Poor sanitation is a major cause of poverty and some preventable diseases like diarrhoea, intestinal worms and dysentery (WHO 2017). The lowest sanitation coverage is concentrated mainly in countries in Sub-Sahara Africa and Southern Asia (Deshpande et al. 2020). Populations living in urban centres in many developing countries lack household toilets and the only toilet facility is the shared toilet systems (Peprah et al. 2015) meant for public use – markets, transport stations and schools. Ghana’s sanitation coverage, as of 2017–2018 was 21%; which is below the millennium development goal (MDG) target of 54% (Appiah-Effah et al. 2019, Ghana Statistical Service 2018). On-site sanitation technology in Ghana serves 85% of the population (Rose et al. 2015), of which 68.2% use public toilets and 19.3% practice open defecation (Ghana Statistical Service 2016). For dry sanitation toilets, 29% of the population use pit or ventilated improved pit (VIP) (Ghana Statistical Service 2016) whereas users of water closets account for 15.4% of the population. Dry on-site sanitation technologies are relatively cheaper, require little or no water and occupy relatively less land, but the facility is usually characterized by malodour and insect nuisance (Obeng et al. 2016), which can discourage users of the facility from patronizing it and rather resort to open defecation (Duke Sanitation Solutions 2016, Obeng et al. 2015).
Malodours are normally indicators that protect humans from potential illness caused by infection through contaminated food and matter (St Croix Sensory 2005). The odours are generally attributed to the evolution of different smell-causing substances (volatile compounds) arising from the anaerobic decomposition of the faecal matter (Mara 1984, Nakagiri et al. 2015, Wagner et al. 1958). The type of volatile compound evolved is also dependent on the age of faecal matter where fresh ones have rancid odour whereas aged ones in latrines have sewage, malodorous smell, like rotten egg due to the anaerobic decomposition process (Nakagiri et al. 2015). The rancid and cheesy odour in dry latrines is associated with the evolution of volatile compounds such as phenylacetic acids, butyric, isovaleric, 2-methyl butyric, isobutyric valeric and hexanoic. Sewage, rotten egg and rotten vegetable odours have been attributed to sulphur-based volatile compounds – arising from protein degradation and activities of sulphur-reducing bacteria (Oh et al. 2000, Persson et al. 1990) – such as dimethyl trisulphide, hydrogen sulphide (H2S), dimethyl disulphide, methyl mercaptan and dimethyl sulphide. Also, skatole, p-cresol, some carboxylic acids, phenol and indole have been associated with farmyard manure-like odours (Lin et al. 2013, Moore et al. 1987, Nakagiri et al. 2015, Sato et al. 2002). That notwithstanding, the sulphur and nitrogen-containing compounds, particularly ammonia (NH3) and H2S, are of particular importance since they are the primary odorous substances and possess a distinctive odour that is readily noticeable even in small concentrations [H2S = 0.005 ppm (Atia et al. 2004); NH3 = 0.05 ppm (van Thriel et al. 2006)] (Ying et al. 2012). In fact, a positive correlation between H2S concentration and user perception of odour have been recorded; otherwise for NH3 concentration (Obeng et al. 2016). It is, therefore, no wonder that recommendations about the odour-irritation threshold concentrations of the NH3 and H2S have been enacted and thus, respectively, ranges from 4 to 8 ppm and from 2.5 to 20 ppm (Schiffman and Williams 2005). Also, to avoid complaints from the facility users, it is recommended that the concentration of the H2S should not exceed (0.05 ppm) 7 µg/m3 for a 30-minute averaging period (WHO 2000).
Many approaches such as pH alteration, specialized/engineered microorganisms usage, microbial growth inhibition and use of biological covers (biocovers) have been investigated to address the malodorous nuances associated with the usage of dry sanitation toilets (Arogo et al. 2001, Ndegwa et al. 2008). Biocovers, in particular, are materials that serve as covers over faecal matter to help suppress gas emissions by either physically limiting the emissions of gases from the surface of the faecal matter or creating a biologically active zone on the top of the biocovers where gases are aerobically decomposed by microorganisms (Atia et al. 2004). Biocovers may be impermeable or permeable to gases depending on the material used. Impermeable biocovers only trap the odorous substances and are therefore normally used in conjunction with other treatment methods such as biofilters or scrubbers (Ndegwa et al. 2008). Examples include glued layers of polyethylene film and tarpaulin (Funk et al. 2004). Permeable biocovers, however, act like biofilters and can trap and subsequently biotransform odorous gases to harmless or less odorous forms (Ndegwa et al. 2008). For instance, H2S evolution can be inhibited via components in the biocovers reacting with and converting the dissolved sulphide into other intermediate forms, or inert metallic sulphides, or bisulphide ions (Atia et al. 2004). The performance of the biocovers is therefore dependent on their physicochemical properties – surface area, porosity, mineral composition, organic matter content and pH amongst many others (He et al. 2011) – and thickness of the applied biocover layer (Atia et al. 2004). It is known that NH3 evolution can be attenuated in low pH (Ndegwa et al. 2008). High organic matter, surface area, porosity and cation exchange capacities (CEC) of waste biocover soil were effective at mitigating H2S evolution via adsorption, principally (He et al. 2011). It is, therefore, no wonder that permeable biocovers with the aforementioned properties have been investigated. These include waste lignocellulosic agricultural biomass – mulched wood material (Hurst et al. 2005), cornstalks, straws and wood chip (Guarino et al. 2006) –, geotextile fabrics (Bicudo et al. 2004), polystyrene foams (Miner and Suh 1997), silicates or clays (Balsari et al. 2006), fly ash (Hurst et al. 2005) and combinations of zeolite and agricultural biomass (Miner and Pan 1995). Lignocellulosic agricultural biomass, for instance, generally contains high organic matter content, helpful as food for microorganisms. Ash is known to contain inorganic constituents especially of alkali and alkaline-earth metals, which renders it highly basic (Sewu et al. 2017) and as such can abate H2S release via its adsorption capability and potential acid-base reactions with acidic H2S (Ducom et al. 2009) when used as a biocover. Another potential biocover seldom researched is biochar.
Biochar, the carbonaceous product of biomass pyrolysis, has gained much popularity as a promising material for different high-value applications such as waste management and climate change mitigation tool (Sewu et al. 2019, Shaheen et al. 2019). Biomass for biochar production can be sourced from locally available agricultural wastes, making it cheap and conducive to the environment (Tan et al. 2017, Tran et al. 2018, Tran et al. 2017). It is hypothesized that owing to the unique physical and chemical characteristics, such as large specific surface area, high porosity, moderate CEC, abundant surface functionality, and excellent thermal, mechanical and chemical stability (Tran et al. 2016, Weber and Quicker 2018), biochar may serve as a potentially excellent biocover to mitigate odour release from dry sanitation toilets.
This study, therefore, investigates the application of biomass, ash and biochar as potential biocovers to attenuate odour evolution from fresh FS generated in dry sanitation toilets. The specific objectives of this study are to (1) determine the on-site odour levels of dry-sanitation public toilets using NH3 and H2S, as the primary odour-indicators; (2) acquire, produce and characterize different materials as potential biocovers for odour mitigation; and (3) evaluate the odour-suppression or odour-removal efficiencies of the as-produced biocovers on fresh human excreta samples from the dry sanitation toilets in a laboratory setting.