Fecal sludge management
Fecal sludge management (FSM) (or faecal sludge management in British English) is the collection, transport, and treatment of fecal sludge from pit latrines, septic tanks or other onsite sanitation systems. Fecal sludge is a mixture of human excreta, water and solid wastes (e.g. toilet paper or other anal cleansing materials, menstrual hygiene materials) that are disposed of in pits, tanks or vaults of onsite sanitation systems. Fecal sludge that is removed from septic tanks is called septage.
FSM is necessary in densely populated areas where a proportion of the population is not connected to a sewerage network, and the covering and rebuilding of pit latrines is not possible. This is the case in most urban areas in developing countries, but such services are also used in developed countries where sewerage systems are unavailable. FSM services are usually provided by formal and informal private sector services providers, local governments, water authorities and utilities. However, in many developing countries FSM services are often unavailable, or if they are available are often informal, unregulated, unhygienic and unsafe. This can lead to surface water and groundwater pollution, the spreading of pathogens into the environment and adverse public health impacts. It can also result in unreliable services with relatively high costs to the households which need them.
Fecal sludge collection services can be made available on a scheduled basis or on a call-for-service basis (also known as on-demand, on-request or non-scheduled services). The collected fecal sludge may be transported to treatment plants using a vacuum truck; a tank and pump mounted on a flatbed truck; a small tank pulled by a motorcycle; or in containers on a hand cart. Mobile or permanent transfer stations can be used to improve the efficiency of fecal sludge transportation by transferring the waste to larger tankers for haulage to treatment. The wider use of multiple decentralized sludge treatment facilities within cities (to avoid long haulage distances) is currently being researched and piloted.
The collected fecal sludge should preferably be processed at dedicated fecal sludge treatment plants, instead of being co-treated with sewage in municipal sewage treatment plants, unless these plants are able to take the additional load, and facilities to separate liquids and solids are available. A variety of mechanized and non-mechanized processing technologies may be used, including constructed wetlands, anaerobic digestion, and waste stabilization ponds. The treatment process can produce useful products such as treated effluent that can be used for irrigation. Another possibility is to use the treated fecal sludge after composting as a soil conditioner or for the production of biogas, charcoal, biodiesel, powdered industrial fuel and electricity. Historically, the term night soil was used for fecal sludge.
Fecal sludge management (FSM) requires safe and hygienic septic tank and pit latrine emptying services, along with the effective treatment of solids and liquids and the reuse of treated produce where possible. It may include a range of options including on-site and offsite treatment, and the dispersal or capture and further processing of the products of the treatment process into such as biogas, compost and energy.
FSM is used synonymously with the term "septage management", but they are not the same. Septage is a specific type of fecal sludge: it is the partially digested fecal solids that accumulates in septic tanks. Fecal sludge from pit latrines may have a lower water content compared to septage and may contain more solid waste.
Pit latrines generate fecal sludge, and these are sometimes classified as dry toilets, although wet pits are also common, especially in Asia. However, other types of dry toilets - those that are designed to be easily emptied without the addition of water - do not generate fecal sludge but generate instead dried feces (in the case of urine-diverting dry toilets) or compost (in the case of composting toilets), for example. In the case of Arborloo toilets, nothing is ever extracted from the pit and, instead, the lightweight outhouse/superstructure is moved to another shallow hole and a tree is planted on top of the filled hole.
Collectively, the collection, transport, treatment and reuse of excreta constitute the "value chain" of fecal sludge management.
Septage or "septic tank sludge" is partially treated sludge that is accumulated and stored in a septic tank or similar onsite sewage facility. Septage is a by-product from the pretreatment of household wastewater (sewage) in a septic tank. After a few years of accumulation, septage is usually pumped out of a septic tank by a vacuum truck. Septic tanks receive blackwater from flush toilets, as well as greywater. This means that septage only contains the kind of solid waste that can be flushed, such as toilet paper. Septage may also include "scum", which is material that floats at the top and contains fats, oil and grease as well as any floating solid waste.
The term "septage" has been used in the United States since at least 1992. It has also been used in projects by the United States Agency for International Development in Asia. Another definition of septage is: "A historical term to define sludge removed from septic tanks."
Septage waste is periodically removed (with a frequency depending on tank capacity, system efficiency, and usage level, but typically less often than annually) from the septic tanks by specialized vehicles known as vacuum trucks. They pump the septage out of the tank, and transport it to a local sewage treatment plant. It can also be used by farmers for fertilizer, or stored in large septage waste storage facilities for later treatment or use on crops.
Worldwide there is an increasing interest and awareness of FSM issues, particularly in Africa and Asia. This is evidenced by some large-scale research and development projects occurring in the area of FSM. Such research is being carried out and funded by several organizations, including the Bill and Melinda Gates Foundation.
In many developing countries, fecal sludge is not properly managed. This may be due to a lack of mandated institutions and low awareness of the impact of poor sanitation; a lack of technical expertise and experience; an inability to source funds for to purchase of vacuum trucks and treatment, as well as a lack of knowledge necessary to initiate and implement successful FSM programs. This can result in the poor performance of onsite sanitation facilities (OSSFs), septic tank and pit latrine overflows, the unsafe emptying of pit latrines, and the dumping of untreated pathogenic fecal sludge into the environment.
Goal 6 of the Sustainable Development Goals for (water and) sanitation has helped to increase awareness of the risks of poor sanitation management along the entire sanitation chain. It has provided a motivation to many countries, cities and organizations to improve the range of improved sanitation services available. Among these are FSM programs designed to:
- Improve the functioning of onsite sanitation systems and to reduce the potential for human contact with fecal-borne pathogens;
- Minimize odors and nuisances, and the uncontrolled discharge of organic matter from overflowing tanks or pits;
- Support the upgrading of onsite sanitation upgrading programs;
- Safeguard public health against indiscriminate disposal of collected fecal sludge;
- Derive agricultural products, including soil conditioner from composting or co-composting materials and energy products such as biogas, charcoal pellets, industrial powdered fuel etc.
- Stimulate economic development, and job creation and livelihood opportunities, while addressing the issues of the social stigma and operator health and safety that continue to impact informal workers.
FSM programs can be drivers of sanitation improvement in different ways. They can encourage OSSF upgrades that reduce (or increase) the frequency of desludging and therefore operational and environmental costs. They can also be drivers of economic development along the sanitation value chain. This can include the provision of materials and parts for new toilets and septic tanks.
This can also include jobs for contractors and equipment installers; for sludge collection personal including drivers and emptiers; and for treatment and reuse systems operators.
This can also include the production and sale of the end-products of the sludge treatment process.
These products may include recycled water for agriculture and industry, soil conditioners, biogas, biodiesel or electricity.
There are currently large numbers of pilot, demonstration and full-scale FSM programs operating in Asia and Africa, however to date few that have demonstrated a sustained impact upon the value chain.
Elements of successful programs
FSM services can be provided as demand based (often called on-request, on-call, on-demand, ad-hoc or non-scheduled) or scheduled (also known as regular) desludging, or a combination of both. Under either mechanism, OSSFs are desludged on a periodic basis or when the household requests it or due to inspection by a competent authority indicates desludging is needed.
An analysis of 20 FSM Innovation Case Studies and research and advocacy of successful programs carried out by Oxfam Philippines has demonstrated that common elements for successful FSM programs include:
- Well formulated and practical policy, rules and regulation: While these are essential they are almost useless, even counterproductive, on their own, and must be supported by complementary factors such as those below;
- Local leadership and clearly mandated and resourced institutions to manage services, even where actual services are delivered by the private sector;
- Partnerships between stakeholders contributes to developing services at scale, building community confidence and achieving sustainability;
- A sustained program of community engagement, marketing and awareness raising is as essential to FSM as sludge treatment – but is frequently under-valued, under-budgeted and sometimes abandoned after an initial period;
- Capacity-building for FSM service providers helps ensure that they can effectively meet all segments of demand and achieve long-term viability. This may include training in both technical matters and business management, and the facilitation of capital formation through grants, equipment leasing, loan guarantees and other financial instruments;
- Tariffs that are pro-poor and representative of operational costs for providing the service;
- Technology that is appropriate to the capacity to operate and maintain the system and the realities of the value chain.
Characteristics of fecal sludge
Fecal sludge (FS) is the human waste, wastewater, trash and debris that accumulates in pit latrines and septic tanks or other onsite sanitation systems. Fecal sludge is an offensive material that contains pathogens, can generate odors and cause surface water pollution, as well as groundwater pollution. Characteristics of fecal sludge may vary widely due to climate, toilet type, diet and other variables. Performing a waste characterization study to understand local conditions provides data that factors into treatment plant sizing, as well as estimating the value of the products that can be derived from the treatment process.
The main physico-chemical parameters commonly measured to characterize fecal sludge include: BOD, total suspended solids, % solids, indication of sand, COD, ammonium, total nitrogen and total phosphorus, Fats, Oil and Grease (FOG), Sludge Volume Index (SVI), pH, alkalinity.
System designers often use default values, such as 2,000 mg/l for BOD and 5,000 mg/l of TSS (which might be an average country-wide) in order to size the treatment system. If no such data exists, or if local conditions are not adequately reflected in assumed values, a waste characterization study can be conducted.
- Methods, techniques and the skill levels of personnel conducting the desludging;
- The efficiency of the different types of equipment used in desludging;
- Seasonality - presence of groundwater or flood water that may infiltrate into tanks and dilute the contents;
- The last time the tank was desludged (age of fecal sludge).
FSM is a critical sanitation service in cities and towns in all countries where households use onsite sanitation systems. Citywide FSM programs may utilize multiple or one treatment facility, use stationary and mobile transfer stations, and engage with micro, small and medium-sized enterprises that may conduct some or all of the services.:99 Programs may be phased in over time to accommodate growing demand.
Peri urban areas
Peri urban areas are often less densely populated than urban centers. Therefore, they have more space and on-site sanitation systems can be effective for solid and liquid treatment. In most such peri-urban areas, it is less likely that they will be connected to a conventional centralized sanitary sewer system in the short or medium term. Therefore, these areas will rely on a mix of onsite-sanitation systems and services, decentralized wastewater management systems, or by condominial or simplified sewerage connected to decentralized or centralized treatment. In all of these situations, FSM is a necessary service to keep the sanitation systems functioning properly.
Rural areas with low population density may not need formal FSM services if the local practice is to cover and rebuild latrines when they fill up. However, if this is not possible, rural areas often lack treatment facilities within a reasonable (say 30 minutes drive) distance; are difficult for tankers to access and often have limited demand for emptying making transport and treatment uneconomic, and unaffordable for most people. Therefore, options such as relocating latrines on-site, double (alternating) pit or Arborloo toilets could be considered. Also sharing decentralized FSM services and sludge treatment between nearby villages, or direct safe removal burial of waste could be considered and organized.
Selecting the operator of FSM services
FSM services are usually provided by formal and informal private sector service providers, local governments, water authorities and utilities. Water utilities with a high percentage of water connectivity (homes with piped water connections) are logical operators of FSM programs. If water is sold to customers through a tariff, an additional tariff to cover FSM services may be added. For larger cities, it is usually the water and sewerage service provider that will be the most appropriate operator.
Local governments may choose to provide services by using their own staff and resources for collection, transportation and treatment. This is often the case in smaller cities or municipalities where the water utility may not have a broad reach. In many cases, cooperation between the city government and the water utility may be strategically advantageous. Dumaguete City, Philippines is one example where the Water District (utility) and Local Government have joint ownership and responsibilities for the FSM program. Organized larger scale FSM programs may be able to provide the service more cheaply and more hygienically than the independent private operators working on an ad hoc basis. Ensuring services are affordable is an important selling point when promoting the program to citizens and encouraging them to participate.
The local private sector is an important player in providing FSM services. In such cases, private sector contractors may work directly for households (under regulation) or bid on desludging contracts let by the city. The private sector can also provide services in operating and maintaining the treatment works, and in processing and selling the commodities resulting from the treatment process. San Fernando City, La Union, Philippines is an example of a local government that has contracted out the treatment facility construction and collection program to the private sector.
Synergy with other programs
FSM is but one aspect of citywide sanitation that also includes:
- Municipal solid waste management;
- Drainage and greywater management;
- Wastewater collection and treatment including effluent overflows from on-site systems where soils based dispersal systems are insufficient to assimilate the volume;
- Water safety; and
- Food safety.
There are important synergies between many of these services and FSM, and investigating co-management opportunities can yield benefits. MSW can often be co-managed with fecal waste, especially when thermal treatment technologies are used. Food waste from restaurants and markets can be co-composted with fecal waste to produce a high value soils amendment. Fats, Oil and Grease (FOG) from commercial grease traps can be added to biodigesters to increase methane production, or used in conjunction with fecal sludge as a feedstock for biodiesel production. Water supply is also closely linked with FSM as it is often the water utility that will manage programs and their customers that will pay for services through tariffs.
Scheduled desludging programs
In Southeast Asia, there is (in 2016) increasing interest in scheduled desludging programs as a means of providing services. A WSP study recommended that efforts to introduce scheduled emptying should focus first on areas where demand was greatest, moving on to other areas when the success of scheduled emptying had been demonstrated in these areas. Analysis of pit and tank desludging records for Palu in Indonesia revealed that existing demand for desludging services varied between sub-districts, with demand being greatest in well-established areas and least in urban fringe areas.
Co-treatment at wastewater treatment plants
Co-treatment of septage at wastewater treatment plants may be considered where the volume of septage removed from on-site facilities is small, as will be the case in situations where most households have access to sewerage. However, the high strength of septage and fecal sludge means that relatively small volumes of both can have a large impact on the organic, suspended solids, and nitrogen loads on a wastewater treatment plant. Possible consequences include an increase in the volume of screenings and grit requiring removal; increased odour emission at headworks; increased scum and sludge accumulation rates; and increased organic loading, leading to overloading and process failure, and the potential for increased odour and foaming in aeration tanks. Because of their partly digested nature, septage and fecal sludge will usually degrade at a slower rate than municipal wastewater. Therefore, their presence is likely to have an adverse impact on the efficacy of treatment processes. The intermittent nature of fecal sludge and septage loading can also amplify the problems identified above.
Despite these possible drawbacks, wastewater treatment facilities with spare capacity are a potential resource to be investigated. Even where co-treatment is not an option, existing wastewater treatment plants may provide land in strategic locations, close to areas of demand for septage management services. Separate preliminary treatment and solids-liquid separation facilities should always be provided for septage/fecal sludge. Solids-liquid separation will reduce both the overall load and the proportion of digested material in the liquid fraction and will thus lessen the possibility that it will disrupt wastewater treatment processes. Separated solids can be treated along with the sludge produced in sedimentation tanks during the wastewater treatment process.:6
Technology components and infrastructure
A formal process should be used for making an informed technology selection for the treatment of the fecal sludge. It is usually a collaborative process conducted by stakeholders, consultants, the operator and the future owner of the facility. The process is based on a long term vision planning with stakeholders as part of citywide sanitation planning. The expected waste flows (volume), their strength, characteristics, and variability in each area need to be known. A formal and transparent process for developing appropriate plans and designs for wastewater and fecal sludge treatment plants will achieve local buy-in and ownership of technology decisions, which is critical for the long term success and sustainability of the program.
Collection vehicles and equipment
If the fecal sludge is liquid enough, it is usually collected by using vacuum pumps or centrifugal style booster pumps. A variety of manual and motorized devices designed to excavate thick and viscous sludge and accumulated trash are also available in the market.
After sitting for years in septic tanks and pit latrines, the accumulated sludge becomes hardened and is very difficult to remove. It is still common that workers enter pits in order to desludge them, even though this practice is generally unsafe and undesirable (in India, this practice is called "manual scavenging"). A number of low-cost pumping systems exist to remove this hardened sludge hygienically from the ground surface, although many of them are still in the experimental stage (e.g. Excravator, Gulper, e-Vac).
Fecal sludge can also be treated inside the tank or pit as well, by use of the "in-pit lime stabilization process", which treats the waste before it is removed from the tank or pit. This methodology was first pioneered by iDE Cambodia in 2010. Once removed, it is transported to onsite or off site treatment and processing facilities.
Some advanced transfer stations and vacuum trucks can dewater fecal sludge to some extent, and this water may be placed in sewer lines to be treated in wastewater treatment plants. This allows more sludge to be dealt with more efficiently and may constitute one of the best cases of co-treatment of fecal sludge in wastewater treatment plants.
Transfer stations are intermediary drop off locations often used where treatment facilities are located too far away from population centers to make direct disposal feasible. In other locations, traffic concerns or local truck bans during daylight hours may make transfer stations feasible. In addition, municipalities where a significant percentage of homes cannot be accessed by tanker truck should utilize transfer stations. Transfer stations are used if:
- More than 5% of the homes are inaccessible by a vacuum truck;
- The treatment plant is too far away from the homes for transport in one haul to be practical;
- Trucks are not permitted on the streets during the day; or
- Heavy traffic during daylight hours impedes the movement of vacuum trucks.
Mobile transfer stations
Mobile transfer stations are nothing more than larger tanker trucks or trailers that are deployed along with small vacuum trucks and motorcycle or hand carts. The smaller vehicles discharge to the larger tanker, which then carries the collected sludge to the treatment plant. These work well in scheduled desludging business models.
Fixed transfer stations
Fixed transfer stations are dedicated facilities installed strategically throughout the municipality that serve as drop off locations for collected fecal sludge. They may include a receiving station with screens, a tank for holding the collected waste, trash storage containers, and wash down facilities. These may be more appropriate for FSM programs using the "call-for-service" business model.
While static transfer stations are fixed tanks, mobile transfer stations are simply tanker trucks or trailers that work alongside the SVVs and actually do the longer haul transferring of the waste from the community to the treatment plant. Mobile transfer stations work best for scheduled desludging programs where there are no traffic restrictions or truck bans, and a relatively large number of homes that are inaccessible to the larger vehicles.
Fecal Sludge is often processed through a series of treatment steps to first separate the liquids from the solids, and then treat both the liquid and solid trains while recovering as much of the energy or nutritive value as possible. Common processes at fecal sludge treatment plants include:
- Fecal sludge reception - where the truck interfaces with the treatment plant and sludge is unloaded.
- Preliminary treatment - to remove garbage, sand, grit, and FOG (fats, oil and grease);
- Primary treatment - simple separation by physical means, or separation with microbial digestion;
- Liquids treatment - for example by using constructed wetlands, waste stabilization ponds, anaerobic digesters; and
- Solids processing - using the solids resulting from fecal sludge treatment for beneficial use where possible.
Constructed wetlands are gaining attention as a low-cost treatment technology that can be constructed in many instances using local materials and labor. For sites with enough land and a ready supply of gravel and sand, this technology offers low cost, scalability, and simple operation.
Simple sludge drying beds can be used for dewatering and drying, as they are a cheap and simple method to dry fecal sludge (they are also widely used to dry sewage sludge). Drainage water must be captured; drying beds are sometimes covered but usually left uncovered.
Drying beds are typically composed of four layers consisting of gravel and sand. The first layer is coarse gravel that is 15 to 20 centimeters thick. Followed by fine gravel that is 10 centimeters thick. The third layer is sand that can be between 10 and 15 centimeters and serves as the filter between the sludge and gravel. Sludge dries up and water percolates to the first layer that is collected at the drainage pipe that is beneath all layers.
Fecal sludges behave differently during dewatering processes than wastewater sludges. The amount of extracellular polymeric substances (EPS) can be an important predictor for faecal sludge dewatering performance. Fecal sludge from public toilets took longer to dewater than sludge from other sources, and had turbid supernatant after settling.
- Thermal processes which can achieve cost effectiveness by eliminating the need for separate processes. They convert the fecal sludge along with certain fractions of sewage sludge or municipal solid waste to produce energy or fuel by using certain sewage sludge treatment technologies.
- Biodiesel can be manufactured by using fats, oils and grease as feedstocks. Research by RTI International is being conducted to use fecal sludge for biodiesel production.
- Electricity can be produced by thermal processes that burn fecal and solid waste together to maintain stable combustion and the heat is used to make steam that drives generators.
Treatment products and reuse options
Composting is a process whereby organic matter is digested in the presence of oxygen with the byproduct of heat. For fecal sludge, the heat deactivates the pathogens while the digestion process breaks down the organic matter into a humus-like material that acts as a soils amendment, and nutrients that are broken down into a form that is more easily taken up by plants. Properly treated fecal sludge can be reused in agriculture (see also reuse of excreta).
Fecal sludge is rich in nitrogen. When they are mixed with materials that are rich in carbon, such as shredded crop wastes, the composting process can be maximized. Proper mixture to achieve a ratio of 20 to 1 to 30 to 1 of carbon to nitrogen is best.
Biogas is a byproduct of the anaerobic digestion process.
Treated effluent can be used for agricultural or landscape irrigation.
These by-products have the potential to offset some of the costs of the program, thereby reducing tariffs for the public. However, value addition all the way to biogas, biodiesel and electricity is difficult to achieve in practice due to technological and operational challenges.
Costs and fees
FSM is considered an entry point for sanitation improvement programs that are led by local governments. Such programs may include tariffs or user fees, promotions campaigns to raise the willingness to pay for the service, and local ordinances that define the rules and regulations governing FSM. In the Philippines, tariffs around US$1 per family per month are generally enough to achieve full cost recovery within a period of 3 to 7 years. Promotional campaigns are used to raise the willingness to pay for services, and local procedures and ordinances provide additional incentives for compliance.
Dumaguete, the Philippines
USAID has supported efforts to introduce scheduled desludging services in some countries in Southeast Asia. The first of these was in Dumaguete in the Philippines. The programme was run jointly by the city government and the Dumaguete City Water District, with the former operating the treatment plant and the Water District conducting the desludging. The cost of the scheme was covered by adding a tariff of 2 pesos (about 5 US cents) to the water bill for each cubic meter of water consumed (about one US dollar per family per month). This approach was possible because around 95% of residents had a connection to the Water District reticulation system. Trucks were to move from neighbourhood to neighbourhood on a scheduled cycle, emptying pits on a regular 3-4 year cycle. This approach requires a database of all pits and septic tanks requiring desludging. However, Dumaguete has by 2018 reverted to an ‘on-call’ system, the cost of which is still covered by the surcharge on the water tariff. It seems that users prefer this small regular payment to having to make large payments when tanks require desludging.
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