode, was determined experimentally as 80 oC. After drying in the range of 1 mg in 25 seconds, this provided constant sludge mass. Table 1 displays the obtained results.
Table 1. Dry mass in waste water sludge (PMV 50 moisture analyzer). Source: Radwag Balances & Scales
|Sludge mass [g]
||Microwave power [%]
||Sludge temperature [oC]
||Dry mass content [%]
||Analysis time [min:s]
Dry mass content determination accuracy was calculated as 0.05% using the following equation:
A = x̅EN 12880 − x̅PMV 50
A – test accuracy
x̅EN 12880 – average value of dry mass of waste water sludge obtained using reference method EN 12880
x̅PMV 50 – average value of dry mass of waste water sludge obtained using the microwave method (PMV 50)
Figure 4 presents the percent deviation of accuracy of successive dry mass content measurements in reference to the value obtained using the reference method EN 12880. The greatest deviation of accuracy was 1.75% and the smallest was 0.19%. This proved that the analyzed product's structure was not homogeneous, which most probably significantly influenced the test accuracy and precision.
Figure 4. Accuracy of determination of dry mass of waste water sludge using microwave radiation (PMV 50 moisture analyzer). Image Credit: Radwag Balances & Scales
The measurement precision of 0.26% of dry mass is for the method based on microwave radiation and presented in terms of standard deviation estimated for series of measurements. It was concluded with 95% probability using this value that the dry mass content of waste water sludge is comprised within a 21.73% - 22.77% range. Figure 5 shows a graphic visualization of the above.
Figure 5. Uncertainty of determination of dry mass of waste water sludge. Image Credit: Radwag Balances & Scales
Usually one determination is carried out during evaluation of the quality of waste water sludge upon dehydration. However, when waste water sludge is a mixture of various substances, the obtained result may not be representative. In this situation, it is recommended that at least three determinations are carried out to provide a more reliable result. This makes analysis duration more tricky. An average analysis time of 5 minutes and 42 seconds was obtained for tests carried out using the microwave radiation. When compared to analysis of the same samples tested using the infrared radiation, this was relatively short as these took around 30 – 40 minutes.
The analysis duration of the microwave method for determining dry mass content of sludge was six times shorter than the method based on the infrared radiation. The accuracy of both methods was comparable. The measurement precision was concluded to be mainly conditioned by analyzed sludge homogeneity. The short analysis duration means that the PMV 50 moisture analyzer can be used to obtain fast reliable results wherever information about the dry mass content is required.
1. Al-Harahsheh M., A.H. Al-Muhtaseb, T.R.A. Magee. 2009. Microwave drying kinetics of tomato pomace: Effect of osmotic dehydration. Chemical Engineering and Processing 48: 524–531.
2. Jakubas M. 2006. Ocena przydatności osadów ściekowych w nawożeniu roślin. Woda-Ścieki-Obszary Wiejskie t6, z.2 (18), 87-97
3. Kamińska A., W. Ciesielczyk. 2011. Kinetyka suszenia mikrofalowego wybranych warzyw i owoców”. Inżynieria i Aparatura Chemiczna 50 (1): 19-20.
4. Napora A., A. Grobelak. 2014. Wpływ osadów ściekowych na aktywność mikrobiologiczną i biochemiczną gleby. Inżynieria i Ochrona Środowiska t 17(4), 619-630
5. PN-EN 12880:2004. Charakterystyka osadów ściekowych. Oznaczanie suchej pozostałości i zawartości wody.
6. Środa K., A. Kijo-Kleczkowska, H. Otwinowski. 2012. Termiczne unieszkodliwianie osadów ściekowych . Inżynieria Ekologiczna 28, 67-81.
7. Soysal Y. 2004. Microwave Drying Characteristics of Parsley. Biosystems Engineering 89 (2): 167-173.
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