Page 129 - Wastewater Solids Incineration Systems

Combustion Technology 97

gas provides an optimal processing zone in the reactor vessel through which all input
material is forced to pass. The reactor vessel operates at atmospheric pressure. The
process requires a minimum feed calorific value of 20 000 kJ/dry tonne to be
autothermal at such low organic concentration. Typical digested biosolids have a
calorific value of 13 700 kJ/dry tonne and would, therefore, need to be dewatered or
dried to a higher solids concentration for the process to be autothermal. The inert
material remains as an ash that needs to be disposed; condensate from the vapor
stream will need to be returned to the sewer or treated. The process has been pilot
tested on various waste streams, including paper and pulp solids and manure. There
are no full-scale operating facilities treating WWTP solids.

10.2.3 SlurryCarb™ Process
The SlurryCarb™ Process is a thermal treatment process to improve dewaterability
of biosolids. EnerTech developed the process, which has been tested with feed solids
concentrations up to 20% (Orange County Sanitation District, 2003). The feed cake is
diluted if the solids concentration is greater than 20% and then macerated to ensure
particle size is smaller than 12 mm (0.5 in). The resulting feed slurry is then pumped
up to the required pressure setpoint of approximately 7 to 10 kPa (1000 to 1500 psi)
and passed through heat exchangers to raise the temperature to approximately 200 to
230°C (400 to 450°F), as shown in Figure 5.21. During the thermal decomposition
reactions, organics in the slurry are broken down and carbon dioxide gas is separated
from the solids. Simultaneously, any chlorine in the slurry is converted to
hydrochloric acid, which is neutralized by the inherent buffering strength of the
slurry before heat treatment. Chlorine is a precursor to dioxins and furans, and this
process enables the chlorine to be washed out of the treated solids in the form of
aqueous salts. The treated solids or char is passed through recovery heat exchangers
used to heat the feed slurry and then washed and dewatered in a centrifuge to a
solids concentration between 50 and 55%. Depending on the reuse options for the
final product, this dewatered cake is either used directly or dried. As a result of the
chemical changes that occur during thermal treatment, the viscosity of the resulting
slurry is greatly reduced and the dewaterability is increased. The product can be fur-
ther dried to 95% solids concentration if required by the reuse options. The product
has a heating value of approximately 15 100 kJ/kg (6500 Btu/lb) at a solids concen-
tration of 95% if undigested sludge is used.
The process does produce an effluent waste stream that is high in ammonia and
organics, which can be controlled by conventional treatment techniques such as