Drying and pasteurisation of vip latrine faecal sludge using a bench scale medium infrared machine.
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Date
2017
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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.
Description
Master of Science in Chemical Engineering. University of KwaZulu-Natal, Durban 2016.