Energy efficiency in sewage treatment

Rene Clarke of Energy Pumps reviews the ways in which energy can be saved on sewage works by choosing the right size and right type of pump

One of the concerns of this modern era is the conservation of our natural fuel resources.

All the time we are being told about the 'Green House Effect' which will lead to the Earth's warming.

To some of us, this seems like a good idea for Britain.

Not so good elsewhere.

We are constantly being reminded of the need to conserve energy.

Our politicians are always seeking to address this problem, but the only real way to conserve our resources is to look at the financial implications of energy costs, and reduce them where possible.

The adage 'greed before need' addresses a thoroughly human trait; conservation of one's own resources.

If we can reduce our resource output to things like power usage, then we are in a position to apply a more generous portion to infrastructure, better conditions etc.

Although Water Companies are not the largest users of power, their contribution to society's general energy needs is significant.

This has increased over the years, as Water Companies have rationalised on manpower.

Now, instead of many operations staff, there is a lot of automated mechanical equipment in place to fulfill the previously human functions.

Hence there are a variety of large energy consumers, especially in the sewage treatment environment.

The main areas of power usage are: pumps; blowers; mixers; screens; centrifuges; heaters; aerators.

Of these, pumps are the largest consumers of power and it is this subject that needs to be appraised due to its complexity.

Over the years, inefficient and unreliable pumping equipment has been installed in the sewage treatment industry.

Due to the large numbers required, one of the main contributors to inefficiency has been auto-desludging pumps.

In many cases the wrong selection has been made concerning type of pump and motor sizes.

The average motor size for an auto-desludging pump on a small sewage treatment works in Britain is 4.0kW.

This size, or larger, is fine for a main treatment works where large volumes of sludge need transferring quickly, commonly to distant holding tanks etc.

However, this size is most common on the smaller satellite works, and most motors have normally been sized according to the efficiency of the equipment they are attached to.

The reasons for this are easily explained.

First, let us look at the different types of pump that are used for primary sedimentation tank desludging.

These are normally positive displacement type pumps, with suction lift capabilities.

Centrifugal and airlift types have not been included, as their installation is limited on PST tanks.

The pump types as shown are categorised into their different types.

This is because the method of pumping differs greatly between each category.

* PUMP TYPES in use within the Sewage Treatment Industry for Automatic desludging of PST tanks.

* Rotodynamic positive displacement.

There are two categories of this type of pump in use within the sewage treatment environment.

Of the two types, progressive cavity is the most popular due to their initial low purchase cost.

Both types of pump are a modern development, and thus call for exacting tolerances to enable them to work.

1) Progressive cavity.

This method relies on a wide friction interface between a metal rotor and a rubber stator.

The rubbing action of the rotor, together with the progressive cavity creates both good suction and delivery performance.

The only problem with this is when wear takes place at the interface; immediate loss of efficiency is encountered.

Figures of only 65% efficiency have been shown for this type of pump.

2) Rotary lobe.

At one time a very popular pump.

However, it became less popular due to its high wear characteristics.

This type of pump also relies on the friction interface between two meshing rotors.

This shows similar wear problems and loss of efficiency as per PC pumps.

Similar efficiencies of only 70% have been shown with these pumps.

Also, due to the interface between the rotor and the stator, or the meshing lobes, these types of pump need a larger motor to enable start up, and their pumping action causes sludge to shear and are generally not suited to sites with high grit and rag content.

The next two categories of pump are modern developments, but are adaptations of older pumping methods.

* Peristaltic positive displacement.

This type of pump relies on the application of pressure to, or squeezing of a flexible hose.

This is a good method of transferring high solids media, without any debris content.

The nature of this action calls for a high start up torque and a high constant load requirement.

This is due to the rigidness of the flexible tube, and the constant forcing of the tube to create a pumping action.

Efficiency figures of up to 75% have been shown with these pumps.

Once again, a larger motor is needed to overcome the inefficiency of the pumping action.

Also the life expectancy of the tube is limited to approximately 1,000 to 1,500 working hours, and is expensive to replace; calling for higher maintenance and downtime costs.

* Close tolerance reciprocating positive displacement or double disk.

This type of pump relies on the interface between a reciprocating disk and a bored cylinder.

This normally consists of two different sized disks that actuate 1x non-return valve.

This is a very energy efficient type of pump.

The only drawbacks with this equipment have been poor suction lift capabilities, due to the wearing out of the disks; as they rub on the inner bore of the cylinder, and can quickly lose effectiveness as there is no adjustment facility.

Efficiency figures of up to 85% have been shown with this type of pump.

* Reciprocating hydraulic positive displacement.

Perhaps the oldest forms of pump type known to mankind.

There are two categories of this type of pump, ram piston and diaphragm.

Both are a Roman invention.

Modern drive systems have been applied to these pumps, i.e electric geared motor and hydraulic drives now replace steam, water or physical input power.

1) Ram piston pumps.

These consist of a reciprocating cylinder that actuates two one way facing non-return valves.

The method of pumping is that the upstroke of the cylinder draws media through the inlet valve, whilst a combination of the working head and application of suction pressure closes the outlet valve.

The reverse occurs when the cylinder is displacing.

The type of valve varies, there are 6 different types, but in the sewage industry, two are commonplace.

These are ball and beak type valves.

Ram pistons need to seal with a small friction interface between a set of seals or packing, and both are normally adjustable to compensate for wear.

The main problem with this type of pump has been blockages of the ball valves, and leaking of the seal / packing after a short time.

These problems have been addressed by Energy Pumps, and have been resolved on our own equipment.

As this type of pump only requires enough energy to actuate the valves and overcome what should be a small amount of friction between the seals and the ram piston, this type of pump is highly energy efficient.

We show an 85% efficiency for the mechanical drive version of these pumps.

2) Diaphragm pumps.

The pumping method is the same as for the ram pumps.

The main difference is that instead of a piston, the primary actuator is a diaphragm, and so no seals are required.

The diaphragm is compressed between an upper and a lower casing, which seals the unit.

Diaphragms take virtually no energy to actuate.

This can easily be done by hand; diaphragm pumps give out effectively as much power as is put in, and so are perhaps the most energy efficient pump type available.

Our in-house tests have shown 87% efficiency at the pump.

Also high reliability and low cost spare parts are a feature of these units.

* Energy costs of automatic desludging pumps.

Based on our experience when replacing other types of pumping equipment, in virtually all cases our kilowatt requirement has been less than half, and in most cases less than a third of the existing installed power.

Therefore, it is straightforward to base a calculation of energy saving costs against what is already installed.

The average running time of a pump on a small or medium sized Sewage Treatment Works is some 6 hours per day.

To make a comparison of energy costs the following criteria can be applied:.

* Small to medium sized sewage treatment works: average motor size running costs: 4.0 kW x 6 hours per day x 365 days per annum @ 6 pence per kWh: (Average) = GBP525 per annum.

Energy diaphragm pump running costs: 1.1 kW x 6 hours per day x 365 days per annum @ 6 pence per kWh: (Average) = GBP144 per annum.

So for each pump installed, an approximate energy saving of GBP381 per annum can be realised.

If this is multiplied by the thousands of pumps installed in this application, then the energy saving per thousand is significant.

The average small sewage treatment works has 2 x primary and 2 x humus tanks, and on larger works this figure is considerably more.

* Large sized Sewage Treatment Works.

The larger STWs normally need higher rated equipment that the small satellite works.

Most applications require a high positive working head to be achieved.

7.5 kW is the average installed power of an auto-desludging pump.

Also, digester feed and thickened sludge transfer is another application to be considered on these sites.

In these cases, we would normally use 5.5 kW as an average motor size.

Our ram piston pump is usually the pump that we would select for most applications on this size of works.

The average running time is 12 hours per day per pump on a large sewage treatment works.

Average motor size running costs: 7,5 kW x 12 hours per day x 365 days per annum @ 6 pence per kWh: (Average) = GBP1,971per annum.

Energy Ram piston pump running costs: 5.5 kW x 12 hours per day x 365 days per annum @ 6 pence per kWh: (Average) = GBP1,445 per annum.

So for each pump installed, an energy saving of up to GBP526 per annum can be realised.

If this is multiplied by the hundreds of pumps installed in this application, then the energy saving is significant.

* Sludge Pipework.

One of the main reasons for higher than should be required energy consumption figures is pipework.

Included are examples of both pump efficiency and pipework sizes to show a comparison between an efficient and an inefficient system.

The main causes of extra working load that pump engineers should size equipment for is the precipitate of fat that forms on the inner walls of the pipework, that reduces the diameter and increases the work effort.

In all too many cases, sludge pipework has been installed with dead zones where grit, stones and the like precipitate.

Also the most common size is 4in or 100mm NB.

In many cases, this can soon reduce to 3in or 80mm NB due to the fat accumulation.

The best reasons for installing 100mm bore pipework is initial cost and that the requirement of some engineers is to try and achieve a self-cleansing velocity.

However, not only has 100mm been installed on the outlet of sludge pumps, this is also installed on the inlet.

6% dry solids sludge will begin to cavitate if drawn at a velocity exceeding 0.3 metres per second.

The following figures show the facts of poor suction pipework design.

Maximum throughput of 6% DS sludge on suction pipework prior to risk of cavitation.

80mm NB pipe: 1.6 l/sec.

100mm NB pipe: 2.5 l/sec.

150mm NB pipe: 5.5 l/sec.

200mm NB pipe: 9.5 l/sec.

Also, the enhanced energy requirement due to the reduced diameter of 100mm / 80mm bore is reflected in the enclosed figures.

The need for smaller bore pipework on the outlet to achieve self-cleansing velocity is from our experience not required.

If sludge is kept at a reasonable thickness, then the homogenous, thixotropic nature will carry along any precipitants like grit etc.

We have also shown that due to the pulsating nature of our type of pump, which gives an increased immediate velocity due to the stop-start action, fat layers within the pipework normally never accumulate to reduce the bore by more than 20%.

A more constant flow pump does not create the peak force or surge required enabling the removal of fat in this manner.

Thus pipework diameters reduced to 40% loss of NB is commonplace in some areas.

A small comment about the necessity to remove precipitation by at least annually jetting out all sludge pipework may be appropriate at this time.

In terms of both energy saving and enhanced pump performance, not to mention less maintenance due to decreased work effort.

The time spent in maintaining an open full NB passage for any type of pump will show good rewards.

Pipework design should always be addressed by all parties concerned when considering an application.

Too often has pipework been installed that does not reflect the best method of transferring sludge in a straightforward manner.

Large diameter 150mm NB should be best practice for flow rates up to 9 l/sec, and 200mm for applications above this is recommended.

The need to maintain as straight a run of pipework as is possible, and removal of unnecessary bends etc should be considered.

Also, removal of dead zones and installation of jetting points are of primary importance; however these are mostly not included, and make for poor upkeep of pumping systems.

The costs of energy are always going to increase.

Fundamental to our future is the conservation of mineral fuels for ourselves and the yet to come generations.

However small a contributions we make to energy saving, be it just switching off a light bulb, will add to the minimisation of expended natural resources upon which we are all so reliant.

The small amounts of saving made, if multiplied by thousands of times are significant and worthwhile aiming for.

Food for thought: A penny saved on energy is two pence gained.

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