THAMES WATER’S CROSSNESS SEWAGE SLUDGE
- In June 1999 the Newcomen Society organised a visit to the new Sewage Sludge Incinerator at Crossness. Bruce Blissett went along: ..
For many years sewage sludge was transported from the Beckton and Crossness sewage works in specially designed ships to be dumped in the North Sea but in order to comply with EU legislation, this method of disposal had to cease by December 1998. Three alternative methods of disposal were considered and these were:
* Distribution on agricultural land as fertilizer
* Dumping at approved land-fill sites
* Incineration with energy recovery
Thames Water found that incineration with energy recovery was the most acceptable and practical option although about 40% of sewage sludge is still sold for use as a soil fertilizer.
In 1994, the local authorities gave approval for incinerators to be built at the Beckton and Crossness works. Her Majesty's Inspectorate of Pollution (now the Environment Agency) authorized Thames Water to operate such incinerators and, in the same year, an AMEC-Lurgi consortium won the £125 million contract to design, construct and commission the two incinerators. The Crossness plant was opened by HRH the Duke of Edinburgh on 4th November 1998.
(i) Sludge Preparation
Raw sewage sludge is blended in the approximate ratio 70:30 with surface-activated sludge (processed sewage sludge which has been oxidised biologically and therefore has a reduced calorific value). A polymeric electrolyte is dissolved in water and added to promote flocculation and allow dewatering to a dry solids content of 4.5%. The precipitated sludge is then forced through large vertical plate filter presses where the solids are trapped and further dewatered by pressurising the filter with compressed air. The target total solids content of the sludge cake prior to incineration is about 32%.
At the end of the dewatering process, each filter plate is separated in turn allowing the cake to fall into a hopper and onto a conveyor which transfers it to the sludge cake silo from which it is fed at a controlled rate into the incinerator.
The incinerator consists of a large diameter squat form cylinder above which a large diameter tube carries the burning cake upwards and onto the boilers.
Hot sand is contained at the bottom of the incinerator and air is forced into and drawn through the sand with a pressure drop of 90 millibars. The incinerator is maintained below atmospheric pressure by means of a fan situated downstream of the boilers. Sludge cake mixes with the hot sand which promotes combustion by heating it and increasing the surface area of the cake exposed to air. Combustion temperatures at sand level are about 850øC rising to 950øC at the very top of the incinerator. Ideally, the cake should burn and ascend from the sand to the boiler inlet in three seconds. These conditions are necessary to ensure that any dioxins coming from the burning sludge are destroyed. It may be necessary to burn additional natural gas in the incinerator in order to maintain combustion at this temperature. If raw sewage only were incinerated, no additional fuel gas would be needed through the sand with a pressure drop of 90 millibars. The incinerator is maintained below atmospheric pressure by means of a fan situated downstream of the boilers. Sludge cake mixes with the hot sand which promotes combustion by heating it and increasing the surface area of the cake exposed to air. Combustion temperatures at sand level are about 850øC rising to 950øC at the very top of the incinerator. Ideally, the cake should burn and ascend from the sand to the boiler inlet in three seconds. These conditions are necessary to ensure that any dioxins coming from the burning sludge are destroyed. It may be necessary to burn additional natural gas in the incinerator in order to maintain combustion at this temperature. If raw sewage only were incinerated, no additional fuel gas would be needed.
(iii) Steam and Electricity Generation
Following the sludge cake incineration, the combustion products pass through two boilers arranged in series; the first of these raises steam at a temperature in excess of 400oC to supply turbine driven alternators while the second boiler generates steam at 150oC for reheating the chimney stack gases.
(iv) Ash, Heavy Metals and Dust Removal
Combustion products emerging from the second boiler are cooled rapidly before entering a cyclone where the ash falls to the bottom of the cylinder for disposal while the remaining gases pass on through a bed of activated lignite coke which absorbs heavy metals such as mercury and any remaining traces of dioxins. Dust remaining in the flue gases is removed by passing the gas through bag filters which are periodically emptied automatically with compressed air. The ash, spent lignite coke and dust from the bag filters are bulked and disposed of in licenced land-fill sites.
(v) Removal of Acid Vapours
Acids are removed in two chambers arranged in series. In the first chamber, which is rubber lined, acid vapours such as those of hydrochloric acid, hydrofluoric acid, oxides of nitrogen (and presumably phosphorus) are washed out by a water spray. The gases then ascend a second gas scrubbing chamber where they are scrubbed with effluent water and sodium hydroxide to maintain a pH value at the bottom of the chamber of 7.5. This second chamber removes any remaining acids including most of the sulphurous acid (sulphur dioxide).
(vi) Analysis and Dispersal of the Cleaned Flue Gases
The cleaned flue gases are reheated with a steam heat-exchanger [see (iii) above] before they are discharged to the atmosphere so that no plume from the top of the chimney stack is visible. Heating the flue gases also causes them to be carried higher into the air by convection so that they will disperse at a higher level and over a larger area.
Certain constituents of the flue gases such as hydrocarbons are continuously monitored while others, such as dioxins, are measured periodically by independent laboratories from flue gas samples. Officers from the Environment Agency are allowed free access to sample the flue gases and to inspect the plant at any time without notice.
The entire incineration plant is housed in an unusual and attractive metal clad building with an outward curving side and an S shaped roof. The chimney at the north end has a convex curved side and although the plant was fully operational, no plume of smoke or steam could be seen coming from it. At the Crossness plant, two parallel sludge incineration lines produce 6 mega-Watts of power which is sufficient to drive the entire sewage works but at Beckton three incinerators produce a surplus of electricity which is sold to the National grid.
Visitors are given a short talk to explain the process before issuing them with hard protective hats for a tour of the plant. During the tour the only time the sludge cake can be seen is when it falls, after filtration, from the separated filter plates into a hopper below. At this point a man, with an implement resembling a long handled spade, dislodges any cake still clinging to the filter plates or remaining in the interconnecting holes.
Almost everything inside the plant seems to be constructed from or encased in galvanized metal. Most floors are made from an open metal mesh which allows through-floor visibility and good ventilation but which made me thankful for the protective hat.
After being shown the flue gas monitoring equipment, the tour ended in the control room from where temperatures, pressures and the general operating status of the entire plant can be monitored and controlled from computer terminals. The engineer demonstrating the computer software and who fielded in detail so many technical questions had soiling
on one side of his overalls but nobody dared ask what it might be.
My grateful thanks go to Malcolm Tucker for his help in completing this account.
This accoiunt appeared in the August 1999 GIHS Newsletter. I'm afraid the flow diagram is not retreivable from the ancient copy we have. Still working on it,.