Faecal sludge drying beds: increasing drying rates for fuel resource recovery in Sub-Saharan Africa

In urban Sub-Saharan Africa, the collection and transport of faecal sludge (FS) typically ends up with FS directly dumped into the urban environment, as safe treatment and disposal options are too expensive or non-existent. Resource recovery from FS treatment, such as dried FS as an industrial fuel, could provide a ﬁ nancial incentive to increase access to FS management services. In Dakar, Senegal, enhanced drying to reduce the footprint of drying beds for fuel production was evaluated. Greenhouses did not increase drying rates over uncovered beds, however, daily mixing of FS on the surface of the beds resulted in a 6 day reduction to achieve 90% total solids (TS). FS was dried to 90% TS in 2 weeks for loading rates of 100 kg TS/m 2 *year, and 3 weeks for 150 kg TS/m 2 *year. The results indicate that with simple but innovative adaptations, footprints of treatment plants could be reduced and/or treatment capacities increased by 20%. FS can be adequately dried in Dakar to produce fuel, meaning 8.25 tons of dried FS could currently be produced daily, contributing 31,403 GJ/year fuel to industries. In addition, this ﬁ nancial incentive could reduce FS that is currently discharged untreated to the environment, and provide an additional 116,705 GJ/year.


INTRODUCTION
Worldwide, the sanitation needs of 2.7 billion (10 9 ) people are met by onsite sanitation technologies such as septic tanks and pit latrines (Strande ). Faecal sludge (FS) is the raw or partially digested, semisolid or slurry, resulting from collection, storage and treatment of blackwater and excreta from onsite technologies, with or without greywater (Strande ). Historically, onsite technologies were only considered as temporary solutions for urban areas, but the reality is they exist in great numbers, are more affordable than sewer-based solutions, and will be required to cope with rapid urbanization in low-income countries (Dodane The biological and energetic potential of FS should be viewed as resources for urban development instead of only a disposal problem, and could provide financial incentives to enhance FS management services (Strande ).
Resource recovery options in Sub-Saharan Africa include solid fuel for combustion, biogas from anaerobic digestion, protein for animal feed, a component in building materials, and soil conditioner (Diener et al. ). The value of FS treatment products varies depending on the local market, however, in Sub-Saharan Africa energy recovery appears to have the greatest financial potential (Diener et al. ).
In contrast to wastewater sludge, information on the use of dried FS as a fuel is lacking (Werther ). FS has an average calorific value of 17.3 MJ/kg dry solids, which is comparable to other commonly used biofuels (Muspratt et al. ). However, reported concentrations of total solids (TS) in FS of 8.9-58 g/l indicate the need for costeffective drying methods to achieve the 90% TS required Drying beds are one of the most commonly employed technologies for sludge dewatering (Tchobanoglous et al. ). They are appropriate technologies for low-income countries, as they have low-operational requirements, and low-capital and operating costs; however, a major drawback is the required footprint. For example, 0.08 m 2 /capita land area was required to achieve 20% TS (Heinss et al. ; Cofie et al. ). The objective of this study was to investigate whether greenhouses or mixing FS on beds could increase drying rates and/or reduce the required footprint, and produce a 90% TS. Characteristics including energy potential and pathogen concentration were also evaluated to determine the viability of energy recovery.

Study area
In Dakar, Senegal, the sanitation needs of 60% of the population, or approximately 1.5 million people, are met by onsite sanitation technologies, which are mostly septic tanks. Currently, 1,500 m 3 FS are delivered daily to three operating faecal sludge treatment plants (FSTP), while it is estimated four times that amount is dumped directly into the urban environment (Cabinet EDE, H  O Engineering ). The process flow consists of screening, settlingthickening and drying beds (BMGF ). Dakar is semiarid with distinct dry and rainy seasons from November to May and from July to October, respectively. The mean annual rainfall is 514 mm and average temperatures range from a low of between 18 and 25 W C to a high between 25 and 31 W C (World Meterological Organization ).

Research facility
This research was conducted over a period of 9 months at a pilot-scale facility at Cambérène Wastewater and FSTP in Dakar. The facility consists of two 17.5 m 3 settling-thickening tanks followed by 12 2 m × 2 m drying beds, as illustrated in Figure 1. The beds have a 10 cm coarse gravel (7-15 mm), 10 cm fine gravel (3-7 mm), and 5 cm sand layer (0.2-0.6 mm). Greenhouses were constructed on the beds, with conventional greenhouse film, that were 1.5 m tall with two openings on the roof for ventilation.

Greenhouses
The results of preliminary experiments with passive ventilation indicated that active ventilation is required for a netdrying benefit with greenhouses. Considerable condensed water vapour was observed on the inside of the walls, which confirmed the need for automated heat vents and/ or ventilators to ensure the removal of moist air (Luboschik

).
As shown in Figure   As shown in Figure 3, ventilated greenhouses did not significantly improve the drying rate during the dry season. The difference in the required time to achieve 90% TS between beds with and without greenhouses was less than 1 day.   speed, and sludge application depth based on initial TS concentrations (Pescod ).

Relevance of study results
As summarized in Table 2, the drying times observed in this study were faster than many previous studies. Pescod () reported drying times of 5-19 days for FS in Bangkok, to achieve 25% TS with an initial concentration of 1.7-6.4%

TS. Strauss et al. () and Cofie et al. () reported similar results for pilot-scale drying beds in Accra and
Kumasi. In Accra, 20% TS was achieved within 10 days with an initial concentration of 1.6-6.7% TS. In Kumasi, 7-8 days were required to achieve 20% TS with an initial concentration of 1.2-5.8% TS. No publications could be found for comparison that evaluated drying times to achieve 90% TS with FS.
Performance of drying beds depends on the desired final TS concentration, the initial TS concentration, characteristics of the sludge, and drainage and evaporation rates

(United States Environmental Protection Agency [USEPA]
). Differences between this study and the ones summarized in Table 2 include that the FS in this study was thickened for 1 week prior to application to drying beds, which increased the initial TS concentration. The drying beds in this study had a sand layer of 5 cm, compared to