Sludge Treatment and Disposal Strategy Study
(ACE Paper 52/99)
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 1).
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 disposal.
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.
FIGURE 1. PROJECTED DAILY SEWAGE & WATER TREATMENT WORKS SLUDGE ARISING
TABLE 1. CURRENT AND PROJECTED SLUDGE ARISING AND LANDFILL ASSIMILATIVE CAPACITY
MAJOR FINDINGS AND RECOMMENDATIONS OF THE STRATEGY STUDY
6. Waterworks 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 measure.
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.
9. Although 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 landfill.
- 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.
- Screening and grit from sewage treatment works should be sent to landfill for disposal.
- Inert water treatment works sludge should continue to be dewatered prior to landfill disposal.
EXISTING 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 the incinerator.
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.
ENVIRONMENTAL 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 and investigation.
THE WAY FORWARD
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.
20. Members are invited to note and comment on the recommended strategy for the treatment and disposal of sludge in Hong Kong.
Environmental Protection Department