Sludge Watch ==> Technical report - problems with drying sludge into pellets

[Maureen Reilly]

www.wef.org/NR/rdonlyres/FD6131D4-F592-4B63-B50B-0997FF1A8454/0/The_Culture_of_Dryers_paperUSA.pdf


This is easier to read in the PDF format above



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The Culture of Dryers Background Dryer technology is not new. It has been 
used for many years in both the food manufacturing and process industries. 
Increased pressures on ‘land bank’ and changing standards for sludge 
disposal together withincreased quantities of sludge resulting from an ever 
expanding capital programme led the water industry inthe UK to look for 
alternative sludge treatment processes. This was further focussed when new 
EC regulations effectively banned the disposal of sludge to sea and the UK 
government increased tax on the use of landfill. Initially incinerators were 
seen as a solution but increasing public opposition to waste incinerators 
ingeneral effectively closed down this option. Dryers in the UK Water 
Industry Sludge drying was seen as a relatively new technology in the water 
industry but with a long track record inother applications. As a result all 
water companies placed a high reliance on the expertise of 
designers,manufacturers/importers and suppliers. In general the dryer was 
bought either as part of overall installationor a package with much 
expertise directly transferred from food industry. In general dryers were 
seen aseconomic in operation, producing a saleable product and above all 
safe. At this point it is worth contrasting the food industry with the water 
industry. •Food IndustryLong history of dust explosions (Flour Mills) 
Closely controlled raw productNo contamination Batch process High Tech•Water 
Industry No history of dust explosionsWidely varying raw productHigh levels 
of contaminants Continuous process Relatively low techIt was against this 
background that several types and designs of dryers were installed at waste 
water treatment plants throughout the UK. Experience of Dryers in the Water 
Industry Overseas plants often given for reference to demonstrate the 
technologies being used. On investigation,later, these plants were sometimes 
found to be much smaller than the proposed plant or not running.Overseas 
plants also have had incidents but they had not been widely reported in the 
UK.To compound matters some of the installers in UK did not have detailed 
experience of operating their own plants and so operation was left with the 
water company itself. The first plants suffered some minor fires in product 
handling equipment and silos but these were considered as of little 
consequence and more of an irritant. There were two main approaches to the 
installation of the plant and equipment: 1)Dryer plant either retrofitted 
into existing buildings or in buildings supplied by others; often withother 
suppliers involved in upstream and downstream facilities 2)Dryer plant, 
buildings and upstream/downstream facilities provided as one package deal
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A series of incidents then raised the profile of dryers both within the 
industry and with the health and safety regulator (HSE). These included: 
During commissioning Feed pumps failed Insufficient product - therefore lack 
of steam and high oxygen levels Smouldering product Explosion in second 
phase of two stage dryerFirst running of dryer Product over dried in first 
stage of two stage dryer Dry product entered second stage, little steam 
Explosion in second stage The critical lesson learnt in these two incidents 
was that oxygen levels are crucial to the safe operation ofdryers and these 
are most likely to be high in start up. During commissioningHigh product 
temperature noted in siloSilo emptied into bagsBags allowed to coolCooled 
bags placed in store Product re-heated! Bags, rapidly, removed from store. 
This made it quite clear that in the right circumstances the product can 
reheat. During operation Sludge had built up at feed end and became dry 
During shut down steam generation reduced Sludge exposed to air (oxygen) 
Smouldering occurred If smouldering product had been drawn into unit 
explosion possible Here the plant was modified to prevent the sludge build 
up and to spray water in the affected area during shut down. During Shut 
Down A horizontal drum type dryer needed maintenance involving emptying of 
the recirculation system. The burner was shut down but the fan left running 
to transport product. Air was blown into the hot dryer and a major explosion 
occurred. Investigations showed the plant operators had witnessed similar 
shutdowns, without incident, duringcommissioning but it was not covered by 
operational procedures and so they were unaware of all the precautions the 
commissioning engineers may have implemented.Damage has not only been 
limited to the plant and equipment. In the above incident the resulting 
explosion was vented at the bag house filter (which was also damaged). The 
explosion relief was within the buildingand the resulting pressure in the 
building blew out part of an external wall. A lesson from this is that where 
explosion venting is within a building the building must be designed for any 
resulting over pressurisation.HSE Involvement Following several of these 
incidents being notified as ‘Dangerous Occurrences’ under the Reporting 
ofInjuries, Diseases and Dangerous Occurrences Regulations (RIDDOR) to the 
HSE meetings were set upwith manufacturers/importers/suppliers, the water 
industry and water industry users. These meetings acted as a catalyst for 
understanding and as a result guidance was produced for HSEinspectors. 
Essentially the advice hinges on the preparation of a ‘Basis of Safety’. 
This sets out the criteria for how safe operation of the dryer can be 
managed.
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As a result of their meeting with the HSE the UK Water Companies set up a 
National Dryers Group tobring together expertise and experience in the 
industry. This group has assisted the HSE in revising their Guidance for 
their own Inspectors. The HSE is continuing it’s involvement with dryers by 
conducting a series of site visits to all UK dryers and contrasting the 
installation to both its basis of safety and the guidance. This is not an 
empty gesture as one plant has been closed down as a result of such a visit. 
Knowledge Gap A knowledge gap has been identified between the dryer 
suppliers and the water industry. The dryersuppliers believed the water 
industry were experts in sewage sludge. On the other hand the industry were 
unaware of the critical parameters applicable to dryer raw feed. What was 
considered normal within the industry was extraordinary to the suppliers. 
For example the suppliers expected fine control of feedmaterials (inc. 
moisture content) and believed as there was grit removal on the plant there 
would be no gritin the sludge. The industry believed the plant was fully 
automatic and could be operated and maintained by ‘normal staff’ Best quote 
“ They were right when they said it would operate fully automatically. What 
they didn’t say was for 15minutes at a time!”. Even simple terms such as 
‘maintenance free’ took on different interpretations. To the users it meant 
fit and forget but one supplier, when pressed, stated it meant “no routine 
visits by the supplier”. The suppliers did not challenge the users view but 
when pressed recommended minimummanning and maintenance significantly above 
initial suggestions. Even such a simple word as ‘Contaminants’ took on new 
relevance. The suppliers viewed substances such as: Hydrocarbons Grit Fibre 
metallic objects and even ferric (iron sulphates) as contaminants to be kept 
far from the dryers. These ‘contaminants’ were seen by the industry as part 
and parcel of sludge and therefore normal. It was only when these were 
understood that a common understanding could be achieved and a 
rationalestablished for the problems being encountered. Critical Factors To 
the water industry the critical thing about sludge was its water content and 
biological make up together with heavy metal content if disposal was to 
land. But for sludge drying a whole new language came into the water 
industry. These describe the critical factors which determine not only is an 
explosion or fire ispossible but how to control the process. Manufacturers 
and suppliers tend to use shorthand for these factors:MOC - Minimum Oxygen 
ContentMPOC - Max Permissible Oxygen Content (MOC -2%) Thermal Stability - 
Basket TestsPmax - Maximum Explosion PressureKst - Pressure Time 
Characteristics (Explosion Violence)MEC - Minimum Explosable 
ConcentrationMIT - Minimum Ignition TemperatureMIE - Minimum Ignition 
EnergyThe operations and process staff within the industry had to learn and 
understand the implications of these factors. A major lesson is that these 
factors change for a given sludge over time and due to outside factors. If 
in addition a plant takes in a new sludge or there is a process change on 
site changes can be sudden and dramatic. Changing from a predominately 
digested sludge to raw sludge can not only change the nature ofthe final 
product but also the safety factors and safety margins within the plant. 
Initial Incidents The critical phase, where the most incidents occur is in 
start up and shut down. Simply mix product, temperature and oxygen. Some of 
the incidents are outlined above.
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Subsequent Incidents Once the critical factors had been established the 
designs and installations improved. However incidents continued to be 
reported which led to the issues being constantly revisited.These included: 
Self heating product – product volumes are critical, segregation needed for 
cooling in bag storesPoor fitting/maintained hatches – maintaining air tight 
joints essential High wear rates (Grit) – expect high wear rates (bends on 
transfer ducts) Dust build up (ducts, buildings) – essential to have regular 
cleaning to high standard (secondary explosionscan be more severe) 
Smouldering – this can occur even at quite low O2 or local to any air 
ingress Insulation effects of product – can affect product cooler 
efficiencyTypical Incidents included:Viewing hatch became obscured by 
product/dust Hatch opened to allow cleaning Oxygen drawn in over hot product 
SmoulderingPrompt action prevented explosion Lessons were learnt from this 
including the need for operator training and awareness (even cleaners!) 
Several fires have occurred in silos Product either: Entered silo with 
excess heat Self heated within siloContinental silos often have inert gas 
systems This demonstrates that transfer of experience is essential, even 
between countries. Several ‘smoulderings’ where product has been stored in 
bulk or in closely packed bags Reasons givenbags sealed preventing cooling 
bags open allowing air ingress moisture content of product (inc. rain) 
Product classified under CHiP Regs for transportation by road. In a more 
recent incident incandescent material produced within dryer and carried by 
product bucketelevator to pellet mill. Sensors picked up problem as minor 
explosion occurred but suppression systemprevented major explosion. Plant 
crash stopped. The product in the elevator buckets then caught fire burning 
through the elevator belt which collapsed intobase of elevator. The 
resulting fire put out by fire brigade The lesson of this incident is that 
whilst explosion suppression can be highly effective it is in essence is 
a‘one shot’ option. The material may continue to burn and you have just used 
up your one defence. It may be necessary to have back up to the suppression 
system to deal with any fires. These incidents are reported to the HSE where 
applicable and discussed at the Dryer User Group. The lessons learnt are 
shared throughout the UK Water Industry. Overseas Incidents Keeping an eye 
on the Internet shows the UK experience is not unique. One from Canada 
created some concern-“Forests were not the only things burning in Ontario. 
People complained about a horrid odour from piles ofsewage sludge pellets, 
which were smouldering in a field where they had been dumped for storage. 
Apparently, the City of Toronto’s sewage sludge pellets had undergone 
spontaneous combustion, a process where the microbial activity in the pile 
caused the pellets to heat to the point of burning. The pellets had been 
heating up in the storage silo and staff had been forced to remove the 
pellets from the storage silo to prevent an explosion or fire.”
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And one from the USA really showed you cannot look at the dryer in 
isolation-A $9million system first of its kind in US was commissioned 
30/10/2000. It was reported that on19/12/2000 Dryer 1 blew up and Dryer 2 
shut down for inspection. Dryer 2 re-started 3/01/2001 but subsequently 
caught fire 18/05/2001. Then both sludge dryers fail -one explodes, then 2nd 
dryer catchesfire. With nowhere to process sludge there was a sludge builds 
up in primary tanks. The scraper bridges faildue to sludge build up. As a 
result sludge carry through inhibits UV plant and the plant fails resulting 
inmultiple violation of state regulations. Current UK situation Dryers still 
represent the only viable sludge treatment/disposal route for many waste 
water treatment works. New dryers are being constructed taking into account 
lessons learnt from the first generation. One site experienced a significant 
explosion which resulted in a forty foot by 10 foot hole in an external 
wallfortunately without injury to personnel. Lessons are being learnt from 
this incident and communicated throughout the industry. The main areas of 
concern is the high level of maintenance and wear rates in existing 
installations. Typically: Dryers in use for some timeRotating screws worn 
away due to abrasive product/poor material selection Wear eroded metal until 
heating oil containment failedHeating oil mixed with product! Checks on 
other dryers revealed high wear rates Again high abrasion Due to different 
design and materials failure occurred in dryer wall Dryer patched with steel 
plate Corrosion of the dryer wall resulted in air ingress. When the dryer 
was shut down air entered whilst the dryer was still hot in a localised 
area. Dust in the dryer became incandescent and gently smouldered until all 
the material in that area combusted. Other Cultures The ‘product’ often goes 
to outside agencies including farmers and agricultural contractors. As 
aresponsible industry we have to ask “Does their ‘culture’ support the risks 
involved with flammable dusts?” And “Can they follow rules for storage and 
spreading?” It really becomes a product liability issue. Where are we now?We 
have come a long way but we have a lot to do to reach the current standards 
of the chemical or food processing industries. We are working with the 
regulators (HSE) as the ‘Water Industry UK’ as the Dryer User Group to share 
experience and knowledge. Many plants are being reviewed and updated to 
current standards to meet new HSE Guidance to Inspectors. Conclusion Dryer 
technology continues to offer a realistic means of cost effective and 
environmentally sound means of treating waste water sludges. In many 
situations it represents the only realistic economic solution. This is often 
the case where no landfill or suitable land bank for disposal. New 
technologies such as co-firing, gassification, and existing technologies 
such as incineration may require dryers as a pre treatment stage. The 
lessons learnt from the process shows what can result from the clash of two 
cultures who use the same words but a different language. The expectations 
of the water industry came from its ‘heavy engineering’ background. This led 
todisappointment when the process was found to require intensive operational 
and maintenance inputs. Manyof the issues related bot to the dryer itself 
but to its interface with the existing sludge process.
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The suppliers on the other hand were used to their equipment being used in 
the relatively high technologyenvironment of the food industry. Because food 
products are strictly controlled and often have a high value added factor it 
is normal for intensive operational and maintenance input. As high 
technology solution to other water treatment issues become available the 
industry will face thesame cultural issues as it experienced with dryers. 
Learning the cultural lessons should help in the smooth,and safe, 
application of new technologies. Christopher Lee Safety Manager United 
Utilities