Greenhouse Solar Dryers: A Cost-Effective Solution to Ensure Safe Application of Faecal Sludge in Agriculture

Non-judicious and long-term application of chemical fertilizers not only deteriorate soil quality but also contributes to climate change effects due to the emission of greenhouse gases during the production and application of these fertilizers. On the other hand, there is an urgent need to look for alternative nutrient sources for food production to feed the growing population.

It is widely known that human excreta is rich in nutrients, specifically Nitrogen and Phosphorous. With the recent thrust on faecal sludge treatment and safely managed sanitation, there is an opportunity to use human excreta as a nutrient source. However, there are concerns of health risks due to the presence of pathogens in faeces. The main cause of concern is the soil-transmitted helminth infections as these are highly resistant to treatment and viable for several years.

In this context, this study was conducted in 4 locations (FSTPs – Faecal sludge treatment plants) of India – Angul, Dhenkanal, Karunguzhi and Devanahalli with the main objective to evaluate the efficiency of polycarbonate-based greenhouse solar dryers in reducing the Helminths eggs in the final treated sludges. Greenhouse solar dryers (GHSD) use passive drying to help increase the temperature and decrease humidity to ensure pathogen kill as well as faster drying.

Scenarios studied under the project:

Following were the assumptions made for the study,

  • Increased temperature and decreased relative humidity inside the GHSD chamber will help in reducing the sludge drying time.
  • Longer exposure of sludge to higher temperature (>50°C) will inactivate Helminths eggs.

GHSD is the polycarbonate sheet installed over the drying beds. This has a parabolic shape to resist wind and to induce greenhouse effect inside the drier. This greenhouse effect inside the drying chamber helps removing the moisture laden air and the moisture content from the drying product (Figure 2).

Solar pasteurisation unit (SPU) follows the same working principle and the structure of the GHSD. However, the height of the roof is less compared to the GHSD. The dried sludge from the GHSD is placed in the SPU. Due to reduced height of the chamber and low moisture content of the sludge, SPU can reach to a higher temperature of more than 60 degrees Celsius which will help eliminating the pathogens (Figure 3).

Galvanised (GI) sheet is one of the most used roofing materials over the sludge drying beds. These are galvanized metals made of thin sheets, coated with zinc. The main purpose of these sheets is to protect the drying beds from getting wet during rainy season.

Below mentioned scenarios were studied under the project,

  • Greenhouse solar dryer (GHSD): Angul and Devanahalli FSTP
  • Galvanised (GI) sheet + Solar Pasteurisation Unit (SPU): Dhenkanal FSTP
  • GI sheet and GHSD: Karunguzhi FSTP.

A Monitoring Technique of Desludging and Decanting Faecal Sludge

Tamil Nadu is a rapidly urbanising state that has been establishing and scaling up sustainable FSSM, leading the way in innovating technologies and operating models in sanitation. Safe collection, handling, and transport of fecal sludge (FS) is an integral part of a septage management programme. This paper documents the use of load axle sensors with GPS technology in the de-sludging vehicle to understand the movement of the vehicle, de-sludging and disposal locations, travel distance and time, and the time for de-sludging and decanting. These learnings help determine the location of current disposal, service area, and planning of decanting facilities.

 As a matter of fact, de-sludging vehicles collect and transport septage to designated decanting facilities, eliminating the need for manual emptying and reducing the risk of human contact with FS. As per the Tamil Nadu Urban Local Bodies (Amendment) Act (2022), the desludging vehicle must install a GPS device to monitor the de-sludging and decanting activities.

However, GPS can only track the movement of the vehicle and not locate de-sludge locations and whether operators were safely decanting the FS at the designated spot. Identification of the desludging and decanting locations with loaded quantities using GPS technology is difficult and a highly time-consuming process. The time required to analyse each vehicle could be a challenge for the ULBs as they scale this monitoring strategy.

Therefore, a study done on this aspect by the Tamil Nadu Urban Sanitation Support Programme (TNUSSP), intended to identify the possibilities of monitoring the de-sludging vehicles using a load sensor with GPS technology.

This monitoring helps identify the service area of the vehicle, the desludging and decanting locations, quantity of FS collected, travel time and distance between desludging and decanting locations, and time for desludging and decanting.  The analysis of travel distance and time aids for planning additional decanting facilities. Additionally, the vehicle was discovered to be loaded overnight and parked, so it is advised to keep the decanting station open at night as well.

Field teams are working on upgrading the system with an ultrasonic load sensor for more accuracy at an affordable cost.  Also, the option of incorporating the sensor output with the FSSM application to plan schedule de-sludging, auto deduction of decanting fee and real-time plant utilization rate, etc., are under progress.

The major challenges associated with scaling this monitoring technique are as follows.

  1. Requirement of an efficient monitoring system within the ULB
  2. Highly priced system: Possibility of vehicle owners with more number of vehicles refusing to adopt
  3. Lower acceptance among de-sludging operators for installation fearing regular monitoring of their activities
  4. Possibility of vehicle operators tampering with device
  5. Difficulty in orientation for desludging operators about proper desludging and decanting due to frequent changes in personnel.

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