Environmental Engineering Reference
In-Depth Information
300 tons/day of source-separated organic waste in which food waste is the major
component. Soil fertilizer and biogas are produced from the process. The process
can be divided into pretreatment, AD, energy recovery, post-treatment of digestate,
and air and waste water treatment. In the pretreatment process, organic waste
undergoes manual sorting, trammel screen, metal separation, and shredding to
separate inorganic and oversized materials, reduce particle size, and adjust the
moisture content in order to facilitate an efficient AD. The inert materials that are
not suitable for bio-conversion are disposed to landfills. The substrate is then
heated to 70°C for 4 hr for pasteurization before entering the digesters. During the
AD, the biogas generated is collected and delivered to the gas-cleaning system for
the removal of impurities such as hydrogen sulfide, water, and particulates,
followed by the CHP generator for onsite energy recovery or exported to the gas
network. After digestion, the digestate passes through a separator for dewatering.
The solid fraction is conveyed to the composting system while the liquid fraction
is pumped to the waste water treatment system. The composting system comprises
enclosed composting tunnels and a maturation area. Air is injected into the floor
of the tunnels and passes through the composting mass to facilitate aerobic
microbial digestion for 8 days. The composting mass undergoes maturation for 14
days before being used as soil fertilizer. On the other hand, the waste water gener-
ated from dewatering of the digestate and the leachate from composting and
biogas drying are diverted to an onsite waste water treatment facility. It is expected
that 0.89 m
3
of waste water is generated per 1 ton of waste. For the air treatment
system, the air is processed by a scrubber for pH adjustment and ozone/UV for
odor treatment.
Today, almost all of the biogas produced worldwide is used for heat and elec-
tricity production. It is also deemed a versatile renewable energy that can be uti-
lized for many applications, including vehicle fuel and town gas. However,
improved biogas quality is needed to meet the specific requirements of these
applications. This mainly involves increasing the methane content in biogas by
removing the impurities such as CO
2
, H
2
S, NH
3
, and solid particles. In Sweden,
Insert residues
Landfill
Biogas
Organic
waste
Combined heat &
power generator
Pretreatment
Anaerobic digestion
Heat & electricity
Digestate
Solid
fraction
Dewatering
Stabilization
Soil conditioner
Liquid
fraction
Waste water
treatment plant
Figure 3.16
The proposed process flow in the Hong Kong Organic Waste Treatment Facility [87].
