Eighteen years old, and still pumping

The Thames Water effluent treatment works at Crawley, West Sussex, uses nine Vogelsang positive displacement rotary lobe pumps to transfer primary sludge from settlement tanks to the next processing stage. Each pump is rated to pump 20m3/h of sludge with about 2m suction lift.

These pumps were installed in 1988 so have been in operation now for more than 18 years, and the only down time has been for planned routine maintenance. There has been no break in service due to breakdowns or any under-performance issues.

For people not used to working in municipal effluent treatment, information about the duty that these pumps perform may not mean very much, so an outline of a typical sewage treatment scheme follows.

Firstly, incoming sewage passes through a fine screen to remove the larger items that people have flushed down toilets, or which have found their way into the drains such as plastics and pieces of wood.

These materials are then chopped up, put into skips and taken to landfill sites.

Incidentally, Thames Water has a campaign in place to encourage customers not to put household rubbish down the drain.

The sewage then flows into channels where heavy particles of grit – a mixture of sand, ash, stones and other small items – will sink. This material is then removed and taken to landfill sites.

The screened sewage flows into large tanks where the remaining heavy solids will sink to the bottom. When fresh sewage or wastewater is added to a settling tank, around half of the suspended solid matter will settle out in about 90 minutes. This collection of solids is known as raw sludge or primary sludge and is said to be fresh before anaerobic processes become active.

Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.

At the Crawley site, these settlement tanks have a sloping bottom and the Vogelsang pumps are connected to off-takes at the low point of the sloping base. This is at a lower level than the pumps so the pumps have to self-prime and draw the effluent up 2m. The separated sludge is transferred to a sludge treatment plant and the remaining liquid – or settled sewage – is treated with bacteria.

One of these pump installations is shown in Fig 1 and 2. Fig 1 shows the external installation adjacent to a settlement tank with a protective cover over the pump and motor. Fig 2 shows the inlet and outlet connections to the pump head and a mechanical pressure relief valve with recycle loop which allows material to be recirculated around the pump if there is any blockage downstream of the pump.

Storage capacity

Typically untreated primary sludge contains 2-8% total dry solids (TS). This primary sludge contains about 3-4% dry solids. The pumps operate for ten minutes every hour on the Crawley site, which has been defined as the optimum time to remove most of the settled sludge without also pumping out too much of the liquid.

The sludge solids consists of grease and fats (6-35% of TS), protein (20-30% of TS), Nitrogen, phosphorous and potash (2.3-7.8% of TS), cellulose (8-15% of TS), Iron (2-4% of TS) and silica (15-20% of TS). The pH will be 5.0-8.0 and the organic acids 200-2,000mg/l as HAc.

This primary sludge is usually grey and slimy and, in most cases, has an extremely offensive odour. Primary sludge can, however, be readily digested under suitable conditions of operation. The specific gravity of these sludge solids is around 1.4 while the specific gravity of the sludge is 1.02.

The quantity of solids entering the WwTW daily may be expected to fluctuate over a wide range. The plant is designed by considering the average and maximum rates of sludge production and the potential storage capacity of the treatment units within the plant.

In this installation, the pumps’ main duty requirements are that they are self-priming and can operate with a suction head, as well as the duty requirements that they can pump semi-solid waste reliably and operate in a stop/start mode. It is also important that the pumps operate reliably.

Different types of pumps can be considered for this application – centrifugal, progressive cavity, reciprocating piston pumps and rotary lobe. The pumps used on this site are rotary lobe type pumps.

Rotary lobe pumps offer several benefits. They are self-priming to 8m and, although not a requirement for this particular site application, they can also run dry for a period of time. We guarantee a 30-minute dry-running time for their rotary lobe pumps, compared with centrifugal and progressive cavity pumps that have a very short or no dry running capability.

The rotary lobe pumps installed are fitted with the original design of rotors rather than the modern HiFlo pulsation-free rotors invented and developed by Vogelsang, and now used as standard on all its rotary lobe pumps, such as those installed on the Thames Water Reading site.

This pulsation-free operation is achieved by putting a twist into each rotor lobe that then achieves a +/-0% pulse compared with the +/-3% pulsation from a progressive cavity pump.

This innovative development in rotary lobe pump design was developed in the Research and Development facility at Vogelsang’s German factory. More than 70 staff work there on new pump and applications development department alone.

A further benefit of these rotary lobe pumps is that the routine maintenance activities can be carried out on the pump head without disconnecting any pipework or removing the pump from its installed position.

Removing the front cover of the pump head allows access to the rotor lobes, mechanical seals and wear plates. So carrying out a full routine service on a pump head will typically take one person less than one hour and no heavy lifting is involved.

The typical expected life for any pump in this type of application used when calculating whole life costs is anywhere up to 25 years. The Vogelsang pumps in this application have operated for more than 18 years now with no breakdowns and show no sign of stopping.

Dean Perry, Thames Water area engineering supervisor responsible for these pumps, says: “There is not a lot to say about them, you want pumps to do their job and work reliably and these ones do just that.”

Feed pressure

On another Thames Water effluent site at Reading, Vogelsang HiFlo rotary lobe pumps are in use at a later stage in the process. Many people will have seen this site when driving along the M4. After being belt thickened and blended, the primary and secondary activated sludges enter the Alpha Biotherm pasteurisation plant’s thermophilic aerobic digester through a macerator, where they are held at 70°C for one hour.

The pasteurised sludge is subsequently pumped to four bottom-fed, egg-shaped, anaerobic digesters, each some 20m high. These are the first of their kind in the UK, constructed entirely from reinforced concrete with external silver cladding. The heat required for the pasteurisation and digestion processes is provided either by digester biogas through CHP engines or three dual fuel (biogas/diesel) boilers.

Originally, centrifugal pumps were used to pump sludge to the pasteuriser plant’s digester. At this stage in the process, the sludge contains 8% dry solids and the centrifugal pumps failed because they could not maintain the required flow and pressure.

These centrifugal pumps were replaced by the Vogelsang rotary lobe pumps. The pumps in this application have a positive feed pressure and have to operate for 40 minutes every hour. The space available for the installation was quite limited so the pump motors were mounted above the pump heads.

This effluent treatment works was opened in January 2005 and Vogelsang and Thames Water are looking forward to writing an article about the performance of these pumps in 2024.

Sludge is a generic term for solids separated from suspension in a liquid by a variety of processes. Most commonly sludge refers to solid waste extracted in the process of sewage treatment; the term sewage sludge common.

However, industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Some examples of other “sludges” that are typically processed using the positive displacement rotary lobe pumps are:

• Molasses – used in animal feed applications
• Animal fats etc in processing plants
• Potato effluent, including peelings, in potato processing plants
• Dairy waste for processing into animal feed

Using rotary lobe pumps in these, and other, applications can provide a number of benefits. For example, due to the compact size of these pumps, they can be installed in locations where some other types of pumps could not be used, and the construction costs of pump houses can be reduced because they occupy a third of the space of progressive cavity pumps.

Vogelsang’s pumps use less energy than centrifugal pumps, offering a cost saving. And maintenance is simplified with the pump being left in-line while maintenance is performed. One additional feature of rotary lobe pumps is that the direction of pumping may be reversed by simply reversing the direction of rotation of the motor, the piping to the pump does not need to be changed.

In the applications discussed here, that is not a specific benefit but in some transfer

applications it can be very useful to pump in either direction as required.

Toby Clarke is managing director of Vogelsang UK.

T: 01270 216600

E: toby@vogelsang.co.uk