Disposal of sewage sludge in Northern Ireland up to the end of 1998 had traditionally concentrated on agricultural land and dumping at sea from Belfast. However the implication of the EC Urban Wastewater Treatment Directive is that sewage sludge quantities are set to increase substantially and at the same time one of the main disposal routes, sea disposal, is now prohibited.
A sewage sludge disposal strategy was developed in 1994 which recognised the important role for incineration. The Best Practicable Environmental Option (BPEO) principle established by The Twelfth Report of The Royal Commission on Environmental Pollution (1988) was applied in the development of the strategy.
By 2010 total sludge production was estimated to be 52,000 tonnes dry solids (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 developments which put greater constraints on land disposal a review of the strategy was undertaken, with a revised sludge production circa. 40,000 tds by 2010 and recommending an expansion of the incineration disposal route by adding a second stream added to the existing stream at Belfast’s Duncrue Street, enabling all of NI wastewater sludges to be incinerated.
In 2003, expansion of incineration capacity was identified as a private finance opportunity, and the Sludge Disposal Service identified as a potential long term Public Private Partnership.
In 2007, a Public Private Partnership (PPP) contract was awarded - Project Omega. This contract includes the delivery of the second stream incinerator and the operation of the disposal service up to 2032 by private sector partnerships.
The contract recognises the need for disposal of sludges through alternative licensed or approved routes at times when the incinerators are unavailable, either through planned or unplanned shutdowns for maintenance or upgrading.
More information on the Project Omega PPP Contract can be found in our Major Infrastructure Investments section of this website.
Technical Information on Incinerators
Sludge Reception and Processing
The sludge reception area consists of six odour-controlled sludge storage tanks. If the incinerators are down for maintenance, there is an odour-controlled emergency cake store. The odour control system extracts the odorous air from the six liquid sludge tanks and the emergency cake store. The odorous air is treated within the two installed biofilters, and the treated air from the biofilters is then passed through an odour-polishing filter that contains activated carbon prior to being vented to air via a 16m stack.
Indigenous sludge with a dry solids content of circa 2% is received into indigenous tank 1 or 2 prior to being pumped into blending tanks 1 and 2 where it is mixed with the liquid sludge that has been received from incoming vacuum tankers.
Liquid sludge with a dry solids content of circa 5% is transported in vacuum tankers from the outlying WwTW sites and is discharged into either the new reception tank or the old reception tanks. It is then screened before being pumped into blending tank 1 or 2, where it is mixed with the arriving indigenous sludges.
The blended sludges of indigenous and liquid sludges received in the vacuum tankers are then passed from blending tanks 1 and 2 into buffer tanks 1 and 2 before being pumped to one of the two site centrifuges where they are dewatered to circa 25% dry solids. The dewatered sludge is then conveyed to the thermal dryer where it is dried to circa 33% dry solids before being incinerated. At this level of dryness the material becomes autothermic i.e. it will burn without the need for additional fuel.
All assets for the collection, treatment and storage of sewage sludges are covered, odorous air extracted and treated via a centralised two-stage odour control unit.
Incinerator No.1
Incinerator No.1 is designed to burn 24,000 tonnes of sewage sludge dry solids a year. It can also burn up to 2,000 tonnes of dry screenings per year but with a reduced sewage sludge throughput. The maximum design feed rate to one incinerator is 3 tonnes dry solids per hour. The feed material to the plant is referred to as sludge cake (nominally 25% solids). Sludge cake can be imported from sewage treatment dewatering centres located within Northern Ireland, however the bulk of the sludge arrives as liquid via the Belfast Wastewater Treatment Works (WwTW) indigenous pumps or via vacuum tankers.
The incinerator is of the fluidised bed type. Within the unit there is an ebullating bed of hot sand, which is held in a fluidised state by means of pre-heated combustion air. Once the dewatered sludge cake enters the fluidised bed and it comes into contact with the hot sand the water content of the sludge is evaporated and the dry sludge burns to maintain sand temperatures of circa 850oC. Combustion of the sludge cake also takes place in the freeboard area above the bed. To control operating temperatures within the incinerator the operator has the option to utilise the water suppression system, if required.
Thermocouples within the fluidised bed and the freeboard zone ensure that sewage sludge is not fed into the incinerator unless the required minimum temperature of 850oC has been reached in the freeboard area. A gas oil burner is used at start up to enable the fluidised bed to reach its minimum operating temperature and also during normal operation to supply supplementary heat to the combustion chamber if required.
Combustion gases from the incinerator pass through a waste heat recovery boiler and then through an electrostatic precipitator to remove particulate matter. This is followed by two stage scrubbing via spray quench and packed column scrubbers, which principally remove acid gases. The exhaust is then passed through a heated absorber unit containing activated carbon and a natural alkali alumino- silicate material, which removes volatile metals and organic compounds (principally mercury and dioxins). A reheat facility heats the exhaust gases to circa 70oC before venting to the atmosphere via a 70m stack.
Steam from the heat recovery system is utilised on the line 2 dryer when line 1 is in operation.
The quench and packed scrubber closed circuit water systems are water-cooled. Final effluent from the adjacent WwTW is used as the cooling medium; this final effluent is not contaminated by the process and is returned to the head of the WwTW. The scrubber effluent which contains acids and metals removed from the incinerator exhaust gases is transferred to a dedicated effluent chemical treatment plant to remove mercury and other heavy metals. Following treatment, the effluent is transferred to the head of the WwTW. Site drainage water is collected and is also returned to the head of the WwTW for treatment.
The sludge from the scrubber effluent treatment plant is dried to approximately 30% dry solids using a plate press and this material is sent off site for disposal to a licensed landfill facility. The ash from the discharge hoppers of the electrostatic precipitator is passed by conveyor to the ash storage silos. To control dust emission the ash is conditioned with final effluent water and trucks are loaded within an enclosed cell. The ash is removed off site and recycled.
Targeted pollutants are continuously monitored both in the emission from the main stack and the effluent that is returned to the head of the wastewater treatment works.
Odorous air from Line 1 is extracted and utilised as process air whilst Line 1 incinerator is operational, on the occasions when Line 1 is offline the odorous air is extracted and discharged to the environment via the Line 1 70m emissions stack
Incineration Facility No.2
The Operator has a second incineration stream utilising similar technology and stack height to that used in the original facility with the following improvements implemented: -
- The second unit burns sewage sludge cake that has been dewatered to circa 25% dry solids prior to arriving at the site.
- During sludge cake unloading the reception pit area is maintained under negative pressure with the extracted air being passed as combustion air to the incinerator with any excess being discharged to atmosphere via the 70m stack.
- The second fluidized bed incinerator is fired on natural gas, to preheat the incinerator and also as a support fuel (when required) by injection through the bed lances.
- Superheated steam from the waste heat boiler is fed to a more efficient axial, condensing turbine/generator set capable of producing up to 3.4MWe output at normal conditions with both streams operating. The power will be fed back to the plant’s own 400 V switchboard to make up to 100% of the stream 1 and stream 2 energy demand.
- Liquor from the quench and packing scrubbers on both incineration plants is received at the SET plant liquor tank, this liquor is cooled as it is pumped through the air blast cooler pipework prior to being chemically treated and filtered through a sand filter before being discharged to the Belfast WwTW for final treatment.
- The ash from the discharge hoppers from stream No 2 electrostatic precipitators is conveyed to the ash storage/treatment area. The ash storage silos have a capacity of 120m3. Dust control during vehicle loading operations is affected by the use of ash conditioning techniques as previously described.
- The bunker building is equipped with a ventilation system to reduce the odour in the bunker and to prevent a higher concentration of CH4 and H2S in the air. The extraction system at the top of the building is connected to the fluidising air fan and to two vent fans. As long as the incineration is in operation, one vent fan is also in operation. When the incineration is out of operation, one vent fan will operate all the time.
- All odorous atmospheres in Line 2 are extracted and utilised as process air whilst Line 2 incinerator is operational, during periods when Line 2 is offline the odorous air is discharged via the Line 2 70 m stack
- Scrubber effluent from stream 2 also passes to the effluent treatment plant for the removal of mercury and other metals.
- As with stream No. 1 the main stack emissions to the atmosphere are continuously monitored for the same suite of pollutants required in the Waste Incineration BAT Conclusions.
Performance Requirements of Incinerators
The Omega contract requires the Contractor to operate the sludge incinerators in accordance with regulatory requirements, and in particular in accordance with the permit issued to the contractor for the operation of the incinerators. The permit is issued by the Northern Ireland Environment Agency under the Pollution Prevention and Control Regulations (Northern Ireland) 2003. The key obligations of the permit relate to the quality of gaseous emissions from the incinerators to the atmosphere and the quality of effluent emissions to the adjacent Belfast wastewater treatment works.