4.2 Feedstock availability

Accurate information about feedstock materials’ quantities and properties is essential for developing treatment facilities. Overestimating their amount or strength can lead to higher investment costs while underestimating them can pose risks to public and environmental health. Thus, it is vital to have accurate data to design treatment facilities that are effective and efficient.

This chapter offers a brief overview of methods to estimate the amount of excreta and faecal sludge available for biogas production. The methods used for estimating feedstock availability vary depending on the type of feedstock and the context in which the biogas plant is utilised. The focus is on situations where biogas is linked directly to the toilet or planned as a semi-centralized unit. To ensure awareness of the potential uncertainty when consulting the available data, the methods are presented in the section for further reading.

If a biogas plant is connected to a toilet, the total input will be determined by several factors. These include the rate of excreta generation, the amount of water used for cleansing or toilet paper, the amount of water used for flushing, and spatial habits that affect where, when and for what purpose people choose to use the toilet.

Estimating each variable accurately can be challenging and time-consuming, especially if no data is available.

To demonstrate this, we can analyse various factors and how they impact certain variables. For instance, let’s take the example of the rate of excrement production. In low-income countries, the amount of faeces produced can vary from 75 to 520 grams of wet weight per person per day, with a median of 250 grams per person per day. The global median volume of urine is 1.4 litres per person per day, with values ranging from 0.6 to 2.6 litres per day. Factors like dietary habits, age, liquid consumption, physical activity, and climate influence the rate of excrement generation. (C. Rose A. P., 2015).

Further, the excreta generation rate will differ for the system where the urine is separated, like urine diversion systems.

The amount of water used will depend on the individual’s cultural practices (whether they wash or wipe), habits (such as how much water is used for washing or flushing in different situations), and the type of flush system being used.

An individual’s spatial habits are influenced by their daily and weekly schedule.

To determine the amount of faecal sludge that requires treatment through the semi-centralized system, one can calculate the accumulation rate for each type of containment. However, there is currently no standardised approach to measuring faecal sludge production. A study compared three methods for quantifying faecal sludge: in-situ, accumulation rate measured with a core sampler (Method 1), accumulation rate determined by the volume emptied by a desludging truck (Method 2), and sludge blanket accumulation rate with a core sampler (Method 3). The study emphasises the importance of accurately defining and reporting terminology and methodology, as demonstrated by the variability of results for onsite systems such as pit latrines and septic tanks (Link).

Additionally, future population projection needs to be considered in the estimation.

As illustrated, determining the quantity of feedstock can be challenging, especially without prior experience values. Therefore, it is helpful to approach the provided data cautiously and understand the estimation method being used. Performing sensitivity analysis could be instrumental in gaining insights into any associated risks and uncertainties.

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