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Despite the trend towards mechanised sludge disposal, LAS International has made a success of doing it nature's way


No-one can deny that over the last few decades there have been considerable advances in the design and implementation of wastewater treatment systems. Manufacturers have met increasingly demanding discharge consents with newer and more tightly controlled systems that would have astounded the wastewater treatment operator of 20 years ago.

These developments have been achieved mostly through a general shift away from a reliance on the biological part of the process - often regarded as less controllable - and more towards mechanical and physical removal, which is seen as being better able to consistently meet the demands made upon wastewater treatment systems.

Whilst this trend has provided the improved treatment desired, there have been less desirable consequences, notably increased operating costs and increased sludge or biosolids production.

One or two companies, however, have bucked this mechanistic trend, realising that biological treatment - essentially a free gift of nature - still had considerable development potential.

One such company, LAS International, has for the last 30 years been designing and building low-cost biological wastewater treatment systems that meet today's consent levels. The LAS Aero-Fac and Accel-o-Fac systems are low-maintenance, low-energy systems which produce no odour and, perhaps more importantly, no sludge.

Because most development work on these systems has been conducted in North America, the processes have received relatively little press in the UK. More recently, however, things have begun to change. Operational costs are becoming more of an issue and sludge disposal is now recognised as probably the industry's biggest challenge.

Perhaps heralding a shift in thinking within the major wastewater operators, Scottish Water has been operating an Aero-Fac system near Dundee for four years, and is shortly expected to start building another two systems designed not only to produce no sludge, but also to take in septic sludge for disposal within the process.

These new plants will utilise Aero-Fac's ability to self-digest sludge in the primary treatment stage, eliminating the need for sludge-handling and disposal, and providing a means of disposing of septic tank sludge collected from the surrounding communities. LAS have now taken this one step further by designing a system for the on-site disposal of sludge produced by conventional treatment systems.

The biological process
The LAS Sludge Digester is designed to be a retro-fit to WwTWs where the costs of sludge-handling is becoming an important issue. Designed using either Accel-o-Fac or Aero-Fac technology, the system utilises the facultative digestive process to take in sludge waste from the treatment works for digestion on-site. Because the LAS Sludge Digester can accept sludge directly from the treatment process, the need for de-watering is removed, as is the requirement for the use of costly treatment chemicals.

The process employs three bacterial zones for complete treatment and odour control. Anaerobic bacteria convert the biosolids into methane, hydrogen sulphide, ammonia and carbon dioxide. A thin layer of facultative bacteria above the anaerobic zone uses available oxygen to stimulate the sludge digestion process. Aerobic bacteria in the upper layer convert gases and reduce soluble BOD, thereby eliminating offensive odour.
Envisioned primarily as a solution for sludge disposal in rural communities, the LAS Sludge Digester is designed, like all LAS systems, to be a low-maintenance process requiring minimal operational input. Systems can be sized to cater for the sludge produced by a single treatment works, or alternatively can be designed to operate as a regional sludge digestion centre, taking in wasted sludge from a number of works within the surrounding area.

With some recent estimates recognising that the cost of sludge disposal can amount to up to 50% of a treatment works' operational cost, it is not difficult to produce a cost/benefit in favour of a system that completely eliminates the need for sludge-handling, de-watering, tankering and disposal over the life of the works.

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