Frequently Asked Questions
Q. How can you minimise the headloss through a Grit King®
Separator?
A. The design programme will calculate the headloss. We recommend
that the inlet velocity is between 1.0 m/s and 1.5 m/s at
peak design flow and it may be necessary to adjust the inlet
pipe diameter to maintain this. The system headloss will vary
accordingly and it will be in proportion to the square of
the inlet velocity.
Q. How do you decide whether to us a gravity system or a pumped
system?
A. In most cases it is obvious which system is to be used.
However it will be necessary to carry out detail hydraulic
calculations to assess if a gravity system is appropriate.
The headloss at the inlet to a Grit-CleanseTM is 1.81 m above
the centreline of the inlet pipe at the design flow of 20
l/s and there is a free discharge at the inlet to a Series
3 Classifier.
Q. Can the Grit King® Process be used for other applications
other than at Sewage Treatment Works inlet works?
A. Yes. We would request that the Client provides a grading
curve and a SG for the material to be separated and this would
provide the information for our Research and Development department
to size the unit. If the material is colloidal or very fine
then vortex separation may not be appropriate.
Q. What is the smallest below ground internals only installation
that a submersible pump will fit into?
A. In the UK we ensure that the minimum diameter of the grit
pot is 900 mm so that a wet well submersible pump will fit.
The diameter of the Grit King® Separator with this grit
pot size is 2.10 m.
Q. What dictates the height and position of the anti siphon
valve on the pump feed to the classifier?
A. There are two reasons for raising the feed pipe above
Top Water Level (TWL) in the separator. Firstly to control
the flow and secondly to prevent siphoning. A detailed hydraulic
calculation is required to calculate the hydraulics and size
the pump.
Q. What materials are used for the construction?
A. In the UK Stainless Steel 304 or 316 are used although
other materials could be utilised.
Q. What is the effect of flows higher or lower than peak design
flow on the performance of the separator?
A. The unit will accept higher or lower hydraulic flows than
the Peak Design Flow however if the flow is higher than the
design flow then the headloss will increase and the performance
reduce. If flows are very low then organics will settle out
with the grit.
Q. When do you recommend using a Grit-Cleanse™ classifier
and not a Series 3 Classifier?
A. If the ratio of Peak Design Flow to Dry Weather Flow
(DWF) is greater than 3 to 1 we recommend the use of a Grit-Cleanse™ Classifier and not the Series 3 Classifier. This is to enable
the grit entrained sewage to be put through a secondary
separation stage prior to the classifier Archimedean screw
thus reducing faecal organics discharged in the grit.
Q. Do you use the spray bar provided on the classifier above
the Archimedean screw for grit washing?
A. Although there is a grit spray bar provided on the Series
3 Classifier it is not connected to a water supply as standard.
We do connect the spray bar on the Grit-Cleanse™ to a potable
or washwater main as standard.
Q. Do you require access to the top of the Grit King®
Separator?
A. There are no moving parts at the top of the Separator
and therefore access is not required at high level for routine
maintenance.
Q. Is the system able to operate on unscreened sewage?
A. We recommend that the Grit King® Process is operated
after screens as in our experience the influent at a sewage
works can contain very large objects. Also if we are operating
with a pumped system rag balling can create a problem. In
special circumstances and depending on the specific circumstances
we would consider installing upstream of screens but this
would not be our preferred option.
Q. Does the system ever block?
A. It is important that the recommended control philosophy
is followed and under these circumstances blockage problems
are unlikely. If the control system is switched off for a
prolonged period with flow continuing to run through the Separator
then grit will continue to be collected in the grit pot and
could build up to a level where it would be difficult to mobilise
again. It is therefore important to be able to isolate the
separator when the process control is switched off. Hydro
do offer at an additional cost a bolted hatch to the bottom
grit pot.
Q. You offer water or air backwash within Grit King® Separator.
Which is preferable?
A. Either. The decision will depend on the availability and
cost of water on site. Some sites have remote inlet works
without a close water supply and in this case the supply of
an airset may be preferable to providing a new water supply
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