Proceedings

The assessment of risk in the selection of post THP sludge liquor treatment

Nemtsov, D., Poznanska-Chapman, K. and Hopkins S., Mott MacDonald Bentley, UK

(free)

Abstract:
The conversion of a Dŵr Cymru Welsh Water WwTW to a regional Sludge Treatment Centre (STC)
using a Thermal Hydrolysis Process (THP) will result in an approximately threefold increase in sludge
throughput. Thermal hydrolysis of sludge has significant impact on the quantity and composition of
sludge liquors, in particular: Ammoniacal Nitrogen (Amm.N), COD, phosphorus and inhibitory
substances. The importance of defining the impact of the predicted increase in Amm.N load on the
main process at the WwTW was identified during the design development. A new Liquor Treatment
Processes (LTP) will be implemented to treat the increased Amm.N loads.

This paper explores the methodology for determining the liquor composition and the risks associated
with selection of an appropriate LTP. The risks explored are: uncertainty in Amm.N and P load, impact
of P on the main process and reliability and robustness of LTP.

Several commercially available Liquor Treatment Processes were considered: Anammox,
nitritation/denitritation and nitrification/denitrification. The identified risks are compared to potential
OPEX savings.

Introduction:

Dŵr Cymru Welsh Water’s (DCWW) strategy for reducing carbon footprint and increasing operational
efficiency includes the conversion of an existing WwTW into a large Sludge Treatment Centre (STC)
treating sewage sludge from across the North Wales region. The existing sludge treatment process
comprises anaerobic digestion (AD) with lime addition; this is to be converted to Advanced AD (AAD)
with THP and also Biomethane to Grid. As a consequence of the conversion, not only will there be an
increased amount of sludge to be treated, but also significant change in quantity and quality of sludge
liquors which will impact upon the performance of the WwTW.

This paper focuses upon the risks associated with treatment of sludge liquors and how these are
balanced against the strategic objective of increasing operational efficiency by reducing costs.
Figure 1 shows the geographic relationship between the new STC where the AAD plant is to be
developed and eight dewatering centres where sludge is screened and dewatered for export to the
STC. Numerous satellite sites export liquid sludge to the dewatering centres and STC.

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