Separate the wheat from the chaff: What should a good grit washer be able to do?

When we launched our COANDA Grit Washer in 1994 and started its success story it was not long before competitors came up with their own grit washers. In the beginning, most of them fortunately did not know what exactly to think about our product. But as the demand from the market increased all our ’friends,’ one after the other, became active, at first in Germany, the ‘birthplace’ of this innovation, later also abroad where there are often no scruples to copy as we all know well.

But no other grit washer would even come close to being comparable with our COANDA Grit Washer in terms of function and efficiency. There are some factors that always must be taken into account to evaluate the quality of a grit washer: separation efficiency, washing efficiency (quality of washed grit), wear resistance. HUBER has meanwhile sold almost 2,000 COANDA Grit Washer units.

Separation efficiency:

Do you want to separate grit or waste space with containers?
Do you want to separate grit or waste space with containers?

The separation efficiency of a grit washer depends on the feeding arrangement, surface loading and effluent quality. But what needs to be available as well is a separation chamber, i.e. a water volume which allows the solids (here grit and organics) to settle quickly. An efficient grit washer achieves a constantly high separation of organic material. We have carried out measurements on our own test stand. Both our results and external measurements at a university prove that central feeding through a vortex chamber and a COANDA Tulip provides optimal preconditions for good separation results. This advantageous feeding arrangement, combined with a large water surface (low surface overflow rate) and a circumferential overfall weir, significantly reduces the velocity from the inlet to the outlet weir so that grit particles > 200 µm are reliably separated. The use of an overflow weir avoids the suction effect, i.e. the acceleration of outflow water. However, even the best dimensioned grit washer will not achieve satisfactory separation results if there is not enough room for sedimentation. It is generally known that a lot of heavy organic particles settle with the fine sand which can only be ‘blown out’ of the plant if the velocity is increased. But this phenomenon would lead to the loss of the fine sand spectrum from 200 to 350 µm. With the use of our patented organics discharge solution heavy organic particles are simply removed at the end of the washing process to guarantee an always optimal room for sedimentation for all particles.

Washing efficiency:

COANDA Grit Washing Plant RoSF 4: Superior function and grit yield
COANDA Grit Washing Plant RoSF 4: Superior function and grit yield

Although our patent gives us comprehensive copy protection and prevents completely uncontrolled copying, each of our competitors tries to somehow get around one distinct patent claim. But, in the end, all grit washers have a conical tank with an inclined grit removal screw, a pressure probe and an organics discharge installed on the tank, without knowing what it is actually good for. Some have tried to replace the stirrer with pressure air without taking into account that the pressure probe will not deliver continuously stable measurements any more. Former colleagues have tried to install only distribution channels in the wash zone where we use our patented perforated plate bottom solution. Their system cannot create a steady fluidized bed with the result that their plant sometimes washes but sometimes by far fails to achieve the guaranteed loss on ignition of < 3%. Others have tried their luck with a fast running stirrer (40 rpm) to achieve fluidisation and wash the grit. At such a speed, however, the pressure probe in the tank is not able to deliver reliable measurements and there is also enormous wear.

Most competitive products do not at all care about a homogenous fluidized bed but stir the settled solids without interruption. This may work as long as there is hardly any grit to wash but if there is more grit the removal screw will discharge the grit along with the organics like a classifier or the grit will leave the plant via the outlet, and it all ends up again with ‘There is no grit!“. There is a litany of how competitors try to wash grit and it would go beyond the scope of this article to mention them all, except that some of our ‘smart’ international competitors copy our COANDA Grit Washer 1:1 at least in appearance but unfortunately have neither the required know-how or any clue about proper control of grit washing processes.

Wear protection:

Grit is a very critical material and causes wear especially when it is moving. After all, complex grit traps are used on many sewage treatment plants with good reason to improve the service life of pumps and pipelines. That is why we attached great importance to wear protection right from the beginning. We avoid high relative speeds of the rotating aggregates (stirrer, screw) of our COANDA Grit Washer and the grit removal screw has high-quality journal bearings on both ends (silicon carbide / chilled cast iron bearings) so that the auger does not scratch the trough walls. Welded conveyance bars, plastic shells or cheap bearings should have no place in grit handling systems. The stirrer in the COANDA Grit Washer always operates in a homogenous grit fluidized bed so that the fluidized grit offers only little resistance to the stirrer arms. But as some wear cannot completely be avoided all stirrer arms of our COANDA Grit Washer units are made of 30 mm diameter full material. Due to the low relative velocity and the massive materials used we can guarantee a service life of about 20,000 operation hours.

Conclusion:

If you are interested in buying a grit washer, check offers for the features a good grit washer should provide:

  • low surface overflow rate
  • circumferential overfall weir
  • separate organics discharge
  • grit washing and removal even during feeding
  • washing function / fluidisation
  • grit removal screw supported on both ends
  • high solids throughput

Rely on our expertise in grit washing. You can be sure to be in best hands (pic. 2). We do not want to sell some hundred kilos of stainless steel to our customers but provide them with a technical solution that really helps them to save a lot of disposal costs. We are true to our word.