Studies on semi-cylindrical solar tunnel dryer for drying wastewater sludge

. Drying plays an important process for wastewater sludge management, as it can minimise the volume of wastewater sludge before disposal, and consequently the cost of storage, handling and transport. In this study, the wastewater sludge was dried by using a solar tunnel greenhouse drying system. The performance of the dryer for drying wastewater sludge has been analyzed through no load and full load test. The three full load drying tests recorded that the temperature inside the tunnel dryer fluctuated around 55±5°C; while the no load drying test, the temperature of the drying tunnel maintained within 60±5°C; as compared to the ambient temperature of 30±5°C. The average moisture content of solar dried sludge decreased from 88.69 - 90.84% to 7.78 – 13.30% in the mixing conditions and 14.78 – 19.52% in the non-mixing conditions, in 5 days. The study suggests that the semi-cylindrical solar tunnel dryer of wastewater sludge has given satisfactory results within five days.


Introduction
Water is an indispensable element for human life.
In recent decades, the rise in global population and human activities lead to an increase in water consumption worldwide.It is predicted that the demand for water will increase by 2050 but the availability of water will be reduced; hence, demand for wastewater treatment plants will be increased [1].Wastewater treatment plants produce treated wastewater that can be reused in agriculture and also as industrial processing water.
Wastewater treatment by biological means in general, and by the activated sludge process in particular, has been widely adopted as the most effective technological method for treating wastewater.However, the application of this method in wastewater treatment plants results in significant production of biosolids (wastewater sludge).Wastewater sludge can be defined as the solid or semi-solid leftover after the treatment of wastewater [2].It is considered as an undesirable by-product in wastewater treatment facilities.
Wastewater sludge is a complicated and heterogeneous substrate containing organic and inorganic materials as well as microorganisms [3].
Wastewater sludge therefore requires special treatment and handling due to its significant volume, organic content, and pathogen load.It is emphasized that solar air dryers have great potential for replacement of conventional scale drying of industrial and agricultural products [9].Besides, affecting saving of fossil fuel, fuel wood, or electricity, solar drying may also be cost effective [10].It was also confirmed that using solar drying plays an interesting role the moisture reduction of wastewater sludge [11].Thus, it is necessary to research and improve the efficiency of drying wastewater sludge using solar devices and apply them in real wastewater treatment plants at low cost.The main objective of the current study is to report results from the experimental study on application of designed solar tunnel dryer for wastewater sludge drying.

Wastewater sludge materials
Wastewater sludge from wastewater treatment plants in Tra Noc Industrial Zone of Can Tho city was used for the experimental study.

Design of the solar tunnel dryer
In the study, a semi-cylindrical solar tunnel dryer was designed at experimental scale (length 2m, width 1m, and height 0.

Temperature
One sensor was kept inside the solar tunnel dryer at the bottom point and one sensor was kept outside the solar tunnel dryer.All temperature data were recorded at an interval of an hour.

Light intensity
A light meter was used to measure luminance flux per unit area, or the amount of visible light present at the dryer place.

Moisture
The moisture content of the sludge was determined by the drying method.Samples of the sludge were taken and weighed.Then they were placed in moisture boxes that were placed in the oven at a temperature of 105 °C for 24 h.The samples were again weighed and the moisture content was determined on wet basis as the formula: M = 100 x (Wet Weight -Dry Weight)/Wet Weight MELAG 405 oven 3

Results and Discussion
The performance of a dryer depends on the duration of drying and the quality of the end product, besides factors such as collector performance and drying temperature.

Variation of temperature and light intensity
Elaborate testing of the solar tunnel dryer was carried out under no load and full load conditions during April 2022 for drying wastewater sludge at the laboratory level.The performance of solar tunnel dryer is discussed under the following subheadings: (i) full load testing of solar tunnel dryer and (ii) no load testing of solar tunnel dryer.

No load testing of solar tunnel dryer
During no load testing, the solar tunnel dryer was empty, i.e., no wastewater sludge was placed in the solar tunnel dryer for drying.The testing on load was done for consecutive 5 days, in period 4, from 20 th to 24 th of April.As shown in Fig. 6 it was observed that the maximum temperature attended inside the tunnel was 69.40 °C at 12:00 hrs while the minimum inside temperature was 48.40 °C at 9:00 hrs in the month of April.

Variation of moisture content
The variation of moisture content in periods 1, 2 and 3 is shown in Fig. 7 In the second period from 10 th to 14 th of April (Fig. 8), the initial wastewater sludge had a moisture content of 89.05%, after the drying process, the remaining moisture content was 13.30% and 15.53%, respectively, in the drying tunnel with mixing and without mixing; Compared with open sun drying, the moisture content was 42.25% and 54.97% under mixing and non-mixing conditions, respectively.In the third period from 15 th to 19 th of April (Fig. 9), the initial moisture content of the sludge was 88.69%, after the drying process, the remaining moisture was as final moisture content within 9-33 days in autumn [11].
Through the three trial drying periods, it was concluded that the reduction in moisture content of wastewater sludge dried in solar tunnel dryer was found to be superior than sludge dried in the open sun environment.

Conclusion
When using solar tunnel dryer to dry wastewater 5m) for drying wastewater sludge at 40 percent moisture content to 10 percent moisture content.Low-cost materials possessing high rigidity, long life and superior thermal characteristics have been used for experiments.The metallic frame structure of the tunnel dryer has been covered with UV stabilized semitransparent polyethylene sheet of 3 mm thickness.A gradient of approximately 5° has been provided along the length of the tunnel to induce natural convection airflow.Steel sheet floor plates have been painted black for better absorption of solar radiation.Three cm thick insulating panel has been provided to reduce heat loss through the floor.It is based on the theoretical calculation for critical insulation thickness for this dryer.The structural components of solar tunnel drier include flat iron (20×3mm), floor, UV stabilized polyethylene film (3mm thick) and drying trays.The dimensions and other design parameters of the solar tunnel dryer are presented in Table 1.The schematic and practical illustration of the solar tunnel drier is given in Fig.1.

Full load testing of
solar tunnel dryer was done for evaluating the performance in actual loaded condition.Wastewater sludge with approximately 90% initial moisture content was taken for study and loaded in the trays of solar tunnel dryer.Wastewater sludge was spread a thin layer of approximately 6 cm thickness in trays of 56 × 36 cm size.Eight trays were loaded onto the tunnel dryer in the three periods 1, 2 and 3.The testing on full load was done for consecutive 15 days the month of April 2022, respectively.As shown in Figs.2-4that maximum temperature inside the solar tunnel dryer fluctuated between 12:00 -13:00 hours.More the, highest temperature was 59.8 °C at 13:00 hrs (period 1), 60.4 °C at 12:00 hrs (period 2), and 60 °C at 12:00 hrs (period 3), against the maximum ambient temperature of 33.04 °C 13:00 hrs (period 1), 33.12 °C at 12:00 hrs (period 2), and 32.22 °C at 12:00 hrs (period 3).Whereas, the minimum temperature inside the solar tunnel dryer was 51, 48.6 and 43.6 °C at 9:00 hrs in periods 1, 2 and 3, respectively, in the month of April against the minimum ambient temperature of 30.24, 30.38 and 29.22 °C in periods 1, 2 and 3, respectively.

Fig. 5 .Fig. 6 .
Fig. 5. Correlation between temperature and light intensity -9, respectively.The two drying conditions were conducted, including (i) the sludge was mixed manually once per day; (ii) the sludge was not mixed.It can be seen that the tunnel drying system removes water content in the sludge than the open sun drying during 5 days of testing.In which, the first period from 5 th to 9 th of April (Fig. 7), that moisture content was reduced from 90.84 percent to 14.57% and 19.52%, respectively, in the mixing and non-mixing conditions, respectively; Meanwhile, open sun drying with mixing and non-mixing conditions achieved the moisture content of 54.21% and 66.04%, respectively.

Fig. 7 .Fig. 8 .Fig. 9 .
Fig. 7. Moisture reduction in period 1 (05/04 -09/04) sludge, both the temperature and intensity of light could affect the drying process.In addition, temperature inside the tunnel dryer showed a positive correlation with light intensity.The greater the temperature is, the faster the drying inside tunnel dryer is heated, and the faster the drying efficiency is.Both light intensity and temperature changed rapidly during the day.Temperature inside the solar tunnel dryer was higher than outside by 20 -28 °C during sunshine hours.The moisture content of wastewater sludge loaded in the dryer during 5 days of testing was reduced from an initial value of 88.69 -90.84% to around 7.78 -13.30% in mixing condition, and around 14.78 -19.52% in non-mixing condition, which is approximately three times lower than the moisture content of the sludge under open sun drying.In conclusion, the performance of studied solar tunnel dryer was quite satisfactory in terms of reducing the mass and the volume of the wastewater sludge within 5 days.