Chemical and Process Engineering




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A S Jessup-Bould:



Waste Water Treatment

End Client: Yorkshire Water




Process analysis and hazardous area classification for a number of treatment works.


During my limited exposure to Yorkshire Water’s AMP3 contract with Earth Tech Engineering I undertook process performance calculation checks for a variety of Yorkshire Water waste water treatment works. I also checked operating information, data sheets, hazardous area classification reports as well as participating in an assessment of BAFF technology.



Example: Snaith WwTW

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Summary of Process

There are two screens at the inlet works, a duty automated and a standby screen that requires manual raking. Cleaning of the automated screen is initiated from a measurement of differential sewage level across the screen.

Captured screenings are transferred through a launder channel to a screenings washing and de-watering unit that consists of a macerator pump and a liquid separation stage. The de-watered screenings are discharged into a skip and the wash-water liquors are returned to the main sewage flow, via the waste liquors pump station.

Grit removal is by a Jeta type grit trap. Settled grit is transferred to a screw classifier by a timer controlled airlift pump arrangement. The screw classifier de-waters and cleans the grit that is then discharged into a skip.

Flow to full treatment is controlled by a modulating penstock in combination with a magnetic flow meter. The vertical position of the door of the penstock is controlled by the flow meter to ensure that only flows up to and including Full Flow to Treatment (FFT) receive primary and secondary treatment. Flows in excess of FFT are diverted to a storm tank by a storm weir, located in the flow splitter chamber.

When the flow, measured by the FFT flow meter, has fallen below 1.6 Dry Weather Flow (DWF), a centrifugal storm return pump, located in a dry well, returns the storm-water to the main flow (flow splitter chamber).

Flows up to and including FFT are divided equally between two Dortmund type primary tanks. The co-settled primary-humus sludge is transferred from the primary tanks to a picket fence thickener to give a sludge with 5% DS. The supernatant gravitates to the waste liquors pump station for return to the head of the works. Progressive cavity pumps (duty/standby) transfer the thickened sludge to the sludge storage tank. The picket fence thickener is vented to atmosphere through a passive carbon filter.

Under normal operation, the settled sewage is divided equally between two bio-filters with random plastic media. The sewage gravitates from the filter distribution chamber to two motorised distributors. Splash plates distribute the flow evenly over the surface of the filter.

The bio-filter effluent gravitates to a pumping station. Under low flow conditions, as detected by the FFT flow meter, a recirculation pump maintains the minimum filter wetting rate. Duty/standby feed pumps transfer the bio-filter effluent to the existing humus tanks for secondary treatment.

The humus solids are removed by settlement in four Dortmund type humus tanks. The settled sludge is transferred to the return liquors pumping station by an automatic airlift de-sludging system.

Final effluent gravitates from the humus tanks to the outfall. A supply of final effluent is taken to provide wash-water for the inlet screen and for site hydrants.



Project Scope

Additional Inlet Screen and Bypass




Automatic Storm Tank Cleaning




Improvements to Filter Bed Recirculation System


Specific responsibilities

Assessment for current and future performance




Process Calculations


Process Parameters

Formula A Flow: 177 l/s




Full Flow to Treatment: 69.1 l/s