According to the Office of National Statistics, by 2028 the UK population will have passed the 70 million mark, and the impact on the water and waste industries will be massive.

Whether rich or poor, people have to eat and drink and, inevitably, produce waste as a result. The treatment and disposal of sewage is already tightly regulated but the burden will increase as the population grows and the volume of land available for disposal shrinks.

Sewage treatment is a very energy-intensive business, yet, in common with all other industries, it needs to reduce energy-consumption to meet legislative and environmental demands. Thus the industry faces a conundrum: it will need more capacity to treat more sludge but, simultaneously, will also have to find ways to reduce its energy consumption.

This was highlighted by a report presented to the House of Commons in April 2008, which showed that treating the ten billion-plus litres of sewage sludge currently produced in England and Wales accounted for 1% (or 6.34 gigawatt hours) of total daily electricity consumption.

Of course, large strides have been taken to reclaim some of this energy. In 2005, waste (including sewage sludge) combustion and biogas accounted for 10.8% and 4.2% respectively of all UK renewable energy. So there is already a clear understanding of the need for more efficient energy recovery. However, the industry should not just be considering how to recover more energy but looking at ways to reduce the quantity consumed.

Several phases of the sludge treatment process lend themselves to improved energy efficiency. A prime candidate is the dewatering process, which can account for as much as 30% of the total energy expended.

Any machine or system that allows this figure to be reduced will also have a significant impact on overall operating costs, and research indicates that one of the quickest wins can be achieved simply by using sensor technology to improve the performance of key equipment. In one study, scientists used sensors to adjust pumping in line with product flow and achieved energy savings of between 30-50% as a result.

Similar savings figures can be achieved by transferring the job of controlling decanter centrifuge performance from the hands of operators to the tentacles of a system known as Octopus. Decanter centrifuges are acknowledged as one of the workhorses of sludge dewatering and their use is becoming more prevalent, with increased demand for ever-drier “cake” to facilitate mechanical handling and sludge disposal. To achieve this, a new generation of centrifuges that operate at higher rates while using less energy has been introduced.

One of the attributes of a good manager is the ability to react flexibly and intelligently to changing circumstances, and the same characteristics are needed in dewatering sewage sludge. A skilled decanter operator reacts to changes in feed concentration and thickness and makes the necessary adjustments to ensure uniform performance and sludge quality. However, manning a dewatering machine 24/7 makes it a costly operation. Generally, the night-time hours, when pay is usually the highest, are those with the lowest manning levels.

Unfortunately, waste production does not necessarily sleep just because we do. The waste stream may slacken but it never stops. With unmanned decanters, however, variations in the sludge feed can go unnoticed, with the result that key performance parameters such as cake dryness, polymer consumption and the solids content of the centrate can be adversely affected.

Infrared light

Many companies are developing systems to counter this problem. One of the first was Octopus, developed by Alfa Laval as an autopilot for decanter centrifuges in sludge dewatering systems. Octopus exploits modern sensor technology to monitor decanter operation on a continuous basis, automatically monitoring and controlling the following key parameters:

  • The suspended solids content and the flow rate of incoming sludge
  • The suspended solids content of the centrate (water removed from the sludge)
  • The rate at which polymer needs to be added to flocculate the sludge
  • The differential speed and conveyor torque of the decanter

The system scatters infrared light into the target stream and then measures the reflected light via two light sensors, one of which measures density while both together measure solids content. It then compiles the data and analyses this using a bespoke software program that makes the appropriate changes in and around the decanter to optimise the process.

A plant operator can set the desired minimum and maximum values for sludge flow rate, polymer dosing, dry-solids recovery and other crucial operating parameters. Major ancillary cost factors, such as polymer costs, energy and transport can also be programmed in.

This provides a sewage treatment works (STW) with various options when it comes to programming Octopus. It can opt for lower costs; to optimise different aspects of the sludge dewatering process, such as cake dryness or polymer consumption, to meet disposal or environmental standards; or a combination of the two.

Since the system automatically monitors and controls the process, it enables dewatering decanters to run unsupervised, thus saving labour costs.

In operation at sites in the Netherlands, the US and the UK, Octopus has achieved “impressive results”. In one installation, it reduced polymer consumption from 12kg/TDS to 8kg/TDS and increased the average solids recovery from 94% to 98%.

Significantly, the best performance was achieved during the night shift when the dewatering function was unmanned. More pertinently, from the perspective of energy savings, it was calculated that Octopus had reduced energy costs by as much as £15 per ton of dry solids.

Better control of the dewatering process is one way of saving money. Another Alfa Laval innovation provides yet another: Power Plates can cut decanter centrifuge running costs by up to 25% by reducing their power consumption.

During centrifugation, a significant amount of the energy used is transmitted to the fluids being processed in the form of rotational velocity. Power Plates work by reclaiming some of this energy before the liquid leaves the bowl and transferring it back in the opposite direction of bowl rotation. As a result, the total effluent discharge velocity is reduced, and as the energy content of the discharged liquid is proportionate to the square of the velocity, even a small velocity reduction can achieve significant energy savings.

Used correctly, Power Plates can provide energy savings of between 10-25%. Obviously, the larger the decanter involved and the greater the initial energy usage, the greater the financial savings that can be made. Under ideal operating conditions, savings will be sufficient to pay back the cost of installation in 12 to 18 months.

In a typical centrifuge installation at a water and wastewater treatment works, an NX945 decanter ran for eight hours a day, processing 60m3/h of effluent. After the installation of Power Plates, the decanter’s energy consumption dropped 20%, from 58kWh to 46kWh. Energy savings were not the only beneficial effect because the plant has also significantly reduced its CO2 emissions and, as a result, the decanter’s environmental impact.

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