Mechanical Dewatering and Drying System (MDDS) for Faecal Sludge Treatment

This option to treat faecal sludge is based on mechanical dewatering and drying. Faecal sludge originating from septic tanks is collected and conveyed to the unit (e.g.: by trucks). The treatment process starts with collecting the sludge in a sump where it is homogenized with 10% lime solution to increase its filterability. Adding lime also suppresses the odour of the sludge. After lime treatment, it is pumped to the dewatering unit. At this point it is in the form of slurry of around 97% moisture content. The dewatering unit comprises of a rotary drum filter and a screw press. The dewatering unit reduces the moisture content of sludge to around 40%. The dewatered sludge then passes through a rotary dryer, which further reduces the moisture content equal to and below 10%. The aspired final product are dried biosolids, which are free of pathogens and can be used as a soil conditioner or manure.

The filtrate obtained either needs transport to a nearby sewage treatment plant or appropriate treatment on-site.

The schematic diagram of MDDS based faecal sludge treatment plant is presented in Fig. 1.

Figure 1. Schematic Flow diagram of MDDS based faecal sludge treatment. Source: POPHALI (n.d.)

In high flooding areas, it is advisable to install MDDS units at some elevation - not only because of its electro-mechanical components, but also for general health safety.

This technology is compact and can be installed and commissioned in a small area. For example, a plant of a capacity of 25m3/d is easily accommodated in an area of 45m2.

Since the entire treatment process is electro-mechanically driven (minimizing the required manpower), continuous power supply is a fundamental requirement for running the plant effectively. Also, the provision of sufficient LPG cylinders for the powering of the rotary dryer is essential for safeguarding its operation.

Other infrastructure requirements are a roof top shade above the units to protect them from direct sunlight and rain. And a sump in the vicinity of the plant for homogenization of sludge with lime.

Attributing to the fact that the entire treatment scheme is electro-mechanical, minimal staff is required to operate the plant.

The most relevant consumables include lime, electricity, and LPG cylinders. Yearly maintenance of units is required by third parties for efficient operation.

According to the authors estimates, a plant of a capacity of 25m3/d would require 1 skilled and 1 unskilled operator who are present during operation times be sufficient to run the plant.

Currently in India, methods which are being used to treat faecal sludge are sludge drying reed beds and anaerobic digestion. At the time this factsheet was compiled, to the knowledge of the authors, this particular set-up of MDDS has not been implemented in India.

Solid and liquid separation of faecal sludge has been used globally for a long period already. Typical methods used for solid-liquid separation are planted drying beds, centrifugation, Imhoff tanks, geobags etc. For sludge treatment, typically composting, vermicomposting, anaerobic digestion, solar drying, shallow trenches etc have been used.

However, the major drawbacks of above-mentioned technologies are their large footprint area and treatment time. Also, whether faecal coliforms are safely removed by the above treatment techniques is still a question.

The information presented in this factsheet is based on a document provided by the authors. It is important to note that the authors did not provide citations for the information presented herein. As a result, we are unable to verify the original sources of the data. This factsheet is intended to offer a summary of the available information, and readers are encouraged to independently verify the details for accuracy.