Evolution keeps Huber on toes
European legislation is having a big impact on the treatment and disposal of sewage sludge. To meet the demands and needs of the times, Huber Technology has developed a range of products for volume reduction and screening of sludge.
Sludge treatment can be subdivided into several clearly defined areas of activity - pasteurisation, drying, disposal to land, landfill and incineration - depending on the strategy of the individual water company. All of these processes, combined with the increasing need to move sludge from smaller sites to sludge treatment centres, require volume reduction or specific screening either for process or transportation reasons.
As sludge treatment and disposal becomes a larger proportion of individual water companies' budgets, so pressures on expenditure increase.
To this end Huber has endeavoured to minimise the capex and opex costs of its sludge-treatment equipment. The small footprint of most units, reducing sludge building dimensions typifies this philosophy. Integral odour control enclosures further minimise required floor space and improve access for maintenance. In particular, Huber's Gravity Belt Thickener has been simplified so that there are only three moving parts, is totally enclosed and compact. Larger gravity belt thickeners are available as two-tier units resulting in a small footprint.
According to Huber, its Drum Dewaterer has low energy and polymer consumption, and yet can produce up to 33% dry solids. These are just some of the examples, Huber says, of how it has designed its sludge treatment equipment to be economic from a capex and opex viewpoint.
Guidelines on odorous emissions are becoming increasingly stringent, with H2S at 5ppm being the current suggested figure. The inherently enclosed design of Huber sludge-treatment equipment, when combined with an appropriate odour extraction system, will meet all current and anticipated odour emission guidelines, without the need for separate enclosures - flanged connections are provided for connection to odour removal ducting.
Increasingly, sludge is being transported from smaller sites to larger works for further treatment, and sludge being offloaded from tankers at sludge-treatment centres is often either inadequately screened or even unscreened.
The Huber Rotamat Sludge Acceptance Plant units are suitable for screening at tanker off-loading points prior to further processing. The screenings are washed to remove biosolids, producing hygienic solids. These units are supplied as compact, free-standing, packaged plants and, with suitable frost protection, are usually sited outside. There are more than 450 of these units installed in the UK.
Finer two-dimensional sludge screening is often required for specific operations. A typical example being sludge fed to dryers where the removal of fine material is necessary. The Huber Strainpress is built for such applications, and many units are installed upstream of dryers and elsewhere. In the UK alone more than 200 units have been installed.
It should be noted that either type of sludge screen can be used, and are suitable for screening all types of sludges in all areas of the process.
For medium to larger works, gravity belt thickeners are still the most common method of thickening wastewater sludge's in the range of 0.5-3% dry solids requiring thickening to 7% dry solids and polymer consumption of about 3-4kg per tonne dry solids.
Huber says it has simplified its Gravity Belt Thickener to the point where there are only three moving parts, resulting in a highly reliable unit requiring the minimum of maintenance. The visual impact is of their small size, enclosed design and all-stainless steel construction.
Hydraulic guiding and belt tensioning systems are replaced by a mechanical belt-guiding and tensioning system that makes no belt contact. The avoidance of belt contact by the guiding system, combined with the low belt tension, prolongs belt life. The odour enclosure is integral to the unit rather than separate from it, reducing the carbon footprint and allowing easy access for maintenance.
Huber Rotamat Drum Thickeners have been developed from the Rotamat Screen and are for thickening sludges in the range 0.5-3% dry solids to 12-13% dry solids although 6-7% is the usual requirement.
With Huber Drum Thickeners, the drum itself does not rotate and is of all-stainless steel construction, including the filter media. The stainless steel mesh is fabricated from strips 2mm wide, with a gap of 0.2mm, and is designed to last the life of the machine.
The unit is inclined at 30˚ and the thickened sludge is transported by screw feed and deposited in an outlet hopper. The tips of the screw feed flights are equipped with brushes, which continually sweep the inside of the drum to prevent internal clogging.
The outside of the mesh is washed intermittently, and so reduces absorbed power. The whole unit is compact, totally enclosed and designed to work with minimum operator intervention. Some of these units have been in operation in the UK for ten years or more. Typical flow rates are to 8- 90m3/h
For smaller sites with sludge flow rates below 40m3/h, there are the Huber Rotamat Disc Thickeners. Essentially, the units comprise an inclined, slowly rotating stainless-steel-mesh disc, which is fed the sludge.
The flocculated, unthickened sludge pools at the lower end of the inclined disc, and water freely drains through the mesh disc. The remaining solids are carried around and up the disc, where it is diverted into an outlet port. These units, like all Huber thickening and dewatering equipment, are of all stainless steel construction and inherently enclosed with a connection for odour extraction ducting.
The Huber Rotamat Drum Dewaterer is designed for smaller works where dewatering is required on site. The unit will dewater sludge as low as 0.5% dry solids without pre-thickening.
Two capacities are available, with flow rates of 12 and 20m3/h. The sludge is firstly thickened in the drum section and transported from the thickening drum by screw feed that compresses the sludge against a restricted outlet, further dewatering up to 25-32% dry solids.
Despite the high dry solids achieved polymer consumption is low at about 3kg/tonne dry solids and energy consumption is also low, at about 4.5kW/h absorbed power for a 12m3/h unit.
These units are also inherently enclosed to minimise odour emissions and are designed to operate with minimum operator intervention. Noise levels are low and in the region of 62dBA at one metre. Flow rate is 12- 20m3/h. Achievable dry solids are up to 32%.
There is a growing requirement for high dry solids, which is met by drying. Drying is now a well-established method of producing high dry solids prior to landfill, disposal to land and incineration. The Huber Low Temperature Dryer can achieve dry solids in the range 30-95% depending on the requirement. Low temperature drying harnesses the capacity of ambient air.
Huber says its engineers have become skillful in harnessing waste heat from CHP plants and other sources.
The company has patented the principal of recovering heat from final settlement tanks by the use of heat pumps. Low temperature drying has other advantages - odour emissions are extremely low and do not require further treatment, acceptable levels of dust emissions without filters, virtual elimination of fire or explosion risk. There are now 22 Huber dryer units installed in Europe.