Improved and promising fecal sludge sanitizing methods: treatment of fecal sludge using resource recovery technologies

The global challenges that face sustainable sanitation services in developing countries are the lack of fecal sludge (FS) management; this is due to the rapid urbanization and population growth as it generates enormous quantities of fecal sludge. The extensive use of unimproved sanitation technologies is one of the main reasons for environmental and public health concerns. In dispersed rural areas, isolated slums or in urban areas where a sewerage system is costly, a decentralized wastewater system can be used. Therefore centralized management of decentralized wastewater systems along with proper institutional framework treatment of fecal sludge can be used to enhance the economies of developing countries from resource recovery. The discovery of new ways to inactivate pathogens contained in human waste is key in improving access to sanitation worldwide and reducing the impact of conventional waste management processes on the environment. The entire FS management system should include on-site sanitary treatment methods, collection, and transportation of FS, treatment facilities as well as resource recovery or disposal of the treated end products. This review paper addresses the hygienization of fecal sludge and improved treatment technologies for safe reuse or disposal of the end products and the significant economic revenues attained from the treatments of fecal sludge.


INTRODUCTION
Sustainable Development Goal number six of the United Nations aims to achieve universal access to 'safely managed' sanitation by 2030. Safely managed sanitation is described as the use of improved facilities with safe disposal in situ or offsite transportation and treatment (Borja-Vega et al. ). Fecal sludge (FS) refers to raw, slurry, or partially digested excreta with or without the combination of gray water that originates from on-site sanitation systems, such as pit latrines, septic tanks, and dry toilets. FS resembles a solid and highly differs in characteristics and consistency (Lindberg & Rost ). FS management (FSM) is challenging due to the lack of accessible sanitation facilities in developing countries (Singh et al. ).
As of April 2020, approximately 93% of the world's population (nearly 7.2 billion people) live in countries with restrictions on movement (Lindberg & Rost ).  (Lalander et al. a, b; Mara ). Given that the demand for improved water supply and sanitation grows globally, treatment technologies that minimize waste and consumption of water and allow water reuse should be considered to achieve SDGs (Gijzen ; Katukiza et al. ).
Human feces are a natural fertilizer that can replace chemical or mineral substances (Factura et al. ; Andreev et al. ). Human feces can play a pivotal role in increasing soil fertility to enhance the crop production due to their nutrient contents (Kimetu et al. ). However, the presence of high concentrations of pathogens and various harmful organisms in feces can affect the soil and crops (Andreev et al. ). Therefore, hygienization of FS is important before using it as a fertilizer to increase the sustainability of soil fertility for agricultural production.
Many technologies are available for the safe management of excreta in the sanitation service chain. Pit latrines, septic tanks, and sewered systems can 'safely manage' excreta, as defined by the SDG. Safe management of household excreta is the containment, collection, and transport of excreta to specified disposal or treatment sites or the safe reuse of excreta depending on local conditions at the household or community level (Anderson et al. ). Figure 1 depicts the service chain of safely managed excreta.
In developing countries, there is a growing interest and awareness of FSM issues which is substantiated through several types of research and projects that have occurred in fecal sludge management. In many developing countries the management of fecal sludge is very poor as it is not properly managed. Several reasons could be addressed for the improper management, perhaps due to the lack of institutional framework, lack of awareness on the effect of poor sanitation, lack of required knowledge to initiate and implement FSM programs, and lack of improper sanitation infrastructures and designing of fecal sludge treatment plants.
This can lead to unsatisfactory operation of on-site sanitation facilities (OSF), overflowing septic tanks and pit latrines as well as unsafe emptying of pit latrines, and discharge of untreated pathogenic fecal sludge into the environment. Therefore this review mainly aims to determine the hygienization of FS, improved treatment technologies for the safe reuse or disposal of FS, and significant economic revenues attained from the treatment of FS.
Globally, the treatment of FS and its benefits in agriculture have attracted substantial interest from researchers.
Nevertheless, many publications have focused on the various uses of FS without considering the problems associated with the collection, transportation, and stabilization processes. In addition, a comprehensive overview of the problems faced by FSMs and new technologies for them is still lacking. This article attempts to fill this gap by reviewing the current advances in research and challenges related to FSM technologies.
The focus is on the problems associated with the collection, transport, and disposal of FS, where the need for improvement is most evident, as well as several treatment processes. There are important points to be considered when selecting treatment technologies; there are different technologies for different treatment purposes, and they can be used alone and/or in combination. There are many factors to consider when selecting the best treatment configurations, including the end-use, treatment goals, potential benefits and limitations, and how to compare costs.

OVERVIEW OF TREATMENT TECHNOLOGIES OF FECAL SLUDGE
Various characteristics of FS make it challenging for treatment. FS must not be discharged into surface water or disposed of in a landfill or treated as wastewater and solid waste due to the presence of excessively high concentrations of contaminants and its high moisture content. Therefore, FS cannot be used in agriculture to enhance production without further treatment. Stabilization of FS must be performed first and its solid and liquid matter must be separated to facilitate treatment (Rashed & Hithnawi ).
The liquid portion of FS can be processed using wastewater treatment technologies, whereas the solid portion is treated to improve its properties either for agriculture reuse or disposal. Available treatment technologies can be used depending on community context and treatment purpose. Properly treated FS can be used efficiently as a sustainable fertilizer in agriculture. Several treatment technologies are used to sanitize the content of pathogenic microorganisms found in FS (Mawioo et al. ).

Intrinsic ammonia
The use of pathogen inactivating action of uncharged ammo-  The high solubility of ammonia (NH 3 ) in water and lipids increases ammonia transport through cell membranes and other cell walls via diffusion. The addition of ammonia will increase the internal pH, disrupt the membrane, and cause the bacterial membrane and cell proteins to degenerate. Hence, the cell of the pathogen will disintegrate and destruct further, ammonia gas rapidly alkalinizes the cytoplasm and causes cell damage. Ammonia treatment Treatment of fecal sludge using lactic acid fermentation

Co-composting
Co-composting is used extensively to process human feces separated from the source (WHO ; Torgbo et al. ).
The partially treated sludge is mixed with organic solid waste fraction after dewatering FS. Well-balanced aeration SSTs operate with a central chamber, which is heated using a low-cost coil at a temperature of 50-60 C through a passive solar heat collector. The solar septic tank design is illustrated in Figure 3. The circulating hot water is generated from the solar water heating device through heat transfer equipment, i.e. the copper coil, and this will increase the temperature inside the septic tank. The fecal pathogens could be effectively inactivated when the effluent passes through the disinfection chamber where the temperature could be more than 55 C (Zhao et al. ). The temperature of the entire tank is then increased using the heat generated from the central chamber contributing to the increased microbial degradation of retained solids.

ECONOMIC VIABILITY OF FECAL SLUDGE TREATMENTS
At present, products of FS treatments are inefficiently used.
FS is commonly buried and dumped into the environment.
Sludge reuse is generally preferable over landfill because  FS is also used without additional treatment on agricultural land and discharged into fish ponds and lakes. These methods of excreta disposal are used mostly in urban residential areas of Africa, Asia, and Latin America (Toledo et al. ). Pathogens are classified into four categories, namely, bacteria, protozoa, helminths, and viruses. Table 2 lists some of the common pathogens that may be excreted in feces and their importance in disease transmission. Therefore, the selected treatment method of FS should be based on the end-use or disposal and handled hygienically. Many urban residents in developing countries (more than 90% in sub-Saharan Africa) do not have access to the sewerage or water supply necessary for its operation.

LIMITATIONS AND CHALLENGES OF FECAL SLUDGE HYGIENIZATION
Only a fraction of the population living in the city center of developing countries has access to sewerage networks and wastewater treatment plants. By comparison, a higher percentage of the population uses septic tanks or pit latrines for collecting the sludge that will be disposed of in a dumping site by the private or public sector. Figure 3 shows the sanitation service chain of on-site sanitation technologies. if the entire service chain, including collection, transportation, processing, and safe disposal, is properly managed. Therefore, untreated FS will end up directly in surrounding areas, contaminate the environment with pathogens, and severely affect public health if an appropriate FSM structure is lacking. Awareness of common challenges associated with resource recovery and ensuring the proper protection of human health and the environment must be addressed.
Therefore, fully understanding key factors in selecting suitable and feasible options of treatment technologies is required to recover resources from FS potentially. Human excreta can play a crucial role in poverty alleviation by increasing soil fertility, enhancing agricultural food production, and using feces for soil amendment.