Sludge Treatment and Disposal 213plant is 1500 kg of dry solids per day. Calculate the required volume of the primary
digester and the hydraulic retention time.
The production of sludge requires
= 500 m3 digester volume.
1500 kg/day
3 kg/m3-dayThe total mass of wet sludge pumped to the digester is
1500kg'day = 37,500kg/day.
0.04Since 1 L of sludge weighs about 1 kg, the volume of sludge is 37,500 Uday or
37.5 m3/day, and the hydraulic residence time ist = (500m3)/(37.5m3/day) = 13.3 days.
The production of gas from digestion varies with the temperature, solids load-
ing, solids volatility, and other factors. mically, about 0.6 m3 of gaskg of volatile
solids added (10 ft3/lb) has been observed. This gas is about 60% methane and burns
readily, usually being used to heat the digester and answer additional energy needs
within the plant. An active group of methane formers operates at 35°C (95OF9, and
anaerobic digestion at this temperature has become known as mesophilic digestion. As
the temperature is increased to about 45°C (115"F), another group of methane formers
predominates, and this process is tagged thermophilic digestion. Although the latter
process is faster and produces more gas, the elevated temperatures are more difficult
and expensive to maintain.
Finally, a word of caution should be expressed about mixing in a primary digester.
The assumption is invariably made that the tank is totally mixed, either mechanically
or by bubbling gas through the tank. Unfortunately, digester mixing is quite difficult,
and some studies have shown that on the average, only about 20% of the tank volume
is well mixed!
All three stabilization processes reduce the concentration of pathogenic organisms
to varying degrees. Lime stabilization achieves a high degree of sterilization, owing
to the pH increase. Further, if quicklime (CaO) is used, the reaction is exothermic
and the elevated temperatures assist in the destruction of pathogens. Aerobic diges-
tion at ambient temperatures is not very effective in the destruction of pathogens.
Anaerobic digesters have been well studied from the standpoint of pathogen via-
bility because the elevated temperatures should result in substantial sterilization.
Unfortunately, many pathogens survive the digestion process with little reduction
in virulence. An anaerobic digester cannot, therefore, be considered a method of
sterilization.