By far the most popular pump type used for leachate extraction and pumping systems is the submersible centrifugal (vortex) type pump, and in particular in the UK the FLYGHT pumps model range has been the most popular, and generally still is. Similar competing versions are manufactured by a number of companies such as KSB, and Hydrostal.
Most waste management industry leachate extraction system pumping systems have been historically specified by engineers familiar with water industry specifications, and they require any pump provided under the specification to be able to handle (or pass) a 60mm, 80mm, or 100mm sphere at pump flow rates which are run with flow rates which are (again according to research carried out on avaoiding sewer blockages) self-cleansing.
Given the requirements for a large solids passing capability means that the Contractor supplying this equipment will select a sewage application type pump. The pumps while complying with the specification, will be quite poor on energy efficiency, and while this specifying policy does result in the provision of pumpsets which rarely block the solids passing criteria may not be as important as the specifier assumes.
Certainly, while a landfill is open and a sbmersible pump is placed at the corner of the active landfill there will be plenty of opportunity for large particle size items to be entrained into the pump volute, but so will a great variety of other materials, such as plastic items, ropes and fibres. In such cases the size of particles passing may have little bearing on the ability to pass such objects through the pump and leachate pipeline subsequently.
From the pump designer’s point of view choosing a particular impeller for a particular wastewater pumping application always entails a compromise between freedom from blockage, efficiency and wear characteristics.
The mechanical engineer will be aware that the two areas in a pump which are particularly prone to clogging are the wear-ring and the leading edge of the impeller. Impellers may be designed in tow forms which are closed and open. In those pumps which have a closed impeller, fibrous materials and small plastics and personal hygiene articles tend to deposit themselves in the gap (clearance) between the pump casing (volute) and the impeller.
Therefore, the aware specifier for leachate extraction pumping systems and pumps in the early life of a landfill should be considering the presence or otherwise of such materials in the flow and specifying for these materials.
However, once a landfill is completed and capped and the leachate underdrainage system is in use, the pumps will not normally be subjected to such objects in the flow, as the landfill (once capped to current EU requirements) will be generating leachate solely from the installed leachate underdrainage system which will effectively filter out all solids in the leachate other than what may be drawn through the pipe system which resemble “French Drains” and in essence filter the leachate before it reaches the extraction well into which the pump will be inserted.
In landfills where sand is used as the leachate drainage layer, the extraction pump may encounter high concentrations of sand and grit.
In such, cases where continual sand and grit entrainment occurs any wear-ring used in the pump’s construction will be abraded by the sand and wear away, and the resulting increased recirculation of flow from the high-pressure side back to the suction in the volute will add to the risk of accelerated wear. However, we are told that there is no concern with vortex type pumps. The design of vortex impellers requires no no wear-ring, eliminating this potentially pump downtime cause. In vortex type pumps there are no spaces where solids might accumulate and therefore this impeller type offers significant advantages in terms of operational reliability.
The other area where problems occur is the impeller eye and the position and shape of the leading edges of the vane. It is here that pieces of textile materials, especially long fibres, sundry household waste, and even small broken pieces of wood can become fixed, reduce the flow, and raise pumping energy costs.
The best impeller designs are shaped so that fibres depositing on the vane are flung off as rapidly as possible. Another factor is the number of vanes in a pump (eg if multi-stage clean water pumps are used) because this and the size of the pump dictates the free passage.