Dear Elizabeth,

Thanks a lot for your question.

The research project aims at understanding the science behind the LaDePa process, with the goal to improve the process. The improvement of the process could be expected to improve the performance of the process, subsequently leading to the decrease of the costs (particularly operating costs, not necessarily the capital cost). The decrease of the operating costs of the LaDePa (that are not low) will make it more attractive, and this can be expected to support its commercialisation. So, I would say that the Master thesis has an indirect impact on the commercialization of the product (as far as the conclusions from the investigation are considered and the recommendations are applied in the modifications of the process).
I think that the LaDePa is a new process with great potential for faecal sludge treatment. However, as emergent technology, it faces issues of acceptance and implantation, but I believe that these can be overcome with research and innovations. For the moment, the costs of the process are relatively high, so it is a priority to reduce the costs or find more cost-effective versions of the LaDePa. This is critical for municipalities or utilities to adopt this technology. I think that the survival of the technology will depend on this.

On my point of view, one of the problems in the development of new technologies is sometimes the lack of understanding of the science behind it (among the typical ones, as lack of capital resources and funding, lack of acceptability or awareness leading to a low adoption of a new technology, etc...). “A week in the library can save you several weeks in the field”, as it is said. I think that this statement should be applied more often (but we don’t have to go the extreme side and overthink the design of a technology) . I don’t think that the problem with LaDePa is not due to a geographical issue. Actually, it has been designed for the faecal sludge from eThekwini municipality, with an average moisture content of 80%. In India, faecal sludge is much more liquid, so the feeding in the LaDePa will be problematic. A previous dewatering or adaptation of the technology to a more liquid sludge will have to be put in place before implementing the technology in India.

I hope that my answers were helpful.

Kindly,
Santiago

Dear Hanjo,

Thanks a lot for your valuable comments.

I hope that the Master thesis will be useful for the improvement of LaDePa process (and for the development of further infrared radiation technologies). I agree with you that this technology deserves more attention and I hope that the Master will be a good platform for its dissemination.

In our study, the thermal diffusivity of the dried sludge is slightly higher than the wet sludge, which means that heat is better conducted in the dried material than in the wet one. So the heat transfer does not slow down when the material is dried. It is just that the moisture evaporation takes too much energy, which does not allow a considerable rise of temperature of the wet material. In fact, drying of a wet solid occurs at the wet-bulb temperature, which depends on the drying conditions (but does not exceed 100°C, which is the temperature of pure water evaporation at 1 atm). Naturally the surface of the material dries faster than the core. When this happens, the temperature of the surface increases until being in thermal equilibrium with the environment (for example, in an environment at 150°C, the dried surface will attain the same temperature), whereas the core will remain at the wet-bulb temperature. Only when this section will be dried, its temperature could rise.
In the case of your solar drying oven in Namibia, I guess that the wet-bulb temperature is 45°C. When the sludge is dried, its temperature should be much higher, maybe not 120°C because there have to be some heat losses. For sure if you reduce the size of your material, you will reduce the heat transfer time, consequently increasing the drying rate and leading to faster drying of the core.

Under typical operating conditions, the LaDePa treats 1000 kg/h of faecal sludge (15% detritus) and dries the product from 80 to 15 % moisture content. This should lead to an energy consumption of approximately 1500 MJ/h. The output energy in the pellets is 4335 MJ/h (considering a production of 300 kg/h of pellets at 15% moisture content, with a higher heating value of 17 MJ/ kg). So, we can see that the use of the pellets as a biofuel would lead to a net positive energy balance. Unfortunately, this calculation was not included in the thesis (but it should!).

Please don’t hesitate to pose further questions.

Kindly,
Santiago

Hi Santiago,

thank you for making your thesis public here. I consider it a technology which does not attract the interest it may eventually deserve. Maybe that changes with your thesis.

As drying progresses, the difference in temperature between the surface and the core increases. A low moisture content also causes lower heat transfer from the pellet’s surface to the core. An interesting finding from this study is that core temperature rises and remains constant at 80 °C for the emitter intensity that was used.

Do I understand your observation right that as the surface of the pellets dries, the heat transfer and thus the increase of core temperature slows down and eventually comes to a standstill which affects further drying of the core?

If so, it coincides with our observations in a solar oven which a colleague built in Namibia and where we observed with temperatures of 120 ⁰C in the container above the FS, the temperature in the FS would not go beyond 45⁰C. This explains the use of the pelletiser: the FS is converted into small bodies which have a large surface compared to the volume ensuring a better drying of the core. Thus the energy needed to dry a certain quantity of FS should be lower than if the FS is in more compact form.

Physical analysis shows that the calorific value of dried pellets is above 10 MJ / kg wet solid at a moisture content below 35 % (wet basis). This is close to the range of most biomass materials but 2 to 3 times lower than that of fossil fuels.

Does your thesis deals with the questions whether the energy needed to produce the dried pellets (as bio-fuel) may be more than the calorific value of the pellets. If so, it wouldn't make sense to dry them for that purpose, or? Sorry, I haven't read your thesis yet. If it deals with this question, please refer me to the right chapter.

Thanks and ciao
Hajo

The LaDePa machine that is central to this research topic has been discussed on the forum previously here (4 pages of discussion):
www.forum.susana.org/280-faecal-sludge-t...-in-ethekwini-durban

It's a machine that makes pellets from faecal sludge using infrared radiation.
As you can read in the other thread, it is a very promising process and seems to work well. Nevertheless, once again, the problem seems to be how to scale this up and to find investors to build or operate such LaDePa machines.

Santiago: I am just wondering if the research that you supervised and which is described in the MSc thesis will help in the commercialisation step of this process in any way? E.g. did you find anything in addition that could help market this process to municipalities or utilities? What do you see as the biggest hurdles in uptake of this technology?

There is by the way a new discussion thread about innovation in the WASH sector where people are also discussing what stops innovation: www.forum.susana.org/161-sanitation-as-a...susana-india-chapter Perhaps the LaDePa process could be a good example? Or perhaps it might be more successful in India than in South Africa? Just speculating; would be keen to hear your thoughts.

Elisabeth

Title of the thesis: Drying and pasteurisation of VIP latrine faecal sludge using a bench scale medium infrared machine

Date of publication: 2017

Author: Simon Mirara

Affiliation: Pollution Research Group, University of KwaZulu-Natal

Supervisors: Dr. Santiago Septien Stringel; MScEng. Anusha Singh; Pr. Chris Buckley

Abstract: The main challenge with Ventilated Improved Pit (VIP) latrines is that they eventually fill up. Solutions therefore have to be sought with the common practice being to cover up the hole and dig a new pit. This however is not sustainable especially in peri-urban areas due to lack of space for the new pits. The other option is to empty the pits and dispose of the sludge.
After the creation of the eThekwini municipality in 1999, over 60 000 ventilated improved pit (VIP) latrines were inherited from the incorporated local entities. In 2009, the municipality set out to empty over 35 000 VIP latrines, which were already full. One of the challenges from this operation was the disposal of the sludge in an environmentally safe way. The initial idea was to dispose of the sludge in wastewater treatment plants but this caused overloading of the treatment plants and so the municipality had to seek alternatives. This led to the concept of the Latrine Dehydration and Pasteurization (LaDePa) machine. This machine is used to process the emptied sludge into dry, pasteurized, pellets, which can be used in agricultural crop production, as a fertilizer or a soil conditioner. The resultant pellets could also be combusted as a fuel. The LaDePa machine uses a screw extruder to form “spaghetti-like” pellets, which are subsequently dried and pasteurised by means of medium infrared (MIR) radiation lamps. Air that flows over the pellets facilitates removal of moisture from the surface of pellets and away from the drying chamber.
Although this machine has been operational, there has been no scientific data on the phenomenological process and on how it affects the characteristics of raw sludge and resultant pellets. An important aspect in processing the pellets in the machine is their processing temperature. At an average drying temperature of 136.5 °C, it was found that core and surface temperature are only isothermal in the initial stages of drying. As drying progresses, the difference in temperature between the surface and the core increases. A low moisture content also causes lower heat transfer from the pellet’s surface to the core. An interesting finding from this study is that core temperature rises and remains constant at 80 °C for the emitter intensity that was used.
Among the parameters, investigated emitter intensity was found to be the most significant in drying and pasteurisation. At a drying temperature of approximately 214 °C, moisture is reduced by 41.5 % (wet basis) in only 8 min while all Ascaris eggs were destroyed. The highest drying temperature investigated was found to be the most efficient in terms of energy consumed. However, fast drying of pellets causes a high thermal gradient between the pellet core and surface, which leads to charring of the pellets. A decrease in height of emitters above the porous steel belt leads to a higher drying rate when emitter power supply and other parameters are kept constant. Reduced airflow leads to decreased drying rate. Small diameter pellets dry out faster and power consumption in their drying is the most efficient compared to larger pellets.
Biological analysis shows that the LaDePa process is very efficient at deactivating helminths. Ascaris eggs, which were the most persistent. They were deactivated either immediately or upon storage when processed at a residence time of more than 8 min and at a temperature about 87 ° C or higher.
The concentration of P, K, Mg and Ca, in raw sludge and the resultant pellets was found to increase with drying. However, the actual content is not affected by drying. For dry pellets to supply P, K, Mg and Ca at the same level as organic fertilizer, the application rate should be 10 times more. The application rate of dry pellets should be, however, about 6 times more than most manure sources. The LaDePa process has a negative effect as it reduces the total soluble N content in sludge. This is as depicted by reduced ammonium, nitrates and nitrites in the resultant pellets. Physical analysis shows that the calorific value of dried pellets is above 10 MJ / kg wet solid at a moisture content below 35 % (wet basis). This is close to the range of most biomass materials but 2 to 3 times lower than that of fossil fuels. Thermal conductivity and heat capacity of dried pellets decreases with increased drying which makes pellets less suitable as a fuel at very low moisture content.
In summary, the LaDePa process was found to be a satisfactory process in drying and pasteurisation of faecal sludge of VIP origin. To ensure that the process is efficient the emitter intensity should be kept relatively high, there should be a high airflow around the pellets, height of emitter above the belt should be low and pellet diameter should be small.

Keywords: faecal sludge; infrared; LaDePa; drying; pasteurization; reuse.