Improper septage management can become a significant burden on the environment and the water resources, which in turn can have a negative impact on human health. Having improved access to sanitation facilities is one of the significant objectives of the United Nations’ Millennium Development Goals (MDGs). If excreta is not managed properly there is a high risk that it will pollute the water, threatening progress towards the MDG of providing access to safe drinking water.
Though septage management is a challenge, it can be converted into an opportunity. Septage contains of high nutrients compared to many other urban waste sources. Our research explores the safe resource recovery and reuse (RRR) potential of septage produced from single pit latrines under the BRAC WASH 1 programme. It should be noted that the sludge originating from double pit latrines and septic tanks relatively stabilized. However, with regards to septage that is obtained from single pit latrines the risk factor is even greater.
One of the key considerations to be taken into account when developing a fecal-sludge treatment process is that, it is applicable to a range of fecal sludge types and is not unduly affected by the characteristics of the feed fecal sludge used. It is imperative, therefore, to ensure that the feed fecal sludge used in the pilot scale drying and co-composting process is representative of the potential feed fecal sludge types that may be encountered in Bangladesh. For example, it is anticipated that the presence of high amount of water during the rainy season and high concentration of salt in some areas may affect the drying and co-composting process, respectively.
Under this research, septage drying strategies such as using filtration sand drying beds (covered/uncovered) for very moist septage, and the effectiveness of the bio-drying (bio-thermal energy) methods for relatively dry septage expected during the dry season are focused upon. The processed (filtered/dried) fecal sludge is mixed with a range of different organic waste materials and co-composted to optimize the composting process.
The produced fecal sludge compost will be upgraded to a high-nutrient fertilizer by blending it with additional nutrient resources. Thereafter, pelletizing activities will be conducted in order to increase the performance of the product and also improve transportability. Evaluation of these products will be done by carrying out a series of agronomic trials using a suitable plant. At this stage it is anticipated that this plant will be a short-cycle crop. However, before trials start, an evaluation of plants will be carried out to determine the type of plant that is most suitable to Bangladesh.
The output from this work will provide recommendations and guidelines on how to dry fecal sludge and conduct co-composting under conditions suitable to Bangladesh, and to produce a highly desirable product, and also to make an evaluation of the performance of the compost using agronomic trials.
Focusing the objectives (optimizing drying process of fecal sludge from single pit latrines in the context of Bangladesh and co-composting the dry fecal sludge of Task-5), a pilot project has been initiated at Purbapara, Gazipur, Bangladesh which is 50 km from the city center of Dhaka.
Drying of Fecal Sludge
At the project site, two identical sand drying beds of concrete structure have been constructed of which each bed has a drying surface area of 12 m2. These beds have been designed for a maximum solid loading rate of 450 kg/m2/yr. The primary intention of these drying beds is to drain the fecal sludge of (FS) before co-composting. Meanwhile, the drying of FS has been carried out under direct sunlight during the dry season (February-March, 2014) to observe the drying phenomena. It has been observed that the designed drying beds are capable of reducing initial moisture content of FS by around 40% within a 15-day drying cycle and around 50% within 30-day drying cycle under open sunlight with a reduction of 90% E. Coli levels. It is noted that the percolate from each of the drying beds does not contain a single helminth egg means it is a very efficient filtration system. Unlike septage, FS from a matured single pit (2 years old) contains less water, which means little hassle in managing percolate from the sand drying bed after effective filtration and post-treatment.
Figure 2. Construction of sand drying bed
Figure 3: Transportation of FS to the project site
Figure 4: Charging of FS on the drying bed
Figure 4. Drying of FS from single pit latrine
Figure 5. Percolate storage
Compost shed and roof preparation
Around 600 ft2 of compost floor was prepared under existing shed for six windrows and two bio-drying beds. Figure 6 shows the civil construction phase of the compost floor.
Figure 6. Floor preparation under compost shed
Co-composting of sand-bed dried fecal sludge
Rice crop residue and cow dung constitute a major portion of traditional biomass in rural areas of Bangladesh. Sawdust is also available both in urban and rural areas. Besides, plenty of organic solid waste generates everyday in 522 cities in Bangladesh. Therefore, these raw materials were selected for co-composting materials for FS. Three windrows (Ws) were made from dry FS and different bulking materials on 11th March 2014 which are shown below:
W-1: Materials used (Dry basis composition)
25% DFS, 25% Cow Dung, 50% Rice Husk
W-2 : Materials used (Dry basis composition)
25% DFS, 25% Cow dung, 30% Rice Straw, 20% Rice Husk
W-3: Materials used (Dry basis composition)
25% DFS, 30% MOW, 45% Rice Husk
Figure 7 shows different bulking materials used for co-composting
Figure 7. Bulking materials for co-composting
Windrows made from sand bed dried FS are shown in figure 8.
Just after a single day, average temperatures of all the three windrows crossed 50oC and maintained an average temperature above 55oC for 26 consecutive days. During this time period about half of the time (13 days) the windrows enjoyed an average temperature ≥ 60oC. Average temperature of the windrows as on June 03, 2014 is around 41oC. Above average windrow temperature 50oC , turning frequency was twice-a-week for these three windrows and below 50oC turning frequency has been reduced to once a week. Turning of windrows is shown in figure 9.
Figure 9. Turning of windrow
At the very initial stage of these windrows, it was observed that the average temperature of the windrows came down at night compared to daytime average temperature. This might be due to reduced ambient temperature at night which might initiate greater heat loss from the windrows to the surrounding environment. To offset this undue heat loss and temperature drop at night, a breathable thin layer of jute fabric was used to cover the windrows at night and during day time the windrows were unveiled. This worked well, with a twofold benefit: i) It reduces heat loss that helps the windrow to maintain temperatures similar to those in day time, ii) evaporated water vapor condenses on the fabric surface and diffuses back to the windrows which reduces the water required for maintaining the optimum moisture content of the windrow.
Through this procedure all the three windrows were at “safe zone” and essentially free from indicative bacteria E. coli.
Bio-drying of FS
Bio-drying of fecal sludge from single pit latrines was also performed. High initial moisture content of FS (83-84%) is a real problem for the bio-drying process. Since bio-drying is an aerobic process, it requires a moisture adjustment at around 60 to 65% to initiate aerobic microbial activity. Therefore, very moist FS was dried on a sand bed through filtration up to a moisture level around 70% and then mixed with saw dust at a desired ratio to bring down the moisture content at a level of 60%. Two identical bio-drying beds made of brick were constructed under the shed each having a dimension of 8’ L X 7’ W X 3’ H. Turning frequency of the bio-drying pile was maintained in such a way as to maintain the pile temperature in the range of 40 to 45oC to avoid excessive degradation of the organic matters. It was evident that it required turning of the windrows twice-a-day to maintain the above-mentioned temperature range. The bio-drying piles were built on April 18, 2014 and it took about 18 days to reduce 15% of its initial moisture content. The drying rate was rather slow due to high average ambient temperature (32oC) and relative humidity (61%) during the bio-drying process. Mixing of saw dust with FS (initially filtered on the sand) in the bio-drying bed and bio-dried FS are shown in figure 10 and 11 respectively.
Figure 10. Mixing of SD with FS
Figure 11. Bio-dried FS
It was found that bio-drying of FS with high moisture content is not feasible for drying FS for co-composting. It requires additional filtration to reduce moisture content for bio-drying with bulking materials. It is not always possible to maintain optimum C/N ratio of the bio-drying pile with high C/N bulking materials at desired initial moisture content to initiate the aerobic drying process. However, it requires frequent manual turning to maintain the temperature for avoiding excessive degradation of organic materials. It is possible to reduce the conductivity of highly saline FS through filtration in sand bed which is impossible if the highly saline FS is not filtered before bio-drying.
Co-composting of Bio-dried FS
However, with the bio-dried fecal sludge, another three windrows were made on May 06, 2014. Windrows were characterized as
W-4: Materials used (Dry basis composition)
50% DFS, 50% SD
W-5 Materials used (Dry basis composition)
25% DFS, 25% MSW, 50% SD
W-6 Materials used (Dry basis composition)
25% DFS, 40% Cow Dung, 35% SD
Figure 12. Left side of the picture shows W-4, W-5 and W-6
Just on the initial day, average temperatures of all the three windrows crossed 50oC and as on June 4, 2014 average temperature of these three windrows was around 55oC. Above average windrow temperatures of 50oC, turning frequency twice-a-week is being maintained.
Modification of Sand Drying Bed for Rainy Season
Very recently these sand drying beds have been modified to conduct and study the drying phenomena of FS during the rainy season in Bangladesh. For this, one of the drying beds has been roofed with corrugated iron sheets with no fence and the other bed has been roofed with transparent poly-carbonate corrugated sheeting and fenced with galvanized corrugated iron sheeting that will serve as cooling pad. This confined arrangement of the drying bed is thought to be operated for rapid drying even during the rainy season. It is thought that FS (gray to black color) will act as black body which absorbs heat, vaporizes water and increases the temperature and humidity inside the confined air volume. As the temperature differential between the confined air and ambient air becomes high, there is every possibility for confined air to dissipate heat energy into the air through the corrugated polycarbonate and iron sheets. Since thermal conductivity of corrugated iron sheets is higher than that of polycarbonate sheets much of the condensation will occur on the surface of the corrugated iron sheet, which results in dehumidification of the confined air. The faster the condensation process the faster will be the evaporation from FS, which makes FS dry within a short time. If this drying process is successful, it will be used throughout the whole year irrespective of seasonal variations.
Modification steps of the existing sand drying beds are shown in figure 13.
Figure 13. Modification of existing sand drying beds for FS drying trial during rainy season