Stelton, H.1, Depala, P.1, Ford, B.2, 1Anglian Water, 2Inverter Drive Systems(free)
Primary and co-settled sludges are commonly removed from settlement tanks using timed, pumped, auto-desludge systems that are neither sensitive to the variable flux of settleable influent material nor sense the thickness of the sludge removed, resulting in thin sludge being produced which requires significant thickening before digestion and are expensive to transport by road tanker. The patented reactive pump controller, developed by IDS, senses the sludge thickness by monitoring the torque of the desludge pump’s motor. A simple calibration of torque against %DS allows desludging to be stopped when sludge becomes too thin with a >30% reduction in sludge volume demonstrated without compromising PST performance. Within Anglian Water increasing the thickness of sludge exported from STWs by 1.5%DS represents a saving of over £4m/yr.
Keywords: Desludge optimisation, pump torque, sludge thickness, tankering
The Anglian Water serves the east of England, a region that covers 20% of the land area of England and Wales with over six million customers. To treat the wastewater produced, Anglian Water has over 1100 sewage works diffusely spread sites across the region. These sites produce 2.2million m3 of sewage sludge per year which has to be transported by road tanker to one of nineteen sludge treatment centres. This poses obvious logistical challenges and significant transport costs, which stand at about £12M/yr.
The sludge tankers are usually provided by Anglian Water’s Road Tankering Service. These can carry 29m3 of sludge and use HACH Solitax probes to record the thickness, or percent dry solids (%DS), of the sludge. Data from each collection is added to a central database giving a picture of the volume and thickness of sludge produced at each sewage works. A review of this information showed the average dry solids content of transported sludge to be 2.7%(Anglian Water 2012), the rest is water. This figure includes all kinds of sludge; primary, secondary and co-settled. Metcalf, Eddy (2003) state that primary and co-settled sludges can be removed from the PSTs at 4-6% dry solids. Achieving an average of 4%DS could reduce the volume of sludge transported across the Anglian region by up to 33% (~700,000m3 ), saving £4M/yr. However, a proportion of the sludge transported is produced in small extended oxidation plants with no primary settlement; outside the scope of this work.
Optimising Removal of Sludge from Primary Settlement (PS) Primary settlement aims to remove readily settleable solids from the wastewater treatment stream. It does this by introducing screened sewage and returned liquors in to the PST at a low velocity and turbulence which allows gravity to settle the solids to the bottom of the tank (Metcalf, Eddy 2003). A key part of the process is efficiently removing the blanket of settled solids (sludge), as if left in the tank too long it risks creating an anaerobic environment in which gas is produced, causing the settled blanket to rise. A rising sludge blanket can increase the solids content of the PST effluent, raising the strength of the settled sewage treated by the secondary process and causing a deterioration in final effluent quality (Metcalf, Eddy 2003).
The sludge removal process is known as desludging and can be performed by opening a valve at the base of the PST or using a pump to remove the sludge. The use of an automated pumped system is increasingly common as sites are visited less frequently. The desludge pump is usually controlled by a timer system which starts the pump at set times and for a set period of time each day; for example five pumps a day for fifteen minutes each. The lengths and timings of these pumps are calculated using the flow rate of the desludge pump and estimating the amount of sludge needing to be removed daily, but given the negative impacts of inadequate sludge removal they are often operated conservatively, leading to the production of thin, low %DS sludge. Preventing the production of this sludge would:
Reactive Pump Controller (RPC) This system uses a standard ABB inverter to control the desludge pump’s motor. The pump is operated at a constant speed while the built in, automatically generated motor model measures the % of maximum torque that the desludge pump’s motor is producing. As the sludge thins, its viscosity drops, causing the torque applied to the motor to fall. If this relationship between %DS and %torque is strong enough it should be possible for the RPC to monitor the thickness of the sludge being pumped and ensure the pump stops before the sludge gets too thin.