Masse, A-L.1
Abstract
Digestion performances are impacted by the quality and the stability of the feed to the digesters. At Reading sewage treatment works STW (210,000PE), unthickend sludge is back mixed in the thickened sludge pump located at the end of the belt thickeners to reach the target dry solid (DS) set by the operator. Monitoring has shown that in this configuration torque of the thickened sludge pump and pressure in the pipework were better parameters to use to control the system than DS meter. At Swindon STW (230,000 PE) a similar system to control the sludge stream was designed.
The run time of the primary settlement tanks desludging pumps is based on the thickness of the sludge extracted. This can produce a sludge of consistent DS, allowing the downstream sludge thickening process to run more consistently. To optimise the polymer use and decrease the operational costs, the polymer dosing rate to the thickeners is based on the flow to the thickening unit, on the DS of the influent and on the total suspended solids of the filtrate as this value changes more rapidly than the sludge DS, allowing a more efficient control. To stabilise the DS of the sludge going to the digesters, a backmixing system is being implemented.
Keywords
Automatic desludging, backmixing, controlled sludge stream, polymer dosing, torque control, turbidity control.
Introduction
The 2020 sludge deregulation is a good opportunity for water companies to re-think their sludge strategy. Being able to free headroom to accept more imports at a reasonable cost will be a key strength in the competition market and it will increase the biogas generation.
Simultaneously to this deregulation, the water companies will have to consider population growth, (and this is particularly true for urban areas) which will bring more wastewater, and consequently generate more sludge that will require treatment. An optimisation of the current assets can make them able to treat larger volume of sludges, and can also extend their life time.
To be ready for the future, wastewater treatment plants (WWTP) will need to increase their automation. With regards to the sludge stream, having a reliable control system able to adapt to changes in the dry solids (DS) of the sludge will reduce the polymer consumption. It will also ensure a stable sludge going to the digester, which will maximize the biogas generation. With a good control of the sludge %DS, blockages in sludge line will also be avoided. The increase automation of the plant will give more time for the operators to focus on others aspects of sludge management, and will also decrease the out of hours calls.
