Characteristics of fecal sludge may vary widely due to climate, toilet type, diet and other variables. Fecal sludge can be grouped by consistency as "liquid" (total solids or TS <5%), "slurry" (TS 5–15%), "semi-solid" (TS 15–25%), and "solid" (TS >25%). Quantities and qualities of fecal sludge and wastewater are very different, with the range of fecal sludge characteristics being 1–2 orders of magnitude higher than wastewater. 
The result of the demographic, environmental, and technical factors that influence characteristics of fecal sludge is a high level of heterogeneity that complicates characterization.
In the absence of actual data, designers often use default values, such as 2,000 mg/L for BOD and 5,000 mg/L of TSS in order to size the treatment system. However, this often results in over-design or under-design of fecal sludge treatment plants. This is because there is often no "standard range of variation" for particular properties, and findings from one study cannot necessarily be used as a base of comparison to another.  
Research has shown that correlations to spatially available data can help predict quantities and qualities of fecal sludge. The relevant indicators for the prediction include income level, users, volume, emptying frequency, and truck size. Using these correlations in characteristics could provide a way to reduce analytical costs for fecal sludge analysis.
Performing a waste characterization study helps to understand local conditions and provides data that factors into treatment plant sizing. It can also help to estimate 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.
Relatively little data exists on pathogen content in fecal sludge. One study from rural Bangladesh determined 41 helminth eggs per g of fecal sludge from pit latrines.
The characteristics of fecal sludge may be influenced by:
* 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).