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Sludge quantities for disposal
At present the annual output of sewage sludge in Northern Ireland is equivalent to 34,000 tonnes of dry solids (tds). Up until the end of December 1998 about half of the sludge was spread on agricultural land and most of the remainder (approx 15,000 tds) was disposed of at sea to a licensed area outside Belfast Lough. A small proportion, some 2,000 tds, was taken to landfill.

Additional quantities will arise as improvements to sewage treatment required under the EC Directive are implemented. By 2010 total sludge production is estimated to be equivalent to 52,000 tds. The 1994 strategy envisaged an incineration capacity of 24,000 tds and 28,000 tds was to be disposed of to agricultural land. (In the light of recent developments which put greater constraints on land disposal a review of the strategy is urgently proceeding.)

Environmental assessment
Emissions to air represented the greatest cause of public concern and Northern Ireland Water therefore carried out extensive studies to assess the likely impact. The impact of emissions from the incinerator depends principally on:-

• Emission standards
• Chimney height
• Prevailing meteorological conditions

The incinerator was designed to the German BimSch V90 emission standards.
The chimney height of 70 metres was agreed by the Regulator (in Northern Ireland, The Alkali and Radiochemical Inspectorate).

The dispersion of the emissions was studied by computer modelling to predict the impact on air quality at ground level.

Impacts were determined as minimal and not significant.

With respect to effluents it was predicted that satisfactory treatment could be achieved by discharging to the adjacent Belfast wastewater treatment works. However it was recognised that this needed to be validated by monitoring post commissioning and therefore provision has been made for the future addition of effluent treatment if required.

Ash was assessed by an independent accredited laboratory against the hazardous properties listed in the Special Waste Regulations (Northern Ireland) and classified as non-special.

Visual impacts have been mitigated by the architectural design of the building which aims to compliment and enhance the harbour environs in which it is located.

Emission limits are as follows:-

Continuously monitored pollutants 

• Pollutant Concentrations Daily (mg/m³) ½ Hourly
• Total Particulate 10 30
• Hydrogen Chloride 10 60
• Sulphur Dioxide 50 200
• Oxides of Nitrogen 200 400
• VOC's (as Carbon) 10 20
• Carbon Monoxide 50 100 (Hourly)

Intermittently monitored pollutants

• Pollutant Concentrations (mg/m³)
• Hydrogen Fluoride 1.00
• Cadium 0.05
• Mercury 0.05
• Total other metals 0.05
• Total Dioxins/Furans 0.1 (ng/m³)

NB: Other metals are; arsenic, chromium, copper, lead, manganese, nickel and tin expressed as the metal, taken together.

The Regulator also requires the supression of a visible plume down to -2 degrees centigrade. A CCTV recording of the stack is a requirement to demonstrate compliance.

Requirements relating to operating conditions, gas, oil, odour control, ash handling and noise are also included. A review of the Regulator's requirements may be undertaken after at least 1 year of operation.

Process Description
The process can be broken down into 3 main stages:-

• Sludge Preparation 
• Sludge Incineration
• Gas clean up

Preparation
Liquid sludge mainly from the adjacent main Belfast treatment works is thickened and dewatered to form a cake with average dry solids (ds) of 25%. Some liquid sludge is also imported for thickening and dewatering. The remaining sludge is imported as cake as 25% ds average.

A weighbridge is provided to monitor incoming and outgoing deliveries.

The cake is dried in a thin film drier using heat from the process to approximately 31% ds which is required for sludge to burn automatically.

Screenings from sewage treatment can be mixed with the sludge up to 10% of the total feed clock prior to passing to the incineration stage.

Sludge cake silos are incorporated in the reception facilities to enable a constant feed rate to be maintained. A separate sludge cake store has also been constructed to allow sludge to be stored during an annual shut down for maintenance. The capacity of the cake store allows for 2 weeks at maximum throughput.

Incineration
The mixed feedstock of sludge and screenings is incinerated in a fluidised bed incinerator at a temperature of at least 850 C to convert the organic matter into gases. The incombustile inorganic matter is carried forward from the incinerator into the waste heat boiler.

A supplementary gas oil fuel system operates during start up and other periods when combustion needs to be supported. Combustion temperature is controlled by varying the evaporation rate in the drier, initial pre-heating of the combustion air, spraying with final effluent and, if required, auxiliary fuel.

Heat generated by the incineration of sludge is recovered in a single pass vertical waste heat boiler and used for power generation, sludge drying, air preheating and flue gas reheating. Power is generated in a back pressure turbine using high pressure steam produced in the boiler. The temperature is rapidly cooled to approximately 220 C in the boiler before the gas clean up phase.

Gas clean up
The flue gas leaving the boiler is cleaned in a twin field electrostatic precipitator (ESP). The ash from the ESP hoppers is pneumatically conveyed to ash silos. For dust free handling and disposal the ash is wetted before discharge to lorries for disposal to landfill.