Controlled Sludge Stream at Swindon STW: Initial results

Massé, A-L. and Shana, A. Thames Water, UK



Sludge treatment accounts for over half of the operational costs of the wastewater treatment. As the
majority of the sewage treatment works (STW) have been build decades ago, they do not always offer
a way to control the sludge stream accurately. Modifying existing assets to make them run at the best
of their capacity can reduce operational costs and increase the biogas generation. At Swindon STW a
new automated control system applied on the sludge stream is currently being implemented, and the
result of this is presented in this paper. The first step consist of desludging the primary settlement tanks
(PST) based on the sludge dry solid (DS) instead of using a timer. A second module adapt the speed
of the thickener feed pump so that the solid flow to the thickener stays constant over time. The third
step optimizes the polymer dosing in real time, based on the filtrate turbidity. As this generates thicker
sludge, a back-mixing system is also provided to ensure the sludge has a consistent DS when going to
the digester. This paper will present the initial results.

With population growth, climate change and stringent new regulation coming into force such as Water
2020 the wastewater industry will be facing important challenges in the next years. It is becoming
increasingly important to be able to operate existing assets at the best of their performance to make the
most of the resources coming in while minimizing energy consumption and operational costs such as
high chemicals consumption. This can be achieved by the implementation of real time control (RTC)
solutions on different parts of the plant.

Increased automation on the sludge stream at Swindon sewage treatment works (STW) has recently
been implemented. It consists of different modules that increased process stability and resilience. The
first automated system works to de-sludge the primary settlement tanks (PSTs) based on sludge
thickness instead of a timer. There are two main objectives associated with this new control. The first
one is to provide a better control of the blanket level in the PST; which will reduce the amount of solids
load being transferred to the aeration lanes, and therefore contribute to reduced aeration costs and
sends more stable sludge to the thickeners. The second objective is to reduce sludge retention time in
the PSTs to improve primary sludge thickening and provide a fresher sludge to the digesters for
enhanced biogas generation. The other automated systems installed or being installed at Swindon are
looking into optimising polymer dosing in real time for the thickening and dewatering equipment (either
based on the thickness of incoming or thickened sludge, or on the filtrate turbidity), as well as providing
a back-mixing solution. This report focuses on the results obtained after PST desludging automation. The control philosophy pre and post automation are presented as well as safeguard systems that
were included in the control software. Desludging pump running times and others relevant parameters
are compared pre and post automation to quantify the benefits

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