Fuel potential of faecal sludge: calori ﬁ c value results from Uganda, Ghana and Senegal

This research tested the viability of using faecal sludge (FS) as solid fuel – an end use that could unlock an environmentally and ﬁ nancially bene ﬁ cial replacement for disposal-oriented FS management, while replacing fossil energy. FS samples were collected from pit latrines, septic tanks, drying beds and stabilization ponds in three cities, Kumasi, Dakar and Kampala. For each sample, the average calori ﬁ c value, solids and water content, and their variation with source and age were determined. The average calori ﬁ c value of untreated FS across the three cities was 17.3 MJ/kg total solids (TS), which compares well with other biomass fuels. The age of FS did not affect its calori ﬁ c value, nor did the reduction in chemical oxygen demand (COD) that occurred while it was in drying beds. The TS content of FS depended on its source but ranged from 1 to 6% for sludge from septic tanks and pit latrines, respectively. Harnessing net energy from FS requires partial drying. The results indicate that suf ﬁ cient drying occurs within two weeks in open-air drying beds, or in a matter of days with simple drying bed innovations.


INTRODUCTION
Provision of adequate, safe and sustainable sanitation coverage is an ever-increasing challenge facing urban areas in sub-Saharan Africa. As urban migration increases, available latrines are receiving more and more users. However, while latrine coverage may remain insufficient, the safe collection and treatment of faecal sludge (FS) from systems that do exist is arguably the weakest link in the sanitation value chain. An estimated 2.4 billion users of on-site sanitation systems generate FS that goes untreated, resulting in pervasive environmental contamination (Koné et al. ). This has led to growing interest in approaches for safe emptying, transport and end use or disposal of FS (Montangero & Strauss ).
This research evaluates a new FS management solution: converting FS to solid fuel for use in industrial kilns and boilers.  (Okuno & Yamada ). In the cement industry, the ash is incorporated into clinker, a pre-cursor to Portland cement, offsetting the need for raw materials like clay, and transforming sludge into a zero-waste feedstock.
The sewage sludge used for fuel is primarily derived from activated sludge systems and may have undergone anaerobic digestion. In either case, stabilization has occurred, which significantly reduces the calorific value of the sludge (Tchobanoglous et  Pre-drying the sludge is a requirement for using it as fuel.
Even if technically viable, the commercial or large-scale viability will be dependent upon identifying cost-effective ways to dry FS. This study serves as a starting point for exploring the use of FS as fuel and the potential for increasing drying rates while minimizing costs. If achieved, industrial fuel is an end use that could unlock an environmentally and financially beneficial replacement for costly, disposal-oriented FS management solutions.

Kumasi, Ghana
With a population of approximately 1.5 million people,

Sampling process
Sampling plans were tailored to the three cities and designed to capture the range of archetypical sanitation systems in each region.
In Kumasi, untreated samples were taken from public and private pit latrines, and septic tanks. These samples were retrieved directly from septic tank emptiers and composite samples were made from three 1-L samples collected at the start, midpoint and end of the truck emptying. Samples were also taken from four different locations in four anaerobic ponds at the Dompoase treatment plant. The process was repeated for five consecutive weeks. In addition, samples were also collected from an offline anaerobic pond that had last been fed six months prior to sampling (Table 1) In Dakar, samples were collected from septic tanks and a fully lined pit latrine. These samples were retrieved directly from cesspit emptiers and again composite samples were made as described above. Samples were also collected from seven open-air drying beds at the Niayes FSTP for three consecutive weeks (

Variation on the basis of FS source and treatment
The  There was no correlation found between the age of FS from different sources and its calorific value (Figure 3).
This combination of findings suggests that the calorific value comes from recalcitrant organic molecules that degrade very slowly, and thus are potentially not oxidized during the breakdown of COD. This is similar to coal, where the chemical bonds are not very susceptible to microbial attack (Kögel-Knabner ).

Harnessing energy from faecal sludge solids
Total solids content in raw faecal sludge The technical and financial viability of turning FS to solid fuel is dependent on both the energy value embodied in the solids and the total solids (TS) content of a given volume of FS. Total solids were found to vary by source.
The TS of FS from unlined pit latrines was 6% of wet weighthigher than that of fully lined pit latrines and septic tanks, which averaged 2.7 and 1% of wet weight, respectively. The higher TS in unlined pits is attributed to liquid leaching to the soil. Conversely, septic tanksdesigned more like holding tanks in many dense urban areashad the lowest solids content because of the  Faso had a concentration of 12,000 (1.2% TS) and 19,000 (1.9% TS), respectively (Koné & Strauss ).

Sludge drying
With TS averaging just 6%, an efficient and effective means of drying the FS is critical. Drying bed experiments in Kampala yielded a solids content of about 30% after two weeks in the beds. In Senegal, sludge averaged 37% solids after one week, 49% solids after two weeks, and 58% solids after three weeks.
In Ghana, FS at all three depths ( . The more commensurate results between the latter study and our own suggests that shallower initial depths improve drying rates.

Rapid prototyping to increase drying speed
Adding polymer and enabling the free water to drain was quite effective at reducing moisture content. This increased the solids content from 5 to 16%, removing 72% of the total water in the sludge. Commercial greenhouse sludge drying beds are capable of providing evaporating rates between 1 and 3.3 kg water/m 2 /day (Bux & Bauman ). By removing 72% of the water through polymerization first, the land requirement is likewise reduced by 72%.
Furthermore, the crude pressing setup removed an additional 46% of the remaining water, increasing the solids content from 16 to 26%. These rapid prototyping results indicate that flocculating with polymer and then pressing out the water would reduce land requirements for drying beds by 85% (Table 2). However, more rigorous experimentation is needed to confirm this result and a cost analysis must be carried out to assess the trade-off between faster drying rates but higher costs of purchasing the polymer.
The combination of polymerization, pressing and drying in thin 1-2 cm layers dried the sludge substantially faster than the first set of drying bed experiments in Ghana. In the previous experiment, it took 15 days for FS to reach approximately 60% solids by weight, whereas 60% solids were achieved in 2.5 days with the rapid prototyping interventions.
One other condition that was briefly tested was drying polymerized sludge on slanted concrete, which allowed water to drain off rather than filter down into the bed. The slanted bed was nearly as dry as the pressed sludge at the end of day 4, suggesting that a slanted concrete surface    This study identified several possible drying bed enhancements. A transparent roof is a simple design intervention to speed drying and thus decrease the land area needed for drying beds. Other promising interventions include flocculating the sludge with polymer and mechanical pressing, which have potential to reduce land requirements by as much as 86%. Furthermore, designing to optimize drainage is key in humid climates, as this is the driving mechanism for water removal. Ultimately, the end value of the fuel must be weighed against the cost of drying to determine the optimal processing conditions, but additional energy for drying FS can be harnessed with minimal recurring costs.