Dewatering: an alternative to belts and centrifuges

As European legislation continues to affect treatment of sewage sludges, so Huber Technology continues to develop and expand its ability to meet the needs of this evolving area of wastewater treatment, says the company

With the increasing numbers and types of products entering the market, end-users are not only taking into account the initial CAPEX price of a piece of equipment but are bringing into the decision matrix the calculated and verified whole-life costs.

To fulfil the requirements of customers, Huber Technology says it has endeavoured to minimise the CAPEX and OPEX costs of its complete range of sludge treatment equipment. This includes designing its range of units with a small footprint to reduce the sludge-building dimensions required and ensure the design includes integral odour control enclosures that further minimise floor space and eliminate the need for further odour control equipment, consequently improving access around the unit to allow routine maintenance to be carried out quickly and safely.

Further examples include the Huber Drum Dewaterer using a very small and efficient motor and gearbox combination so that it has an extremely low energy requirement against other dewatering types of equipment, according to the company. The unit, it says, also performs very well in terms of requiring a very low polymer consumption while producing up to a 33% dry solids end product.

New way of working
There is a well used saying within the industry that needs to change, says Huber, which is: "We have always done it that way before." To make sure we achieve the efficiencies and reductions in respect of CAPEX and OPEX and carbon reduction a new open and receptive way of working must be adopted. We have to look at alternatives to the old methods of sludge dewatering such as belt presses, and centrifuges and look to the more energy efficient and reduced whole-life cost equipment on the market today. One product that must be considered is the Huber RoS range of sludge dewatering equipment.

Huber's RoS3 Drum Dewaterer, or Screw Press, offers a reliable, stainless steel alternative to belt presses and centrifuges The smaller size has a capacity up to 10m3/hr (subject to feed sludge type) and the larger size up to 20m3/hr.

The RoS3 size two unit has a single, 1.5kW VSD for the auger and a tiny 0.04kW drive for the spray bar. On a case study of 44m3/hr of mixed municipal sludge at 3% DS feed, a typical centrifuge solution worked out as being over 6.5 times more expensive than a Huber RoS3 or belt press solution when considering power use alone, the company says.

The RoS3 will typically have a similar polymer demand to a belt press of around 5kg/tDS. It is common knowledge that centrifuges require significantly more polymer, perhaps up to twice as much, compared to this as a result of the high rate of shear induced by the rotation speed.

The RoS3 is fully enclosed so all odours are contained and can be removed by odour extraction systems when connected to the available flanges on the unit.

The Drum
Dewaterer can cope with process changes without the operator having to clean up spills afterwards. If a polymer goes off or if sludge has sat inside dewatering after an emergency stop, it will simply start-up again in auto without complaint, once the issue is resolved.

It is a very good solution where the feed sludge may contain elements that could
otherwise potentially blind a belt, for example.

The RoS3 therefore requires lower operator attendance time than alternative dewatering technologies. The machine's ability to cope with greasy and blinding sludges not only comes from its fully enclosed design but also from the edgewire
construction of the inner drum. The slot-like apertures are less prone to blockage than the weave on a belt, for example, yet still allow the RoS3 to achieve a solids capture rate of 95%.

Final effluent can, of course, be used as wash water and a variety of optional nozzle types are available to cater for FE of differing cleanliness. But where potable water is to be used and is of limited availability, the water demand associated with an RoS3 is significantly lower than that of a belt press, Huber claims. On the same 44m3/hr case study, a typical belt press solution would require over 30m3/hr if running continuously, whereas an RoS3 solution would typically require less than a tenth of that.

The RoS3 can achieve high cake drynesses as required, comparable with those from centrifuges and belt presses - 25% DS cake is a typical requirement on municipal applications but in some areas the success of the unit with mixed sludges has led to 27% DS that the Drum Dewaterer can readily achieve. Due to the ease of parameter adjustment (such as auger rotation speed and the duration between washing sequences), the cake dryness can be wetter than this if required. High cake drynesses have been observed from RoS3 applications even with high proportions of secondary sludge in the mix.

The RoS3I industrial variant of the Drum Dewaterer incorporates a larger main drive and pneumatically controlled release valve that allows the degree of dewatering to be reduced if the sludge is becoming too dry (such that it could potentially damage the wedge-wire). There are a number of UK references on
applications that include paper production, nonmetal recycling from cars and kitchen appliances and sludge composting. Globally, this machine has also been used to dewater manure/slurry, oil and distillery sludges among others.

The newer RoS3Q is derived from the RoS3I and is designed for smaller flows. The smaller Q280 unit would typically accept around 2m3/hr of a thick feed sludge and the larger Q440 around 4m3/hr. Up to double these flow rates may be possible with smaller solids loads and particularly dewaterable sludges.

Recent trials in Arizona found the RoS3Q achieved impressive performance for small flow rates of biological/low-fibre sludges - 20-22% DS cake was produced from a feed of 1.5% DS of secondary sludge from an SBR plant. The unit can be positioned horizontally or on an incline subject to the desired throughput relative to the desired output cake dryness.

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