Treatment and Disposal Strategy Study
In May 1997,
the Environmental Protection Department (EPD) commissioned the Sludge
Treatment and Disposal Strategy Study (the Study) to formulate an
integrated strategy for sludge management. The purpose of this paper
is to brief members on the outcome of study and to seek members'
views on the proposed sludge management strategy.
2. Sewage treatment
and water treatment processes produce sludge as a by-product. Generally
speaking, more sludge will be produced if higher level of treatment
is required. At present, all sewage sludge and water treatment works
sludge generated in Hong Kong are mechanically dewatered at individual
treatment works and taken to landfills for final disposal. However,
dewatered sludge still contains a significant amount of water and
can cause stability and operational problems at landfill. To overcome
these problems, it is necessary to mix the sludge with ten times
its volume of solid waste during disposal. The landfills in Hong
Kong currently receive about 16,900 tonnes of solid waste daily,
representing a maximum theoretical assimilative capacity of 1,690
tonnes for dewatered sludge.
3. The progressive
implementation of various upgrading programmes for sewage and water
treatment works in Hong Kong (e.g. the Strategic Sewage Disposal
Scheme (SSDS)) will greatly increase the quantity of sludge requiring
landfill disposal (Figure 1). This increase would exceed the maximum
assimilative capacity of the landfills from 2008 onwards (Table
4. The situation
would be further aggravated by the various waste reduction measures
(e.g. incineration and recovery of municipal solid waste and re-use
of construction and demolition materials) to reduce the quantity
of solid waste requiring landfill disposal to 12,000 tpd by 2008.
It is therefore necessary to examine alternative methods for sludge
5. The Waste
Reduction Framework Plan presents an integrated strategy for the
management of solid waste in Hong Kong. The Plan aims to minimise
the amount of waste destined for landfill disposal and hence reduce
the associated costs and extend the life of the existing strategic
landfills. The Plan seeks to encourage the prevention, re-use and
recycling of wastes, backed up by bulk waste reduction such as waste-to-
energy incineration and composting. Although the Plan is targeted
at achieving significant reduction in the quantity of municipal
solid waste (MSW) destined for disposal, it has indicated that the
incineration option may also be selected as a waste reduction option
for sludge management.
1. PROJECTED DAILY SEWAGE & WATER TREATMENT WORKS SLUDGE ARISING
1. CURRENT AND PROJECTED SLUDGE ARISING AND LANDFILL ASSIMILATIVE
FINDINGS AND RECOMMENDATIONS OF THE STRATEGY STUDY
sludge is generally inert and inorganic in nature and has limited
potential for recycling or bulk reduction by incineration. The current
treatment and disposal arrangement at landfill is the most cost-effective
7. Sewage sludge
in Hong Kong has a high chloride content associated with the use
of seawater for flushing. This places constraint on the beneficial
reuse of sewage sludge as compost or soil conditioner. However,
it also has a very high calorific value which makes it possible
to be incinerated without additional fuel.
8. The Study
has examined various recycling, treatment and disposal options for
managing sewage sludge. These included composting, oil from sludge,
drying, incineration and marine disposal.
recycling through composting or land application are often considered
as environmentally beneficial options, the high chloride content
in sludge, the lack of market for compost and the need for a larger
area to conduct these operations in Hong Kong have ruled out their
feasibility. Export to the Mainland was also considered, but the
associated transport and handling costs have far exceeded the potential
benefits. Other innovative technologies such as oil from sludge
were still at experimental stage and generally unproven for large
scale application. Marine disposal is no longer acceptable internationally
and cannot be adopted as the basis of our sludge management strategy.
10. The study
concluded that thermal treatment such as drying and incineration
are more suitable for Hong Kong in view of their ability to:-
- Reduce water
from sludge to avoid causing any potential stability problem at
- Reduce the
sludge volume by up to 90% in line with the Waste Reduction Framework
Plan to save landfill space.
11. Sewage sludge
incineration was recommended because it has the following additional
benefits over drying:-
- It has the
ability to generate some 10MW surplus energy and is therefore
a more cost effective option than drying which necessitates the
input of energy.
- It is a well
proven technique and less susceptible than drying to the corrosion
problem associated with the high chloride content sludge.
- It has a
better volume reduction capability and can save around 35% more
landfill space than drying.
12. The major
recommendations of the Study are summarised below:
- All sewage
sludge, septic tank sludge and waste of similar physical and calorific
properties (e.g. grease trap waste) should be dewatered and incinerated
prior to final disposal at landfill.
and grit from sewage treatment works should be sent to landfill
- Inert water
treatment works sludge should continue to be dewatered prior to
AND NEW INCINERATION FACILITIES
13. In recommending
sewage sludge incineration, the Study also reviewed the potential
interaction with all other existing and planned waste incineration
facilities in Hong Kong. The high temperature incinerator at the
Chemical Waste Treatment Centre is specially designed to destroy
organic chemical waste. We plan to modify the incinerator to burn
clinical waste. However, it has limited capacity to treat the significant
volume of sewage sludge, even it is technically feasible to modify
14. The Waste
Reduction Framework Plan indicated our intention to build two 1
million tonnes per annum capacity waste-to-energy incinerator (WEIF)
to burn municipal solid waste. Although it is technically feasible
to co-incinerate sewage sludge with municipal solid waste, it is
not recommended because it has only been practised on a limited
scale overseas and it is not a well proven technology for adoption
at this stage. However, we will explore the possible co-location
of the WEIF with the sludge incinerator.
15. We also
plan to build an animal cremator to handle animal carcasses, which
are mainly disposed of at landfills at present. Co-incineration
of animal carcasses with sewage sludge or municipal solid waste
is not proven and operationally not practicable. We need to build
a dedicated cremator in the next few years so that we can stop the
landfill disposal arrangement, which is environmentally unsatisfactory.
CONSIDERATION AND SITING OF THE SLUDGE INCINERATION FACILITY
16. A preliminary
environmental review indicated that a modern and well managed sludge
incineration facility equipped with proper flue gas control system
will not cause adverse environmental impact. The new facility will
adopt the state-of-the-art technology to ensure the dioxin emission
does not exceed the most stringent standards adopted in the developed
countries. We will also conduct an independent review on dioxin
in order to allay public concerns, and we will brief relevant parties
on the findings in 2000.
17. While proven
engineering solutions exist to overcome any potential problem associated
with sludge incineration, the local community has expressed preference
to locate any new incineration facilities in remote areas with good
air dispersion characteristics. We will take this into account.
We will also explore the co-location potential of the sludge incinerator
with other waste incineration facilities currently under planning
18. Once the
proposed strategy to incinerate sewage sludge is confirmed and adopted,
we will conduct more sludge characteristic tests to help determine
the most appropriate incineration technology and gas cleaning system.
19. Our tentative
programme is to initiate the Feasibility Study in mid 2000, award
the contract by 2005 and commission the incineration facility by
2008. An Environmental Impact Assessment (EIA) would be carried
out during the project feasibility study in accordance with the
Environmental Impact Assessment Ordinance.
are invited to note and comment on the recommended strategy for
the treatment and disposal of sludge in Hong Kong.