How Can I Calculate FS Generation?

For effective Faecal Sludge Management (FSM), it is important to determine the expected quantities as well as the qualities of the Faecal Sludge (FS). FS quantity can be estimated using either the sludge collection method (based on the volume of containment and the emptying frequency) or the sludge production method (based on the FS accumulation rate and the emptying frequency). As FS accumulation rates vary greatly between locations it is important to determine the local FS accumulation rates before calculating FS quantities and it is worth investing resources to do so.  

Relevance/Importance

Designing adequate FS Emptying and Transportation (E&T) services and Faecal Sludge Treatment Plants (FSTP) relies on an appropriate quantification of the expected FS to be handled. Estimates that are too low will underestimate FS E&T services or overload the FSTP, resulting in poor performance and public health hazards. Estimates that are too high will overestimate FS E&T services and underload the FSTP, leading to higher costs and wasted resources.  

Biological factors affect accumulation; they include the degradation of organic matter, the growth of microorganisms and nutrient cycling which are affected by many parameters including varying levels of oxygen, water content and temperature. Physical processes affecting quantity include the infiltration and inflow of groundwater or the liquid fraction in containment and the infiltration of soil and sand (affected by construction quality, soil type and groundwater level). Other factors include how the containment is designed, constructed, used and maintained. 

Making community-wide estimates based on factors and processes within each containment is unrealistic due to time, financial and other practical constraints. Instead, different methods can determine the total amounts of FS to be managed on a larger scale. 

As well as the quantities of FS, its qualities (characteristics) must also be considered when planning and designing an FSTP.  

Overview

The following two theoretical methods can be used to calculate FS quantity: 

1. Sludge collection method and  

2. Sludge production method  

The sludge collection method (Strande et al. 2021) is used where the volume of all the containments is known and when the goal is to determine the total sludge production. This method is particularly appropriate during an acute response phase where containments are standardised and desludging is done on demand. 

In the long term, where different types and volumes of containments with different filling rates co-exist, the sludge production method (Strande et.al, 2014) is used to plan for scheduled desludging and to calculate the expected sludge loading in a treatment plan. This particularly applies to the stabilisation phase or when the affected population is living alongside host communities. 

The sludge production method (Strande et.al, 2014) requires more data, especially for calculating the sludge accumulation rate that is influenced by multiple factors; it may therefore require specialised external support. 

In humanitarian contexts, neither method is necessarily the best suited and it might be appropriate to apply both methods to see which one is a better fit. Starting with the sludge collection method is recommended in the acute phase. Plan for the sludge production method combined with measuring FS quality in the long term (when a desludging frequency is more or less established for FSM). 

Sludge Collection Method: uses the quantity and volume of on-site collection technologies (e.g. Single Pits, or Septic Tanks) combined with the current demand for services to calculate the emptying frequencies.  

To estimate the emptying frequency of FS, consult the transport companies (both formally and informally). The consultations may result in a correction to the expected volumes collected in each emptying. For example, only 80% of a septic tank with a volume of 3m³ may be emptied (i.e. 2.4m³ instead of 3m³) as the sludge at the bottom is too thick. Additionally, the company may add 10% of water to dilute the sludge so the volume would be 2.64m³.  

Data required for the sludge collection method are context-specific and are estimated using primary data sources (e.g. Surveys, Focus Group Discussions, or Interviews) or secondary data sources (e.g. reports or documents). 

The following example illustrates the sludge collection method: in a camp, 4000 households (each with an average of 5 household members totalling 20,000 people) have a single pit of 3m³ which is emptied every second year. The emptying volume would be 4,000 x 3 = 12,000m³. The sludge volume to be desludged can then be estimated: 12,000m³ / 2 years / 52 weeks/year = 115.5m³/week. 

Sludge Production Method: uses the sludge accumulation rate and the desludging interval. It is calculated with the following formula: 

Where: 

• V₆: Volume of sludge delivered and treated 
• V₅: Volume of sludge collected but not delivered and treated 
• V₄: Volume of sludge not collected 
• V₃: Volume of sludge accumulated 

Where: 

• V₂: Volume of sludge produced 
• V₁: Volume of excreta produced 
• V_BPC: Change in volume due to biological, physical and chemical processes 

The most critical factor here is V₃ which depends on many biological, physical and chemical factors as well as other elements such as the design, construction, use and maintenance of the containment. Design also refers to the containment type, mainly whether it has an outlet (e.g. septic tank and cesspit) and if it is sealed or not. Previous studies have identified that FS accumulation rates vary between containment types, toilet types, whether there is water connectivity or not and between household and non-household sources. These factors must therefore be considered and verified through sampling; this requires expert support. 

First, evaluate what information is locally available and what can be collected elsewhere. Information required includes the types of containment (and ideally approximately how many of each), toilet types, whether there is a water connection or not, the source (household versus non-household), number of users per containment, containment size and the desludging frequency (if available). If historic data on desludging is available, the FS accumulation rate can be estimated relatively easily with the following formula: 

If historic data on desludging is unavailable then the accumulation rate has to be estimated by a more detailed assessment of sludge production at the household level (excreta + urine + flushing and kitchen water + solid waste) and the type of containment (lined/unlined, effluent/closed). Based on this information the table below allows an approximation to be made. Assess if there are patterns between containment types, toilet types, water connection and household versus non-household sources (for example with boxplots, as done in Prasad et al. 2021). Analyse which factor varies the most (e.g. there are often significant differences between containment types but in some locations there could be more significant differences between the water connectivity, or a combination of these differences). Determine a median accumulation rate for each category. 

The following example illustrates the sludge production method: 

In a camp, 4000 households (20,000 people) have a septic tank which is emptied every second year. 

• FS accumulation rate for pit latrine = 0.027m³/cap/year (total volume measured with core sampler according to Prasad et al., 2021). 
• FS volume (only solids) = 20,000 capita * 0.027m³/capita/year * 2 years = 520m³  
• Assuming that not only sludge is emptied but also the supernatant: 520m³ * 3 = 15,600m³  
• Sludge volume to be emptied in a year =15,600 / 2 = 7,800m³/year 

The Sludge volume to be treated can then be estimated: 15,600m³ / 2 years / 52 weeks/year = 150m³/week. This is approximatively 25% more than with the sludge collection method (see above). The difference can be explained through the loss of sludge due to degradation over time and infiltration of liquids from unsealed containments. The 150 m²/week represent the total FS volume to be managed. Although the ultimate goal is for all FS to be delivered to an FSTP, it is not realistic to assume that all FS produced will initially be collected and transported for discharge at an FSTP (V4+ V5). Thus, this method will result in an overestimation of the potential volumes to be delivered to an FSTP.  

For more detailed information on determining FS quantities and qualities, refer to chapter five of Methods for Faecal Sludge Analysis (Velkushanova et al, 2021). 

Process & Good Practice

• Determine and select localised accumulation rates. There is no universal number for the FS accumulation rate. Therefore, it is essential to calculate it using the information available on the type of containment (septic tank or pit, lined/unlined) and the soil and groundwater conditions (if available) as well as the use of water in the households 
• Ask local actors for information on the local situation. Informants could be existing emptying services, NGOs or local authorities 
• Assess the existing literature if no information at all is available to calculate local accumulation rates, e.g. Methods for Estimating Quantities and Qualities of Faecal Sludge (Prasad et al. 2021). Choose the scenario closest to your situation and use those figures 
• Be aware that all FS generated might not reach an FSTP. However, for planning purposes, it is good practice to consider the total volume and work towards a full collection and delivery using financial and legal incentives and by building up to a fully working sanitation service chain.